University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln
Dissertations & Theses in Earth and Earth and Atmospheric Sciences, Department Atmospheric Sciences of
8-2011
The Application of Biostratigraphy and Paleoecology at Southern Ocean Drill Sites to Resolve Early to Middle Miocene Paleoclimatic Events
Ryan Farmer University of Nebraska-Lincoln, [email protected]
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Farmer, Ryan, "The Application of Biostratigraphy and Paleoecology at Southern Ocean Drill Sites to Resolve Early to Middle Miocene Paleoclimatic Events" (2011). Dissertations & Theses in Earth and Atmospheric Sciences. 19. https://digitalcommons.unl.edu/geoscidiss/19
This Article is brought to you for free and open access by the Earth and Atmospheric Sciences, Department of at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Dissertations & Theses in Earth and Atmospheric Sciences by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. The Application of Biostratigraphy and Paleoecology
at Southern Ocean Drill Sites to Resolve Early to
Middle Miocene Paleoclimatic Events
By
Ryan K. Farmer
A THESIS
Presented to the Faculty of
The Graduate College at the University of Nebraska
In Partial Fulfillment of Requirements
For the Degree of Master of Science
Major: Earth and Atmospheric Sciences
Under the Supervision of Professor David M. Harwood
Lincoln, Nebraska
August 2011
The Application of Biostratigraphy and Paleoecology at Southern Ocean Drill Sites to
Resolve Early to Middle Miocene Paleoclimatic Events
Ryan K. Farmer, M.S.
University of Nebraska, 2011
Advisor: David Harwood
The diatom biostratigraphy and paleoceanography of Ocean Drilling Program (ODP) Site 744 on the Southern Kerguelen Plateau, southern Indian Ocean are documented for the early to middle Miocene to improve chronostratigraphic age control for the Southern Ocean and Antarctic region. Paleoenvironmental fluctuations in the Southern Ocean are inferred from changes in fossil diatom abundance, preservation, and assemblage composition. A robust, new age model for Holes 744A and 744B is constructed using Constrained Optimization (CONOP) model ages for diatom biostratigraphic datum levels and new magnetic polarity data, which enables assessment of a nearly continuous record of paleoenvironmental change from ~20.25 to 13.75 Ma, and the dating of specific paleoenvironmental events identified in the diatom record. This study spans stratigraphically from the lowest occurrence of Thalassiosria spinosa var. aspinosa (~20.26 Ma) up to the lowest occurrence of Nitzschia denticuloides (~13.73 Ma), including the interval known as the Mid-Miocene Climatic Optimum (MMCO) ~17 to 15 Ma. Two stratigraphic breaks are recognized at Site 744 during the study interval. Ages interpreted for diatom events at Site 744 confirm the utility of CONOP modeled ages and indentify that Total Range Model and Hybrid Range Model ages presented the greatest utility in age model development at this site. A total of 78 diatom taxa are reported here, more than doubling previous reports, which enables age calibration of many new biostratigraphic datum levels.
iii
Acknowledgements
I would like to thank the Department of Earth and Atmospheric Sciences at the
University of Nebraska-Lincoln for the financial support that allowed me to conduct my thesis research. This project was also supported by NSF Cooperative Agreement OPP
0342484 to the University of Nebraska. I owe much thanks to Aaron Gewecke, Nicole
Pierson, and Teresa Franta, for their hard work and help in preparing materials for analysis. Thanks go out to my committee members: Drs. Tracy Frank and David Watkins and to Drs. Steve Bohaty and Fabio Florindo for their valuable input and suggestions that allowed this research to develop and progress in a timely manner. I would like to acknowledge the work of Rosemary Cody who provided unpublished, updated CONOP model outputs for the diatom ages presented herein. I acknowledge and thank the staff of the Integrated Ocean Drilling Program repository on the campus of Texas A&M for allowing us to collect the materials necessary to complete this project. A special thanks goes out to my advisor Dr. David Harwood for his encouragement and knowledge, in addition to providing me with a challenging project that introduced me to the world of diatoms. Lastly, I would like to thank my friends, my girlfriend Chesney, and family for their support throughout the years. 1
1. Introduction
Paleoenvironmental information from siliceous marine microfossils at Southern
Ocean sites is limited for the early to middle Miocene due to a poor understanding of diatom paleofloras and poor preservation at many sites. This study investigates how changes in fossil diatom floras reflect a response to climate perturbations during the early to middle Miocene. Of particular interest is the Southern Ocean and Antarctic record of
(1) the Mid-Miocene Climatic Optimum (MMCO; ~17 to 15 Ma), an interval identified in deep-sea records that is considered to be the warmest period of the Neogene (Savin,
1977; Majewski, 2002), and (2) the diatom response to transient climate excursions during the Miocene (e.g. the ‘Mi’ events of Miller et al., 1991). This new record from
ODP Site 744 allows for the development of new diatom proxies that reflect paleoceanographic changes in surface-water masses at high southern latitudes.
1a. Miocene Southern Ocean Diatom Biostratigraphy
Studies of Miocene Southern Ocean diatom floras began with McCollum (1975), who reported results from Deep Sea Drilling Project (DSDP) Leg 28. Since then, studies of Miocene Southern Ocean and high-latitudes floras have been conducted by: Schrader
(1976), Gombos (1976), Weaver and Gombos (1981), Ciesielski (1983), Gersonde
(1990), Gersonde and Burckle (1990), Baldauf and Barron (1991), Harwood and
Maruyama (1992), Censarek and Gersonde (2002), Arney et al. (2003), Bohaty et al.
(2003) and Whitehead and Bohaty (2003a). These studies serve as the framework for diatom taxonomic concepts and paleoenvironmental associations utilized in this study.
2
Antarctic diatom records are available from the McMurdo Sound region from: the
Dry Valley Drilling Project (DVDP) (McKelvey, 1981; Ishman and Reick, 1992; Winter and Harwood, 1997); the McMurdo Sound Sediment and Tectonics Studies (MSSTS)
(Harwood, 1986); the Cenozoic Investigations in the Western Ross Sea project (CIROS)
(Harwood, 1989); the Cape Roberts Project (CRP) (Harwood et al., 1998; Scherer et al.,
2001; Barrett et al., 2001; Harwood and Bohaty, 2002); and most recently, the ANDRILL
Southern McMurdo Sound Project AND-2A drillcore (Olney et al., in prep.).
New records from the Wilkes Land margin (IODP Exp 318) and revised records paired with CONOP diatom age modeled results (this study), will further advance our understanding of the state of Southern Ocean diatom biostratigraphy during the Miocene.
For further information about the Neogene diatom biostratigraphy used here and a detailed look at the current state of Neogene diatom biostratigraphy, see Yanagisawa and
Akiba (1990), Harwood and Maruyama (1992), Censarek and Gersonde (2002), Cody et al. (2008), and Olney et al. (in prep.).
The recent development of refined chronostratigraphic controls for the Neogene
(Lourens et al., 2004), and biostratigraphic synthesis of Southern Ocean diatom records
(Cody et al., 2008), provide the opportunity to reevaluate existing Southern Ocean drillsites that contain a complex history of climatic and paleoceanographic change. This study presents a high-resolution analysis of diatom biostratigraphy, which is the basis of a revised age model for Ocean Drilling Program (ODP) Site 744. A parallel study by
Florindo et al. (in prep.) presents new magnetic polarity data that amplifies the robustness of this new age model for Site 744.
3
1b. Prior research on ODP Site 744 materials
The present study builds on the ODP Leg 119 shipboard studies at Site 744 reported in Barron, Larsen, et al. (1989), diatom studies of Baldauf and Barron (1991), and magnetic polarity stratigraphy reported by Keating and Sakai (1991), by applying the
Astronomical Tuned Neogene Time Scale (ATNTS) chronostratigraphic ages (Lourens et al., 2004) and CONOP diatom biostratigraphic ages (Cody et al., 2008). This report doubles the number of diatom taxa reported previously at Site 744 and integrates new magnetostratigraphic data (Florindo et al., in prep.). Baldauf and Barron (1991) report 29 diatom taxa for the core interval between 80.30 and 98.25 mbsf in Hole 744A and 24 diatom taxa for the core interval between 56 and 79.6 mbsf for Hole 744B. This study documents and describes 51 taxa at Hole 744A (Table 1a) and 69 taxa at Hole 744B
(Table 1b) over the same stratigraphic interval, but at a higher resolution sample spacing.
The lower and middle Miocene strata recovered at ODP Site 744 are mixed pelagic biosiliceous-carbonate oozes that were deposited at relatively shallow water depths (presently at 2300 mbsl) on the Southern Kerguelen Plateau (Fig. 1) (Shipboard
Scientific Party, 1989). A low-resolution isotope stratigraphy has been established for the lower to middle Miocene succession recovered at this site (Woodruff and Chambers,
1991; Barrera and Huber, 1991). The carbonate record at Site 744 allowed Majewski
(2002; 2003) to document changes in planktonic foraminifera assemblages and assess their relationship to changing paleoceanographic conditions during the early and middle
Miocene. Majewski (2002) documented trends in foraminiferal faunal composition and
4 diversity through time, in addition to analyzing select benthic and planktonic foraminifera
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Figure 1. Location of ODP Site 744 (red star) on the Southern Kerguelen Plateau (61°34.66’S, 80°35.46’E; water depth of 2300 mbsl) (after Florindo et al., in prep.). The extent of the mean sea ice coverage in summer (mssis; red line), mean sea-ice coverage in winter (mwsiw; white line), and average present day position of the Polar Front (PF; yellow line) are noted. Inset Map: The present day position of the Polar Frontal Zone (PFZ) and ODP Site 744 (red star).
18 13 for δ O and δ C ratios. Based on these'./0%+&12 data, no significant change in sea-surface temperatures were interpreted between 17 to 14.2 Ma at Site 744 (Majewski, 2002).
However, the revised diatom record for this study indicates that sea-surface conditions
5 were much more dynamic during this interval than previously indicated. The present study and that of Florindo (in prep.), establishes the chronostratigraphic framework to compare foraminiferal and diatom assemblage changes at Site 744, and relate inferred paleoenvironmental events into a global context.
