Calcareous nannofossil and foraminiferal trace element records in the Sorbas Basin: A new piece of the Messinian Salinity Crisis onset puzzle

摘要

The Messinian Salinity Crisis (MSC) was an extreme event that affected the Mediterranean Sea during the late Miocene, leading to massive evaporite deposition across the basin. Here we focus on the Perales section (Sorbas Basin, Western Mediterranean), using calcareous nannofossil (CN) and foraminiferal geochemical analyses to trace the paleoenvironmental dynamics culminating in the MSC onset. Orbital and tectonic forcing drove the CN fluctuations that correlated with the lithological quadripartite precessional cycle. Our integrated analysis reveals that cyclical sapropel deposition was triggered by an increase in marine productivity, followed by an increase in freshwater input and the development of a Deep Chlorophyll Maximum (DCM). The overlying marl records the protracted freshwater input that led to the shallowing of the DCM, decreasing organic carbon export and promoting seafloor re-oxygenation. Stratification, acting as a barrier, trapped part of the river-sourced nutrients below the photic zone. The subsequent gradual decrease in temperature promoted the disruption of the stratification, mixing nutrients upward into the photic zone and promoting diatom proliferation and preservation. When the reservoir of dissolved silica was exhausted, bioturbated marls were deposited at the top of the cycle. In the last pre-evaporitic cycle (UA34) the usual lithological cyclicity is obliterated; however the precessional footprint is revealed by CN fluctuations, suggesting that sapropel-like deposition lasted for most of the cycle. This cycle also records a succession of CN abundance peaks (MSC onset bioevent), as already recorded in several Mediterranean sections approximately at the MSC onset, suggesting that the same paleoenvironmental changes triggered the evaporitic phase in the whole Mediterranean. This bioevent marks the last step of water exchange reduction with the Atlantic (restriction pulse), which increased the sensitivity of the Mediterranean to freshwater input and associated nutrient delivery, culminating in a further increase in marine productivity.