Spatial and Temporal Variability in Branched Glycerol Diakyl Glycerol Tetraethers (brGDGTs) in a Varved Tropical Lake System: Implications for High-Resolution Paleotemperature Reconstructions

Abstract:

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) provide a potentially valuable tool for reconstructing paleotemperature variations in small to medium sized lake systems. However, empirical lake surface sediment calibrations of the brGDGT-temperature relationship yield uncertainties of several degrees, which limits the application of this approach to orbital and millennial scale reconstructions. Little has been done to assess the relative importance of within-lake processes on the variability in brGDGT climate reconstructions and uncertainties in mean annual air temperature estimates. To address this, we examined brGDGT variations in the surface sediments and in a high-resolution varved sediment sequence from anoxic Lake Bosumtwi, Ghana. Lake surface sediment samples show significant variations in brGDGT distributions in shallow water, but yield temperature estimates that are generally consistent with measured lake water temperatures in sediments from throughout the deepest part of the lake. Down-core temperature reconstructions show multidecadal to centennial scale trends that are broadly consistent with a cooler Little Ice Age and recent warming. However, on shorter timescales reconstructed temperature variations are much larger than expected (1-2°C) based on instrumental air temperature data, complicating the interpretation of the brGDGT temperature data. We hypothesize that these variations are related to within-lake processes that affect brGDGT production such as the depth and timing of mixing events.