A 6200-Year Environmental History of the Pearl River Marsh, Louisiana. Xu Li Louisiana State University and Agricultural & Mechanical College

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LSU Historical Dissertations and Theses Graduate School

1994 A 6200-Year Environmental History of the Pearl River Marsh, Louisiana. Xu Li Louisiana State University and Agricultural & Mechanical College

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University Microfilms International A Bell & Howell Information Company 300 North Zeeb Road. Ann Arbor, Ml 48106-1346 USA 313/761-4700 800/521-0600

A 6200-year environmental history of the Pearl River Marsh, Louisiana

Li, Xu, Ph.D.

The Louisiana State University and Agricultural and Mechanical Col., 1994

UMI 300 N. ZeebRd. Ann Arbor, M I 48106

A Dissertation

Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in Partial fulfillment of the requirement for the degree of Doctor of Philosophy

The Department of Geography and Anthropology

by Xu Li B.S., Nanjing University, China, 1983 M.S., Chinese Academy of Sciences, 1986 May 1994

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. To my wife Kewen

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGEMENTS

I would like to thank my advisor, Dr. Kam~biu Liu, for

providing me with the crucial sediment core sample, C-14

dating funding and all the laboratory facilities for pollen

and diatom analyses in this study. He also reviewed and

edited the drafts of this dissertation. I am forever

indebted to his enlightening guidance during my study in

the Louisiana State University.

I also sincerely thank my committee members, Drs.

Bruce Williamson, Nina Lam, Gregory Stone and Keith

Henderson for their sparkling ideas, useful suggestions and

kind supports in completing this study.

Dr. Michael Sullivan of the Mississippi State

University provided important justification and valuable

critics for the diatom species identification. His

assistance is greatly appreciated.

Dr. Gregory Stone offered great help in providing me

with the persons names whom I might contact to get

available data. I also deeply appreciate his untiring

tutoring for my study.

Miriam Fearn offered me the valuable Loss-on-Ignition

data file for Pearl River Marsh core which constitutes one

of the most important parts of this research. She also

patiently taught me to use the TILIA software package to

draw all the pollen and diatom diagrams of this

dissertation. Kari Gathen and Miriam Fearn let me access

iii

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the analytical result of the Horn Island lake sediment core

which has proved to be very important for testing the

hypothesis of this study.

Steve Underwood of the Department of Natural

Resources, Louisiana, provided me with the saltwater

intrusion data associated with Hurricane Andrew. His

information enhanced the precision of my sampling targets

in the most recently hurricane affected area of coastal

Louisiana. Drs. Denise Reed and Quay Dortch of the

Louisiana University Marine Consortium also provided me

with information about Hurricane Andrew and other storm

related seawater intrusions to the coastal marshes. Their

help is all important to my study.

Michael Parsons of the Louisiana State University and

Xuan Li of the University of New Orleans generously let me

use the reference books for diatom identification. Five

field trips to collect surface samples from the coastal

marshes and swamps were successfully conducted with the

help of my friends Baoshun Fu, Yisheng Song, and my wife

Kewen Huang. Their friendship is deeply appreciated.

This study is supported by a National Science

Foundation research grant (grant number: SES-9122058) to

Dr. Kam-Biu Liu and a research grant (grant number: 5197-

93) of the Geological Society of America to me.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS

ACKNOWLEDGEMENTS...... ill % LIST OF TABLES...... vii

LIST OF FIGURES...... viii

ABSTRACT..... J...... xi

CHAPTER 1. INTRODUCTION...... 1

2. LITERATURE REVIEW...... 6

3. GEOGRAPHIC BACKGROUND OF THE STUDY AREA...... 30 1) Settlement History...... 30 2) Geomorphology...... 30 3) Climate...... 31 4) Hydrology...... 31 5) Vegetation...... 33

4. METHODOLOGY...... 44 1) Pollen Analysis...... 46 2) Diatom Analysis...... 47 3) Loss-on-Ignition...... 48 4) C—14 and Cs-137 Dating...... 48 5) Discriminant Analysis...... 49

5. MODERN POLLEN RAIN IN THE PEARL RIVER MARSH..... 51 1) Provenance and Distribution of Main Pollen Taxa...... 51 2) Discussion...... 62 3) Discriminant Analysis...... 63

6. MODERN DIATOM DISTRIBUTION IN THE PEARL RIVER MARSH...... 71 1) Freshwater Diatoms...... 72 2) Marine and Brackish Water Diatoms...... 82 3) Discussion...... 85

7. POLLEN ANALYTICAL RESULTS OF THE PEARL RIVER MARSH SEDIMENT CORE...... 87 1 ) Pollen Zone I ...... 2) Pollen Zone 3) Pollen Zone Ill...... 4 ) Pollen Zone IV...... 5 ) Pollen Zone 6 ) Pollen Zone 7 ) Pollen Zone VII...... 8 ) Pollen Zone VIII......

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 9) Pollen Zone IX...... 97 10) Pollen Zone X ...... 97 8. DIATOM ANALYTICAL RESULTS OF THE PEARL RIVER MARSH SEDIMENT CORE...... 99 1) Diatom Zone 1...... 102 2) Diatom Zone II...... 102 3) Diatom Zone III...... 103 4) Diatom Zone IV...... 103 5) Diatom Zone V ...... 104 6) Diatom Zone VI...... 105 7) Diatom Zone VII...... 105 8) Diatom Zone VIII...... 106 9) Diatom Zone IX...... 106 10) Diatom Zone X ...... 107

9. SEA LEVEL CHANGES DURING THE PAST 6200 YEARS 108 '1) Hypothesis...... 108 2) Sea Level Changes...... 110 3) Discussion...... 121

10. FLUVIAL DISCHARGE CHANGES DURING THE PAST 5900 YEARS...... 128 1) Hypothesis...... 128 2) History of Fluvial Discharge Changes...... 129 3) Discussion...... 137

11. HURRICANE ACTIVITY IN THE LITTLE ICE AGE...... 145 1) A Review of the Little Ice Age...... 145 2) Hypothesis...... 148 3) Hurricane Andrew as a Modern Analog...... 154 4) Pollen and Diatom Analytical Results...... 160 5) The Mvrica Rise...... 167 6) The Study of Horn Island...... 170 7) Historical Hurricane Records in the Gulf of Mexico Coastal region...... 175

12. CONCLUSION...... 178

BIBLIOGRAPHY...... 183

APPENDIXES A: Main pollen types in the Pearl River Marsh area ...... 196 B: Main diatom types in the Pearl River Marsh area ...... 198

VITA...... 208

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF TABLES

2-1. Dates of the 'Homeric Minimum' cold period in North America and other parts of the world ...... 14

3-1. Climate data at Bay St. Louis, Mississippi, east of the Pearl Riv-er Marsh...... 32 5-1. Standard canonical discriminant function coefficients for the surface samples of the Pearl River Marsh...... 66

5-2. The most and the second most probable vegetation types for the surface samples of the Pearl River Marsh as predicted by discriminant analysis...... 69

7-1. C-14 dates of the Pearl River Marsh core...... 90

7-2. The most and the second most probable vegetation types for the samples of the Pearl River Marsh core...... 92

11-1. Saffir/Simpson hurricane scale with central barometric pressure...... 151

12-1. Summary table of the history of environmental changes in the Pearl River Marsh area...... 179

vii

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OP FIGURES

1-1. Map of the Pearl River Marsh and the location of the sediment core, PR1...... 4

2-1. Diatom diagram from Lianvatn of Frosta, Norway...... 9

2-2. Sediment column, C-14 dates, and percentage diagram of different diatoms found in the Shine profile, Puget Lowland, Washington...... 10

2-3. Stratigraphy cross section in Cooper River-Grove Creek area, South Carolina...... 12 2-4. Mean sea level curve for southwest Florida based on beach ridge study...... 13

2-5. Diatom diagram of Otter Harbor, Cape Breton Island, Nova Scotia...... 15

2-6. Simulated annual precipitation (P) and precipitation minus evaporation (P-E) for the last 18000 years in eastern North America...... 19

2-7. Pollen diagram of Bigbee, eastern Mississippi...... 21

2-8. Oxygen Isotope measurements of a single species of ostracod from a 7.7 m core taken from Lake Miragone, Haiti...... 25

2-9. Pollen diagram of San Joaquin Marsh, southern California...... 27

3-1. Monthly water and sediment discharges of the Pearl River at Bogalusa station, MS...... 34

3-2. Vegetation map of the Pearl River Marsh area...... 35

3-3. Representative photograph of the swamp forest in the Pearl River Marsh area...... 37

3-4. Representative photograph of the fresh marsh in the Pearl River Marsh area...... 40

3-5. Representative photograph of the brackish marsh in the Pearl River Marsh area...... 42

3-6. Representative photograph of the brackish marsh in the Pearl River Marsh area...... 43

4-1. Stratigraphy of the Pearl River Marsh core, PR1...... 45

viii

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5-1. Locations of the surface samples in the Pearl River Marsh area...... 52

5-2. Pollen diagram of the Pearl River Marsh surface samples ...... 53

5-3. Pollen diagram of the surface samples from Baritaria .Basin, south-central Louisiana...... 54

5-4. All-group scatterplot showing the distribution of the group centroids for the Pearl River Marsh surface samples...... 68

6-1. Freshwater diatoms of the Pearl River Marsh surface samples ...... 73

6-2. Marine and brackish water diatoms of the Pearl River Marsh surface samples ...... 74

6-3. Diatoms in the surface samples of a salt marsh at Cocodrie, south-central Louisiana...... 75

7-1. Positions of the C-14 dates of the Pearl River Marsh core and the inferred average sedimentation rates..88

7-2. Cs-137 measurement of the uppermost 20 cm sediment of the Pearl River Marsh core, PR1...... 89

7-3. Pollen diagram of the Pearl River Marsh core...... 91

8-1. Freshwater diatoms of the Pearl River Marsh core...100 i 8-2. Brackish water and marine diatoms of the Pearl River Marsh core...... 101

9-1. Diagnostic syndromes of regression and transgression layers...... Ill

9-2. Beach ridges in the Chenier Plain of Louisiana.....116

9-3. Sea level change curves in the offshore areas of coastal Louisiana...... 125

9-4. Sea level change curve derived from the study of the Pearl River Marsh core...... 127

10-1. Hypothetical diagnostic syndromes of large fluvial discharge layers...... 130

10-2. Large fluvial discharge layers in the Pearl River Marsh core...... 131

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 10-3. Simulated surface winds at 9000, 6000 years B.P. and at present in eastern North America...... 139

10-4. Fluvial terrace at the Tunica Hills, Louisiana....140 > 10-5. Meander scars of the Sabine River in southwestern Louisiana...... 143

11-1. Hypothetical diagram showing that the Little Ice Age is expected to contain fewer hurricane layers in the sediment core...... 149

11-2. Hypothetical diagram showing the salt water intrusion to the fresh marsh by hurricane surge...... 152

11-3. Gage height and salinity records at Houma Navigational Canal during Hurricane Andrew...... 155

11-4. Location of the test core at Du Large Gas Field, Houma, Louisiana...... 156

11-5. Pollen response to Hurricane Andrew at Du Large Gas Field, Houma, Louisiana...... 158

11-6. Diatom response to Hurricane Andrew at Du Large Gas Field, Houma, Louisiana...... 159

11-7. Pollen diagram of uppermost 84 cm sediment of the Pearl River Marsh core...... 161

11-8. Freshwater diatoms of the uppermost 84 cm sediment of the Pearl River Marsh core, PR1...... 162

11-9. Marine and brackish water diatoms of uppermost 84 cm sediment of the the Pearl River Marsh c o r e ...... 163

11-10. Location of Horn Island, Mississippi...... 171

11-11. Diatom diagram of the Fearn Lake sediment core from the Horn Island, Mississippi...... 172

11-12. Recorded historical hurricane in the U.S. Gulf of Mexico Coastal Region...... 177

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ABSTRACT

An 8.81 m sediment core from the fresh marsh area of

the Pearl River delta was examined for pollen, diatom, and

organic content, to reconstruct the history of the late-

Holocene environmental changes. The data show that prior to

5900 years B.P., the coring site was occupied by a swamp

forest. Afterwards, the swamp was replaced by a marsh

vegetation as a result of postglacial transgression. Since

5900 years B.P. the marsh has remained brackish for most of

the time, characterized by abundant marine diatoms and

brackish marsh pollen. However, the brackish water

environment was interrupted at least two times when the

local salinity became low enough to support oligohaline

communities. The two regressions correspond to C-14 dates

of 3400 to 2200, and 1400 to 1100 years B.P. The most

prominent regression is the one from 3400 to 2200 years

B.P., which is correlatable with a major neoglacial event.

This regression is also compatible with the results of sea

level studies in other areas of the U.S.

The study results indicate that there were more large

fluvial discharge layers during the time of the mid-

Holocene than during the late-Holocene. This probably

corresponds to a climatic regime of increased precipitation

during the the mid-Holocene. These results support the

climate modeling prediction of increased precipitation

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. during the the mid-Holocene. These results support the

climate modeling prediction of increased precipitation

during the the the the mid-Holocene in the southeastern

U.S. The study indicates that the climatic response to

Hypsithermal warming in the Gulf coastal region was

probably different from the other regions of the U.S. such

as the Midwest.

Temperature depression during the early part of the

Little Ice Age probably was severe enough to affect

hurricane formation as indicated by the extreme low

percentages of marine and brackish water diatoms in the

sediment core from the Pearl River Marsh. The hypothesis is

further substantiated by the study of a lake sediment core

from Horn Island, Mississippi.

xii

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 1. INTRODUCTION

Recent palynological studies have improved our

understanding of Holocene environmental changes. It is

widely accepted that the mid-Holocene in eastern North

America was characterized by a warm Hypsithermal climate

(Wright 1976). However, in the late-Holocene starting from

about 3500 years B.P., the climate became generally cooler S and was interrupted by several cold episodes known as the

neoglaciations. The most recent one is referred to as the

Little Ice Age. These climatic changes might cause

corresponding variations in a number of inter related

parameters such as vegetation, sea level, fluvial discharge

and hurficane activities. Many uncertainties regarding

these factors still remain unresolved.

One of the controversial issues is the late-Holocene

sea level changes in the Gulf of Mexico coastal region.

According,to some earlier studies, the sea level in this

region and the Atlantic Coast has generally been rising

since the last deglaciation (Nelson and Bray 1970; Curray

1965; McFarlan 1961; Coleman and Smith 1964; Redfield 1967;

Emary and Milliman 1970; Emery and Garrison; Emery et al

1967; Dillon and Odale 1978). However, recent studies

suggest that the sea level had undergone several

significant regressions during the late-Holocene (Stapor et

al 1991; Tanner and Donoghue 1993; Tanner 1988, 1991, 1992,

1993; Tanner and Demirpolat 1988; Davis 1991; Davis et al

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1992; Eronen et al 1987). The regressions have been

attributed to Neoglacial cooling. This study intends to

shed some light on this debate. Another objective of this study is to establish a

proxy record of fluvial discharges in response to climatic

changes during the past 6000 years. Available evidences

predict two scenarios of the mid-Holocene Hypsithermal

climate in this region. One indicates that the Hypsithermal

climate was drier than that in the late-Holocene as

suggested by Whitehead and Sheehan (1985). The other

suggests a wetter scenario as proposed by the climatic

simulation results (Kutzbach 1987). The climatic changes

might affect the fluvial discharges, hence the sedimentary

environment of fluvial, lacustrine and coastal systems.

This study intends to shed light on this problem.

The last objective of this study is to reconstruct a

proxy record of major hurricane activity during the past

millennium.in the study area, and to test the hypothesis

that there was a reduction in hurricane landfalls during

the Little Ice Age as a consequence of global cooling. As a

corollary, hurricane activities before and after the Little

Ice Age are expected to have been heightened to the warmer

climates.

Diatom and pollen analyses are powerful tools in

reconstructing Quaternary vegetation and climate. However,

not much work has been done in the coastal region around

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the Gulf of Mexico. There is little diatom literature

concerning Holocene environment in this region. Most pollen

studies are from inland areas which are not sensitive to sea level changes and hurricane strikes (Whitehead and

Sheehan 1985; Delcourt 1979, 1980; Delcourt and Delcourt

1977; Delcourt et al 1985, 1985; Watts 1975, 1992). There

was only one palynological study concerning the late-

Holocene coastal marsh vegetation in this region (Chmura

1990). Unfortunately, its C-14 dating control is

insufficient to provide precise environmental information,

since there is only one radiocarbon date for that 5 m long

sediment core.

Sea level change, fluvial discharge and hurricane

landfall may affect marsh salinity, which will be reflected in the diatom and pollen assemblages associated with the

marsh environment. This study involves a pollen and diatom

analysis of an 8.81 m sediment core, PR1, taken from the

fresh marsh zone of the Pearl River delta, southeastern

Louisiana (Figure 1-1). The core contains a sedimentary

record spanning the last 6200 years. The coring site, about

7 km away from the nearest beach and at less than 1.5 m

above sea level is strategically located in the seaward

side of the fresh marsh, immediately adjacent to the

brackish marsh. This location should make it sensitive to

any salinity changes in the substrate. The reason for

choosing a fresh marsh site instead of a brackish one is

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4

-7— —-j Wb'QaTf?^^ 7fT- EARCftWHg ls=fi IWIlgU^^NAGE

'•■'SIT -^Weems'-,*- "fe/a/jtf.

’•OoUjV r>*yr-&L — fi_— JXyOoL.-f— Zfr- T

KILOMETERS 1 0 1 2 3 4 5

Figure 1-1. Map of the Pearl River Marsh and the location of the sediment core.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. that the contrast in hydrological conditions between the

fresh marsh and the sea is more pronounced, so that clearer

signals of environmental changes can be derived.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 2. LITERATURE REVIEW

The principal methodology employed in this study is

diatom and pollen analyses. This chapter reviews the

theoretical background and applications in these two

techniques.

Diatoms are unicellular planktonic or benthic algae

which are widely distributed in natural aquatic

environments. The life cycles of diatoms are variable, but

usually very ephemeral. One dominant population may be

totally replaced by another within a single growing season

in response to changes in hydrological conditions. The

shapes and decorations of the siliceous valves of the

individual diatom frustles are unique for each taxon, which

serve as the basis for identification. The silica valves

are relatively resistant to chemical alterations after

deposition, so they can be preserved for a long geological

time.

The most important property of diatoms for this study

is that many species have distinct preferences for

salinity. There are several diatom classification systems

which are not quite consistent with each other (Foged 1974,

1981, 1984; Patrick and Reimer 1966; Patrick 1977). Usually

diatoms maybe classified into three major groups: (1)

polyhalobiens (marine diatoms, in water with salt

concentration of 3-4%); (2) mesohalobiens (brackish water

diatoms, in water with salt concentration of 0.5-2%); and

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 7

(3) oligohalobiens (freshwater diatoms, in water with salt

concentration of <0.5%). The oligohalobiens may be

subdivided into three categories: (a) halophilic, best

developing in water with a very low salt concentration; (b)

indifferent, referring to those species not sensitive to a

little salt; (c) halophobs, those species living in very

pure water and dislike salt (Patrick and Reimer 1966;

Patrick 1977).

Diatoms have been used in a number ways to reconstruct

paleo-environments (CLIMAP Project Members 1976; Burckle

1972; Sancetta and Robinson 1982; Burckle 1984; Gasse 1989;

Ritchie and Koivo 1975) . They have been acknowledged as good indicators for relative sea level changes since

different diatom assemblages from a stratigraphic sequence

can be compared and their associated salinities may be

inferred (Palmer 1978). This approach has been widely used

in northern Europe where isostatic rebound after

deglaciation had resulted in significant sea level fall.

As a consequence many former offshore areas were separated

from the open sea. Diatom analysis has been used to

identify this geological event. For example, Kejemperud

(1981) analyzed the diatoms in sediments from eight basins

at Frosta, Norway, and found that all the basins had a

succession from a flora influenced by marine and brackish

water to a freshwater flora after they were separated from

the sea. The first period after isolation was characterized

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 8

by a maximum in productivity of brackish diatoms (Figure

2-1) .

In a similar study, diatom is used to determine the

isolation times of the basins from the open sea (Hyvarinen

1980). In that study, Melosira islandica was found to be

abundant before the isolation, but after 8000 years B.P.

Melosira italica. Tabularia fenestrata. and Nitzschia

scalaris increased greatly. In Eronen's (1974) study of the

history of the Litorina Sea, diatom was also used to

identify the isolation of lakes from the sea.

The application of diatom analysis in sea level study

is also documented in North America. One notable example is

the study of a late-Holocene core from the Puget Lowland in

Washington (Eronen et al 1987) (Figure 2-2). An 8.45 m core

from that area provided a relative sea level curve since

7600 years B.P. The result indicates that from 5000 to 3400

years B.P., the relative sea level rise resulted in an

accumulation of sediments rich in marine and brackish water

diatoms, such as Melosira sulcata. Hvalodiscus laevis.

Caloneis westii. and Nitzschia siama. Afterwards, from 3400

to 2200 years B.P. there was a significant regression

characterized by fresh and fresh/brackish water diatoms

such as Fraoilaria construens var. venter. F. virescens.

Navicula nusilla. and Nitzschia frustulum. Since 2200 years

B.P., the sea level has been generally rising except for

the past 500 years when freshwater diatoms again became

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 9

Polyholoboin Mesohotoboui H.pht Indifferent oltgohalobous

['■■••’•••■i.lc u y W i l l Clay g y ttja £ 1 ^ 1 Fine detritus g/tt|a Coarse detritus gyttja

Hatched area «* 5 x exaggeration

Figure 2-1. Diatom diagram from Lianvatn of Frosta, Norway (from Kjemperud 1981).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 10

SHINE PEAT AREA DIATOM DIAGRAM

—A .5 P

7 20 : SO

,tS 0 r 70 & > * : * > > > :

^ ■ 2140 : 70 *0 .■*-

■ 2 5 3 0 :5 0 tjzg™ 3140 *9 0 cwpo i 3 4 0 0 :8 0 f ^SS 5iIo fl 4 3 2 0 :7 0 H 20 I 4 6 4 0 :7 0 :?p2« i 4900:100 $ai»o UiJ 5090:30 «if»uw4i 5690:50 ! -sriT”i« 7630:90 210*"

r*-esn Water flT\ Peo* j*505| Clay Gyttja D ia to m s

-'■esn & Srackisn 5 5 3 3 4 G y ttia j2.7*ZtJ1{ Sandy Gy*«ja water Oiatoms

Marine & Srackisn | » H C lay U t it J Sandy Clay water Oiotoms

j-7«7] Pebbles k x w k ) volcanic Ash °oor m Diatoms I • * * -1 4 Sand □ S e d im e n t A b se n t

Figure 2-2. Sediment column, C-14 dates, and percentage diagram of different diatoms found in the Shine profile, Puget Lowland, Washington (after Eronen et al 1987). There are two distinctive freshwater diatom intervals, 3400 to 2100 and after 400 years B.P., indicating sea level regressions.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 11

dominant. One limitation of this study is that the sediment

of the last two millennia was interrupted by episodes of

peat accretion with poor diatom preservation which was

unable to provide enough environmental signal.

In Copper River and Grove Creek area, South Carolina,

Brooks et al (1979) identified two regressive peat layers with fresh/brackish water diatoms sandwiched between the

salt-marsh clayey sediments. The two regression layers were

radiocarbon dated to 2330±40 and 3100±100 years B.P.(Figure

2-3) .

Both diatom studies from Washington and South Carolina

indicate a low sea level stage from 2200 to 3400 years B.P.

