Abstract
The distribution of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in soils, peats, and lake sediments has been shown to correlate with mean annual air temperature (MAAT) and has provided valuable new climate reconstructions. Here we use an improved chromatographic method to quantify the fractional abundances of 5- and 6-methyl isomers in surface sediments from 65 East African lakes spanning temperatures 1.6–26.8 °C, and investigate the relationships between these fractional abundances and temperature, lake pH, and other environmental variables. We find that temperature exerts a strong control on brGDGT distributions, including the relative abundances of 5- and 6-methyl isomers, whereas other environmental variables, including lake pH, are weakly correlated to the fractional abundances of the brGDGTs. The distributions of brGDGTs in our lake sediments differ from those of soils and peats, leading to temperature offsets if soil- and peat-based brGDGT temperature calibrations are applied. We develop new calibrations for MAAT for use in lake sediment based upon the MBT′5Me and Index 1 ratios, as well as a multivariate regression of brGDGT fractional abundances on temperature using stepwise forward selection. We obtain root mean square errors (RMSE) between ∼ 2.1 and 2.5 °C for these calibrations, highlighting the potential for brGDGTs to provide precise temperature reconstructions using lake sediment cores. Calibrations for lake pH perform more poorly, likely due to weak correlations between pH and brGDGT distributions in East African lakes. These results indicate that quantification of 5- and 6-methyl isomers separately in lake sediment can improve paleoclimatic reconstructions.
Highlights
Paleoclimate reconstructions provide fundamental insight into the mechanisms governing long-term climate change, and constitute the sole test-bed to investigate the ability of global climate models (GCMs) to simulate climate under boundary conditions that differ strongly from the present (Braconnot et al, 2012; Otto-Bliesner et al, 2014)
We developed new calibrations for mean annual air temperature (MAAT) and pH based upon MBT’, MBT05ME, Index 1, CBT0, and CBT05ME by regressing these indices from our lake sediment data on observed MAAT and lake water pH (SW and BW), and compare the calibrations’ error statistics (RMSE, r2, etc.) to those from calibrations based on global soils, peat, and previously published East African lake calibrations
This contrasts with soils in which the abundances of 6-methyl isomers are strongly correlated to pH, suggesting that the bacteria producing brGDGTs respond differently to temperature and chemical gradients in soils vs lakes
Summary
Paleoclimate reconstructions provide fundamental insight into the mechanisms governing long-term climate change, and constitute the sole test-bed to investigate the ability of global climate models (GCMs) to simulate climate under boundary conditions that differ strongly from the present (Braconnot et al, 2012; Otto-Bliesner et al, 2014). Ies for past temperature that have sufficient sensitivity and precision to provide robust tests of GCM simulations. This is the case in the tropics, where traditional temperature proxies such as tree-rings and ice core datasets are rarely available.
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