The preservation of organic matter as biosignatures over geologically long timescales is strongly affected by interactions that occur between fluids, organic species, and minerals during diagenetic processes. We seek to better understand how mineral surfaces affect the degradation of organic species in these processes by investigating the reaction of the sugar model cis-1,2-cyclohexanediol at 200°C in water both with and without the presence of the mineral goethite (α-FeOOH). The temperature was chosen to represent plausible diagenetic conditions that organic compounds can encounter. We determined after 264 hours in water alone, that 85.2 % and 5.8 % of the original cis-1,2-cyclohexanediol dehydrated respectively into cyclohexanone and cyclopentanecarboaldehyde. However, when a loading of 20 g/L goethite was added, only 32.6 % and 0.6 % of the cis-1,2-cyclohexanediol dehydrated to respectively form cyclohexanone and cyclopentanecarboaldehyde over the same timespan. The addition of goethite to water increased the pH and, therefore, might be expected to inhibit the dehydration of cis-1,2-cyclohexanediol; however, we conducted pH-adjusted experiments that determined the fluid pH exerts a limited effect on the amount of dehydration that occurs in the presence of goethite. We hypothesize it is more likely that the goethite surface inhibits the dehydration pathway of cis-1,2-cyclohexanediol through surface protonation. The ability of a mineral surface to inhibit reactions under diagenetic temperature underscores the importance of minerals in the process of preserving molecular biosignatures in the geologic record.

Goethite surface inhibition of the dehydration of cis-1,2-cyclohexanediol at 200°C

2019 Spring National Meeting

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