Stable Carbon and Nitrogen Isotopic Studies of Devonian Land Plants--An Indicator of Paleoclimate and Paleoenvironmental Changes

Home , Archaeopteridales, Drepanophycus, Hampshire Group, Leclercqia (plant), Pertica, Price Formation

Stable Carbon and Nitrogen Isotopic Studies of Devonian Land Plants

--An Indicator of Paleoclimate and Paleoenvironmental Changes

A dissertation submitted to the

Graduate School

University of Cincinnati

In partial fulfillment of the requirements for the degree of

DOCTOR OF PHILOSOPHY

Department of Geology

McMicken College of Arts and Sciences

July 5th, 2012

Zhenzhu Wan M.S., Peking University, 2007 B.S., Peking University, 2004

Dissertation Committee

Thomas J. Algeo, PhD, Chair Carlton E. Brett, PhD Aaron F. Diefendorf, PhD Patricia G. Gensel, PhD Warren D. Huff, PhD J. Barry Maynard, PhD Stephen E. Scheckler, PhD

Abstracts

Analysis of >400 specimens of 11 taxa of Devonian land plants (Drepanophycus,

Sawdonia, Psilophyton, Leclercqia, Pertica, Rhacophyton, Haskinsia, Pseudosporochnus,

Tetraxylopteris, Archaeopteris, and Genselia) revealed stable carbon isotopic (δ13C) variation ranging from -30 to -20‰, with a distribution similar to that of modern C3 land plants. Some of this variation is related to secular trends, i.e., specimens of Middle Devonian age are generally 3-

4‰ heavier than those of Early or Late Devonian age, but the substantial δ13C variation among taxa of a given age (up to ~6‰ for median δ13C) is evidence of genetic, metabolic, or environmental influences. Where plant genera from a single depositional system show differences in median δ13C values (up to 5‰), genetic or metabolic factors may have played a role. However, both inter- and intrageneric differences in median δ13C values between depositional systems are indicative of environmental controls. A review of environmental controls on modern plant C-isotopic variation suggests that water availability (i.e., mean annual precipitation, or MAP) and salinity stress were probably the dominant influences on Devonian plant δ13C values.

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Acknowleagements

Many thanks to Dr. Tom Algeo for his teaching in and outside the classroom, the freedom to do research projects that I liked (even though not sure to be successful), the tolerance of my distraction to things that were not closely related to research. I saw a lot from all the field trips led by Dr. Carl Brett, always surprised by his “encyclopedia” brains, and encouraged by his high geologic spirits. I really appreciate Dr. Aaron Diefendorf’s help in organic geochemical area, and I benefited a lot from discussion with him about a wide range of research. Thanks to Dr. Pat Gensel, who accommodated me during my visits to her lab, showed me techniques of experiments, and gave me rare paleobotany literature and nice fossils to work on. I am very grateful to Dr. Warren Huff in many ways, and he is always nice and ready to help, supplying me with detailed guidance in the sixth floor labs, showing how to be an organized and creative geologist. I sincerely thank Dr. Barry Maynard for his instructions in courses I took from him, solving problems from different angles, gathering useful information effectively, and also his strong care and support. I am so indebted to Dr. Stephen Scheckler for his wealth collection of fossils, abundant knowledge of both geology and paleobotany, that I could always turn to him if I have questions; and I also thank him for his careful editing, as early as my proposal of the preliminary exam.

Thanks to Drs. Christopher Berry, Walter Cressler, William Stein, and Honghe Xu for identification and supply of plant fossils, and also thanks to Peabody Museum of Yale, State Museum of New York, Smithsonian Museum for supply of compression fossils of Devonian land plants. Field and lab assistance from Xinda Hu, Hengye Jing, Teng Teng, N. Doug Rowe and Xinlin Wang is greatly appreciated. Thanks a lot to Dr. Harold Rowe and Dr. Peter Sauer for stable carbon and nitrogen isotopic analysis lab work.

Many thanks to various support and opportunities provided by the UC Department of Geology. Kate Cosgrove, Mike Menard, Tim Phillips, Krista Smilek, Dani Adams and Jenna Schroer would always be there when I needed something. Truly thanks to Dr. Arnie Miller who is there ready to help from the very first day of my time at UC, supported me in various ways. I benefited a lot from discussions with Dr. Dave Meyer and Dr. Attila Kilinc. And many thanks to colleague graduates that helped and inspired me in different aspects: Joanne Ballard, Kate Bulinski, Tanya del Valle, Jason Dortch, Gianna Evans, Kelsey Feser, Sharm Giri, Sarah Kolbe, Nadeesha iv

Koralegedara, Nathan Marshall, Gary Motz, Tashia Pierce, Trish Smrecak, Julia Wise, Jackie Wittmer, and Jay Zambito.

I am so grateful to my family and friends in China, for their support throughout graduate school. Thanks to all the encouragement and comfort from 12 times zones away, and I would not be able to do this without them.

This study was funded by graduate research grants from American Association of Petroleum Geologists, Geological Society of America, Paleontological Society, the Society for Sedimentary Geology, Palaeontological Association, the UC Department of Geology, the UC Graduate Student Governance Association, and the UC University Research Council.

Table of Contents:

Chapter 1: Introduction (p. 1-2)

Chapter 2: Environmental Influences on the Stable Carbon isotopic Composition of Devonian

Land Plants. (p. 3-44)

Chapter 3: Stable Carbon Isotopic Composition of Plants: An Indicator of Devonian

Paleoclimate Change. (p. 45-100)

Chapter 4: Stable Nitrogen Isotopic Studies of Devonian Plants and Paleosols. (p. 101-146)

Chapter 5: Conclusion (p. 147-149)

Chapter 1

Introduction

The Devonian Period is one of the most important geologic intervals for the development of land plants. Between ~417 and 354 million years ago, the terrestrial ecosystem expanded enormously, from limited amounts of small, mostly non-vascular plants located exclusively in wet lowlands (Early Devonian) to abundant vegetation including trees that carpeted continental interiors including drier upland areas (Late Devonian). The northern part of the Appalachian

Basin was located in the tropical to subtropical climate belt of the southern hemisphere at this time, a region with habitat suitable for early terrestrial floras. This study examines 11 Devonian land plant genera (Drepanophycus, Sawdonia, Psilophyton, Leclercqia, Pertica, Rhacophyton,

Haskinsia, Pseudosporochnus, Tetraxylopteris, Archaeopteris, Genselia) from the northern part of the Appalachian Basin, as well as from other areas including China, Germany, Greenland, and

Venezuela.

Carbon (C) and nitrogen (N) are important nutrients for plants, and along oxygen (O) and hydrogen (H) they constitute the bulk of plant biomass. During burial and diagenesis, most oxygen and hydrogen are lost in the process of fossilization of land plant tissue, leaving carbon and nitrogen as the major constituents of plant fossils. Each of these elements has two stable isotopes (12C and 13C for carbon, and 14N and 15N for nitrogen). Variation in the proportion of these isotopes can record substantial information about the history of formation and preservation of land plant tissues. The present study examines C- and N-isotopic variation among Devonian land plants with the goals of reconstructing aspects of their paleoecology, contemporaneous climate changes, and soil microbial cycling.

The stable carbon isotopic composition (δ13C) of plants is different from that of atmospheric

13 CO2, which is the source of most plant carbon. Differences in plant and atmospheric δ C are due to fractionation during photosynthesis that is related to genetic and environmental factors

(e.g., CO2 concentration, temperature, and water availability). Chapter 2 examines patterns of

δ13C variation among Devonian land plant genera in order to determine the dominant genetic and environmental controls on this variation. These influences are recorded in δ13C variation as a function of paleolatitude, proximity to paleocoastlines, and inter- and intra-generic differences among Devonian plant taxa.

The Devonian was a period of major changes in global climate that have been documented in earlier studies. Chapter 3 examines secular variation in land plant δ13C during the Devonian, and its relationship to independent proxies of Devonian climatic conditions, including paleoatmospheric δ13C (reconstructed from the marine carbonate δ13C record) and paleotemperatures (reconstructed from conodont δ18O data). This analysis reveals a strong relationship of land plant δ13C to both paleoatmospheric δ13C and paleotemperatures, although with significant time lags that may reflect relative forcings in the global climate system.

Nitrogen is usually a limiting element during plant growth, and the stable nitrogen isotopic composition (δ15N) of plants is an indicator of nitrogen sources. Chapter 4 examines variation in the N-isotopic composition of both plant fossils and their sediment matrices with the goal of identifying sources of fixed N used by Devonian land plants as well as the extent to which a soil microbial community existed in Devonian terrestrial ecosystems.

Chapter 2

Environmental influences on the stable carbon isotopic composition of Devonian land

plants

Zhenzhu Wan

Department of Geology, University of Cincinnati, Cincinnati, Ohio 45221-0013 USA

Email address: [email protected]

Abstract

Analysis of >400 specimens of 11 taxa of Devonian land plants (Drepanophycus,

Sawdonia, Psilophyton, Leclercqia, Pertica, Rhacophyton, Haskinsia, Pseudosporochnus,

Keywords: Mean annual precipitation (MAP); Appalachian Basin; Devonian; Land plants;

Carbon isotopic composition

Introduction

Modern C3 plants make use of the enzyme rubisco to fix carbon via the Calvin-Benson cycle (Raven et al, 2005). This leads to an average carbon isotopic fractionation of about -18‰ during photosynthesis, yielding organic material with a δ13C value averaging ca. -25‰ but ranging from about -20 to -30‰ (Bacon, 2004). Devonian land plants are thought to have been exclusively C3 plants, and they exhibit a mean and range of δ13C values similar to those of modern C3 plants (see results below). Given their common photosynthetic pathway, variations in

δ13C in both modern and Devonian C3 plants would appear related to genetic and environmental factors (e.g., temperature, atmospheric pCO2, and water availability).

Much research on modern plants has shown that environmental factors, especially mean annual precipitation (MAP), are important influences on plant C-isotopic compositions. For C3 plants, many studies have documented a negative relationship between δ13C and MAP (figure 1; e.g., Diefendorf et al., 2010; Gouveia and Freitas, 2009; Hatté et al., 2001; Leffler and Enquist,

2002; Liu et al., 2005; Miller et al., 2001; Macfarlane et al., 2004; Roden et al., 2005; Song et al.,

2008; Ning et al., 2008; Stewart et al., 1995). This relationship is generally thought to be non- linear, with less sensitivity of C-isotopic composition to variations in MAP at higher values of

MAP (figure 1; Diefendorf et al., 2010). The many published studies offer a wide range of slopes

for this relationship, from about 3 to 12‰ over the range of 0-1000 mm yr-1 MAP, although the most commonly reported slopes are ~4-5‰ (figure 1).

The relationship of plant δ13C to MAP has been quantified as follows. O’Leary (1981),

Farquhar and Richards (1984) proposed an equation about the C-isotopic fractionations of plants:

Isotope effect = k12 / k13 = R(source) / R(product) (1) where R is the ratio of 13C to 12C, and isotope effect, k12 / k13 is the ratio of rate constants for reactions of the respective isotopic substances. Farquhar et al. (1982) and Farquhar (1983) gave an approximate expression for the overall isotope effect, α, during carbon fixation by leaves of a

C3 plant species:

α = Ra / Rp = 1.0044 * [(pa - pi) / pa] + 1.027 * (pi / pa) (2)

where Ra and Rp are the abundance ratios of the carbon in the atmospheric CO2 and of the carbon fixed in the leaves, pi / pa is the ratio of intercellular and atmospheric partial pressures of CO2,

12 13 1.0044 is the ratio of the conductances to diffusion of CO2 and CO2 (Vogel, 1980), and 1.027 is the approximate value of the isotope effect associated with carboxylation (Farquhar et al.,

1982). Hence, the discrimination is given as:

-3 Δ = α -1 = (4.4 + 22.6 * pi / pa) * 10 (3)

13 13 = δ Ca – δ Cp (4)

13 13 where δ Ca and δ Cp are expressions of the stable carbon isotopic compositions of the atmosphere and plants. Therefore:

13 13 -3 δ Cp = δ Ca – (4.4 + 22.6 * pi / pa) * 10 (5)

13 Over short time periods, pa and δ Ca are stable, and there is a negative correlation between pi and

13 δ Cp. pi is mainly controlled by leaf surface stomatal conductance.

13 The parameters pi and δ Cp can be related to MAP through water-use efficiency (WUE).

In general, higher precipitation and humidity result in lower water stress for plants, allowing

13 plant stomata to be open for longer intervals and, thus, yielding higher pi and lowerδ Cp values.

13 The relationship between WUE and δ CP is quantified as (Hubick et al, 1986):

13 13 WUE = pa * (1 - Φ) * (b – δ Ca + δ Cp) / [1.6 * (ei - ea) * (b - a)]

(6)

where ei and ea are ambient and intercellular pressure of water vapor; Φ is the proportional of net daytime fixation of carbon in the shoots lost by respiration; a is the fractionation caused by diffusion from atmosphere to plants, and b is the fractionation associated with carbon dioxide fixation.

Gutschick (2004) quantified relationships among WUE, humidity, and CO2 concentration as follows:

WUE = pa / [m * esat * hs * (1 - hs)] (7)

where m is commonly very close to 10 for all unstressed C3 plants, even desert shrubs; esat is saturated water vapor pressure, which is closely related to temperature; and hs is relative humidity at the leaf surface, which is similar to atmospheric humidity. Equations 6 and 7 show that there is a quantitative relationship between atmospheric humidity and plant δ13C. Under

13 normal conditions, δ Cp will exhibit negative covariation with both pa and hs, and the latter two parameters (pa and hs) will therefore exhibit positive covariation with each other.

13 Environmental factors other than MAP have been linked to variation in δ Cp in some studies, but in general there is no scientific consensus regarding such controls. Altitude effects

13 are small, at most, as elevation changes between 50 and 6000 meters yield an increase in δ CP of only 1-2 (Song et al, 2008). Latitude exerts a stronger effect, with C3 plants showing an average decrease of 3-4‰ between the equator and poles (Körner et al., 1988; Marshall and Zhang, 1994;

Li et al., 2009). Neither altitude nor latitude is a direct environmental control, however, and their influence must come through other factors, such as increased UV irradiation with altitude or reduced light levels with latitude. Altitude and latitude also correlate with variations in water availability (MAP) and temperature (MAT), so these factors may play a role in controlling

13 associated δ CP variation. For example, precipitation generally decreases with increasing

13 elevation (Sokol and Bližňák, 2009), a pattern which could account for higher δ CP values with

13 altitude (cf. figure 1). On the other hand, the decrease in δ CP with latitude cannot be related to

MAP, because higher latitudes are generally associated with lower precipitation; other factors, such as light levels, must assume greater importance.

13 An additional environmental control that may exert a strong influence on δ CP in some regions is soil salinity. Research on two C3 plants, Puccinallia ciliate and Trifolium michelianum,

13 showed that δ CP values increase with (1) higher soil salinity (by up to 2‰) and (2) longer growth period in a saline environment (by up to 1‰) (figure 2; Mostajeran and Renel, 2007).

Additionally, more saline conditions resulted in the δ13C of roots becoming heavier than that of shoots (by up to 1‰). Another study of Trifolium michelianum suggested that soil salinity effects could be even larger, with 13C enrichment by up to 5‰ at salinities of 400 mM (Winter and

Holtum, 2005). These results have been confirmed by other experimental and in-situ studies of a range of modern plants (Brugnoli and Lanteri, 1991; Poss et al., 2000; Wei, 2007), although a

few studies (Malamud-Roam and Ingram, 2001; Kemp et al., 2009) have inferred that the δ13C of coastal plants is only weakly affected (<1‰) by salinity variations. The influence of salinity on

13 δ CP is generally thought to be due to reduced stomatal conductance and lower pi / pa ratios at higher salinities, mimicking the effects of low precipitation (Farquhar et al., 1989; Brugnoli and

Lanteri, 1991; Poss et al., 2000).

Paleoclimate and Paleoenvironments

The northern Appalachian Basin was at low paleolatitudes in the southern hemisphere during the Early Devonian, Venezuela was south of the Appalachian Basin, Xinjiang (China) and

Germany were slightly south of the paleoequator, and Greenland was positioned slightly north of the paleoequator (figure 3). Laurussia shifted northward during the Devonian but by only about

5˚ from the Early to Late Devonian, resulting in limited change in the climate zones inhabited by fossil plants of the present study. During this whole time period, they were within the tropical/subtropical climate belt.

Until the Middle or Late Devonian, plants were found mainly along rivers or in wet lowland areas, and the interior upland regions of continents were mostly devoid of vegetation

(Algeo et al., 1995; Retallack and Huang, 2011). The depositional environments represented by the collection sites of this project were mostly river floodplains. During the Early to Middle

Devonian, land plants were clustered in lower floodplain and coastal delta habitats. From the

Middle to Late Devonian, land plants began to occupy more upland regions, and fossil plant deposits can be found in upper floodplain settings. Some of the fossils were buried in situ, while others were transported downstream. Since most fossils of this project are well preserved and relatively intact, it is unlikely that they were transported any long distance.

The depositional environments associated with individual fossil plant taxa are summarized in table 1. Following are brief descriptions of the most important locales yielding fossil plants used in the present study:

1. Red Hill

The Late Devonian Red Hill locality in north-central Pennsylvania contains an

Archaeopteris-dominated plant fossil assemblage. Red Hill is a kilometer-long roadcut exposure of the Duncannon Member of the Catskill Formation, an upper alluvial plain facies of the

Catskill Delta Complex. The outcrop consists of nearly horizontal layers of fluvial sandstones, siltstones, mudstones and paleosols. The fossil flora is typical of Late Devonian (Famennian) subtropical to tropical floodplain forest vegetation. Archaeopteris is dominant and Rhacophyton is abundant. Sedimentological analysis of the main plant-fossil bearing layer at Red Hill indicates that it was an abandoned channel that became an oxbow lake, subsequently filled by silt deposits over a relatively short period of time. A seasonal wet-and-dry climate is indicated by well-developed paleovertisols at Red Hill and a paleolatitude reconstruction of 20° South. At least during the wet season, vegetation grew in profusion along the freshwater littoral zone of the oxbow lake. Presence of charcoal interspersed with plant fossils indicates that low-intensity fires occurred in this landscape, perhaps as regular events during the dry season (Cressler, 1999).

2. Elkins

This outcrop consists of fossiliferous, deltaic sediment of the Hampshire Formation near

Elkins, Randolph County, West Virginia. The Hampshire Formation represents the fluvial to deltaic facies of terminal portions of the Catskill deltaic complex that prograded into northern

Pennsylvania, Maryland, West Virginia and Virginia during the Late Devonian. Elkins locality is near the westernmost and youngest Devonian extension of this progradation. The beds consist of

medium bedded to finely laminated mudstones, cross-stratified sandstones, and coals. They are interpreted as shoreline deposits, some of which represent newly emergent delta top-set beds or interdistributary splays, while others show evidence of storm-wave action and contain marine elements. Plants of this outcrop are thought to be colonizers on levee tops and along the leading edges of small delta lobes. (Rothwell et al., 1989).

3. Valley Head

The Valley Head outcrop consists of black and gray siltstone and fine sandy shales belonging to the Famennian Hampshire Formation. Plant fossils of Rhacophyton and

Archaeopteris are abundant (Andrews and Phillips, 1968). The Valley Head and Elkins localities have similar fossil compositions and paleoenvironments (Rothwell et al., 1989).

