Nutrient utilisation and weathering inputs in the Peruvian upwelling region since the Little Ice Age

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 (δ³⁰Si_opal) and nitrogen (δ¹⁵N_sed)), as well as in ocean circulation and material transport (authigenic and detrital radiogenic neodymium (ϵ_Nd) 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%, δ¹⁵N_sed values of ~5‰ and intermediate δ³⁰Si_opal 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 ³⁰Si_opal 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 δ¹⁵N_sed of up to ~7‰, and by the highest degree of silicate utilisation with δ³⁰Si_opal reaching values of +1.1‰. At the same time, detrital ϵ_Nd 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 δ³⁰Si_opal and δ¹⁵N_sed signatures suggests that during the past 650 years the δ¹⁵N_sed 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).