Refining the global branched glycerol dialkyl glycerol tetraether (brGDGT) soil temperature calibration
Reason for embargo
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: MAAT soil = 40.01 x MBT’5me − 15.25 (n=350, R2 22 = 23 0.60, RMSE = 5.3 °C) and growing degree days above freezing (GDD0 soil) = 14344.3 x MBT’5me - 4997.5 (n=350, R2 24 = 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 x !"#!!" ! − 14.50 (n=177, R2 30 = 0.76, RMSE = 4.1 °C) and GDD0 soil’ = 13498.8 x !"#!!" ! − 4444.5 (n=177, R2 31 = 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 rang
This research was funded through the advanced ERC grant `The greenhouse earth 412 system' (T-GRES, project reference 340923). R.D.P. acknowledges the Royal Society 413 Wolfson Research Merit Award.
This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this record.
Available online 10 February 2017