Development of global temperature and pH calibrations based on bacterial 3-hydroxy fatty acids in soils

Abstract

Abstract. Gram-negative bacteria produce specific membrane lipids, i.e. 3-hydroxy fatty acids with 10 to 18 C atoms. They have\nbeen recently proposed as temperature and pH proxies in terrestrial\nsettings. Nevertheless, the existing correlations between pH or temperature and\nindices derived from 3-OH FA distribution\nare based on a small soil dataset (ca.\xa070 samples) and only applicable\nregionally. The aim of this study was to investigate the applicability of\n3-OH FAs as mean annual air temperature (MAAT) and pH proxies at the global\nlevel. This was achieved using an extended soil dataset of 168 topsoils\ndistributed worldwide, covering a wide range of temperatures (5\nto 30\u2009∘C) and pH (3 to 8). The response of 3-OH FAs to temperature\nand pH was compared to that of established branched glycerol dialkyl glycerol tetraether (GDGT)-based proxies\n(MBT 5Me/CBT). Strong linear relationships between 3-OH-FA-derived\nindices (RAN15, RAN17 and RIAN) and MAAT or pH could only be\nobtained locally for some of the individual transects. This suggests that\nthese indices cannot be used as palaeoproxies at the global scale using\nsimple linear regression models, in contrast with the MBT 5Me and CBT.\nHowever, strong global correlations between 3-OH FA relative abundances and\nMAAT or pH were shown by using other algorithms (multiple linear regression,\nk-NN and random forest models). The applicability of the three\naforementioned models for palaeotemperature reconstruction was tested and\ncompared with the MAAT record from a Chinese speleothem. The calibration\nbased on the random forest model appeared to be the most robust. It\ngenerally showed similar trends with previously available records and\nhighlighted known climatic events poorly visible when using local 3-OH FA\ncalibrations. Altogether, these results demonstrate the potential of 3-OH\nFAs as palaeoproxies in terrestrial settings.\n