Evolution of C4 Photosynthesis & Grasslands

The proposed study is to establish stable carbon isotope (δ^13C) records of terrestrially-derived floral biomarkers from globally dispersed ocean sites to determine the history of the Cenozoic expansion of the C4-photosynthetic pathway. The Cenozoic evolution of the C4-photosynthetic pathway was likely driven by specific environmental conditions and thus ultimately reflects a proxy record of global climate change. To date, the history of C4 photosynthesis has been evaluated from only specific terrestrial sites using the stable carbon isotopic analyses of fossil equid bioapatite (Cerling et al, 1997) and pedogenic carbonates (Quade et al, 1989; Latorre et al, 1997). Compilations of these d^13C records suggest that C4 grasses rapidly expanded during the late Miocene/Pliocene (Cerling et al, 1997). Given the advantages that C4 plants possess over C3 plants under low atmospheric CO2 partial pressure (pCO2) (see below), Cerling et al. (1997) argues that a large decline in pCO2 between 8-4 Ma forced an abrupt increase in C4 grasses in low latitudes and a decline in overall C3 floral success. A rise in C4 dominated grasslands at this time is probable, but it is far from certain that this event was caused by a decrease in pCO2. There are several lines of evidence that disavow a pCO2 control on C4 plant expansion during this time. Recent paleo-pCO2 records indicate that, during the Miocene, pCO2 steadily increased from ~180 parts per million by volume (ppmv) at 14 Ma to stabilize at concentrations between 250 and 320 ppmv during the late Miocene (Pagani et al, 1999); far below the 500 ppmv threshold concentration needed for the Hatch-Slack cycle (C4) to gain an adaptive advantage over the Calvin cycle (C3). Hence, C4 development should have occurred earlier than the late Miocene (Pagani et al, 1999; Pearson & Palmer, 2000). These pCO2 records further indicate that changes in carbon dioxide were not synchronous with C4 expansion, suggesting pCO2 was not the primary factor forcing ecological change (Pagani et al, 1999; Pearson & Palmer, 2000). Indeed, numerous studies have documented global changes in seasonality, evaporation, and aridity preceding and accompanying C4 expansion (Flynn & Jacobs, 1982; Quade et al, 1989; Cerling & Quade,1990; Latorre et al, 1997) and thus allow the potential that other factors controlled the expression of C4 grass expansion during the late Miocene. Finally, our preliminary compound-specific isotopic analyses of extracted organic compounds from ocean sediments indicate C4 flora constituted as much as 30% of available terrestrial organic carbon during the early Miocene (23.8-16.4 Ma), in agreement with recent results from Fox and Koch (2003), which demonstrated C4 plants composed between 12-34 % of total flora biomass throughout the Great Plains of North America during the early Miocene.

Approaching storm above the South African bushveld Site-specific records do not provide global or large-scale regional assessment of the influence of C4 photosynthesis on total terrestrial productivity. I propose an innovative study to determine the history and factors that controlled the development of the C4-photosynthetic pathway by establishing and coupling regional and global carbon isotopic records from terrestrially-derived organic matter entrained in marine sediments. Terrestrially-derived organic carbon transported to marginal and pelagic marine environments represents an integrated signal of terrestrial plant material on a continental-scale and thus has an advantage over evaluating isotopic compositions of soil carbonates and fossil mammal tooth enamel from individual localities (Quade et al, 1989; Cerling et al, 1997; Latorre et al, 1997). In this study, extracted organic matter from marine sediments from high and low latitude Deep Sea Drilling Project/Ocean Drilling Program (DSDP/ODP) sites will be used to establish the stable carbon isotopic composition (δ^13C) of terrestrially-derived, high-molecular weight n-alkanes (organic molecules derived primarily from higher-plants). Gas-chromatography coupled with isotope-ratio mass-spectrometry will be used to analyze δ^13C of individual n-alkanes. Low latitudinal sedimentary n-alkanes are characterized by both C3 and C4 contributions and thus represent an integrated δ^13C signature. High latitudes (>60^o) are virtually devoid of C4 floral inputs, therefore, the δ^13C record defines a pure C3 end-member. Further, the carbon isotopic composition of planktonic foraminifera from well-stratified ocean basins, as well as higher-plant organic matter (coalified and fossilized wood, etc) from marine sediments will be analyzed via duel-inlet mass-spectrometry and used to determine δ^13C of atmospheric carbon dioxide throughout the Cenozoic. δ^13C of floral material can then be determined from δ^13C of atmospheric CO2 and known carbon isotopic fractionation between atmospheric CO2 and plant biomass carbon. From this, I will determine relative proportions of C3/C4 flora on a global scale, and as a result, constrain the timing of Cenozoic C4-photosynthetic pathway evolution.