4. Gilboa

Plant-bearing strata belonging to the upper part of the middle Givetian Panther Mountain

Formation are located in a cliff face on the west flank of Brown Mountain, Gilboa, New York.

Plant remains, including Leclercqia, Archaeopteris and Drepanophycus, are preserved both as carbonized compressions and as pyrite petrifactions (Banks et al., 1972). Shoreline deposits of eastern New York are characterized by sandstone facies that record a shifting mosaic of shoreline environments including fluvial-estuarine channels and floodplains, tidal channels and flats, and storm-dominated estuarine mouth shoals (Miller, 1991). The arrangement of mudstones, coarse sandstone and tree stumps suggest that the site was a natural levee subject to successive destructive floods interspersed with stable periods in which soils accumulated and the vegetation recovered. Plants at Gilboa may have included ground cover, shrubs, and medium-sized trees.

5. New Brunswick

This outcrop is on a beach ~10 miles east of Campbellton, New Brunswick, near Pin Sec

Point. It consists of a conglomerate of limestone cobbles embedded in a sandstone matrix. Most of the cobbles are barren, but a few contain plant remains. Based on analysis of dispersed spores, it was noted that the rocks between Campbellton and Dalhousie are early to late Emsian in age

(Trant and Gensel, 1985). Specimens of Psilophyton and Drepanophycus were mainly from Pin

Sec point, a site on the south shore of the Restigouche River approximately 2 miles west of

Dalhousie, New Brunswick. Plant remains are highly carbonized and preserved totally flattened, or in part three dimensionally, depending on grain size of the enclosing sediment (Li et al., 2000).

6. Gaspé

The Gaspé Peninsula, Quebec, Canada contains strata ranging in age from Emsian to

Frasnian. The most fossil-rich sediments belong to the Battery Point Formation, which is composed of three members: the Petit-Gaspé Member (Emsian), the Cap-aux-Os Member, and the Fort Prével Member (Emsian to Eifelian). They record a transition to a more proximal alluvial-fan and flood plain system. Drepanophycus is found here, but there are few well preserved cuticles, and the matrix rock usually occurs in a grayish black slate which is with associated greywacke and conglomerate (Dorf and Cooper, 1943; Stubblefield and Banks, 1978;

Elick et al., 1998).

7. Pulaski

The Price Formation at Pulaski is Early Mississippian (Tournaisian) in age and may be correlative with the lower portions of the Pocono Formation of Pennsylvania and West Virginia, the Cuyahoga Formation of Ohio and the New Providence Formation of Kentucky (Gensel and

Skog, 1977). Kreisa and Bambach (1973) interpreted the paleoenvironment of the Price

Formation as a westward prograding deltaic shoreline complex that at times supported a coal

swamp forest. Delta sediments derived from erosion of highlands during the late phases of the

Acadian Orogeny. Plant fossils are preserved as impressions or coalified compressions in either a buff-colored siltstone or a gray to gray-buff fine-grained sandstone.

8. Ashland

The Ashland locality is in Greene County, New York, and consists of sandstones of the

Frasnian Oneonta Formation, which is correlative with the Ithaca Formation in the Genesee

Group of the Finger Lakes Stage, Senecan Series (Paleobiology Database). Tetraxylopteris fossils are abundant, and plant fossils are preserved as molds, impressions, or compressions

(Bonamo and Banks, 1967; Paleobiology Database).

Fossil Plant Taxonomy and Paleoecology

All of the fossil plant taxa examined in this study have been the subject of at least partial morphological reconstructions (figure 4A). They were members of early land plant groups, including the pteridophytes, spermatophytes, zosterophylls and lycopsids (figure 4B). These taxa were mainly of Devonian age, with overlapping ranges that covered part or all of the Early,

Middle, or Late Devonian, although Genselia had a range extending into the Lower

Carboniferous. A brief summary of their characteristics by genus follows:

1. Psilophyton

Psilophyton is one of the most completely known members of the Trimerophytophyta group, existing throughout the Devonian. It has been found in the northeastern United States

(Maine and New York), Canada (Gaspé), and the Czech Republic. Variation within the genus is significant (Taylor et al., 2009). Naked stems of P. forbesii were marked by longitudinal striations, and ellopsoidal sporangia were produced in pairs on fertile lateral branches.

2. Rhacophyton

Rhacophyton belongs to the order of Rhacophytales, which existed during the Late

Devonian. It has been found around the world in Upper Devonian strata (Taylor et al., 2009).

According to Andrews and Phillips (1968), this plant grew to ~1m height as bushes, and its

“fronds” consist of quadreseriate lateral branches (also termed tetrastichous, i.e., borne in four ranks or rows) that are attached to the main axis in two vertical rows.

3. Pertica

Pertica is considered a trimerophyte based on the spiral arrangement of its branches in a clockwise direction from base to apex (Kasper and Andrews, 1972). It had numerous sporangia gathered as dense clusters in the fertile area of ultimate branchlets (Kasper and Andrews, 1972;

Taylor et al., 2009). It existed from the Early to the Middle Devonian. Fossils of this genus have mostly been found in northern area of the Appalachian Basin (Maine, New Brunswick, and

Quebec).

4. Genselia

Genselia may have been a seed fern (Knaus, 1995; Taylor et al., 2009). This genus has bipinnate fronds, non-bifurcate rachises, and elongated sporangia (Knaus, 1994, 1995; Skog and

Gensel, 1980; Taylor et al., 2009). It is found in the Lower Carboniferous Pocono and Price

Formations of Pennsylvania, Maryland, West Virginia and Virginia, in the Appalachian Basin of

North America.

5. Tetraxylopteris

Tetraxylopteris is one of the Aneurophytales, belonging to Progymnospermopsida, and existed from the Middle to the Late Devonian. To date, it is known from only two areas: the

Catskill Delta in New York, and the Campo Chico Formation in northwestern Venezuela

(Hammond and Berry, 2005). Beerling and Fleming (2007) think that the non-laminate proto- leaves of Tetraxylopteris were precursors to true leaves.

6.Archaeopteris

Archaeopteris belongs to Archaeopteridaceae family, Progymnospermophyta division, mainly existing from Late Devonian to Early Carboniferous. It was a major component of many

Devonian-Carboniferous ecosystems, and this genus is known from numerous Northern

Hemisphere localities in North America, Russia, China, Morocco, and Europe (Meyer-Berthaud et al., 1999), as well as from the Southern Hemisphere, e.g., Australia (Beck and Wright, 1988) and Colombia (Berry et al., 2000). By the middle of the late Frasnian, monospecific archaeopterid forests had become the dominant vegetation type in lowland areas and coastal settings over a vast geographic area (Algeo et al., 2001). Archaeopteris has been thought as a tall tree that looked like some modern conifers, and it had pseudomonopodial branching in the lateral branch systems (Beck, 1962). Archaeopteris is usually preserved as impression and compression fossils.

7. Sawdonia

Sawdonia is one of the early Zosterophylls, belonging to the Lycophytes, and was widespread during the Devonian (Kenrick and Crane, 1997). This plant is pseudomonopodially branched with circinate tips (Steward, 1983).

8. Drepanophycus

Drepanophycus belongs to Drepanophycaceae family, Lycopodiophyta division, which existed through almost the entire Devonian Period, and was found in eastern Canada, northeastern USA, China, Russia, and Europe. Stems ranged from several mm to several cm in diameter, several cm to a meter in length, and they were erect or arched and sometimes dichotomizing. Leaves are unbranched thorn-shaped microphylls, several mm long with a single middle prominent vascular thread, and they were arranged spirally or randomly on the stem.

Characteristics of epidermal cells and stomata are usually preserved very well, and

Drepanophycus is typical genus on which research of stomatal index and stomatal density has been undertaken (Li and Edwards, 1995; Li et al., 2000).

9. Leclercqia

Leclercqia is a known member of the Protolepidodendraceae family, Lycopodiophyta division, which existed from the Early to the Middle Devonian. It is thought to be a slender, herbaceous plant. Fossils of Leclercqia have been found in Australia, North America, and

Europe (Taylor et al., 2009). The functions of ligules are suggested as secretion and accumulation of water, mucilage, enzymes and (or) nutrients, or superficial conduction of water

(Pant et al., 2000). Usually, the presence or absence of ligules has been used to define some groups of lycopsids (Taylor et al., 2009).

10. Haskinsia

Haskinsia is a widespread herbaceous lycopsid, from South and North America to China

(Berry and Edwards, 1996; Xu et al, 2008; Taylor et al, 2009) during the Middle to Late

Devonian. Haskinsia used to be regarded as a species of Drepanophycus (Grierson and Banks,

1983), and then it was placed with the Protolepidodendrales because of the presence of deltoid- shaped sporophylls (Taylor et al., 2009).

11. Pseudosporochnus

Pseudosporochnus existed from the Middle Devonian to Early Carboniferous, and it has been found in Europe and Norther America (Taylor et al., 2009). It was interpreted as about 2 to

4 meter tall trees (Berry and Fairon-Demaret, 2002).

Material and Methods

Plant fossil collection

More than 400 specimens of Devonian land plants, and matrix samples in which they were preserved, were analyzed for stable carbon isotopic composition. Other than samples collected from Red Hill, PA by Wan, the majority of plant fossils were supplied by paleobotanical colleagues, including Drs. Christopher Berry, Walter Cressler, Patricia Gensel,

Stephen Scheckler, William Stein, and Honghe Xu. Nearly all of the fossil specimens collected belong to one of 11 Devonian plant genera: Drepanophycus, Sawdonia, Psilophyton, Leclercqia,

Pertica, Rhacophyton, Haskinsia, Pseudosporochnus, Tetraxylopteris, Archaeopteris, Genselia

(figure 4).

Plant fossil preparation

For this project, well-preserved compression fossils that were identified taxonomically to at least the genus level were macerated for sample extraction. Pieces of the fossil plant were peeled from the surface of individual specimens, after which any rock matrix adhering to the sample was digested successively in hydrochloric acid (HCl) and hydrofluoric acid (HF), with

three rinses in distilled water in between steps. Organic fragments of the plant fossils were picked out of the residue and rinsed in distilled water.

Plant fossil samples were dried and weighed for C-isotopic analysis. Analyzed specimens weighed between 0.1 and 10 mg. They were wrapped in tin capsules of the same standard.

Samples of this project were processed through EA-IRMS analysis in two stable isotope laboratories: (1) Department of Earth and Environmental Sciences, University of Texas at

Arlington (UTA), and (2) Department of Geological Sciences, Indiana University. Results are displayed as δ13C value of each sample relative to the PDB (Pee Dee Belemnite, based on

Cretaceous marine fossil, Belemnitella americana in South Carolina) standard.

Analytical reproducibility was evaluated by analyzing one set of 5 samples twice at

Indiana University, yielding nearly identical results. Interlaboratory comparison was undertaken by analyzing another set of 15 samples at both Indiana University and the University of Texas at

Arlington. On average, the C-isotopic composition of samples only differed by <0.2‰ on average, documenting that the data produced by the two laboratories are effectively equivalent.

Results

Among major Devonian plant taxa studies of this project (Figure 6), Drepanophycus,

Sawdonia and Psilophyton all went through from Early to Late Devonian; there are Leclercqia samples in Lower and Middle Devonian, Archaeopteris in Middle and Upper Devonian,

Rhacophyton in Middle and Upper Devonian and Lower Carboniferous, Pertica in Lower

Devonian, Haskinsia in Middle Devonian, Pseudosporochnus and Tetraxylopteris in Upper

Devonian. Genselia was found in Lower Carboniferous. Median δ13C values of three taxa all show an increase from Early to Middle Devonian, followed by decrease from Middle to Late

Devonian, only except that some Middle Devonian Psilophyton specimens have lower values:

Drepanophycus (-26.2‰, -25.4‰, -26.0‰), Sawdonia (-27.5‰, -24.3‰, -27.8‰) and

Psilophyton (-25.0‰, -25.9‰, -24.8‰). Median δ13C value of Leclercqia increases from Early

(-24.0‰) to Middle (-22.4‰) Devonian. Median δ13C value of Rhacophyton decreased from

Middle (-24.9‰) to Late Devonian (-25.3‰). Median δ13C value of Archaeotperis increased from Frasnian (-26.1‰) to Famennian (-25.3‰). Plant taxa with median δ13C values of one time period are: Pertica (-25.4‰), Haskinsia (-22.3‰), Pseudosporochnus (-26.5‰), Tetraxylopteris

(-25.9‰), and Genselia (-23.1‰).

Seven plant taxa with specimens found from more than one location regions are placed by fossil regions (Figure 7). It is obvious that within an individual fossil region, Different taxa

δ13C values have various ranges, and δ13C distributions are more compact within an individual taxon than from different taxa. Average δ13C values of Drepanophycus are: -25.8‰ (Germany), -

24.8‰ (Xinjiang, China), -25.9‰ (Quebec), -26.5‰ (New Brunswick), -25.3‰ (Maine) and -

25.6‰ (New York); Sawdonia: -27.1‰ (Quebec), -26.4‰ (New Brunswick), and -24.1‰

(Venezuela); Psilophyton: -24.2‰ (Quebec), -24.0‰ (New Brunswick), -23.9‰ (Maine) and -

25.8‰ (New York); Leclercqia: -22.8‰ (Xinjiang, China), -24.6‰ (New Brunswick), and -21.9‰

(New York); Rhacophyton: -25.0‰ (New York), -23.8‰ (Pennsylvania) and -24.6‰ (West

Virginia); Haskinsia: -22.2‰ (Xinjiang, China) and -24.1‰ (Venezuela); and Archaeopteris: -

24.8‰ (Xinjiang, China), -26.2‰ (Greenland), -28.1‰ (Quebec), -28.4‰ (New Brunswick), -

26.2‰ (New York), -25.1‰ (Pennsylvania), -25.8‰ (West Virginia) and -26.0‰ (Venezuela).

Seven fossil regions (New Brunswick, New York, Pennsylvania, Quebec, Venezuela,

Virginia, and West Virgnia), with plant fossils found more than two occurrences, are placed by geological time periods of Devonian (Figure 8). It displays a secular pattern of fossil plant δ13C

value distribution in each fossil region, from Early to Late Devonian. δ13C value of every time period has a wide range: from -29.7‰ to -22.0‰ in Early Devonian, from around -25.2‰ to -

20.0‰, and from -29.4‰ to -21.8‰ in Late Devonian, and the median values show an increase from Early to Middle Devonian, then a decrease from Middle to Late Devonian.

Discussion

Although plants were usually transported by streams and rivers before they were buried, fossil plants have been thought as indicators of environment in which they grew, providing information about paleoclimate including precipitation, temperature, and soil conditions (Beck,

1962; Algeo et al., 2001; Hammond and Berry, 2005; Xu et al., 2008). As root systems were not advanced enough to absorb sufficient water until Middle Devonian (Algeo et al., 2001; Hotton et al., 2001), early land plants are thought to have grown primarily in lowland areas with easy access to water, e.g., along river banks, lake margins, and deltas.

Genetic and metabolic controls

Botanical research shows that there are morphology and anatomy differences among plant taxa because of genetic and metabolic controls (Raven et al, 2005), different plant taxa display the variation of δ13C values throughout the whole Devonian, as well as in different paleogeographic regions (Figure 6 and 7).

Drepanophycus, Sawdonia and Psilophyton all went through from Early to Late

Devonian (Figure 6), and plant δ13C show a similar trend: increase from Early to Middle

Devonian, followed by decrease from Middle to Late Devonian; only except that some Middle

Devonian Psilophyton specimens have lower values. Although without samples of the whole

Devonian, δ13C of Leclercqia obviously increased 1.6‰ from Early to Middle Devonian, and

δ13C of Rhacophyton decreased about 0.6‰ on average. The consistent changing trend of different taxa during Devonian may have been mainly the results of global climate change, for they were from different area and their local weather and other conditions wouldn’t be the same

(Figure 3).

Major fossil regions of this project spread from subtropical area in the south hemisphere to slightly north of the equator (Figure 3). Even though from the same geographic area, different plant taxa would have obvious δ13C variation (Figure 7), which may have mainly been the results of genetic differences, for local weather and other factors were probably very similar.

Psilophyton and Sawdonia both appeared in Quebec and New Brunswick (Figure 7), but average

δ13C value of Sawdonia is about 2‰ lower than Psilophyton in both regions. It is the same with

Archaeopteris and Rhacophyton, for average δ13C value of the former is about 1‰ lower than the latter in both Pennsylvania and West Virginia. It could also shows that average variation within the same taxa from different regions, is not as obvious as variation within the same region but from different taxa (Figure 7), which may indicate that genetic differences surpass the differences resulting from geographic distances.

Precipitation controls

The influence of water availability on plant δ13C may be indicated by geographic patterns of C-isotopic variation. As shown in map of Late Devonian δ13C value distribution area,

Archaeopteris was found in various locations: Red Hill, PA; Elkins and Valley Head, WV; and northwestern Venezuela, and average δ13C values of each area are: -25‰, -25.6‰, -26.1‰, which decreased southward from Pennsylvania to Venezuela. Since Archaeopteris existed at almost the same geologic time from three different geographic areas, it indicates that

precipitation levels were increasing southward, from Pennsylvania to West Virginia and North-

West of Venezuela, if other environmental factors had minor effects on C-composition.

In addition to Archaeopteris, other plant taxa from Late Devonian show geographic variation in C-isotopic compositions. δ13C values of Late Devonian Sawdonia exhibit a decrease from north to south: Gaspe samples are about -27‰ on average, while Valley Head, WV samples are about -30‰. In the same areas, however, Rhacophyton yielded nearly constant δ13C values, i.e., -25.0‰ versus -25.2‰.

Secular pattern of plant δ13C value of major fossil regions (Figure 8) suggests that precipitation differences, resulting from geographic differences, may have caused plant δ13C variation among different localities. Quebec and New Brunswick have been relatively close to each other, and their δ13C ranges are almost the same (Early Devonian); it is similar with

Virginia and Pennsylvania in Late Devonian. While average δ13C value in Quebec is about 2‰ lower than that of New York and Pennsylvania, about 3‰ lower than that of Venezuela (Late

Devonian), for Venezuela was further south, New York and Pennsylvania were in the middle, and Quebec was closer to the equator (Figure 3). This example shows that the further south, the higher the plant δ13C value, and the reason might be that humidity was lower in the south because of less precipitation.

Salinity controls

As there is a positive relationship between plant δ13C values and salinity levels (Brugnoli and Lanteri, 1991; Poss et al., 2000; Mostajeran and Renel, 2007), changes in plant δ13C values would indicate the changes of environmental salinity. Besides variation among species, δ13C values of organic matter in salt marshes could tell changes of environmental information, e.g.,

the variability over annual cycles (Cloern et al., 2002)—reflecting plant growth and senescence, and rates of sea-level rise (Middelburg et al., 1997)—indicated by marsh-accretion rates.

Since Sawdonia fossils were found in sandy shale (Andrews and Philips, 1968), they are thought to have grown in coastal area (Gensel, 1992; Gensel and Edwards, 2001). On average,

Sawdonia δ13C values of Early (around 407 Ma, New Brunswick, Canada), Middle (around 389

Ma, Venezuela) and Late Devonian (around 367 Ma, New Brunswick) are -26‰, -24‰ and -

28‰. Early and Late Devonian samples were collected from the same geographical area, if other environmental factors had minor influences, habitat salinity level of Early Devonian would have been obviously higher than that of Late Devonian in New Brunswick, which may indicate that salinity level around 407 Ma was higher than that of around 367 Ma, so did sea level of the two time periods.

