Geochemical signatures of the Namibian diatom belt: Perennial upwelling and intermittent anoxia

Abstract

[1] 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.

Sediment redox tracers in Strait of Georgia sediments – Can they inform us of the loadings of organic carbon from municipal wastewater?

“…GVRD-1, -6 and -7 also show slight Mn 246 enrichments in the top 3-5 cm. GVRD-2 shows a very minor enrichment in the top 10 cm, but 247 the average Mn concentration (537 ± 3 g g -1 (SE)) is consistent with the low crustal average for 248 granite (Taylor, 1964;Borchers et al, 2005). GVRD-3 shows no detectable enrichment, with 249 surface values actually depressed (498 ± 5 g g -1 ) relative to deeper layers of the core (540 ± 6 g 250 g -1 ).…”

Selenium as paleo-oceanographic proxy: A first assessment

“…The major supply of Se to modern marine sediments is the deposition of organic detritus at the seafloor (Wrench and Measures, 1982;Cutter and Bruland, 1984;Ohlendorf, 1989;Bö ning et al, 2005;Borchers et al, 2005). Selenium in organic compounds is mainly under the -II oxidation state (Rother, 2012).…”

A dissolved cobalt plume in the oxygen minimum zone of the eastern tropical South Pacific

“…Bottom water deoxygenation restores Co fluxes to the water column (Johnson et al, 1988;Sundby et al, 1986), but Co is also incorporated into sulfide minerals, analogous to the "Goldilocks" mechanism for benthic Fe release where flux is maximized when redox conditions are low enough to promote oxide dissolution but still high enough to avoid pyrite burial (into which cobalt is incorporated; Morse and Luther, 1999;Scholz et al, 2014). Cobalt burial in pyrite is evident in the high Co / Al content of Black Sea sediments (Brumsack, 2006) and sulfide-rich pockets of Namibian sediments near Walvis Bay (2.9 × 10 −4 g g −1 , Borchers et al, 2005), despite more widespread Co depletion in suboxic (but not sulfidic) terrigenous sediments underneath the Benguela upwelling region (Bremner and Willis, 1993). Prevailing suboxic conditions along the Namibian coast ultimately lead to an extensive dCo plume that reaches across the South Atlantic basin (Noble et al, 2012).…”