2. Southern Ocean Setting
ODP Site 744
ODP Site 744 was drilled on the Southern Kerguelen Plateau (61°34.66’S,
80°35.46’E; water depth of 2300 mbsl) in an effort to recover complete Oligocene and
Neogene stratigraphic sections from the Indian sector of the Southern Ocean (Shipboard
Scientific Party, 1989). Sediments at Site 744 consist primarily of biogenic components, dominated by calcareous nannofossils that were deposited in a pelagic setting with minimal terrestrial influence (Shipboard Scientific Party, 1989). Carbonate content is relatively high (> 90%) during the early to middle Miocene section at Site 744, but decreases to 80 % in two discrete intervals (Ehrmann, 1991). High siliceous microfossil abundance correlates with lower percentages of nannofossils in the sediment (Shipboard
Scientific Party, 1989). Diatoms dominate the siliceous component, with radiolarians, silicoflagellates, and sponge spicules being absent or present only in trace amounts
(Shipboard Scientific Party, 1989). The site has been above the carbonate compensation depth (CCD) since at least the late Eocene, with the oldest portions of the core containing carbonate microfossils that are devoid of dissolution features (Shipboard Scientific Party,
1989). Low sediment recovery in Cores 744A-8H (62.8%) and 744A-9H (12.1%) led to
6 the drilling of Hole 744B, which achieved excellent recovery (101.6%) in its nine cores
(Barron, Larsen et al., 1989).
3. Materials and Methods
3a. Sample Collection and Preparation
For this study Cores 744B-7H, 744B-8H, 744B-9H, 744A-10H, and 744A-11H were sampled at 10-12 cm intervals, where possible, by using 5 cc plastic cylindrical tubes at the Integrated Ocean Drilling Program Repository on the campus of Texas A&M
University. Coeval stratigraphic intervals from Cores 744B-7H, 8H, and 9H were examined, due to incomplete recovery of Cores 744A-7H, 8H, and 9H.
Approximately 0.4 cc of material was extracted from individual 5 cc sample tubes, dried, weighed, and placed into a 15 ml centrifuge tube. Samples were treated with hydrochloric acid to remove calcium carbonate, and then treated with hydrogen peroxide to remove organic matter. Once the reaction ceased, the samples were washed with deionized water and then rinsed and centrifuged repeatedly until a neutral pH was attained. After completing the washing process, vials were placed on a vortex stirrer to homogenize and disperse the sample. Strewn slides were prepared by transferring an aliquot (~ 1.5 ml) of the suspended sediment to a partially filled (with deionized water)
15 ml petri-dish containing two 22 x 22 mm coverslips. Sufficient time was allowed for diatoms to settle to the bottom, covering the petri-dish and coverslips. Water was removed from the petri-dishes through capillary action using paper towels and evaporation over 24 hours. Coverslips were then heated on a hotplate and mounted to glass slides using Norland Optical Adhesive #61 (refractive index of 1.56) and analyzed
7 using 40x (dry), 60x (oil), and 100x (oil) objective lenses on an Olympus BH2 light microscope.
3b. Biostratigraphic Approach
Individual diatom species abundance for each slide were recorded by examining the entire slide using a 40x objective lens as follows:
A = abundant; multiple specimens every 1 to 2 fields-of-view (FOV);
F = frequent; >1 specimen in every 5 FOV;
R = rare; 1 specimen per 5 FOV;
VR = very rare; < 10 specimens in the entire coverslip area;
X= present; no quantitative assignment; r = reworked; no quantitative assignment.
Relative diatom abundance for each slide was recorded using a 40x objective lens as follows:
H = High; little to no void space on the slide; individual specimens are often overlapping and at multiple focal planes on the slide;
M = Moderate; more void space over the entirety of the slide; specimens are more evenly dispersed at a single focal plane; individual specimens may overlap but not to an extreme degree;
L = Low; slide is dominated by void space.
Preservation of diatoms in each strewn slide was determined as follows:
G = good; individual specimens exhibit minimal dissolution or fragmentation; diagnostic and delicate features are preserved allowing identification to species level;
M = moderate; individual specimens may show evidence of dissolution or fragmentation; some specimens cannot be identified to the species level;
P = poor; individual specimens exhibit significant fragmentation and/or dissolution.
8
The first occurrence (FO) for an individual diatom species was placed at a minimum depth (mbsf) of the sample slide on which it was observed. A maximum FO depth for an individual diatom species was placed down-section at the depth of the preceding sample slide. The last occurrence (LO) for an individual diatom species was placed at a maximum depth of the slide where it was last seen. A LO minimum depth was determined by placing it between the depth of the last sample slide where the species was seen and the depth of the next up-section sample.
3c. GRAPE Data
Shipboard Gamma Ray Attenuation Porosity Evaluation (GRAPE) data were used in this study to identify potential core intervals with higher diatom abundances. Whole- core GRAPE density values are a useful proxy for identifying core intervals with high or low siliceous microfossil abundance. At Site 744, core intervals with high diatom abundances exhibited lower GRAPE density values (Fig. 3) when compared to intervals with greater abundance of carbonate microfossils and higher GRAPE density values.
This correlation results from diatoms using amorphous silica, which has a lower grain density than that of calcium carbonate, to build their skeleton. Consequently, fluctuations in GRAPE density values strongly parallel fluctuations in relative diatom abundances
(Fig. 3) and can be used as a proxy to identify zones of high or low diatom abundance at
Site 744.
4. Composite Depth
9
The cores in the study interval from Holes 744A and 744B were placed on a single composite depth scale (Fig. 2). The composite depth assignments in the overlapping intervals of Cores 744A-7H to 9H and Cores 744B-7H to 9H were estimated due to poor core recovery and coring disturbances in Cores 744A-8H and 9H. However, several tie-points were established between the cores using both shipboard and newly generated magnetic susceptibility and polarity data (Barron, Larsen, et al., 1989; Keating and Sakai, 1991; Florindo et al., in prep.), and shipboard gamma-ray attenuation (GRA) density data (Barron, Larsen, et al., 1989). Depths for the finalized composite section are reported here in meters composite depth (mcd). However, this does not include the discussion of Diatom assemblage Zone(s) (DZ), where changes in the diatoms present and their respective abundances are reported in mbsf relative to Hole 744A or Hole 744B.
The reader is directed to Florindo et al. (in prep.) for further details about the construction of the composite depth scale seen in Figures 2 and 3.
10
Age (Ma) 0123$456&$,77! 13.734 13.605 14.095 14.877 15.160 14.194 14.784 15.974 13.015 13.183 13.369 14.581 15.032 16.268 16.303 16.472 17.740 18.056 18.524 18.748 19.722 20.040 20.213 20.439 16.543 16.721 20.709 17.235 17.717 17.533 n r n r r r .1r .2r A A C C
GPTS of Gradstein r r n.1r C A n.1n n.2n n.1n n.2n n.3n A A A C C C A A C6n C6r et al. (2004) C5Br C5 C5Dn C5En C5Er C6
0123$456&$,778 C5A C5A C5ABn C5ABr C5 C5 C5ADn C5ADr C5D C5D C6 C5Bn.1r C5Bn.2n C5 C5 C5 C6 C6 13 14 15 16 17 18 19 20 21 )* 50
Tr age CONOP Hybrid age CONOP Avg age CONOP 55 FO/FCO )) LO LO Nitzschia sp. 17 FO N. denticuloides
%# FCO A. ingens var. +* nodus 60 FO D. simonsenii 0.5 cm/k.y. LO A. lewisianus &# LO R. marylandicus
1 cm/k.y.
+) FO D. lauta 65
2 cm/k.y. FO N. grossepunctata
LO C. kanayae var. pacifica ,* 70 9#:;5$%=2<( FO A. ingens LO F. maleinterpretaria
FO Nitzschia sp. 17 FO D. maccollumii FO E. antarctica FO C. pennatum ,) 75 $# FO C. kanayae var. pacifica and FO A. lewisianus
9#:;5$%<=>=2<($,77! LO T. praefraga -* 80 !"#
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.* 90 !!# LO T. spumellariodes
.) FO T. praefraga FO F. maleinterpretaria '' FO T. spumellariodes FO R. marylandicus FO C. miocenicus /** 13 14 15 16 17 18 19 20 21 -90 -60 -30 0 30 60 90 456&$0123?1';?@1$%A( !"#$%&'( Figure 2. Age-depth plot for Site 744 with a meters composite depth (mcd) scale used to compare Holes 744A and 744B. Diatom datums are used to constrain magentostratigraphic interpretations, which are correlated to the Astronomical Tuned Neogene Time Scale (ATNTS) of Lourens et al. (2004).
11
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Figure 3. Core recovery for Holes 744A and 744B with a meters composite depth (mcd) scale. Diatom abundance (red line) for the study interval at Site 744 in meters composite depth (mcd) is also shown. An interpreted disconformity is noted at 73.23 mcd. H = High diatom abundance, M = Moderate diatom abundance, and L = Low diatom abundance. Gamma Ray Attenuation Porosity Evaluation (GRAPE) values are shown for Holes 744A (blue (shipboard data); yellow (gint filter)) and 744B (orange (shipboard data); green (gint filter)). Intervals with increased opal concentrations are noted in gray and correspond to high diatom abundance intervals (red line).
5. Results
5a. Overview
Core recovery was excellent in many intervals at Site 744. A high-resolution sample set allowed for the collection of diatom presence and abundance data of 78 diatom taxa between 56.04 and 100.25 mcd. The stratigraphic occurrence of individual
12 diatom species is reported in Tables 1a and 1b, and key diatom species are illustrated in plates I-VI. The core depths of the FOs and LOs of selected diatom taxa are presented in
Tables 2 and 3. Newly-developed diatom records for the lower to middle Miocene intervals of Holes 744A and 744B are paired with new paleomagnetic data (Florindo et al., in prep.), which provide the basis for a revised age-depth model (Fig. 2).
Diatom data produced in this study are described below. The first section describes the nature of the diatom assemblage at the site, including preservation, abundance, and the depth-habitat of the taxa. The section on age presents the construction of a new age model for Holes 744A and 744B, which integrates diatom biostratigraphy (this study) with new magnetic polarity results of Florindo et al. (in prep.). The biostratigraphic approach and interpretation of ages of new diatom datum levels is then presented, and an array of DZs are described in order to identify distinct changes in the abundance and composition of the diatom floras at Site 744. Variations in the siliceous microfossil assemblage are inferred to result from paleoenvironmental changes in the Southern Ocean.
5b. Diatom Assemblage Characteristics
The majority of the diatom taxa recorded from Site 744 samples are planktonic species (Tables 1a; 1b), but in several discrete intervals both neritic taxa, e.g. Paralia sulcata, and benthic diatoms, e.g. Cocconeis and Rhabdonema species are present.
Additionally, several intervals containing reworked diatoms are present (noted by an “r” in Tables 1a; 1b). The following diatom abundance data is presented from the base of
13
Core 744A-11H (at 100.25 mcd) up through younger core material at 56.04 mcd in Core
744B-7H.
