Further Test for Stable Carbon Isotopic Fractionation of Biomarkers during Diagenetic Processes and Examination of Possible Causes

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ABSTRACTOCE-0526111The stable carbon isotopic composition of biomarkers has been widely used to study global carbon cycling and the paleoceanographic record. Among specialists, however, there has been substantial disagreement regarding the impact of diagenetic processes on the carbon isotopic signatures of sedimentary biomarkers. Assumptions abound, but there have been few actual studies to determine how and to what extent the carbon isotopic composition of biomarkers derived from terrestrial higher plants can change during diagenesis. In this study, researchers at the University of Georgia will attempt to assess and to identify the major processes by which the C isotopic composition of plant-derived biomarkers changes during sedimentary diagenesis. There are two primary objectives. The first is to study stable carbon fractionation of various lignin-derived phenol biomarkers from C3 and C4 vascular plants during degradation by incubating natural plant materials in different environmental regimes. The second goal is to test two specific hypotheses related to isotopic fraction mechanisms: (1) that the intrinsic carbon fractionation at a reaction site of a molecule may be kinetics-dependent and responsible for the most of molecular isotopic alteration during degradation, but the heterogeneous 13C distributions within molecules (between reaction and non-reaction sites) may significantly affect the molecular isotopic fractionation; and (2) that the same biomarker molecules bound in different intracellular components (e.g., cell membrane vs. metabolic storage tissue) may carry distinct carbon isotopic compositions because they are biosynthesized at different growth stages, which may result in molecular fractionation when they degrade at different rates and through different pathways.This research could have a number of broader impacts in the fields of organic geochemistry, chemical sedimentology and paleoceanography. Results from this study could potentially set a new criterion for evaluating the applicability of aquatic and terrestrial plant-derived biomarkers and their isotopic compositions in carbon cycling and sedimentary record. Comprehensive understanding for current cycling and past record will help the scientific community to predict the trend of global climate change. The project will also provide for the support and training of a postdoctoral associate and of graduate and undergraduate students. The principal investigator also plans to involve summer high-school interns in this research program.