Reconstruction of past nutrient utilization in the Eastern Angola Basin based on sedimentary 15N/14N ratios

Abstract

The δ15N records of two sediment cores from the Angola Basin have been used to investigate past changes in nutrient utilization in the surface waters of this highly productive region. Low 15N/14N ratios in the cores (from >3000 m water depth) generally correspond to low sea surface temperatures and high sedimentary organic content, demonstrating a link between upwelling, productivity, and the degree of nitrate depletion. Glacial δ15N was lower by 0.5–0.7‰, and paleoproductivity was elevated by around 30% relative to interglacial periods, indicating that although productivity was higher, nitrate was less depleted during glacial than interglacial periods. More pronounced than the glacial/interglacial changes in δ15N values of both cores are precession‐related (23 kyr) fluctuations. These cyclic δ15N variations, which are in phase with those of paleoproductivity and sea surface temperature, indicate that changes in trade wind driven upwelling intensity and the advection of cold, nutrient‐rich water from the south drive nutrient availability and productivity off the southwest coast of Africa. No strong evidence was found from sedimentary δ15N values for denitrification in the water column in the Angola Basin during the past 180 kyr.