Geochemistry of Sediments from the Peru Upwelling Area: Results from Sites 680, 682, 685, and 688

Abstract

Sediments from four sites drilled during Ocean Drilling Program (ODP) Leg 112 exhibit chemical variability that distinguishes diatomaceous muds in shallow water (Site 680) from those muds of deeper water (Sites 682, 685, and 688). Differences in the concentrations of elements precipitated or mobilized during epigenesis and diagenesis, in particular barium abundance, show promise as indicators for depositional environment. The distribution of barium is related to detrital components in Site 680 and related to epigenetic and diagenetic processes in deep-water sites (Sites 682, 685, and 688), making it a potential indicator for water depth in siliceous, organic carbon-rich sediments. Bromine abundance relative to organic carbon may be inversely related to the age of the sections. We document that bromine (in solids) and bromide (in dissolved) phases do not behave conservatively, but that they are instead related to organic matter remineralization. On the basis of differences in the abundances of detrital and diagenetic components, we can statistically discriminate between the shelf and slope deposits of the Peru forearc basins. A significant deficit in detrital accumulation is apparent in upper Miocene through Pliocene sediments at Sites 682 and 685 on the lower slope, as compared to a steady, if erratic, increase of detrital influx during the Pleistocene. Our data on the concentration of elements in the detrital component on the lower-slope sediments further suggest that substantial sources of clastic material deposited on the lower slope have not yet been identified, because published estimates of river runoff and clastic sedimentation on the upper slope and shelf fall short by at least an order of magnitude.