文摘
Chilean margin sedimentary N isotope records have been the focus of paleoceanographic studies examining the extent of water-column denitrification in the eastern South Pacific in the past. Here we use 15N/14N of nitrate and surface sedimentary N along the Chilean coast to investigate the relative contributions of water-column denitrification and surface nitrate assimilation by phytoplankton to the sedimentary N isotope record. Off northern and central Chile, subsurface enrichment of 15NO3- is associated with the lowest oxygen concentrations and maximum nitrate deficits, the product of water-column denitrification, locally at 21°S and through the transport of denitrified waters to the south. While elevated, the δ15N of pycnocline nitrate shows no distinct trend with distance from the OMZ and is nearly homogenous within the shallow subsurface layer, presumably due to lateral circulation along the margin. Moreover, an isotopically depleted and relatively uniform layer exists within the shallow subsurface, possibly as a result of the remineralization of newly fixed nitrogen that may work to further homogenize the δ15N of the upwelling nitrate. Whereas the high δ15N of sedimentary N in the region is clearly a product of denitrification and its isotopic imprint on nitrate along the margin, the northward increase in sedimentary δ15N from higher southern latitudes also reflects the degree of surface layer nitrate consumption by phytoplankton. The northward increase in sedimentary δ15N corresponds to a regional decrease in the surface nitrate concentration, and isotopic fractionation during nitrate assimilation is apparent in shallow nitrate δ15N. A comparison of the δ15N in shallow subsurface nitrate and sedimentary N suggests that, north of 30°S, nearly complete nitrate consumption causes the δ15N of sediments to converge on that of the nitrate supply, such that denitrification should be the dominant signal in downcore δ15N records from these latitudes. Moreover, the lateral homogeneity of the denitrification signal in the subsurface within this region suggests that such records will provide robust reconstructions of denitrification intensity.
