Deep water nutrient and oxygen gradients in a modern coastal upwelling zone and their paleoceanographic implications

Dissolved phosphate and oxygen data from the waters at >300 rn depth beneath the Peru upwelling system imitate observed onshore/offshore surface productivity data. Positive onshore phosphate gradients and negative onshore oxygen gradients are greater in deeper water (up to 1000 m), reflecting the decreasing degree of horizontal mixing at depth. Mass balance reconstructions of surface productivity, nutrient remineralization, and onshore-offshore chemical gradients suggest that the lack of wide continental shelves during glacial climates would lead to more pronounced dissolved nutrient and oxygen gradients in the deep water adjacent to continental margins. Horizontal •i13C gradients in sediments beneath ighly productive coastal upwelling zones may have been as high as 0.5-1.0%o per 100 krn, suggesting caution in the interpretation ofbenthic •il3C samples adjacent to continental margins during low sea level stands.