The role of adsorption in sediment-water exchange of phosphate in North Sea continental margin sediments

Abstract

The effect of adsorption on the sediment-water exchange of PO, was investj gated in sediments from four different types of sedimentary environments in the southern and eastern North Sea in August 1991 and February 1992. Nonlinear adsorption isotherms for oxidized sediment from eight stations indicate that North Sea sediments differ widely in their capacity to adsorb PO,. A good correlation between the value of the adsorption coefficient and NH,-oxalate-extractable Fe was observed. A combination of the adsorption data with Porewater PO, profiles, solid phase results, and measured and calculated rates of sediment-water exchange of PO, for 15 stations in both August and February indicates that adsorption plays an important role in controlling sediment-water exchange of PO, during at least a part of the year in three of the four North Sea environments. At most stations, PO, adsorption constrains the flux of PO, to the overlying water. At one station in the depositional environment of the Skagcrrak, however, desorption is responsible for the maintenance of a flux of PO, to the overlying water. A one-dimensional reactiondiffusion model, describing Porewater PO, and solid phase P profiles, was de*deloped and applied to results for two stations. The model results show that both enhanced retention and enhanced release of PO, can be adequately described when simultaneous equilibrium and first-order kinetic reversible adsorptive reactions are assumed.A substantial proportion (lo-50%) of pelagic primary production reaches the sea floor in continental margin environments (Jorgensen 1983). When this organic material is mineralized in the sediment, a release of dissolved PO, to the pore water results. This PO, can be retained in the sediment, but it may also escape to the overlying water, where it once again can become available for uptake by phytoplankton (e.g. Howarth et al. 1995).Phosphorus (P) retention and release in marine sediments on short time-scales are usually considered to be redox-dependent. When an oxidized surface layer is present, substantial amounts of PO, can be retained in the sediment through adsorption to Fe oxides (e.g. Krom and Berner 1980b, 198 1;Sundby et al. 1992;Jensen et al. 1995). This adsorption process generally results in a buffering of Porewater PO, concentrations to low values in the oxidized sediment zone (Froelich 1988;Sundby et al. 1992), thus allowing only limited diffusive transport of PO, to the overlying water. When, in contrast, the oxidized surface layer is thin or absent, the PO, released from organic matter and from Fe oxides upon their reduction can escape to the overlying water. This was first described for lake sediments by Einsele (1936) and Mortimer (1941Mortimer ( , 1942. Redox-dependent sediment PO, re-I Present address: Department of Geochemistry, Institute of Earth Sciences, Utrecht University, Budapcstlaan 4, 3584 CD Utrecht, The Netherlands. AcknowledgmentsWe thank the crew of RV Pehgia for their help during the cruises and A. J. J. Sandcc for the grain size analy...

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