The dates are compatible with results from a recent study

of beach ridges in Florida (Stapor et al 1991) (Figure 2-4)

and pollen result in Southern California (Davis 1992)

(Figure 2-9; discussion late in this chapter). The

regression during this time interval was probably

correlated with the periods of Homeric Minimum (2880 to

2600 years B.P.) and the Greek Minimum (2460 to 2260 years

B.P.) of solar activities (Landsheidt 1985). The Homeric

Minimum had been widely documented in North America and

other parts of the world, which was correlated with a major

Neoglaciation event (Davis 1992) (Table 2-1).

In Cape Breton Island, Nova Scotia, Canada, a diatom

study documented the late-Holocene sea level rise for that

region (Miller et al 1993) (Figure 2-5). The result

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 12

H IG H M ARSH

A 8 A16 AtO A14 A1S

RIVER ~ - /

gf— KU (4135165) (m) V e t1'!'!—■ MO (500S i MO)

PEAT W IT H FRESH A N D Uj] BRACKISH WATER DIATOMS

□ SALT M A R SH CLAYS

Figure 2-3. Stratigraphy cross section in Cooper River-Grove Creek area, South Carolina (from Brooks et al, 1979 and Palmer et al 1986) . The upper two peat layers are chronologically correlatable with the 'Greek Minimum' and 'Homeric Minimum' of 14 C anomalies.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 13

14 YEARS BP (uncorrectad Cl

3000■ 2000 1000

WULFERT

LA COSTA

SANIBEL I BUCK KEY SANIBEL II

Figure 2-4. Mean sea level curve for southwest Florida based on beach ridge study (after Stapor et al 1991).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 2-1. Dates of the ’Homeric Minimum' cold period in North America and other parts of the world (after Davis 1992 with modification).

Location Age Reference (years B.P.)

San Joaquin Marsh, CA 2830 Davis 1992 Diamond Pond, OR 2700 Mehringer and Wigand 1986 Rattlesnake Cave, ID 2790 Davis et al 1986 Sevier Lake, UT 2560 Oviatt 1988 Bonfire Shelter, TX 2780 Bryant 1978 Norway 2595 Nesje and Kvamme 1991 China 2920 Grove 1988 New Guinea 2930 Hope and Peterson 1975

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 15

• Halophoba.- ••■Halophile •

e o . ■ 03 u a o — * ■*-2 a as . a. n

£ •— •— o o o .2 -5. 2 .5 § i ID ID Zona

510 * 601

Zone 2 780 * 8 0 1 E -E 4

10S0 * 80 8 Transition

Zone 1 4020 * 90II Q aJ 20 20 2 d 20 ' 20 20 20 20 20

Figure 2-5. Diatom diagram of Otter Harbor, Cape Breton Island, Nova Scotia (after Miller et al 1993) . Due to the great variation in sedimentation rates, a depositional hiatus might exist between 4000 and 1000 years B.P., which removed the intermediate regression sediment.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. suggests that at 4000 years B.P., the relative sea level

was significantly lower than that of the present. Abundant

freshwater diatoms, such as Diatoma hiemale v. mesodon.

Eunotia spp., Fracrilaria virescens. Tabularia flocculosa

were present in the sediment core. Since 950 years B.P.,

the marine diatom Skeletonema costatum became dominant. The

result suggests that the sea level was 1.5 m lower than the

present at 950 years B.P. The average rate of sea level I rise was 16 cm per century which was much lower than the

rate of 30-35 cm per century determined by local tidal

gauge. The 'Homeric Minimum' was not found in this study.

One major problem with this study is that the sedimentation

rate was not constant. The sedimentation rate was generally

low from 4000 to 1000 years B.P. Afterwards it increased

about twenty fold. A sedimentation hiatus might exist in

the interval from 4000 to 1000 years B.P. The absence of

evidence for the 'Homeric Minimum' was possibly attributed

to such a hiatus caused by the regression process.

Generally, in all these diatom studies, an increase in

the abundance of various marine species is associated with

a transgression due to sea level rise, whereas an increase

in freshwater diatom abundance is related to regression due

to sea level fall (Palmer and Abbott 1986).

One problem with diatom analysis is that diatom i frustles sometimes may be dissolved, especially in a highly

reduced environment like coastal marshes where hydrogen

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. sulphide content is high (Lewin 1961; Eronen 1974). Lewin

(1961) found that living cells of freshwater diatoms have a

lower rate of dissolution of silica walls than those dead

cells because aluminum in living cells can combine with

silica and therefore reduce its solubility. Since

solubility is partially dependent on the surface area

available, recent diatom silica is dissolved faster than

fossil diatom's because there is a reduction of specific

surface area upon aging. Diatom fossilization may lead to

recrystallization of the silica in the diatom and result in

stability of the diatom frustles. In Eronen's (1974) study

of the history of Litorina Sea, he suggests that the diatom

poverty phases are associated with the dark gyttja layers

which are rich in sulphide. He suggests that the formation

of sulfide tends to dissolve the diatoms according to

Lewin's (1961) experiment.

Pollen analysis was applied to complement the diatom

analysis in this study. Pollen are usually produced in

large quantity and can be well preserved in reducing

environment like marshes. The shape, size and sculpture of

pollen are usually identifiable to the genus or family

level. In the southeastern United States, pollen analysis

has long been regarded as an effective tool in

reconstructing paleoclimate (Delcourt and Delcourt 1977;

Delcourt 1979, 1980; Delcourt et al 1983; Watts 1979, 1980;

Whitehead and Sheehan 1985).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. A controversial issues about Holocene climate in the

Gulf of Mexico coastal region concerns the occurrence of

the Hypsithermal. It has been accepted that the mid-

Holocene Hypsithermal was characterized by a dry and warm

climate in the Northeast and Midwest as exemplified by the

eastward extension of the Prairie Peninsular. According to

computer modelling result, the reduced precipitation was

caused by the stronger westerlies around the heat low after the melting of the continental ice sheet (Webb et al 1987).

However the circulation pattern in southeastern U.S. was

different from Midwest and Northeast at the time of the

Hypsithermal according to the modeling result. The

strengthened southerly flow could actually bring more

precipitation to the Gulf of Mexico coast region, hence a

wetter Hypsithermal here might be present (Kutzbach 1987)

(Figure 2-6). Therefore there were two possible climatic

scenarios in the Gulf of Mexico coastal region regarding to

the Hypsithermal.

The climatic simulation results are not well

substantiated by fossil evidences. To the west of the Gulf

of Mexico coastal region, a drier Hypsithermal is supported

by the studies in western Texas. According to pollen and

archeological evidences, the Pecos Valley in west Texas

experienced a relatively arid climate during the time of

the mid-Holocene (Patton and Dibble 1982; Bryant 1966;

Bryant and Shafer 1977). The climate favored the

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 19

-180 -150 -120 -90 -60 -30 9 0 /\-yy.

60 -

NC —

18,15 NE

3 0 - SE

6 ,3 .0

Annual P -E (mm/day) Annual Precip (mm/day)

NE NC SE NE NC SE

3 - 0 .7 - .2- 1.2 - 3 .3 - 3 .2 - 4 .3 -

6 - taD.

12-

1 5 -

I-1 I-1 £ 10% Deviation Figure 2-6. Simulated annual precipitation (P) and precipitation minus evaporation (P-E) for the last 18000 years in eastern North America (after Webb et al 1987). The data indicate that the SE North America experienced a positively biased P and P-E from 9000 to 3000 years B.P., suggesting a wetter Hypsithermal.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. development of ephedra, mesquite, and cactus which

gradually replaced the early pine forest. Also due to the

arid climate, bisons were largely driven away from that

area, and the local inhabitants had to rely on small games

(Bryant 1966).

At Tunica Hills of northeastern Louisiana, pollen and

macrofossil evidence indicates that the mid-Holocene

vegetation was characterized by Faqus. Magnolia. Ouercus.

Liriodendron. Ostrva. Ulmus. Fraxinus, Nvssa and Vitis

(Delcourt and Delcourt 1977). These are all common species

in the modern mesic forest; their presence therefore argue

against scenario of a significantly dry Hypsithermal

climate. However, the result does not clearly enough to

support the scenario of a significantly wetter Hypsithermal

either.

In eastern Mississippi, Whitehead and Sheehan's (1985)

palynological study of sediments from an oxbow lake

documented the local Holocene climate and vegetation

(Figure 2-7). They indicate that the early Holocene

vegetation was typical mesic forests dominated by Ouercus.

Carva. Ulmus. Ostrva/Carpinus. Celtis, Fraxinus. Platanus.

and Betula. The mid-Holocene Hypsithermal was very

distinctive, characterized by a xeric Ouercus-Carva forest.

During the mid-Holocene (from about 8000 to 3000 years

B.P.), sedimentation rate in the oxbow lake was

dramatically reduced and the lake was shallow enough to

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. eo H

Nnnarbnrca! pollen Nnnarbnrca! Arboreal pollen (after (after Whitehead and Sheehan 1985). The data suggest a dry Figure 2-7. Pollen diagram of Bigbee, eastern Mississippi climate during the Hypsithermal. However, there is no C-14 date between 8760 and 2680 years B.P. which made the prediction questionable. i £ i i

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 22

support a shrubby and herbaceous community, therefore a

drier Hypsithermal was inferred. During the late-Holocene

the climate became wet again and there was an extensive

development of Nvssa and Pinus forests. Nevertheless, there

was only 2 0 cm of mid-Holocene sediment, and there was no

intermediate radiocarbon dates to bridge the 6000-year time

interval from 8760 to 2680 years B.P., so that the result

must be considered with caution.

The Hypsithermal climate in central Alabama was

different from the prediction from eastern Mississippi. A

pollen record from Cahaba Pond, St. Clair County, central

Alabama shows that the early Holocene vegetation was xeric

type with Ouercus and Carva predominant (Delcourt et al

1983). A mesic forest dominated by Nvssa-Ouercus-Pinus- Liauidambar-Carva has existed there since 8000 years B.P.

which possibly supports the wetter Hypsithermal scenario.

Nevertheless the study could not differentiate the climatic

conditions between mid- and late-Holocene. Perhaps the

climatic change from mid- to late-Holocene was not

significant enough to affect the main composition of the

local forests.

At Goshen Spring, Pike County, southern Alabama, a dry

climate during the early Holocene favored a forest

dominated by Ouercus. Carva and Pinus (Delcourt 1980).

This forest existed from the late glacial time to about

5000 years B.P. After 5000 years B.P., the early forest was

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 23

largely replaced by the widespread southern pine. However,

this study did not offer much information about the

difference of climatic conditions between mid- and late-

Holocene. In addition, the uppermost three radiocarbon

dates, 26000, 5620 and 1342 years B.P., of the sediment

core for that study are widely spaced, so that the

chronology of the sediment is questionable.

A pollen study of Lake Annie, south central Florida,

suggests that prior to about 5000 B.P., the upland

vegetation was virtually treeless with high percentages of

herbs and xeric shrubs. After about 5000 years B.P. there

was an extensive development of Okefenokee Swamps, the

Everglades and other swamps, which might be related to

increased precipitation. On the other hand, this swamp

development may be alternatively explained as a result of

sea level rise (Watts 1976, 1980; Wright 1976).

Study of meander scars of the Sabine River, western

Louisiana suggests that the fluvial discharge during the

mid-Holocene Hypsithermal was much larger than today's; the

mid-Holocene is characterized by large meander scars

(Alford and Holmes 1985). These large meander scars were

probably associated with interglacial warm climate when

tropical storms were more frequent thus bringing more

precipitation to the Gulf of Mexico Coastal region (Alford

and Holmes 1985). Nevertheless, the large meander scars

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 24

were not directly C-14 dated. The chronology of this study

was based on inferred ages derived from other studies.

Convincing result of increased precipitation during

the Hypsithermal comes from an oxygen isotope and pollen

study in Haiti. It indicates that the climate was

increasingly mesic from about 8200 to 3200 years B.P. After

3200 years B.P., there was a considerable loss of mesic

forest trees and the vegetation became dominated by dry

forest taxa and abundant weeds (Hodell et al 1991) (Figure

2-8). However, Haiti is far from the Gulf Coastal region.

The geographical setting and climatic controls between

these two places are different.

It seems that the Hypsithermal climate in the Gulf of

Mexico coastal region remains a mystery. The precipitation

regimes between mid- and late-Holocene could not be

differentiated based on the current results of pollen

studies in this region. It is probable that the

precipitation change from mid- to late-Holocene, about 10%

difference according to Kutzbach (1987), was not pronounced

enough to result in significant changes in the overall

character of the vegetation in this region. More evidence,

particularly from physical climatic proxies, is needed to

resolve this problem. The Pearl River Marsh study may

provide supplementary proof to decipher the Hypsithermal

climate in this region.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 5

*>180 (°oo) Pollen

P 6

P5 200C 3.C00 P4 Q_m V) CO P3 0 >. 5C00 f c o .o CO o P 2 cu

i i

0 25 50 75 ICO 125 150 175 Insolation at 10° N (Aug. minus Feb.)

Figure 2-8. Oxygen Isotope measurements of a single species of ostracod from a 7.7 m core taken from Lake Miragone, Haiti.The ratio of evaporation to precipitation is high to the left side of the figure. The data indicate a 'mesic1 climate during the mid-Holocene (after Hodell et al 1991).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. In addition, a number of pollen studies in coastal

wetlands have been conducted in the U.S. Most of them are

concerned with regional vegetation changes (Heusser 1963,

1975; Mudie and Byrne 1980; Clark and Patterson 1984, 1985;

Clark 1986;1 Clark et al 1986; Miller et al 1993).

Application of pollen and foraminifera in reconstructing

sea level changes was used by Newman and Munsart (1968) in

the Wachapreague Lagoon, Virginia. They found that prior to

4400 years B.P., regression affected the area. There was

neither pollen nor foraminifera in the sediment, probably

because of oxidation. There was a major transgression

between 4400 and 1000 B.P. when the lagoon consisted of

open bays and tidal flat, characterized by abundant

foraminifera and arboreal pollen. Tidal marshes began to

form about 1000 years B.P when extensive herbaceous pollen

were found in the sediment. One problem with this study is

that the sediment core was not well dated, because there

was only one C-14 date for this sediment core.

One of the best examples of pollen application in sea

level study comes from San Joaquin Marsh, southern

California (Figure 2-9). Pollen stratigraphic study of a

well dated 6.87 m core indicates that from 7000 to 4500

years B.P., the site was a freshwater habitat,

characterized by the pollen of Cyperaceae, Liliaceae,

Tvoha-Sparqaniumm. Umbelliferae and Salix, indicating that

the sea level was still low. As sea level rose, the site

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 27

SflN JOHQUIN MRR5H o' S9°i^e°c

sea a i s 283 9 40 a aao o 102030 0 to 0 10 .v.*Kv ~£z**riwK'M’ XvXvIv W’MvXW !■!•!•!•!

Xv.v,

POLLEN .PERCENT o< = lS

Figure 2-9. Pollen diagram of San Joaquin Marsh, Southern California. Shaded regions represent freshwater intervals or regressions (Davis 1992).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 28

was converted to a salt marsh, dominated by halophytic

taxa, such as Chenopodiaceae/Amaranthaceae. However, there

were four fresh marsh episodes, 4000 to 3500, 3000 to 2500,

2200 to 2400, and after 560 years B.P., when the salt marsh

pollen were partially replaced by the fresh marsh

components. The establishment of fresh marshes was possibly

related to regressions. These fresh episodes were broadly

correlatable, with the ages of the noeglaciations such as

the period of Homeric Minimum and the Little Ice Age (Davis

1992, Davis et al 1992). The sea level changes derived from

the San Joaquin Marsh study are consistent with the diatom

data from Washington (Eronen et al 1987; Figure 2-2) and

South Carolina (Brooks et al 1979; Figure 2-3).

In the Louisiana coastal area, some palynological work

was conducted recently (Darrell and Hart 1970; Chmura and

Liu 1990; Chmura 1990). Perhaps the most significant work

is the study in the Barataria Basin by Chmura (1990). She

conducted some research about the modern pollen

distribution and their significance to climatic

reconstruction and concluded that different marshes have

distinctive pollen assemblages, . Specifically, the

freshwater marshes are characterized by Polypodiaceae

spores, Taxodiaceae/Cupressaceae/Taxaceae, Salix and Vicma

pollen, whereas the salt and brackish marshes are

represented by higher percentages of Gramineae, Ambrosia

and Chenopodiaceae/Amaranthaceae pollen. However, there is

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. only one C-14 date for the entire 5 m core in that study,

so that the core was poorly dated. Moreover, for unknown

reason, only the lower 2.9 m sediment of the core was

studied for pollen. The environmental reconstruction based

on that study was far from sufficient.

One limitation for pollen analysis is that pollen

grains are usually difficult to be identified to the

species or even generic level. For example most plants of

the Gramineae family have similar pollen. It is impossible

to distinguish between Soartina alterniflora (typical salt

marsh species) and Panicum virqatum (typical fresh marsh

species), based on pollen morphology and size, but they are

ecologically very different. In this study,

paleoenvironmental interpretation will be based on the

whole pollen assemblage rather than individual taxa and

supplemented by diatom data.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 3. GEOGRAPHIC BACKGROUND OF THE STUDY AREA

1) Settlement History

The area was populated by aboriginal Mushagen Indians.

Significant European settlements began in early 19th

century (Newton 1972; Harry 1971). In the late 1800s and

early 1900s the bottomland forests supported a lumber

industry. The swamps yielded a good amount of cypress and

water tupelo, while in the surrounding upland areas, oaks

and hickories were removed. By 1930, the lumbering was

stopped because of the establishment of Honey Island

National Park. Today the Pearl River Marsh area is

protected by the Louisiana Department of Wildlife and

Fisheries as a Wildlife Management Area (White 1979).

2) Geomorphology The Pearl River forms part of the border between

Louisiana and Mississippi. Its drainage basin is

approximately 386 km long, 80 km wide, and include an area

of 23,000 km2. Elevations range from 198 m to sea level. At

30°10'N and 89°40'W, the Pearl River Marsh is a fluvial

estuarine lowland located at the mouth of the Pearl River.

On the Pearl River Marsh are multiple distributary

channels which flow into Little Lake and then into Lake

Borgne before entering the Gulf of Mexico. Many of the

channels are stagnant at low water, but flow swiftly during

periods of high river stages. Historically the East Pearl

was the main channel. However, between 1850 and 1878, due

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 31

to artificial dredging and some natural processes, the East

Pearl River was eventually cutoff from the main drainage

basin and the West Pearl River became the main channel

(Collins and Howell 1879).

The geology of the drainage basin includes Quaternary

and Tertiary clastic sediments. Unlike central Louisiana,

the Pearl River Basin was too far from the ancestral

Mississippi to have significant loess deposits (Hudson

1993).

3) Climate

The climate in this area is humid subtropical, typical

of that along the northern Gulf of Mexico Coast. Summer is

long, hot and humid; winter is short and mild (Table 3-1).

Winter and spring storms account for 50% of annual

precipitation. Summer thunderstorms account for 30%. The

remaining 20% is in the fall. The area is subject to

hurricane induced tidal surges and wave actions. Hurricane

Camille in 1969 created 2 to 3 m tides which severely

affected the area (U.S. Army Corps of Engineers 1985).

4) Hydrology

According to a 22 year hydrological record from 1967

to 1989 in Bogalusa, Mississippi, the Pearl River has a

mean annual water discharge of l.02xl01° m3, and a mean

annual sediment discharge of 1.32X109 kg (data calculated

from Hudson 1993). There are strong seasonal, annual and

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 32

Table 3-1. Climate data at Bay St. Louis, Mississippi, east of the Pearl River Marsh (after U.S. Army Corps of Engineers 1985) .

Temperature (°C) Annual mean 24.5 Summer mean (June to August) 32.3 Winter mean (December to February) 17.2 Extreme high 43.8 Extreme low -9.4

Precipitation (mm) Annual mean 1600 Wettest month (July) 172 Driest month (October) 60 Extreme high 2577 Extreme low 728

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 33

decadal variations in both water and sediment discharges

(Figure 3-1).

5) Vegetation The vegetation in the study area is strongly affected

by various environmental gradients (Figure 3-2). Perhaps

the most important factors are topography and salinity. In

the well drained areas, upland forests are predominant,

whereas the waterlogged deltaic area is mainly occupied by

various coastal marshes and swamps. The coastal marshes are

differentiated into fresh and brackish types according to

soil salinity and vegetation communities.

(1) Upland forests

The upland vegetation in coastal Louisiana and

Mississippi is mostly pine forests. The most common species

are pine (Pinus palustris and P. taeda). Hardwood forests,

primarily composed of oak (Ouercus virqiniana. 0. geminata.

0. chapmanii. O. incana. 0. laurifolia. O. mvrtifolia. 0.

falcata. 0. marilandica), sweetgum (Liouidambar

stvraciflua). Hickory (Carva spp.), ash (Fraxinus spp.),

elm (Ulmus spp.), and Magnolia (Magnolia grandiflora). do

exist but most of the original forests have been cleared

for settlement or agriculture (Duncan and Duncan 1987).

(2) Swamps

Extensive bottomland hardwood forests and cypress-

tupelo forests develop in the ecotone between upland and

coastal marsh vegetation (Figure 3-2). Soil salinity in the

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 34

^ A** c5> ./V

# v r

1970-

1975 -

1980

1985 -

1990 - 1000 1500 2000

Figure 3-1. Monthly water and sediment discharges of the Pearl River at Bogalusa station, MS (zero values of sediment discharges mean lacking of data; data from U.S.G.S.).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 35 znjwv a-±

.; .w i

E r i t c h i

Brackish Marsh-X^'v ~ ^>3: JVhams

t flk*5 '.’>.

'Cs&iaH fiotni V'C.-i ’-cy ^ ic y^Bor Jaun* Point

KU.OMETERS I

MILES I Figure 3-2. Vegetation map of the Pearl River Marsh area (vegetation boundaries refering White 1983 and Taylor 1992).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. swamp area is close to zero. The soil is generally acidic,

pH values range from 4.8 in the bottomland forests to 6.5

in the cypress-tupelo swamps (White 1983). The most common species include bald cypress (Taxodium distichum), water

tupelo (Nvssa aguatica), and swamp tupelo (Nvssa svlvatica

var. biflora), ironwood (Carpinus caroliniana), sweetgum

(Licruidambar stvraciflua) , swamp red maple (Acer rubrum

var. drummondii) (White,1983). Other common arboreal plants include river birch fBetula nigra), hickory fCarva spp),

button bush (Cephalanthus occidentalis), beech (Fagus

grandifolia). ash (Fraxinus_spp.), passsumhaw (Ilex

deciduous), wax myrtle (Mvrica cerifera), oak (Quercus

lvrata. 0. michauxii. 0.nigra. 0. laurifolia), black willow

(Salix nigra), and American elm (Ulmus americana) (White

1983) (Figure 3-3).

(3) Fresh Marsh

Due to the diluting effect of the Pearl River

discharge, typical salt marsh is not developed in the Pearl

River delta. The marsh may be divided into fresh and

brackish types according to soil salinity and vegetation.

However, considerable controversy exists in the

classification of the Pearl River Marsh. According to

Chabreck and Linscombe (1978) most part of the study area

is classified as intermediate marsh, perhaps because of the

frequent occurrence of bulltongue (Sagittaria lancifolia)

and wiregrass (Spartina patens). This marsh type range from

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. u

Nyssa aquatica. and Taxodium distichum Figure 3-3. FigureRepresentative 3-3. photograph theof swamp forest in the Pearl River Marsh area. The swamp forest is characterized forestswamp is The RiverMarsharea. Pearlthe by

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 38

the edge of the swamps to the northern bank of Little Lake.

