The delta N-15 of nitrate in the Southern Ocean: Nitrogen cycling and circulation in the ocean interior

We report analyses of the nitrogen isotopic composition of nitrate in the e astern Indian and Pacific sectors of the Southern Ocean. In this paper, we focus on the subsurface data as well as data from the deep waters of other ocean basins. Nitrate delta(15)N is relatively invariant in much of the aby ssal ocean (i.e., below 2.5 km), with a value of 4.8 +/- 0.2 parts per thou sand observed in Lower Circumpolar Deep Water, North Atlantic Deep Water, a nd central Pacific deep water. The isotopic invariance of deep ocean nitrat e stems fundamentally from the completeness of nitrate utilization in most of the global surface ocean, the Southern Ocean surface being an important exception. In the Subantarctic Zone (north of the Polar Frontal Zone) the n itrate delta(15)N of Upper Circumpolar Deep Water is similar to 0.7 parts p er thousand greater than that of Lower Circumpolar Deep Water. This isotopi c enrichment appears to result from denitrification in the low-latitude wat er masses with which Upper Circumpolar Deep Water communicates. The isotopi c enrichment of Upper Circumpolar Deep Water is diminished in the Antarctic , probably because of the remineralization of sinking organic N, which has a low delta(15)N in the Antarctic. Relative to the other water masses of th e Southern Ocean, the Subantarctic thermocline has a very low nitrate delta 15N for its nitrate concentration because of exchange with the low-latitud e thermocline, where this isotopic signature appears to originate. This sig nature of the low-latitude thermocline has two probable causes: (I)mixing w ith low-nitrate surface water and (2) the oxidation of newly fixed N.