Speciation and burial flux of phosphorus in the surface sediments of the Eastern Mediterranean

The distribution of various forms of P in surface sediments across the Eastern Mediterranean and in Saharan dust and Nile particulates as a potentially important source of sediment to the basin has been determined using the SEDEX procedure (Ruttenberg, 1992). The distribution of most phases could be described by a rather simple mixing between Adriatic sediment and Saharan dust across the Ionian Sea and between Saharan dust and Nile particulates across the Levantine basin. The relative proportions of P between the various phases were similar to that found previously in other marine systems with P_aut>P_Fe>P_det>P_ex. However, P_org in the Eastern Mediterranean was ∼ 15 to 20 percent which was lower than that found in many other marine sediments. P_Fe was higher (∼36 percent) in the areas adjacent to the Nile delta than elsewhere across the basin. Saharan dust was a source of P to surface waters. There was, however, no evidence of uptake of phosphate by Saharan dust in Levantine Deep water (LDW). By contrast Nile particulates adsorb phosphate from LDW probably into an Fe-bound phase. The organic C/P ratio in the deep part of the basin was 136 which is typical of low sedimentation rate ocean sediments and indicates that the organic P being buried is likely to be refractory. However on the Israeli shelf, where Nile particulates predominate, the organic C/P was also low (∼164) which is unusual for a sediment with a relatively high sedimentation rate (0.005-0.3 cm/yr). It was suggested that this was due to the unusual nature of the Nile system in which inorganic particulates were jetted out into the basin beyond the plume of increased primary productivity. The burial flux of total P in the Eastern Mediterranean was calculated as 5.5 * 10¹² mmoleP/yr. Previous estimates of the P budget of the eastern basin have used the net flux of P through the straits of Sicily to calculate the terrestrial and atmospheric input to the basin. These estimates, which ignored sediment burial of P, need to be increased by a factor of ∼2. However 3.0 * 10¹² mmoleP/yr of this input was from the river Nile, which as a result of the completion of the Aswan dam in 1965, no longer supplies sediment to the basin.