On the other hand, Groat et al (1984) classify the same

area as fresh marshes.

There is still another classification which is totally

different from the previous two. Based on their intensive

studies of the Pearl River Marsh, White (1979, 1983) and

Taylor (1992) define the fresh marsh area as dominated by

switch grass (Panicum virqatunO and the brackish marsh as

dominated by wire grass (Spartina patens).

In this study, the fresh marsh follows the delineation

of the latter two authors. It includes the area north of

the Salt Bayou and the Mill Bayou to the edge of the swamps

(Figure 3-2) . Soil salinity in this area is generally very

low, averaging 0.6 ppt. The soil is still acidic, with an

average pH of 6.2 (White 1983). In this area, the most

important high plant species is switch grass (Panicum

virqatumt . Other common species include aster (Aster spp.),

hedge bindweed (Calvsteaia sepiuml , sawgrass (Cladium

iamaicense), spikerush (Eleocharis macrostachva), marsh

pennywort (Hydrocotyle verticillata), marsh mallow (Ipomoea

saqittata), climbing hempweed (Mikania scandensl, mock

bishop's-weed (Ptilimnium capilaceumf. big cordgrass

(Spartina cvnosuroides) , goldenrod (Solidago sempervirens)

and cow pea (Vigna luteola). Also present but less common

are alligator weed (Alternanthera philoxeroides), sedge

(Carex spp., Cyperus spp.), water grass (Echinochiloa

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 39

waterit, needle rush (Juncus roemerianusf, seashore mallow

(Kosteletzkva virainica), cutgrass (Leersia orvzoidesf,

royal fern (Osmunda regales), switch grass (Panicum

virgatum), maidencane (P. hamitomom), common reed

(Phragmites communis),frog-fruits (Phvla lanceolataf,

smartweed (Polygonum punctatum). marsh pink (Sabatia spp.),

bulltougue (Saqittaria lancifolia), lizard tail (Sarurus

cernuus), leafy three cornered grass (Scirpus robustus),

foxtail (Setaria glauca), wiregrass (Spartina patens), bald

cypress (Taxodium distichlis) and marsh fern (Thelvpteris

palustris) (Taylor 1992; White 1979; 1983; Brewer and Grace

1990) (Figure 3-4).

(4) Brackish Marshes

Next to the fresh marshes, brackish marshes are

developed extensively on Weems Island, Hog Island, and the

peripheries of Little Lake and the Rigolets of the Pearl

River delta. Both soil salinity and pH values in these

areas are high, averaging 1.5 ppt and 6.6, respectively.

(White 1983). Local vegetation is dominated by wiregrass

(Spartina patens). Other common species include aster

(Aster spp.), hedge bindweed (Calvsteqia sepiumf . spikerush

(Eleocharis macrostachva), marsh mallow (Ipomoea

sagittata), climbing hempweed (Mikania scandens), frog-

fruits (Phvla lanceolatat, bulltongue (Saqittaria

lancifolia), and cow pea (Viqna luteola). In addition, some

less common species include alligator weed (Alternanthera

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. O

Panicum virgatum. Figure 3-4. Representative photograph of the freshmarsh in characterized by the Pearl River Marsh area. The fresh marsh vegetation is

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4 1

philoxeroidesf. sedge (Carex spp., Cvperus spp.), saltgrass

(Distichlis spicataf, water grass (Echinochiloa wateri),

needle rush (Juncus roemerianus), seashore mallow

(Kosteletzkva virginica), cutgrass (Leersia orvzoides),

maidencane (Panicum hemitomon), frog-fruits (Phvla

lanceolata), smartweed (Polygonum punctatum), marsh pink

(Sabatia spp.), and foxtail (Setaria glauca) (Taylor 1992;

White 1979, 1983) (Figure 3-5, 3-6).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. to

Sagittaria landfolia. in in the Pearl River Marsh area. This brackish marsh is characterized by Figure 3-5- Representative photograph of the brackish marsh

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. W

. Spartina patens in in the Pearl River Marsh area. This brackish marsh is characterized by Figure 3-6. Representative photograph of the brackish marsh

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 4. METHODOLOGY

An 881 cm sediment core, PR1, was taken from the

fresh marsh area by a Livingstone sampler in September,

1992. From 850 to 881 cm, the sediment is composed of

coarse sand which is then graded to heavy sticky clay to

777 cm. From 777 to 751 cm is a whole piece of wood of

unknown tree species. From 751 to 468 cm the sediment is

generally composed of peaty clay interrupted by occasional

thin sand layers. Three distinctive sand layers are at

levels 503, 637, 642 cm. From 468 cm upward, the sediment

is more organic with two clay bands at 73 and 407 cm

(Figure 4-1).

Forty surface sediment samples were collected along a

transect from the swamp forest to the Gulf of Mexico coast

in order to study the distribution of pollen and diatom on

the marsh surface to provide modern analogs to the past.

Most of the surface samples are partially decomposed

organic detritus. The samples are about 1 km apart from

each other, from two transects in the Pearl River Marsh

area, one along the West Pearl River, another along the

West Middle River. Because of limited accessibility to the

marsh area, most samples are about 30 m away from the

fluvial channels. Due to the absence of salt marsh in the

Pearl River Marsh today, four more surface samples were

collected from the salt marshes near Louisiana University

Marine Consortium (LUMCON), Cocodrie, Louisiana.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4 5

Depth Stratigraphy (m)

0 1

1 -

LEGEND 2 "

3 -

4 -

5 -

6 ~

7 -

8 -

Figure 4-1. Stratigraphy of the Pearl River Marsh core, PR1.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 46

Both the core and the surface samples collected in the

field were intensively analyzed in the laboratory. Based on

the C—14 chronology, the uppermost 84 cm of the core was

intensively analyzed for diatom and pollen analysis at

approximately two centimeter intervals to focus on the

environmental changes during the Little Ice Age. Each

sample approximately represents about 30 years of

accumulation according to the average sedimentation rate.

1) Pollen Analysis

One hundred and forty pollen samples were analyzed,

110 of which are from the sediment core, and the remaining

30 are surface samples from swamps, fresh marshes, and

brackish marshes. Pollen extraction followed the standard

procedures described by Faegri and Iverson (1975). The

basic treatments involve:

1. HC1 to dissolve carbonate and Lvcopodium marker

tablets for calculating pollen concentration.

2. KOH to remove organic acid.

3. HF to dissolve silicate minerals.

4. Acetolysis solution to dissolve lignin.

5. Silicone oil to make slide.

Pollen slides were examined with an Olympus optical

microscope at 40x10 magnification. At least 300 pollen

grains were counted for most samples. In some unusual

samples where pollen concentrations are very low, 250

grains were counted.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 47

2) Diatom Analysis

One hundred and fifty eight diatom samples were

analyzed for this study. One hundred and seventeen are from

the core and 40 are surface samples. Diatoms are plentiful

in most of the samples. There are a number of possible ways

for diatom sample preparation. The basic requirement is to

remove the organic material in the sediments. A

conventional treatment of sediment is digesting the organic

material by hydrogen peroxide (H202) . This method was

applied to the sediment collected. It is proved to be very

effective. The procedure is outlined as follows.

1. 0.9 ml sediment is taken from the center of the

core and transfer* to a 50 ml beaker. 2. 10 ml H2o2 is added to the beaker. After stirring

gently, the mixture is heated on a hot plate with low

temperature. As soon as bubbling occurs, remove from stove.

Let the mixture stay for two hours for thorough oxidation.

3. Add 30 ml distilled water and settle for 1 hour,

then decant the suspension.

4. Repeat step 3 again.

5. A drop of residue is transferred by a pipette to a

slide, then dry it on the hot plate at low temperature and

then seal it with a drop of Permont to make a microscopic

slide. Diatom identification and counting were performed at

100x10 magnification with immersion oil because some

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 48

diatoms are very small. At least 300 diatom individuals

were counted for each sample. In unusual cases, only 250

were counted. Due to great variations in diatom size, a

broken diatom valve was also counted as an individual if it

was large enough to be identifiable.

3) Loss-on-Ignition Analysis

In order to measure the relative amounts of organic

and clastic materials in the sediments, Loss-on-Ignition

(LOI) analysis was conducted by Miriam Fearn as part of Dr.

Kam-biu Liu's hurricane paleoclimate research project. A

sediment sample was first dried at 105°C to measure the

water content, then put in a furnace and heated at 550°C to

determine the percentage of organic material and mineral residue. The latter provides a crude measurement of the

clastic material abundance (Dean 1974). The LOI data for

the Pearl River Marsh core are made available to me by Dr.

Kam-biu Liu and Miriam Fearn.

4) C—14 and Cs-137 Dating

Six samples from the Pearl River Marsh core, PRl, were

radiocarbon dated to provide an absolute chronology. All

the samples are peat sediments. Three of them are dated

with accelerator mass spectrometry (AMS) technique in order

to get more-precise results, which used only 1 cm of the

sediment core each. The other three were derived by

conventional method which involved about 10 cm sediment for

each sample.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 49

Cesium (Cs)-137 is a recent element created by

explosions of atomic bombs. It gradually increased in

abundance since early 1950' to the peak value in 1963 when

atmospheric nuclear testing was most intensive, then it

gradually decreased in abundance. The element may be

incorporated with sediments and provide a relatively

accurate means of dating recent sediments. Five continuous

samples for the uppermost 20 cm of the Pearl River Marsh

core were measured for Cs-137 activity.

5) Discriminant Analysis

Discriminant analysis had been used previously in

paleo-environmental studies (Birks and Peglar 1980; Liu and

Lam 1985; Chmura 1990). This approach was applied to the

pollen and diatom data derived from the modern and

stratigraphic samples. The technique first assembles the

pollen or diatom information of the modern surface samples

from different vegetation types, and calculate a number of

discriminant functions. It then uses the same functions to

assign each fossil pollen or diatom sample a 'most probable

vegetation type1 depending on the probability of group

membership. In addition, a 'second probable vegetation

type' is also, calculated for each sample. The similarity

between a fossil sample and a 'typical' modern sample is

represented by the 'probability of modern analog'(Liu and

Lam 1985). A SSPS/PC software package is available from the

Computer Mapping Sciences Laboratory in the Department of

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 50

Geography and Anthropology, Louisiana State University, which made this statistical analysis feasible.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 5. MODERN POLLEN RAIN IN THE PEARL RIVER MARSH

The study of surface samples is important to the

understanding of the modern pollen distribution in relation

to their parent plants. They are the key to interpret the

pollen analytical result of the sediment core. Thirty

surface samples from the Pearl River Marsh are analyzed for

pollen, of which 5 are from swamp forests, 8 from fresh

marshes, and 17 from the brackish marshes (Figure 5-1). A

total of 49 pollen types were identified from these surface

samples. The main taxa are shown by Figure 5-2. The salt

marsh samples taken from Cocodrie, south central Louisiana

were not analyzed for pollen, since Chmura (1990) had

already done an intensive study nearby (Figure 5-3).

1) Provenance and Distribution of the Main Pollen Taxa

(1) Pine (Pinus) The highest Pinus pollen percentages occur in the

swamp surface samples. This is attributed to the extensive

pine forests growing in the vicinities of the Pearl River

Marsh area. There are at least five species of pine in

Louisiana and Mississippi. The most likely source of the

pine pollen rain is Pinus palustris. P. taeda and P.

elliottii (Duncan and Duncan 1978; Chmura 1990). Pine

pollen is also very abundant in the brackish marsh samples,

but generally less abundant in the fresh marsh samples.

This is perhaps due to the dispersal ability of pine pollen

and the relatively low pollen production of brackish marsh

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ubstation

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KILOMETERS I

Figure 5-1. Locations of the surface samples in the Pearl River Marsh area.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 w U Fresh Swamp Brackish (100 qroins/mi) (100 20 20 1500 500 1000

20 20 20 ' 2 0 20 20 40 20 20

20 <^v Figure 5-2. Figure 5-2. Pollen diagram of the Pearl RiverMarsh samples surface sum (pollen as total pollen counted). C J Y J 20 J0 50 20 ' 40 60 20 40

H 10 F28 f27 F25 FOl F02 TOO F03 004 BOS B06 609 620 610 8 BM 815 637 033 'A05 '.‘/06 007 605 824 B23 922 821 318 %t)7 W

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 54

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 55

plants. Comparable results are also reported in the

Barataria Basin, south central Louisiana, where pine pollen

is over represented in the brackish marsh samples (Chmura

1990). Thus pine pollen is a good indicator of the regional

vegetation, but having only limited value for inferring

marsh or swamp vegetation types.

(2) Oak (Quercus)

Ouercus laurifolia. 0 . nigra and 0 . lvrata are

reported in the Pearl River swamp area, which are perhaps

the most important pollen sources of this pollen taxon.

Other oak species that may contribute pollen to the Pearl

River Marsh include live oak (0. virginianaf which is

widely distributed in Louisiana and Mississippi, sand live

oak (0. geminata), chapman oak (0. chapmanii), bluejack oak

(0. incanaf, myrtle oak (0 . mvrtifoliaf, southern red oak

(0 . falcataf, and blackjack oak fO. marilandicaf which are

common on the sandy soils of Mississippi.

The distribution pattern of Ouercus pollen is similar

to that of Pinus. Both are overwhelmingly abundant in the

swamp surface samples and very common in the brackish marsh

areas. This is due to the relatively high pollen

productions and dispersal abilities of these two taxa.

(3) Ash (Fraxinusf

It seems that there is no obvious trend in the

distribution of Fraxinus pollen. The main pollen sources of

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this pollen taxon are possibly F. Pennsylvania. F. profunda

and F. Carolina which are all common swamp forest species.

(4) Hickory (Carva)

There are four species of Carva in the bottomland

swamp and upland vegetation of Louisiana (Chmura, 1990).

The frequency of this pollen type is generally low in most

surface samples.

(5) Elm (Ulmusl

Ulmus americana is reported in the bottomlands with

moderate density. The abundance of this pollen type is

relatively low in all vegetation types.

(6 ) Ironwood (Carpinus)

Carpinus caroliniana is widely distributed in the

swamps. The pollen type is frequently found in most

surfaces samples. However, like many other arboreal taxa,

the pollen percentage of Carpinus pollen in the swamp

forest is overshadowed by the great abundance of Pinus and

Ouercus pollen.

(7) Willow (Salix)

Salix nigra is a local swamp forest species which may

be the main pollen source of this pollen taxon. It is

abundant only in one swamp surface sample. Pollen

percentages decrease from swamps to brackish marshes.

(8 ) Cypress (Taxodium) I The most abundant pollen source of TCT (Taxodiaceae-

Cupressaceae-Taxaceae) type in coastal Louisiana is

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Taxodium distichum. so that Taxodium is used to represent

this pollen type instead of TCT. This pollen type is very

common in the swamp surface samples, but generally less

abundant in fresh or brackish marsh samples, suggesting it

is a fairly good indication of swamp forest. Chmura (1990)

found a lot of TCT pollen which she suggested are the

product of T. distichum. but such pollen type is not very

abundant in the Pearl River Marsh samples. This is perhaps

because the bald cypress forest in the Pearl River Marsh is

less extensive than in the south central Louisiana.

(9) Water-tupelo (Nyssa)

Nvssa pollen is supplied by N. aouatica and N.

svlvatica var. biflora which are two of the most common

species in the cypress-tupelo forests. This pollen type is

especially abundant in swamp surface samples, suggesting

that it is a good indicator of swamps.

(10) Myrtle (Mvrica)

The primary pollen source of Mvrica is wax myrtle (M.

cerifera) which is a shrub or tree growing up to 10 m tall.

This species is reported in the cypress-tupelo forest

(White 1979, 1983) in the study area. The plant may also

appear in fresh and intermediate marshes as well as on sand

dunes and natural levees (Chmura 1990; Duncan and Duncan

1987), but according to White (1979, 1983) and Taylor

(1992), the species is not found in the fresh marsh area of

the study area.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 58

The distribution of Mvrica pollen is relatively

uniform in all habitats at low percentages.

(11) Swamp loosestrife (Decodon)

Decodon verticillatus is the only possible source of

Decodon pollen. The species is not reported from the Pearl i River Marsh, but it is a common marsh species in coastal Louisiana (Duncan and Duncan 1987). Its pollen occurs in

many fresh marsh samples at low abundance.

(12) Gramineae By conventional pollen analysis, most grass pollen is

not identifiable to genus or species. Pollen analytical

results of surface samples in the Pearl River Marsh area

indicate that Gramineae pollen is abundant in all

vegetation types. Panicum viraatum is perhaps the main

pollen source of the Gramineae pollen in fresh marsh. Other

Gramineae pollen in the fresh marsh may come from Spartina

cvnosuroides. Phracrmites communis. Echinochiloa wateri.

Leersia orvzoides. and Panicum hemitomon. In the brackish

marsh samples, most Gramineae pollen is perhaps derived

from Spartina patens. Other sources may include Distichlis

spicata. Echinochiloa wateri. Leersia orvzoides. Panicum

hemitomon. and Sataria glauca. among others.

Gramineae pollen is most abundant in the brackish

marshes and least abundant in the swamps. This is because

grasses ate the principal components of the brackish

marshes, but less common in fresh marshes and least

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 59

abundant in swamps. The phenomenon is also recorded in

Baritaria Basin, Louisiana (Chmura 1990). The property

makes this pollen taxon a good indicator of brackish marsh

environment.

(13) Ragweed (Ambrosia)

Ragweed is not recorded in either White's (1979) nor

Taylor's (1992) survey. Ambrosia pollen is probably part of

the regional pollen rain. The abundance of this pollen type

is low in all the surface samples.

(14) Compos itae

Compositae pollen is also abundant in both fresh and

brackish marsh surface samples. The highest percentages

seem to be in the ecotone between fresh and brackish

marshes. Solidaqo sempervirens. and Aster spp. are perhaps

the main contributors to this pollen type. Because

Artemisia and Ambrosia pollen are separable from the

Compositae family, these taxa are calculated separately.

(15) Chenopodiaceae/Amaranthaceae

It is very difficult to separate the pollen of this

two families. Alternanthera philoxeroides is reported in

both fresh and brackish portions of the Pearl River Marsh

(Taylor,1992). In addition, Saliconia spp. are common herbs

in the coastal salt marsh area, although they are not found

in the Pearl River Marsh area. Pollen percentages of this

pollen type are generally low, however they seem to

increase toward the high salinity samples.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 60

(16) Polygonaceae

Polygonum virgatum is reported in both the fresh and brackish marshes in the study area. In the surface samples

it seems that this pollen type occurs in both fresh and

brackish marshes with relatively low percentages.

(17) Umbelliferae

Hvdrocotvle verticilata is very abundant in the fresh

marsh area with limited occurrence in the brackish marsh area. Its pollen percentages are low in most surface

samples.

(18) Cyperaceae

The pollen of Cyperaceae family is also impossible to

separate into subdivisions. Eleocharis macrostachva and E.

cellulosa are recorded as the common plants in both fresh

and brackish environments (Taylor, 1992). In addition,

Carex spp., Cvoerus odoratus. Cvperus flavescens. Scirpus

robustus. S. olnevi and Cladium iamaicense are all possible

contributors to this pollen type. The pollen type is

abundant in most surface samples, but more abundant in the

brackish marshes. This trend is similar to that of

Gramineae.

(19) Cattail (Tvpha latifolia and Tvpha angustifolia)

Tvpha latifolia pollen is designated to represent the

Tvpha pollen with tedrad structure, while T. angustifolia

represents the single-grain Tvpha pollen. Both plant

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 61

species are not recorded by White (1979, 1983) and Taylor

(1992). Their frequency of occurrence is generally low.

(20) Bulltongue (Saglttaria)

It is almost certain that most of the Sagittaria

pollen are derived from the extensive growth of Sagittaria

lancifolia in the Pearl River Marsh area. The highest

Sagittaria pollen percentages are in the brackish marshes

which correspond to their population maximum.

(21) Cow pea (VignaJ. Vigna luteola is the parent plant of this pollen type.

It is widely distributed in moist places lagoon shores,

beaches, dunes, thin woods, roadside in Louisiana and

Mississippi. Pollen of Vigna type occurs more common in

brackish marshes.

(22) Royal fern (Osmunda).

Most Osmunda spores are perhaps derived from Osmunda

regalis which is a common fresh marsh species in this

region (White, 1979; Chmura, 1990). The highest Osmunda

percentages are in the fresh marsh samples, indicating that

it is a good representative of fresh marsh habitats.

(23) Polypodiaceae

Thelvoteris palustris is recorded in the fresh marsh

area of the Pearl River Marsh (White 1979). Its spores,

identified only as Polypodiacaeae, in the Pearl River Marsh

surface samples are mainly found in swamp and fresh marsh

samples, although its frequency is extremely low. This

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spore type is very abundant in the fresh marsh surface

samples in Baritaria Basin, south-central Louisiana (Chmura

1990). In this study, Polypodiaceae is regarded as a fresh

marsh indicator, similar to Osmunda spore.

2) Discussion

Generally, it seems that the pollen distribution in

the Pearl. River Marsh surface samples follows certain

patterns. Most arboreal pollen, such as Pinus and Ouercus.

are perhaps part of the regional pollen rain, except for

Taxodium and Nvssa which are good indicators of the local

swamp forests. Herbaceous pollen are abundant in both fresh

and brackish marshes, however their compositions in the two vegetation types are different. The brackish marsh samples

are rich in Gramineae, Cyperaceae, Sagittaria and Vigna

pollen, whereas the fresh marsh samples are characterized

by high abundance of Osmunda spores as well as a low amount

of Decodon pollen. Polypodiaceae may also be regarded as a

fresh marsh indicator, due to its common occurrence in

fresh marsh samples in south central Louisiana (Chmura

1990). This trend is important to the interpretation of the

fossil pollen results.

In addition, there is some trend in the distribution

of pollen concentrations of the surface samples (Figure 5-

2). Total pollen concentrations are generally lower in the

brackish marshes than in the swamps and fresh marshes.

There is no straight forward explanation to the changes in

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 63

concentration. One possible explanation is pollen

production. The brackish marsh plants may have lower pollen

production'than the swamp and fresh marsh plants. Another

reason may be that some pollen in the brackish marsh area

are resuspended and removed by tidal water. Still another

possible reason may be the variation in sediment

composition, especially the water content of the samples.

Therefore, pollen concentration values only have limited

value as a criterion to interpret pollen result.

3) Discriminant Analysis The pollen analytical results of the surface samples

indicate that the relation between the different pollen

taxa and their parent plants varies. Considerable amount of

Pinus and Ouercus pollen is transported to the marsh area

due to their superior dispersal ability and high pollen

production. Such pollen types are good representatives of

the upland vegetation, but have little value in

reconstructing marsh vegetation. On the other hand, cypress

and tupelo and most herbaceous pollen are closely related

to their local habitats. These are the key taxa for

understanding the changing marsh history. In order to

assemble the various information provided by these pollen

taxa, discriminant analysis is a very effective statistical

technique.*

The initial step in applying discriminant analysis to

the Pearl River Marsh samples is to select the pollen types

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. as discriminant variables. Two criteria are applied to this

selection. First, they should be present in the local or

extra-local communities. Second, they should have fairly

high percentages in both the surface samples and the

sediment core. Pinus and Ouercus are excluded from the

analysis to reduce the 'noise' in the pollen data, because

they mostly come from the regional pollen rain. Fraxinus.

