Alkenone-based sea surface salinity and temperature reconstructions across late Pliocene sapropels from the Eastern Mediterranean Sea

The Eastern Mediterranean sapropels were deposited during periods of increased monsoonal runoff, enhanced export productivity, salinity stratification and deep-sea anoxia since the Miocene. The most pronounced sapropels, with total organic carbon (TOC) contents up to 30%, formed during the late Pliocene (3.16-2.94 Ma). It remains enigmatic which environmental conditions caused the deposition of these high-TOC sapropels.

Here, we aim to record spatiotemporal changes in sea surface salinity (SSS) and sea surface temperature (SST) associated with sapropel formation. We use compound-specific hydrogen isotope analysis of C_37:2 alkenones (δ²H_alkenone) as a proxy for SSS. This proxy is, in contrast to δ¹⁸O_foram, independent of temperature and can be applied to organic-rich samples that lack foraminifera. In addition, we use alkenone-based Uk'37 to estimate SSTs. We compare the δ²H_alkenone signatures across the strongest late Pliocene sapropels from ODP sites 967 and 969 to that of a slightly younger, lower-TOC sapropel.

The δ²H_alkenone value in marls is comparable to modern-day Mediterranean δ²H_alkenone values and across the strongest sapropels, the δ²H_alkenone shift of 15-35‰ is comparable to that recorded in sapropel S5. This suggests that freshwater forcing was not exceptionally strong during the deposition of the late Pliocene sapropels. However, in the marls prior to the high-TOC sapropel at 2.95 Ma, δ²H_alkenone is ~20‰ more depleted and reconstructed SSTs are higher.

We discuss the salinity forcing on sapropel development and investigate if and how initial warmer and fresher surface waters could have affected the formation of this particularly high-TOC sapropel.