Alkenones, as tool for reconstructing surface water temperature and salinity changesduring the Miocene-Pliocene transition in the northern Mediterranean basin

Alkenones are unsaturated long-chained ketones produced by haptophyte algae living in the photic zone of the water column. In cultures of Emiliania huxleyi, the alkenones’ degree of unsaturation between C₃₇ alkenone isomers (UK37’) is proportional to thetemperature of the water in which this haptophyte lives. Besides the UK37’ providing a paleotemperature proxy in the marine realm, the identification of long-chain alkenones in modern lacustrine waters and sediments allowed the recognition of non-marinehaptophyte algae able to produce alkenones. At different salinities, the haptophytes produce a different measure of alkenones molecules in particular: the abundance of C_37:4 against the other C₃₇ isomers; the different C_38Et/C_38Me ratio, and the differentratio between ∑C₃₇/∑C₃₈ isomers.

One of the best laboratories to explore and extend the usage of alkenones as a tool for reconstructing changes in sea surface temperature (SST) and salinity (SSS) is the Mediterranean domain during the Miocene - Pliocene, a boundary marked by the abrupt transition from uppermost Messinian brackish and shallow waters of the Lago-Mare event (5.55-5.33 Ma) to lower Pliocene open marine (Zanclean; 5.33 Ma). Here, we will further address the challenge of using alkenones coupled with a novel combination of commonly used proxies to reconstruct the highly varying water surface condition affecting the Mediterranean Sea during the Miocene to Pliocene transition. Therefore, we studied three time-equivalent sections from the northern part of the basin.

The integration of micropaleontological data with inorganic and organic geochemical ones allowed to evaluate of the changes of the alkenones producers across the Miocene to Pliocene transition and to test the reliability of the C₃₇/C₃₈ ratio, the C_37:4% andC_38Et/C_38Me ratio as paleoenvironmental proxies.

Our results indicate that alkenone-based proxies can be confidently used for tracing SSS changes at the Miocene-Pliocene transition. Their reliability is supported by comparison with the micropaleontological assemblages (body fossils). In particular, the Pliocene sediments are dominated by alkenone marine producers, which records limited SSSs variations, confirmed by the abundant anddiversified calcareous plankton assemblage. For the marine Pliocene, our calculated SSTs (average 20 °C) and SSSs (35-38 PSU) are in the range of the values for that time.

In contrast, for the Lago Mare phase, the scarcity or absence of in situ body fossils of marine calcareous plankton and the mixing of freshwater and marine alkenones producers suggest more significant salinity fluctuations, with a general increase in salinity approaching the Miocene-Pliocene boundary. For the Lago Mare event, our reconstructed SSTs are in the range of those in Pliocene, and the reconstructed salinities for the latestMiocene samples (33 to 36 PSU) are not so different from the ones in Pliocene. Although the presence of marine primary producers inthe Lago Mare sediments is not conclusive concerning establishing a marine-typical salinity environment before the Pliocene in the Mediterranean, the calculated SSSs reflect the reconstruction proposed by alkenones. The high SSSs values and the presence of marine primary producers in the latest Lago Mare may suggest sporadic influxes of oceanic waters.