Diatom abundances between 100.25 and 56.04 mcd were moderate, with the exception of discrete zones with high or very low diatom abundance (Fig. 3; Fig. 4). Core
744A-11H, from 100.25 to 91.75 mcd, had the lowest diatom abundances of this study, with diatom abundance never exceeding moderate levels, and contained many intervals with very low diatom abundances (Fig. 3). Continuing up-section from the base of Core
744A-10H, diatom abundance is more varied and several intervals of high diatom abundance occur near 80.80 mcd (Fig. 3). Core 744B-9H has low to moderate diatom abundance from the base of the core at 79.6 mcd up to 72.98 mcd, where there are several up-section intervals of high diatom abundance (Fig. 3) and moderate to good preservation. Core 744B-8H yielded some of the highest levels of diatom abundance
(Fig. 3) and the best preservation in this study. However, at 62.62 mcd in Core 744B-8H, there is a substantial drop in diatom abundance (Fig. 3) and preservation (Table 1b) that continues until the base of Core 744B-7H (at 59.4 mcd). From the base of Core 744B-7H at 59.4 mcd up to the top of this study at 56.04 mcd, diatom abundance is high, with the exception of a few discrete intervals with low diatom abundance (Fig. 3) and poor to moderate preservation (Table 1b).
14
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Figure 4. Photomicrographs of diatom assemblages in early to middle Miocene samples from Site 744 within intervals characterized by high diatom abundance, and contrasting assemblage compositions. Photo A is dominated by Actinocyclus ingens, and Photo B is dominated by pennate diatoms.
15
5c. Age Model
A chief objective of this study and that of the parallel study by Florindo et al. (in prep.) was to produce a robust age model (Fig. 2) for the early to middle Miocene interval of Site 744 using diatom biostratigraphy and magnetostratigraphy data. This work builds upon chronostratigraphic refinement reported for the late Eocene and Oligocene intervals of Hole 744A by Roberts et al. (2003), by extending that record up to the middle
Miocene.
An initial, working age model for the early to middle Miocene interval of Site 744 was developed by establishing the depth of diatom biostratigraphic datum levels for the
FO and LO of selected diatom species. Ages were assigned to diatom events by utilizing three CONOP modeled age outputs. The Total Range Model (TRM) and Average Range
Model (ARM) diatom biostratigraphic outputs are presented and described in Cody et al.
(2008). A new Hybrid Range Model (HRM) (Cody, unpublished data) that accommodates and adjusts for natural up-section reworking of diatoms is also applied here. Model ages for select diatoms from all three CONOP models are provided in Table
2. TRM and HRM ages for 25 diatom events are plotted in Figure 2.
16
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
Table 2. Compilation of diatom biostratigraphic events at Site 744 with comparison to Southern Ocean (40°S latitude and higher) CONOP model diatom ages. TRM = Total Range Model; HRM = Hybrid Range Model; ARM = Average Range Model. “*” indicates diatom events environmentally controlled and (r?) indicates diatom events suspected of having been reworked.
17
The biostratigraphic constraints provided by these diatom ages enables the assignment of magnetic polarity chrons and subchrons to the polarity intervals identified within the Site 744 section (Fig. 2); all of which are correlated to the ATNTS of Lourens et al. (2004). The resulting tie-points define age-depth relationships that reflect changes in sediment accumulation rates through the early to middle Miocene interval and the presence of previously unrecognized short stratigraphic breaks (Fig. 2). One stratigraphic disconformity, spanning ~ 800 kyr is inferred at 73.23 mcd (744B-9H-2, 133-134 cm) based on the truncation of several diatom events (Table 1b) and missing portions of magnetic polarity Chrons C5Br and C5Cn.1r (Fig. 2). Through the study interval, sedimentation rates averaged 1 cm/kyr at Site 744. However, sedimentation rates were as high as 2 cm/kyr, e.g. from 73.23 to 82 mcd, and as low as 0.7 cm/ kyr, e.g. from 82 to
83.8 mcd, in discrete intervals (Fig. 2).
The model ages provided by the CONOP method (Cody et al., 2008) represent a regional synthesis of many Southern Ocean sites positioned at 40°S latitude and higher
(Fig. 5). The ages of diatom events at Site 744 were recalculated using the interpreted age model, resulting in site-specific age calibrations for diatom events. Site-specific diatom ages are expected to differ slightly from the CONOP compilation due to local environmental controls on species distribution and migration in biogeographic ranges that control species occurrences on a spatial and temporal scale. Interpreted diatom Ages (IA) for Site 744 are presented on the far right column of Table 3 and include the FO and LO for many diatom taxa that are not yet included in the CONOP database. Comparison of the IA for those taxa currently included in the CONOP compilation (Table 3) allows for a
18 confirmation of CONOP model ages for the early to middle-Miocene interval (see discussion, Application of CONOP Model Ages at Site 744).
Figure 5. The location of Antarctic shelf and Southern Ocean sites currently included within the CONOP diatom biostratigraphic database. Modern oceanographic boundaries are noted. Ocean Drilling Program (ODP) drill-sites discussed in this paper are noted with red stars. Figure modified from Cody et al. (2008).
19
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
20
5d. Standard Diatom Zones vs. Datum Levels
Standard Southern Ocean diatom zonations, such as those used by Harwood and
Maruyama (1992) were not used during this study. Instead, a greater number of specific diatom events (Table 2) were used to create a robust biostratigraphic framework for this site. The resolution of standard diatom biostratigraphic zonations is too coarse for the
(~10 cm) sample spacing used in this study, and many diatom events identified at Site
744 (Table 2) might not have been recognized using standard zonal schemes. Although biostratigraphic zones are not applied, there is considerable value in interpreting the diatom record in terms of a series of environmentally controlled assemblages. The following section discusses diatom paleoecology in the context of discrete stratigraphic intervals that are interpreted to reflect paleoenvironmental conditions that can be inferred by changes in the assemblage composition of diatom flora of Site 744.
5e. Diatom Assemblage Zonation (DZ)
The following section presents the stratigraphic succession of Diatom assemblage
Zone(s) (DZ) (Fig. 6). Fourteen zones are defined in the study interval for Site 744, from
Zone A at the base of our study interval (98.25 mbsf), up to Zone N (56.04 mbsf) at the top of our study interval. The DZs (Fig. 6) reflect distinct changes in abundance and composition of the diatom flora present at Site 744.
Ocean Drilling Program Site 744 DIATOM ZONES DIATOM 1. Actinocyclus ingens 1. nodus Actinocyclus ingens var. 2. Araniscus lewisianus 3. Araniscus umbonatus 4. sp. 1 5. Bogorovia veniamini 6. Bogorovia 7. Cavitatus miocenicus pennatum 8. Corethron 9. Coscinodiscus rhombicus 10. Crucidenticula ikebei pacifica Crucidenticula kanayae var. 11. 12. Denticula norwegica 13. Denticulopsis lauta 14. Denticulopsis maccollumii 15. Denticulopsis sp. 1 16. Denticulopsis simonsenii 17. Eucampia antarctica 18. Fragilariopsis maleinterpretaria 19. Mediaria splendida 20. Nitzschia denticuloides 21. Nitzschia grossepunctata 22. Nitzschia sp. 1 1976 23. Nitzschia sp. 17 sensu Schrader, 24. Nitzschia sp. 3 25. Raphidodiscus marylandicus 26. Rocella gelida bukyri 27. Thalassiosira sp. cf. T. 28. Thalassiosria fraga 29. Thalassiosira praefraga 30. Thalassiosira spinosa aspinosa 31. Thalassiosira spinosa var. 32. Thalassiosira spumellariodes Depth (mbsf) Core recovery Section Core Hole Polarity Chron Age (Ma) 50 7 55 top of this 5 13.73 study C5ACn 20. 6 N cc 60 14.19 1 C5ADn 21. 14.58 2. 16. 2 25c. M C5ADr 3 31b.
4 8 L 65 14.78 13. 744 B 24. 5 C5Bn.1n 19. K C5Bn.1r 14.88 6 15.03 cc C5Bn.2n J 1 15.16 70 C5Bn.2r 14. I 2 23. 25c. 16.30 disconformity 3 1. 15. H C5Cn.2n 8.
4 9 G 75 5 16.47 3. 11. C5Cn.2r 12. 17. F 6 16.54 4. 10. C5Cn.3n Core Break 80 1 16.71 E 2 C5Cn.3r 17.24 3 C5Dn 85 4 10 17.53 25b. 5 C5Dr D 6 18.05 cc 90 25b. 1 C5En A 2 A 744 18.52 5. 28. C5Er F 3 18.75 29. C R 18. 25a. 4
95 11 C6n VR 32. 5 B X 30. 6 7. 9. 22. 25a. 100 ? ? 6. 26. 27. 31a. A
Figure 6. A stratigraphic succession of Diatom assemblage Zone(s) (DZ) from Zone A at the base of our study interval, up to Zone N at the top of our study interval. Assemblage zones are noted in meters below seafloor (mbsf) and reflect distinct changes in abundance and composition of the diatom flora present at Site 744. A disconformity is interpreted at 71.83 mbsf. 21 22
DZ-A (base: 98.25 and top: 97.71 mbsf)
Zone A is characterized by abundant specimens of Bogoroiva veniamini and
Thalassiosria spinosa var. aspinosa (Fig. 6), with minor occurrences of Rosiella paleacea
(Table 1a). Diatom preservation in this zone is poor to moderate and diatom abundance is moderate. Nitzschia sp. 1 FO is in the upper part of this zone, but only in trace abundance. Rocella gelida is also present but in very rare to trace amounts. Specimens of
Stellarima spp. are rare to frequent within this zone. Lastly, the zone is characterized by the absence of Cavitatus miocenicus and Coscinodiscus rhombicus, which appear in the overlying assemblage zone.
DZ-B (base: 97.71 and top: 96.22 mbsf)
Zone B is characterized by the presence of Cavitatus miocenicus and
Coscinodiscus rhombicus. Diatom abundance and preservation are similar to Zone A.
The abundance of Thalassiosria spinosa var. aspinosa and Stellarima spp. decreases notably from underlying Zone A to only trace amounts in Zone B. Raphidodiscus marylandicus is notably present in this zone and occurs with frequent abundance.