There is no evidence that other plant taxa used to grow in coastal area or salt marshes, so sea level rising or not had little influence on those plants. Besides genetic differences, environmental factors, e.g. precipitation would have had larger influences on C-composition of other plant taxa.

As quantitatively demonstrated in Equations 6 and 7, humidity and plant δ13C are negatively related. Since precipitation is usually the major control of atmospheric humidity

(Diefendorf et al, 2010), precipitation and plant δ13C are also negatively related. So, fossil plant

δ13C values could be considered negatively related to precipitation during Devonian, and δ13C value increase would mean less precipitation, and vice versa.

Since most plant specimens of this project have been collected from the northern part of the Appalachian Basin, climate differences might have played a very minor role in this case. As genetic, geographic, precipitation and salinity controls have all been discussed above, the major

reasons of δ13C values of plant taxa in Devonian (Figure 6) may have been taxonomic differences and water availability in a certain area (salinity is considered too). Reconstruction of plant distribution in Devonian (Figure 9) is mainly based δ13C values (Figure 6, 7 and 8)—the lower the δ13C value, the more water availability, and the closer to a water source, and vice versa.

Environmental information of former studies, for example, specimen petrology and stratigraphy features, has also been applied to the reconstruction (Andrews et al., 1968; Scheckler, 1986;

Knaus, 1995; Boyce and Knoll, 2002).

Conclusions

This study has examined the stable carbon isotopic compositions (δ13C) of 11 Devonian plant taxa and their relationships to environmental factors. Distribution reconstruction of

Devonian land plants is based on water availability, mainly considered as distances to water sources, such as river, lakes, etc., for it is rather hard to tell rainfall differences in the northern part of Appalachian Basin. It indicates that possibly because of evolution and adaptation, different plant taxa would maintain a certain difference in δ13C values even in the same geographic area, for example, in New Brunswick, New York and Xinjiang, Drepanophycus δ13C value is always about 1.5 to 2.5‰ lower than that of Leclercqia on average. It is also indicated that global climate changes had more influences than local climate when calculating a large amount of specimens, for example, δ13C values of Archaeopteris from two areas with a shorter distance, Pennsylvania and West Virginia, are very similar to each other, while δ13C values of specimens from Quebec, are much lower than those of Pennsylvania and West Virginia samples.

Acknowledgments

Many thanks to Drs. Christopher Berry, Walter Cressler, Patricia Gensel, N. Doug Rowe,

Stephen Scheckler, William Stein, and Honghe Xu for identification and supply of plant fossils, and also thanks to Peabody Museum of Yale, State Museum of New York, Smithsonian Museum for supply of compression fossils of Devonian plants. Field assistance from Xinda Hu, Teng

Teng, and N. Doug Rowe is greatly appreciated. Thanks a lot to Harold Rowe and Peter Sauer for stable carbon isotopic analysis lab work. This study was supported by graduate research grants from American Association of Petroleum Geologists, Geological Society of America,

Paleontological Society, Society for Sedimentary Geology, and University of Cincinnati.

References

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Figure captions

Figure 1. Average δ13C vs. mean annual precipitation (MAP) (Kohn, 2010).

Figure 2. Data source from Mostajeran and Rengel (2007). Curves P1 to P6 are δ13C values of

Puccinallia ciliate with different salinities, and P1-aerated shoot, P2-hypoxia 10-day shoot,

P3-hypoxia 20-day shoot, P4-aerated root, P5-hypoxia 10-day root, P6-hypoxia 20-day root;

curves T1 to T6 are δ13C values of Trifolium michelianum with different salinities, and T1-

aerated shoot, T2-hypoxia 7-day shoot, T3-hypoxia 14-day shoot, T4-aerated root, T5-

hypoxia 7-day root, T6-hypoxia 14-day root.

Figure 3. Paleogeographic area of the Appalachian Basin (star), Xinjiang of China (Square),

Venezuela (hexagon), Germany (triangle) and Greenland (ellipse) in Late Devonian (Blakey,

Paleogeography).

Figure 4. (A) Reconstructions of main Devonian plant genera of the present study; (B) their

phylogenetic relationships.

Figure 5. (A-B) Photographs of Archaeopteris specimens; (C-D) SEM photos of a Psilophyton

fossil stem; (E-F) SEM photos of cuticles of Drepanophycus.

Figure 6. δ13C distributions of Devonian fossil plants by genus and age. In each column, colored

squares represent the δ13C values of individual samples, black dots the median value, and

rectangles the 16th-84th percentile range. D1, D2, and D3 represent the Early, Middle, and

Late Devonian, respectively, and D22, D31, and D32 represent of the Givetian, Frasnian,

and Famennian stages, respectively; C is the Early Carboniferous Period.

Figure 7. Fossil plant δ13C values from different geographic regions.

Figure 8. Secular pattern of fossil plant δ13C values during Devonian.

Figure 9. Reconstruction of the ecological distribution of Devonian land plants: ①- Drepanophycus, ②-Sawdonia, ③-Psilophyton, ④-Leclercqia, ⑤-Pertica, ⑥- Rhacophyton, ⑦-Haskinsia, ⑧-Pseudosporochnus, ⑨-Tetraxylopteris, ⑩-Archaeopteris, ⑪ Genselia, which is of Early Carboniferous age, but is shown in the Late Devonian diagram above, witha distribution similar to that of Archaeopteris .

Figure 1.

-25.0 δ13C‰ P1 P2 -26.0 P3

P4 -27.0 P5

P6 -28.0 T1

T2 -29.0 T3

T4 -30.0 0.00 0.50 1.00 1.50 2.00 2.50 3.00 T5 Salinity (g NaCl/ kg Soil) T6

Figure 2.

Figure 3.

Figure 4.

Figure 5.

Figure 6.

Figure 7.

Figure 8.

Figure 9.

Table 1. Geologic environmental information of fossil plants

Taxon Location Formationa Environmenta

Archaeopteris Red Hill, PA Catskill Terrestrial indet

Valley Head, WV Hampshire Fine channel fill

Elkins, WV Hampshire Flood plain to swamp

Burtville, PA Catskill Terrestrial indet

Prattsville, PA Oneonta Terrestrial indet

Hancock, PA Enfield Terrestrial indet

North-west Venezuela Campo Chico Terrestrial

Quebec, Canada Hugh Miller Terrestrial

Cliffs

Drepanophycus New Brunswick, Canada Campbelton Coastal and fluvial-lacustrine intermontane

environments

Gaspe, Quebec, Canada Battery Point Terrestrial

Piscataquis County, ME Trout Valley Terrestrial

Summit of Route 10, NY Panther Terrestrial indet

Mountain

Genselia Route 738, Pulaski, VA Price Terrestrial indet

Haskinsia North-west Venezuela Campo Chico Terrestrial

Xinjiang, China Hujiersite Coal swamp

Leclercqia Gilboa, NY Panther Marine indet

Mountain

Northern New Brunswick Campbellton Coastal and fluvial-lacustrine intermontane

environments

Xinjiang, China Hujiersite Terrestrial

Pertica Dalhousie Junction, New Campbellton Coastal and fluvial-lacustrine intermontane

Brunswick environments

Gaspe, Quebec Battery Point Terrestrial

Trout Brook, ME Trout Valley Terrestrial

Pseudosporochnus Ashland, NY Oneonta Terrestrial indet

Psilophyton New Brunswick, Canada Campbellton Coastal and fluvial-lacustrine intermontane

environments

Gaspe, Quebec, Canada Battery Point Terrestrial

Traveler Mountain, ME Trout Valley Terrestrial

Rhacophyton Boone Mountain, PA Pocono Terrestrial indet

Elkins, WV Hampshire Shoreface

Valley Head, WV Hampshire Fine channel fill

Sawdonia New Brunswick, Canada Campbellton Coastal and fluvial-lacustrine intermontane

environments

Gaspe, Quebec Battery Point Terrestrial

North-west Venezuela Campo Chico Terrestrial

Valley Head, WV Hampshire Fine channel fill

Tetraxylopteris East Ashland, NY Oneonta Terrestrial indet

West Cave Mountain Oneonta Terrestrial

quarry, NY

North-west Venezuela Campo Chico Terrestrial (Hammond and Berry, 2005)

aInformation of geologic formation is cited from the Paleobiology Database

(http://paleodb.org/cgi-bin/bridge.pl)

Chapter 3

Stable carbon isotopic composition of plants:

an indicator of Devonian paleoclimate change

Zhenzhu Wan

Department of Geology, University of Cincinnati, Cincinnati, Ohio 45221-0013, USA

Email address: [email protected]

Abstract

Research on recent geologic epochs demonstrates a relationship between the stable carbon isotopic composition of plants and atmospheric pCO2. The former is linked to temperature and the stable carbon isotopic composition of marine carbonate, representing a proxy for photosynthetic fractionation of ancient plants that may reveal information about

13 paleoclimate, especially paleotemperature and paleoatmospheric pCO2. This study analyzed δ C variation in nine common fossil plant genera through the Devonian Period (418-360 Ma):

Drepanophycus, Psilophyton, Leclercquia, Sawdonia, Genselia, Pertica, Archaeopteris,

Rhacophyton, and Tetraxylopteris. These fossils exhibit a wide range of δ13C values, from -20‰ to -30‰, although each taxon had its own narrower δ13C range. Significantly, all taxa show similar temporal trends for the Devonian as a whole, or for the part of the Devonian represented by any individual taxon: δ13C values are higher in the Middle Devonian by 3-4‰ relative to the

Early and Late Devonian, implying lower atmospheric pCO2 and cooler climates during the

13 Middle Devonian. The derived proxy Δ C‰(Atm-Plant) (i.e., photosynthetic fractionation) matches well with secular variation in (1) an independent O-isotope-based paleotemperature record and (2)

13 δ C of atmospheric CO2 for the Devonian, according to cross correlation analysis. This analysis

13 also reveals phase shifts of 4.5±4 Myr between Δ C‰(Atm-Plant) and paleotemperature, 9(-4,+3)

13 13 Myr between paleotemperature and δ CAtmopheric CO2, and 8±5 Myr between Δ C‰(Atm-Plant) and

13 13 δ CAtmopheric CO2. These relationships suggest that Δ C‰(Atm-Plant) and paleotemperature are

13 leading indicators of ancient climate change, and that the δ C of atmospheric CO2 is modified more slowly in response to climate-induced changes in C-cycle fluxes and reservoir masses.

Keywords: Photosynthetic fractionation; Paleotemperature; Appalachian Basin; Land plants;

Atmospheric pCO2

Introduction

Land plants first appeared during the Ordovician Period, but the most important developments in terrestrial ecosystems took place during the Devonian Period (Gensel and

Edwards, 2001). Carbon is one of the most fundamental and abundant elements in plants, which is acquired from atmospheric CO2 via photosynthesis. However, the stable carbon isotopic composition (δ13C) of plants is always different from that of the atmospheric source owing to

13 13 photosynthetic fractionation (Δ C‰(Atm-Plant)) (Farquhar and Richards, 1984). Δ C‰(Atm-Plant) can vary among different plant taxa as a consequence of both environmental and climatic

13 controls. Secular variation in Δ C‰(Atm-Plant) potentially can provide information regarding changes in paleoclimate variables such as temperature and atmospheric pCO2.

Although the stable carbon isotopic composition of Devonian marine organic matter has been documented in multiple studies (Maynard, 1981; Lewan, 1986; Stephens and Sumner, 2003;

Zhao et al., 2010), only limited research has been undertaken on the isotopic composition of

Devonian land plants. Lichens and liverworts, respectively, were analyzed by Fletcher et al.

(2004) and Graham et al. (2010), as well as a few taxa of vascular land plants by Beerling and

Royer (2002, and Beerling et al., 2002). To date, there has been no systematic study of carbon isotopic variation among higher land plants of the Devonian Period.

The present project investigated variation in the stable carbon isotopic composition of land plants of Early to Late Devonian age in the Appalachian Basin and elsewhere (figure 1).

The main goal was to determine whether (1) secular and/or taxon-specific isotopic variation existed, and, if so, (2) how such variation was related to coeval changes in paleoclimate. About

500 samples of Devonian plant fossils were collected, mainly from the northern part of the

Appalachian Basin (figure 2), and well-preserved pieces of organic material were processed for stable carbon isotopic information. Herein, we document (1) secular and taxon-specific patterns of δ13C variation in Devonian land plants, and (2) demonstrate a close relationship to contemporaneous climate changes as proxied by independent O-isotope-based paleotemperature

13 data and δ CAtmospheric CO2 trends.

Paleobotany and Paleoenvironments

Samples of well-studied major plant genera were analyzed in this study. The ranges of the various taxa overlapped to varying degrees (table 1). The most important genera analyzed include:

1) Drepanophycus, a herbaceous lycopod (Stewart, 1983), with dichotomous branching microphylls that are usually spirally arranged. The stomatal density and stomatal index of

Drepanophycus has been used as an indicator of Devonian climate change (Li, et al., 2000).

2) Psilophyton, a member of the Trimerophytopsida (Stein, 1993) that had dichotomously and laterally branch stems, which are usually spiny (Stewart, 1983). Most Psilophyton specimens are found in strata of Emsian, Eifelian, and early Late Devonian age (Doran et al., 1978).

3) Leclercqia, a known member of the Protolepidodendraceae family, Lycopodiophyta division, which existed from the Early to the Middle Devonian. It is thought to be a slender, herbaceous plant. Fossils of Leclercqia have been found in Australia, North America, and

Europe (Taylor et al., 2009).

4) Sawdonia, a zosterophyll with pseudomonopodial branches having circinate tips

(Hueber, 1971b). It is found throughout the Devonian, being first reported in the Siegenian

(Hueber, 1971a), becoming abundant in the Emsian (Gensel, 1992), and surviving into the Late

Devonian (Cressler, 1999).

5) Genselia, may have been a seed fern (Knaus, 1995; Taylor et al., 2009). This genus has bipinnate fronds, non-bifurcate rachises, and elongated sporangia (Knaus, 1994, 1995; Skog and Gensel, 1980; Taylor et al., 2009). It is found in the Lower Carboniferous Pocono and Price

Formations of Pennsylvania, Maryland, West Virginia and Virginia, in the Appalachian Basin of

North America.

6) Pertica, is considered a trimerophyte based on the spiral arrangement of its branches in a clockwise direction from base to apex (Kasper and Andrews, 1972). It had numerous sporangia gathered as dense clusters in the fertile area of ultimate branchlets (Kasper and Andrews, 1972;

Taylor et al., 2009). It existed from the Early to the Middle Devonian. Fossils of this genus have mostly been found in northern area of the Appalachian Basin (Maine, New Brunswick, and

Quebec).

7) Archaeopteris, one of the first free-sporing plants with gymospermous wood, which was the source of early coal beds (Cressler, 1999). With tall stems and compound leaves,

Archaeopteris exhibited both vegetative and reproductive characteristics of the

Progymnospermopsida (Scheckler, 1978).

8) Rhacophyton, belongs to the order of Rhacophytales, which existed during the Late

Devonian. It has been found around the world in Upper Devonian strata (Taylor et al., 2009).

According to Andrews and Phillips (1968), this plant grew to ~1m height as bushes, and its

“fronds” consist of quadreseriate lateral branches (also termed tetrastichous, i.e., borne in four ranks or rows) that are attached to the main axis in two vertical rows.

9) Tetraxylopteris, is one of the Aneurophytales, belonging to Progymnospermopsida, and existed from the Middle to the Late Devonian. To date, it is known from only two areas: the

Catskill Delta in New York, and the Campo Chico Formation in northwestern Venezuela

(Hammond and Berry, 2005). Beerling and Fleming (2007) think that the non-laminate proto- leaves of Tetraxylopteris were precursors to true leaves.

Most of the fossil plants studied in this project are from outcrops of northeast of US in the Appalachian Basin. The northern Appalachian Basin was in the southern hemisphere during the Devonian (figure 1). During the Devonian (418-360 Ma), the study area shifted slightly toward the paleoequator, from about 35⁰S in the Early Devonian to 30⁰S in the Late Devonian.

Throughout the Devonian, the study region was located in the subtropical climate zone

(DiMichele et al., 2008). The climatic conditions of the fossil plants used in the present study was thus similar throughout the Devonian.

Until the Middle or Late Devonian, plants were found mainly along rivers or in wet lowland areas, and the interior upland regions of continents were mostly devoid of vegetation

(Algeo et al., 1995; Retallack and Huang, 2011). The depositional environments represented by the collection sites of this project were mostly river floodplains. During the Early to Middle

Devonian, land plants were clustered in lower floodplain and coastal delta habitats. From the

Material and Methods

Plant fossil preservation

Fossils used in this project were preserved in relatively good condition, and can be taxonomically identified at least to the genus level. Most samples consist of compression fossils of stems and leaves that are intact or nearly so (figure 4A-B). During the carbonization process

(Guo et al., 2010), most water in the plant tissue was lost, and the remaining (mainly carbon) material was preserved as a black organic film. Even though compressed as flat layers, some of the anatomical characteristics of the fossil plants are still evident, and the tracheid cell structures and clearly be seen under high magnification (figure 4C-D). For example, cuticle cells of

Drepanophycus and cell external forms and walls still keep the original shapes of the living

plants (figure 4E-F), which shows that pre-burial bacterial decay was limited and that little deformation of the fossils has happened after burial. The δ13C values of each plant taxon at a given outcrop tend to be quite consistent (mostly <1‰ variation; see below), indicating that there were no random diagenetic effects on the plant fossils during burial.

Thermal maturity of study samples

The thermal maturity of Devonian sediments in the northern part of the Appalachian

Basin is moderate to high (Friedman and Sanders, 1982; Karig, 1987; Rowan, 2006; Repetski et al., 2008). Maximum burial depths in the study area have been estimated between 4 and 10 km, with burial temperatures between 150 and 170⁰C (Friedman and Sanders, 1982; Dorobek, 1989;

Repetski et al., 2008). Some uncertainty exists, however: Friedman and Sanders (1982) estimated that the Middle Devonian Gilboa Formation within the eastern Catskill Mountains of New York

State was buried to 6.5 km, which implies a level of organic metamorphism (LOM, a single scale that synthesizes several existing indices of organic maturity (Hood et al, 1975)) of 16. In contrast,

Richard (1981) reconstructed a maximum burial depth of 3-3.5 km, with burial temperatures at the base of the Upper Devonian in New York State never having exceeded ~120⁰C.

In New Brunswick and Gaspe (Nova Scotia) in eastern Canada, maximum burial depths of Late Devonian sediments may have been as deep as 12 km (Utting and Hamblin, 1991).

Different wells in that area show a maximum burial temperature ranging from 50 to 280⁰C

(Bertrand and Malo, 2001; Chi et al., 2001).

In East Greenland, maximum burial depths of Devonian strata ranged from 2 to 12.5 km

(Higgins et al., 2008), and maximum burial temperatures from 80 to 250⁰C (Leslie and Higgins,

2008).

