The distribution of dissolved species in interstitial waters of sediments sampled by Ocean Drilling Program (ODP) Leg 175 along the southwest African margin indicates that organic matter remineralization and carbonate dissolution/precipitation reactions dominate diagenetic processes in this region. Degradation of organic matter in the shallowest sediments generates extremely high alkalinities as well as large concentrations of dissolved ammonium and phosphate. For example, Site 1084 (Cape Basin), with an average of 8 wt% organic carbon and maximum values approaching 20 wt%, records the second highest alkalinity and dissolved ammonium concentrations observed in the history of Deep Sea Drilling Project/ODP drilling. These high alkalinities drive the shallow precipitation of authigenic carbonate phases, resulting in depletions of dissolved Ca2+ that overwhelms the Ca2+ release by dissolution of biogenic calcite (that is indicated by increases with depth in dissolved strontium). Dolomite precipitation causes significant negative excursions of dissolved magnesium from background downcore trends. These excursions are coincident with minima in dissolved Ca2+. Diatom dissolution causes dissolved silica concentrations to be higher than in seawater, and downcore trends in dissolved silica usually correspond to variations in sedimentary diatom abundance. At many sites, a shallow increase in dissolved chloride appears to record a diffusional signal of glacial seawater.
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Reproduced online:
31 August 2007