Chilean margin sedimentary N isotope records have been the focus of paleoceanographic studiesexamining the extent of water-column denitrification in the eastern South Pacific in the past. Here weuse 15N/14N of nitrate and surface sedimentary N along the Chilean coast to investigate the relativecontributions of water-column denitrification and surface nitrate assimilation by phytoplankton to thesedimentary N isotope record. Off northern and central Chile, subsurface enrichment of 15NO3 isassociated with the lowest oxygen concentrations and maximum nitrate deficits, the product of water-column denitrification, locally at 21 1S and through the transport of denitrified waters to the south.While elevated, the d15N of pycnocline nitrate shows no distinct trend with distance from the OMZ andis nearly homogenous within the shallow subsurface layer, presumably due to lateral circulation alongthe margin. Moreover, an isotopically depleted and relatively uniform layer exists within the shallowsubsurface, possibly as a result of the remineralization of newly fixed nitrogen that may work to furtherhomogenize the d15N of the upwelling nitrate. Whereas the high d15N of sedimentary N in the region isclearly a product of denitrification and its isotopic imprint on nitrate along the margin, the northwardincrease in sedimentary d15N from higher southern latitudes also reflects the degree of surface layernitrate consumption by phytoplankton. The northward increase in sedimentary d15N corresponds to aregional decrease in the surface nitrate concentration, and isotopic fractionation during nitrateassimilation is apparent in shallow nitrate d15N. A comparison of the d15N in shallow subsurface nitrateand sedimentary N suggests that, north of 301S, nearly complete nitrate consumption causes the d15 Nof sediments to converge on that of the nitrate supply, such that denitrification should be the dominantsignal in downcore d15 N records from these latitudes. Moreover, the lateral homogeneity of thedenitrification signal in the subsurface within this region suggests that such records will provide robustreconstructions of denitrification intensity.
Controls on sedimentary nitrogen isotopes along the Chile margin.
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