DZ-C (base: 96.22 and top: 91.1 mbsf)
Zone C is characterized by the prevalence of the several species of the genus
Thalassiosira. Bogorovia veniamini is present in only trace amounts, compared to Zones
A and B, and the LO of all Bogorovia species at Site 744 occurs within this zone. Diatom preservation varies from poor to moderate throughout this zone, and diatom abundance ranges from low to moderate. Thalassiosira spinosa var. aspinosa increases in
23 abundance and Thalassiosria spinosa is present up to the top of this zone. The FO of
Thalassiosria spumellariodes occurs in Zone C (Fig. 6) and increases in abundance through this zone. The FOs of both Thalassiosira praefraga and Thalassiosria fraga are at the top of this zone (Fig. 6). Specimens of Stellarima spp. reappear at 91.74 mbsf and continue to the top of this zone. Fragilariopsis maleinterpretaria FO is in this zone and slightly precedes the expansion in abundance of Cavitatus miocencus and the brief appearance of Bogorovia sp. 1 at the top of the zone. Several taxa alternate as the most dominate taxon through Zone C, with shifting abundance of Azpeitia oligocenicus and
Cavitatus species (Table 1a). There appears to be a dominance shift from Azpeitia oligocenicus at 93.61 mbsf to Cavitatus species, and followed by a shift from Cavitatus species back to Azpeitia oligocenicus dominance at 91.92 mbsf (Table 1a).
DZ-D (base: 91.1 and top: 85.10 mbsf)
Zone D is characterized by the abundant occurence of Thalassiosira praefraga and T. fraga throughout this zone. Coscinodiscus rhombicus occurs in higher abundances toward the top of this interval. Diatom preservation and abundance are moderate throughout this zone. A substantial drop in the abundance of Thalassiosira spinosa var. aspinosa and the absence of T. spinosa distinguishes this zone from the underlying Zone
C. The LO of T. spumellariodes occurs in the lowermost portion of this interval.
Raphidodiscus marylandicus reappears (Fig. 6) with a substantial expansion from its previous occurrence. Zone D also exhibits alternations in abundance of taxa, with a shift in the abundance of Azpeitia oligocenicus and Cavitatus species. The zone begins with a dominance of Azpeitia oligocenicus and changes to Cavitatus species dominance at 89.64
24 mbsf (Table 1a). Azpeitia oligocenicus becomes dominant again at 86.17 mbsf (Table
1a).
DZ-E (base: 85.10 and top: 80.30 mbsf)
Zone E is characterized by increased abundance of Thalassiosira spinosa var. aspinosa, a substantial drop in the abundance of T. praefraga, and higher numbers of
Nitzschia sp. 1, relative to Zone D. Coscinodiscus rhombicus is absent in this zone. The top of the zone is constrained by a core break between Holes 744A and 744B, and marks the point in our study where this report describes material from Hole 744B rather than
Hole 744A (Fig. 6). Diatom abundance tends to be moderate throughout, but in discrete intervals abundance ranges from low to high. Diatom preservation is moderate in most cases, but intervals of poor to good preservation also occur. Raphidodiscus marylandicus is absent from this zone (Fig. 6) and Thalassiosira sp. cf. T. bukyri (sensu Harwood and
Maruyama, 1992) increases from trace amounts in lower zones and expands in abundance to the top of this zone. Specimens of Stellarima spp. reappear within this zone at 82.34 mbsf, and continue up through the remainder of the zone. Zone E also exhibits an alternation between dominance of Azpeitia oligocenicus from the base of the zone (85.10 mbsf) up to 83.84 mbsf, with a shift to dominance of Cavitatus jouseanus and C. linearis between 83.84 and 82.34 mbsf, and a return to dominance of Azpeitia oligocenicus between 82.34 and 80.30 mbsf (Table 1a).
DZ-F (base: 78.2 and top: 75.03 mbsf)
25
Zone F is characterized by the LO of Nitzschia sp. 1, Thalassiosira fraga, T. praefraga, and the last abundant occurrence of T. spinosa var. aspinosa until it reappears upsection in Zone M (Fig. 6). Thalassionema nitzschioides increases in abundance during this zone (Table 1b). The FO of several taxa, including: Araniscus umbonatus,
Denticula norwegica, Eucampia antarctica, and Crucidenticula ikebei are noted within this zone. Diatom abundance from 78.2 to 77.34 mbsf is low (Fig. 3) and tends to be poor to moderately preserved, often showing signs of dissolution. Diatom abundance and preservation is moderate in the upper part of this zone from 77.34 to 75.0 mbsf.
DZ-G (base: 75.03 and top: 73.59 mbsf)
Zone G is characterized by an increase in abundance of Fragilaropsis maleinterpretaria, Crucidenticula ikebei, C. kanayae var. pacifica, Araniscus umbonatus relative to Zone F, as well as the FO of A. lewisianus, and LO of Thalassiosira sp. cf. T. bukyri. The zone is further characterized by the absence of Eucampia antarctica and
Denticula norwegica, which are present in underlying Zone F. The zone also shows a decline in diatom preservation and abundance, both of which are relatively moderate.
DZ-H (base: 73.59 and top: 71.83 mbsf)
Zone H is characterized by a decrease in abundance of the genus Crucidenticula relative to the underlying zone, and the FO of Corethron pennatum. Additionally,
Eucampia antarctica returns in this interval along with varieties E. antarctica var. 1 and
E. antarctica var. 2. The FO of Actinocyclus ingens is recorded at the top of this zone but only in trace amounts; it is not consistently present until Zone L (Fig. 6). Diatom
26 preservation in the zone is mostly moderate with diatom abundances being variable. For the majority of the zone, diatom abundances are moderate but there are several intervals of high diatom abundance and fewer intervals with low diatom abundances. A disconformity at 71.83 mbsf (73.23 mcd) is coincident with the top of this zone (Fig. 6).
DZ-I (base: 71.83 and top: 69.98 mbsf)
Zone I is characterized by a shift in abundance from the dominance of Corethron pennatum and Cavitatus miocenicus, to an increase in Crucidenticula species (Table 1b) relative to Zone H. Diatom abundance tends to be moderate with a few discrete intervals of high or low abundance. Preservation of diatoms is moderate throughout. Zone I is further characterized by the FO and LO of Denticulopsis sp. 1 (see plate IV; 33, 34, and
37), the FO of Nitzschia sp. 17 sensu Schrader (1976), consistent fragments of Rouxia spp., and the reappearance of abundant Raphidodiscus marylandicus (Fig. 6).
Additionally, there is an alternating trend between assemblages with abundant Azpeitia tabularis/endoi or Azpeitia salisburyana within the zone. A. salisburyana is dominant from the base of the zone up to 71.36 mbsf (Table 1b), where there is a shift to the dominance of A. tabularis/endoi to the top of the zone.
DZ-J (base: 69.98 and top: 67.59 mbsf)
Zone J is characterized by a drop in the abundance of Nitzschia sp. 17 sensu
Schrader (1976), a substantial decrease in the abundance of Fragilariopsis maleinterpretaria, and Denticulopsis sp.1 disappears corresponding to the FO of
Denticulopsis maccollumii (which is likely related to Denticulopsis sp. 1). Diatom
27 abundance is variable throughout the zone, with abundance ranging from low to high.
Diatom preservation can be poor to moderate at the lower portion of the zone, with it becoming consistently moderate towards the upper range of the zone. The abundance of
Crucidenticula kanayae var. pacifica drops substantially relative to the underlying zone, and Denticula norwegica reappears for a brief period during an interval with a high abundance of diatoms (Fig. 6).
DZ-K (base: 67.59 and top: 65.57 mbsf)
Zone K is characterized by the LO of Crucidenticula ikebei and C. kanayae var. pacifica within the lower part (66.69 mbsf) of the zone, as well as the presence of
Eucampia antarctica (65.98 mbsf) and Denticula norwegica (65.98 to 66.53 mbsf) in the upper interval of the zone (Fig. 6). Diatom preservation and abundance tends to be moderate through this zone.
DZ-L (base: 65.57 and top: 64.34 mbsf)
Zone L is characterized by the first consistent occurrence of Actinocyclus ingens, in addition to a peak in the abundance of Araniscus lewisianus (Fig. 6). The zone is further characterized by an increase in abundance in Nitzschia sp. 17 sensu Schrader
(1976), an increase in abundance of Thalassionema nitzschioides (Table 1b), the FO of
Nitzschia sp. 3, the FO of Denticulopsis lauta, and the FO and LO of Mediaria splendida.
Diatom abundance is high compared to other zones, and several ‘acmes’ of diatom abundance correspond to low GRAPE values (Fig. 3) within Zone L. Diatom preservation is moderate throughout.
28
DZ-M (base: 64.34 and top: 59.63 mbsf)
Zone M is characterized by the continued presence of Actinocyclus ingens, the increase in abundance of Denticulopsis lauta relative to the underlying interval, the LO of
Raphidodiscus marylandicus, the FO of Denticulopsis simonsenii and the absence of
Mediaria splendida. Diatom abundance is moderate to high in the lower portion of this zone (base to 62.18 mbsf), but remains low to the top of the zone. Diatom preservation is moderate through 62.18 mbsf, but then drops to poor to moderate in the upper part of the zone. Denticulopsis maccollumii and Nitzschia sp. 17 sensu Schrader (1976) continue up from the underlying zone and Araniscus umbonatus slightly increases in abundance (Fig.
6). Fragments of Rouxia spp. are rare to frequent in this interval and trace fragments of
Nitzschia sp. 3 are also seen. Additionally, Thalassiosira spinosa var. aspinosa reappears in low numbers and is assumed to be reworked. An up-section switch from the dominance of Azpeitia tabularis/endoi to Azpeitia salisburyana occurs at 62.37 mbsf
(Table 1b).
DZ-N (base: 59.63 and top: 56.04 mbsf)
Zone N is characterized by the high abundance of Actinocyclus ingens and A. ingens var. nodus (Fig. 6), and a substantial increase in several diatom abundances from the underlying zone. Diatom abundance fluctuates throughout the zone, with an ‘acme’ in total diatom abundance from the base of the zone up to 58.8 mbsf that corresponds to low GRAPE values (Fig. 3). Above this level, the zone exhibits moderate diatom abundance, although there are a few low abundance intervals in the zone. Preservation of
29 diatoms tends to be moderate throughout, although a few poor to moderate intervals occur in Zone N. This zone also contains the FO of Nitzschia denticuloides, and a noticeable increase in Nitzschia grossepunctata and Crucidenticula nicobarica (Table
1b). The FO of both Nitzschia grossepunctata and Crucidenticula nicobarica occur below this zone, but their occurrence is discontinuous until this zone.