Ulmus. Carpinus. Salix, Licruidambar. Carva and Alnus are

also excluded because their pollen percentages are

generally low and have no clear distribution pattern. Only

Taxodium. Nvssa. and Mvrica are chosen from the arboreal

pollen taxa, since they are good representatives of the

swamp and marsh communities and have relatively high

percentages in the sediments. Most common herbaceous pollen

are included, including Decodon. Gramineae, Compositae,

Chenopodiaceae / Amaranthaceae, Polygonaceae, Umbelliferae,

Cyperaceae, Tvpha latifolia and T. angustifolia. Sagittaria

and Osmunda. These taxa are likely to be locally present in

the marsh community and have fairly high percentages in

both the surface samples and the sediment core. Ambrosia

pollen is excluded because of its generally low occurrence

in the marsh samples and because it is regarded as an

indicator of forest disturbance due to European settlement

(Webb and Bryson 1972). Vigna is excluded because the

frequency of occurrence in the core is very low, despite

greater abundance in the surface samples. Its low frequency

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. in core samples may be due to poor preservation. The

phenomenon is also recorded in Baritaria Basin where Vigna

pollen is common in the surface samples but only high at

one level of the sediment core (Chmura 1990). Percentages

of Polypodiaceae are erratically high in some levels of the

core, but generally very low in the surface samples, so

this spore type is also excluded due to uncertainties about

modern analogs. The 13 coefficients for the discriminant

analysis are based on the percentages of 14 taxa. These

include Taxodium. Nvssa. Mvrica. Decodon. Gramineae,

Compositae, Chenopodiaceae/Amaranthaceae, Polygonaceae,

Umbelliferae, Cyperaceae, Tvpha latifolia. T. angustifolia.

Sagittaria and Osmunda. However, Mvrica is excluded from

the analysis. This is because Mvrica is extrordinarily high

in the uppermost 25 cm sediment of the core which was

probably caused by human activities (see Mvrica rise' in

Chapter 11 for detail). This exclusion also garantees that

the total pollen percentages do not add up to 100% in order

to avoid some probable statistical problem.

By stepwise discriminant analysis, 9 of the 13

variables are selected by the SPSS/PC+DSCRIMINANT program

to calculate the canonical discriminant functions, until

the F level is insufficient for further computation (Table

5-1). The four taxa not included in the canonical

discriminant function include Gramineae, Polygonaceae,

Cyperaceae, and Tvpha angustifolia. They will be discussed

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Table 5-1. Standardized canonical discriminant function coefficients for the surface samples of the Pearl River Marsh.

FUNC 1 FUNC 2

TAXODIUM .71247 -.49000

NYSSA .55633 -.10749

DECODON .24470 .44301

TUBULIFL .69040 .53755

CHENOPOD -.54064 .02019

UMBELIFE -.47316 -.19800

TYPHA LATIFOLIA .33299 -.37809

SAGITTAR -.46287 -.22455

OSMUNDA .51308 .91150

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 67

separately with the other pollen types not included in the

discriminant analysis.

Based on the canonical discriminant functions, the

surface samples are reclassified according to their

discriminant scores, in order to test the validity of the

these functions. The output indicates that 93.3% of the

surface samples are correctly classified. Both the

territorialtmap and the scatterplot of the surface samples

on the canonical functions show that the group centroids of

the three vegetation types are widely separated (Figure 5-

4). This suggests that each vegetation type has a

statistically distinctive identity. The probability of

modern analog is generally high for the surface samples,

with an average value of 0.5566. The lowest values occur in

the samples taken from the ecotonal areas (Figure 5-2).

In order to use the information generated by the

canonical discriminant functions, both the most probable

and the second most probable vegetation types are

considered. The probabilities of the second most probable

vegetation type are usually much lower than those of the

first one. Both the most and the second most probable

vegetation type are shown by Table 5-2. The term

'vegetation zonal index' (VZI) was coined by Liu and Lam

(1985) to represent this characteristic of the surface

samples. In the Pearl River Marsh study, the same term is

adopted. Here the vegetation zonal index for a typical

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 68

Out

4.0

Function 1 333 33*3

-4.0

Out X

Out -6. 0 -4.0 -2.0 .0 2.0 4.0 6.0 Out

Function 2

Figure 5-4. All-group scaterplot showing the distribution of group centriods of the Pearl River Marsh surface samples (l=swamp, 2=fresh marsh, 3=brackish marsh, and *=group centroids).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 69

Table 5-2. The most and the second most probable vegetation types for the surface samples of the Pearl River Marsh as predicted by discriminant analysis. (l=swamp, 2=fresh marsh, and 3=brackish marsh).

Sample Most Second Sample Most Second

W02 1 2 B07 3 2 W05 3 1 B09 3 2 WO 6 1 2 B08 3 2 W07 1 2 B24 3 2 W10 1 2 B23 3 2 F28 2 3 B22 3 2 F27 2 3 B21 3 2 F25 3 2 B20 3 2 F01 2 1 B18 3 2 F02 2 3 BIO 3 1 F00 2 3 Bll 3 2 F03 2 3 B14 3 2 B04 3 2 B15 3 2 B05 3 2 B37 3 2 B06 3 2 B38 3 2

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 70

swamp sample with 100% probability of group membership is

defined as 1, a typical fresh marsh sample as 2, and a

typical brackish marsh as 3. Samples having intermediate

probabilities are calculated by the equation:

VZI=I(most)xP(most)+1(2nd)xP(2nd)

I (most): index of most probable vegetation type

(swamp=l;.fresh marsh=2; brackish marsh=3)

P(most): probability of group membership of the most

probable vegetation type

I (2nd): index of the 2nd probable vegetation type

(swamp=l; fresh marsh=2; brackish marsh=3)

P(2nd): probability of group membership of the 2nd

probable vegetation type

For instance, if a sample has 80% probability as fresh

marsh and 20% probability as brackish marsh, the zonal

index would be 2.2.

Both the vegetation zonal index and the probability of I modern analog are included in the pollen diagram (Figure 5-

2 ) .

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 6. MODERN DIATOM DISTRIBUTION IN THE

PEARL RIVER MARSH

Diatom analysis is another crucial part of this study.

Sullivan (1976, 1978, 1983) conducted a number of modern

diatom studies in the Gulf of Mexico coastal region. His

work mainly focuses on the effects of eutrophication on

diatom populations, but does not deal with long term

environmental changes directly. However, his research

provides an inventory of the modern diatom flora in this

region which is very useful for the diatom species

identification in this study, especially in the surface

samples.

Forty diatom surface samples were analyzed for this

study. Diatoms are abundant in all of the samples and the

species diversity is very high. Sullivan (1978) encountered

119 diatom species in just five salt marsh samples. However

due to limited reference materials, not all of the diatoms

in the Pearl River Marsh surface samples were identified to

species. Some diatoms were only identified to genus or

counted as unknown or just given the genus names. One

hundred and seven diatom species were identified from the

surface samples. Since there is a considerable difference

between the modern and fossil diatom floras (discussed

later), special attention was paid to the species which are

common in the sediment core. The discrepancy is possibly

caused by differential dissolution of diatom valves during

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 72

fossilization (Lewin 1961? Eronen 1974). Also due to this

difference, an attempt to apply discriminant analysis to

the diatom samples was aborted since the probabilities of

modern analog for most fossil diatom assemblages are zero.

Two diagrams are plotted to represent the main diatom

species in the surface samples. One of them includes the

main freshwater diatoms (Figure 6-1). Another includes the

main brackish and marine diatoms (Figure 6-2). In addition,

in order to understand the diatom distribution in salt

marshes, four more surface samples were taken from

Cocodrie, south-central Louisiana (Figure 6-3). Since the

diagrams do not show the percentages of unknown and

indeterminable diatoms, the percentages of fresh and

brackish/marine diatoms may not add up to 100% in some

levels. The general ecological requirements and the

distribution of the main diatom taxa in the study area are

discussed below.

1) Freshwater Diatoms

The freshwater diatoms (Figure 6-1) here refer to the

oligohalobiens which typically occur in water salinities of

less than 0.5%, including: a) the true fresh water diatoms

(halophob); b) the diatoms that can withstand a small

amount of salt (indifferent); and c) the diatoms that are

best developed in water with low salinity (halophil)

(Patrick and Reimer 1966).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. w Fresh Swamp Broddsh -/ __

*“ ^ ^ *“ ^q- ^ ^ ^ 4 v /VV/// // x . 4 ■V* ^ 4#V* * - Figure 6-1. Freshwater surface samples diatoms (percentages astotal diatoms of counted). the Pearl River Marsh .. •5s' •5s' N£* v ^ WSsS//SI// y

i TO rs w r a m ra ra t ra tu BA m n n nt 127 BA S* 807 809 900 8< >23 122 121 CO M no hi H2 914 119 937 IS 107 wa W9 ¥10

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FA 40 40 80 40 vC> & o- P . P ^ 4 t . <<> r y 'A c£" .. .. <0- A a F FP • O s P3' a -'V ^-o

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.

Figure 6-3. marshsurface saltFigurethe at samples 6-3. a Diatoms of in counted). Cocodrie, Cocodrie, south-central LA (percentages as total diatoms

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(1) Achnanthes temperii

Ecology: Distributed from extreme upper- to middle-

tidal portions of coastal rivers (Patrick and Reimer 1966).

Distribution: found in both brackish and fresh marsh

samples at very low abundance. (2) Bacillaria paradoxa

Ecology: In fresh as well as brackish water (Dodd

1987; Heurck, 1962).

Distribution: Most abundant in brackish marshes, but

also common in fresh marshes.

(3) Campvlodiscus echeneis

Ecology: In freshwater with high conductivity or

brackish water (Hustedt 1930; Heurck 1962).

Distribution: Found in brackish marshes at very low

abundance.

(4) Cocconeis placentula

Ecology: A widespread eurytopous species epiphytic on

aquatic plants and other objects. More commonly found in

circumneutral to alkaline waters. Oligohalobe

(indifferent); Cosmopolite (Patrick and Reimer 1966; Foged

1984).

Distribution: Found in fresh as well as brackish marsh

samples at low to medium percentages.

(5) Cvclotela meneqhiniana

Ecology: Oligohalobien (halophil). Best developed in

water with small amount of salt (Patrick and Reimer 1966).

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Distribution: Found in all surface samples but seems

most abundant in fresh marshes of the study area.

(6) Cvmbella

One species identified is possibly Cvmbella mueleri.

but only genus name is provided because of insufficient

specimens.

Ecology: Most taxa of this genus are restricted to

fresh water (Patrick and Reimer 1975).

Distribution: Occasionally found in swamp and fresh

marsh surface samples.

(7) Dioloneis elliptica

Ecology: Fresh water to slightly brackish water; found

in bogs, lakes and springs (Patrick and Reimer, 1966).

Distribution: Found in surface samples from all three

vegetation types of the Pearl River Marsh at low

percentages. Slightly more abundance found in fresh marsh

area.

(8) Diploneis ovalis

Ecology: Fresh to slightly brackish water, sometimes

in damp places (aerophil) (Patrick and Reimer 1966)

Distribution: Found very frequently in swamp and fresh

marsh samples, but greatly reduced in abundance in the

brackish marsh samples.

(9) Eoithemia

One species is identified as E. adnata var. minor.

Frequency of occurrence is generally low.

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Ecology: Most species of Epithemia are found in fresh

water habitats (Patrick- and Reimer 1975).

Distribution: Very rare in all vegetation types of the

Pearl River Marsh area.

(10) Eunotia

Nine species of Eunotia are identified in the Pearl

River Marsh surface samples. The most common ones are E.

maior. E. pectinalis. E. pectinalis var. minor, and E.

pectinalis var. ventricosa. Other less common species are

E . curvata. E. diodon. E. flexosa. E . praerupta and E.

triqiba. In the diatom diagram (Figure 6-1), E. flexosa. E.

maior. E. pectinalis. E. pectinalis var. minor, and E.

pectinalis var. ventricosa are displayed separately from

Eunotia spp. which represents the diatoms unidentifiable to

species but belonging to this genus.

Ecology: The species of this genus are usually found

in water with low calcium and chloride contents, and often

in oligotrophic or dystrophic water (Patrick and Reimer,

1966).

Distribution: Very abundant in the swamp surface

samples of the study area, and scarcely found in other

habitats.

(11) Fragilaria construens var. venter

Ecology: Widely distributed, seems to prefer water of

fairly low nutrient content (oligotrophic to mesotrophic)

(Patrick and Reimer, 1966).

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Distribution: Found in both fresh and brackish marsh

samples of the study area at low percentages. It was found

very abundant in a freshwater lake sediment core taken from

Horn Island, Mississippi (Discussed later in Chapter 12).

(12) Gomphonema

The genus is easily identifiable with its wedge-shaped

frustules, but difficult to be separated into species, so

that only genus name is given to this diatom type. Five

possible species are included under this genus, G.

acuminatum. G . intricatum. G. lanceolatum.and G. parvulum.

Ecology: The ecological requirement of this genus is

mostly fresh habitats with only a few exceptions (Patrick I and Reimer, 1975).

Distribution: Found in the swamp and fresh marsh

surface samples at low frequencies.

(13) Melosira qranulata and M. qranulata var. anqustissima

Ecology: In fresh to slightly brackish water.

Distribution: Found in all surface samples. Abundance

varies in different locations.

(14) Navicula capitata

Ecology: Seems to tolerate a wide range of water

chemistry (Patrick and Reimer 1966).

Distribution: In fresh and brackish marshes at low

percentages.

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(15) Navicula eleaans

Ecology: In fresh to brackish water (Patrick and

Reimer 1966).

Distribution: In both fresh and brackish marshes of

the study area at low percentages.

(16) Navicula placenta

Ecology: Fresh water, often associated with moss as an aerophil species (Patrick and Reimer 1966).

Distribution: Found in swamp and fresh marsh surface

samples of the study area at low abundance.

(17) Navicula pusilla

Ecology: Seems to prefer fresh water of high mineral

content or slightly brackish water, aerophil, often found

in cool temperate area (Patrick and Reimer 1966).

Distribution: Present only at low percentages; more

commonly found in fresh marshes and swamps than in brackish

marshes.

(18) Navicula tripunctata

Ecology: Widely distributed in fresh water and in

slightly brackish water (Patrick and Reimer 1966).

Distribution: Very abundant in fresh and brackish

marsh samples.

(19) Nitzschia amphibia

Ecology: Freshwater diatom (Heurck 1962).

Distribution: Found in both fresh and brackish marshes

of the Pearl River delta at low percentages.

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(20) Nitzschia brevissima

Ecology: In fresh to brackish water (Hustedt 1930).

Distribution: Found in both fresh and brackish marshes

at high percentages.

(21) Pinnularia

12 species of Pinnularia are identified from the

surface samples. The most common ones includes P. biceps.

P. microstauron. P. maior. P. viridis. P. nobilis. Other

species includes P. abauiensis. P. acrosphaeria. P.

acuminata. P. boqotensis var. undulata. P. braunii var.

amphicephala. P. diverqens. P. qibba. P. subcapitata. P.

substomatophora. and P. sudetica.

Ecology: Fresh water with low mineral content (Patrick

and Reimer 1975).

Distribution: Common in the swamp and fresh marsh

samples, but also in brackish marsh samples occasionally.

(22) Rhopalodia aibberula

Ecology: Seems to prefer water with some chloride

(halophil), but may be found in water of low conductivity.

It is a widely tolerant species (Patrick and Reimer 1975).

Distribution: Very abundant in the ecotonal zone

between swamps and fresh marshes, but also present in most

samples at low percentages.

(23) Stauroneis phoenicenteron

Ecology: Oligohalob, pH indifferent— has a wide range

of ecological tolerance (Patrick and Reimer 1966).

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Distribution: In swamp surface samples at very low

frequencies.

(24) Tabularia tabulata Ecology: Most species of Tabularia are in freshwater

(Patrick and Reimer 1966).

Distribution: Highest percentages found in swamp and

fresh samples.

(25) Terosinoe musica

Ecology: Oligohalobe (indifferent); alkaliphil;

euryhaline; in running freshwater (Foged 1984).

Distribution: Occasionally occurs in fresh and

brackish marshes.

2) Marine and Brackish Water Diatoms

The brackish water diatoms here refer to mesohalobiens

living in water with salt concentration of 0.5-2%. The

marine diatoms represent the polyhalobiens adapted to water

with salt concentrations of 3-4% (Patrick and Reimer 1966).

Since many species can survive under both hydrological

conditions, the brackish and marine diatoms are shown in

one group (Figure 6-2). In addition, the diatoms identified

from the salt marsh samples of Cocodrie, south-central

Louisiana, are presented in Figure 6-3.

(1) Achnanthes haukinana

Ecology: Most commonly found in slightly to moderately

brackish water, also reported from inland fresh water with

relatively high mineral content (Patrick and Remier 1966).

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Distribution: Very abundant in the brackish marsh

samples; absent in swamp and fresh marsh samples.

(2) Actinocvclus beaufortianus

Ecology: Marine diatom (Sullivan 1983; Hustedt 1955).

Distribution: Occasionally found in brackish marsh

area in the Pearl River Marsh. Surface samples taken from

south central Louisiana show increased abundance of this

species in salt marsh (Figure 6-3). (3) Actinoptvchus undulutus

Ecology: Marine diatom (Heurck 1962).

Distribution: In brackish marshes with low

percentages. Also found in the surface samples from salt

marshes of south central Louisiana at very high abundance

(Figure 6-3).

(4) Biddulohia lavis Ecology: Marine and brackish water (Wolle 1890).

Distribution: Occasionally very abundant in the

brackish marsh samples.

(5) Coscinodiscus

Most species of the genus are marine diatoms. Two

species are identified in the surface samples, C.

excentricus and C. radiatus.

Ecology: Marine diatoms (Foged 1984).

Distribution: C. excentricus is found in both fresh

and brackish marshes. C. radiatus is limited to the

brackish marsh samples. Both species are very abundant in

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the salt marsh samples of south-central Louisiana (Figure

5-3). In the diatom diagram, Coscinodiscus spp. represents

all the species undifferentiated.

(6) Diploneis bombus

Ecology: Brackish water (Patrick and Reimer 1966).

Distribution: In brackish marshes at low percentages.

(7) Diploneis smithii

Ecology: In slightly brackish to brackish water.

Distribution: Found in brackish marshes at very low

percentages.

(8) Navicula maculata

Ecology: Brackish water (Patrick and Reimer 1966).

Distribution: Found in brackish marsh samples.

(9) Navicula pereqrina

Ecology: Seems to prefer water with high mineral

content or brackish water (Patrick and Reimer 1962).

Distribution: Occasionally found in samples from all

three major vegetation types of the study area.

(10) Nitzschia qranulata

Ecology: Brackish water (Foged 1974).

Distribution: Occasionally found in brackish marsh

samples. Also very abundant in salt marsh samples from

south-central Louisiana.

(11) Nitzschia scalaris

Ecology: Mesohalobe to polyhalobe; Cosmopolite? (Foged

1984).

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Distribution: Found in both fresh and brackish marshes

at low abundance.

(12) Paralia sulcata

Ecology: Polyhalobe (Foged 1984).

Distribution: Found in brackish marshes at low

percentages.

3) Discussion

In general, it seems that there is some regularity in

the zonal distribution of diatoms in the Pearl River Marsh

area. The swamp samples are characterized by the freshwater

diatoms Eunotia spp, E. maior. E. oectinalis. E. pectinalis

var. minor. E. pectinalis var. ventricosa. Pinnularia spp,

P. biceps, and P. maior. The fresh marsh samples contain a

relatively high percentages of Cvclotela meneahiniana. Diploneis elliptica. D. ovalis. Gomphonema spp., Melosria

granulata var. angustissima. Rhopalodia qibberula. and

Tabularia tatulata. Many of these diatoms were also found

in the swamp samples, indicating the overall freshwater

habitats. The fresh and brackish marshes also share many

common freshwater species such as Bacillaria parodoxa.

Cvclotela meneghiniana. Navicula tripunctata. and Nitzschia

breveissima. This is possibly due to the low salinity in

the Pearl River Marsh -- only 1.5 ppt in the brackish

marshes (White 1983). However, there is a general trend

that the bfrackish marsh samples contain more brackish and

marine species, such as Achnanthes haukinana. Achtinocvclus

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. beaufortianus. Coscinodiscus s p p ., C. excentricus. C.

radiatus. Diploneis bombus. Nitzschia granulata. N.

scalaris and Paralia sulcata. These diatoms become more

prominent in the salt marsh samples at Cocodrie, south

central Louisiana. This pattern of zonal distribution of

diatoms along the environmental gradient is essential to

the interpretation of the fossil diatom results.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 7. POLLEN ANALYTICAL RESULTS OF THE

PEARL RIVER MARSH SEDIMENT CORE

The 8.81 m sediment core taken from the Pearl River

Marsh was intensively sampled for pollen at 10 cm

intervals, except for the uppermost 84 cm where 2 cm

sampling intervals were used. A total of 65 pollen taxa

were identified from this core. The bottom 71 cm of the

core is composed of sandy sediment (see Figure 4-1 in Chapter 4) with very low value of Loss-on-Ignition as well

as very low pollen and diatom concentrations. This section

is skipped for pollen and diatom analyses. The sampling

procedure paid special attention to the levels where

organic contents are low because such levels are crucial to

the identification of large fluvial discharge events. Mean sedimentation rates are derived from the C-14 dates and Cs-

137 measurement (Figure 7-1; 7-2; Table 7-1). The rate is

used to calculate the estimated age for each sample of the

core.

The canonical discriminant functions derived from the

surface pollen samples are automatically applied to the

classification of the pollen assemblages in the core by the

SPSS/PC+DSCRIMINANT program. Both the first and the second

most probable vegetation types are assigned to each pollen

sample (Table 7-2). The pollen diagram is shown as Figure

7-3. It is divided into ten pollen zones according to the

vegetation zonal indices and the actual pollen assemblages.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 88

0.6

200 1.0

a o B 400

600 - 0 mm/year

800 0 1000 2000 3000 4000 5000 6000 Years B.P

Figure 7-1. Positions of the C-14 dates of the Pearl River Marsh core and the inferred average sedimentation rates.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. without prohibited reproduction Further owner. copyright the of permission with Reproduced iue -. s17 esrmn, f h uprot 0 cm 20 uppermost the of measurement, Cs-137 7-2. Figure eieto h er ie as oe PR1 core, Marsh River Pearl the of sediment Depth (cm) - -15.0- - 10.0 -5.0- 20.0 0.0 0.0 0.5 s17 ciiy (pCI/g) Activity Cs-137 1.0 89 Table 7-1. C-14 dates of the Pearl River Marsh core

Sample No. Depth (cm) Age (years B

Beta-654 39* 25-26 210±60

Beta-65440* 73-74 1050±70

Beta-60235 117-132 1240160

Beta-65441 267-275 2730170

Beta-60236* 471-472 4490160

Beta-60237 736-746 5830190

* samples dated by accelerator mass spectrometry (AMS) technique

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-A 1. k 1

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54 (0

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I..i Ih i m h h 04 10 W * a) 7 tr [-’ +)a s c a> a> 54 a 3 54 o»

1 a § S s RI

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 92

Table 7-2. The most and the second most probable vegetation types for the Pearl River Marsh core (l=swamp, 2=fresh marsh, and 3=brackish marsh).

