PRESERVATION POTENTIAL OF SEDIMENTARY ORGANIC BIOMARKERS IN THE PRESENCE OF CLAY MINERALS

The preservation of primary organic molecular and isotopic signatures after sediment burial is critical for paleoclimate and petroleum research. However, organic matter may undergo numerous chemical transformations after deposition – the degree of transformation is based upon the reactivity of individual compounds, environmental conditions, and the presence of catalysts or substrates such as minerals. Here, we investigate leaf wax n-alkane distributions and isotopic changes during post-depositional thermal maturation of modern sedimentary organic matter in the presence of two clay minerals (kaolinite and montmorillonite). To do this, we used a series of laboratory pyrolysis experiments heating total lipid extract in the presence of clay minerals at temperatures of 100 - 300°C in closed, anhydrous vessels. We measured changes in molecular distributions of n-alkanes as well as their compound-specific hydrogen and carbon isotopic compositions (δD and δ¹³C, respectively). Results show that clay minerals can inhibit apparent changes in indices of molecular distribution such as average chain length (ACL) of n-alkanes, whereas significant decreases in carbon preference index (CPI) are observed. Shifts in distribution at higher temperatures coincide with an increase in total amount of n-alkanes, indicating a higher rate of secondary production than any decomposition rate that may be superimposed. Isotopic results show preservation of δD and δ¹³C below 200°C in the presence of clay minerals. Comparison of results with experiments in the absence of clays supports the notion that clays may preserve hydrophobic bituminous matter in a natural setting more so than catalyzing its decomposition. Differences in kinetics of reaction and secondary products highlight the importance of mineralogy on signal preservation and suggest that deposition in climates conducive to kaolinite or montmorillonite formation may impart differential artifacts in the sedimentary organic record.