6. Discussion
6a. Paleobiogeography
It has been well documented that Antarctica became thermally isolated during the late Paleogene with the opening of Tasmanian gateway and the Drake Passage (Exon et al., 2004; Stickley, 2004). In the geological record, the beginning of strong provincialism of diatom floras is seen at this time, which continues in the Southern Ocean today. In the modern Southern Ocean, diatom distribution is largely controlled by latitudinal temperature gradients, the spacing of which is set by polar conditions on the Antarctic continent (Anderson, 1999). Today, the Polar Frontal Zone (PFZ) acts as a biogeographic boundary to diatom floras, and is thought to have acted in a similar manner throughout the Neogene (Abelmann et al., 1990). During glacial intervals, temperature gradients strengthen across the Polar Front, while during interglacial intervals, latitudinal temperature gradients relax and isotherms become more diffuse.
In order to assign paleoecological affinities to the early and middle Miocene diatom floras recovered at ODP Site 744, the records of diatom occurrences in ODP drillcores spanning a range of high-latitudes were reviewed to infer cold-water and warm- water diatom floras, respectively. ODP Site 1165 is located adjacent to the Antarctic
30 margin of Prydz Bay (Fig. 6), and ODP Site 747 is located on the central Kerguelen
Plateau (Fig. 6). These diatom records, combined with the record of ODP Site 744 presented here, represent a latitudinal transect that spans ~ 5 degrees of latitude. The diatom record from Site 744 reflects an oscillating influence from both the cold-water and warm-water regions, in response to changing oceanographic conditions that are highly influenced by glacial/interglacial variability on Antarctica.
ODP Site 1165, located on the Antarctic continental rise near Prydz Bay at
64°22.78′S, 67°13.14′E (Shipboard Scientific Party, 2001), ~ 742 km to the south of Site
744, recovered a coeval early to middle Miocene record between 288.90 and 753.77 mbsf at ODP Holes 1165B and 1165C. The diatom flora from ODP Site 1165 (Whitehead and
Bohaty, 2003a; Bohaty unpublished notes) shares several floral elements with those at
ODP Site 744. However, one notable difference is the greater number and persistence of neritic taxa (e.g. Paralia sulcata) and benthic diatoms (e.g. Cocconeis species,
Grammatophora species, etc.) at Site 1165. Other taxa considered to represent early and middle Miocene Southern Ocean cold-water flora include: Eucampia antarctica, E. antarctica var. 1 and E. antarctica var. 2, Corethron pennatum, and Denticula norwegica. Thus, intervals with consistent occurrences of these diatoms at ODP Site 744 are inferred to represent times of cooler oceanographic conditions and the northward advancement of surface water isotherms.
ODP Site 747 at 54°49’S, 76°48’E, present water depth of 1697.7m, ~ 283 km to the north of Site 744 (Fig. 6), on the Central Kerguelen Plateau (Schlich, Wise, et al.,
1989) recovered a coeval diatom record, as reported by Harwood and Maruyama (1992).
Although preservation is poorer overall at Site 747, several diatoms are in common with
31 the floras from Site 744. These species are used here to infer warm-water influence at
Site 744. Diatoms present at Site 747, and absent or rare at ODP Site 1165 include:
Raphidodiscus marylandicus, Azpeitia tabularis, Thalassionema nitzschioides, and various Crucidenticula species, which are inferred to reflect warm waters. Intervals with consistent occurrence of these diatoms at ODP Site 744 are inferred to represent times of warmer oceanographic conditions and the southward advance of surface-water isotherms.
Guided by the paleontological associations noted above, several assemblage shifts can be identified within the Site 744 section that are suggestive of changes in surface- water conditions, and possibly broad climatic cycles in the Southern Ocean and Antarctic region. Narrow sample spacing (~10 cm) allowed the documentation of many abrupt (≈10 kyr) changes in diatom abundance and assemblage composition (Fig. 3; Fig. 6) that may have missed at more traditional sample spacing. Some of these changes are reflected in the designation of DZs (DZ-A to DZ-N, Fig. 6) and are suggestive of climatic and oceanographic changes at Site 744.
Floral shifts are interpreted as reflecting the strengthening or weakening of oceanic isotherms near Site 744, due to specific diatom assemblage associations with warm or cooler water conditions. Alternatively, diatom assemblage changes may be reflective of changes in thermohaline circulation (THC) and the addition of heat and nutrients through North Atlantic Deepwater influx, as the strength of THC and NADW influx into the Southern Ocean are thought to have affected Southern Ocean sea-surface temperatures during the Pliocene (Whitehead and Bohaty, 2003b).
32
These ‘cold’ and ‘warm’ water diatom associations allow inferences to be made about environmental preferences of select diatom taxa, and enhance the ability to identify paleoclimatic and paleoceanographic changes at Site 744 and other Southern Ocean sites.
6b. Cooler Assemblages
In the Southern Ocean today, diatoms show distinct provincialism with regard to temperature and nutrient requirements. The same can be assumed for the Miocene, as substantial floral changes are noted throughout the study interval at Site 744. Many extant ‘cold-water’ flora, e.g. Corethtron pennatum and Eucampia antarctica are present at Site 744 and other higher latitude coeval sites, e.g. ODP Site 1165. Additionally,
Denticula norwegica, Porosira spp., and Synedropsis spp., all co-occur within similar assemblages and give credence to a cold-water affiliation. These taxa are rare or absent to the north at Site 747 (Harwood and Maruyama, 1992) toward the warmer waters of the
PFZ. Four of the DZs defined above contain diatom assemblages suggestive of cooler water conditions at Site 744; these are discussed below in stratigraphic order from oldest to youngest.
The majority of taxa within DZ-F are considered to be indicative of cooler water conditions, e.g. Denticula norwegica and Eucampia antarctica. However, Crucidenticula species are also present, and Yanagisawa and Akiba (1990) point out that the genus
Crucidenticula is generally considered to be a warm-water taxon. Crucidenticula specimens are present at such low abundances during Zone F that the overall assemblage may be indicative of cooler water conditions at Site 744. Occurrences of Crucidenticula
33 spp. and Thalassionema nitzschioides both occur in the upper portion of Zone F and represent the initiation of a warmer-water assemblage seen within the overlying Zone G.
In DZ-H, Corethron pennatum is present and is generally considered to be suggestive of cooler water conditions (Armand et al., 2005). The presence of Eucampia antarctica (and several varieties) and the low abundance of Actinocyclus ingens, further suggests that Zone H is a time of cool water conditions at Site 744.
Within DZ-J, the abundance of Crucidenticula kanayae var. pacifica drops significantly and Denticula norwegica reappears within a thin interval associated with high diatom abundance (Fig. 3). This may suggest that the upper range of Zone J was representative of a period of cooler conditions at Site 744, but not as strongly in other zones suggestive of cooler conditions.
Zone DZ-K reflects a decrease in the numbers of Crucidenticula ikebei and C. kanayae var. pacifica with the reappearance of Eucampia antarctica and Denticula norwegica. This transition further supports an interpretation of continued cooling at Site
744 from the top of DZ-J up through DZ-K.
6c. Warmer Assemblages
Provincialism is seen in several extant diatoms in the Southern Ocean today with warmer water affinities, such as Azpeitia tabularis and Thalassionema nitzschioides.
Crosta et al. (2005) showed that these diatoms occur at their greatest abundances in warmer subantarctic/subtropical waters and are rare or absent at higher latitude sites. In
Miocene sediments of the Southern Ocean, other diatoms that co-occur with these extant warm-water taxa include: Crucidenticula spp., Actinocyclus ingens, Actinocyclus ingens
34 var. nodus, and possibly, Nitzschia sp. 3 and Mediaria splendida. From this, the abundance of these taxa is inferred to be indicative of warmer conditions during the
Miocene at Site 744.
Several of the DZs described above contain diatom assemblages that are suggestive of warmer conditions at Site 744. An increase in the abundance of
Crucidenticula species and Thalassionema nitzschioides within DZ-G, coincides with a general absence of cooler-water taxa recorded in the underlying DZ-F, e.g. Eucampia antarctica and Denticula norwegica. Today, the presence of Thalassionema nitzschioides is interpreted as an indicator of warmer conditions in the Southern Ocean (Crosta et al.,
2005; Romero et al., 2005). The relative increase in these warm-water taxa, paired with the absence of cooler water taxa, strongly suggest that Zone G is representative of a period of warming at Site 744.
The first abundant occurrence of Actinocyclus ingens occurs in DZ-L (Fig. 6) along with an increase in the abundance of Thalassionema nitzschioides (Table 1b). This is interpreted to represent a time of change in oceanographic conditions at Site 744 because of several intervals of high diatom abundance where a variety of distinct taxa occurs such as, Nitzschia sp. 3, and the only occurrence of Mediaria splendida at Site 744
(Fig. 6). Through association with other taxa, Nitzschia sp. 3 and Mediaria splendida are inferred to reflect warm water conditions, or alternatively, could be a response to increased surface-water nutrient levels due to the upwelling of nutrient enriched waters.
Zone N is interpreted to represent one of the warmest intervals in our study at Site
744. This assemblage is dominated by an abundance of Actinocyclus ingens and A. ingens
35 var. nodus, along with abrupt increases in the abundance of Crucidenticula nicobarica, which suggests warmer conditions at Site 744.
6d. Diatom Abundance and GRAPE Density
Several intervals are noted where diatom abundance and GRAPE density changes substantially over short stratigraphic intervals (Fig. 3). Figure 3 shows the variability of diatom abundance and GRAPE density data at Site 744 with respect to a composite depth for Holes 744A and 744B. Diatom abundance was low to moderate from the base of Core
744A-11H (at 100.25 mcd) up-section to 84 mcd (Fig. 3). At 84 mcd, diatom abundance substantially increases and fluctuates between moderate to high until the core break at
80.8 mcd (Fig. 3). GRAPE density values are lower during this interval (84 to 80.8 mcd).
From 79.6 mcd up to the disconformity at 73.23 mcd, diatom abundance was low to moderate (Fig. 3). Directly up-section of the disconformity (at 73.23 mcd), diatom abundance is high and GRAPE density is low until diatom abundance drops substantially around 71 mcd (Fig. 3). At 71 mcd, diatom abundance and GRAPE density are highly variable and continue to fluctuate widely until ~ 58.5 mcd (Fig. 3). This interval (71 to
58.5 mcd; Fig. 3) shows the highest variability in diatom abundance and GRAPE density during the study interval at Site 744. The sudden, significant changes in diatom accumulation during this interval are perhaps related to changing temperature and/or surface-water nutrient conditions over the site. From 58.5 mcd up to the top of the study at 56.04 mcd, diatom abundance is moderate with the exception of one low diatom abundance at ~ 57.6 mcd, where GRAPE density values are also elevated (Fig. 3).