Analytical methods

Prior to isotopic analysis, well preserved and identified compression fossils were macerated for sample extraction. First, different compression parts (carbon pieces) of the fossil plant were peeled; second, rock matrix attached was digested successively in hydrochloric acid

(HCl) and hydrofluoric acid (HF), with 3 rinses in distilled water between steps. Organic carbon fragments of fossil plants were picked from the residue and rinsed 3 times in distilled water.

Carbon samples were dried and then weighed for carbon isotopic analysis. Samples of this project were analzed by EA-IRMS analysis in two stable isotope laboratories: Department of

Geological Sciences, Indiana University at Bloomington (IU), and Department of Earth and

Environmental Sciences, University of Texas at Arlington (UTA). Results are displayed as δ13C values of each sample relative to the PDB standard (based on a Creataceous marine fossil

Belemnite americana, from the Pee Dee Formation in South Carolina). In order to test for possible systematic differences in results between the two laboratories, a set of 12 sample pairs were processed in both labs (Figure 4). Most pairs of analyses differed by <0.5‰, and the C- isotopic composition of samples only differed by <0.2‰ on average. The same sample analyzed twice in the same lab could yield a 0.2‰ difference, documenting that the data produced by the two laboratories are effectively equivalent.

Data analysis techniques

The present study makes use of multiple environmental-climatic proxies, including the

13 13 δ C values for Devonian fossil plants (δ CPlant) generated herein (figure 5), and previously

13 published records of C-isotopic composition of marine carbonate fossils (δ CCarb) (figure 6A;

18 Simon et al., 2007) and the O-isotopic composition of conodont apatite (δ OConodont) (figure 6B, green lines; Joachimski et al., 2009). These three independent proxies were used to calculate the ”derived” variables discussed below, including (1) paleotemperature (T) (figure 6B, red

13 13 lines), (2) the δ C of atmospheric CO2 (δ CAtm) (figure 7A), and (3) photosynthetic

13 fractionation, i.e., the C-isotopic discrimination of plants relative to atmospheric CO2 (Δ C(Atm-

Plant)) (figure 7B). The latter variables were calculated as follows:

18 18 T (⁰C) = 113.3 – 4.38 * (δ Ophos. - δ Owater) (1)

13 13 δ CAtm = δ CCarb – [11.98 – (0.12 x T)] (2)

13 13 13 Δ C(Atm-Plant)= δ CAtm – δ CPlant (3)

Equation 1 is based on Kolodny et al (1983) and equation 2 on Romanek et al. (1992). As shown by the equations above, paleotemperature is dependent exclusively on conodont δ18O, the δ13C

13 18 composition of atmospheric CO2 is a function of both δ CCarb and δ OConodont, and

13 13 18 photosynthetic fractionation is a function of δ CCarb, δ CPlant, and δ OConodont. For the purpose

13 of constructing records of secular proxy variation through the Devonian, values of δ CCarb

(figure 7A) and paleotemperature (figure 7B) were determined at 0.5-million-year intervals.

Results

Stable carbon isotopic compositions

The δ13C values of Devonian plant fossils range between -20‰ and -30‰ (figure 5). The range of δ13C values is much narrower for an individual taxon during a given time interval (i.e.,

Early, Middle, or Late Devonian; figure 6B). In the Early Devonian, average δ13C values for all taxa are -25.8±2.0‰, while δ13C values for Drepanophycus are -26.8±0.9‰, Leclercqia -

25.3±1.2‰, Psilophyton -24.6±0.8‰, and Sawdonia -25.5±0.8‰. In the Middle Devonian, average δ13C values for all taxa are -22.2±2.2‰, while δ13C values for Leclercqia are -22.5±1.5‰ and Sawdonia are -24.3±0.3‰. In the Late Devonian, average δ13C values for all taxa are -

26.2±4.1‰, while δ13C values for Sawdonia are -28.2±2.3‰, Rhacophyton -25.1±0.6‰, and

Archaeopteris -25.3±0.8‰. Taxon-specific differences in δ13C values are likely to be due to as- yet unidentified genetic, metabolic, or ecophenotypic controls.

Collectively, the fossil plant stable carbon isotopic dataset exhibits a strong pattern of secular δ13C variation (figure 5, heavy black line). On average, fossil plant δ13C increased by 3 to

4‰ from the Early to the Middle Devonian, and then decreased by about 4‰ from the Middle to the Late Devonian. Most taxa that have a wide stratigraphic range follow this general pattern, resulting in taxon-specific curves that are generally parallel to each other although offset by up to

5-6‰ (figure 5, colored lines) owing to the taxon-specific effects discussed above. For example,

Leclercqia and Sawdonia exhibit systematically higher and lower δ13C values, respectively, than

Drepanophycus (and most other taxa) throughout the Devonian. Secular variation cannot be recognized in taxa limited to a single time slice, e.g., Archaeopteris, which is found only in the

Late Devonian.

No systematic variation in δ13C values was detected when analyzing different parts of a single plant. Most of the analyzed samples represent stem material, while ~10% of samples are leaf material. Where paired stem-leaf analyses were undertaken, the δ13C values are similar

(generally <0.5‰ difference) and exhibit no systematic offset. For example, an Archaeopteris specimen from Elkins, West Virginia (sample ID ARCH-PG-019) yielded δ13C values of -25.8‰ and -25.2‰ for stems and -25.4‰ and -25.2‰ for leaves, and a Drepanophycus specimen

(sample ID DREP-PG-008) yielded δ13C values of -26.7‰, -26.2‰, and -24.8‰ for stems and -

26.5‰ and -26.4‰ for leaves.

No systematic variation in δ13C values was detected as a function of geographic site of collection. For example, Early Devonian samples of Drepanophycus yielded values of -25.6‰ in the northern Appalachian Basin, -25.8‰ and in Germany. Early Devonian Drepanophycus samples from different locales thus yielded substantially less δ13C variation (<0.2‰) than between Drepanophycus samples of Early Devonian (-26.2‰) versus Middle Devonian age (-

25.4‰). One factor that may have limited site-specific δ13C variation is that almost all samples derive from the peri-equatorial belt (figure 1). However, the present dataset includes an insufficient number of samples from outside the northern Appalachian Basin to permit a rigorous test of the influence of geographic locale on sample δ13C.

Photosynthetic fractionation between atmospheric CO2 and plants

It is not immediately clear whether the pattern of secular variation in Devonian plant δ13C

13 13 documented herein (figure 6) was due to changes in the δ C of atmospheric CO2 (δ CAtm) or to

13 13 changes in the degree of photosynthetic fractionation (Δ C(Atm-Plant)). δ CAtm is known to have varied through time (Romanek et al., 1992), including a decrease of 1.5‰ during just the past two centuries as large amounts of 13C-depleted carbon from fossil fuels have entered the atmosphere (Long et al., 2005). This issue was investigated by calculating changes in Devonian

13 13 δ CAtm (equation 2) using the marine carbonate δ C record (Simon et al., 2007), the seawater

13 paleotemperature record (Joachimski et al., 2009), and known temperature-dependent δ CCarb-

13 13 δ CAtm fractionation (Romanek et al., 1992). The resulting δ CAtm record (figure 7A) exhibits frequent and rapid changes over a ~3‰ range during the Devonian, largely mirroring changes in

13 13 the δ CCarb record (figure 6A). On average, δ CAtm values were 1-2‰ higher in the Early and

Late Devonian relative to the Middle Devonian. A significant point is that this secular pattern is the opposite of that of fossil plant δ13C, which exhibits lower values in the Early and Late

13 Devonian than in the Middle Devonian (figure 5). Thus, secular variation in δ CAtm cannot

13 account for that in δ CPlant, and the opposing trends mean that their influence on calculated

13 values of photosynthetic fractionation (Δ C(Atm-Plant)) is additive.

13 Secular trends in photosynthetic fractionation (Δ C(Atm-Plant)) during the Devonian (figure

13 13 7B) were determined from the δ CPlant (figure 5) and δ CAtm records (figure 7A) per equation 3.

13 Δ C(Atm-Plant) is 17-19‰ in the early to mid-Early Devonian, declining gradually during the late

13 Early and early Middle Devonian to a minimum of 15-17‰. Thereafter, the Δ C(Atm-Plant) record exhibits a sharp rise to 17-20‰ in the late Middle Devonian, which is sustained (with minor

13 variation) throughout the Late Devonian. For the Devonian as a whole, average Δ C(Atm-Plant) thus varies over a range of 4-5‰ (figure 7B).

Discussion

Carbon isotopic variation in land plants

Land plants exhibit compound-specific C-isotopic variation that needs to be taken into consideration in evaluating δ13C variation in fossil plants. In general, the δ13C values of whole- leaf is about 2‰ more depleted than cellulose (Benner et al., 1987), and cellulose values are 0.25

to 3‰ greater than whole-tissue δ13C values (O’Leary, 1988; Marino and McElroy, 1991); lignin ranged from 4.2 to 2.0‰ less than whole-plant tissue values (Benner et al., 1987). Lipids can be highly isotopically variable (5 to 10‰ less than whole-plant tissue), but they usually decay fast and are rarely preserved (Park and Epstein, 1961). Cellulose and lignin are among the most resistant compounds and commonly dominate the fraction preserved in sedimentary rocks (Arens et al., 2000). In general, individual compounds retain their characteristic C-isotopic compositions during burial and diagenesis, even though the bulk δ13C of the plant fossil changes owing to differential preservation of compounds (Benner et al., 1987). Assuming similar fractionations among the compounds of different fossil plants and similar patterns of post-burial alteration, the

C-isotopic composition of preserved plant material should contain a record of contemporary climate conditions (i.e., paleotemperature and atmospheric pCO2).

Climate controls on secular δ13C variation in Devonian land plants

Some environmental factors have influences on δ13C values of plants. Arens et al. (2000) measured 519 samples of 176 C3 plant species, and they think that much of the δ13C value variation was results of atmospheric δ13C value changes (from -6.4‰ to -9.6‰), which indicates the close correlation between C- isotopic composition of atmospheric CO2 and contemporary plants. However, experiments of their study took place in a controlled lab with highly depleted

13 CO2 (δ C is as low as -45‰), which may not be seen in natural environments.

There are various local conditions (e.g. pH, soil nutrients) that would have some effects on the carbon isotopic composition of terrestrial plants (Retallack and Huang, 2011), however, data of this project show the two main controls seeming to be taxonomy and global environment

(mainly CO2 levels and temperature). So comparison will take place under factors that are known

from geochemical models, for example, (1) atmospheric CO2 levels (they are put in a certain time span, and the CO2 is known from calculations of other observational parameters) (Berner,

2006), (2) paleo-geographic locations (all the outcrops spread in tropical and subtropical latitude area during Devonian, and most of the fossils plants grew in humid lowland or delta habitats)

(Figure 1).

Ever since plants first appeared on land in Late Ordovician, they dramatically changed the whole terrestrial ecosystem (Gensel and Edwards, 2001). During the same time, changes happened beyond the terrestrial ecosystem, because of development of plants, e.g. atmosphere and marine carbonate (Algeo et al., 1995; Algeo et al., 2001; Brett et al., 2011). Due to rapid colonization and expansion in Devonian, plants had fairly strong influences on global climate, which also influences (or controls) the growth and development of land plants. Within many factors, atmospheric CO2 level plays one of the major roles in climate change, and thus it has important effects on plants. During Devonian, terrestrial plants developed and expanded rapidly, needs of large amount of carbon pulled down CO2 level of atmosphere because burial of organic carbon (Algeo et al., 1995; Algeo and Scheckler, 1998). On the other hand, change of atmospheric CO2 level drove the global temperature change (Simon et al., 2007; Joachimski et al., 2009), and further influenced both marine and terrestrial ecosystems. Therefore, different spheres of the Earth would affect each other in various ways, and rapid development and expansion of terrestrial plants in Devonian, not only changed the looks of the land, but also influenced features of atmosphere and marine to some extent. 9-million-year lag time of

13 atmospheric CO2 δ C value and temperature curve (figure 8B, 8C, and figure 9), together with the drop of the former from Early to Middle Devonian, indicate an accelerated absorption of lighter carbon by plants through photosynthesis, which further suggests accelerated accumulation

of plant biomass burial. During this time period, 1. as a result of root development (Algeo and

Scheckler, 1998), plants were able to expand to relatively higher lands, and freed themselves from low land areas (e.g. river banks, delta). 2. size and biomass of individual plants rapidly increased, for an advanced root system would be able to support higher and heavier shoots and

13 supply the whole plant with more nutrients and water. Thus, rapid drop of atmospheric CO2 δ C value from Early to Middle Devonian would support fast development of land plants and corresponding change of landscape (Algeo et al., 2001).

Rapid decrease of CO2 levels led to obvious temperature drop, about 2 ⁰C on average from Early to Middle Devonian (figure 6B, 8C, Joachmiski et al., 2009), which might have caused the relatively cooler period of late Middle Devonian (Royer et al., 2004; Joachimski et al.,

2009). From Middle to Late Devonian, average temperature increased by about 2 ⁰C (Joachimski et al., 2009), for the CO2 level stopped dropping and slightly increased (Berner and Kothavala,

2001). Sea level generally was at high stand during this time (Algeo et al., 2007; Brett et al.,

2011), which might mainly the results of tectonic effects, e.g. orogeny activities. Proxy fractionation between atmospheric CO2 and organic carbon of plant fossils increased during the

13 same time period (figure 7B), which agrees with temperature rising and atmospheric CO2 δ C increasing.

δ13C value of well-preserved fossil plants could indicate the change of paleoclimate. Most of the fossils of this project were collected from the Appalachian Basin (table 2), which was moving towards the north in Devonian, in subtropical and tropical area of the south hemisphere.

Thus, the relatively cooler time of Middle Devonian didn’t affect much of the growth and expansion of plants during that time period. While CO2 is considered as the primary driver of

Phanerozoic climate (Berner and Kothavala, 2001; Royer et al., 2004), δ13C value of plant

13 13 carbon is a direct result of δ C and concentration of atmospheric CO2, which makes plant δ C value an indicator of climate change. In this project, plant δ13C curve and proxy of fractionation between atmospheric CO2 and plants curve both match well with the temperature curve (figure

6B, 8C; Joachimski et al., 2009) (greenhouse to ice house, and to greenhouse climate again) , and they further agree with CO2 level change of Devonian (Berner and Kothavala, 2001) and

13 atmospheric CO2 δ C curve.

Temperature and proxy photosynthetic fractionation curves correlated to each other between 417 Ma and 355 Ma. Despite geologic time gaps among Devonian plant fossils, running

13 average curve (figure 7a, purple, table 3 data) of Δ C‰(Atm-Plant) of proxy photosynthetic fractionation value between atmospheric CO2 and plants, matches paleo-temperature curve of

Devonian (figure 6B) (Joachimski et al., 2009) in shape (r value of cross correlation is about 0.67 when lag time is 3.5 Ma ), which means that they keep the same increase and decrease trends during the whole Devonian time. Within the geologic time periods with records of both, proxy photosynthetic fractionation and temperature dropped to the lowest point around 393 and 390 Ma, and then rose again thereafter and arrived at another peak around 365 and 370 Ma, which was followed by another decrease (figure 8A, 8C). However, because of lack of continuance of fossil records, proxy photosynthetic fractionation curve doesn’t have the obvious rise and fall in Early

Devonian that is demonstrated in the paleotemperature curve.

Relationships between photosynthetic fractionation, atmospheric pCO2, and temperature during the Devonian

Via calculation of cross-correlation (Davis, 2002), the time lag between two curves is

13 4.5±4 Ma (figure 9, red line): in general, every increase or decrease of the Δ C‰(Atm-Plant) curve

falls into the lag time range, no matter earlier or later than the paleo-temperature curve. This

4.5±4-million-year lag indicates that there were some long-term effects between temperature, and proxy fractionation between atmospheric CO2 and plants, which indicates that these two parameters change on the same pace, while resolution differences between fossil records and paleotemperature may cause some lag time periods on curves. During a certain time period, the higher temperature- meaning the higher the atmospheric CO2 level, the bigger the fractionation of ambient CO2 and absorbed CO2 through photosynthesis of plants, thus there is a positive correlation between the two. Temperature may be immediately reflected by δ18O[‰V-SMOW] of conodonts, and it would be the same with fractionation between ambient CO2 and organic carbon of plants. Since ages of plant fossils are only average values of strata, the lag may be results of imprecise of geologic ages.

13 Although the δ C‰ of atmospheric CO2 curve (figure 8B) doesn’t match the paleo-

13 temperature curve (figure 8C) as well as Δ C‰(Atm-Plant) curve (figure 8A) does—the maximum r value of cross correlation is about 0.54, general changing trends of δ13C value of Devonian atmospheric CO2 and temperature of Devonian are rather similar, and they would increase and decrease correspondingly to each other: there are two peaks in Early Devonian on both curves, then they both declined into Middle Devonian and rose in Late Devonian with two peaks, which was both followed by another decline. Temperature (figure 8C, red line) and value of

13 atmospheric CO2 δ C (figure 8B, purple line) both arrived at their first two peaks around 418

Ma and 412 Ma, after which they both decreased and stayed a rather low value time, across

Middle Devonian. Late Devonian, they both reached another two peaks around 373 Ma and 370

Ma, and then both dropped towards 360 Ma.

Cross-correlation (Davis, 2002) indicates that, the lag is around 9(-4, +3) Ma (figure 9,

13 blue line), which means that lag time is not uniform during Devonian, but atmospheric CO2 δ C value seemed one step (2 to 3 million years on average) ahead of the paleo-temperature during the whole Devonian time. Theoretically, temperature would follow the change of CO2 concentration right away in geologic time scales, for CO2 is the main greenhouse gas in atmosphere, levels of which are proxies of temperatures (Royer, 2006; Orombelli et al., 2010).

13 And δ C value of atmospheric CO2 would also directly reflect the level change of atmospheric

13 CO2. So, if every other factor is under control, changes of temperature and atmospheric CO2 δ C value should be at the same pace. During the whole Devonian, especially Early to Middle

Devonian, CO2 concentration dropped dramatically because of the increase of land plant biomass

(Algeo et al., 2001; Berner and Kothavala, 2001), so lighter carbon (12C) was absorbed selectively by plants via photosynthesis. In this case, even if the CO2 levels had kept the same,

13 13 atmospheric CO2 δ C would have dropped. Thereafter, this additional factor drove that δ C drop faster than the concentration of atmospheric CO2, especially from Early to Middle

Devonian. So the lag here might just be the result of rapid development of terrestrial plants, which enhanced the drop of δ13C value.

13 The curves of proxy photosynthetic fractionation and atmospheric CO2 δ C value also match closely (mathematical function to define similarity among curves). Both curves (figure 7B; figure 7A, purple line, showing general trend) are U-shaped. Low-value time period of both curves were around Middle Devonian, and the values were relatively higher around Early and

Late Devonian.

There is a bigger lag between proxy photosynthetic fractionation and atmospheric CO2

δ13C curves, which is around 8±5 Ma (figure 9, yellow line), showing by calculation of cross-

correlation (Davis, 2002). Since atmospheric CO2 is the direct source of plant organic carbon, paces of the two are supposed to be exactly the same. However, there is an obvious lag between

13 the two, which may be also the results of time resolution. δ C values of atmospheric CO2 are calculated results of marine carbonate δ13C value and temperature, both of which are at the resolution of 0.5 million years; while the imprecise range of plant fossils could be up to 10 million years.

