Phosphorus dynamics in the Black Sea and Baltic Sea

Abstract

The dynamics of the key nutrient phosphorus (P) in hypoxic and anoxic marine basins are still incompletely understood. This thesis focuses on the cycling of P in two of such basins: the Baltic Sea and the Black Sea. Water column particulates and sediments from the deep basin of the Black Sea were collected during seagoing expeditions with the Dutch research vessel Pelagia, whereas sediments from the Baltic Sea were retrieved during the IODP Baltic Sea Paleoenvironment Expedition 347. A wide range of analytical techniques was applied to study the chemical composition of solutes, bulk sediments, sediment minerals and water column particulates, including sequential extractions for P and iron (Fe) and synchrotron-based X-ray absorption spectroscopy (XAS). Our results show that sedimentary P burial, the only long-term removal pathway of P in the marine realm, is strongly controlled by the environmental conditions (e.g. salinity and oxygen) in the water column and sediment and varies between different seas. The results show that organic P, authigenic Ca-P (apatite) and P associated with calcium carbonate (CaCO3­) are the major P burial phases in sediments in the sulfidic deep basin of the Black Sea. Carbonate-bound P, which may already be present in the water column or that may form in the surface sediments, can thus be an important sink for P in CaCO3-rich sediments of anoxic, sulfidic basins. This P pool should be considered as a potential P sink (and P source in case of CaCO3 dissolution) when reconstructing P dynamics in past oceans. In the CaCO3-poor sediments of the Baltic Sea, vivianite (Fe(II)3(PO4)2·8H2O) was found to act as a key P burial pool. In the deepest basin, the Landsort Deep (437 m water depth), a strong shelf-to-basin shuttling of manganese(Mn)- and Fe-oxides and high inputs of P in settling organic matter were shown to stimulate the formation of vivianite and other P-bearing minerals in the sediments. In the shallow Ångermanälven estuary (< 90 m water depth), in contrast, refreshening and re-oxygenation of the basin, caused by gradual isostatic uplift, promoted burial of P (as vivianite) in the sediment. These salinity-related changes in redox conditions and P burial are highly relevant in light of current climate change as increased freshwater input linked to global warming may enhance coastal P retention. This could contribute to oligotrophication in both coastal and adjacent open waters. The post-depositional formation of vivianite at the lake-marine transition in sediments of a hypoxic basin in the Baltic Sea, the Bornholm basin, was also investigated. The results show that vivianite precipitates at the interface where downward diffusing phosphate(PO4) from the organic-rich, brackish-marine sediments meets upward diffusing dissolved Fe from the lake sediments. This formation of vivianite strongly alters the sedimentary P records in the Bornholm Basin and other parts of the Baltic Sea. The possibility of post-depositional vivianite formation should thus be considered when using sediment P proxies to reconstruct paleo-environmental conditions in coastal basins.