An East-West transect of N2 gas Across the Eastern Tropical North Pacific Oxygen Deficient Zone

The Eastern Tropical North Pacific Oxygen Deficient Zone (ODZ) is the largest marine ODZ and has most of its volume under oligotrophic surface waters. However, the actual westward extent of anoxic waters is not well known. In April 2018, we traversed a diagonal transect from the Mexican coast (106^oW, 19^oN) to the edge of the ODZ (128^oW, 15^oN), sampling every 2^o longitude. While the ODZ had a total thickness of 640m (110m to 750m) at 106^oW, it narrowed to 220m thickness (180m to 400m) from 120^oW to 126^oW. A secondary chlorophyll maximum in the ODZ persisted from 110^oW until 120^oW. Low chlorophyll in these surface waters allowed deeper light penetration, and the secondary chlorophyll maxima to exist at deeper depths (140-160m) than typically seen (100-130m). The presence of primary producers inside the ODZ may compensate for low carbon flux rates from surface waters. Nitrite was present at all anoxic stations with higher concentrations closer to shore (3.5 μM at 106^oW versus 0.9 μM at 126^oW). Biological N₂ gas concentrations matched N loss calculated from nutrients (N*), and their maxima remained at ~15 μM throughout this transect, including at the hypoxic station (128^oW). The thickness of the N₂ peak changed with the thickness of the ODZ. Higher biological N₂ in each depth profile corresponded with more depleted biological δ¹⁵N-N₂,. However, at the hypoxic station (128^oW), [biological N₂] and δ¹⁵N positively correlated. We believe that the negative correlation of [biological N₂ gas] and δ¹⁵N combined with the presence of nitrite, indicated that N₂ production was occurring throughout the anoxic stations of this transect. The large volume of oligotrophic anoxic waters and their predicted future expansion makes understanding carbon limited anoxic N cycling critical. Here we show that anoxic waters extend over more than 20^o longitude in the ETNP and that N₂ production likely occurred throughout this area.