Refining the global branched glycerol dialkyl glycerol tetraether (brGDGT) soil temperature calibration

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.