An experimental field study to test the stability of lipids used for the TEX86 and U37K′ palaeothermometers

We report a one year field experiment to study sedimentary degradation of isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs) and long-chain alkenones, lipids used for the TEX86 and U37K′ sea surface temperature (SST) proxies, respectively. Homogenised surface sediments from the Namibian upwelling region were set out on moorings in oxic (Oder Bay and Arkona Bay; Baltic Sea) and sub-oxic to anoxic (Gottland Basin; Baltic Sea) waters and in oxic Atlantic waters off Cape Blanc for one year to assess the influence of oxygen exposure on GDGTs and alkenones. No selective degradation effect on both TEX86 and U37K′ proxies was evident under anoxic and sub-oxic conditions. Under oxic conditions, GDGT concentrations and the TEX86 did not significantly change as well. However, selective degradation of alkenones occurred at all oxic sites resulting in a small but significant increase of the U37K′ index by up to 0.06 units, corresponding to a temperature increase of up to 1.8 °C. Our results demonstrate that selective alkenone degradation is a relatively fast process and complement earlier evidence for selective degradation of alkenones in the laboratory, water column and sediments. In contrast, our results suggest that the TEX86 is relatively resistant to selective oxic degradation at least on relatively short (years) time scales.