13 If it is not a coincidence that Δ C‰(Atm-Plant) curve, paleo-temperature curve, and

13 atmospheric CO2 δ C curve matches well during the whole Devonian time, there would be some

13 correlation among the three curves. Also, change of atmospheric CO2 δ C was 4-5 million years earlier than that of paleo-temperature, and change of paleo-temperature is 4-5 million years earlier than that of proxy photosynthetic fractionation. Because of a similar lag in the aspect of time during Devonian among three curves, there would possibly some relationship among causes.

13 18 13 Raw data of δ C Carbonate and δ OConodont have almost the same cross correlation of δ CAtmospheric

13 CO2 and paleotemperature (table 3); but δ CPlant is different possibly for low resolution (table 2).

Three main players connected here are Devonian plants, atmospheric CO2 and temperature. Growth of plants is affected by both local habitat (nutrients, humidity) and global

(CO2 levels etc) factors. However, geographic influences seem to be averaged out (figure 8, 9), which indicates a moderate to strong global trend. Plants and atmospheric CO2 directly interact with each other, for CO2 is the key compound that plants need to grow and survive, and stable carbon composition of plants is controlled directly by intercellular CO2 and atmosphere CO2

13 levels (Farquhar et al., 1982; Bacon, 2004). So CO2 concentration and δ C value of plants are with the cause and effect correlation. Change of atmospheric CO2 levels would directly cause the change of stable isotopic carbon composition of plants.

Conclusions

By comparing with other climate parameters (stable carbon isotopic composition of

13 atmospheric CO2 and paleotemperature of Devonian), Δ C‰(Atm-Plant) (calculated from fossil plant stable carbon isotopic composition) changes provide one more dimension of paleo-climate studies. Curves from secular pattern of Devonian plants agrees with paleotemperature trends of former studies (Joachimski et al., 2009), and also match changes of δ13C value curve of atmospheric CO2, for they increase and decrease correspondingly during Devonian. Thus,

13 comprehensive data of ancient Δ C‰(Atm-Plant0 could be a very useful tool as an independent indicator of paleoclimate change. Although there are lag time periods among all of the three parameters, which might be involved with local factor effects, or sampling resolution, the general pattern agrees with other ancient climate models.

Acknowledgments

Many thanks to Christopher Berry, Walter Cressler, Patricia Gensel, N. Doug Rowe,

Sauer for stable carbon isotopic analysis lab work. This study was supported by graduate research grants from American Association of Petroleum Geologists, Geological Society of

America, Paleontological Society, Society for Sedimentary Geology, and University of

Cincinnati.

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Figure captions

Figure 1. Redrawn paleomap of Early Devonian (Scotese, Paleomap Project). Red star shows the

position of the northern Appalachians study area.

Figure 2. Major outcrop locations of fossils in the Appalachian Basin.

Figure 3. (A-B) Photographs of Archaeopteris specimens; (C-D) SEM photos of a Psilophyton

fossil stem; (E-F) SEM photos of cuticles of Drepanophycus.

Figure 4. δ13C values for 12 paired samples from various fossil plants for purposes of

comparison of results between the University of Texas at Arlington (UTA) and Indiana

University (IU) stable isotope laboratories.

Figure 5. Secular variation in δ13C of fossil plants keyed to taxon. Taxon-specific curves (light

colored lines) and general Devonian secular trend (heavy black line). The Devonian trend

was calculated from all data in A using an inverse-distance-squared weight function.

Figure 6. (A) Devonian marine carbonate δ13C values and secular trend (from Simon et al., 2007).

(B) Devonian conodont δ18O values and seawater paleotemperature secular trend (from

Joachimski et al., 2009).

13 13 Figure 7. (A) Secular trend in δ C of atmospheric CO2 (δ C(Atm), blue; smoothed curve, purple)

calculated from marine carbonate δ13C trend (figure 5A) per equation 2. (B) Secular trend in

13 13 photosynthetic fractionation (Δ C(Atm- Plant)) calculated from trends in plant δ C (figure 5)

13 and δ C(Atm) (figure 6A) per equation 3.

13 Figure 8. Comparison of secular trends for (a) photosynthetic fractionation (Δ C(Atm- Plant)), (b)

13 13 δ C of atmospheric CO2 (δ C(Atm)), and (c) seawater paleotemperature (T).

13 Figure 9. Cross-correlation analysis between A. photosynthetic fractionation (Δ C(Atm- Plant)) and

13 13 13 δ C of atmospheric CO2 (δ C(Atm)), B. seawater paleotemperature (T) and δ C(Atm)), C. T

13 and Δ C(Atm- Plant). .

Tables

Table 1. Formation of plant fossil locations.

Table 2. δ13C values of plant fossils of this study.

Table 3. Data cited and calculated from Simon et al. (2007) of stable carbon isotopic composition and from Joachimski et al. (2009) of paleotemperature of Devonian.

Figure 1.

Figure 2.

Figure 3.

-23 Lab difference

UTA data δ13C‰

-24

-25

-26

-27

IU data δ13C‰ -28 -28 -27 -26 -25 -24 -23

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Table 1

Taxon Location Formationa

Archaeopteris Red Hill, PA Catskill

Valley Head, WV Hampshire

Elkins, WV Hampshire

Burtville, PA Catskill

Prattsville, PA Oneonta

Hancock, PA Enfield

North-west Venezuela Campo Chico

Quebec, Canada Hugh Miller Cliffs

Drepanophycus New Brunswick, Canada Campbelton

Gaspe, Quebec, Canada Battery Point

Piscataquis County, ME Trout Valley

Summit of Route 10, NY Panther Mountain

Genselia Route 738, Pulaski, VA Price

Leclercqia Gilboa, NY Panther Mountain

Northern New Brunswick Campbellton

Xinjiang, China Hujiersite

Pertica Dalhousie Junction, New Brunswick Campbellton

Gaspe, Quebec Battery Point

Trout Brook, ME Trout Valley

Psilophyton New Brunswick, Canada Campbellton

Gaspe, Quebec, Canada Battery Point

Traveler Mountain, ME Trout Valley

Rhacophyton Boone Mountain, PA Pocono

Elkins, WV Hampshire

Valley Head, WV Hampshire

Sawdonia New Brunswick, Canada Campbellton

Gaspe, Quebec Battery Point

North-west Venezuela Campo Chico

Valley Head, WV Hampshire

Tetraxylopteris East Ashland, NY Oneonta

West Cave Mountain quarry, NY Oneonta

North-west Venezuela Campo Chico aInformation of geologic formation is cited from the Paleobiology Database (http://paleodb.org/cgi-bin/bridge.pl)

Table 2

Age average Taxon δ13C‰ Fossil ID Specimen ID Sample type (Ma)

