Reconstruction of surface ocean nitrate utilization in the Southeast Atlantic Ocean based on stable nitrogen isotopes.

Abstract

Stable nitrogen isotopes measured in bulk sediments and sinking particles have been used to evaluate changes in relative nutrient utilization in surface waters of the subtropical southeast Atlantic. In order to determine the dominant controls on the nitrogen isotopic composition of organic matter (OM) in this region at the present time, samples of sinking particulate matter collected with a sediment trap and surface sediments in seven sea to shore transects in the Angola and Cape Basins were analyzed. Variations in the degree of surface nitrate depletion during the Late Quaternary were also investigated based on g15N in two deep-sea cores from the Angola Basin.The g15N signal observed in sinking partic1es at the Walvis Ridge over a one year period mirrored sea surface temperatures (SST) and particulate flux rates with two distinct g15N decreases of ~2 in austral fall and spring. Low g15N values coincided with low SST, indicating that the low temperatures were brought about by recently upwelled, nutrient-rich water, a supposition substantiated by wind stress data from the literature. The simultaneous occurrance of maxima in particle flux rates with the decreases in g15N values and SST minima provides additional evidence that the g15N signal in sinking OM in the study area records the extent to which the surface nitrate pool has been utilized as well as the productivity variations driven by euphotic zone nitrate availability.