Core intervals with high diatom abundance are likely related to periods of
36 increased nutrients in the surface-waters that may or may not be related to temperature changes at Site 744. However, during intervals with low diatom abundance, preservation often suggests that these were warm intervals in the core, as diatoms tend to show signs of dissolution and these intervals tend to show an increase in the abundance of calcareous microfossils (Shipboard Scientific results, 1989).
6e. Global Events
This study documents a history of Southern Ocean paleoenvironmental changes from ~20.25 to 13.75 Ma, including the interval known in deep-sea sections as the Mid-
Miocene Climatic Optimum (MMCO; ~17 to 15 Ma; Savin, 1977; Majewski, 2002) and the Mi-2 event (~16.3 Ma; Miller et al., 1991). The MMCO was a time of global climate warming and the warmest period of the entire Neogene (Savin, 1977; Flower and
Kennett, 1993; Majewski, 2002). Majewski (2002) noted that the MMCO is first apparent at Site 744 in the foraminferal record at 80 mbsf (~19 Ma) by the progressive increase in foraminiferal taxa (Tenuitellinata pseudoedita to Globorotalia zealandica) associated with warming surface-water conditions. At 75 mbsf (~17.5 Ma) the warmest surface-water conditions were reached at Site 744 (Majewski, 2001) and continued until c. 13.5 Ma (~ 55 mbsf), when surface-water temperatures may have been as high as 10°C
(Majewksi, 2002). However, previous foraminiferal isotope data are low-resolution, and diatom abundance and warm/cold affinity diatom assemblages indicate more complex climatic conditions at Site 744 during this interval. Diatom abundance is highly variable from 69.7 up to 58.6 mbsf (~71 to 58.6 mcd; Fig. 3), and multiple cool-water DZs, e.g.
DZ-H, DZ-J, and DZ-K, are present during this interval (Fig. 6). These diatom results
37 suggest that climatic conditions at Site 744 were much more dynamic during the MMCO, and surface-water conditions were much more variable than previously reported. Higher resolution foraminiferal isotope records are needed to elucidate the true state of climatic and surface-water conditions at Site 744 during the MMCO.
Miller et al. (1991) defined the Mi-2 event by increased δ18O values of benthic foraminifera, and it is thought to represent a transient glacial event. It is represented at
Site 744 by the presence of a disconformity that spans ~ 800 kyr at 73.23 mcd. An associated array of cold-water diatoms in DZ-H, are present above and below the disconformity at 73.23 mcd.
6f. Application of CONOP Model Ages at Site 744
CONOP model ages for the Southern Ocean diatom biostratigraphic synthesis
(Cody et al., 2008) are generally in good agreement with the age calibration of specific diatom events at Site 744 (Table 3). The Total Range Model (TRM) and Hybrid Range
Model (HRM) ages (Cody et al., 2008; Cody unpublished data) proved to be of the greatest utility in constructing the age model and in constraining correlations of magnetic polarity reversals to the global time scale (ATNTS, Lourens et al., 2004). For many taxa, the Average Range Model (ARM) ages were too broad to be applied reliably to our age model (Table 3). However, in some instances the ARM proved to be the most useful in constructing the age model, e.g. the FO of Actinocyclus ingens (Table 2). In the case of
A. ingens, data incorporated into the TRM and HRM outputs included erroneous reports of A. ingens age-ranges, resulting from the misidentification of a Cestodiscus species as
Actinocyclus ingens. Below ~ 95 mcd, CONOP model ages tended to be less robust due
38 to the scarcity of Southern Ocean diatom records available from this period, and the early part of the early Miocene was outside the targeted age range of the data compiled for the original CONOP dataset (Cody et al., 2008; Cody unpublished data). Additionally, some of the diatom events reported in CONOP and Table 2 of this study are likely environmentally dependent, e.g. Eucampia antarctica and Mediaria splendida, so the timing of their occurrences will vary from site to site.
7. Conclusions
This study greatly improves the early and middle Miocene diatom record for a poorly known interval (~20.25 to 13.75 Ma) of the Southern Ocean’s history. The diatom record at ODP Site 744 shows variation in diatom assemblages through time that are indicative of temperature changes (cooling/warming) and nutrient flux in Southern Ocean surface-waters during the early to middle Miocene. A new robust age model is presented for ODP Site 744 that will serve as a reference point for subsequent biostratigraphic studies and offers new biostratigraphic events that are linked to chronostratigraphic tiepoints for incorporation into the CONOP database.
Of the CONOP model ages, the TRM and HRM (Cody et al., 2008; Cody unpublished data) ages were the most applicable for specific diatom events at Site 744
(Table 3). In rare cases, the ARM ages were the most applicable (Table 3), but this is likely a result of taxonomic misidentification in previous studies affecting TRM and
HRM outputs. Because the current CONOP model synthesis for Southern Ocean sites only extends back to 18 Ma (Cody et al., 2008), diatom datums below 95 mcd at Site 744 are poorly constrained by the current CONOP model. This study provides robust age
39 interpretations and calibration for diatom events between ~20.25 and 13.75 Ma, which will provide new information for extending the CONOP model results further back in time.
Diatom abundance varies greatly over the ~ 6.5 myr study window at Site 744
(Fig. 3). Core 744B-8H showed the greatest variability in diatom abundance with some of the lowest and highest diatom abundance throughout our study range at Site 744 (Fig. 3).
High and low diatom abundance is reflective of changes in diatom productivity, and thus paleoceanography, from ~20.25 Ma to 13.75 Ma at Site 744. Diatom abundance and floras present are representative of varying surface-water conditions at the site, resulting from variations in latitudinal temperature gradient are associated with changing conditions on the Antarctic continent. The diatom assemblages at Site 744 show wide changes in environmental variability, in some cases, corresponding to known global events, e.g. Mi-2 event and cold-water assemblages, but present a higher frequency of environmental variation than previously reported for the MMCO and other deep-sea proxies. Diatom abundance events may parallel previous isotope and GRAPE results
(Fig. 3), but more quantitative work would be required for the diatom record, and increased sample size of carbonate microfossils is needed, to accurately compare these records.
40
8. Diatom Flora List
The following is a list of diatom taxa or taxonomic groups observed within ODP Holes 744A and 744B. Detailed synonomies for the taxa seen in this study are not presented here. The reader should refer to the works of Yanagisawa and Akiba (1990), Harwood and Maruyama (1992), Ramsay and Baldauf (1999), Censarek and Gersonde (2002), Barron (2005), and Olney et al. (in prep.) for detailed species references.
Diatom Taxa
Actinocyclus curvatalus Janisch 1878 in Schmidt et al. (1874-1959) (Plate V, Fig. 6)
Comments: Valve has a fine mesh with labiate processes and associated hyaline zones on the margin; areolae rows are slightly curved.
Actinocyclus ingens Rattray, 1890
Actinocylcus ingens var. nodus Baldauf 1980, in Baldauf and Barron (Plate V, Fig. 1)
Actinocyclus octonarius var. tenella (Brébisson, 1854) Hendey (1954) (Plate V, Fig. 3)
Actinoptychus senarius Ehrenberg, 1843
Araniscus lewisianus (Greville) Komura, 1998 (Plate I, Figs. 1-3, 7)
Araniscus umbonatus Komura, 1998 (Plate I, Fig. 12)
Asteromphalus oligocenicus Schrader and Fenner, 1976
Asteromphalus sp. A (Plate I, Fig. 6) sensu Scherer et al., 2001
Asteromphalus symmetricus Schrader and Fenner, 1976
Azpeitia oligocenica Jousé (1974) Sims, in Fryxell et al., 1986 (Plate III, Figs. 2, 4)
Azpeitia sp. 2 (Plate III, Fig. 13)
Comment: Azpeitia specimens with a substantially hyaline valve face, i.e. more silica between pores, were put into this category.
Azpeitia salisburyana (Lohman) Sims in Sims et al., 1989 (Plate III, Fig. 7)
Azpeitia tabularis/endoi (Grunow, 1884) Fryxell and Sims, in Fryxel et al., 1986 (Plate III, Figs. 3, 12)
41
Bogorovia veniamini Jousé, 1974 (Plate VI, Fig. 16)
Comments: sensu Barron (2005) Plate 9, Figs. 2, 3
Bogorovia sp. 1 (Plate VI, Figs. 13, 15)
Comments: Bogorovia sp. 1 appears for a very brief stratigraphic interval at Site 744. Specimens were smaller than B. veniamini and tended to have the two hyaline ridges in the center of the valve face as seen in Plate VI, Fig. 13.
Cavitatus sp. cf. C. jouseanus (Plate VI, Fig. 8)
Comments: Some Cavitatus specimens shared morphological features of both C. linearis and C. jouseanus, as illustrated in Plate VI, Fig. 8.
Cavitatus jouseanus (Sheshukova) Williams, 1989 (Plate VI, Fig. 7)
Comment: Cavitatus specimens whose valve is distinctly “cuneate” were recorded here.
Cavitatus linearis s.l. (Sheshukova-Poretzkaya) Akiba and Yanagisawa in Akiba et al., 1993 (Plate VI, Fig. 5)
Comments: Cavitatus linearis s.s., as well as specimens that were not distinctively “cuneate” and had broadly rounded valve margin ends in the fashion of C. linearis, were recorded here.
Cavitatus miocenicus (Schrader) Akiba and Yanagisawa in Akiba et al., 1993, (Plate VI, Fig. 17)
Comment: Specimens included here were only in the strict sense, that had square or “coke bottle” ends, versus the rounded and broadly rounded ends of C. jouseanus or C. linearis respectively; specimens were also elongate.
Cavitatus rectus Akiba and Hiramatsu in Akiba, Hiramatsu and Yanagisawa, 1993 (Plate VI, Fig. 11)
Cavitatus sp. 1 (Plate VI, Fig. 19)
Comment: Specimens included in this category appear to be a more heavily silicified, shorter, Cavitatus species that most resembles C. miocenicus.
Cestodiscus pulchellus Greville, 1866 (Plate V, Fig. 2)
Cestodiscus sp. 1 (Plate V, Fig. 5)
Comments: Cestodiscus specimens with a large hyaline central area with pores that
42 decrease in size towards the valve margin were placed here. Specimens were often highly fragmented with mainly the central hyaline portion of the valve preserved.
Cestodiscus sp. 2 (Plate V, Fig. 10)
Comments: Cestodiscus specimens that had three long intersecting rays and absence of a central ring of pores as seen in C. puchellus were tracked here.
Cestodiscus sp. 3 (Plate V, Fig. 4)
Comments: Cestodiscus specimens with a flat valve margin, a distinct central pore that has a hyaline ring, and radiating lines of pores that are slightly curved giving the valve face a slightly “torqued” appearance were placed here.
