Abstract
Phosphorus (P) is a key nutrient for marine organisms. The only long-term removal pathway for P in the marine realm is burial in sediments. Iron (Fe) bound P accounts for a significant proportion of this burial at the global scale. In sediments underlying anoxic bottom waters, burial of Fe-bound P is generally assumed to be negligible because of reductive dissolution of Fe(III) (oxyhydr)oxides and release of the associated P. However, recent work suggests that Fe-bound P is an important burial phase in euxinic (i.e. anoxic and sulfidic) basin sediments in the Baltic Sea. In this study, we investigate the role of Fe-bound P as a potential sink for P in Black Sea sediments overlain by oxic and euxinic bottom waters. Sequential P extractions performed on sediments from six multicores along two shelf-to-basin transects provide evidence for the burial of Fe-bound P at all sites, including those in the euxinic deep basin. In the latter sediments, Fe-bound P accounts for more than 20% of the total sedimentary P pool. We suggest that this P is present in the form of reduced Fe-P minerals. We hypothesize that these minerals may be formed as inclusions in sulfur-disproportionating Deltaproteobacteria. Further research is required to elucidate the exact mineral form and formation mechanism of this P burial phase, as well as its role as a sink for P in sulfide-rich marine sediments.
Highlights
Phosphorus (P) is a key nutrient for marine organisms and its availability may limit primary production, on both short and long timescales
The released PO4 can precipitate in the form of authigenic carbonate fluorapatite at depth, or diffuse upward where it can again be bound to Feoxides in the oxic surface sediment [2,5]
In marine sediments overlain by oxic bottom water, organic P, Feoxide bound P and authigenic Ca-P are typically the major sedimentary phases contributing to burial of P [2,5]
Summary
Phosphorus (P) is a key nutrient for marine organisms and its availability may limit primary production, on both short and long timescales. Organic P is the dominant form of P deposited at the sediment-water interface [4]. Part of this organic P is degraded in the sediment and dissolved phosphate is subsequently released to the bottom water or pore water (mainly HPO422 in seawater; termed PO4). In sediments overlain by oxic bottom waters, PO4 can be bound to iron (oxyhydr)oxides ( termed Fe-oxide bound P) in the surface sediment. In marine sediments overlain by oxic bottom water, organic P, Feoxide bound P and authigenic Ca-P are typically the major sedimentary phases contributing to burial of P [2,5]
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