Phosphorus burial and associated sediment geochemistry in the Gulf of Finland: the role of inter-decadal redox variability

Abstract

Due to the anthropogenic loading of nutrients since the beginning of the 1900’s, eutrophication related hypoxia has become a problem in the Baltic Sea. Efforts have been made to significantly reduce the anthropogenic loading but internal feedback mechanisms in the phosphorus cycle prevent a recovery to pre-industrial conditions. In recent years, several studies have been made into phosphorus cycling in the deep basins of the Baltic Sea to understand and tackle the details of this feedback mechanism. In particular, phosphorus burial in deep basin sediments has recently been better characterized. However, phosphorus burial in sediments close to the halocline has been less intensively studied.

This study investigates phosphorus burial in the Gulf of Finland, a shallow and highly eutrophied subbasin of the Baltic Sea. A large area of the sediments of the Gulf of Finland underlie halocline waters, making them susceptible to variable redox conditions. We focus on the impact of inter-decadal redox changes on sedimentary chemistry during the late 20th century, which strongly influence phosphorus burial. Bulk sediment analysis, pore water and phosphorus speciation was done on three cores taken along a transect near the halocline, where oxic to seasonally hypoxic conditions prevail. Sediment age dating (210Pb) was done to couple sedimentary signals to bottom water oxygen data over the past 50 years. Furthermore, bottom water oxygen data from the Gulf of Finland were compared to the evolution of bottom water oxygen concentrations in the deep Baltic Proper. By doing so we link the dynamics of the shallow Gulf of Finland to the better studied deep basins of the Baltic Sea.

Although Fe-oxide bound P in the surface sediments is high, confirming large seasonal PO42- release into the water column due to reductive dissolution of Fe-oxides, we also found high background Fe-P concentrations at these sites, which we interpret as vivianite. We found the rate of vivianite formation to be coupled to Corg¬ concentrations, and therefore peaks in Fe-P occurred during periods of increased hypoxia. Similar background Fe-P concentrations have so far only been found at permanently anoxic sites. Our findings show that, not only in the deep basins but also in shallow areas close to the halocline, Fe-P can be an important long-term burial phase for phosphorus. Hence this is significant information for Baltic Sea nutrient budget calculations and models.