Depth Most 2nd Depth Most 2nd Depth Most 2nd (cm) (cm) (cm) 0.0 2 3 114.9 2 1 450.0 2 1 3.1 3 2 124.0 2 1 460.0 2 1 5.2 2 3 130.1 2 1 472.0 2 1 7.3 3 2 132.0 2 1 480.0 3 2 10.4 3 2 140.2 3 2 490.0 2 3 13.6 2 3 150.4 3 2 500.0 2 3 15.7 2 3 160.5 3 2 510.0 2 3 17.8 2 3 170.7 3 2 520.0 2 3 20.9 2 3 180.8 2 3 530.0 2 3 23.0 2 3 191.0 2 3 540.0 2 3 26.1 3 2 201.1 3 2 550.1 2 3 29.3 3 2 211.3 3 2 560.2 3 2 31.3 3 2 221.4 3 1 570.3 3 2 33.4 3 2 227.0 1 3 580.5 3 2 35.5 3 2 231.6 3 1 590.6 3 2 37.6 3 2 241.7 2 1 600.7 3 2 39.7 3 2 251.9 2 3 610.8 2 1 41.8 3 2 262.0 2 3 620.9 3 2 46.0 3 2 270.0 3 2 631.0 3 2 48.1 3 2 276.0 2 3 641.2 3 2 50.1 3 2 280.0 2 3 651.3 3 2 52.2 3 2 290.0 3 2 661.4 3 2 54.3 3 2 300.0 3 2 671.5 3 1 56.4 3 2 310.0 3 2 681.6 2 3 58.5 3 2 320.0 3 2 690.0 2 3 62.7 3 2 330.0 2 3 693.0 3 2 64.8 3 2 340.0 2 3 700.0 3 2 66.9 3 2 350.0 2 3 710.0 3 2 68.9 3 2 360. 0 3 2 720. 0 3 2 71.0 3 2 370.0 3 2 730.0 3 2 74.2 3 2 380.0 3 2 740.0 3 2 76.3 3 2 390.0 3 2 750.0 3 2 79.4 3 2 400.0 2 3 776.0 1 2 81.5 3 2 410.0 3 2 783.0 1 3 83.6 3 2 420.0 2 3 790.0 1 3 94.0 3 2 430.0 3 2 810.0 1 3 104.5 2 1 440.0 2 1

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 93 * 1) Pollen Zone I From 810 to 753 cm, the estimated age is from 6200 to 5900

years B.P. In this zone, arboreal pollen are dominant.

Taxodium and Nvssa are the most prominent components.

Taxodium is more important in the lower part of this

section, whereas Nvssa is more abundant upward. The

vegetation zonal indices for all four samples in this zone

are 1, indicating a swamp vegetation. The sediment of the

upper 25 (cm of this zone is wood coming from a log with ft high LOI value, possibly suggesting the existence of a r swamp forest at the site. Both pollen and macrofossil

evidences thus indicate that this pollen zone represents a

swamp. In addition to the swamp forest pollen, the same

pollen zone contains relatively high percentages of Carva.

Pinus. Ouercus. Fraxinus. Salix and Licmidambar. These

arboreal pollen further indicate a forest vegetation in the

swamp or in the upland.

2) Pollen Zone II

From 753 to 688 cm, the estimated age is from 5900 to

5600 years B.P. There is a high percentage of Mvrica

pollen at the bottom of this zone, possibly indicating a

transition from swamp to a shrubby vegetation. This

transitional pollen assemblage is quickly replaced by a

brackish marsh pollen assemblage dominated by Gramineae,

Cyperaceae, and Chenopodiaceae/Amaranthaceae. Other less

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 94

important pollen taxa in this zone include Compositae,

Polypodiaceae, Umbelliferae, Saoittaria and Polypodiaceae.

Vegetation zonal indices for most pollen samples in this

zone are close to 3 with high probability of modern analog.

The general appearance of this zone represents a brackish

marsh type.

From this zone upward, herbaceous pollen became the

principal components of the pollen assemblage. This zone

represents the initiation of the coastal marsh vegetation

at the coring site. The former Taxodium. Nvssa. Carva and

Ulmus pollen are greatly reduced in this zone. Instead,

Pinus and Ouercus become dominant which were probably

derived from the upland vegetation. Their percentages do

not change significantly upward until the top of the core,

although random variations do exist.

3) Pollen Zone III

From 688 to 550 cm, the estimated age is from 5600 to

4900 years B.P. In comparing with the previous zone, the

principal pollen taxa are still Gramineae and Cyperaceae.

However, this zone contains less Chenopodiaceae/

Amaranthaceae but more Osmunda which is a common fresh

marsh indicator. Vegetation zonal indices are low at the

bottom of this zone but are not low enough to be classified

as fresh marshes at the upper part of this zone. The

overall appearance of this zone still represent a brackish

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 95

marsh vegetation, although it seems true that the marsh is

less saline than before.

4) Pollen Zone IV

From 550 to 435 cm the estimated age is from 4900 to

4200 years B.P. This pollen zone starts with a particularly

high peak of Polypodiaceae and ends with an enormous peak

of Decodon. both are fresh marsh indicators. In addition,

Osmunda is common in all samples of this zone. Although

vegetation zonal indices at the lower part of this zone are

high, these samples should not be regarded as brackish

marshes because Polypodiaceae, a fresh marsh indicator, is

not included in the discriminant analysis. Vegetation zonal

indices of two samples in the middle part of this zone are

also high which may represent the real salt marsh

situation. This zone generally indicates a fresh marsh

environment which is interrupted by a short interval of

slightly saline environment.

5) Pollen Zone V

From 435 to 345 cm, the estimated age is from 4200 to

3400 years B.P. The dominant pollen types are Gramineae and

Cyperaceae. Other common taxa include Saqittaria. Compositae, Umbelliferae, Tvoha anqustifolia and Mvrica.

The typical fresh marsh indicators, Decodon. Osmunda. and

Polypodiaceae, decreased significantly, indicating a

brackish marsh environment.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 96

6) Pollen Zone VI

From 345 to 235 cm, the estimated age is from 3400 to

2400 years B.P. In the middle of the zone, Cyperaceae is

dominant. Gramineae, Saqittaria. Tvpha anqustifolia.

Umbelliferae, and Compositae remain as important components

of the pollen assemblages. There is strong evidence that

this zone represents a fresh marsh environment because the

presence of abundant Osmunda spores. Vegetation zonal

indices indicate that the marsh changed from fresh to

brackish and then switch back to fresh again.

7) Pollen Zone VII

From 185 to 135 cm, the estimate age is from 2400 to

1400 years B.P. In this zone, Osmunda decreases dramatically to very minor importance. The most common

pollen types are Gramineae, Cyperaceae, Tvpha anqustifolia.

Saqittaria and Compositae. In addition, Chenopodiaceae/

Amaranthaceae has a single high value in the middle of the

zone. Two samples in the middle part have low vegetation

zonal indices, which probably due to slightly increased

percentages of Decodon and Osmunda; they represent a brief

return to fresh marsh condition. However, generally

speaking, this pollen zone represents a typical brackish

marsh with high vegetation zonal index values and high

probabilities of modern analog.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 97

8) Pollen Zone VIII

From 135 to 100 cm, the estimated age is from 1400 to

1100 years B.P. This narrow zone is characterized by a

prominent peak in Decodon pollen, accompanied by very high

percentage of Osmunda. Other pollen types, Gramineae,

Cyperaceae, Saqittaria. Compositae, Umbelliferae are less

frequent. Vegetation zonal indices in this zone fall

uniformly to 2. This zone represents a fresh marsh

vegetation that does not resemble any modern fresh marsh

community indicated in the surface samples because the

probabilities of modern analog of this zone are all zero.

9) Pollen Zone IX

From 100 to 24 cm, the estimated age is from 1100 to

2 00 B.P. Gramineae is dominant, especially in the lower

part of this zone. Other common pollen types are

Cyperaceae, Compositae, Saqittaria and Mvrica. Osmunda and

Decodon declined to very low percentages. The vegetation

zonal indices of all the samples are close to 3 with high

probabilities of modern analog. The overall appearance of

this zone represents a brackish marsh vegetation.

10) Pollen Zone X

The last zone of the core includes the uppermost 24 cm

of sediments. The bottom of this zone is 200 B.P., and the

top is quite recent since abundant Cs-137 activities is

detected from 10 cm upward (Figure 7-2). This zone is

characterized by high percentages of Mvrica and Osmunda.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Gramineae and Compositae are still common, but Cyperaceae decreases considerably. Viqna has a high value only in the

topmost sample, but generally low in the rest of the zone

and in the whole core. Most samples' vegetation zonal

indices in this zone are close to 2 which represents a

fresh marsh vegetation. The probabilities of modern analog

are generally low at the lower part of the zone because of

the unusually high percentage of Mvrica. However, they

increase remarkably at the top, which is expected from a

surface sample representing the modern vegetation.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 8. DIATOM ANALYTICAL RESULTS OP THE

PEARL RIVER MARSH SEDIMENT CORE

A total of 102 fossil diatom species were identified

from the Pearl River Marsh core, PR1. The diatom data are

presented in two diagrams, one for freshwater species

(Figure 8-1), and one for brackish water and marine diatoms

(Figure 8-2). An attempt to apply the discriminant analysis

to the diatom samples was aborted because the probability

of modern analog for' most of the samples are close to zero.

A comparison between the surface samples and the cored

samples shows that their principal diatoms are different.

The species Bacillaria paradoxa. Cvclotela meneahiniana.

Diploneis ovalis, Navicula tripunctata. Nitzschia

brevissima, and Nitzschia obtusa are very abundant in the

surface samples, but their frequencies of occurrence are

minimal in the stratigraphic samples where Diploneis

elleptica. Pinnularia spp., Nitzschia scalaris.

Actinocyclus beaufornianus are dominant. It is this

difference that explains the 'no-analog1 situation. The

cause for this difference is intriguing. It seems unlikely

that the diatom community has changed so dramatically

recently, because the pollen and sediment records indicate

that the basic environment in the area has remained a

'marsh' since about 6000 years B.P. One possible

explanation is that the fossil diatom flora has changed

during diagenesis due to partial dissolution (Lewin 1961;,

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br> % j k $ %. %i *

CO -P 0) <1)

§ 3 8 g

I M I 11 I P IT 'H 'I I

e I O © r* CO •« +1 "H o o n C\ CNi> •c

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 101 Transgression Transgression Regression Transgression R e g r e s s io n

■/ 40 40 S3 W /* # 60 60 80 - °1 50- <5? 100 150- 600- 200 250 300- 650- 350- 400- 450- 500- 550 700- 750 800 850J

f 1000; 2000I 4000 3000- 5000 6000-

b Figure 8-2. Brackish water and marine diatoms of the Pearl RiverMarsh core (percentages as total diatoms counted). 210±60 - 210±60 5830190a 4490180- 1240160s 2730170 1050±70 - - 1050±70

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 102

Eronen 1974; Eronen et al 1987; see Chapter 2 for

discussion). The core is divided into ten diatom zones

according to the diatom assemblages. They are independent

from the'pollen zones, but the pollen and diatom zones seem

to show a high degree of correspondence.

1) Diatom Zone I

From 810 to 753 cm, the estimated age is from 6200 to

5900 years B.P. This zone is characterized by the abundance

of fresh water diatoms. The most common species is Eunotia

maior. In addition, E. pectinalis. E. pectinalis var.

minor. E. pectinalis var. ventricosa and other fresh water

species such as Pinnularia spp. and Tabularia tabulata are

also common. These are all typical diatom species found in

the modern swamp surface samples. There are few marine and

brackish water diatoms, expect for Nitzschia scalaris which

occurs at very low percentages. It is evident that this

zone represents a swamp.

2) Diatom Zone II

From 753 to 705 cm, the estimated age is from 5900 to

5700 years B.P. The most important diatoms in this zone are

the marine and brackish species: Actinocvclus beaufotianus.

Fresh water diatoms are also very common in the diatom

assemblages. Most of them are the halophil or indifferent

types, such as Campvlodiscus echeneis. Rhopalodia

gibberula. Diploneis elliptica. Cvclotela meneqhiniana.

Diploneis ovalis. Navicula pusilla. The halophobic fresh

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water diatom, Eunotia spp., disappeared. This zone seems to

represent a marsh environment with moderate salinity, i.e.

brackish marsh.

3) Diatom Zone III

From 705 to 605 cm, the estimated age is from 5700 to

5200 years B.P. Fresh water diatoms are the dominant

components in the diatom assemblages. There are three major

successive communities during this half millennium. The

first one is dominated by Nitzschia amphibia. It is

replaced by Fragilaria construens var. venter. which is in turn replaced by Tabularia tabulata and Epithemia spp.

These rapid diatom community replacements imply that the

hydrological conditions were not constant, i.e. the

physical or chemical properties of the water varied through

time, which were suitable for the growth of different

diatoms. Other fresh water species include Bacillaria

paradoxa. Cvclotela meneqhiniana. Diploneis elliptica.

Pinnularia spp., Rhopalodia gibba. R. gibberula. The

brackish water/marine diatoms, Actinocvclus beaufortianus

and Nitzschia scalaris. are drastically reduced. In

general, it seems that the marsh hydrological conditions

became less saline than in Diatom Zone II.

4) Diatom Zone IV

From -605 to 475 cm, the estimated age is from 5200 to

4500 years B.P. The percentages of the main diatoms

fluctuate very frequently in this zone, as evident in the

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pronounced sawtooth-shaped diatom curves. Among the marine

diatoms, Actinocvclus beaufortianus is the most abundant,

but its prominence is frequently interrupted by peaks in

fresh water diatoms, particularly by Diploneis elliptica

and Rhopalodia gibberula. In addition, species diversity in

this zone is high. Halophobic fresh water species, for

example Eunotia spp., E. maior. E. pectinalis. E.

pectinalis var. minor. E. pectinalis var. ventricosa.

Pinnularia spp., Pinnularia maior. P. viridis. and

Rhopalodia gibba. are very frequent in this zone. Some of

them continue the same trend already started in the

previous zone. In general, this zone represents a

hydrological condition which is very unstable, switching

frequently between fresh and saline conditions.

5) Diatom Zone V

From 475 to 345 cm, the estimated age is from 4500 to

3400 years B.P. This zone still has high abundance of

Actinocvclus beaufortianus. There are two isolated peaks of

the brackish water species Paralia sulcata and Navicula

maculata at the lower portion of this zone. In addition,

Coscinodiscus spp., Nitzschia granulata. and N. scalaris

are also common brackish species. Few halophobic fresh

water species are present. Some halophilic or indifferent

species include Campvlodiscus echeneis. Achnanthes

temperei. Diploneis elliptica. Navicula pusilla. Rhopalodia

gibberula and Terpsinoe musica. Since the diatom

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communities change so frequently, the overall appearance of

this zone still represents a relatively unstable brackish

water environment.

6) Diatom Zone VI

From 345 to 215 cm, the estimated age is from 3400 to

2200 years B.P. The brackish water and marine diatoms are

gradually replaced by freshwater diatoms which include many

halophobic species, such as Eunotia spp., Eoithemia spp.,

Gomphonema spp., and Pinnularia spp. At the lower part of

this zone, marine species are still frequent. However, they

are replaced by Nitzschia scalaris which usually requires

brackish environment. In general, this zone represents a

predominantly fresh marsh environment.

7) Diatom Zone VII

From 215 to 135 cm, the estimated age is from 2200 to

14 00 years B.P. Nitzschia scalaris increased from Diatom

Zone VI to reach its first maximum in the lower part of

this zone. However, this peak is truncated by a sudden

increase of fresh water diatom, Diploneis elliptica. which

is then replaced by dramatic increase in Actinocvclus

beafortianus. Besides Diploneis elliptica. some other fresh

water species are also present in this zone, including

Achnanthes temperei. Bacillaria parodoxa. Cocconeis

placenta. Cvclotela meneqhiniana. Diploneis ovalis.

Navicula placenta. Pinnularia spp., Rhopalodia gibberula.

Tabularia tabulata. and Terpinoe musica. Typical halophobic

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species are almost absent. Generally speaking, this

suggests a very brackish hydrological condition which is

only interrupted by a very short episode of fresh water

environment.

8) Diatom Zone VIII

From 135 to 100 cm, the estimated age is from 1400 to

1100 years B.P. Actinocvclus beaufortianus is greatly

reduced in this zone and remains low in percentages ever

after. The most common diatoms in this zone are fresh water

species, such as Diploneis elliptic. Pinnularia spp.,

Rhopalodia gibberula. Navicula pusilla. Among the brackish

water and marine diatoms, Coscinodiscus spp., C.

excentricus. Paralia sulcata and Nitzschia scalaris are the

most important ones. In comparing with the previous zone,

this one obviously represents a much less saline

hydrological condition.

9) Diatom Zone IX

From 100 to 24 cm, the estimated age is from 1100 to

200 years B.P. This zone is characterized by the

extraordinary richness of the brackish species, Nitzschia

scalaris. Among the freshwater diatoms, Diploneis

elliptica. Pinnularia spp., P. maior. and P. viridis. are

also very abundant. However, the overall impression of this

diatom assemblage suggests a brackish hydrological

condition.

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10) Diatom Zone X This zone includes the uppermost 24 cm sediment of the

core. The estimated age is from 200 years B.P. to the

present. Nitzschia scalaris became very insignificant in

this zone. Freshwater species are dominant, especially

Diploneis elliptica. Rhopalodia gibberula. Navicula

placenta. and N. pusilla. There is an increase in the

percentages of marine diatoms, Coscinodiscus spp., C.

excentricus. C. radiatus in the middle part of this zone,

indicating a temporary rise in salinity. The uppermost 7 cm

of sediment contain many species which are abundant only in i, the surface samples, Bacillaria parodoxa. Diploneis ovalis.

Nitzschia brevissima. Navicula tripunctata. but are seldom

found in the rest of the sediment core.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 9. SEA LEVEL CHANGES DURING THE PAST 6200 YEARS

1) Hypothesis

Due to the flat topography, the coastal marsh area is

very sensitive to sea level change. Because the various

marshes are situated very close to sea level, only a slight

sea level rise will inundate many parts of the brackish

marshes and convert the fresh marshes to more saline ones.

There have been a considerable number of studies about sea

level changes in the Gulf of Mexico region (Emery &

Garrison 1967; Nelson & Bray 1970; Curry 1965; McFarlan

1961; Coleman & Smith 1964; Pirazzoli 1991). Generally,

they show that sea level in this region has been rising

continuously during the past 6000 years. Most of those sea

level change reconstructions are based on the study of

topographic features on the continental shelf. They are

broadly accurate for late Pleistocene and early Holocene

when the magnitudes of sea level changes were more

pronounced. However, the geological time control for late

Holocene, especially the last one or two millennia, is

relatively poor. Recently, a different picture is proposed (Stapor et

al 1991; Tanner and Demirpolat 1988; Tanner 1991; 1992;

Tanner and Donoghue 1993). These studies show that the

extensive development of beach ridges in the Gulf of Mexico

coastal region is related to sea level changes. They

108

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rekindled another interest in the late Holocene sea level

research for the Gulf of Mexico coastal region.

There are ten zones in both the pollen and diatom diagrams of the Pearl River Marsh sediment core, which are

broadly consistent with each other, but their boundaries

differ considerably at some levels. Level by level

correlation is even more difficult, especially for the

section from about 340 to 700 cm. The reason for this discrepancy is possibly that diatom and high plants respond

to environmental changes at different rates due to their

different life cycles. Diatoms should be more sensitive to

environmental changes due to their short life cycles. This

is theoretically possible and also proved by the research

in south-central Louisiana about the pollen and diatom

responses to Hurricane Andrew of 1992 (see Chapter 11 for

detail). This assumption is important in interpreting the

pollen and diatom analytical results.

The study of sea level changes in the estuary area is

complicated by large fluvial discharge events. It seems

more straight forward to predict a regression layer from

the biostratigraphy. The hypothesis is that a sea level

drop will result in high percentages of both fresh water

diatom and fresh marsh vegetation at the coring site; at

the same time, marine and brackish components should become

less freqyent. In regression, the predictors of large

fluvial discharge and sea level changes are consistent with

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each other, both are related to 'fresh conditions'.

Moreover, since sea level change involves a relatively long

time, the sediment incorporated in such a period should be relatively thick which is another criterion for regression.

On the other hand, a transgression should result in

more diatoms and plants of saline environment at the site,

but the situation is complicated by the large fluvial

discharges which are fresh water events. In this situation,

the predictors of large fluvial discharge and sea level

change are contradictory to each other, because fluvial

discharge is related to freshwater whereas sea level is

related to brackish water. The situation becomes even more

complicated when considering the different response time of

diatoms and vegetation to hydrological changes.

Nevertheless, a transgression layer is still decipherable

by two criteria: 1) considerable stratigraphic thickness;

2) frequent occurrence of marine/brackish water diatoms

and/or brackish marsh pollen (Figure 9-1).

2) Sea Level Changes

(1) From 6200 to 5900 years B.P.

The oldest sediment in the Pearl River Marsh core is

about 6200 B.P. according to the estimated age. Both pollen

and diatom (Pollen Zone I and Diatom Zone I) show that the

local environment of the ancestral Pearl River Marsh was

occupied by a vegetation similar to the modern swamps. The

swamp was dominated by Taxodium and Nvssa with abundant

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Ill

Regression layer

1) Fresh water diatom and fresh marsh pollen 2) thick layer

Transgression layer

large fluvial discharge layer 1) m arine/brackish water diatom and/or brackish marsh pollen

2) thick layer

.SNSSSNSN ///////// ///////// . /////////s \ \ s s \ \ \ s * \ \ S \ \ N S \ ' /sss/ss/s N\\NSN\\' ///////// . \ S \ \ S S \ ' / / / / / / / / / k \ N \ \ S \ V \ ‘ k \ \ S S y \ S ////////\ S S \ \ \ - //////// S \ \ \ y \ S \ //////// S y \ y y y S ' //////•// y y y y y \ y y v S \ \ \ \ \ \ %J / / / / / / / / y y y y y \ y y y //////// \ S \ S y \ S \ \ //////// N y \ \ \ \ \ ‘ y //«'////\ y y s y s \ / / / / / / / / y y y y y \ y \ s / s / / s s

/ X / / / y \ y \ y XXX/ SS//SS.+ / y y y y y y y y •* /•/•/•xxxxy

Figure 9-1. Diagnostic syndromes of regression and transgression layers. The transgression layer is complicated by large fluvial discharge layers.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 112

Carva. Pinus. Ouercus. Ulxnus and Fraxinus growing in the

site or in the adjacent uplands. The swamp supported a

freshwater diatom flora dominated by Eunotia spp. and

Pinnularia spp., both are typical halophobic species,

preferring zero salinity environment. This indicates that

the sea level was much lower than the level of the swamp.

The situation lasted at least 300 years and terminated at

5900 B.P. when a major environmental shift occurred.

(2) From 5900 to 3400 years B.P.

This period represents the continuous postglacial sea

level rise. It may be divided into three stages, according

to the response of diatom and pollen to the environmental change. The first stage includes the time span from 5900 to

5600 years B.P. The swamp forest was for the first time

being replaced by a brackish marsh vegetation dominated by

the herbaceous plants of Gramineae, Chenopodiaceae/

Amaranthaceae, and Cyperaceae. At the same time, the local

diatoms was replaced by a brackish water community together

with saline water species Actinocvclus beaufortianus. and

freshwater species Rhopalodia qibberula. Achnanthes

temperei. Campvlodiscus echeneis. Cvclotela meneghiniana.

Diploneis elliptica and Navicula pusilla. It seems that the

transition from swamp to brackish marsh environment is very

abrupt due to the absence of an intermediate fresh marsh

condition. This perhaps imply that the rate of sea level

rise was too fast for the fresh marsh to develop.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 113

The second stage was from 5600 to 5200 years B.P.

Vegetation zonal indices indicate that the marsh remained

brackish, only occasionally interrupted by fresh marsh

components, such as Osmunda and Polypodiaceae. Although the

diatom analytical results indicate that the brackish and

marine species were largely replaced by freshwater species, the typical marine species Actinocvclus beaufortianus was

still present at low percentages, indicating brackish water

condition. Sea level studies in the offshore areas of the

Gulf of Mexico do not suggest any regression during this

time interval either (Nelson and Bray 1970; Mcfarlan 1961;

Coleman and Smith 1964; Redfield 1967).

