oa Hopanoid Distributions Differ in Mineral Soils and Peat: A Re-Evaluation of Hopane-Based PH Proxies

Hopanoids are predominantly produced by bacteria and commonly found in terrestrial and marine environments. In modern environments, hopanoids mostly occur in the biological 17ß,21ß(H) configuration. Over geological time (106 to 108 years), thermal degradation changes their stereochemistry to the mature 17?,21ß(H) configuration. However, in modern acidic peat-forming environments, the ‘thermally mature’ C31 17a,21ß(H)-homohopane dominates over the biological ßß stereoisomer, with an increase in the relative abundance of the aß 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 remains 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. In our soil dataset, there are relatively few thermally mature aß hopanes – even within acidic mineral soils – and there is no relationship between hopane isomerisation ratios and pH. This suggests that the C31 hopane ßß/(aß + ßß) should only be employed as a quantitative pH proxy in peats.