Controls on sedimentary nitrogen isotopes along the Chile margin

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 ¹⁵ NO₃ ^- 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.