Geochemical signatures of the Namibian diatom belt: Perennial upwelling and intermittent anoxia
A gravity core from the Nambian diatom belt (83 m water depth) was analyzed for major (Al, Ca, Fe, K, Mg, P, Si, Ti) and minor elements (Ag, As, Ba, Bi, Cd, Co, Cr, Cu, Mn, Mo, Ni, Rb, Re, Se, Sr, Tl, U, V, Zn, Zr), as well as C and S. The major element geochemistry is dominated by the contribution of biogenic phases (opal and minor amounts of carbonate and phosphate), whereas terrigenous components of eolian and fluvial origin form only a minor admixture of <15%. Examination of the Fe/S/C‐relationship reveals a completion of pyritization close to the sediment‐seawater interface. Higher than average shale Fe/Al ratios reflect the presence of highly reactive iron oxides in terrigenous source material from the Namibian desert and suggest little loss of Fe within the OMZ. The low total Fe content likely represents an important factor responsible for sporadic H2S eruptions. The upwelling system off Namibia may be regarded as an intermediate system, combining high productivity typical for upwelling areas with the sporadic presence of H2S in the water column. By examining the source‐sink relationship of individual trace metals, two different trends are discernible: Mo, Re, and U accumulate following seawater availability, whereas Ag, Cd, Cu, Ni, Tl, and Zn require a biological preconcentration step. Trace metals like As, Ba, Cr, Se, and V show an intermediate behavior. Metal enrichment is favored by high interstitial water sulfide concentrations close to the sediment‐seawater interface, leading to slightly elevated Co contents as opposed to extremely low Mn/Al ratios, reflecting the overall reducing character of the Namibian system. Maximum contents of Cu, Ni, and Zn are significantly lower than those reported previously. This may in part be related to the absence of fecal material and a stronger dilution by biosiliceous material at this location compared to the seaward flank of the diatom belt. Keywords: Namibia upwelling system, H2S eruption, pyritization, trace metals, major elements.
Geochemistry, Geophysics, Geosystems