Sedimentary facies and high resolution primary production inferences from laminated diatomacous sediments off northern Chile (23°S)

Abstract

The oceanographic conditions at the Mejillones bay (23°S) are strongly influenced by an important wind-driven upwelling cell of the Humboldt Current System (HCS), as well as by the presence of the Oxygen Minimum Zone (OMZ) in this region. Laminated diatomaceous sediments in this bay were analysed using different qualitative and quantitative techniques on the sediment cores 33C and 32B, to interpret the main physical ocean–climate factors driving sedimentation processes. The composition of these sediments is dominated by the high concentration of resting spores of the genus Chaetoceros sp., which denotes the strong influence of the upwelling events and primary production on hemipelagic sedimentation processes within the basin. The laminae, characterized by thickness ranging from 1 mm to several cm, are associated with changes in density (/porosity) due to the different concentration and composition of phytoplankton remains and organic matter. Dark laminae are generally characterized by higher concentration of Chaetoceros r.s., TOC, organic matter, carbonates and lithic minerals with respect to the adjacent lighter laminae, representing periods of strong upwelling events, high primary production rates and intensified hypoxia. Contrarily, relatively lighter laminae are characterized by diminished concentration of Chaetoceros r.s. and relative increases of the concentration of centric and pennate diatoms, producing a more porous sediment. Some of these light laminae could be associated with short-lived major oceanographic ENSO-like events. The variability of the sedimentary fluxes observed in both cores represents mostly ocean–climate trends at decadal, interdecadal and secular time periods. The secular trends of the sedimentary fluxes suggest an intensification of the upwelling events and primary production from the second half of the 19th century, as well as intensified processes during the 20th century. These changes seem to occur contemporaneously with ocean–climate variations in the southwestern tropical Pacific Ocean, suggesting a major basin scale climate change during this period.