Soil and Air Temperature Calibrations Using Branched GDGTs for the Tropical Andes of Colombia: Towards a Pantropical Calibration.

Abstract:

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are bacterial cell membrane lipids that, when preserved in sedimentary archives, can be used to infer continental paleotemperatures. Although commonly used global calibrations capture a relationship between the distribution of brGDGTs and temperature, they underestimate temperatures for tropical regions as much as ~16 °C. Furthermore, some global calibrations reach saturation at around 24-25 °C, and, in general, they have root-mean-squared errors (RMSEs = 3.8-4.9 °C) that are too large for them to resolve the small variations in paleoclimate that are typical in tropical regions. We present an in situ regional calibration of soil brGDGTs along altitudinal transects on both flanks of the Eastern Cordillera of Colombia in the northern tropical Andes that spans ~3200 m, 17 °C in mean annual soil temperatures, and 19 °C in mean annual air temperatures. We use the Bayesian Information Criterion (BIC) to obtain these calibrations, which favors parsimony: small uncertainty in temperature with few brGDGTs. These new soil and air regional calibrations yield root-mean-square errors of 1.5 °C and 1.9 °C, respectively. In order to improve temperature reconstructions within the tropics, we combined the Eastern Cordillera of Colombia results with existing data from elsewhere in the tropics (23.5°). The integrated data (n = 175) fit a linear calibration with a RMSE of 2.7 °C, and a nonlinear calibration with a RMSE of 2.2 °C. These calibrations allow for a more precise and reliable reconstruction of past temperatures in the tropics than global calibrations.