Archaeopteris 379.9 -26.15 ARCH-CB-001 001a Stem

Archaeopteris 379.9 -26.03 ARCH-CB-001 001c Leaves

Archaeopteris 379.9 -25.68 ARCH-CB-002 002a Stem

Archaeopteris 379.9 -26.37 ARCH-CB-003 003a Stem

Archaeopteris 385.3 -26.39 ARCH-CB-004 004a Stem

Archaeopteris 383 -28.38 ARCH-NYSM-034 034a Plant

Archaeopteris 379.9 -26.58 ARCH-NYSM-035 035a Plant

Archaeopteris 366.85 -26.40 ARCH-NYSM-036 036a Plant

Archaeopteris 366.85 -23.17 ARCH-NYSM-037 037a Plant

Archaeopteris 366.85 -26.43 ARCH-NYSM-038 038a Plant

Archaeopteris 379.9 -25.33 ARCH-NYSM-039 039a Plant

Archaeopteris 366.85 -25.71 ARCH-NYSM-040 040a Plant

Archaeopteris 372.25 -28.32 ARCH-NYSM-041 041a Plant

Archaeopteris 366.85 -28.50 ARCH-NYSM-043 043a Plant

Archaeopteris 379.9 -25.86 ARCH-NYSM-044 044a Plant

Archaeopteris 379.9 -27.62 ARCH-NYSM-045 045a Plant

Archaeopteris 372.25 -27.94 ARCH-NYSM-046 046a Plant

Archaeopteris 372.25 -26.01 ARCH-PG-005 005a Stem

Archaeopteris 366.85 -25.27 ARCH-PG-017 017a Stem

Archaeopteris 366.85 -25.39 ARCH-PG-017 017b Leaf

Archaeopteris 366.85 -25.1 ARCH-PG-018 018a Stem

Archaeopteris 366.85 -25.6 ARCH-PG-018 018b Stem

Archaeopteris 366.85 -23.21 ARCH-PG-018 018c Leaf

Archaeopteris 366.85 -24.14 ARCH-PG-018 018d Leaf

Archaeopteris 366.85 -26.12 ARCH-PG-018 018e Stem

Archaeopteris 366.85 -25.8 ARCH-PG-019 019a Stem

Archaeopteris 366.85 -25.2 ARCH-PG-019 019b Stem

Archaeopteris 366.85 -25.2 ARCH-PG-019 019c Leaf

Archaeopteris 366.85 -25.4 ARCH-PG-019 019d Leaf

Archaeopteris 366.85 -27.01 ARCH-PG-020 020a Stem

Archaeopteris 366.85 -25.78 ARCH-PG-020 020b Stem

Archaeopteris 372.25 -27.53 ARCH-PM-050 050a Plant

Archaeopteris 372.25 -27.49 ARCH-PM-053 053a Plant

Archaeopteris 379.9 -26.70 ARCH-PM-054 054a Plant

Archaeopteris 372.25 -27.36 ARCH-PM-055 055a Plant

Archaeopteris 366.85 -28.73 ARCH-PM-056 056a Plant

Archaeopteris 372.25 -28.04 ARCH-PM-057 057a Plant

Archaeopteris 372.25 -28.56 ARCH-PM-058 058a Plant

Archaeopteris 383 -28.31 ARCH-SM-029 029a Plant

Archaeopteris 383 -28.64 ARCH-SM-029 029b Plant

Archaeopteris 366.85 -27.1 ARCH-SM-030 030a Plant

Archaeopteris 383 -25.78 ARCH-SM-031 031a Plant

Archaeopteris 383 -25.92 ARCH-SM-031 031b Plant

Archaeopteris 383 -26.1 ARCH-SM-032 032a Plant

Archaeopteris 379 -27.90 ARCH-SS-052 052a Plant

Archaeopteris 379.9 -25.63 ARCH-WS-007 007b Stem

Archaeopteris 379.9 -25.69 ARCH-WS-007 007d Stem

Archaeopteris 379.9 -26.36 ARCH-WS-007 007e Leaf

Archaeopteris 379.9 -25.77 ARCH-WS-007 007f Leaf

Archaeopteris 379.9 -26.9 ARCH-WS-008 008b Stem

Archaeopteris 379.9 -25.81 ARCH-WS-008 008d Stem

Archaeopteris 379.9 -26.29 ARCH-WS-010 010b Stem

Archaeopteris 379.9 -26.38 ARCH-WS-010 010d Plant

Archaeopteris 379.9 -24.9 ARCH-WS-011 011b Stem

Archaeopteris 379.9 -25.57 ARCH-WS-011 011d Stem

Archaeopteris 379.9 -25.6 ARCH-WS-022 022a Stem

Archaeopteris 379.9 -25.58 ARCH-WS-022 022b Stem

Archaeopteris 366.85 -24.51 ARCH-WS-023 023a Leaf

Archaeopteris 366.85 -24.21 ARCH-WS-023 023b Leaf

Archaeopteris 366.85 -24.34 ARCH-WS-023 023c Stem

Archaeopteris 366.85 -24.61 ARCH-WS-023 023d Stem

Archaeopteris 366.85 -24.71 ARCH-WS-023 023e Stem

Archaeopteris 366.85 -21.75 ARCH-WS-025 025a Stem

Archaeopteris 366.85 -25.17 ARCH-WS-026 026a Stem

Archaeopteris 366.85 -24.32 ARCH-WS-026 026b Leaf

Archaeopteris 366.85 -25.5 ARCH-WS-027 027a Stem

Archaeopteris 366.85 -24.68 ARCH-WS-027 027b Leaf

Archaeopteris 379.9 -25.65 ARCH-WS-028 028a Stem

Archaeopteris 379.9 -26.53 ARCH-WS-049 049a Plant

Archaeopteris 366.85 -26.36 ARCH-WS-051 051a Plant

Archaeopteris 366.85 -24.25 ARCH-WZ-021 021a Stem

Archaeopteris 366.85 -25.98 ARCH-WZ-021 021b Stem

Drepanophycus 391.4 -24.50 DREP-NYSM-018 018a Plant

Drepanophycus 388.55 -25.32 DREP-NYSM-019 019a Plant

Drepanophycus 406.75 -28.46 DREP-NYSM-020 020b Plant

Drepanophycus 406.75 -26.61 DREP-PG-001 001a Stem

Drepanophycus 406.75 -26.16 DREP-PG-002 002a Stem

Drepanophycus 402.25 -26.70 DREP-PG-003 003a Stem

Drepanophycus 372.25 -26.51 DREP-PG-004 004a Stem

Drepanophycus 400 -26.2 DREP-PG-008 008a Stem

Drepanophycus 400 -24.8 DREP-PG-008 008b Stem

Drepanophycus 400 -26.5 DREP-PG-008 008c Leaf

Drepanophycus 400 -26.4 DREP-PG-008 008d Leaf

Drepanophycus 400 -26.7 DREP-PG-008 008e Stem

Drepanophycus 406.75 -27.5 DREP-PG-009 009a Stem

Drepanophycus 406.75 -27.3 DREP-PG-009 009b Stem

Drepanophycus 406.75 -26.7 DREP-PG-009 009c Leaf

Drepanophycus 406.75 -27.7 DREP-PG-009 009d Stem

Drepanophycus 406.75 -26.0 DREP-PG-009 009e Leaf

Drepanophycus 406.75 -26.2 DREP-PG-009 009f Leaf

Drepanophycus 406.75 -24.0 DREP-PG-010 010a Stem

Drepanophycus 406.75 -23.4 DREP-PG-010 010b Leaf

Drepanophycus 406.75 -23.5 DREP-PG-010 010c Leaf

Drepanophycus 406.75 -25.46 DREP-PM-021 021a Plant

Drepanophycus 400 -25.48 DREP-PM-023 023a Plant

Drepanophycus 406.75 -26.03 DREP-PM-024 024a Plant

Drepanophycus 406.75 -27.92 DREP-SM-016 016a Plant

Drepanophycus 402.25 -26.3 DREP-SM-017 017a Plant

Drepanophycus 402.25 -26.20 DREP-SM-017 017b Plant

Drepanophycus 402.25 -25.79 DREP-WS-011 011a Stem

Drepanophycus 402.25 -25.33 DREP-WS-011 011b Stem

Drepanophycus 402.25 -24.22 DREP-WS-011 011c Stem

Drepanophycus 402.25 -24.42 DREP-WS-011 011d Stem

Drepanophycus 402.25 -26.51 DREP-WS-012 012a Stem

Drepanophycus 402.25 -26.46 DREP-WS-012 012b Stem

Drepanophycus 402.25 -26.89 DREP-WS-013 013a Stem

Drepanophycus 402.25 -28.14 DREP-WS-013 013b Stem

Drepanophycus 406.75 -25.76 DREP-WS-014 014a Stem

Drepanophycus 406.75 -26.02 DREP-WS-014 014b Stem

Drepanophycus 402.25 -25 DREP-WS-015 015a Stem

Drepanophycus 402.25 -25.16 DREP-WS-015 015b Stem

Drepanophycus 391.4 -23.32 DREP-XH-022 022a Plant

Colpodexylon 391.4 -22.49 COLP-XH-001 001a Plant

Colpodexylon 391.4 -22.84 COLP-XH-002 001a Plant

Haskinsia 388.55 -23.73 HASK-CB-001 001a Stem

Haskinsia 388.55 -20.82 HASK-CB-002 002a Stem

Haskinsia 388.55 -23.04 HASK-XH-003 003a Plant

Haskinsia 388.55 -21.58 HASK-XH-004 004a Plant

Leclercqia 391.4 -23.76 LECL-PG-001 001a Stem

Leclercqia 391.4 -20.81 LECL-PG-002 002a Stem

Leclercqia 391.4 -22.85 LECL-PG-003 003a Stem

Leclercqia 406.75 -24.03 LECL-PG-004 004a Stem

Leclercqia 406.75 -25.78 LECL-PG-005 005a Stem

Leclercqia 406.75 -26.11 LECL-PG-005 005b Stem

Leclercqia 402.25 -23.6 LECL-PG-006 006a Stem

Leclercqia 402.25 -23.86 LECL-PG-006 006b Stem

Leclercqia 402.25 -23.9 LECL-PG-006 006c Leaf

Leclercqia 402.25 -23.5 LECL-PG-006 006d Leaf

Leclercqia 402.25 -24.92 LECL-PG-006 006e Stem

Leclercqia 402.25 -24.3 LECL-PG-006 006f Leaf

Leclercqia 388.55 -20.34 LECL-PG-007 007a Stem

Leclercqia 388.55 -22.17 LECL-PG-008 008a Stem

Leclercqia 388.55 -21.92 LECL-PG-008 008b Stem

Leclercqia 388.55 -21.08 LECL-PG-008 008c Stem

Leclercqia 388.55 -20.75 LECL-PG-008 008d Stem

Leclercqia 388.55 -22.72 LECL-PG-009 009a Stem

Leclercqia 388.55 -22.65 LECL-PG-009 009b Stem

Leclercqia 388.55 -21.8 LECL-SM-010 010a Plant

Leclercqia 388.55 -22.68 LECL-SM-010 010b Plant

Leclercqia 391.4 -23.31 LECL-XH-011 011a Plant

Leclercqia 391.4 -22.41 LECL-XH-012 012a Plant

Leclercqia 388.55 -23.35 LECL-XH-013 013a Plant

Pertica 402.25 -25.01 PERT-PG-001 001a Leaves

Pertica 406.75 -26.25 PERT-PG-002 002a Stem

Pertica 402.25 -26.6 PERT-PG-003 003a Stem

Pertica 402.25 -25.7 PERT-PG-003 003b Stem

Pertica 402.25 -25.6 PERT-PG-003 003c Leaf

Pertica 402.25 -25.5 PERT-PG-003 003d Leaf

Pertica 402.25 -24.8 PERT-PG-003 003e Fertile

Pertica 402.25 -25.3 PERT-PG-003 003f Fertile

Pertica 402.25 -26.25 PERT-PG-004 004a Stem

Pertica 402.25 -26.14 PERT-PG-004 004b Stem

Pertica 406.75 -24.55 PERT-PM-008 008a Plant

Pertica 402.25 -25.54 PERT-PM-009 009a Plant

Pertica 402.25 -25.88 PERT-PM-010 010a Plant

Pertica 402.25 -25.17 PERT-PM-011 011a Plant

Pertica 406.75 -24.7 PERT-SM-005 005a Plant

Pertica 406.75 -24.70 PERT-SM-005 005b Plant

Pertica 406.75 -24.82 PERT-SM-006 006a Plant

Pertica 406.75 -25.14 PERT-SM-006 006b Plant

Pertica 406.75 -25.73 PERT-SM-007 007a Plant

Pertica 406.75 -25.26 PERT-SM-007 007b Plant

Pseudosporochnus 379.9 -26.61 PSEU-NYSM-008 008a Plant

Pseudosporochnus 379.9 -26.47 PSEU-NYSM-009 009a Plant

Pseudosporochnus 379.9 -26.54 PSEU-SS-004 004a Stem

Pseudosporochnus 379.9 -26.32 PSEU-SS-004 004b Stem

Pseudosporochnus 379.9 -27.51 PSEU-SS-005 005a Plant

Pseudosporochnus 379.9 -26.09 PSEU-SS-006 006a Plant

Pseudosporochnus 379.9 -27.02 PSEU-SS-007 007a Plant

Pseudosporochnus 379.9 -25.91 PSEU-WS-001 001a Stem

Pseudosporochnus 379.9 -25.57 PSEU-WS-001 001b Stem

Pseudosporochnus 379.9 -26.32 PSEU-WS-002 002b Stem

Pseudosporochnus 379.9 -26.58 PSEU-WS-002 002d Stem

Psilophyton 406.75 -27.29 PSIL-NYSM-014 014a Plant

Psilophyton 366.85 -26.71 PSIL-NYSM-015 015a Plant

Psilophyton 366.85 -24.80 PSIL-NYSM-016 016a Plant

Psilophyton 406.75 -26.27 PSIL-NYSM-021 021a Plant

Psilophyton 406.75 -25.54 PSIL-PG-001 001a Stem

Psilophyton 406.75 -23.74 PSIL-PG-002 002a Stem

Psilophyton 402.25 -24.42 PSIL-PG-003 003a Stem

Psilophyton 402.25 -24.68 PSIL-PG-004 004a Stem

Psilophyton 402.25 -24.6 PSIL-PG-007 007a Stem

Psilophyton 402.25 -24.9 PSIL-PG-007 007b Stem

Psilophyton 400 -24.24 PSIL-PG-008 008a Stem

Psilophyton 400 -23.64 PSIL-PG-008 008b Stem

Psilophyton 387.6 -26.94 PSIL-PM-018 018a Plant

Psilophyton 406.75 -22.06 PSIL-PM-019 019a Plant

Psilophyton 402.25 -23.31 PSIL-PM-020 020a Plant

Psilophyton 406.75 -25.15 PSIL-SM-011 011a Plant

Psilophyton 406.75 -25.13 PSIL-SM-011 011b Plant

Psilophyton 406.75 -24.67 PSIL-SM-012 012a Plant

Psilophyton 406.75 -24.34 PSIL-SM-012 012b Plant

Psilophyton 406.75 -25.42 PSIL-SM-013 013a Plant

Psilophyton 406.75 -25.30 PSIL-SM-013 013b Plant

Psilophyton 402.25 -21.53 PSIL-WS-006 006b Stem

Psilophyton 402.25 -21.86 PSIL-WS-006 006d Stem

Psilophyton 388.55 -25.89 PSIL-WS-009 009a Stem

Psilophyton 388.55 -25.95 PSIL-WS-009 009b Stem

Psilophyton 406.75 -26.13 PSIL-WS-010 010a Stem

Psilophyton 406.75 -26.31 PSIL-WS-010 010b Stem

Psilophyton 402.25 -27.45 PSIL-WS-017 017a Plant

Rhacophyton 366.85 -25.03 RHAC-PG-001 001a Stem

Rhacophyton 366.85 -25.4 RHAC-PG-001 001b Stem

Rhacophyton 366.85 -25.0 RHAC-PG-002 002a Stem

Rhacophyton 366.85 -24.47 RHAC-PG-002 002b Stem

Rhacophyton 366.85 -25.55 RHAC-PG-002 002c Leaf

Rhacophyton 366.85 -25.5 RHAC-PG-002 002d Leaf

Rhacophyton 391.4 -25.3 RHAC-PG-003 003a Stem

Rhacophyton 391.4 -24.5 RHAC-PG-003 003b Stem

Rhacophyton 391.4 -24.6 RHAC-PG-003 003c Leaf

Rhacophyton 391.4 -25.4 RHAC-PG-003 003d Leaf

Rhacophyton 366.85 -25.28 RHAC-PG-004 004a Stem

Rhacophyton 366.85 -25.52 RHAC-PG-005 005a Stem

Rhacophyton 366.85 -24.39 RHAC-PM-014 014a Plant

Rhacophyton 372.25 -22.40 RHAC-PM-015 015a Plant

Rhacophyton 372.25 -25.25 RHAC-SM-011 011a Plant

Rhacophyton 372.25 -25.56 RHAC-SM-011 011b Plant

Rhacophyton 372.25 -25.58 RHAC-SM-012 012a Plant

Rhacophyton 372.25 -25.40 RHAC-SM-012 012b Plant

Rhacophyton 372.25 -25.42 RHAC-SM-013 013a Plant

Rhacophyton 366.85 -29.18 RHAC-SS-016 016a Plant

Rhacophyton 366.85 -25.62 RHAC-SS-017 017a Plant

Rhacophyton 366.85 -29.40 RHAC-SS-018 018a Plant

Rhacophyton 366.85 -23.63 RHAC-WS-006 006a Stem

Rhacophyton 366.85 -23.72 RHAC-WS-006 006b Stem

Rhacophyton 366.85 -24.03 RHAC-WS-006 006c Stem

Rhacophyton 366.85 -24.04 RHAC-WS-006 006d Stem

Rhacophyton 366.85 -24.92 RHAC-WS-007 007a Stem

Rhacophyton 366.85 -25.19 RHAC-WS-007 007b Stem

Sawdonia 388.55 -23.99 SAWD-CB-001 001a Stem

Sawdonia 388.55 -24.54 SAWD-CB-001 001b Stem

Sawdonia 406.75 -24.93 SAWD-PG-002 002a Stem

Sawdonia 406.75 -25.27 SAWD-PG-002 002b Stem

Sawdonia 402.25 -26.39 SAWD-PG-003 003a Stem

Sawdonia 366.85 -27.8 SAWD-PG-005 005a Stem

Sawdonia 366.85 -29.2 SAWD-PG-005 005b Stem

Sawdonia 366.85 -25.7 SAWD-PG-005 005c Leaf

Sawdonia 366.85 -27.5 SAWD-PG-005 005d Leaf

Sawdonia 406.75 -28.76 SAWD-PG-006 006a Stem

Sawdonia 406.75 -28.8 SAWD-PG-006 006b Stem

Sawdonia 406.75 -27.5 SAWD-PG-006 006c Leaf

Sawdonia 406.75 -27.6 SAWD-PG-006 006d Leaf

Sawdonia 402.25 -27.48 SAWD-PG-007 007a Stem

Sawdonia 402.25 -29.49 SAWD-PG-007 007b Stem

Sawdonia 402.25 -30.51 SAWD-PG-008 008a Stem

Sawdonia 406.75 -25.91 SAWD-PM-013 013a Plant

Sawdonia 378.35 -25.71 SAWD-PM-014 014a Plant

Sawdonia 406.75 -23.86 SAWD-SM-011 011a Plant

Sawdonia 406.75 -23.94 SAWD-SM-011 011b Plant

Sawdonia 406.75 -28.02 SAWD-SM-012 012a Plant

Sawdonia 406.75 -27.69 SAWD-SM-012 012b Plant

Sawdonia 402.25 -28.54 SAWD-WS-004 004b Stem

Sawdonia 402.25 -28.31 SAWD-WS-004 004d Stem

Sawdonia 402.25 -28.49 SAWD-WS-009 009a Stem

Sawdonia 402.25 -29.13 SAWD-WS-009 009b Stem

Sawdonia 402.25 -26.51 SAWD-WS-010 010a Stem

Sawdonia 402.25 -26.23 SAWD-WS-010 010b Stem

Traxylopteris 379.9 -23.93 TETR-CB-001 001a Stem

Traxylopteris 372.25 -25.10 TETR-NYSM-017 017a Plant

Traxylopteris 379.9 -29.28 TETR-NYSM-018 018a Plant

Traxylopteris 379.9 -25.50 TETR-NYSM-019 019a Plant

Traxylopteris 379.9 -25.70 TETR-NYSM-020 020a Plant

Traxylopteris 372.25 -22.30 TETR-PG-002 002a Stem

Traxylopteris 379.9 -25.9 TETR-PG-011 011a Stem

Traxylopteris 379.9 -25.9 TETR-PG-011 011b Stem

Traxylopteris 379.9 -26.2 TETR-PG-011 011c Leaf

Traxylopteris 379.9 -26.0 TETR-PG-011 011d Leaf

Traxylopteris 379.9 -27.13 TETR-PM-026 026a Plant

Traxylopteris 379.9 -27.35 TETR-SS-022 022a Plant

Traxylopteris 379.9 -28.10 TETR-SS-023 023a Plant

Traxylopteris 379.9 -28.09 TETR-SS-024 024a Plant

Traxylopteris 379.9 -27.19 TETR-SS-025 025a Plant

Traxylopteris 379.9 -25.37 TETR-SS-027 027a Plant

Traxylopteris 379.9 -24.94 TETR-WS-003 003b Stem

Traxylopteris 379.9 -24.94 TETR-WS-003 003d Stem

Traxylopteris 379.9 -27.88 TETR-WS-007 007b Stem

Traxylopteris 379.9 -27.85 TETR-WS-007 007d Stem

Traxylopteris 379.9 -26.32 TETR-WS-012 012a Stem

Traxylopteris 379.9 -26.27 TETR-WS-012 012b Stem

Traxylopteris 379.9 -24.8 TETR-WS-013 013a Stem

Traxylopteris 379.9 -25.01 TETR-WS-013 013b Stem

Traxylopteris 379.9 -25.76 TETR-WS-014 014a Stem

Traxylopteris 379.9 -25.88 TETR-WS-014 014b Stem

Traxylopteris 379.9 -25.43 TETR-WS-015 015a Stem

Traxylopteris 379.9 -25.4 TETR-WS-015 015b Stem

Traxylopteris 379.9 -26.45 TETR-WS-016 016a Stem

Traxylopteris 379.9 -25.56 TETR-WS-016 016b Stem

Traxylopteris 379.9 -26.39 TETR-WS-021 021a Plant

Table 3

Smoothed Air Age (Ma) Carbonate δ13C‰ Temperature (⁰C) Fractionation Factor Air δ13C‰ δ13C‰ 420 29.10 8.49

-6.88 419.5 1.50 30.10 8.37 -6.87 -6.89 419 0.40 31.00 8.26 -7.86 -6.90 418.5 0.45 31.55 8.19 -7.74 -6.92 418 2.55 32.00 8.14 -5.59 -6.95 417.5 2.85 32.00 8.14 -5.29 -6.98 417 2.00 31.75 8.17 -6.17 -7.02 416.5 1.53 31.40 8.21 -6.68 -7.04 416 0.53 30.70 8.30 -7.77 -7.05 415.5 0.02 30.40 8.33 -8.31 -7.03 415 0.25 30.20 8.36 -8.11 -6.99 414.5 0.46 30.20 8.36 -7.90 -6.92 414 0.60 30.50 8.32 -7.72

-6.83 413.5 1.02 31.05 8.25 -7.23 -6.73 413 2.10 31.40 8.21 -6.11 -6.63 412.5 2.58 31.40 8.21 -5.63

412 2.43 30.95 8.27 -5.84 -6.55

411.5 2.44 30.25 8.35 -5.91 -6.50

411 2.44 29.30 8.46 -6.02 -6.48

410.5 2.43 28.60 8.55 -6.12 -6.49

410 2.38 28.05 8.61 -6.23 -6.54

409.5 2.37 27.45 8.69 -6.32 -6.62

409 2.30 26.90 8.75 -6.45 -6.73

408.5 2.10 26.50 8.80 -6.70 -6.85

408 1.70 26.40 8.81 -7.11 -6.98

407.5 1.40 26.45 8.81 -7.41 -7.12

407 1.29 26.50 8.80 -7.51 -7.25

406.5 1.21 26.70 8.78 -7.57 -7.37

406 1.12 27.00 8.74 -7.62 -7.47

405.5 1.01 27.50 8.68 -7.67 -7.56

405 0.91 27.60 8.67 -7.76 -7.64

404.5 0.80 27.25 8.71 -7.91 -7.69

404 0.73 26.75 8.77 -8.04 -7.73

403.5 0.78 26.00 8.86 -8.08 -7.74

403 1.00 25.25 8.95 -7.95 -7.74

402.5 1.20 24.50 9.04 -7.84 -7.73

402 1.39 23.80 9.12 -7.73 -7.71

401.5 1.50 23.25 9.19 -7.69 -7.69

401 1.62 23.00 9.22 -7.60 -7.66

400.5 1.70 22.75 9.25 -7.55 -7.64 -7.61 22.70 9.26 -7.47 400 1.79 -7.60 399.5 1.88 22.70 9.26 -7.38 -7.59 399 1.93 22.75 9.25 -7.32

-7.59 398.5 1.87 22.80 9.24 -7.37 -7.58 398 1.56 22.90 9.23 -7.67 -7.58 397.5 1.40 23.00 9.22 -7.82 -7.58 397 1.33 23.05 9.21 -7.88 -7.56 396.5 1.60 23.00 9.22 -7.62 -7.55 396 1.60 22.95 9.23 -7.63 -7.52 395.5 1.60 22.80 9.24 -7.64 -7.49 395 1.74 22.60 9.27 -7.53 -7.45 394.5 1.74 22.45 9.29 -7.55 -7.41 394 1.73 22.20 9.32 -7.59 -7.38 393.5 1.83 22.05 9.33 -7.50 -7.35 393 2.17 21.90 9.35 -7.18 -7.33 392.5 2.50 21.70 9.38 -6.88 -7.32 392 2.75 21.40 9.41 -6.66 -7.32 391.5 2.55 21.20 9.44 -6.89 -7.33 391 2.40 20.95 9.47 -7.07 -7.33 390.5 1.54 20.75 9.49 -7.95 -7.33 390 1.80 20.60 9.51 -7.71 -7.32 389.5 1.74 20.55 9.51 -7.77 -7.31 389 1.74 20.60 9.51 -7.77 -7.28 388.5 1.83 20.80 9.48 -7.65 -7.26 388 2.18 21.30 9.42 -7.24 -7.23 387.5 2.54 21.75 9.37 -6.83 -7.20 387 2.75 22.30 9.30 -6.55 -7.18 386.5 2.55 22.90 9.23 -6.68 -7.16 386 2.40 23.20 9.20 -6.80

-7.15 385.5 1.50 23.50 9.16 -7.66 -7.14 385 1.80 23.55 9.15 -7.35 -7.13 384.5 1.70 23.50 9.16 -7.46 -7.12 384 2.50 23.30 9.18 -6.68 -7.10 383.5 1.85 23.20 9.20 -7.35 -7.08 383 1.60 23.10 9.21 -7.61 -7.05 382.5 1.43 23.10 9.21 -7.78 -7.03 382 2.70 23.20 9.20 -6.50 -7.00 381.5 2.96 23.70 9.14 -6.18 -6.98 381 2.40 24.10 9.09 -6.69 -6.95 380.5 2.18 24.60 9.03 -6.85 -6.93 380 2.02 25.20 8.96 -6.94 -6.90 379.5 1.80 26.00 8.86 -7.06 -6.86 379 1.54 26.95 8.75 -7.21 -6.82 378.5 1.30 27.80 8.64 -7.34 -6.76 378 1.10 28.50 8.56 -7.46 -6.70 377.5 1.30 28.90 8.51 -7.21 -6.64 377 2.20 29.25 8.47 -6.27 -6.58 376.5 2.00 29.75 8.41 -6.41 -6.54 376 2.86 30.40 8.33 -5.47 -6.50 375.5 2.50 31.10 8.25 -5.75 -6.48 375 2.12 30.80 8.28 -6.16 -6.47 374.5 1.79 30.00 8.38 -6.59 -6.47 374 1.69 29.80 8.40 -6.71 -6.47 373.5 1.68 29.80 8.40 -6.72 -6.48 373 1.69 30.00 8.38 -6.69

-6.48 372.5 1.68 30.25 8.35 -6.67 -6.48 372 1.64 30.60 8.31 -6.67 -6.47 371.5 1.60 31.00 8.26 -6.66 -6.46 371 1.73 31.25 8.23 -6.50 -6.46 370.5 2.05 31.25 8.23 -6.18 -6.45 370 2.22 31.10 8.25 -6.03 -6.46 369.5 2.20 30.70 8.30 -6.10 -6.48 369 2.01 30.20 8.36 -6.35 -6.50 368.5 1.93 29.70 8.42 -6.49 -6.54 368 1.88 29.25 8.47 -6.59 -6.58 367.5 1.88 28.70 8.54 -6.66 -6.63 367 1.90 28.20 8.60 -6.70 -6.69 366.5 1.96 27.80 8.64 -6.68 -6.75 366 2.05 27.60 8.67 -6.62 -6.83 365.5 2.09 27.50 8.68 -6.59 -6.91 365 1.98 27.45 8.69 -6.71 -6.99 364.5 1.74 27.40 8.69 -6.95 -7.08 364 1.80 27.35 8.70 -6.90 -7.16 363.5 2.10 27.30 8.70 -6.60 -7.24 363 1.20 27.28 8.71 -7.51 -7.30 362.5 0.52 27.25 8.71 -8.19 -7.35 362 0.65 27.20 8.72 -8.07 -7.37 361.5 1.50 27.10 8.73 -7.23

361 27.00 8.74

360.5 26.90 8.75

360 26.75 8.77

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Chapter 4

Stable nitrogen isotopic studies of Devonian plants and paleosols

Zhenzhu Wan

Department of Geology, University of Cincinnati, Cincinnati, Ohio 45221-0013

[email protected]

Abstract

Studies of modern soils show that stable nitrogen isotopic composition (N-composition, represented by δ15N values) in the soil column is related to many environmental factors, e.g., mean annual precipitation (MAP) and mean annual temperature (MAT). Soil δ15N is also related to vegetation types and mycorrhizal groups, which themselves are influenced by MAP and MAT.

In this study, fossil specimens of Devonian land plants from the Appalachian Basin, Greenland,

Venezuela, and China were analyzed for bulk-organic δ13C and δ15N values. In addition, the paleosol matrix of some specimens was analyzed. The fossil plants yield high C:N ratios (20 to

120), whereas the sample matrices yield low C:N ratios (0 to 30), indicating that organic matter in the latter was not derived mainly from higher terrestrial plants but, rather, probably represents soil bacterial biomass. The average difference of δ13C values between plant fossils and matrix is only about 0.6‰, much closer than the difference between fossil plant and matrix δ15N values

(about 3‰). The N-isotopic composition of plant fossils (mainly between -1 and 4‰) is more consistent than that of the sample matrices (between -5 and 4‰), which is consistent with nitrogen derived from a rich community of soil microbes, including N-fixing cyanobacteria that

15 15 yield δ N values close to or below that of atmospheric N2 (0‰). The δ N data of both the fossil

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plants and sample matrices do not show a secular trend through the Devonian, unlike fossil plant

δ13C data.

