Global Temperature and pH Calibrations Based on Bacterial 3-HYDROXY Fatty Acids in Soils

Summary 3-hydroxy fatty acids (3-OH FA), produced by Gram-negative bacteria, were recently proposed as promising temperature and pH proxies in terrestrial settings. Nevertheless, the existing correlations between pH/temperature and indices derived from 3-OH FA distribution are based on a small soil dataset (ca. 70 samples) only applicable regionally. The aim of this study was to investigate the applicability of 3-OH FAs as mean annual air temperature (MAAT) and pH proxies at the global level using an extended dataset of 168 surficial soils from 8 globally distributed elevational transects. Strong linear relationships between 3-OH FA-derived indices and MAAT/pH could only be obtained locally, for some of the individual transects. In addition to linear regressions, non-parametric, machine learning models were used to improve the global relationships between 3-OH FA distribution and MAAT/pH. Strong global correlations between MAAT/pH and 3-OH FA relative abundances were obtained by using multiple linear regression, k-NN and random forest models. The applicability of the k-NN and random forest models for paleotemperature reconstruction was tested with the MAAT record from a Chinese speleothem. The calibration based on the random forest model appeared to be the most robust. These results demonstrate the potential of 3-OH FAs as paleoproxies in terrestrial settings.