Award: OCE-1851361

Intellectual Merit

 New approaches have been developed for using autonomous platforms to study the biogeochemistry of naturally occurring oxygen deficient zones in the ocean (ODZs). This first involved development and acquisition of floats (Fig. 1) that are capable of acquiring data during Lagrangian drifts at relatively shallow depths to determine rates of biogeochemical processes.  By drifting at near constant depth/density, variations in biogeochemical properties with time may interpreted as reflecting in situ rates.  Second, commercially available optode oxygen sensors were calibrated and validated for very low oxygen measurement necessary for ODZ regions (details below).  Optodes are typically not calibrated for O₂ concentrations <5 umol/kg even though this a critical range for microbial process key to ODZ biogeochemistry.  Both lab and field results showed a precision of better than 0.1 umol/kg.  Third gas tension devices (GTD) were developed as unique sensors for measuring the production of biogenic N₂ gas produced by microbial N-loss, a key process for the ocean’s global nitrogen budget that occurs in ODZs.  GTD’s were deployed on floats as well as from a ship’s CTD and these data were essential for interpretation of the float-based data.

Two cruises were made to the Eastern Tropical N Pacific (ETNP) ODZ. The first (SR2011) took place between Dec 2020 and Jan 2021 and served as a survey and testing cruise (Fig. 2).  The second and longer one (SR2114) took place between Dec 2021 and Jan 2022 along a track between Costa Rica and San Diego, USA (Fig. 3). During this cruise, 18 autonomous floats were deployed and 50 hydrographic stations were made spanning the southern boundary, core, and northern boundary of the ETNP ODZ.  Overall, our ship and float surveys of the ETNP ODZ system have revealed their complex geography and the likely role of mesoscale eddies in their structure and evolution. This places related work on the microbiology of these systems in a much clearer geographical, physical and chemical context. We found mesoscale physics play a significant role in ODZ biogeochemistry of the ETNP. During our two cruises, satellite imagery was used in real time to target mesoscale features for sampling. During SR2011, an eastward flowing jet with elevated chlorophyll was studied. During SR2114, 2 coastal anticyclones, an offshore anticyclone and a high chlorophyll jet of coastal origin were observed. 

Mesoscale physics impacted ODZ biogeochemistry in at least two distinct ways. The first involved vertical excursions in density layers and the depth of the oxycline of up to 75 m as seen in a large offshore anticyclone during SR2114 (Fig. 3). An unusually deep secondary chlorophyll maximum (200 m) was likely also produced by the downward forcing of density surfaces at eddy center (Fig. 4). Greater O₂ penetration to deeper density surfaces in this feature also suggests enhanced vertical mixing. The second route for mesoscale impact is associated with the transport of high productivity (high chlorophyll) upwelled coastal waters 100’s of km offshore by jets forced by periodic strong winds entering the ETNP through the mountain gaps in Tehuantepec and Papagayo (Fig. 3). Presumably through enhanced downward organic carbon flux, highest rates of N-loss were measured offshore at a station within a chlorophyll jet. These results challenge the current paradigm that near-shore waters support the highest N loss rates. High N₂O concentrations in a similar feature during SR2011 suggest an impact on sea-air fluxes of this greenhouse gas. Mesoscale features also appear responsible for subsurface O₂ intrusions along specific density surfaces into the geographic interior of the ODZ (Fig. 5). 

In order to measure ODZ relevant oxygen concentrations, we have also shown that typically oxygen optode sensors used on autonomous floats yield oxygen concentrations in the range of 2-4 µmol-1kg at zero oxygen concentration and are thus unable to detect the functionally anoxic regions. We have developed a new optode calibration formula applied it to optodes/floats we deployed in this project as well as developed a method to correct for sensor drift during float deployment.

Broader Impacts

This work demonstrates how the structure and extent of the oceanic ODZs could be monitored by an array of BGC Argo floats in a changing ocean. Graduate students at UMassD were trained in start of the art analytical techniques as well as had extensive at-sea experience. This project entailed extensive national and international collaboration with researchers at WHOI, Montana State, U. South Carolina, Texas A&M Corpus Cristi, U. Victoria, CICIMAR, U. Southern Denmark, U. Victoria, U. Barcelona , National U. of Costa Rica, and the Guatemala Env. Ministry.

Last Modified: 05/27/2025
Modified by: Mark A Altabet