The third stage was from 5200 to 3400 years B.P.

During this stage, marine and brackish water diatoms, such

as Actinocvclus beaufortianus. were very abundant,

suggesting continued transgression. Their dominance was

frequently interrupted by freshwater diatoms, such as

Diploneis elliotica. Rhopalodia cribberula. Achnanthes

temperei and Campvlodiscus echeneis; or by other brackish

and marine diatoms, such as Paralia sulcata. Nitzschia

granulata and Navicula maculata.

According to pollen analytical results, the responses

of the vegetation to the contemporaneous environmental

changes was not quite consistent with diatoms, except for

the first stage. This was possibly caused by the different

response time between diatom and pollen. Nevertheless there

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is one similar pattern about diatoms and pollen, both

indicate that the marsh was frequently switched between

fresh and brackish types. However the freshwater episodes

seems too short to be regarded as regressions. They might

be related to occasional large fluvial discharges which

will be discussed in next chapter.

The continuous transgression scenario from 5900 to

3400 years B.P. is also consistent with studies in the

offshore area of the Louisiana coast (Nelson and Bray 1970;

McFarlan 1961; Coleman and Smith 1964; Redfield 1967; Gould

and McFarlan 1959).

(3) From 3400 to 2200 years B.P.

Both pollen and diatom analytical results indicate

that from 3400 to 2200 years B.P., the coring site became a

fresh marsh type. Among the diatoms, the typical marine

species Actinocvclus beaufortianus was greatly reduced to

insignificant abundance. Typical fresh water diatoms, such

as Eunotia spp.. and Pinnularia spp., became dominant.

Vegetation zonal indices of this period are generally low,

with frequent occurrences of fresh marsh indicators, such

as Decodon and Osmunda. Although during the middle of this

period, the zonal indices were high, it may or may not mean

a real brackish marsh since the principal components of the

local vegetation was Cyperaceae which may occur in both

fresh and brackish marsh communities. Generally speaking,

it seems that this period corresponds to a sea level drop

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or regression, according to the diagnostic syndromes

described above. Most studies of sea level changes in the Gulf of

Mexico do not show any regression during this time (Nelson

and Bray 1970; McFarlan 1961; Coleman and Smith 1964;

Redfield 1967). Nevertheless, the early study of the

Chenier plain of Louisiana by Gould and McFarlan (1959)

indicates that there was a significant regression from 3500

to 2200 years B.P. which led to the subsequent extensive

development of the first set of beach ridges in the Chenier

plain. This is evidenced by the C-14 dates from Little

Pecan Island, Little Chenier, High Island, Creole Ridge,

Back Ridge in the eastern Chenier (Figure 9-2). Such beach

ridges are also found in northwestern and southwestern

Florida, suggesting a significant lower-than-present sea

level from about 3 000 to 2000 years B.P. (Stapor et al

1991; Tanner 1993) (see Figure 2-4 in Chapter 2).

This regression was also documented in South Carolina

where two peat layers with fresh/brackish water diatoms

were sandwiched between the salt-marsh clayey sediments.

The regression layers were radiocarbon dated to 2330+40 and

3100±100 years B.P. (Brooks et al 1979) (see Figure 2-3 in

Chapter 2).

Supporting evidences concerning this regression were

further available from the U.S. Pacific coast. At San

Joaquin Marsh, southern California, there are three

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.

UK — — Ill (after Gould(after and McFarlan Penland1959; and Suter 1989). Figure 9-2. Beach ridges the in Chenier Plain of Louisiana categories: a) 2800 to 2200 years B.P.; b) 1400 to 1100 years B.P; and to 600 years100 B.P. Most of the C-14 dates theof beach ridges fall into three GULFOF MEXICO

ight owner. Further reproduction prohibited without permission. 117

regression sediment layers dated between 3800 and 2200

years B.P. (Davis 1992, Davis et al 1992). The regression

layers are rich in pollen of fresh marsh plants, such as

Liliaceae. In the Puget Lowland of Washington, fresh and

fresh/brackish water diatoms such as Fraqilaria construens

var. venter. F.. virescens. Navicula pusilla. and Nitzschia

frustulum suggest a regression from 3400 to 2200 years B.P.

(Eronen et al 1987) (see Figure 2-2 in Chapter 2).

Recently, a study of paleosalinity in a Maya wetland

in Balize suggests that the former open-water mesohaline

lagoon was converted to a oligohaline lagoon at 3400 years

B.P. (Alicala-Herrera et al 1984). The authors provided a

number of possible scenarios for this turnover. However, it

seems that the regression at 3400 years B.P. may provide a

better interpretation, since a sea level fall may result in

the isolation of the lagoon and therefore reduce the

salinity.

Although it is difficult to tell the exact magnitude

of the sea level change, the data from the Pearl River

Marsh are generally consistent with the results from South

Carolina, Florida, California and Washington. The

regression during this time interval was correlatable with

an episode of late-Holocene neoglaciations. The advancement

of glaciers from 3400 to 2200 years B.P. had been

documented in Alaska, Baffin Island, Northern Scandinavia,

the Alps, the Himalayas, New Guinea, Patagonia and New

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 118

Zealand (Grove 1988). Associated with this cold episode,

there were two minima of solar activities, the Homeric

Minimum and the Greek Minimum (Stuiver & Kra 1986). The

Homeric Minimum had been widely documented in North America

and other parts of the world (Davis 1992) (see Table 2-1 in

Chapter 2). The regression recorded by the Pearl River

Marsh core, PR1, was probably a consequence of global

cooling. The low abundance of marine and brackish water diatoms

in this period may be alternatively explained by an

increase in fluvial discharge. If this were the case, Loss-

on-ignition values should have decreased due to the

increased sediment load (see Chapter 10 for detail). However, this conditions cannot be satisfied based on Loss-

on-Ignition data, therefore the alternative must be

rejected.

(4) From 2200 to 1400 years B.P.

The period from 2160 to 1350 years B.P. is undoubtedly

a high sea level stage because of the abundant marine and

brackish water diatoms and the generally high vegetation

zonal indices of the pollen stratigraphy. This

transgression may be divided into two stages. The earlier

one is characterized by abundant brackish diatom, Nitzschia

scalaris, while the latter one is dominated by Actinocvclus

beaufortianus. The two stages were separated by a sharp

peak of freshwater diatom, Dipioneis elliotica. This peak

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was probably more related to a large fluvial discharge

rather than a regression since it happened only at one

level. Nevertheless, this event was very important to the

local diatom community. It removed most of Nitzschia

scalaris and created a favorable habitat for Achnanthes

beaufortianus.

The sea level studies in Florida support this

reconstruction. Analytical results of the beach ridges

there indicate that there was a high sea level stand

centered at about 1500 to 2000 years B.P. (Stapor et al

1991; Tanner 1993; see Chapter 2, Figure 2-4), which

corresponds to the maximum of marine diatom, Actinocvclus

beaufornianus. at the Pearl River Marsh.

(5) From 1400 to 1100 years B.P.

From 1400 to 1100 years B.P., most marine diatoms were again replaced by fresh water species. At the same time,

pollen analytical results suggest that there was a short

period of fresh marsh condition existed when Decodon and

Osmunda increased significantly. This represents another

regression event. This regression corresponds to the Roman

Minimum of solar activities, based on

(Stuiver & Kra 1986). It was associated with another major

neoglaciation event, but the duration and magnitude of this

event was smaller than the one from 3400 to 2200 years B.P

based on the C-14 dates and sediment thickness. Glacial

advances associated with this event were recorded in

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Alaska, Baffin, Northern Scandinavia, Western Alps,

Himalaya, New Guinea and New Zealand (Grove 1988). The

result from the Pearl River Marsh is again compatible with

the result of the sea level studies in Florida, where there

was a low stand of sea level at about 1500 to 1100 years

B.P. (Stapor et al 1991; Tanner 1993).

This regression temporally corresponds to the

formation of another beach ridge set at the Chenier Plain,

characterized by the C-14 dates from Oak Grove Ridge, Grand

Chenier Ridge,- and Pecan Island (Gould and McFarlan 1959)

(Figure 9-2).

An alternative explanation for the low abundance of

marine and* brackish water diatoms during this period is the

increase in fluvial discharge. However, Loss-on-Ignition

values in this period were especially high which argue

against the alternative explanation.

(6) From 1100 years B.P. to present

In general, from 1100 years B.P., transgression is the

dominant trend. Based on the diatom community and

vegetation changes, this period may be divided into two

stages interrupted by a short episode of low sea level.

From 1100 to 400 years B.P., marine diatom

Actinocvclus beaufortianus occurred only occasionally, the

most abundant species was Nitzschia scalaris which is a

common brackish water species. Pollen analytical result

suggests that during this stage, the coring site was

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occupied by a brackish marsh with high vegetation zonal

indices.

( There was a major environmental change at about 200

years B.P. Nitzschia scalaris was mostly removed. There was

a significant increase in marine diatoms, Coscinodiscus

spp.which still represent a relatively high sea level. The

two transgression events during the past 1100 years were

separated by a small regression from 400 to 200 years,

characterized by the minimal percentages of marine diatoms.

This short' period of sea level fall was probably resulted

from the Little Ice Age. The regression associated with

this noeglacial event was documented in the studies from

Florida (Stapor et al 1991; Tanner 1993), California (Davis

1992; Davis et al 1992), and Washington (Eronen et al

1987) . 3) Discussion -

(1) The Pearl River Marsh vs. the Chenier Plain

Based on the studies of the submerged and subaerial

portions of the Mississippi River delta, a model for sea

level ch&nge and coastal geomorphic development during the

past 7000 years was proposed by Penland (Penland 1990;

Penland and Suter 1989). Without direct radiocarbon dating

control, he suggests that during the first stage from 7000

to 4000 years B.P., there was a sea level still stand about

6 m below the present which led to development of the mid-

Holocene delta- plain. Afterward, there was a rapid sea

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level rise from 4000 to 3000 years B.P. (Penland 1990). In

another occasion he indicates this sea level rise was from

3000 to 2500 years B.P. (Penland and Suter 1989). This sea

level rise resulted in the transgressive submergence of the

mid-Holocene delta plain and the formation of the most

landward beach ridges in the Chenier Plain. Late Holocene

from 2500 years B.P. to present was characterized by a

general regressive processes with the development of the

modern delta plain and the subsequent beach ridges in the

Chenier plain. He suggests that the Little Cheneir-Little

Pecan Island trend in the Chenier plain represents the high

stand of the Holocene sea level. This is contrast with the

model proposed by Gould and McFarlan (1959) which suggests

that the edge of Pleistocene prairie terrace marks the high

sea level stand.

However the study of the Pearl River Marsh strongly

disagrees with Penland's model in several points. First,

the existence of the still stand sea level between 7000 and

4000 years B.P. cannot be substantiated. Both pollen and

diatom data from the Pearl River Marsh core indicate that

at 5900 years B.P., there was a transition of vegetation

from swamp to brackish salt marsh at the study site,

suggesting a sea level rise rather than a still stand.

Moreover, the diatom and pollen analytical results indicate

that from 5900 to 3400 years B.P., the Pearl River Marsh

area has been under a transgressive environment. This

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general trend is consistent with the numerous studies

conducted in the offshore areas of the Gulf of Mexico

(McFarlan 1961; Coleman and Smith 1964; Redfield 1967;

Nelson and Bray 1970).

The most controversial discrepancy between Penland's

model and the Pearl River Marsh study concerns the rapid

sea level change during 3000 to 2500 years B.P. According

to Penland's model, this time corresponds to a rapid sea

level rise, while the result from the Pearl River Marsh

suggests this period happened to be in a regressive phase.

There was no available study results to support the

hypothesis of sudden accelerated sea level rise during 3000

to 2500 years B.P. as proposed by Penland. However the

regression from 3400 to 2200 years B.P. has been documented

by many studies (Stapor et al 1991; Tanner 1993; Davis

1992; Davis et al 1992; Eronen et al 1987; Gould and

McFarlan 1959) .• This regression was broadly compatible with

the timing of the Neoglacial events. The high stand from

3000 to 2500 years B.P. as postulated by Penland is

therefore problematical. The Little Chenier-Little Pecan

Island beach ridge set in the Chenier plain was formed

during the middle of this regression instead of at the

onset of it. The study of the Pearl River Marsh supports

the result of the early study at the Chenier Plain which

suggest the edge of the Pleistocene Prairie Terrace marking

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 2 4

the position of Holocene high sea level (Gould and McFarlan

1959) (Figure 9-2).

(2) Problem of subsidence

Since the study site is in a coastal marsh, it is

important to estimate the magnitude of subsidence. Studies

in the offshore areas of Louisiana unanimously agree that

at 5900 years B.P., the 'eustatic' sea level was about 8.5

m lower than the present (Nelson and Bray 1970; McFarlan

1961; Coleman and Smith 1964; Redfield 1967) (Figure 9-3).

The eustatic sea level in the Pearl River Marsh should be

the same as in the offshore areas of Louisiana at 5900

years B.P. In the Pearl River Marsh core, the stratigraphic

level marking the switch from swamp to brackish marsh occur

755 cm below the modern marsh surface and is extrapolated

to be at 5900 years B.P. It suggests that there is only

about 1 m difference between the eustatic sea level in the

Gulf of Mexico and the relative sea level in the Pearl

River Marsh for the past 5900 years B.P. It further

indicates that subsidence in the study sites was minimal.

This is very different from the Mississippi deltaic region

where subsidence has been recently accelerated to about 10

to 12.1 mm a year (Penland and Rmsey 1990).

One probable explanation fo the non-subsidence in the

Pearl River Marsh area is that the bottom of the core is

composed of white coarse sandy sediments with very low

value of Loss-on-Ignition (see Figure 4-1 in Chapter 4).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 2 5

-10 £

-20

ut ° -30 H*

< -40

Nalscn ft Brqr. 1979 MeTarlan, 1961 Colmnan 6 Snltit, 196a -50 Rodflald. 1967 T T T 10 A G E YR BP x 103

Figure 9-3. sea level change curves in the offshore areas of coastal Louisiana. These earlier studies indicate a) a general rising sea level since the mid-Holocene; b) the eustatical sea level at 5,900 years B.P. was about 8.5 m lower than present (adapted from Pirazzoli 1991).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Such sediments are similar to the sandy soils near the NASA

center, northeast of the marsh. The Pearl River Marsh was

likely developed on such a sandy substrate which has little

potential for compaction. This result is also consistent

with the sea level study of Penland and Ramsey (1990)

(Figure 9-4a). It indicates that the contribution of

subsidence to sea level rise in Biloxi of Mississippi is

close to zero, although in Pontchartrain Basin of Louisiana

it is as high as 40%. Geographically, Pontchartrain Basin

belongs to the Mississippi drainage system which explains

the high subsidence rate. However the Pearl River has a

totally different drainage system from the Mississippi. The

situation in the Pearl River Marsh is perhaps more similar

to that in Biloxi than in the Mississippi delta. Because

subsidence is minimal, a tentative sea level curve for this

region is reconstructed (Figure 9-4b). It is based on the

assumption that the accretion of brackish marsh, as

identified by pollen and diatom analyses, has been

synchronous with transgressions, therefore the three

transgressions are plotted as solid lines with high

confidence level. However, the magnitudes of regressions

seem not so easy to measure. Therefore they are plotted as

dashed lines with less confidence level. The average sea

level rise rate during the past 6000 years is about 0.13

cm/yr which is close to the sea level rise rate in Biloxi

during past century, 0.15 cm/yr (Penland and Ramsey 1990).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 2 7

O iii Z :> 60 o 3 55 a w> K O 5 2 4 0 OZ uju u z UJ Q 5CO 20 tn3

MISSISSIPPI

0 brackish marsh^**-^^ 1050±70/®r^210±60

2 brackish marsh ‘»»«® fresh marsh 1240±60

fresh marsh

•4 brackish marsh

.22 -5 4490+60 2730±70 CO tn® -6c 7 5830±90 8 swamp

6000 Years B.P.

Figure 9-4. Sea level change curve derived from the study of the Pearl River Marsh core. From (a) it indicates the contribution of subsidence to sea level rise at Biloxi, east of the Pearl River Marsh, is close to zero (after Penland and Ramsey 1990), therefore the sea level rise in the study area should mainly caused by eustatic effect (see text for detail).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 10. FLUVIAL DISCHARGE CHANGES DURING

THE PAST 5900 YEARS

1) Hypothesis

Loss-on-Ignition (LOI) analysis provides a

quantitative measurement of the relative abundances of

organic and inorganic components. In the study area, the

organic materials are usually derived from the

autochthonous dead plant tissues, whereas at least two

factors may affect the percentages of inorganic materials.

One is marsh salinity. The primary factor affecting LOI

values is the inorganic sediment load by fluvial discharge.

Large discharge should be related to low LOI values,

whereas small scale fluvial discharges may not provide too

much clastic sediment to the coring site, so that the LOI

values should not be as low. The other factor contributing

to the inorganic materials at the study site are overwash

deposits by storm surges.

Because fluvial discharges are freshwater events,

hypothetically fluvial discharge layers may be separated

from overwash deposits by their relatively high abundance

of freshwater diatoms. Since fluvial discharges are short

in time duration, vegetation may not be as sensitive to

them as diatom, therefore pollen data have only limited

usage to predict fluvial discharge events. The diagnostic

syndromes for a large fluvial discharge layer are: (1)

abundant fresh water diatoms; and (2) low values of LOI

128

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 129

(Figure 10-1). However, it must keep in mind that such a

layer should represent a period of large fluvial discharges

rather than a single flooding event.

Nevertheless, in reality, there is another difficulty.

It seems difficult to set two grand means to define the low

and high values of the freshwater diatoms and of LOI. The

first attempt was to use the mean values of the surface

samples, but it turned out to be invalid since the fossil

diatom flora is quite different from the modern diatom

flora. A similar problem was encountered when trying to

average the diatom and LOI values of the uppermost 1 m of

the core as the mean. The final choice is to use the

average freshwater diatom and the LOI values from 751 cm

(the level of marsh initiation) upward to the surface of

the sediment core as the grand means, which are 66.4% and

33.8%, respectively. Any level having a total freshwater

diatom percentage higher than 66.4% and a LOI value lower

than 33.8% is classified as a large fluvial discharge

layer. All the large fluvial discharge layers are

represented by Figure 10-2.

2) History of Fluvial Discharge Changes

From 6200 to 5900 years B.P. the coring site at the

Pearl River Marsh was occupied by a swamp. The magnitude of

fluvial discharge was impossible to predict due to the

completely different local environment from today's. After

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large fluvial discharge layer

1) freshwater diatom and/or fresh marsh pollen Late-Holocene 2) low value of loss-on-lgnition

Mid-Holocene

Figure 10-1. Hypothetical diagnostic syndromes of large fluvial discharge layers. More such layers are expected to be found in mid-Holocene stratigraphy than in late-Holocene due to the wetter Hypsithermal.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 131

Z10±60

1050±7D

1240±6QB

27M ±70B

449 0±SD

583049DB

Figure 10-2. Large fluvial discharge layers in the Pearl River Marsh core, PR1. They are the levels having LOI values <33.8% and total fresh water diatom percentage >66.3%. The result indicates that more such layers were present in the stratigraphy prior to 4500 years B.P.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 132

5900 years B.P., the fluvial discharge events in the study

area may be divided into 12 stages (Figure 10-2).

(1) Stage 1 During the first stage, from 5900 to 5400 years B.P.,

corresponding to 752 to 655 cm of the core, the average LOI

value was 22.7%, less than the grand mean of 33.8%. Fresh

water diatoms, such as Rhopalodia gibberula. Campvlodiscus

echeneis, Cvclotela meneghiniana. Diploneis elliptica.

Nitzschia amphibia and Fraqilaria construens var. venter

were dominant in the diatom assemblages. The total

freshwater diatom percentages at most levels were higher

than the grand mean of 66.4%. This stage represents a

period with frequent large fluvial discharges according to

the diagnostic syndromes of fluvial discharge layers.

(2) Stage 2

From 5400 to 5200 years B.P., this stage corresponds

to the depth of 655 to 615 cm of the core. Mean LOI value

decreased to an extreme low to 9.9%. The principal

freshwater diatoms were Bacillaria paradoxa. Cvclotella

meneghiniana. Epithemia spp., Fraqilaria construens var.

venter. and Tabularia tabulata. The total freshwater diatom

percentage at all levels in this stage was higher than the

grand mean of 66.4%. This stage indicates a period of

intensive fluvial activities. The fluvial discharges during

this stage were probably larger than the previous one.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. (3) Stage 3

From 5200 to 4800 years B.P., corresponding to the

depth of 615 to 535 cm of the core, total percentages of

the freshwater diatoms were generally lower than 66.4%, and

•*- mean LOI value was 33.8%, equal to the grand mean. The

general appearance of this stage suggests a period of

weaker fluvial discharges than the previous two stages.

(4) Stage 4

From 4800 to 4500 years B.P., depth interval 535 to

474 cm, mean LOI value was 15.9%, much less than the grand

mean of 33.8%. It was the second lowest value since 5900

years B.P. Moreover, for most levels, total fresh water

diatom percentages were higher than the grand mean of

66.4%. The main freshwater diatoms were Rhopalodia

gibberula-. Diploneis elliptica. and Cocconeis placentula.

Both conditions satisfy the diagnostic syndromes of large

fluvial discharges.

(5) Stage 5

Corresponding to the period from 4500 to 4100 years

B.P., the mean LOI value from 474 to 425 cm was 58.1%, much

higher than the grand mean of 33.8%. Total freshwater

diatom percentages at most levels were lower than the grand

mean of 66.4%. Both characteristics indicate that the

fluvial discharge events during this stage became smaller

in magnitudes.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 134

(6) Stage 6 From 4100 to 3400 years B.P. or from 425 to 350 cm,

the mean LOI value decreased to 26.5%, lower than the grand

mean of 33.8%. The dominant freshwater diatoms were

Achnanthes temoerei. Campylodiscus echeneis. and Diploneis

elliptic. However, their total percentages at most levels

of this stage were slightly higher than the grand mean of

66.4%. Therefore only two large fluvial discharge layers

were detected from this stage, indicating the fluvial

discharges were larger than in the previous stage but still

incomparable to the ones in stage 1,2, and 4.

(7) Stage 7 Corresponding to the period from 3400 to 2800 years

B.P., the mean LOI value from 350 to 280 cm of the core was

38.4 % which was slightly higher than 33.8%. Although the

percentages of total freshwater diatoms at many levels of

this stage were significantly higher than the grand mean of

66.4%, it does not indicate a period of large fluvial

discharge events because of the generally high LOI values.

These high percentages of freshwater diatoms were probably

caused by a sea level fall because this stage was within a

period of regression (see Chapter 9 for detail).

(8) Stage 8

From 2800 to 2600 years B.P., corresponding to the

depth inte'rval of 280 to 262 cm of the core, this stage had

a mean LOI value of 26.3%, lower than the grand mean of

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 33.8%. At the same time, freshwater diatoms such as Diploneis elliptica. D. ovalis. and Rhopalodia aibberula

were dominant. The total percentages of freshwater diatoms

were higher than the grand mean. Both conditions suggests

this short stage corresponds to a period of relatively

large fluvial discharges. This stage was still within the

time span of the grand regression from 3400 to 2200 years

B.P. The high freshwater diatom abundances in this stage

was probably caused by the double effect of both regression

and large fluvial discharges.

(9) Stage 9- •

From 2600 to 1900 years B.P., or from 262 to 190 cm of

the core, the mean LOI value became 42.4%, much higher than

the grand mean of 33.8%, although at some levels, the total

freshwater diatom percentages were higher than the grand

mean of 66.8%. The general characteristics suggest that

large fluvial discharges were rare events in this stage.

