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. Gram-negative bacteria produce specific membrane lipids, i.e. 3-hydroxy fatty acids with 10 to 18 C atoms. They havebeen recently proposed as temperature and pH proxies in terrestrialsettings. Nevertheless, the existing correlations between pH or temperature andindices derived from 3-OH FA distributionare based on a small soil dataset (ca. 70 samples) and only applicableregionally. The aim of this study was to investigate the applicability of3-OH FAs as mean annual air temperature (MAAT) and pH proxies at the globallevel. This was achieved using an extended soil dataset of 168 topsoilsdistributed worldwide, covering a wide range of temperatures (5to 30 ∘C) and pH (3 to 8). The response of 3-OH FAs to temperatureand pH was compared to that of established branched glycerol dialkyl glycerol tetraether (GDGT)-based proxies(MBT'5Me/CBT). Strong linear relationships between 3-OH-FA-derivedindices (RAN15, RAN17 and RIAN) and MAAT or pH could only beobtained locally for some of the individual transects. This suggests thatthese indices cannot be used as palaeoproxies at the global scale usingsimple linear regression models, in contrast with the MBT'5Me and CBT.However, strong global correlations between 3-OH FA relative abundances andMAAT or pH were shown by using other algorithms (multiple linear regression,k-NN and random forest models). The applicability of the threeaforementioned models for palaeotemperature reconstruction was tested andcompared with the MAAT record from a Chinese speleothem. The calibrationbased on the random forest model appeared to be the most robust. Itgenerally showed similar trends with previously available records andhighlighted known climatic events poorly visible when using local 3-OH FAcalibrations. Altogether, these results demonstrate the potential of 3-OHFAs as palaeoproxies in terrestrial settings.

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