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

Abstract

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 (δ ¹³ C) and oxygen (δ ¹⁸ O) 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 ∼1‰ negative shift in planktonic δ ¹⁸ O 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.