Early diagenesis of phosphorus in continental margin sediments

<p>Most of the organic material in the oceans that reaches the sea floor is deposited on continental margins and not in the deep sea. This organic matter is the principal carrier of phosphorus (P) to sediments. A part of the organic material is buried definitely. The other part decomposes, resulting in a release of dissolved HP0 <sub>4</sub><sup>2-</SUP>to the pore water. This HP0 <sub>4</sub><sup>2-</SUP>either returns to the overlying water and becomes available for uptake by phytoplankton, or is retained in the sediment in an organic or inorganic form.<p>Quantification of the P release from and P retention in sediments on relatively short time scales of days to years is necessary for a correct understanding of the nutrient dynamics in regional seas such as, for example, the North Sea. An accurate assessment of the modem global ocean burial flux of reactive P (i.e potentially bioavailable P) and the burial flux in the geological past is important for understanding the global oceanic P cycle. This, in turn, can provide insight in possible controls on organic C burial and atmospheric concentrations of C0 <sub>2</sub> and 0 <sub>2</sub> , because P may limit oceanic primary production and thus determine the amount of organic material in the oceans on geological time scales.<p>The research presented in this thesis concentrates on the short-term processes controlling sediment P release and retention in temperate, non-upwelling, continental margin environments. The research commenced with a laboratory study on the effect of organic matter deposition and macrofauna on sediment-water exchange and retention of P in Fe oxide-poor, sandy sediments (Chapter 2). A suspension of dead algal cells <em>(Phaeocystis</em> sp.) was applied to sediment in experimental systems (boxcosms), either once or every week during 19 weeks. The results demonstrate that deposition of organic matter on this type of sediment enhances pore water concentrations and sediment-water exchange of HP0 <sub>4</sub><sup>2-</SUP>. The enhanced HP0 <sub>4</sub><sup>2-</SUP>release was due to microbially mediated mineralization of the organic material and due to direct release of HP0 <sub>4</sub><sup>2-</SUP>from the algal cells (lysis). A major portion of the algal material remained at the sediment-water interface and this organic layer probably regulated the sediment-water exchange of HP0 <sub>4</sub><sup>2-</SUP>directly. The activity of the macrofauna was mainly limited to reworking of the sediment. The effect of the macrofauna on the sediment-water exchange Of HP0 <sub>4</sub><sup>2-</SUP>was negligible. In the boxcosms to which organic material was added only once, the concentration of NaOH-extractable sediment P increased following the addition, especially in the presence of macrofauna.<p>Sorption of P to Fe oxides is the most important short-term process responsible for the retention of P in sediments. Using a combination of differential X-ray diffraction (DXRD) and extraction procedures, the character of the Fe oxides that bind P in 4 North Sea sediments was studied (Chapter 3). The results indicate that poorly crystalline ferrihydrite and akageneite were present in the fine sediment fraction (< 10 μm) of surface samples from all locations. Combination of these results with bulk sediment extractions of Fe and P and sorption characteristics for P provides evidence for the dominant role of poorly crystalline Fe oxides for the binding of P in these North Sea sediments. These poorly crystalline Fe oxides are suggested to act as both a temporary and permanent sink for P.<p>The redox conditions in continental margin sediments can vary both seasonally and spatially. To obtain more insight in the redox conditions in North Sea sediments, the Mn and Fe cycle at 15 locations in 4 different sedimentary environments was studied in 2 contrasting seasons (Chapter 4). The quality and quantity of the organic matter deposited in each environment was found to determine whether sediments become sufficiently depleted of 0 <sub>2</sub> and NO <sub>3</sub><sup>-</SUP>to allow for (1) Fe and Mn reduction and (2) escape of dissolved Fe <sup>2+</SUP>and Mn <sup>2+</SUP>to the overlying water. A steady- state diagenetic model describing solid phase and pore water metal profiles was developed and applied to Mn and Fe data for 11 and 3 stations, respectively. The model results demonstrate that (1) reversible sorption in combination with sediment mixing can enhance diffusive transport of dissolved metals; (2) precipitation of Fe <sup>2+</SUP>and Mn <sup>2+</SUP>in the form of reduced authigenic minerals can explain the reversal of the pore water Fe <sup>2+</SUP>and Mn <sup>2+</SUP>gradients at depth at many stations, and (3) in most North Sea sediments, Fe and Mn oxides do not play an important role as redox intermediates in organic C oxidation (accounting for < 4 %); only in the depositional environment of the Skagerrak, metal oxide reduction may contribute substantially to organic C oxidation (-20%).<p>Reversible sorptive reactions can both constrain and enhance the flux of HP0 <sub>4</sub><sup>2-</SUP>from the sediment to the overlying water. The role of sorption in sediment-water exchange of HP0 <sub>4</sub><sup>2-</SUP>in North Sea sediments was investigated for 15 locations in 2 seasons (Chapter 5). P sorption data, pore water HP0 <sub>4</sub><sup>2-</SUP>profiles, solid phase results and measured and calculated rates of sediment- water exchange of HP0 <sub>4</sub><sup>2-</SUP>were combined. Sorption was found to play an important role in controlling sediment-water exchange of HP0 <sub>4</sub><sup>2-</SUP>during at least part of the year in 3 of the 4 North Sea environments. At most stations, adsorption limits the flux of HP0 <sub>4</sub><sup>2-</SUP>to the overlying water. At one station in the Skagerrak, however, desorption is responsible for the maintenance of a flux of HP0 <sub>4</sub><sup>2-</SUP>to the overlying water. A onedimensional reaction-diffusion model describing the sedimentary P cycle was developed and applied to the results for 2 stations, The model results show that both enhanced retention and enhanced release due to sorption can be adequately described when simultaneous equilibrium and first-order reversible sorptive reactions are assumed.<p>P bound in authigenic minerals may not be solubilized again, whereas Fe-bound and organic P can still be released upon deep burial. Therefore, more insight in the extent of authigenic P mineral formation in continental margin sediments is important. A combination of pore water and solid phase analysis was used to determine whether authigenic carbonate fluorapatite (CFA) is currently forming at two locations on a North Atlantic continental platform (Chapter 6). Results of selective extractions suggest that an authigenic P phase is forming at the expense of Fe-bound P at both stations. A steady-state diagenetic model for the P cycle was developed and applied to the data of I station. The model results indicate that CFA formation can account for the observed increase of authigenic P with depth at this station. Furthermore, the results show that an intense cycling of P between Fe-bound P and pore water HP0 <sub>4</sub><sup>2-</SUP>at the redox interface can create conditions beneficial for CFA formation. This internal P cycle is driven by downward, bioturbational transport of mainly in-situ formed Fe-bound P into the reduced sediment zone. Losses from the internal P cycle due to CFA formation and HP0 <sub>4</sub><sup>2+</SUP>diffusion are compensated for by sorption of HP0 <sub>4</sub><sup>2-</SUP>released from organic matter to Fe oxides. Fe bound P thus acts as an intermediate between organic P and CFA. Burial of CFA can account for between 25 and 70% of the total burial flux of reactive P and thus may act as an important sink for P in this low sedimentation, continental margin environment