Terrestrial and marine dynamics on the brink of the Messinian salinity crisis: A wet scenario from the northern Mediterranean

The ongoing debate on the causes and modalities of the Messinian Salinity Crisis (MSC) from 5.971 Ma needs to be implemented by an in-depth knowledge of the events and changes, on a global to regional scale, that have preceded its onset. Here we use palynological (pollen and dinocysts) and micropaleontological (foraminifers and calcareous nannoplankton) quantitative analyses from the Govone sedimentary succession (Northern Italy, western Mediterranean) to reconstruct the cyclical changes in atmosphere, surface, and deep-water hydrographic conditions during the Late Miocene, prior to the onset of the MSC (from ca 6.6 Ma) and across it (up to 5.96 Ma). On land, plant communities were dominated by arboreal life forms typical of warm and humid conditions. However, mid- to high altitude coniferous taxa attest to a sequence of cold to cool conditions, some coinciding with glacial obliquity-controlled phases, notably those across the MSC onset (i.e. TG34 and TG32). Dominant trees were Taxodioideae, especially in freshwater swamps, during warm and humid phases. The latter alternate with relatively less warm, and less humid phases characterised by the reduction of Taxodioideae, according to the precession-controlled sedimentary cyclicity (sapropel-marl couplets). The absence of any significant expansion in the herbaceous cover is evidence of no substantial increases in aridity at insolation minima. In the marine realm, the precession-controlled cycles correspond to prevalent alternations between warm water, oligotrophic (sapropel, insolation maxima) and cool water, eutrophic (marls, insolation minima) conditions. The progressive decrease of oceanic/outer neritic dinocyst taxa, along with a more continuous occurrence of the inner neritic ones, confirm the progressive restriction of Mediterranean and Atlantic connections, despite oceanic influxes, the latest at 6.14 Ma and 6.05 Ma. On the other hand, the occurrence of freshwater episodes, according to dinocysts, does not exclude possible connections with Paratethys, at 6.18 Ma and 6.07 Ma. All calcareous nannoplankton and foraminifera apparently disappear from the sedimentary record before the onset of the MSC, at ca 6.02 Ma; however, dinocysts allow the documentation of the history of pre-evaporitic conditions up to the beginning of the MSC onset, when lagoons, marked by frequent salinity changes, expanded in the inner neritic zone of the Govone intermediate depth basin, during ca 60 kyrs. Plant assemblages attest a long-lasting increase of seasonality (from 6.2 Ma) as well as a peculiar inversion in the dominant vegetal patterns of the sapropel-marl couplets, between ca 6.02 Ma and 5.96 Ma, corresponding to the uppermost four cycles. The significant restriction of the marine environment, after 6.02 Ma, possibly promoted atmospheric changes marked by reduced vs enhanced activity of the Mediterranean storm-track in phase with precession. This, in turn, regulated the amounts of runoff in the Northern Italian sectors from the Alps.