University of Illinois Urbana-Champaign

Evolution and ecology of C4 grasses: insights from carbon isotope analysis of grass pollen grains

Urban, Michael Allen

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URBAN-DISSERTATION-2015.pdf

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https://hdl.handle.net/2142/89020

Description

Title
Evolution and ecology of C4 grasses: insights from carbon isotope analysis of grass pollen grains
Author(s)
Urban, Michael Allen
Issue Date
2015-12-01
Director of Research (if dissertation) or Advisor (if thesis)
Hu, Feng Sheng
Doctoral Committee Chair(s)
Hu, Feng Sheng
Committee Member(s)
  • Johnson, Thomas M
  • Leakey, Andrew
  • Marcot, Jonathan
Department of Study
School of Integrative Biology
Discipline
Ecology, Evolultion, & Conservation Biology
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Date of Ingest
2016-03-02T19:34:01Z
Keyword(s)
  • Paleoecology
  • C4 grass
  • Miocene
  • stable carbon isotopes
  • moving wire
Abstract
C4 grasses account for >25% of global primary productivity and dominate tropical, subtropical, and warm-temperate grassland ecosystems. Because of limitations of conventional morphological and geochemical proxies, identification of C4 grasses in the fossil record is difficult. Thus, testing long-standing hypothesis regarding the patterns and drivers of the origins and expansion of C4 grasses has not been possible. I addressed this limitation by utilizing Single Pollen Isotope Ratio AnaLysis (SPIRAL), a technique that reliably distinguishes C4 from C3 grasses in the fossil record via δ13C values of individual pollen grains. Before applying the technique to fossil samples, I refined the original SPIRAL method by expanding its training dataset and developing a hierarchical Bayesian model that increases precision of C4 grass estimates. I applied the refined technique to samples from the Oligocene and Miocene in Western Europe, a region of the world with no previous documented evidence of C4 grasses, yet with climatic conditions similar to regions where C4 grasses are common today. In Oligocene samples from France, I found that >70% of total grasses were C4. During the Miocene, SPIRAL results show 21-71% of total grasses from a site in Spain were C4. Thus C4 grasses were present in Western Europe as early as the earliest Oligocene, ~14 million years earlier than previous isotopic evidence of the first C4 plants. The Early Oligocene was also a period with extremely high pCO2, suggesting the precipitous drop in pCO2 by the end of the Oligocene did not trigger the appearance of C4 photosynthesis. The relatively high proportion of C4 grasses during the early Oligocene to middle Miocene was likely facilitated by warm and dry climatic conditions. This interpretation is further supported by spatial variability in C4 grass abundance in Spain during the early to middle Miocene. C4 grasses were abundant in southern Spain during the early Miocene, when aridity was high, and decreased after the early-middle Miocene as the region became cooler and precipitation patterns more seasonal. This pattern contrasts with the late-Miocene expansion of C4 grasses in many other regions of the world, highlighting spatial variability in the global expansion of C4 dominated grasslands. My results help elucidate the development of grasslands in Europe, provide information on the spatiotemporal dynamics of C4 grasses in earth's history, and offer insight into the relative drivers of C4 grass expansion.
Graduation Semester
2015-12
Type of Resource
text
Permalink
http://hdl.handle.net/2142/89020
Copyright and License Information
Copyright 2015 Michael Urban

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Graduate Theses and Dissertations at Illinois
Dissertations and Theses - Ecology, Evolution, and Conservation Biology