Corethron pennatum (Grunow) Ostenfeld in Van Heurck, 1909 (Plate I, Fig. 13)
Comments: Synonym Corethron criophilum
Coscinodiscus marginatus Ehrenberg emend. Sancetta, 1987
Coscinodiscus marginatus var. 1 (Plate I, Fig. 10)
Comments: Coscinodiscus maginatus specimens that had a distinctly ‘domed’ valve were assigned here.
Coscinodiscus lewisianus var. rhomboides Barron, 1985
Coscinodiscus rhombicus Castracane, 1886 (Plate I, Figs. 4, 5, 12)
Coscinodiscus sp. 1 (Plate I, Fig. 10)
Comments: Coscinodiscus specimens placed here have four to six pores in the central area that are noticeable larger than the surrounding pores and those on the valve margin.
Crucidenticula kanayae var. pacifica Yanagisawa and Akiba, 1990 (Plate IV, Figs. 3, 4, 8, 13a, 13b, 14a, 14b, 15a, 15b)
Comments: Many of the specimens originally assigned to C. nicobarica by Baldauf and Barron, 1991 are represented here. Specimens of C. kanayae var. pacifica are more heavily silicified and have coarser punctation than C. nicobarica.
Crucidenticula ikebei Akiba and Yanagisawa, 1986 (Plate IV, Fig. 11)
Comments: C. ikebei specimens have a distinctively “lanceolate” valve, with finer punctation (Plate IV, Fig. 11 but not Plate IV, Fig. 13a&b) than that of C. kanayae var. pacifica.
43
Crucidenticula nicobarica (Grunow) Akiba and Yanagisawa, 1986 (Plate IV, Figs. 1, 2)
Comments: Not in the sense of Barron and Baldauf, 1991; Crucidenticula specimens that were less-heavily silicified and smaller punctation than C. kanayae var. pacifica were placed here.
Crucidenticula sawamurae Yanagisawa and Akiba, 1990 (Plate IV, Fig. 9)
Comments: Crucidenticula sawamurae appears briefly at Site 744 and differs from Crucidenticula kanayae var. pacifica by its slightly finer punctation on the valve face.
Denticula norwegica Schrader, in Schrader and Fenner, 1976 (Plate IV, Fig. 10)
Denticulopsis lauta (Bailey) Simonsen, 1979 (Plate IV, Figs. 6, 7)
Denticulopsis maccollumii Simonsen, 1979 (Plate IV, Figs. 32, 35)
Denticulopsis simonsenii Yanagisawa and Akiba, 1990 (Plate IV, Figs. 5a, 5b)
Denticulopsis sp. 1 (Plate IV, Figs. 33, 34, 37)
Comments: Specimens of Denticulopsis that have central hyaline patch with finer space septa were placed here; specimens in this category may be related to D. maccollumii but can be distinguished based on the finer spacing of their septa and a central hyaline patch that tends to be narrower than that of D. maccollumii.
Eucampia antarctica (Castracane, 1886) Mangin, 1914
Eucampia antarctica var. 1 (Plate III, Figs. 6, 8)
Comments: This variety of E. antarctica has not been formerly described and is characterized by its “kidney-shaped” appearance; sensu Eucampia antarctica var. A in Whitehead and Bohaty, 2003a.
Eucampia antarctica var. 2 (Plate III, Fig. 9)
Comments: This variety of E. antarctica has not been formally described and is characterized by rotation (up to 90°) on the valvar plane; sensu Eucampia antarctica var. C in Whitehead and Bohaty, 2003a.
Fragilariopsis maleinterpretaria (Schrader, 1976) Censarek and Gersonde, 2002 (Plate IV, Figs. 19-21)
Comments: Specimens formerly assigned to the species Nitzschia maleinterpretaria Schrader, (see Schrader, 1976, p. 634, pl. 2, figs. 9, 11-19, 21, 24) are included here.
44
Genus et sp. indent. # 1 (Plate I, Fig. 8)
Comments: An unidentified centric diatom that is unadorned in appearance and appears to be a solid silica circle with a rim.
Grammatophora spp. (Plate II, Figs. 16, 18)
Hemiaulus incisus Hajós, 1976 (Plate III, Figs. 1, 10)
Comments: Hemiaulus specimens that resembled H. incisus but with inflated elevations were also recorded here with more “typical” forms of this taxon.
Hemiaulus polycystinorum Grunow, 1884 (Plate III, Fig. 11)
Hemiaulus spp. (Plate III, Fig. 5)
Comments: All Hemiaulus specimens that could not be identified down to species level were tabulated as a combined group.
Lisitzinia ornata Jousé, 1978 (Plate V, Fig. 9)
Macrora barbadensis (Deflandre) Hanna, 1932 (Plate V, Fig. 7)
Comments: Specimens previously known as Pseudorocella barbadensis are included here.
Mediaria splendida Sheshukova, 1962 (Plate VI, Fig. 6)
Misc. benthics (Cocconeis sp. etc.)
Comments: Miscellaneous benthic diatoms seen in discrete intervals at Site 744, e.g. Cocconeis sp. were tabulated as a combined group.
Nitzschia sp. 1 (Plate IV, Figs. 18, 26, 38)
Comments: Specimens of Nitzschia sp. 1 have a very distinct raphe on one side of the valve face and finer punctation than Nitzschia sp. 2 or Fragilariopsis maleinterpretaria.
Nitzschia sp. 2 cf. N. maleinterpretaria (Plate IV, Figs. 27-30)
Comments: Specimens were separated from Fragilariopsis maleinterpretaria (Nitzschia maleinterpretaria sensu Schrader 1976), and have finer punctation with a distinct raphe on one side of the valve face.
Nitzschia sp. 17 sensu Schrader, 1976
45
Nitzschia sp. 3 (Plate IV, Figs. 12, 16)
Comments: This variety is the same in appearance as Nitzschia sp. 17 (sensu Schrader, 1976) except for its unusually large size; specimens range up to ~ 60 to 100 µm in length.
Nitzschia denticuloides Schrader, 1976 (Plate IV, Figs. 24, 25)
Nitzschia sp. cf. N. denticuloides? (Plate IV, Fig. 23)
Comment: Specimens are rare and stratigraphically older than known range of N. denticuloides. Septa have a “curved” or “tilde-like” appearance across the valve face, in contrast with the straight septa across the valve face seen in typical N. denticuloides.
Nitzschia grossepunctata Schrader, 1976 (Plate IV, Figs. 22, 31, 36)
Paralia sulcata (Ehrenberg) Cleve 1873 (Plate I, Fig. 9)
Paralia crenulata (Grunow) Gleser in Gleser et al., 1992
Pleurosigma sp. (P. angulatum?)
Porosira spp. (Plate V, Fig. 8)
Comments: Specimens of Porosira spp. occurred in discrete intervals and were included here, resembling both P. pseudodenticulata and P. glacialis.
Proboscia spp.
Pyxilla spp.
Raphidodiscus marylandicus Christian, 1887 (Plate V, Fig. 11)
Rhabdonema spp. girdle bands (Plate II, Figs. 4, 7, 11, 13, 14)
Rocella gelida (Mann) Bukry, 1978 (Plate I, Fig. 11)
Rocella vigilans Fenner, 1984
Rosiella paleacea (Grunow) Desikachary and Maheshwari, 1958
Rouxia spp. (Plate VI, Figs. 3, 4, 14)
Comments: Fragments of Rouxia species seen throughout the study interval are included here. In very rare cases whole specimens were preserved.
46
Stellarima spp.
Comments: Specimens of Stellarima species occurred in discrete intervals and were included here.
Synedropsis sp. 1 (Plate VI, Figs. 1, 2, 9, 10, 12)
Comments: Specimens of an unidentified Synedropsis sp. were included here. The valve is similar in morphology to Cavitatus miocenicus and Cavitatus sp. 1 (this study), but is less heavily silicified, and strut-like marks are noticeably present on margins of the valve.
Thalassionema nitzschioides (Grunow) Mereschkowsky 1902
Thalassionema nitzschioides var. parva Heiden, in Heiden and Kolbe, 1928 (Plate VI, Fig. 20)
Thalassiosira sp. cf. T. bukyri Barron, 1983 (Plate II, Fig. 3)
Comments: sensu Harwood and Maruyama, 1992
Thalassiosira fraga Schrader in Schrader and Fenner, 1976 (Plate II, Figs. 10a, 10b)
Thalassiosira nansenii Scherer in Scherer and Koç, 1996 (Plate II, Fig. 17)
Comments: Specimens of T. nansenii were present in isolated intervals of Site 744 and were often highly fragmented.
Thalassiosira praefraga Gladenkov and Barron, 1995 (Plate II, Figs. 8, 9)
Comments: T. praefraga specimens were differentiated from T. fraga, by the lack of a well-developed strutted margin and the non-linear pattern of areolae on the valve face.
Thalassiosira spinosa Schrader, 1976 (Plate II, Figs. 5, 6)
Thalassiosira spinosa var. aspinosa, (Plate II, Figs. 12a, 12b, 15a, 15b)
Thalassiosira spumellaroides Schrader, 1976 (Plate II, Figs. 1, 2)
Trinacria spp.
Comments: Trinacria excavata and undifferentiated Trinacria spp. were included here.
47
9. Plate Captions
Plate I (scale bar = 20 µm)
Figs. 1-3, 7, Araniscus lewisianus. Fig. 1, sample 744B-9H-5, 50-51 cm; Fig. 2, sample 744B-9H-1, 108-109 cm; Fig. 3, sample 744A-11H-4, 57-58 cm; Fig. 7, sample 744B- 9H-5, 97-98 cm. Figs. 4, 5, Coscinodiscus rhombicus. Fig. 4, sample 744A-10H-6, 35- 36 cm; Fig. 5, sample 744B-10H-4, 147-148 cm. Fig. 6, Asteromphalus sp. A sensu Scherer et al., 2000, sample 744B-9H-3, 122-123 cm. Fig. 8, Genus et sp. indent. #1, sample 744B-8H-2, sample 146-147 cm. Fig. 9, Paralia sulcata, sample 744A-11H-3, 116-117 cm. Fig. 10, Coscinodiscus sp. 1, sample 744B-9H-1, 98-99 cm. Fig. 11, Rocella gelida, sample 11H-5, 24-25 cm. Fig. 12, Araniscus umbonatus, sample 744B- 9H-5, 58-59 cm. Fig. 13, Corethron pennatum, sample 744B-9H-3, 55-56 cm.
48
1.
3.
2.
7.
6.
4. 5. 8. 9.
12. 13.
