Paleoenvironmental change in a precession-paced succession across the onset of the Messinian salinity crisis: Insight from element geochemistry and molecular fossils

In marginal Mediterranean sub-basins, the early phase of the Messinian salinity crisis (MSC) is recorded by cyclic successions of gypsum and shales, which in deeper parts of the sub-basins make lateral transition into organicrich shales, marls, and carbonates. The cyclic stacking pattern of the gypsum-bearing sequences is assumed to reflect periodic paleoenvironmental change induced by precession-driven climate perturbations, with the assumption that shales reflect humid climate (precession minima) while gypsum reflects arid conditions (precession maxima). However, this correlation has not been verified to date, mostly because of the scarcity of microfossils, the most commonly used tools for the reconstruction of precession-driven paleoenvironmental change. Such change can, instead, be reconstructed through the study of the deeper water counterparts of gypsum (marly and carbonate layers) with geochemical indicators (major and trace elements, molecular fossils), which provide insight on climate and aquatic productivity. We used this approach to study a section from the Piedmont Basin (NW Italy) where the onset of the MSC is archived in a sequence of organic-rich sediments. This sequence displays distinct lithological cyclicity, evidenced by the repetition of couplets of organic-rich shales and marls, either bioturbated (in the pre-MSC part of the section) or laminated (during the MSC). The influence of orbitally-driven (precession) climate oscillations is demonstrated by fluctuations of Ti/Al, Si/Al, Mg/Al, K/Al, Zr/Al, and Ba/Al ratios that are in phase with lithological cyclicity. These fluctuations are interpreted to reflect alternation of humid (shales, deposited during precession minima) and arid (bioturbated and laminated marls, deposited during precession maxima) phases, dominated by fluvial and aeolian transport of detrital material, respectively. The cyclicity of the element ratios is mirrored by changes in organic carbon content and molecular fossil inventory. In particular, the distribution of long-chain n-alkanes and their degree of preservation reveal that humid phases at times of precession minima were typified by the maximum input of degraded terrestrial organic matter driven by enhanced riverine runoff, which promoted water column stratification. Coeval increase in Ba content, a common paleoproductivity proxy, agrees with enhanced nutrient supply during humid periods, promoting phases of eutrophication in the basin. Lithological and geochemical changes are observed in MSC sediments deposited at times of precession maxima, evidenced by the replacement of pre-MSC bioturbated marls by laminated marls rich in filamentous fossils corresponding to the remains of probable colorless sulfide-oxidizing bacteria. Such changes reflect an intensification of water column stratification after the onset of the MSC, possibly related to the combined effect of persistent freshwater inflow and basin isolation, preluding the advent of gypsum precipitation.