(10) Stage 10

From 1900 to 1300 years B.P., corresponding to the

depth interval of 190 to 125 cm of the core, the mean LOI

value was 29.8%, lower than the grand mean of 33.4%.

However, generally high percentages of marine diatoms, such

as Actinocyclus beaufortianus and Nitzschia scalaris in

this stage sdggest that the decrease in LOI values were

probably caused by the development of salt marsh at the

site as a result of sea level rise because this stage was

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contemporaneous with the main part of the transgression

from 2200 to 1400.

(11) Stage 11

From 1300'to 800 years B.P., corresponding to 125 to

60 cm of the core, the mean LOI value was as high as 60.7%,

much higher than the grand mean of 33.8%, indicating a very

tranquil environment without large fluvial discharge

disturbances. The dominant diatom, Nitzschia scalaris. a

brackish water species, further attests to the rarity of

freshwater events related to fluvial discharges.

(12) Stage 12 Stage 12 spans the period from 800 years B.P. to the

present, corresponding to the depth interval of 60 cm to

the core top. In comparing with the LOI values of the

previous stage, the mean LOI value decreased to 35.4%,

close to the grand mean of 33.8%. There was a considerable

increase in the freshwater diatom percentages. Their total

percentages at many levels exceeded the grand mean of

66.3%. In general this stage represents a period when large

fluvial discharges became more frequent. Because the

sampling interval corresponding to this stage is switched

to 2 cm, the large fluvial discharge layers in Figure 10-2

seems to be consecutive in this stage. These layers were

interrupted at the time interval from about 400 to 150

years B.P. which corresponding to the main part of the

Little Ice’Age (see Chapter 11 for more detail).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 137

3) Discussion

(1) Mid-Holocene Hypsithermal precipitation

In humid subtropical regions, such as the Pearl River

Marsh area, precipitation in a given basin is positively

correlated with fluvial discharge (Muller and Larimore

1975). Therefore the large fluvial discharge layers

identified from the Pearl River Marsh core should represent

a changing history of precipitation in this region for past

5900 years. Based on the history of fluvial discharge

changes, it is evident that during mid-Holocene from 5900

to 4500 years B.P., there were more large fluvial discharge

layers than during late-Holocene (Figure 10-2). It is

therefore logical to infer that precipitation within the

Pearl River drainage basin during mid-Holocene was more

than that during late-Holocene. This conclusion is further substantiated by the change

in sedimentation rates of the Pearl River Marsh core (see

Chapter 7, Figure 7-1). In Figure 7-1, it is evident that

the sedimentation rate during the time interval from 5900

to 4500 was higher than the rest part of the core. This

high sedimentation rate during the mid-Holocene was perhaps

maintained by the large fluvial discharges which provided

more sediment load to the marsh area.

Based on the fluvial discharge change, this study

indicates that the mid-Holocene Hypsithermal climate was

wetter than that during the late-Holocene in the middle

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part of the U.S. Gulf coastal region. This inferred

precipitation change over the past 5900 years is generally

consistent with the climate modeling prediction of

increased precipitation for the southeastern U.S. during

the mid-Holocene (Kutzbach 1987). The trend of climatic

changes during the past 5900 years in this region was

different from that in the Midwest and Northeast where the

mid-Holocene was characterized by a drier climate. The

higher precipitation during the mid-Holocene in the Gulf of

Mexico region probably resulted from a strengthened

southerly flow from the Gulf of Mexico (Webb et al 1987,

Kutzbach 1987) (Figure 10-3).

There is still two other evidences to support the

proposition of increased precipitation during mid-Holocene.

Discovery of abundant macrofossil of mesic forest trees at

the Little Bayou Sabra in the Tunica Hills of Louisiana

does not support the scenario of a drier climate during the

mid-Holocene. Dated to 5295±295 years B.P., these

macrofossils include Faqus grandifolia. Magnolia acuminata.

Ouercus spp. O. nigra. 0. alba. O. virginina. Liriodendron

tulipifera. Ostrva virginiana. Ulmus alata. Fraxinus

pennsvlvartica. Nvssa sp. Liguidambar stvraciflua. Corvlus

sp., and Cornus sp. (Delcourt and Delcourt 1977). Moreover,

stratigraphic studies of fluvial terraces at the Tunica

Hills suggest a major change in fluvial activities at the

transition from mid- to late-Holocene (Figure 10-4). The

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July 9 ka 9 0 90 irm

6 0 6 0

3 0 3 0

Jan 9 ka

180 ■150 120 •90 ■60 3 0 July 6 ka 9 0 zzzz 90 zzzzfcs

6 0 60

30 30

Jan 6 ka

180 -150 -120 -90 -50 -30 i______I______I______L July 0 ka 9 0 90 u n U / M

. 'tLa7s t 60

Jan 0 ka

2 0 m /s 10 m /s 5 m /s

Figure 10-3. simulated surface winds at 9000, 6000 years B.P. and at present in eastern North America. The length and orientation of the arrows indicate wind speed and direction (after Webb et al 1987).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 140

m Sand

Figure 10-4. Fluvial terrace at the Tunica Hills, Louisiana The surface of Terrace 1 was dated at 3457+366 years B.P. The formation of Terrace 1 was probably due to a eastward shift of the Mississippi River or a reduced fluvial discharge during the late-Holocene (after Delcourt and Delcourt 1977) .

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. surface of Terrace 1 was dated at 3065±110 years B.P. by

Alford et al (1983) and 3457±366 years B.P. by Delcourt and

Delcourt (1977). One possible cause for the incision of

this river channel during the late-Holocene was reduced

fluvial discharge resulting in the formation of a climatic

terrace (Budel- 1982). Although base level change may offer

an alternative explanation since there was a regression

between 3400 and 2200 years B.P. based on this and other

studies, the magnitude of this regression was probably in

the order of 1 or 2 meters, so that its effect on the

Little Bayou Sabra should be minimal. There is still

another alternative explanation, which concerns the change

of local base level caused by the meandering of the

Mississippi River (Delcourt and Delcourt 1977). This

scenario is plausible according to Alford et al (1983). The

possible reason is that the Little Bayou Sabra and the

Mississippi River are separated by the Pleistocene

escarpment of Mississippi River Valley. There is a major

gradient change from the Little Bayou to the fluvial

discharge plain. The local base level for Little Sabra

Bayou should be the fluvial discharge plain rather than the

Mississippi River per se. There is an 1000-year difference

between the switching point of fluvial discharges at 4500

years B.P. in the Pearl River Marsh and the terrace

incision at the Tunica Hills at about 3500 years B.P. This

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 142

may be caused by a time lapse of the fluvial geomorphic

feature to the climatic change.

The wet Hypsithermal hypothesis is also consistent

with the prediction of paleoclimate study in southwestern

Louisiana. Based on the occurrence of large meander scars

formed by the Sabine River, a study suggests that the

fluvial discharge and precipitation during the mid-Holocene

Hypsithermal were probably much higher than today's (Alford

and Holmes 1985) (Figure 10-5). According to indirect

chronological control, they interpret that the large

meander scars were formed during an interglacial warm

climate due to more frequent tropical storms which brought

increased precipitation to the Gulf of Mexico coastal

region. Such large meander scars are not unique to the

Sabine River since many streams in the Gulf of Mexico

coastal plain have distinctly large meander scars. However,

such features are rarely found in the interior lowlands

(Gagliano and Thom 1967). These large meander scars could

not be the products of the glacial age climate because the

glacial ages in this region was dry (Alford and Holmes

1985). The geographical distribution of these large meander

scars strongly suggests that they were formed in

interglacial times and unique to the coastal region and

probably related to tropical storms (Alford and Holmes

1985). A wet mid-Holocene climate is a likely explanation

for the development of such geomorphic features.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 4 3

Figure 10-5. Meander scars of the Sabine River in southwestern Louisiana. The meander scars on the left side of the map was much larger than that of today. The large meander scars were probably associated with tropical storm precipitation under a mid-Holocene warmer climate (after Alford et al 1983).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 4 4

(2) Fluvial discharge frequency There is some regularity in the LOI curve of the Pearl

River Marsh core, PR1. The 1000-year period is the most

obvious one. During the past 5900 years, there were six

stages with relatively low Loss-on-Ignition values,

including stages 1, 3, 5, 7, 9, and 11. They were separated by six stages of high Loss-on-Ignition values, including

stages 2, 4, 6, 8, 10, and 12. The average length for each

couple of low-and-high cycle is about 1000 years. This

periodicity might be a reflection of fluvial discharge

change in this area, however more evidences are needed to

substantiate this periodicity.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 11. HURRICANE ACTIVITY IN THE LITTLE ICE AGE

The study of hurricane history will only focus on the

Little Ice Age for two reasons. First, stratigraphic

analysis of the Pearl River Marsh core, shows that the

study site has been a brackish marsh during most part of

the past 6000 years. This makes the site suboptimal for the

intended hurricane reconstruction study because the latter

require a fresh marsh site. Second, hurricanes are short

term events,' so that very close stratigraphic sampling is

needed to detect them. Sampling the whole 881 cm core of

the Pearl River Marsh at every 2 cm for pollen and diatom

analyses need to analyze 881 samples, which is beyond the

scope of this dissertation research.

1) A Review of the Little Ice Age

It is widely accepted that there was a considerable

decrease in temperature in many regions during the Little

Ice Age. Glacial advance associated with this neoglaciation

was documented in Alaska, the Baffin, the Scandinavia, the

Alps, the Himalayas, New Guinea, Patagonia, and New Zealand

(Grove 1988). In England, the Little Ice Age was so cold

that the River Thames in London 'was frozen at least 11

times in the 17th century'(Lamb 1982). Sea ice incidence

around Iceland reached the maximum value from A.D. 1600 to

1900 (Lamb 1982). The glaciers on the eastern Alps,

according to historical records, started to advance from

1600 and did not retreat until 1850 (Grove 1982) . China

145

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 4 6

also experienced a remarkable cooler climate than in the

present century. A succession of winters with damaging

frost between 1654 and 1676 caused the abandonment of

orange cultivation in the Jiangxi Province in South China,

where oranges had been growing for centuries (Lamb 1982).

In North America, reports of early settlers and

explorers from Europe show that there were some notably

severe winters. The winter of 1607-1608 in Maine was so

severe with persistent northerly winds and frosts that many

people died, and ice was found on the edges of Lake

Superior in June 1608 (Lamb 1982). The cold front advanced

earlier to the Gulf of Mexico than today. This might have

terminated the first voyage of Columbus (Millas 1968).

Particularly significant to this research is the study

of glaciers in the low latitudes. In Ecuador, some glaciers

are developed on the Cordillera Occidental and Cordillera

Oriental near the equator. Documentary records in Quito

archives, dating from the early years of Spanish rule,

suggest that in the sixteenth century conditions were more

severe than today's, with permanent snow on the crests that

are now free of perennial snow (Grove 1982; Hastenrath

1981). The ice-covered area is an order of magnitude

smaller now than it was at the maximum of the Little Ice

Age, and the regional snow-line has risen intermittently by

about 300 m over the last 250 years (Grove 1982; Hastenrath

1981). Supporting evidence also come from the Quelccaya ice

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. cap (13°56'S, 70° 50'W) in Peru of South America. Results

there indicate that there was a clear temperature

depression during the Little Ice Age in this tropical area

as illustrated by oxygen isotope analysis (Thompson 1992).

It sqems safe to suggest that the Little Ice Age was a

global event because many regions of the world under

different climatic and topographic conditions were

affected. The causes of the Little Ice Age are still

controversial. One explanation evokes variations in solar

radiation. It had been documented that there was a

significant decrease in solar activity during the Little

Ice Age. The prolonged absence of sunspots from A.D. 1645

to 1715 was known as the Maunder Minimum, named after E.

W. Maunder, a 19th century astronomer in the solar

department of the Greenwich Observatory (Eddy 1976).

However, the Maunder Minimum alone seems too short in time

duration to explain the three century cooling of the Little

Ice Age, thus other factors might also have been involved.

Volcanic activities were proposed by Lamb (1970) and Hughes

(1979). The persistent dusts produced optical effects that

reflected more solar radiation and thus cooled the earth. A

dust veil index (DVI) was proposed by Lamb (1970, 1972) to

measure the effect of volcanic eruptions on the atmosphere.

Considering 250 eruptions of the last five centuries, he

found that the total global frequency of eruptions of

magnitude DVI>1000 averaged five per century between A.D.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 148

1500 and 1900, but during the 20th century there were

scarcely any. This implies a global wave of volcanic

activities during the Little Ice Age.

2) Hypothesis

The, hypothesis in this study is that the temperature

decrease during the Little Ice Age might have reduced the

thermal energy in the tropical oceans, thus resulting in

fewer hurricanes formed in the Atlantic Ocean (Figure 11-

D-

One of the' crucial prerequisites for hurricane

formation is that the ocean surface temperature must be

warmer than 26.5°C in order to provide a source of heat to

warm the air moving across the ocean. Moreover, the warm

ocean surface temperature must extend downwards for 60-70

m, to a depth sufficiently great to prevent cooler water

from being brought to the surface by the storm (Anthes

1982; Gray 1990; Emanuel 1988). The temperature decrease

during the Little Ice Age, especially in the tropical

region, should therefore suppress hurricane formation.

On the other hand, most of the evidences regard

regarding to the Little Ice Age cooling are from

terrestrial areas which are different from the oceanic

areas in thermal response. According to CLIMAP Project

Members (1976), the tropical ocean surface temperature was

less sensitive to climatic change than temperature over the

continental landmass. For example, during the Last Glacial

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 4 9

Little Ice Age (Fewer hurricane layers)

Hurricane layer

Figure ll-l. Hypothetical diagram showing that the Little Ice Age is expected to contain fewer hurricane layers in the sediment core.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Maximum, the temperature decrease in the tropical Atlantic

Ocean temperature decrease was only 0 to 2°C, while in the

adjacent Colombian Andes and Amazon lowlands, a temperature

depression of at least 6°C has been documented (van der

Hammen, 1960; Liu and Colinvaux, 1985). If the Last Glacial

Maximum had so little effect on the tropical Atlantic

Ocean, it is also possible that the climatic cooling in the

Little Ice Age has insignificant effect on the same ocean

realm and associated hurricanes, so that the hypothesis of depressed hurricane activities during the Little Ice Age

remains to be tested.

In order to test the hypothesis, the uppermost 84 cm

of the Pearl River Marsh core, PR1, was intensively sampled

at every 2 cm interval for pollen and diatom analyses. The

rationale is that hurricanes impact the coastal freshwater

communities by short term sea water intrusions generated by

tidal surges under low barometric pressure and strong wind

(Table 11-1 and Figure 11-2). Each episode of salt water

intrusion may only last a few hours or no more than a few

days depending on the strength of the storm and the local

topography. However, the physical and chemical conditions

of this seawater may result in alteration of the local

hydrological, edaphic and biological environment, and the

associated diatom and possibly vascular plant communities

(Brewer and Grace 1990; Conner et al 1989) . Such salt water

intrusion to the coring site in the Pearl River Marsh is

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Table 11-1. Saffir/Simpson hurricane scale with central barometric pressure (after Gray 1990).

Category Central Pressure Wind Speed Surge (mbar) (km/h) (m)

1 >980 119-153 1.0-2.0 2 965-979 154-177 2.0-2.5 3 945-964 178-209 2.5-4.0 4 920-944 210-249 4.5-5.0 5 <920 >249 >5.5

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 152

Huzricaam Siaztga

Sail: Pulsa

BcacteLstii

Figure 11-2. Hypothetical diagram showing the salt water intrusion to the fresh marsh by hurricane surge.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 153

possible because, the local marsh surface is very flat and

only about 1 m above sea level, whereas the storm surge

associated with a category 1 hurricane is typically more

than 1 m high according to the Saffir/Simpson scale. For

instance, water level of 6 m high was recorded in Clermont

Harbor, Mississippi, ca. 2 km away from the beach when

hurricane Camille made landfall in 1969 (Roberts 1969).

Hurricane Andrew increased water level by 1.5 m at the

Houma Navigati,onal Canal, 80 km east of the hurricane track

and 20 km inland from the Gulf of Mexico (Underwood 1993,

unpublished data). Diatom and pollen analysis was applied to identifying

the hurricane layers in the sediment core. Hypothetically,

a hurricane layer will be represented by a sudden increase

in marine and brackish water diatoms in a background of

freshwater diatoms, at the same time pollen of brackish

marsh plants will replace fresh marsh plants. However, due

to various other disturbances, (e.g. fluvial, biotic), the

hurricane associated brackish water/marine diatoms and

brackish marsh pollen may be reworked and result in

contamination of the stratigraphically adjacent freshwater

assemblages. This "blurring" effect may obscure the

individual hurricane event, especially when considering

hurricane is such a short term catastrophic event.

Moreover, the different response time of diatoms and pollen

to the change of hydrological conditions make the situation

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more complicated. Due to this deficiency, other supporting

evidences are essential to test the hypothesis of decreased

hurricane activities during the Little Ice Age. Therefore,

the study result of a freshwater lake on Horn Island,

Mississippi, is cited in this chapter (Liu, Fearn, and

Gathen 1993, unpublished data).

3) Hurricane .Andrew as a Modern Analog Hurricane Andrew is especially important to this study

since it was the most recent intense hurricane affecting

coastal Louisiana. On August 26, 1992, Hurricane Andrew

made landfall in south-central Louisiana. Peak wave height

exceeded 6 m at the storm center (Stone et al 1993). The

storm surge was monitored at Houma Navigational Canal,

about 80 km east of the hurricane track. Gage record

indicates that water level increased about 1.5 m when the

hurricane passed by (Figure 11-3). The high water level was

not caused by the heavy precipitation associated with the

hurricane since there was a synchronous rise in water

salinity, suggesting that the water came from the sea.

A short core was taken from Du Large Gas Field, Dulac,

Houma in July 8, 1993, about one year after the hurricane

strike (Figure 11-4). The coring site is about 3 km west of

the Houma Navigational Canal and 20 km inland from the Gulf

of Mexico coastline. The local vegetation is dominated by

Panicum hemitomon. It is an intermediate marsh similar to

the Pearl River Marsh (Chabreck and Linscombe 1978). Two

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HOUMA NAVIGATIONAL CANAL

14.5-

13.5-

12.5-

11.5-

26 27 28 AUGUST 1992

SALINITY GAGE HEIGHT

Figure 11-3. Gage height and salinity records at Houma Navigational Canal during Hurricane Andrew (from Underwood, unpublished data)_

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 5 6

1 KILOMETER

Figure 11-4. Location of the test core at Du Large Gas Field, Houma, Louisiana.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 157

stratigraphic samples were studied for pollen and diatom.

One was at the surface; the other was 3 cm below the

surface which probably represents the situation prior to

Andrew. The results are shown in Figure 11-5 and 11-6.

Pollen analytical results indicate that one year after

the hurricane strike, the local pollen assemblage was not

the same as it was before, but the pollen assemblage does

not show significant removal of the fresh marsh indicators,

such as Osmunda and Polypodiaceae (Figure 11-5). This

suggests that the site remains an intermediate marsh as it

was before the hurricane. This conclusion is consistent

with some other studies about hurricane impacts on coastal

plant communities (Morgan et al 1958; Chabreck 1973;

Rejmanek et al 1988; Conner et al 1989). Most studies

indicate that hurricanes indeed can impact coastal

vegetation, but plants generally recover very quickly, and

some species may even be unaffected. •

The response of diatoms to this hurricane seems to be

much more pronounced than that of pollen (Figure 11-6).

Before the hurricane, the diatom community was dominated by

fresh water species, such as Eoithemia spp., Eunotia spp.,

Pinnularia spp., Achnanthes temperi. Navicula pusilla. and

N. pereqrina. Brackish water species were also present,

including Nitzschia obtusa and N. scalaris. but their

percentages were much lower than the freshwater diatoms'.

One year after the hurricane, the sediment supported a

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U-, A IP ’ § r r 1 20 3J 0-1 o E d> CL Q Gas Field, Louisiana. Gas Field, Houma, Figure 11-5. Figure 11-5. Pollen responseHurricaneto Andrew at Du Large

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Figure 11-6. Figure 11-6. Diatom responseHurricaneto Andrew at Du Large Gas Louisiana. Field, Houma,

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 160

totally different diatom community. The once dominant fresh

water species were almost totally removed. The principal

new freshwater species include Rhopalodia aibberula.

Navicula tripunctata. Tabularia tabulata and Cvclotela

meneghiniana. Brackish water species increased

significantly, such as Nitzschia obtusa. N. sioma. and

Navicula maculata. but Nitzschia scalaris decreased. In

general, the result suggests that diatom communities are

more sensitive to hurricane disturbance than are

macrophytic vegetation communities.

4) Pollen and Diatom Analytical Results

The uppermost 84 cm segment of the Pearl River Marsh

core, PR1, was intensively sampled at an average interval

of about 2 cm for both diatom and pollen analyses. The

analytical results are shown in Figure 11-7, 11-8, and 11-

9. According to C-14 dates and Cs-137 measurements (see

Figure 7-1 and 7-2 in Chapter 7), the estimated age of this

section corresponds to the past millennium which

encompasses the Little Ice Age, and the two warmer periods

before and after it. The calendar year chronology is

calculated based on the inferred C-14 dates following the

calibration of Stuiver and Kra (1985).

Marsh development during this interval may be divided

into two phases according to the pollen analytical results.

From A.D. 900 to 1750 (Pollen Zone 1), the local vegetation

was dominated by Gramineae, Cyperaceae, and Compositae.

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* 'jb *"*1 tio ate ~ ate tio *"*1 'jb * pollen counted). Figure 11-7. Pollen diagram of uppermost 84 cm sediment of the Pearl River Marsh core, PR1 (percentages as sumtotal of Jo 5 » 21CHA0B - - - 0 0 * A.D. 1ROO \7U0- 1600 1500 1 120a 1000 1900- 1300- lioo-

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<9 % c O % . \ X % 0 • ■ % % .C_ '■tyw %. in T3 v % x h « S c % • % % > - % . v \ « s

' V % > Si ® w * * s WA -5-4+J » 5 <0 * P.’O® « - _ in

i « % % ? * ■ & r . O (0 %. £ O ' v ~ % . 4, % fib. * (0 3 -p •a c a) o a) -p ptu yC fe,^y <5 ( 0 cu V % V V v 5 w A xin : <9 8 o h

ft* ■

<-4 O 0( D BO 3 0< CP 04 -H 3 &4 w

° 8 8 8 8 8 8 •— n n §m w r« § 1 |

i I § | 1 I §i 1 i

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. u> H

diatoms counted). Figure 11-9. Marine and brackishwater diatoms of the Pearl River Marsh core (uppermost 84 cm; percentages as total - - 1000 1200 woo- 1100- A.D. 1700- 1600- 1500- 1800- 1300- 1900-

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 164

Vegetation zonal indices indicate that the marsh was

brackish throughout this period. After A.D. 1750 there was

a major environmental change, represented by the sudden

rise in Mvrica and Osmunda in Pollen Zone 2. The marsh

became fresh and gradually evolved to its present

condition. Mvrica started to decline, and Vigna and

Sagittaria increased after about A.D.1850.

Diatom analytical results for this section are not

quite consistent with the pollen results. Instead of being

two zones, the Diatom stratigraphy may be divided into

three zones. Among the marine and brackish diatoms, Diatom

Zone 1 is dominated by brackish water diatom Nitzschia

scalaris. Diatom in this zone is consistent with the pollen

result. Both indicate a brackish marsh condition. The

estimated age of Diatom Zone 1 is from A.D. 900 to 1550.

according to the average sedimentation rates.