Key words: Microorganism activity; N-containing compounds; Appalachian Basin; Terrestrial plants

Introduction

Nitrogen is essential for all forms of life, and it is important for plant growth, because cellular metabolites, especially nucleic acids and proteins, need plenty of nitrogen (Morot-

Gaudry, 2001; Bashkin, 2002; Talbot, 2002). Because most plants (except leguminous plants) can take up nitrogenous minerals such as ammonium and nitrates only from the soil, nitrogen is usually the growth-limiting element in the soil system (Morot-Gaudry, 2001; Howarth and

Marino, 2006). Nitrogen cycling in the environment is involved with various biological and non- biological processes: fixation, mineralization, nitrification, and denitrification. Nitrogen shows

+ - - up in various forms of organic nitrogen—ammonium (NH4 ), nitrite (NO2 ), nitrate (NO3 ), nitrous oxide (NO2), nitric oxide (NO), and inorganic nitrogen gas—N2 (Bengtson, 2010).

Studies of stable nitrogen isotopic composition (N-composition) of modern soils show that δ15N of the soil column is strongly influenced by MAP and MATet al.et al.et al. (Martinelli et al., 1989; Ambrose, 1991; Handley et al., 1999; Amundson et al., 2003; Cambell et al., 2009;

Hobbie and Ouimette, 2009; Liu et al., 2011; Peri et al., 2012). Other influences include microorganism activities (Mayland and McIntosh, 1966; Delwiche et al., 1979; Virginia and

Delwiche, 1982; Handley and Raven, 1992; Hobbie and Ouimette, 2009), vegetation types

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(Peterson and Fry, 1987), soil landscape (Dijkstra et al., 2003), soil texture (Delawiche and Steyn,

1970; Papanicolaou et al., 2003), and soil nutrient levels (Fry, 1991).

The N-composition of paleosols has received only limited study. In the Early Devonian, seasonal rainfall and flooding events would have influenced mineral nutrient changes, which may have affected N-fixing cyanobacteria (Hillier et al., 2008). Other paleosol studies are more concerned about soil landscape, redox conditions and nitrogen cycling (Driese et al., 1997;

Driese and Ober, 2005; Nordt et al., 2011), e.g. waterlogged and well-drained soils from Middle

Devonian, soil landscape composition from Lower Pennsylvanian, and original organic matter accumulation and decomposition of early N-cycling from Cretaceous. Doubtlessly, multiple factors influenced changes in Devonian Paleosols, but only the factors that operated most strongly (e.g., MAP and MAT) can be reconstructed on the basis of present data.

This study will focus on the N-composition of Devonian land plants and that of the soil matrix of these fossil plants, which are mainly from the Appalachian Basin, Greenland,

Venezuela, and China. I attempted to figure out N-composition changes between fossil plants and the matrix, and possible reasons for such variation. Also, I investigated secular variation of

δ15N during the Devonian, which might indicate changes in MAP and MAT through time. By comparing C-composition differences between plant fossils and matrix samples to that of N- composition, it may be possible to determine differential post-depositional cycling of these elements in the soil system.

N Isotopic Fractionation

The terrestrial N cycle consists of various chemical reactions that can cause significant

15 nitrogen isotopic fractionations (table 1). For example, atmospheric N2 is N-depleted relative to

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organic N (including sedimentary N), a situation resulting from a greater expressed discrimination in the organic N to N2 (via denitrification) reaction than of a diazotrophy during accumulation of the reduced N (Handley and Raven, 1992). In terrestrial ecosystems, the N-cycle provides a long-term loop of interdependent N fractionations, including both isotopic enrichments and depletions. However, short-term sampling of plant N reveals that plant-δ15N is a function of the δ15N-value of the source N. For most plants, the source is soil. In general, residual soil N is enriched in 15N by the loss of depleted mineral N compounds due to nitrification, denitrification, ammonia volatilization and leaching of depleted nitrate-N.

Paleosol Development during Devonian

Although paleosols are not commonly preserved, some soil profiles or evidence of soil existence can be found from early in geologic history, including the Archean (Serdyuchenko,

1968; Reimer, 1986). Because of vascular plant evolution on land, soils became better developed during the mid-Paleozoic (Algeo et al.., 1995; Algeo and Scheckler, 1998). Ever since the rise of land plants, fossils (especially plant fossils) and paleosols are associated with each other, and they would influence each other, and both reflect some of these environmental factors.

Carbonate in paleosols could be indicators of atmospheric CO2 concentration variation.

Driese and Mora (1993) did research in the aspect of morphology and geochemistry of vertic claystone paleosols of Devonian Catskill Formation in central Pennsylvania, and in this study, they were able to categorize the carbonate into three generations with different characteristics, which would each estimate an individual paleoatmopheric level of CO2. Retallack and Huang

(2011) studied paleosols from the Middle Devonian of New York, and they estimated annual precipitation and temperature from soil characteristics. After studying paleosols of Crooked Fork

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Group (Lower Pennsylvanian) in eastern Tennessee, USA, Driese and Ober (2005) think that climate changes (possibly driven by Milankovitch cycles) had strong influences on evolution of soil landscape and compositions, such as vertic paleosols dominated in seasonally wet floodplains and delta area, while sideritic gley paleosols dominated in poorly drained, always wet swamps. Driese et al. (1997) did research on Catskill Delta Complex forest trees of Middle

Devonian (Givetian), which was in a gray-green, pyritic, grey siltstone paleosol interpreted as waterlogged—reducing environment and less grading, in the coastal-margin area. And there is another paleosol profile in the area, red, sandy paleosol interpreted as well-drained—oxidizing environmental and well graded, in the proximal to active alluvial channels.

The development of deeper, more complex root systems in conjunction with the spread of land plants had profound consequences for Devonian soils and landscapes (Algeo et al.., 2001).

Shallow rooting systems of the Early Devonian had relatively little impact (Algeo et al., 2001).

Elick et al. (1998) found evidence that Emsian plant roots, from the Batter Point Formation of

Gaspé Bay, Québec, Canada, could go as deep as 1 m in alluvial areas, which indicates increased landscape stabilization and development of paleosol morphologies. Because of the deep roots and large stature, they think that Early Devonian plants initiated a steep decline in paleoatmospheric pCO2.

Geologic Record of Soil N Cycling

Nitrogen fixation has a long history documented in aquatic and terrestrial environments.

Nitrogen fixation in aquatic systems has been documented as long ago as the Archean.

- - Cyanobacteria are able to fix nitrogen into NH3, NO2 or NO3 , which can be absorbed by plants and converted to organic matter (Bashkin, 2002). They are considered to be the main N-fixing

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microbes in both fresh water and marine systems, and there is evidence that they have existed for more than 2.8 billion years ago (Olson, 2006). Hillier et al. (2008) think that cyanobacteria would fix nitrogen after flooding events associated with seasonal rainfall and involving with increases in mineral nutrients.

For early soil profiles, nitrogen cycling was slow at the beginning, for the processes depend on accumulation and decomposition of original organic matter (Nordt et al., 2011). They analyzed a series of Cretaceous paleosol profiles, concluding that, although there were plenty of nutrients preserved in feldspars and micas, low exchange of acidity and limited hydrolytic weathering would happen in the early environments; so not many elements were released in a bio-available form. In this case, nitrogen cycling might depend on accumulation and decomposition of a soil organic matter pool, which was not steady at this early stage.

There are all kinds of N-containing compounds in the soil system, and relationship of certain compounds could indicate changes in climate. In the Chinese Loess Plateau from Shanxi

+ Province, China, the distribution of fixed NH4 -N of Luochuan loess is closely related to granularity of aeolian dust, matching paleoclimate variation, which could be regarded as a useful indicator of the winter monsoon record (Rao et al., 2004). Also, Calderoni and Schnitzer (1984) extracted humic acids from six paleosols with ages from about 6,000 to 29,000 yr from northeast of Naples, Italy, and they found that amino acid-N and ammonia-N decreased with increasing age, but some “unknown” N increased (for identifiable nitrogen was converted into complex polymeric compounds), which is useful information for studies of buried soil history.

Stable C and N isotopic composition, along with other proxies, can provide insight regarding the chemical and physical mechanisms of soil stabilization. The dark horizon at 6 m depth of Holocene loess deposits in the central Great Plains of the United Staes (Chaopricha et

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al., 2010), is referred as the Brady soil, and organic matter C:N ratio of the Brady soil has a narrow range, which possibly represents microbial production. Papanicolaou et al. (2003) studied the Palouse Basin, and think that elemental measurements of carbon/nitrogen (C/N) are potentially powerful tools for identifying soil origin in watershed; stable carbon and nitrogen isotopic compositions of soils reflect the types of plants growing on them and the source of nitrogen, respectively. This study clearly indicates that stable isotope tracers have the ability to discriminate between sources of sediment and are potentially powerful tools for sediment fingerprinting.

Burial Diagenetic Changes to Sediment δ15N

After burial, there are diagenesis effects, e.g., as in a modern vertisol from central Texas, which exhibits oxidation of organic carbon, illitization of smectites, etc. (Driese et al., 2000).

Metamorphic alternation would happen at the temperature of 200⁰C or above (Retallack, 1997).

Jia (2006) studied the Ordovician Cooma metamorphic complex, which lies in the southeastern

Australian Paleozoic Lachlan Fold Belt, part of a major orogenic belt. It was noticed that nitrogen concentrations decrease and δ15N increases with increasing metamorphic grade, which can be explained as a continuous release of nitrogen depleted in 15N, and it caused an enrichment of 15N in the residual nitrogen of the rock. With higher metamorphic alteration, the only remaining clues of a paleosol might be a highly aluminous bulk composition and a mineralogy dominated by kyanite, sillimanite, garnet, or corundum (Reimer, 1986).

Material and Methods

Plant fossil preparation

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In this study, more than 400 specimens of Devonian land plants were analyzed for stable carbon isotopic composition, and some samples for nitrogen. Other than samples collected from

Red Hill, Pennsylvania, by myself, the majority of plant fossils were supplied by paleobotanical colleagues: Drs. Christopher Berry, Walter Cressler, Patricia Gensel, Stephen Scheckler, William

Stein, and Honghe Xu.

Well preserved and identified compression fossils were macerated for sample extraction.

First, different compression parts (fossil pieces) of the fossil plant were peeled; second, rock matrix attached was digested successively in hydrochloric acid (HCl) and hydrofluoric acid (HF), with at least 3 times rinse in distilled water in between. The residue was then examined for organic fragments, which were picked out and rinsed in distilled water. Samples of the rock matrix enclosing some fossil plant specimens were collected within 1 cm area of the plant compression fossils and were ground to a powder using a ball mill.

Stable isotopic analyses

Plant fossil samples were dried and weighed for C-isotopic analysis. Analyzed plant specimens weighed between 0.1 and 10 mg; powdered matrix samples weighed between 5 to 10 mg. Each sample was acidified with sulfuric acid in a fume hood, in order to eliminate inorganic carbon. After the samples were dried in the hood, they were tightly wrapped in the silver capsules, and the carbon and nitrogen isotopic compositions were analyzed using EA-IRMS equipment. Samples of this project were processed through EA-IRMS analysis in two stable isotope laboratories: (1) Department of Earth and Environmental Sciences, University of Texas at Arlington (UTA), and (2) Department of Geological Sciences, Indiana University. Results are displayed as δ15N value of each sample.

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Analytical reproducibility was evaluated by analyzing one set of 5 samples twice at

Indiana University, yielding nearly identical results. Interlaboratory comparison was undertaken by analyzing another set of 15 samples at both Indiana University and the University of Texas at

Arlington. On average, the C-isotopic composition of samples differed by <0.2‰, which could be lower than the difference between two analyses of the same sample, documenting that the data produced by the two laboratories are effectively equivalent.

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Results

Figure 2 shows δ13C and δ15N of fossil plants and sample matrices. Red squares show the distribution of δ13C and δ15N values of plant fossils, and they separately range from -26.8 to -

20.8‰, -1 to 3.8‰. Blue diamonds show the distribution of δ13C and δ15N values of the matrix, and they separately range from -27.6 to -21.2‰, –4.8 to 6.8‰. On average, δ13C values of the matrix are 0.8‰ more depleted than those of the plant fossils, while δ15N values of the matrix are about 1.8‰ more depleted than those of the plant fossils.

Figure 3 shows C/N versus δ13C of fossil plants and sample matrices. Solid triangles represent the relationship of δ13C values and C/N ratio of plant fossils, and they separately range from -27 to -21‰, and from 5 to 120. Open triangles represent those of the matrix, and they range from -27.8 to -21‰, and from 0 to 40. Plant fossil and matrix δ13C values have similar ranges, while C/N ratios of the two vary a lot, the average value of the matrix (15, and pretty consistent) is much lower than that of the plant fossils (60, scattering in a wider range).

Figure 4 shows C/N versus δ15N of fossil plants and sample matrices. Solid diamonds are

δ15N values and C/N ratio of plant fossils, and they separately range from -1 to 3.5‰, 5 to 120.

Open diamonds represent those values of the matrix, and they range from -4.2 to 3.5‰. On average, plant fossils have a higher δ15N value (1.5) than that of the matrix (0.5), and the δ15N value range of plant fossils is much narrower than that of the matrix. The average C/N ratio of the plant fossils is about 60, much higher than that of the matrix, only about 15.

Figure 5 shows the δ13C of fossil plants versus that of matrix from the same sample. Plant fossils and the corresponding matrix have almost identical stable carbon isotopic composition.

On average, δ13C values of plant fossils are slightly more depleted in 13C that those of the matrix, at about 0.6‰. Figure 6 shows the δ15N of fossil plants versus that of matrix from the same

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sample. In contrast to C-composition, δ15N values of the plant fossils and corresponding matrix seem not to be related at all. δ15N values of plant fossils range from -2.0 to 4.0‰, while the matrix δ15N value has a much wider range, from -4.0 to 8.0‰.

Figure 7 shows secular variation in plant δ15N values during the Devonian. Average δ15N value of Devonian samples is around 2

, while the range is between -1 and 4‰. It appears that plant δ15N exhibits no significant trend during the Devonian, unlike plant δ13C, which is markedly enriched in the heavy isotope of carbon (13C) during the Middle Devonian relative to the Early and Late Devonian (see chapter 3).

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Discussion

The Terrestrial N Cycle

As the nitrogen cycle in modern soil system shown in Table 2 and Figure 1, atmospheric nitrogen gas (N2) is the major reservoir of nitrogen, constituting about 78% by volume of Earth’s atmosphere, which is very inactive and which most organisms cannot use. Since most plants

- + must absorb nitrogen in a “fixed” form, such as NO3 , NH4 , (NH2)2CO (urea), the fixing process—transforming nitrogen gas into nitrogenous compounds, is very important (Gray, 2009;

Bengtson, 2010). Of the nitrogen distribution (reservoir), on the land especially, dead organic matter is a major source of sediment N, more than 80%. The distribution of my results indicates different sources of nitrogen: organic matter from plant litter, and activities of microorganism communities.

- In nature, besides little amount of NO3 fixed by lightning, most forms of nitrogen are fixed via certain microbes (Morot-Gaudry, 2001), most of which are free-living cyanobacteria.

Biological nitrogen fixation needs a lot of enzymes and a large amount of ATP. Ammonia is the first stable product of the fixation process, and it will be quickly incorporated into protein of plants, or oxidized into nitrate by soil bacteria (the genera Nitrosomonas and Nitrobacter), which

- is called nitrification (Lewis, 1986). NO3 is easily leached in the soil, and also lost via the process called denitrification—reduced by soil anaerobic bacteria into N2, N2O and NOx. Along with development of land plants in Devonian, flora of the Appalachian Basin would have been able to get nitrogen from the soils.

- + Since both NO3 and NH4 would easily dissolve in water and be washed away (Gray,

2009), in the soil system, nitrogen exists in the form of organic nitrogen, such as biomass of

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microbes, plants and animals. When plants die, organic nitrogen will return to the environment though the process of decaying, with participation of all kinds of microorganisms, which is called mineralization (Morot-Gaudry, 2001). At the end, degraded organic nitrogen will either be consumed by microorganisms as energy source, or released into soil horizons in the form of

+ - - ammonia. Free NH4 would either be re-absorbed by plants, or be turned into NO2 , then NO3 through bacterial oxidation (Lewis, 1986). In the fossil plants, nitrogen would possibly be contained in the organic matter left. While in the matrix, results indicate there might have been microorganism activities, besides plant organic nitrogen.

Nitrogen isotopic composition of terrestrial soil system

15N, the rare stable isotope of nitrogen (0.366% of total nitrogen), is now being used widely as a tracer or integrator of studies of nitrogen cycling in organisms and ecosystems

(Peterson and Fry, 1987; Fry, 1991; Robinson, 2001). In the fixation of nitrogen, oxidation of ammonium ion to nitrite by Nitrosomonas and the assimilation of ammonium ion by the several species examined all showed some discrimination in favor of the lighter isotope 14N (Delwiche and Steyn, 1970; Talbot, 2002). During nitrogen decomposition in soils, loss of 14N is faster than that of 15N (Mariotti et al., 1980), which leads an increase of δ15N values. So, in studies of paleosols and fossils, δ15N might also be an integrator of the paleo-ecosystems.

Among many environmental factors, it is generally thought that mean annual precipitation (MAP) and mean annual temperature (MAT) are primary influences on soil N- isotopic composition. The correlation between soil δ15N and mean annual temperature and precipitation, latitude, altitude, and soil pH, was examined by Handley et at (1999), and they concluded that while soil δ15N was correlated with rainfall and latitude, more data are required

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before we can describe general relationships. For example, δ15N values are higher in hot and arid environments than in cool, wet ones (Ambrose, 1991); in a study of N-isotopic composition of leaves and soil of both tropical and temperate forests (Martinelli et al., 1989): foliar δ15N values from tropical forests averaged 6.5‰ higher than from temperate forests; the average δ15N values for tropical forest soils, either for surface or for depth samples, were almost 8‰ higher than temperate forest soils. On the other hand, these results may have provided a line of evidence that

N is relatively abundant in many tropical forest ecosystems. Some researchers even suggest that climate is the primary factor controlling soil δ15N (Handley et al., 1999; Amundson et al., 2003).

Peri et al. (2012) studied 33 Nothofagus forests stands within Patagonia, Southern Argentina.

Foliar and soil δ13C and δ15N natural abundance declined with increased moisture supply. They conclude that a decline in water use efficiency at wetter sites promotes both the depletion of heavy C and N isotopes in soil and plant biomass. Differences in isotopic N and C contents of leaves and soils observed across the study sites was related mainly to water availability (e.g., precipitation, climatic water balance) and marginally to temperature (e.g., MAT and annual temperature range). Amundson et al. (2003) showed that soil δ15N decreases with decreasing mean annual temperature and increasing annual precipitation. Globally, plant δ15N values are

15 15 more negative than soils, but the difference (δ Nplant- δ Nsoil) increases with decreasing MAT

(and secondary increasing MAP), suggesting a systematic change in the source of plant-available

+ - N (organic/NH4 versus NO3 ) with climate.

General relationships between plant litter, soil organic matter, humidity and temperature can be summarized as follows: 1. δ15N values increase from plant litter to fine soil organic matter;

2. δ15N values increase with larger relative humidity and temperature, variation from -5.9 to -

0.3‰; 3. under controlled climate, significant nitrogen isotope differences in δ15N values

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15 (Δδ Nplant-soil) between plant litter and bulk soil organic matter were observed, which could reveal the effect of humidity on transferring process of nitrogen from plant to soil in arid and semi-arid ecosystems (Liu et al., 2011). For modern soil from the Loess Plateau, the TON/TN

(total organic nitrogen/ total nitrogen) values range from 0.95 to 0.99, which suggests that soil nitrogen is primarily present in soil organic matter (Liu and Zhang, 2009). The soil δ15N value is typically higher than that of the original input into organic matter due to isotope fractionation during decomposition (Amundson et al., 2003).

