Membrane tetraether lipids of planktonic Crenarchaeota in Pliocene sapropels of the eastern Mediterranean Sea

The distribution of glycerol dibiphytanyl glycerol tetraethers (GDGTs), predominantly derived from planktonic Crenarchaeota, were studied in three age-equivalent Pliocene sapropels as well as in the homogeneous intervals, below and above the sapropel. In both the homogeneous intervals and sapropels the dominance of GDGT-0 and crenarchaeol among the GDGTs indicated their origin from planktonic Crenarchaeota as it was found in many other marine sediments. In the homogeneous intervals highest GDGT abundances (normalized to TOC) were measured at the easternmost study Site 967 of the eastern Mediterranean Sea and decreased at the central and western Sites 969 and 964. Within the three studied sapropels large variations in GDGT abundance were observed. The newly established sea surface temperature (SST) proxy, the TEX86 index, derived from GDGTs of planktonic Crenarchaeota was used to estimate past SST. Comparison with previous obtained SST data using the alkenones from haptophyte algae revealed substantial differences. Whereas the U37K′-based SSTs were almost constant at ca. 25 °C in both the homogeneous intervals and the sapropels, the TEX86-based SSTs were 26–29 °C in the homogeneous intervals and decreased to 15–17 °C in the sapropels. The TEX86-based SST values outside the sapropel probably reflect summer SST based on a comparison with the present-day Mediterranean Sea. The surprisingly low TEX86-based SST estimates for the sapropels showed similarities with those obtained for the contemporary euxinic Black Sea. The distribution of marine Crenarchaeota in the modern Black Sea, i.e. thriving at the deeper and colder chemocline, suggests that this leads to a reduction in TEX86, resulting in artificially low SST estimates. A similar situation is inferred for the Pliocene eastern Mediterranean during sapropel deposition since the SST anomaly co-occurs with the build-up of a shallow chemocline.