| dc.description.abstract | At present the Black Sea is the largest basin in the world with permanently anoxic deep waters and it has been anoxic for roughly the past 8000 years. The inflow of saline water from the Mediterranean through the Bosporus in what was until then an oxic, fresh-water, lacustrine environment led to a density driven stratification of the water column, which inhibits mixing. Consequently, the water column became divided between oxic surface waters and euxinic deep waters. This lake-marine transition influenced the phosphorus (P) and iron (Fe) speciation in the sediment. While P bearing Fe oxides were formed and preserved under the oxic lacustrine condition, the euxinic marine conditions instead favor the formation and preservation of FeS2, which does not form a significant sink for P. At the same time, infiltration of HS- into the lake sediment forms a sulfidization front above which Fe oxides are reduced by their reaction with HS-, forming FeS and FeS2 within the lacustrine deposits, releasing P into the porewater.
As P is an essential and often limiting nutrient and Fe oxides form an important sink for P, it is important to know how the Fe-P cycle is influenced by the interaction with HS- and how this may affect P availability. Therefore, it is the aim of this study to determine the influence of the lake-marine transition and the sulfidization front on the sedimentary Fe and P record in the Black Sea. Furthermore, this paper aims to determine how sediments from the deep basin may differ from slope sediments in the Black Sea as a result of differences in sedimentation rates and the presence of an active Fe shuttle. In order to do so, two different cores collected during the 2013 PHOXY cruise on the R/V Pelagia are examined. One of the cores PHOX 4 was collected from the western slope (ca 400 mbss) of the Black Sea, the other (PHOX 12) from the deep basin (ca. 2000mbss). Both cores were subjected to total solid phase analysis, a sequential P extraction (SEDEX; Ruttenberg 1992) and two sequential Fe extractions (Claff et al., 2010 and Poulton and Canfield, 2005).
The data show that the lake-marine transition causes a shift in the importance of different sedimentary Fe and P pools. Where the oxic lacustrine system favored the preservation of Fe oxides and carbonates, the marine system favors the formation of FeS2. The high concentrations of Fe and Fe-bound P in the surface sediments of both cores and of Fe oxides in PHOX 4 is the result of an Fe shuttle that transports Fe oxides and associated P from the shelf via the slope towards the deep basin of the Black Sea, undergoing continuous reduction via the reaction with HS-. Furthermore, Organic Matter (OM), and consequently Org. P, is only preserved in the marine environment, resulting in higher concentrations of org P in the marine sediments, compared to the lacustrine deposits, despite the anoxic conditions which favor the recycling of org P.
Additionally, the downward diffusion of HS- into the lake sediments results in a replacement of Fe oxides that were previously present by FeS2 through the reaction of Fe with HS-. The PO43- released as a consequence of the reduction of the Fe oxides by HS-, diffuses upward and downward to below the S.F. were it can be react with Fe2+ in the porewater forming Fe-P minerals such as vivianite. As a result, enrichments in Fe-bound P are present below the S.F. | |
