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

Clara A Fuchsman, University of Maryland Center for Environmental Science Horn Point Laboratory, Cambridge, MD, United States and Allan Devol, University of Washington, School of Oceanography, Seattle, WA, United States
Abstract:
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 (106oW, 19oN) to the edge of the ODZ (128oW, 15oN), sampling every 2o longitude. While the ODZ had a total thickness of 640m (110m to 750m) at 106oW, it narrowed to 220m thickness (180m to 400m) from 120oW to 126oW. A secondary chlorophyll maximum in the ODZ persisted from 110oW until 120oW. 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 106oW versus 0.9 μM at 126oW). 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 (128oW). 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 (128oW), [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 20o longitude in the ETNP and that N2 production likely occurred throughout this area.