Open AccessGeochemical signatures of the Namibian diatom belt: Perennial upwelling and intermittent anoxia
Author(s) -
Borchers S. L.,
Schnetger B.,
Böning P.,
Brumsack H.J.
Publication year - 2005
Publication title -
geochemistry, geophysics, geosystems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.928
H-Index - 136
ISSN - 1525-2027
DOI - 10.1029/2004gc000886
Subject(s) - geology , upwelling , seawater , terrigenous sediment , geochemistry , oxygen minimum zone , water column , carbonate , trace element , environmental chemistry , scavenging , diatom , trace metal , mineralogy , sedimentary rock , oceanography , metal , chemistry , organic chemistry , biochemistry , antioxidant
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 H 2 S 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 H 2 S 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.