Trace element signatures of Chilean upwelling sediments at ~ 36°S

Abstract

With this contribution we continue the discussion on trace element (TE) cycling in Chilean upwelling sediments from within and below the oxygen minimum zone (OMZ) [Böning et al. (2005) Marine Geology 220, 1–21]. Low authigenic (i.e. non-lithogenic) contents of TE (Ag, Cd, Mo Re and Se) were interpreted to result from the distinct seasonality in productivity-induced organic flux, bottom water oxygenation as well as dilution by terrigenous matter. While all sediments were reducing enough to become depleted in Mn highest enrichments in Mo, Cd and Re occur in sediments in contact with the OMZ. Here, for a meaningful discussion, total contents of As, Bi, Co, Cr, Cu, Ni, Pb, Sb, Tl, V, U and Zn were corrected for the lithogenic background by determining TE/Al ratios from local river samples. The latter showed significantly lower TE/Al ratios than average shale, crust or andesite which are classically used for background correction. Hence, the precise assessment of the lithogenic TE contribution for the calculation of authigenic TE contents allows to examine the source–sink relationship of individual authigenic TE. For Chilean sediments four TE groups may be distinguished: (i) Mo, Re, U and Sb likely originate from sea water and accumulate via diffusive supply across the SWI, (ii) Cd, Cr, Cu, Ni and Zn require biologically-induced pre-concentration within biogenic material, (iii) Ag, As, Se, Tl and V show an intermediate behaviour and (iv) Bi and Pb are contributed by anthropogenic activity. Highest enrichments in Ni, Cu, U, As and V occur in sediments in contact with the OMZ. The distinct depletion in Co (along with Mn) indicates the overall reducing, but non-sulfidic character of the Chilean OMZ. The degree of sedimentary sulfate reduction seems to be the most important control on Chilean and Peruvian near-coastal enrichment of Cd, Mo and U.