Northern Adriatic Runoff During Pliocene Sapropel Deposition: Implications for Mediterranean Deoxygenation

The Piacenzian (Pliocene) warm period featured extreme low-oxygen conditions and organic-rich (sapropel) sedimentation in the Mediterranean Sea, yet the processes operating along northern borderlands and marginal basins remain poorly constrained. We investigate Piacenzian Sapropel A at Fiumana, an expanded, shallow-water succession from the Northern Adriatic, a key source of eastern Mediterranean deep-water ventilation. Using lamina-scale sedimentology, SEM-EDS analyses, calcareous nannofossil assemblages, high-resolution foraminiferal records, and species-specific stable carbon (delta 13C) and oxygen (delta 18O) isotope measurements, we reconstruct seasonal- to centennial-scale environmental variability during sapropel deposition. Increased abundance of reworked nannofossils, thick terrigenous layers, monospecific coccolith lenses, and a similar to 1 parts per thousand negative shift in planktonic delta 18O indicate enhanced continental runoff. Laminated facies record alternating intervals of runoff-driven terrigenous input (humid phases) and enhanced late-winter mixing that stimulated diatom productivity. In contrast, sand-rich intervals reflect episodic strengthening of bottom currents under cooler and/or drier conditions, indicating transient enhancement of deep-water formation that was insufficient to restore bottom-water oxygenation. The succession documents pronounced hydroclimatic variability characterized by alternating flood- and drought-like conditions with recurrent, non-seasonal freshwater discharge that likely reduced deep-water formation and weakened ventilation of the eastern Mediterranean. Our analysis suggests that sapropel conditions may have persisted longer at Fiumana than in the deep Ionian Sea, implicating a strong sensitivity of the northern Adriatic to precession-driven hydroclimatic forcing under warm Piacenzian boundary conditions. These results suggest that hydroclimatic variability in the Northern Adriatic likely influenced Mediterranean deep-water ventilation and (de)oxygenation during sapropel conditions.