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

Abstract

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 (C7–C40) 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°C), the distribution of pure n-alkane mixtures predictably shifted towards a higher proportion of shorter chain lengths (< C20), with no odd-over-even preference. During the heating process, the δ2H values of long-chain n-alkanes (> C20) became 2H-enriched by up to 12‰, while the δ2H of short chain n-alkanes were 2H-depleted by up to 28‰. When repeating these experiments with the total lipid extract from soils, we observed 2H-enrichment of individual n-alkanes and mean-weighted δ2HnC27–31 of soil n-alkanes under both ambient air (up to 15‰ at 300°C over 72h) and Ar-purged, anhydrous, closed-system (up to 10‰ at 300°C over 720h) 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°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°C. The magnitude of change is greatest for shorter chain n-alkanes and at temperatures above 150°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°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.