STABLE ISOTOPIC AND MOLECULAR ALTERATION OF ORGANIC BIOMARKERS IN SOIL DURING SIMULATED DIAGENESIS: IMPACT OF VARYING MOISTURE, TEMPERATURE, ATMOSPHERE OXYGEN AND LIGHT

Organic molecular and stable isotopic records are increasingly utilized to understand terrestrial climatic and tectonic evolution and have become a key component of the library of geochemical paleoenvironmental proxies. While there is considerable research on the process of thermal cracking of organic molecules, there is still a degree of uncertainty over how C and H isotopes of commonly utilized biomarkers change as a function of increased temperature, pressure and water availability during burial diagenesis. This study focused on evaluating carbon and hydrogen isotopic alteration of normal alkanes during simulated diagenesis to understand how the pressure-temperature pathways common to sediments from tectonically active sedimentary basins impact organic molecular records.

We conducted artificial maturation experiments under hydrous and anhydrous conditions over a range of temperatures and pressures consistent with shallow burial diagenesis (25-350 °C) to develop new constraints on the potential for isotopic alteration during thermal cracking and post-depositional exchange in n-alkanes and n-alkanoic acids. Consistent with prior work, our results show that the distribution of n-alkanes dramatically change above ~150 °C, with a reduction in carbon preference index from 10.8 to 1.6. However, despite a shift in compound distributions, the isotopic compositions of hydrogen and carbon of target molecules are stable until ~200 °C in an anhydrous environment. Hydrous experiments show the production of secondary organic molecules that are exchangeable to H and co-elute with long-carbon chain n-alkanes (e.g. n-C29). These data provide new insight into potential alteration of primary isotopic signatures of normal alkanes that are preserved in sedimentary basins and can aid in reconstruction of paleoenvironments.