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Abstract:
For this study two sediment cores from the Peruvian shelf covering the time period between the Little Ice Age (LIA) and present were examined for changes in productivity (biogenic opal concentrations (bSi)), nutrient utilisation (stable isotope compositions of silicon (δ30Siopal) and nitrogen (δ15Nsed)), as well as in ocean circulation and material transport (authigenic and detrital radiogenic neodymium (ϵNd) and strontium (87Sr/86Sr) isotopes).
For the LIA the proxies recorded weak primary productivity and nutrient utilisation reflected by low average bSi concentrations of ~10%, δ15Nsed values of ~5‰ and intermediate δ30Siopal values of ~0.9‰. At the same time, the radiogenic isotope composition of the detrital sediment fraction indicates dominant local riverine input of lithogenic material due to higher rainfall in the Andean hinterland. These patterns were most likely caused by permanent El Niño-like conditions characterised by a deeper nutricline, weak upwelling and low nutrient supply. At the end of the LIA, δ 30Siopal dropped to low values of +0.6‰ and opal productivity reached its minimum of the past 650 years. During the following transitional period of time the intensity of upwelling, nutrient supply and productivity increased abruptly as marked by the highest bSi contents of up to 38%, by δ15Nsed of up to ~7‰, and by the highest degree of silicate utilisation with δ30Siopal reaching values of +1.1‰. At the same time, detrital ϵNd and 87Sr/86Sr signatures documented increased wind strength and supply of dust to the shelf due to drier conditions. Since about 1870, productivity has been high but nutrient utilisation has remained at levels similar to the LIA, indicating significantly increased nutrient availability.
Comparison between the δ30Siopal and δ15Nsed signatures suggests that during the past 650 years the δ15Nsed signature in the Peruvian upwelling area has to a large extent been controlled by surface water utilisation and not, as previously assumed, by subsurface nitrogen loss processes in the water column, which only had a significant influence during modern times (i.e. since ~AD 1870).