Plate I 10. 11. 49
Plate II (scale bar = 20 µm)
Figs. 1, 2, Thalassiosira spumellaroides. Fig. 1, sample 744A-11H-3, 116-117 cm, focal plane is on the valve margin. Fig. 2, sample 744A-11H-3, 72-73 cm, focal height results in light concentrating at valve center. Fig. 3, Thalassiosira sp. cf. T. bukyri. Fig. 3, sample 744A-10H-5, 95-96 cm. Figs. 4, 7, 11, 13, 14, Rhabdonema spp. girdle bands. Fig. 4, sample 744B-8H-2, 118-119 cm; Figs. 7, 11, sample 744B-9H-1, 35-36 cm; Fig.13, sample 744B-7H-6, 69-70 cm; Fig. 14, sample 744B-8H-6, 136-137 cm. Figs. 16, 18, Grammatophora sp. Fig. 16, sample 744B-7H-6, 130-131 cm; Fig. 18, in girdle view, sample 744B-8H-2, 3-4 cm. Figs. 5, 6, Thalassiosira spinosa. Fig. 5, sample 744A-11H-4, 118-119 cm; Fig. 6, sample 744A-11H-5, 24-25 cm. Figs. 8, 9, Thalassiosira praefraga. Fig. 8, sample 744A-11H-2, 19-20 cm; Fig. 9, sample 744A- 11H-1, 15-16 cm. Figs. 10a, 10b, Thalassiosira fraga, same specimen at two focal planes, sample 744A-10H-4, 86-87 cm. Figs. 12a, 12b, 15a, 15b, Thalassiosira spinosa var. aspinosa. Figs. 12a, 12b, same specimen at two focal planes, sample 744A-11H-6, 33-34 cm; Figs. 15a, 15b, same specimen at two focal planes, sample 744A-11H-6, 27-28 cm. Fig. 17, Thalassiosira nansenii, sample 744A-11H-1, 88-89 cm.
50
2.
1.
6.
4. 5.
8. 9.
3. 7. 10a.
11.
10b. 12a. 12b.
14. 13. 15a. 15b. 16. 17. Plate II 18. 51
Plate III (scale bar = 20 µm)
Fig. 1, Hemiaulus incisus, sample 744B-7H-6, 19-20 cm. Figs. 2, 4, Azpeitia oligocenicus. Fig. 2, sample 744B-9H-3, 147-148 cm; Fig. 4, sample 744A-11H-4, 2-3 cm. Figs. 3, 12, Azpeitia endoi/tabularis. Fig. 3, sample 744B-8H-5, 103-104 cm; Fig. 12, sample 744B-8H-5, 87-88 cm. Fig. 5, Hemiaulus sp. A sensu Harwood and Maruyama, 1992, sample 744B-8H-5, 119-120 cm. Figs. 6, 7, Eucampia antarctica var. 1. Fig. 6, sample 744B-8H-4, 3-4 cm; Fig. 8, sample 744B-8H-4, 127-128 cm. Fig. 7, Azpeitia salisburyana, sample 744A-11H-6, 9-10 cm. Fig. 9, Eucampia antarctica var. 2, sample 744B-9H-1, 133-134 cm. Fig. 10, Hemiaulus sp. cf. Hemiaulus incisus, sample 744A-11H-5, 118-119 cm. Fig. 11, Hemiaulus polycystinorum, sample 744A-10H-5, 36- 37 cm. Fig. 13. Azpeitia sp. 2, sample 744A-11H-5, 101-102 cm.
52
2.
3.
1.
4.
6. 7.
12. 8. 5.
11.
10. 9. Plate III 13. 53
Plate IV (All scale bars = 20 µm)
Figs. 1, 2, Crucidenticula nicobarica, sample 744B-7H-6, 37-38 cm. Figs. 3, 4, 8, 13a, 13b, 14a, 14b, 15a, 15b, Crucidenticula kanayae var. pacifica. Fig. 3, sample 744B-9H- 2, 13-14 cm; Fig. 4, sample 744B-9H-1, 98-99 cm; Fig. 8, 744B-9H-2, 133-134 cm; Figs 13a, 13b, same sample at two focal planes, sample 744B-9H-3, 136-137 cm; Figs. 14a, 14b, same sample at two focal planes, sample 744B-9H-2, 86-87 cm; Figs. 15a, 15b, same sample at two focal planes, sample 744B-9H-4, 110-111 cm. Figs. 5a, 5b, Denticulopsis simonsenii, same sample at two focal planes, sample 744B-7H-6, 7-8 cm. Figs. 6, 7, Denticulopsis lauta. Fig. 6, sample 744B-8H-4, 72-73 cm; Fig. 7, sample 744B-8H-3, 108-109 cm. Fig. 9, Crucidenticula sawamurae, sample 744B-9H-3, 147- 148 cm. Fig. 10, Denticula norwegica, sample 744B-9H-5, 134-135 cm. Fig. 11, Crucidenticula ikebei, sample 744B-9H-4, 9-10 cm. Figs. 12, 13, Nitzschia sp. 3. Fig. 12, sample 744B-8H-4, 100-101 cm; Fig. 13, sample 744B-8H-4, 72-73 cm. Fig. 17, Nitzschia sp. 17 sensu Schrader, 1976, sample 744B-9H-2, 19-20 cm. Figs. 18, 26, 38, Nitzschia sp. 1. Fig. 18, sample 744A-10H-1, 125-126 cm; Fig. 26, sample 744A-11H-2, 54-55 cm; Fig. 38, sample 744A-11H-5, 81-82 cm. Figs. 19-21, Fragilariopsis maleinterpretaria, sample 744B-9H-2, 108-109 cm. Figs. 22, 31, 36, Nitzschia grossepunctata. Fig. 22, sample 744B-7H-5, 29-30 cm; Fig. 31, sample 744B-8H, 119- 120 cm; Fig. 36, sample 744B-7H-5, 86-87 cm. Fig. 23, Nitzschia sp. cf. N. denticuloides, sample 744B-9H-1, 108-109 cm. Figs. 24, 25, Nitzschia denticuloides, sample 7H-5, 68- 69 cm. Figs. 27-30, Nitzschia sp. 2. Fig. 27, sample 744B-9H-1, 149-150 cm; Fig. 28, sample 744B-9H-2, 103-104 cm; Fig. 29, sample 744B-9H-1, 108-109 cm; Fig. 30, sample 744B-9H-1, 4-5 cm; Figs. 32, 35, Denticulopsis maccollumii. Fig. 32, sample 744B-8H-5, 119-120 cm; Fig. 35, sample 744B-9H-1, 27-28 cm. Figs. 33, 34, 37, Denticulopsis sp. 1. Figs. 33, 37, sample 744B-9H-2, 19-20 cm; Fig. 34, sample 744B- 9H-1, 108-109 cm.
54
1. 2.
3. 4. 6. 5a. 5b. 8. 7. 9.
10.
Plate IV 11.
14a. 14b. 15a. 15b.
13a. 13b.
22. 23.
19. 20. 21. 25. 24.
18. 17. 26. 27. 28. 29. 30.
38. 12. 16. 31. 32. 33. 34. 35. 36. 37. 55
Plate V (scale = 20 µm)
Fig. 1, Actinocyclus ingens var. nodus, sample 744B-8H-5, 5-7 cm. Fig. 2, Cestodiscus puchellus, sample 744A-10H-4, 40-41 cm. Fig. 3, Actinocyclus octonarius var. tenella, sample 744B-8H-6, 149-150 cm. Fig. 4, Cestodiscus sp. 3, sample 744B-9H-1, 67-68 cm. Fig. 5, Cestodiscus sp. 1, sample 744B-8H-5, 48-49 cm. Fig. 6, Actinocyclus curvatulus, sample 744B-9H-5, 48-49 cm. Fig. 7, Macrora barbadensis, sample 744A-10H-1, 74-75 cm. Fig. 8, Porosira spp., sample 744B-9H-1, 149-150 cm. Fig. 9, Lisitzinia ornata, sample 744A-11H-5, 131-132 cm. Fig. 10, Cestodiscus sp. 2, sample 744A-11H-6, 105- 106 cm. Fig. 11, Raphidodiscus marylandicus, sample 744A-11H-5, 34-35 cm.
56
2.
1.
5. 4. 3.
7. 8.
6.
9.
Plate V 10. 11. 57
Plate VI (All scale bars = 20 µm)
Figs. 1, 2, 9, 10, 12, Synedropsis sp. 1. Fig. 1, sample 744A-10H-6, 2-3 cm; Fig. 2, sample 744A-11H-1, 121-122 cm, scale bar is equal to 20 µm; Fig. 9, sample 744A-10H- 7, 58-59 cm; Fig. 10, sample 744A-10H-3, 34-35 cm; Fig. 12, sample 744B-9H-2, 108- 109 cm. Figs. 3, 4, 14, Rouxia spp. Fig. 3, sample 744A-10H-4, 136-137 cm; Fig. 4, sample 744B-8H-5, 48-49 cm; Fig. 14, sample 744B-8H-2, 146-147 cm. Fig. 5, Cavitatus linearis, sample 744A-11H-2, 149-150 cm. Fig. 6, Mediaria splendida, sample 744B-8H-5, 103-104 cm. Fig. 7, Cavitatus jouseanus, sample 744A-11H-3, 91-92 cm. Fig. 8, Cavitatus sp. cf. C. jouseanus, sample 744A-11H-6, 51-52 cm. Fig. 11, Cavitatus rectus, sample 744A-11H-5, 13-14 cm. Figs. 13, 15, Bogorovia sp. 1. Fig. 13, sample 744A-11H-1, 141-142 cm; Fig. 15, sample 744A-10H-2, 148-149 cm. Fig. 16, Bogorovia veniamini, sample 744A-11H-5, 24-25 cm. Fig. 17, Cavitatus miocenicus, sample 744A- 10H-4, 136-137 cm. Fig. 19, Cavitatus sp. 1, sample 744A-11H-5, 131-132 cm. Fig. 18, Cavitatus sp. cf. C. jouseanus, sample 744A-11H-1, 105-107. Fig. 20, Thalassionema nitzschioides var. parva, sample 744B-9H-1, 108-109 cm.
58
1.
9. 3.
2. 4. 5. 10.
11. 6.
12. 13. 8.
7.
18. 19.
15. 20. Plate VI 16. 17. 14. 59
10. Figure Captions For Supplemental Tables
Table 1a. Diatom occurrence, preservation, and abundance data for diatoms observed in Cores 744A-10H and 11H.
Table 1b. Diatom occurrence, preservation, and abundance data for diatoms observed in Cores 744B-7H, 8H, and 9H.
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