Carbon isotope ratio of Cenozoic CO2: A comparative evaluation of available geochemical proxies

The carbon isotope ratio (delta C-13) of plant material is commonly used to reconstruct the relative distribution of C-3 and C-4 plants in ancient ecosystems. However, such estimates depend on the delta C-13 of atmospheric CO2 (delta C-13(CO2)) at the time, which likely varied throughout Earth history. For this study, we use benthic and planktonic delta C-13 and delta O-18 records to reconstruct a long-term record of Cenozoic delta C-13(CO2). Confidence intervals for delta C-13(CO2) values are assigned after careful consideration of equilibrium and non-equilibrium isotope effects and processes, as well as resolution of the data. We find that benthic foraminifera better constrain delta C-13(CO2) compared to planktonic foraminiferal records, which are influenced by photosymbiotes, depth of production, seasonal variability, and preservation. Furthermore, sensitivity analyses designed to quantify the effects of temperature uncertainty and diagenesis on benthic foraminifera delta C-13 and delta O-18 values indicate that these factors act to offset one another. Our reconstruction suggests that Cenozoic delta C-13(CO2) averaged -6.1 +/- 0.6 parts per thousand (1 sigma), while only 11.2 million of the last 65.5 million years correspond to the pre-Industrial value of -6.5% (with 90% confidence). Here delta(13)C(CO2)also displays significant variations throughout the record, at times departing from the pre-Industrial value by more than 2%. Thus, the observed variability in delta C-13(CO2) should be considered in isotopic reconstructions of ancient terrestrial-plant ecosystems, especially during the Late and Middle Miocene, times of presumed C-4 grassland expansion.