Calibrating branched GDGTs in bones to temperature and precipitation: Application to Alaska chronological sequences

Our recent study suggests that branched glycerol dialkyl glycerol tetraethers (brGDGTs) in ancient bones may have great potential for reconstructing in-situ paleoclimate conditions. However, one major problem with reconstructing past climate and environmental conditions using brGDGTs in ancient bones is the lack of calibrations specific to bones. Here we investigate the distribution of brGDGTs in 191 modern weathered bone samples in China with a broad range of mean annual air temperature (MAAT: −3.3-25.6 °C) and mean annual precipitation (MAP: 163-2062 mm), and present the first bone-specific brGDGT temperature and precipitation calibrations. We find that the best correlation with temperature is found in bones with a higher ratio of 6-methyl over 5-methyl brGDGT isomers, suggesting a preferential production of 6-methyl brGDGTs by bacteria inside bones. The bone-specific temperature relationship is distinct from published mineral soil calibrations (e.g., with virtually no influence from soil pH changes). We hypothesize that the lower calibration residual values result from more homogenous and semi-closed environment for bacterial growth inside the bones than soil environments. We also demonstrate that, in semi-arid and semi-humid regions (MAP: 200-710 mm), brGDGT distributions in cancellous bones with relatively high percentages of 6-methyl isomers are strongly correlated with mean annual precipitation. Application of our bone-specific brGDGT calibrations to Alaskan chronological sequences yields new insights to high latitude temperature and precipitation variations for the past 30,000 years.