Biogenic silica and diatom thanatocoenosis in surface sediments below the Peru–Chile Current: controlling mechanisms and relationship with productivity of surface waters

Abstract

Based on 76 surface sediment samples collected between ca. 22° and 44°S along the Chilean coast in the southeast Pacific Ocean, we report on the north–south distribution of opal content and diatom concentration, and the quantitative and qualitative composition of the preserved diatom community. Latitudinal differences in the siliceous signal reflect the influence of two main mechanisms of surface water production: coastal upwelling off northern and central Chile till ca. 38°S, and nutrient enrichment by the input of cold, southern, non-upwelling-associated waters south of ca. 38°S. The poleward increase in biogenic opal and diatom values parallels well the north–south increase of pigment concentration in surface waters as recorded by satellites. Between 22° and 33°S, where low pigment concentrations are measured, opal content and diatom concentration are mostly low (except at 22°–23°S). In contrast, both higher opal and diatom values at 34°–38°S and 41°–42°S correspond well with higher pigment estimations. Preserved diatom populations document the present-day dominant hydrographical conditions. Nutrient enrichment of surface waters due to coastal upwelling results in intensive production of diatoms off northern and central Chile, reflected by the predominance of the richly-composed association of Chaetoceros spores north of ca. 38°S. The diatom association shows that the area between 35° and 38°S is transitional. The abrupt decrease of Chaetoceros spores south of ∼38°S off Chile coupled with the enhanced contribution of Thalassiosira spp., representatives of high-productivity, low-temperature waters, points to the presumed high nutrient supply by the iron-limited, nutrient-rich Antarctic Circumpolar Water. The contribution of benthic diatoms reflects some lateral transport from the near-shore area into deeper positions south of 38°S.