Hydrogen isotope and molecular alteration of n-alkanes during heating in open and closed systems

We conducted a series of heating experiments using pure n-alkane mixtures and natural soil organic extracts to evaluate isotopic and molecular changes during cracking and exchange associated with thermal alteration. Experiments included controlled heating of pure n-alkane mixtures (C-7-C-40) and a total lipid extract from a modern wetland sediment under both ambient air and anhydrous, closed-system conditions. In an argon-purged, closed-system (at temperatures of 60-300 degrees C), the distribution of pure n-alkane mixtures predictably shifted towards a higher proportion of shorter chain lengths (< C-20), with no odd-over-even preference. During the heating process, the delta H-2 values of long-chain n-alkanes (> C-20) became H-2-enriched by up to 12%, while the delta H-2 of short chain n-alkanes were H-2-depleted by up to 28%. When repeating these experiments with the total lipid extract from soils, we observed H-2-enrichment of individual n-alkanes and mean-weighted delta H-2(nC27-31) of soil n-alkanes under both ambient air (up to 15% at 300 degrees C over 72 h) and Ar-purged, anhydrous, closed-system (up to 10% at 300 degrees C over 720 h) conditions. Carbon preference indices (CPI) decreased during the thermal alteration process in tandem with isotopic changes. The average chain length (ACL) of the n-alkanes decreased by more than 1 unit during heating, even at temperatures as low as 60 degrees C, when in the ambient air. This highlights the potential for differences in sediment lithology and gas permeability on an outcrop scale to alter primary organic signatures during thermal alteration. In the presence or absence of oxygen, the hydrogen isotopic composition of pure n-alkane mixtures and natural extracts change from the original composition, though for most compounds this was less than 10-15% at temperatures below 150 degrees C. The magnitude of change is greatest for shorter chain n-alkanes and at temperatures above 150 degrees C. We conclude that degradation and/or production during heating, such as may be expected during sediment burial or contact heating, could drive shifts in the measured hydrogen isotope composition of sedimentary leaf wax biomarkers. This is especially rapid at temperatures in excess of 200 degrees C, but the magnitude of isotopic change is generally in the range 10-15% before the loss of compounds reaches levels that are below analytical relevance for isotope work. Collectively, our results highlight the need for careful consideration of the depositional environment (particularly for information pertaining to oxygen availability) when interpreting molecular and/or isotopic information from sedimentary alkanes. In addition, they also demonstrate the general fidelity of hydrogen isotopes of n-alkanes under shallow burial temperatures with only modest isotopic enrichment prior to complete loss of long carbon chain compounds. (C) 2017 Elsevier Ltd. All rights reserved.