Nitrous oxide and methane in the Atlantic Ocean between 50°N and 52°S: Latitudinal distribution and sea-to-air flux

We discuss nitrous oxide (N(2)O) and methane (CH(4)) distributions in 49 vertical profiles covering the upper similar to 300 m of the water column along two similar to 13,500 km transects between similar to 50 degrees N and similar to 52 degrees S during the Atlantic Meridional Transect (AMT) programme (AMT cruises 12 and 13). Vertical N(2)O profiles were amenable to analysis on the basis of common features coincident with Longhurst provinces. In contrast, CH(4) showed no such pattern. The most striking feature of the latitudinal depth distributions was a well-defined "plume" of exceptionally high N(2)O concentrations coincident with very low levels of CH(4), located between similar to 23.5 degrees N and similar to 23.5 degrees S: this feature reflects the upwelling of deep waters containing N(2)O derived from nitrification, as identified by an analysis of N(2)O, apparent oxygen utilization (AOU) and NO(3)(-), and presumably depleted in CH(4) by bacterial oxidation. Sea-to-air emissions fluxes for a region equivalent to similar to 42% of the Atlantic Ocean surface area were in the range 0.40-0.68 Tg N(2)O yr(-1) and 0.81-1.43 Tg CH(4) yr(-1). Based on contemporary estimates of the global ocean source strengths of atmospheric N(2)O and CH(4), the Atlantic Ocean could account for similar to 6-15% and 4-13%, respectively, of these source totals. Given that the Atlantic Ocean accounts for around 20% of the global ocean surface, on unit area basis it appears that the Atlantic may be a slightly weaker source of atmospheric N(2)O than other ocean regions but it could make a somewhat larger contribution to marine-derived atmospheric CH(4) than previously thought. Crown Copyright (C) 2008 Published by Elsevier Ltd. All rights reserved.