Temperature and precipitation patterns can influence vegetation type and microbial activity, and therefore indirectly influence litter quality, the prevalence of different mycorrhizal associations, and soil nitrogen dynamics. However, climatic factors (MAP and MAT) and nitrification do not directly affect patterns of soil δ15N with depth in temperate and tropical soils

(Hobbie and Ouimette, 2009). These climatic factors can therefore influence 15N patterns of bulk soil and vegetation (Handley et al., 1999). Foliar δ15N values have been correlated with climate across a wide range of study sites, with foliar δ15N (and bulk soil δ15N) lower in colder, wetter areas compared to warmer, drier sites, and lower at temperate than at tropical sites (Handley et al., 1999, Martinelli et al., 1999, Amundson et al., 2003). Vertisols are usually rich in clay with high shrink-swell potential, and twelve vertisol pits of the Coast Prairie of Texas were studied to determine influences of mean annual precipitation (MAP) on chemistry of the soils (Driese et al.,

2005). They think that bulk chemistry of paleosols can be a way of interpreting geological record, such as the vertisol characteristics indicated a higher MAP shift of the Appalachian Basin during

Late Mississippian.

Isotopic composition in the soil column, across soil organic matter pools, is thought to be consistent (Cambell et al., 2009). Isotopic signature is reflective of microbial induced nitrogen

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decomposition, and microbial activities are largely influenced by temperature and moisture.

Mean annual temperature would influence isotopic indicators of soil organic matter turnover at their study sites, as well as 10 other regional and global soil carbon and nitrogen isotopic studies.

So, δ15N at various depths differed among the soils examined (Delawiche and Steyn, 1970), and the differences are a function of soil texture or texture-dependent factors. A study of decay of red pine (Pinus resinosa Ait) shows (Melillo et al., 1989) that small changes in N isotopic composition were observed during litter decay; larger changes were observed with depth in the soil profile. Other than depth, soil particle size is also related to δ15N: investigations of the surface soil from the Loess Plateau and Qinghai-Tibetan Plateau in China (Liu et al., 2011) revealed that the δ15N values of litter and soil organic matter increased with decreasing particle size of the soil. As shown in paleosol layers preserved, root traces can sometimes be seen, color and texture of the soil columns could indicate the possible oxidizing or reducing environments, as well as the plant fossil specimens.

Since samples of this project were from the lowland area, possible N-fixers for the plants could have been similar to bacteria of modern time. Nitrogen isotope distribution is presumed as the indicator of nitrogen fixation, for most nitrogen fixers are depleted in 15N, and there are capability differences among different fixers (Delwiche et al., 1979). For example, there are obvious mycorrhizal system differences: averaging 9.6±0.4‰ in ectomycorrhizal system, and

4.6±0.5‰ in arbuscular mycorrhizal systems (Hobbie and Ouimette, 2009). However, it has been

15 shown that N2-fixing species had lower N abundance than the other plants on most sites examined despite large differences between sites in vegetation, soil, and climate (Virginia and

15 Delwiche, 1982). The mean N abundance of N2-fixing plants varied little between sites and was

15 close to that of atmospheric N2. The N abundance of presumed non- N2-fixing species was

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highest at coastal sites and may reflect an input of marine spray N having relatively high 15N abundance.

In the fixation of nitrogen, the oxidation of ammonium ion to nitrite by Nitrosomonas and the assimilation of ammonium ion by the several species examined all showed some discrimination in favored of the lighter (14N) isotope. Leguminous plants fixing nitrogen have a nitrogen isotope composition similar to that of the atmosphere and non-leguminous plants or legumes which are not fixing N have a nitrogen isotope composition similar to that of the medium in which they are grown (Hoorman, 2011). The studies of entry of N2 into the soil N pool via the process of non-symbiotic N fixation by desert algal crust organisms, observe the fixation rate and N forms entering into the soil N pool. After 21 days of incubation, there was little effect on the relative distribution of N in the several fractions (Mayland and McIntosh,

1966).

There is a geographical gradient of nitrogen isotopic composition in the large scale. The survey of nitrogen stable isotope signatures of leaves and roots of 16 plant species growing under natural conditions in a meadow and a forest understory (Dijkstra et al., 2003), showed differences in nitrate and ammonium availability. The range of Δ15N[leaf-root] values was -0.97 to +0.86‰, leaf δ15N value range is -1.04 to +1.08‰. Precipitation (Handley et al., 1999) and substrate development (Vitousek et al., 1989; Hobbie et al., 1998; Martinelli et al., 1999) are put forward as an explanation for geographically large-scale gradients in nitrogen isotope composition. Enzymes of the nitrogen assimilation pathway, such as nitrate reductase and glutamine synthetase discriminate against the heavier nitrogen isotope (Mariotti et al., 1982;

Ledgard et al., 1985; Yoneyama and Kaneko, 1989). Stable isotope data can contribute both source-sink (tracer) and process information (Peterson and Fry, 1987). Therefore, no matter in

117

large or small scales of study, it is more promising to study other isotopic ratios together with

15N:14N, e.g. 13C:12C or 18O:16O (Högberg, 1997), for the environments are always involved with multiple factors.

15 + - Plant δ N in the organic matter-> NH4 ->NO3 sequence would become increasingly depleted (table 3 and figure 8).

In general, it indicates that the lower the latitude, the smaller the fractionation difference between soil system and the plants (figure 9). As the Appalachian Basin was in the tropical/ subtropical area of the south hemisphere, δ15N differences between plant fossils and the matrix are supposed to be around -1.7 to -6.1‰, if the conditions had been the same. However, results of our data show a different result, which could have been the activities of microorganism activities, or changes after burial.

Implications for my study

Warmer climate and higher precipitation could have enhanced the development land vegetation in the Appalachian Basin, as well as microorganism communities in the soil system. If both plant organic matter (plant fossils) and the soil (matrix) have preserved the record, δ15N value distribution would reflect the changes of MAT and MAP during the Devonian.

C:N ratio differences between matrix and fossil plants

Although there is a minor fraction of organic matter in the sediments, the sedimentary record of organic composition, e.g. C/N ratio, can be used to reconstruct paleoenvironments and soil microbial ecology (Meyers, 1994, 1997; Joergensen et al., 2011). Most soil fungi decompose

118

recalcitrant organic residues high in cellulose and lignin, and they store C and N at the ratio of

~10:1 (Hoorman, 2011).

In modern soils, microorganisms comprise about 3-8% of the organic residue, and the

C:N ratio of these microorganisms is mainly <10 (Hoorman and Islam; Miller). As shown in table 4, the main microorganism species are fungi. Primary source of organic matter in the soil is plant litter (Liu and Zhang, 2009). As shown figure 7, the matrix C:N ratio is between 0 and 40, with the average value 15; fossil plant C:N ratio is between 5 and 120, with the average value 60.

Since the C:N ratio of organic matter from higher plants is usually >20, one may conclude that: 1.

Plant tissues with higher C:N ratios were preferentially preserved, e.g. cellulose, lignin; 2. A lot of microorganism activities have decomposed part, if not all of the plant litter, and there is mixed organic matter in the matrix: plant litter residue and microorganisms.

As shown in figure 8, δ15N values of fossil plant and most of the matrix samples span the range of -0.5 to +4.0‰, however, about a quarter of the matrix samples yielded δ15N values between -5.0 and -0.5‰. It could be the results of microorganism activities, which drove the matrix toward more 15N depleted, for they preferentially chose lighter nitrogen. According to changes of N-isotopic composition, there should have been intense microorganism activities during the process of decomposition and burial. However, δ13C values of the matrix and fossil plants are consistent, and on average, the matrix is only about 0.6‰ more depleted than fossil plants. There could be two possible reasons: 1. Carbon was quite abundant, and only a little portion of organic carbon from plant litter was consumed, or 2. There has been no obvious δ13C shift during the decomposition of microorganisms.

δ15N values of matrix and plant fossils along geologic time

119

It is hard to distinguish precipitation and temperature (as well as other factors) differences in the Appalachian Basin, however, changes of δ15N values along different geologic time, might show changes of environmental factors—MAP and MAT, to some extent. As shown in figure 7, there had been no obvious increase or decrease in the values of δ15N during Devonian, there could be following two explanations: 1. The combined effects of MAP, MAT and other factors hadn’t changed much during Devonian, which as a result, didn’t cause much variation in

δ15N value of fossil plants; 2. Original δ15N values of Devonian have been changed or buffered after more than 360 million years, no matter they have recorded the climate changes or not.

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Conclusions

1. There have been intense microorganism activities after the burial of plant litter, during

the decay process, and after. Changes happened in soil caused a wide range of variation

in δ15N values of organic matter in the matrix (soil), relative to a rather consistent range

of fossil organic matter.

2. However, δ13C values of plant fossils and matrix are closely related, and they are almost

identical (with an average difference of 0.6‰).

3. Reasons for variation in δ15N values, but consistence in δ13C values: 1. Plant fossils and

organic matter in the soil are both resisting tissues, such as cellulose, lignin, etc. Other C-

rich organic matter has been consumed in the matrix; 2. Nitrogen is a limiting element

for microorganisms, and original N-isotopic composition has been changed largely

during the process of consumption of microorganism activities; 3. Nitrogen cycling is

much more complicated than carbon in the soil system, much more possible N-containing

compounds, each step with some amount of fractionation. At the end, it showed up with a

wide range of δ15N distribution.

4. Secular pattern of δ15N distribution hadn’t changed much during Devonian. It indicates

that either 1. Factors influencing δ15N values didn’t vary much during Devonian, which is

hard to tell, for there are no other accurate ways of telling the climatic history during

Devonian. Or 2. δ15N is not a valid proxy of paleoclimate, in this case, the climate

variation and changing trends during Devonian, for it is not so clear about nitrogen

cycling in modern soil cycling system, not to mention that of the Devonian soil system.

Other research also indicated that after late diagenesis, bulk isotopic signal of the 367 Ma

sample could not be used to paleoecological reconstructions. By studying Paleozoic

121

immature rocks (345 Ma, 367 Ma), Ostertag-Henning and Ostertag-Henning (2000) think

that N-isotopic signals have been altered, and they attempted to unravel transformations

of different N forms during diagenesis. While modern samples show a considerate 1 per

mil difference, the Paleozoic kerogen and sulk sediment exhibits nearly identical δ15N

values.

Acknowledgments

Many thanks to Christopher Berry, Walter Cressler, Patricia Gensel, N. Doug Rowe,

Teng and N. Doug Rowe is greatly appreciated. Thanks a lot to Harold Rowe and Peter Sauer for stable carbon isotopic analysis lab work. This study was supported by graduate research grants from American Association of Petroleum Geologists, Geological Society of America,

Paleontological Society, Society for Sedimentary Geology, and University of Cincinnati.

122

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Figure captions

Figure 1. Nitrogen cycle diagram (Bengtson, 2010). Figure 2. δ13C and δ15N values of plant fossils and matrix (in general plant δ13C values are 1‰ higher than those of matrix). Figure 3. δ13C values and corresponding C/N ratio of the matrix and plant fossils. Figure 4. δ15N values and corresponding C/N ratio of the matrix and plant fossils. Figure 5. δ13C values of plant fossils and corresponding matrix. Figure 6. δ15N values of plant fossils and corresponding matrix. Figure 7. Plant δ15N values through the Devonian. Figure 8. Soil nitrogen transformations based on the conceptual model (Schimel and Bennet, 2004). The transformations are: a, depolymerization; b, gross mineralization; c, volatilization; d, nitrification; e, denitrification; f, microbial immobilization; g, death of soil microbes. Figure 9. Estimated range in the effect of individual state factors on the soil δ15N value of soil N (Amundson et al., 2003). Figure 10. (A) Estimated geographical distribution of soil δ15N values to 50 cm. (B) Estimated 15 geographical trends in Δ δ Nplant-soil. Global mean annual temperature and precipitation (0.5 X 0.5 degree grids) data are obtained from Willmott and Matsuura (2000) (Amundson et al., 2003).

133

Figure 1.

134

Figure 2.

135

Figure 3.

136

Figure 4.

137

Figure 5.

138

Figure 6.

139

Figure 7.

140

Figure 8.

Figure 9.

141

Figure 10.

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Table 1. Fractionations against 15N associated with different soil N processes

Process Fractionation (‰)a N2 fixation -2 to 2 Assimilation -1 to 1.6 Nitrification 12 to 35 Denitrification 0 to 33, 26 Ammonia volatilization 20 to 27 Mineralization -1 to 1 Ion exchange -1 to -8 Enzymatic hydrolysis 10 to 24 N transfer, ECM fungi to plant host 8 to 10 N transfer, AM fungi to plant host 0 to 3.5b

a 15 + Positive values indicate that the reactant is enriched in N (e.g., NH4 in nitrification) and the

15 - product is depleted in N (e.g., NO3 in nitrification). ECM ectomycorrhizal, AM arbuscular mycorrhizal. b Based on maximum difference between non-mycorrhizal and mycorrhizal plants.

(Hobbie and Ouimette, 2009)

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Table 2. Global inventory of nitrogen (Handley and Raven, 1992)

Location, chemical nature N Tmol-1 Atmosphere: 8 N2 2.7 × 10 N2O 930 + NH3, NH4 0.07-0.29 - NOx, NO3 0.04-0.21 Total atmosphere 2.7 × 108 Land: Plants organic N 0.86 Animal organic N 0.14 Dead organic N 5.4 × 104 + 3 NH3, NH4 7 × 10 - - 3 NO2 , NO3 3 × 10 Total land 6.4 × 104 Sea: 6 N2 1.4 × 10 Plant organic N 57 Animal organic N 12 Dead organic N 2.4 × 104 + NH3, NH4 710 - - 4 NO2 , NO3 4.3 × 10 Total sea 1.5 × 106 Sediment 5.4 × 107 Crust 4.6 × 107 Mantle 4.0 × 109 Total 4.4 × 109

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Table 3. Observed discriminations from transformation in the nitrogen cycle (Evans, 2008)

Transformation Process Discrimination (‰) Gross mineralization b 0-5 Nitrification d 0-35 + NH4 <->NH3 equilibrium 20-27 Volatilization c 29 N2O and NO production during nitrification d 0-70 N2O and N2 production during denitrification e 0-39 - NO3 immobilization f 13 + NH4 immobilization f 14-20

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Table 4. Relative number and biomass of microbial species at 0-15 cm depth of soil (Dailer

et al., 2010; Hoorman and Islam, 2010; Miller, 2010; Hoorman, 2011).

Microorganisms Number /g of soil Biomass (g/m2) C:N ratio δ15N (‰)

Bacteria 108-109 40-500 3 to 10 xx

Actinomycetes 107-108 40-500 xx

Fungi 105-106 100-1500 10 xx

Algae 104-105 1-50 4 to 10 0 to 10

Protozoa 103-104 Varies 2 to 12

Nematodes 102-103 Varies xx

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Chapter 5

Conclusions

Well-preserved plant fossils can be indicators of the climatic and environmental conditions in which they grew, providing information about such features as precipitation, temperature, and soil conditions. In this study, the stable carbon and nitrogen isotopic compositions of 11

Devonian land plant fossils (Drepanophycus, Psilophyton, Leclercquia, Sawdonia, Genselia,

Pertica, Archaeopteris, Rhacophyton, Tetraxylopteris, Haskinsia, and Pseudosporochnus) were determined with the goals of reconstructing the ecological distributions of various plant genera, local environmental factors influencing their growth, secular climatic variation, and the development of soil microbial communities during the Devonian.

Chapter 1 examines differences in the C-isotopic composition of various Devonian plant genera, revealing both genetic and environmental controls on plant δ13C. These fossils exhibit a wide range of δ13C values, from -20‰ to -30‰, although each taxon had its own narrower δ13C range in a small area. Through studies of both paleobotany and paleoenvironment aspects, conclusions of Chapter 1 are made as follows: 1. Even though growing during the same geological time period, and in a rather close paleogeographical area, different plant taxa still maintained C- isotopic differences from each other; 2. It is indicated that C-isotopic compositions of some plant taxa are more sensitive than others, for example, the amount change of Sawdonia C-isotopic value is much bigber than that of Drepanophycus and Psilophyton in different geologic time periods; 3. Despite differences of plant taxa, geographical distances, C-isotopic composition of most plants would show the same changing trend in Devonian; 4. When precipitation of different outcrops in northern part of the Appalachian Basin couldn’t be distinguished, reconstruction of

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Devonian land plant distribution would rely on water availability, mainly considered as distances to water sources, such as river, lakes, etc.

Chapter 2 examines secular variation in land plant δ13C and in the degree of

13 photosynthetic fractionation (Δ C‰(Atm-Plant)) through the Devonian Period. Most plant taxa show similar temporal trends for the Devonian as a whole, or for the part of the Devonian represented by an individual taxon: δ13C values are higher in the Middle Devonian by 3-4‰ relative to the Early and Late Devonian, implying lower atmospheric pCO2 and cooler climates during the Middle Devonian. The degree of photosynthetic fractionation correlates with secular variation in (1) an independent O-isotope-based paleotemperature record and (2) δ13C of atmospheric CO2 for the Devonian, according to cross correlation analysis. This analysis also

13 reveals phase shifts of 0±4 Myr between Δ C‰(Atm-Plant) and paleotemperature, 2.5(-4,+3) Myr

13 13 between paleotemperature and δ CAtmopheric CO2, and 4.5±1 Myr between Δ C‰(Atm-Plant) and

13 δ CAtmopheric CO2. These relationships suggest that photosynthetic fractionation and paleotemperature are leading indicators of ancient climate change, and that the δ13C of atmospheric CO2 is modified more slowly in response to climate-induced changes in C-cycle fluxes and reservoir masses. Thus, comprehensive data of ancient Δ13C‰(Atm-Plant) could be a useful tool as an independent indicator of paleoclimate change.

Chapter 3 examines N-isotopic variation in Devonian plant fossils and their sediment matrices, and its relationship to C-isotopic variation in the study samples already documented in chapters 1 and 2. The Corg:N ratios of the plant fossils (40-400) show that they consist of compressed organic matter from higher land plants that has been little affected by bacterial decay.

In contrast, the Corg:N ratios of the sediment matrix (<10) show that its organic matter has been heavily bacterially reworked and is now dominated by bacterial biomass. The N-isotopic

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composition of the plant fossils shows only limited variation (mostly +2 to +6‰), a range of values that is typical of modern C3 plants. Although a large proportion of the matrix samples show a similar N-isotopic range, a significant subset of samples (n = 10, or ~10% of the total analyzed) yielded δ15N values that are <0‰. These light values are indicative of N fixation by soil cyanobacteria, and the present results are the earliest documented example of such bacteria in a paleosol environment. In contrast to δ13C (chapter 2), plant δ15N shows no coherent pattern of secular variation through the Devonian.

The present study demonstrates that a systematic analysis of stable carbon and nitrogen isotopic variation in Devonian plant fossils can yield substantial information concerning the ecological distribution of early land plants, paleoclimate, and soil microorganism communities.

Geochemical analysis of plant fossils is thus a useful tool for the study of early terrestrial environments.

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