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 by 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. ii iii 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. v 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) vi 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. 1 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. 2 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, Tetraxylopteris, Archaeopteris, and Genselia) revealed stable carbon isotopic (δ13C) variation ranging from -30 to -20‰, with a distribution