Major Changes in Ocean-Continent Dynamics Across the Iberian Margin During the Miocene-Pliocene Boundary: Insights from Calcareous Nannofossils, Reac-Tive Phosphorus and C:n Proxy

The Messinian Salinity Crisis (MSC) represents a major paleoceanographic event with profound impacts on the Mediterranean basin, global climate, and regional environments. The Iberian Margin (IM), characterized by high sedimentation rates that record orbitally-driven fluctuations in terrigenous input and biogenic carbonates, serves as a high-resolution archive of continent-ocean interactions. Despite its strategic location, the specific implications of the MSC on these interactions remains poorly understood. Phosphorus (P) is a key limiting nutrient for oceanic productivity primarily supplied to the oceans via the chemical weathering of continental rocks and subsequent riverine delivery. Consequently, shifts in the continent-ocean relationship during this event could have significantly influenced regional primary productivity. To reconstruct the evolution of these interactions during the late Miocene to early Pliocene at the IM, this study integrates high-resolution calcareous nannofossil assemblages, phosphorus sequential extraction (P-SEDEX), C/N ratios, and X-ray fluorescence (XRF) core scanning. Analyses were conducted on samples from IODP Expedition 397 (Hole U1587). Our results reveal high sedimentation rates between 5.800 and 5.516 Ma. During this interval, reactive P and calcareous nannofossil abundance exhibit marked high productivity pulses with a 20-kyr periodicity. Furthermore, C/N ratios indicate alternating marine and terrestrial organic matter sources, suggesting a tight continent-ocean coupling. Conversely, the interval between 5.516 and 5.315 Ma, coinciding with the Mediterranean “Lago-Mare” phase, shows a sharp decline in Ti flux, reactive P, and nannofossil accumulation during the hole interval. From 5.315 Ma onwards, Ca/Ti ratios stabilize, but sedimentation rates remain 50% lower than in the Late Miocene. During the early Pliocene, productivity appears significantly reduced, and C/N ratios indicate a predominantly marine origin for organic matter. These findings suggest that the MSC fundamentally reorganized the depositional and biological systems of the Iberian Margin, permanently altering the regional continent-ocean relationship.