Stable Isotopic and Molecular Signatures of Organic Biomarkers and Their Alteration During Thermal Maturation

Organic biomarkers are produced by living organisms and can be preserved in the sedimentary environment, providing a wealth of information about conditions during the period of biosynthesis and processes that alter original compositions during sedimentary burial. The geochemical signature of these compounds can thus be related to paleoclimate, post-depositional processes and hydrocarbon source or migration. In recent decades, advances in analytical techniques have enabled the analysis of novel compounds at trace concentrations and expanded the use of organic biomarkers for various disciplines. However, despite the rapid growth of the field of organic geochemistry in the last several decades, there are two fundamental challenges in the application of biomarkers for tracing compounds through the earth system. First, there is uncertainty in the factors that affect the production and distribution of distinct biomarkers or their isotopic signatures, and second, the original signatures generated during biosynthesis may change as the organic matter is buried and heated in the subsurface. This study focuses on evaluation and quantification of several distinct biomarkers that are found in the modern soil system and factors that affect their distribution or isotopic composition during the process of thermal maturation. Firstly, two classes of organic biomarkers that are commonly utilized to reconstruct past environmental conditions are evaluated along the southeast margin of the Tibetan Plateau to assess how these proxies record changes in elevation despite variations in ecosystem. This work specifically examines the variation in hydrogen isotope compositions of straight-chain normal alkanes (n-alkanes) and the distribution of branched glycerol dialkyl glycerol tetraethers (brGDGTs) along this spatial gradient to assess the fidelity of these signatures as a record of changes in the environmental conditions associated with modern elevation change. Secondly, while there is significant application of the organic biomarkers as a proxy for terrestrial geochemistry, the preservation of their initial geochemical signature during thermal maturation is examined. In this study, a series of heating experiments were conducted using pure n-alkane mixtures and natural soil organic extracts to quantitatively measure isotopic and molecular changes during cracking and exchanges associated with thermal alteration. Experiments include controlled heating of pure n-alkane mixtures and total lipid extract from a modern wetland sediment under both ambient air and anhydrous, closed-system conditions. Molecular distribution and isotopic compositions of carbon and hydrogen are monitored along the artificial heating process. Finally, kinetic parameters of the temperature dependent reactions are determined and new constraints are estimated on 1) molecular and isotopic alteration during thermal cracking and post-depositional exchange in n-alkanes, and 2) changes in the distribution of branched glycerol dialkyl glycerol tetraethers (and calculated temperature) during burial diagenesis. In total, these experiments will result in a more complete understanding of the preservation potential of primary isotopic and organic molecular signatures and enable more precise application of compound-specific stable isotopes and organic molecular distributions to a range of climatic, tectonic and hydrocarbon research questions.