Hopanoid distributions differ in mineral soils and peat: a re-evaluation of hopane-based pH proxies

Abstract

Hopanoids are produced by bacteria and are commonly found in terrestrial and marine environments. In modern environments, hopanoids mostly occur in the biological 17β,21β(H) configuration. Over geological time (106 to 108 years), thermal degradation changes their stereochemistry to the thermally mature 17α,21β(H) configuration. However, in modern acidic peat-forming environments, the ‘thermally mature’ C31 17α,21β(H)-homohopane dominates over the biological ββ stereoisomer, with an increase in the relative abundance of the αβ stereoisomer at lower pH. Based on this pH dependency, hopane isomerisation ratios have been used to reconstruct pH in ancient peat-forming environments. However, the environmental controls on hopane isomerisation remain poorly constrained and it is unclear whether this proxy is also applicable in mineral soils. Here, we analysed hopane distributions in mineral soils characterised by a wide range of mean annual temperature and pH. We show that mineral soils are dominated by diploptene, an unsaturated C30 hopanoid synthesised by a wide range of bacteria. In our soil dataset, there are relatively few thermally mature αβ hopanes – even within acidic mineral soils – and there is no relationship between hopane isomerisation ratios and pH. We propose that mineral protection in these soil environments selectively protects hopanoids from rapid degradation and subsequent isomerisation in modern samples. This provides a plausible explanation for the lack of 17α,21β hopanes in modern acidic mineral soil and suggests that the C31 hopane ββ/(αβ + ββ) should only be employed as a quantitative pH proxy in peats. Moving forward, we propose that hopane isomerisation ratios can help fingerprint the delivery of (acidic) peat into the marine realm and build upon other biomarker-based proxies developed to trace the input of terrestrial OC into the marine realm.

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ORCID for Gordon N. Inglis: orcid.org/0000-0002-0032-4668

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