Postdoctoral Fellowship: OCE-PRF: Reconstructing CO2 Levels for the Late Cretaceous through Paleocene using Sedimentary Compositions of Molecular and Isotopic Proxies

This project focuses on reconstruction of ancient atmospheric CO2 levels providing crucial evidence about past ‘greenhouse’ conditions that is critical to forecasting and modeling Earth’s future climate. This project investigates organic molecules derived from ancient organisms that are preserved in ocean sediments. The composition of these molecules records environmental information that includes evidence of CO2 levels enabling estimation of its atmospheric abundance through geological history. The study centers on examination of molecules in sediment cores recovered from the Transkei Basin off South Africa's coast that were collected during Expedition 392 of the International Ocean Discovery Program (IODP) in 2022. The sediments were deposited from 79 to 61 million years ago, when CO2 levels were much higher than today but also rising, so this time intervals provides an analog for Earth's future climate. Thus, the project will advance our understanding of past CO2 levels and seek to refine and expand the methods used for their determination. It also comprises initiatives designed to broaden scientific participation. PI-Doiron will engage K-6 students through collaboration with Centralia Elementary School District in Southern California and partner with South African colleagues to create K-12 outreach materials related to the findings from IODP Expedition 392. Doiron and her mentors will also lead a workshop to discuss the range of molecular and isotopic data that have proven useful for reconstructing past climates with the aim of educating members of the scientific community interested in Earth's climate history about these tools. Specifically, this research project will assess the comparative utility of different lipid biomarker classes as pCO2 proxies by examining the del13C values of phytane, pristane, alkenones, and glycerol dialkyl glycerol tetraethers (GDGTs). Phytane is a phytoplankton-associated pCO2 proxy assumed primarily to represent the phytol side chain of chlorophyll a, and thus records CO2 incorporated into biomass during photosynthesis. Yet, there are caveats associated with the direct application of this proxy in the sedimentary record because of the potential for phytane to derive from other sources and fulfill a different biochemical function. Specifically, archaeal lipids (e.g., diphytanyl ethers) can contribute to the phytane pool, and thereby compromise the phytane pCO2 signal. This proposal seeks to examine the veracity of the phytane proxy by obtaining multiple pCO2 records from other source-specific biomarkers in immature sediments. It will assess and compare the following components: (i) phytane, which is derived from breakdown of phytoplankton chlorophyll or from archaeal diethers; (ii) C37 alkenones, that are source-specific lipids of haptophyte algae; (iii) pristane, a breakdown product formed either from the phytyl side chain of chlorophyll a or from tocopherols; and (iv) GDGTs, which are membrane lipids in archaea. It will focus on Campanian to Paleocene sediments from the Transkei Basin recovered from IODP Expedition 392. In summary, this study can make a critical contribution toward enhancing the application of different biomarker-based pCO2 proxies and generating evidence for pCO2 concentrations during the Late Cretaceous through the Paleocene where data are sparse yet directly relevant to future climate scenarios. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.