Abstract
Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are increasingly used to reconstruct past terrestrial temperature and soil pH. Here we compare all available modern soil brGDGT data (n=350) to a wide range of environmental parameters to obtain new global temperature calibrations.We show that soil moisture index (MI), a modeled parameter that also takes potential evapotranspiration into account, is correlated to the 6-methyl brGDGT distribution but does not significantly control the distribution of 5-methyl brGDGTs. Instead, temperature remains the primary control on 5-methyl brGDGTs. We propose the following global calibrations: MAATsoil=40.01×MBT5me′−15.25 (n=350, R2=0.60, RMSE=5.3°C) and growing degree days above freezing (GDD0 soil)=14344.3×MBT5me′−4997.5 (n=350, R2=0.63, RMSE=1779°C).Recent studies have suggested that factors other than temperature can impact arid and/or alkaline soils dominated by 6-methyl brGDGTs. As such, we develop new global temperature calibrations using samples dominated by 5-methyl brGDGTs only (IR6me<0.5). These new calibrations have significantly improved correlation coefficients and lower root mean square errors (RMSE) compared to the global calibrations: MAATsoil′=39.09×MBT5me′−14.50 (n=177, R2=0.76, RMSE=4.1°C) and GDD0 soil′=13498.8×MBT5me′−4444.5 (n=177, R2=0.78, RMSE=1326). We suggest that these new calibrations should be used to reconstruct terrestrial climate in the geological past; however, care should be taken when employing these calibrations outside the modern calibration range.
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