Nitzschia scalaris is greatly reduced and freshwater

diatoms, such as Diploneis elliptica and Rhopalodia

gibberula. are absolutely abundant in Diatom Zone 2. The

estimated age for Diatom Zone 2 zone is A.D. 1550 to 1850,

corresponding to the Little Ice Age. One possible

explanation to the decline of marine and brackish water

diatoms in Diatom Zone 2 is the reduced hurricane

activities during this period, according to the hypothesis

discussed above. Alternatively, the high percentages of

freshwater diatoms in Diatom Zone 2 may be explained by a

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sea level fall or an increased fluvial discharge. The regression scenario for the Little Ice Age has been

discussed before. Based on the stratigraphic thickness and

the high vegetation zonal indices, it seems that the

regression during that time was not as pronounced as the

two other regressions from 3400 to 2200 and from 1400 to

1100 years B.P (see Chapter 9 for more detail). The 'increased fluvial discharge' scenario seems

unlikely either because the overall Loss-on-Ignition values

during the Little Ice Age were not significantly low which

is the most important criterion for detecting large fluvial

discharges. Moreover, meteorological data of 1830s, the

late part of the Little Ice Age indicate that the Gulf of

Mexico coastal region experienced a climate generally drier

than today' s,except for the interval from April to June

when precipitation was slightly higher (Wahl 1968). In

addition, there is another proxy geological evidence which

suggests a dry climate during the Little Ice Age based on

the study of meander scars of the Sabine River in southwest

Louisiana (Alford and Holmes 1985). The result there

suggests that the fluvial discharge of the Old River (i.e.

former Sabine River which was active until the 19th

century) during the Little Ice Age was only 5% of that of

the present Sabine River (see Figure 10-7 in Chapter 10).

Therefore, the increased discharge alternative may be

excluded.

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The estimated age for Diatom Zone 3 corresponds to

A.D. 1850 t o ‘the present, which corresponds to the period

of climatic warming after the Little Ice Age. The increase

of marine dia'tom, Coscinodiscus s p p . , in the sediment was

possibly related to an increase in hurricane frequency*

However, it is impossible to identify individual hurricane

layers based on the biostratigraphy. The lack of

biostratigraphic resolution may be due to physical

disturbance and bioturbation at the coring site. The

hurricane associated brackish and marine diatoms and

brackish marsh pollen might have been reworked and mixed

with the adjacent freshwater assemblages.

Another possible explanation for the change in diatom

assemblage may be sea level change. In Florida, Stapor et

al (1991) and Tanner and Donoghue (1993) suggest that there

was a low sea level from 1300 to 1800 A.D. roughly

contemporaneous with the Little Ice Age, followed by a sea

level transgression. The increase in brackish and marine

diatoms in Diatom Zone 3 might caused by a post Little Ice

Age transgression.

There is still another possible explanation for the

sudden rise of marine diatom in Diatom Zone 3. Between 1850

and 1878, the East Pearl River was eventually cut off from

the main drainage basin and the West Pearl River became the

main channel (Collins and Howell 1879). This channel shift

greatly reduced the fresh water discharge of the East Pearl

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River and might possibly result in deeper penetration of

the salt water wedge into the upper reaches of the channel

during high tides. But the coring site is located midway

between the old and the new channels, about 2 km away from

each, thus the shift of river channels should not have too

much effect on the local environment.

5) The Mvrica Rise In comparing Pollen Zones 1 and 2, it is clear that

there is a sharp rise of Mvrica pollen. These changes in

pollen assemblages may be explained by the settlement

history in this area.

The Mississippi River delta had probably been

discovered by Alvarez de Pineda during his exploration of the Gulf Coast in 1519 (Harry 1971). In 1682, early French

explorers descended the Mississippi from the Great Lakes.

They erected a cross at the river mouth and claimed the

territory in the name of Louis XIV. In 1723, New Orleans

became the capital of Louisiana, superseding Biloxi.

Significant European settlement began in the early 19th

century. The first population census in 1810 in former

Louisiana counted 76,556 people (Harry 1971). In the late

1800s and early 1900s the bottomland hardwood forests

supported a lumber industry. The swamps yielded a great

deal of cypress, while in the surrounding upland areas,

oaks and hickories were harvested (White, 1979).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. In pollen zone 2, the Mvrica rise seems synchronous

with the decrease in Taxodium pollen. Mvrica is a common

shrub in the understory or in the secondary forest. One

probable for this Mvrica rise is the development of the

lumbering industry in the swamp areas where the cypress

forests were removed. However, based on radiocarbon dating,

the Mvrica rise happened at A.D. 1750 which is about a

century earlier than the significant development of

lumbering industry in this area. There is a probability of

other explanations. The date of the sudden rise of Mvrica

date is correlatable with the widely cultivation of wax

myrtle in this region during the early phase of European

settlement. In 1752, 'myrtle wax had become one of the

important products of Louisiana. One of the most successful

of the planters, says that he had just planted two thousand

wax-myrtles in addition to his old plants and that he had

sold part of his crop of the preceding year for three

thousand livres and still had the larger part of it in his

barn' (Rowland and Godfrey 1932). At that time, the salary

of a master craftsman might earn 500 livers or less

(McCarry 1989). This indicates that the myrtle wax business

made a very good profit to the local residents.

The following is a brief description of this plant by

an 18th century French traveller du Pratz (1774):

'The inyrtle wax-tree is one of the greatest blessings

with which nature has enriched Louisiana ..... The tree

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thrives as well as in the shade of other trees as in the

open air; in watery places and cold countries as well as in

dry ground-' An English botanist William Bartram in 1778 provided

the following account of the wax myrtle (Rowland and

Godfrey 1932):

'In my excursions about this place (Mobile), I

observed many curious vegetable productions, particularly a

species of Myrica (Mvrica inodoraf. This very beautiful

evergreen shrub, which the French inhabitants call the wax

tree, (cirier), grows in wet sandy ground about the edges

of swamps, it rises erect nine or ten feet...... It is in

high estimation with the inhabitants for the production of

wax for candles, for which purpose it answers equally well

with bees-wax, or preferable, as it is harder and more

lasting in burning.1

There was a major decrease in Mvrica pollen at about

1800 and it became unimportant at about 1900. This probably

reflects the gradual abandonment of wax myrtle plantations

because of the depreciation of myrtle wax due to other more efficient lighting materials, such as electricity, because

the incandescent electric lamp invented by Edison in 1879

was soon used in many western countries (Dierdorff 1971).

This time corresponds to the first decrease of Mvrica

pollen in the Pearl River Marsh core.

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The Mvrica rise at 1800 A.D. may be as significant as

the contemporaneous Ambrosia rise which is an important

pollen stratigraphic marker for the European settlement in

eastern North America. It has the potential of serving as

an criterion for pollen-stratigraphic correlation.

6) The Study of Horn Island

It seems that the study of Pearl River Marsh sediment

core alone cannot provide sufficient evidence to test the

hypothesis of decreased hurricane activities during the

Little Ice Age. The main problem here is that the

individual hurricane layers were obscured by other factors

such as bioturbation and sea level changes. More evidence

is essential in order to test this hypothesis. Perhaps the

best evidence comes from the stratigraphic study from

Fearn Lake in Horn Island, Mississippi, by Liu, Fearn and

Gathen (unpublished data).

Located at 88°37'W and 30°15'N, Horn Island is a

barrier island about 10 km offshore from the Mississippi

coastline, and about 75 km east of the Pearl River Marsh

(Figure 11-10). A sediment core taken from Fearn Lake on

the island was studied for stratigraphy, Loss-on-Ignition

and diatoms. The results are shown in Figure 10-11.

The trend of changes in diatom assemblages is similar

to that of the uppermost 84 cm of the Pearl River Marsh

core, PR1. The two C-14 dates show that brackish water and

marine diatoms, such as Parlia sulcata. Nitzschia scalaris.

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n * - . - , . 11-,10. Location of Horn Island, Mississippi. 11-,10. F i g u r e

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0) V\ p n 0 a\ 0 01 P s P § c c a) p e > l a) Ul ■o 5 & * 4 C a) (0 M iO 3 PI C P s ta 0 a) u IP P a) -C •p p a a. W - l

a *<

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Navicula maculata. and Amphora ovalis. are very abundant in

Diatom Zone 1, corresponding to the time before A.D. 1450.

From A.D. 1450 to 1700 years B.P., corresponding to Diatom

Zone 2, the birackish and marine diatoms were largely replaced by the freshwater diatom Fraailaria construens

var. venter. After A.D.1700, corresponding to Diatom Zone

3, percentages of brackish water and marine diatoms

increased again. At the same time, the percentage of the

freshwater diatom Fraailaria construens var. venter

decreased. Like the situation in the Pearl River Marsh, it is impossible to distinguish individual layers with

abundant marine diatoms from the adjacent levels in all

three diatom zones. Even in samples taken from the sand

layers, the marine diatom percentages did not show any

higher than normal values. The great abundance of brackish

water and marine'diatoms in Diatom Zone 1 and 3 may be

explained by either transgression or by more hurricane

activities. On the other hand, the high freshwater diatom

percentages in Diatom Zone 2 are probably related to

regression and/or reduced hurricane activities. The fluvial

discharge factor may be excluded here.

In order to differentiate the two possible causes,

hurricane and sea level, for the changes in diatom

assemblages, the stratigraphy and LOI values are closely

examined. Four sand layers are identified in this core,

corresponding to levels with relatively low values of LOI.

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According to the pioneer study on hurricane climate

reconstruction in Alabama, these sand layers were possibly

generated by hurricanes that were strong enough to

transport the overwash deposits to the coring sites (Liu

and Fearn 1993) . One thick sand layer occurs at 65 to 70 cm

in the core with an inferred age of A.D. 1400. The other

three thinner ones occur at depths of 9, 17 and 23 cm. The

C-14 dates suggest that they were deposited after A.D.

1700. The uppermost sand layer was deposited by Hurricane

Camille of 1969, because the highest Cs-137 activity,

representing 1963, was detected right below this sand

layer. Nevertheless, no sand layers were found in the

period from about A.D. 1450 to 1700, the early part of the

Little Ice Age.

The magnitude of regression during the Little Ice Age

in this region, if there was any, was perhaps in the order

of one meter or less which is much less than the height of

storm surges created by hurricanes. The absence of sand

layers during the early part of the Little Ice Age was

probably attributable to a decrease of in hurricane

activities. The lack of contemporaneous sea water incursion

from storm surges induced by strong hurricanes may in turn

explain the higher percentages of freshwater diatoms in

Diatom Zone 2 at Fear Lake in Horn Island Core.

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7) Historical Hurricane Record in the Gulf of Mexico Coast

Region

The hypothesis of decreased hurricane activities is

further substantiated by the historical records of

hurricane landfalls in the U.S. Gulf coastal region. Five

hundred years ago, at the onset of the Little Ice Age,

Columbus and his sailors discovered the New World. It seems

amazing to the modern climatologists that Columbus ventured

into the Atlantic in September which is the most dangerous

hurricane season today. However, he was very lucky in his

first voyage because he stayed at sea for 33 days without

encountering any hurricanes. Following Columbus a lot more

ships sailed across the Atlantic Ocean safely. Columbus

alone made four trials. One implication of the early

sailors' success stories is that hurricane frequency and

intensity then might not be as high as that of today.

Documented hurricane records appeared after the

Spaniards found the New World (Tannehill 1938; Millas

1968). These records for the Gulf of Mexico coast are

generally consistent with the hypothesis of depressed

hurricane activities during the Little Ice Age. In order to

reduce the bias caused by instrumental development and

colonial history, the data presented here only include the

hurricanes making landfalls, which should be more reliable

than the maritime hurricane records (Dunn and Miller 1960).

Three diagrams were plotted to illustrate the recorded

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hurricanes in (1) Florida; (2) Louisiana, Mississippi, and

Alabama; and (3) Texas (Dunn and Miller 1960; Neumann et al

1992) (Figure 11-12). The data show that:

1) The documented total hurricanes increased in

frequency from the 18th century to the 20th century.

2) The documented extreme and major hurricanes

(probably the category 3, 4 and 5 ones) also became more

frequent toward the 20th century. 3) There were no documented hurricanes in this region

in the 17th century, although there were several counts

during the 16th century. Although these patterns may be biased by incomplete

historical records, population growth and the gradual

advancement in the technology of hurricane detections, it

is hard to explain the complete absence of documented

hurricanes in the 17th century. This period corresponds

with the time of lowest brackish and marine diatom

percentages in the Pearl River Marsh and the Horn Island

sediment cores, as well as a lack of sand layers in the

Horn Island core. This time also corresponds to the main

part of the Maunder Minimum (from A.D. 1645 to 1715), and

it had been widely documented as being the coldest episode

of the Little Ice Age (Lamb 1982). All these evidences

suggest that at least during the coldest part of the Little

Ice Age, hurricane activities in this region were

depressed.

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& C L & '

*600 1700* 1800 1850 1S00 - 1S00

1600 1700 1800 I860 1900

« ” fT I. “ f I “ I1 •3 | I X = 03/

3 0 2 / <

1 01- o*£

1600 1700 1800 1850 :900

gggj Es n m a Ma n a n a s ^jjgMsforHuitanu 2 2 other Humcaneo Figure 11-12. Recorded historical hurricane in the U.S. Gulf of Mexico coastal region. (a) Florida; (b) Alabama, Mississippi and Louisiana; Jc) Texas (data from Dunn and M iller 1990; and Neumann at al 1992).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 12. CONCLUSION

The Pearl River Marsh sediment core was intensively

studied by means of diatom, pollen, and Loss-on-Ignition

analysis (Table 12-1). The results indicate that the study

area was occupied by a swamp forest at about 6000 B.P. The

swamp was dominated by Taxodium and Nvssa at the coring

site, with Carva. Pinus. Ouercus. Fraxinus. Ulmus. Salix

and Liauidambar growing on the surrounding uplands. The swamp was replaced by a brackish marsh at 5900 years B.P.

as a result of the postglacial transgression. Since then

the marsh has.remained brackish for most of the time, as

reflected by the predominance of marine/brackish water

diatoms and brackish marsh pollen. However, the brackish

water environment was interrupted at least twice when both

diatom and pollen results suggest that the marsh salinity

became low enough to support fresh marsh communities. The

first fresh marsh conditions occurred from 3400 to 2200

years B.P.; and the second one from 1400 to 1100 years B.P.

Minor discrepancy in zonation exists between the

diatom and pollen stratigraphies. Level by level comparison

is even more difficult, especially for the period from 5900

to 3400 years B.P. This is probably due to the different

response time between the diatoms and vegetation to

environmental changes, particularly to short term events

such as fluvial discharges and hurricanes. Diatoms should

respond faster than vegetation.

178

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Table 12-1. Summary table of the history of environmental changes in the Pearl River Marsh area.

Age Pollen Result Large Diatom Result Mean Loss-on- Sea Level (B.P.) Zone Vegetation Zone Vegetation Ignition (%) Floodings X Fresh X Brackish 35.4 Transgression IX Brackish IX Brackish

VUI Fresh v m Fresh 60.7 Regression

vn Brackish vn Brackish 29.8 Transgression -2000 Less 42.4 Frequent

VI Fresh VI Fresh 26.3 Regression -3000 38.4

V Brackish 26.5 -4000 V Brackish 58.1 rv Fresh 15.9 Transgression rv Brackish -5000 33.8 More in Brackish m Fresh 9.9 Frequent ii Brackish ii Brackish 22.7

i Swamp i Swamp N/A

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The changes in diatom and plant communities during the

past 6000 years are explained in the context of sea level

and fluvial discharge changes. The general brackish marsh

condition during this period, especially from 5900 to 3400

years B.P., suggests a transgressive scenario which is

compatible with published sea level data from the early

studies in the offshore areas of the Gulf of Mexico

(McFarlan 1961; Coleman and Smith 1964; Redfield 1967;

Nelson and Bray 1970). Nevertheless, the sea level rise

does not seem to be unidirectional. The two fresh marsh

conditions from 3400 to 2200 and from 1400 m to 1100 years

B.P. were possibly the results of regressions. These

regressions, particularly the one from 3400 to 2200 years

B.P., are contemporaneous with the late-Holocene neoglacial

events. They are not unique to the study area, but broadly

comparable with the recent study results from other coastal

areas of the U.S., such as in Florida (Stapor et al 1991;

Tanner 1993), South Carolina (Brooks et al 1979), southern

California (Davis 1992; Davis et al 1992), and Washington

(Eronen et al 1987).

Based on the diatom and Loss-on-Ignition data the

fluvial discharge in the Pearl River Marsh since 5900 years

B.P. may be divided into 12 stages. The data suggest that

the fluvial discharge of the Pearl River from 5900 to 4500

years B.P. was generally larger than that during the past

4500 years. Two extremely large fluvial discharge periods

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 8 1

were dated from 5400 to 5200 and from 4800 to 4500 years

B.P., when’Loss-on-Ignition values of sediment in the Pearl

River Marsh core were especially low with abundant fresh

water diatoms. This study supports the climate modeling

results that there was more precipitation during the mid-

Holocene Hypsi'thermal period in the U.S. Gulf of Mexico

coastal region (Kutzbach 1987).

Study of hurricane history during past several

centuries is very important to evaluate climate modeling

predictions about hurricane intensity change caused by the

greenhouse effect (Emanuel 1987). The study pays special

attention to the Little Ice Age. Climatic cooling during

this period probably depressed the hurricane activities in

the Gulf- of Mexico region. The hypothesis is supported by

stratigraphic data from Fear Lake on Horn Island, where

more hurricane-generated sand layers are observed in the

sediment after A.D. 1660 years B.P., but none was found in

the early part of the Little Ice Age. The distinctly low

abundance of marine and brackish water diatoms for the

Little Ice Age in the sediment cores taken from the Pearl

River Marsh and from Fear Lake on the Horn Island was

probably also due to a reduced hurricane incidence,

although a sea level fall during the Little Ice Age may

provide an alternative explanation. Historical hurricane

records in the Gulf of Mexico coast also seem to support

the hypothesis of depressed hurricane activities during the

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Little Ice Age. However, because it is impossible to differentiate the individual hurricane layers based on the

biostratigraphy, more studies are needed to substantiate

the proposition.

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. DIX A: MAIN POLLEN TYPES IN THE PEARL RIVER MARSH AREA

1, Pinus PR-0-2-N1

2-3. Ouercus PR-0-7-N1

4. Taxodium PR—0-10-N1

5. Nvssa PR—7—1-N1

6. Mvrica PR-0-3—N1

7. Carva PR-3-1-N1

8. Decodon PR—100—1—N1

9. Gramineae PR—0—16-N1

10-11. Polygonaceae PR-0-12-N1

12. Viana PR-0—6-N1 13. Compositae PR-0-13-N1

14. Ambrosia PR-0-17-N1

15. Chenopodiaceae PR-0-1-N1

16. Cyperaceae PR—0—14—N1

17. Typha PR—0—15—N1 18. Umbelliferae PR—3-2-N1

19-20. Saaittaria PR-0-9-N1

21. Polypodiaceae PR—0-12-N1

22. Osmunda PR-0—11-N1

( X 1 0 0 0 )

196

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPENDIX B: MAIN DIATOM TYPES IN THE PEARL RIVER MARSH AREA

1 . Eunotia flexuosa Breb. & Kutz. PR-23 0—118-N2

2. E. maior (W. Sm.} Rhabh. PR-13-7-N4

3. E. oectinalis (O.F.Mull.f Rabh. PR—230—113—N2 4. E. nectinalis var. ventricosa Grun. PR—230—117—N2

5. E. Dectinalis var. minor (Kutz.fRabh. PR—23 0—13—N4

6. Pinnularia so. PR—230—111—N2

7. P. viridis (Nitzsch) Ehr. PR-1—14—S

8. P. boaotensis v. undulata(M.Peraa.) Boyer PR-1—20—S

9. P. maior Kutz. PR-1-32-N3

10. P. nobilis Ehr. PR—23 0—115-N2

(cont'd)

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m m

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11. Gomohonema so. PR—230—116—N2

12. Meridion circulare v.constrictumfGr.)Cl. PR—520-122-N2

13. Rhooalodia cribba (Ehr.) 0. Muller PR—660-14—N4

14. R. aibberula (Ehr.) 0. Muller PR-1-9-N3 i 15. Surirella so. PR-1-29-N3

16. Eohithemia so. PR-640-12-N4 17. Terosinoe musica Ehr. PR—1—18—N3

18. Tabularia tabulata (C.A.Acr.) Snoeiis PR-64 0-21-N4

19. Stauroneis ohonicenteron Ehr. PR-230-121-N2

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(cont'd)

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I ® ! ? ® ! 1 ■ '' .;*.V-VV\ -v * . 7 i t 3 f{- lill 1 IS#

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20. Achnanthes inflata (Kutz.) Grun. PR-310-10-N4

21-22. A. temperei M. Perag. PR-71-4-N4

23. Bacillara paradoxa Gmelin PR-1-22-N4

24. Diploneis elliptica (Kutz.) Cl. PR-1-5-N3

25. D. ovalis (Hilse) Cl. PR-1-35-S

26. Cvmbella sp. PR-1-9-N3 27. Cvclotela meneghiniana Kutz. PR-1-27-N3

28. Cocconeis placentula Ehr. PR-1-12-N3

29. Fraqilara construens var.venter (Ehr.)Grun.PR-640-25-N4

30. Navicula pusilla W. Smith PR-1-21-N3

31. N. placenta Ehr. PR-1-15-N4

32. N. capitata Ehr. PR-1-26-N3

33. N. tripunctata (O.F.Muller) Bory PR-1-26-N4

34. N. eleqans W.Sm. PR-1-36-S

35. Melosira qranulata v. anqustisima O.Muller PR-1-25-N3

36. Nitzschia amphibia Grunow PR-670-20-N4

37. N. trvblionella Hantzsch PR-1-19-N3

38. N. brevissima Grun. PR-1-17-S

39. Campylodiscus echeneis Ehr. PR-410-11-N4

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H i

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40. Achnanthes haukiana Grun. PR—1-15-N3

41. Dimloneis bombus Ehr. PR—1-6—N3

42. D. smithii (Brev.) Cl. PR—690-1—N3

43. Navicula maculata fJ.W.Bailf Cl. PR—440—1—N5

44. Nitzschia cjranulata Grunow PR-8-2-N5

45. N. scalaris W. Smith PR—670—19—N4

46. N. obtusa W. Smith PR—1-16-N4

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47. Actinocvclus beaufortianus (Hust.)Sulliv. PR-160-1-S

48. Actinoptvchus undulata (Bail.) Ralfs PR—13—9-N4

49. Coscinodiscus excentricus Ehr. PR-13-5-N4

50. C. radiatus Ehr. PR—13—9—N4

51. Paralia sulcata (Ehr.) Kutz. PR-470-24-N4

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. VITA

Xu Li was born in China on June 21, 1962. He studied

physical geography at Nanjing University, China and was

awarded an B.S. in 1983. He studied palynology at the

Academy of Sciences of China, Nanjing Division and earned a

M.S. in 1986. After completing master degree, he worked as

a research associate in Qingdao Institute of Marine

Geology, the Ministry of Geology and Mineral Resources of China. In 1990, he entered the Ph.D. program in the

Department of Geography and Anthropology at Louisiana State

University, Baton Rouge, Louisiana and is now a candidate

for the Ph.D. degree.

208

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Candidate: Xu Li

Major Field: Geography

Title of Dissertation: a 6200-Year Environmental History of the Pearl River Marsh, Louisiana

Approved:

Major Professor and Chairman £-1

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