Probing large paleoenvironmental variability of Mediterranean during the Miocene-Pliocene transition via advanced multivariate statistical analysis on lipid biomarker multiproxy.

The quantitative reconstruction of paleoclimatic and paleoenvironmental conditions in regions and in time periods characterized by recurrent and significant fluctuations is challenging. An example of strong paleoenvironmental change occurred in the Mediterranean Basin across the Miocene – Pliocene boundary (5.33 Ma), marked by the restoration of normal marine conditions after the 'Lago-Mare' terminal phase of the Messinian salinity crisis. Environmental conditions during the Lago-Mare phase are still uncertain due to the controversial body fossil record, consisting of freshwater to brackish assemblages (ostracods, dinocysts, mollusks, and foraminifera), as well as marine microfossils (otoliths of marine fishes, calcareous plankton).

However, two scenarios were suggested to describe this transition: 1) a catastrophic and sudden sea level rise causing the drastic change from freshwater to marine deep environments; 2) a gradual sea level rise, characterized by a fast to gradual transition from brackish to marine environments.

To quantify the changing conditions during the Miocene–Pliocene transition, we used a multivariate statistical approach to interpret a large array of terrestrial and aquatic molecular-based indices, in a sedimentary succession of the Northern Mediterranean (Maccarone section, Central Italy). The statistical procedure was specifically developed to address the complexities emerging from the heterogeneous dataset.

An illustrative example suggests that using the TEX₈₆, U^K₃₇’, and MBT´_5ME paleothermometers, we obtained different values and trends in the changing Mediterranean during the study interval. While the study acknowledges the validity of U^K₃₇’ as a paleothermometer in variable environments, it highlights that TEX₈₆and MBT´_5ME are sometimes compromised by other sources, such as reworked sediment or other organisms that produce the same lipid inventory. In these cases, these proxies provide information about environmental processes rather different than temperatures.

Cluster analysis supports a stepwise evolution during the Miocene-Pliocene transition, besides redundancy analysis (RDA) indicates that the water column structure changed from stratified (Tetrahymanol) during the Messinian to mixed during the Zanclean. A second gradual change is instead related to terrestrial vegetation modifications, indicating a gradual coastal environment reconfiguration after a marine transgression with the distancing of the costal line and  a reduction of wetland aquatic plants signal. Finally, molecular fossils are also influenced by cyclical changes, not related to the Messinian salinity crisis demise but linked to astronomical-driven climatic cycles.