Multi-proxy constraints on sapropel formation during the late Pliocene of central Mediterranean (southwest Sicily)

- We discuss the formation of Pliocene sapropels in Sicily using a multi-proxy approach.
- Recorded increases in SST indicate that sapropels were formed under warmer conditions.
- Biomarkers/nannofossils show distinct trophic conditions for two sapropel series.
- The first is due to better organic matter preservation and thermohaline stratification.
- The second is due to enhanced productivity in a weakly-stratified water column.

The late Pliocene (Piacenzian) in the Mediterranean region was punctuated by short-lived episodes of widespread deposition of organic-rich sedimentary layers known as sapropels. The causes of their formation remain a long-standing debate in the science community, and require disentangling the roles of climatic/oceanographic processes that triggered higher primary productivity or enhanced organic matter preservation. The lack of data, especially of sea temperatures at sufficient temporal resolution, is one of the main challenges to solve this debate.Here, we present new organic geochemistry and micropaleontological data from the late Pliocene at Punta Grande/Punta Piccola sections (southwest Sicily) that allow untangling the mechanisms that favored the formation of two sapropel series (noted S and A) in the central Mediterranean area during this period. Sea surface (SSTs) and subsurface temperatures were estimated using three distinct organic geochemical proxies namely the alkenone unsaturation index (UK′37), the long-chain diol index (LDI) and the tetraether index (TEX86). Reconstructed SSTs are relatively stable throughout the late Pliocene and ∼4 °C higher than modern Mediterranean SSTs, which is consistent with the climatic conditions inferred for this period from paleoclimate modeling. An increase in SST is, however, recorded by UK′37 and LDI proxies across each sapropel horizon, supporting that the two sapropel series S and A were formed during warmer climate conditions. The comparison of SST data with variations in accumulation rates of total organic carbon and lipid-biomarkers (alkenones, long-chain alkyl diols, archaeal and bacterial tetraethers), and with changes in calcareous nannofossil assemblages, indicates that the studied sapropels might have formed under different environmental conditions. The first series of sapropels (S), deposited between 3.1 and 2.8 Ma, is likely due to a better preservation of organic matter, induced by the development of a strong thermohaline stratification of the water column and to oxygen-depleted bottom waters. Higher terrestrial input that occurred between 3.1 and 2.8 Ma may interestingly explain the large discrepancies observed between TEX86 and UK′37-LDI temperature values during this period. The second series of sapropels (A), deposited between 2.7 and 2.6 Ma, is more likely due to enhanced primary productivity in a weakly-stratified water column.