Low oxygen eddies in the eastern tropical North Atlantic: Implications for N 2 O cycling.

Nitrous oxide (N2 O) is a climate relevant trace gas, and its production in the ocean generally increases under suboxic conditions. The Atlantic Ocean is well ventilated, and unlike the major oxygen minimum zones (OMZ) of the Pacific and Indian Oceans, dissolved oxygen and N2 O concentrations in the Atlantic OMZ are relatively high and low, respectively. This study, however, demonstrates that recently discovered low oxygen eddies in the eastern tropical North Atlantic (ETNA) can produce N2 O concentrations much higher (up to 115 nmol L-1 ) than those previously reported for the Atlantic Ocean, and which are within the range of the highest concentrations found in the open-ocean OMZs of the Pacific and Indian Oceans. N2 O isotope and isotopomer signatures, as well as molecular genetic results, also point towards a major shift in the N2 O cycling pathway in the core of the low oxygen eddy discussed here, and we report the first evidence for potential N2 O cycling via the denitrification pathway in the open Atlantic Ocean. Finally, we consider the implications of low oxygen eddies for bulk, upper water column N2 O at the regional scale, and point out the possible need for a reevaluation of how we view N2 O cycling in the ETNA.