Response of phosphorus burial and post-depositional diagenesis to postglacial climate change in the coastal system

The phosphorus (P) burial and diagenesis in coastal systems determine the reactive P flux to the ocean, influencing primary productivity and nutrient element cycles. To understand the fate of P in coastal areas and its association with postglacial climate change, we determined P species, Fe-(oxyhydr)oxides (Fe-oxides), major elements, nutrient elements (C and N), and vivianite minerals characteristics in sediment cores from the Qiantang River (QR) estuary in eastern China. Our findings reveal that the P cycle was predominantly controlled by postglacial sea-level changes in the QR estuarine system. Fe-bound P (P_Fe) dominates in fluvial and paleo-estuarine sediments, comprising ∼79 % of total P pool, which mainly exists in the form of vivianite, particularly enriched below the estuarine-marine transition. This enrichment is linked to increased bottom-water sulfate concentrations and reduced sedimentation rates in response to marine transgression around 8.5 ka BP, promoting the sustained anaerobic oxidation of methane and reductive dissolution of Fe-oxides. In contrast, Ca-associated P (mostly carbonate fluorapatite) formed at the expense of P_Fe and organic P in shallow marine and modern estuarine deposits, acting as a major P sink during high sea-level episodes. The high P_Fe deposition event is linked to a paleo-flood event at the Younger Dryas-Holocene transition. Flood-driven surface sediments oxygenation facilitated the coprecipitation of Fe-oxides and P, notably lowering Fe/P ratio in P_Fe speciation. Our work highlights the long-term impact of postglacial climate change on the coastal P cycle through sea-level fluctuation, environmental perturbation, and abrupt climate events.