Siliceous microorganisms in the upwelling center off Concepción, Chile (36°S): Preservation in surface sediments and downcore fluctuations during the past ∼150 years

Abstract

We analyzed the temporal patterns of siliceous microorganisms in the water column and their representation in the underlying surface sediments between September 2002 and November 2005, at a fixed station on the shelf off Concepción, Chile (Station 18, 36° 30.80′S and 73° 07.75′W), connecting our results to instrumental records of temperature, salinity, coastal upwelling and freshwater input. The goals of the study were to understand the seasonal representation of siliceous microorganisms in surface sediments and to evaluate the use of this information in the interpretation of past climatic/oceanographic conditions in the area. The implications of seasonal and preservational biases are discussed. Additionally, a 30-cm core collected at the same site and spanning the last ∼150 years of sedimentation was also studied in order to provide a record of historical siliceous productivity changes. Firstly, the analyses focused on diatoms since they were numerically the most prominent microorganisms in the water column and the surface sediments, independent of season and year. In both settings, maxima of the key diatom genera Chaetoceros and Skeletonema coincided with the spring–summer upwelling period. For the autumn–winter non-upwelling period, significant plankton–sediment discrepancies were observed, with enrichment of moderately robust taxa in the sediments, as well as freshwater diatoms and phytoliths tracing the increased river discharges in winter. Secondly, the downcore analysis revealed a marked decrease in total diatom accumulation rates since the late 19th and throughout the 20th century, which was accompanied by increasing concentrations of lithogenic particles and freshwater diatoms. An alkenone-based sea surface temperature reconstruction in the same core (past ∼150 years) and instrumental data show that these changes occurred simultaneously with a general trend of increasing temperatures in the upwelling area off Concepción. Taken together, these signals suggest an intensified influence of ENSO-like variability in the ocean-climate system off central-southern Chile.