National Science Foundation


PROJECT #1: Denitrification in OMZs


Title:               Collaborative Research: Autonomous Lagrangian Floats for Oxygen Minimum Zone Biogeochemistry

PI:                   Craig McNeil

Source:            National Science Foundation (NSF) OCE-1153295

Amount:          $999,760 plus supplement of $199,758

Period:             1 June 2012 to 31 May 2017 (on NCE)


Intense oxygen minimum zones (OMZ) of the world’s oceans, though constituting a small fraction of total oceanic volume (0.1‰), host critical biogeochemical processes and are central to understanding the ocean’s N cycle and its biogeochemical and isotopic signatures. OMZ’s are sites for a large portion of marine combined N loss to N2 (25 to 50%) and dominate the ocean N isotope budget through cogeneration of 15N and 18O enriched NO3-. Major outstanding issues include the magnitude of this N sink,the stoichiometry between NO3- loss and the production of biogenic N2, the microbial pathways leading to N2 production, as well as the interaction between these OMZ processes and the surface export of organic matter as well as physical circulation. At stake are assessment of the current balance between oceanic N sources and sinks and prediction of OMZ responses to future climate change. Autonomous platforms have great potential for continuously monitoring OMZ’s and observing their unique biogeochemical processes on time/space scales not typically resolvable by ship-board observations. We propose to develop a sensor suite for such platforms appropriate to OMZs, install it on two Lagrangian floats with a 0 to 500 m depth range, and verify its performance against standard methods as part of a 6 month deployment in the OMZ south of Baja, Mexico. Two key chemical signatures of an OMZ are very low (0.001–10 μmol/kg) oxygen concentrations and supersaturated N2 concentrations due to conversion of NO3- to N2 by denitrification and/or anammox stimulated by these nearly anoxic conditions. We propose to develop a new method of operating an existing oxygen sensor (Seabird SBE-43) to yield improved accuracies at low concentrations and to develop a new type of gas tension sensor (for N2) that can operate at the depths of OMZ’s. NO3- concentrations and possibly NO2- will be measured by a Satlantic UV spectrophotometer. Together, these sensors will remotely measure N2, O2 and NO3- with an accuracy goal of 0.3, 0.3 and 1 μmol/kg respectively. These measurements will be validated against high precision N2/Ar by mass spectrometry, standard nutrient analyses, and Winkler O2 measurements. We hypothesize based on recent observations that the subsurface N loss rate is highly patchy, responding to time varying inputs of organic matter from above. We will test this hypothesis by including optical backscatter (particles) and fluorescence (chlorophyll) sensors on our floats with the expectation that variations in O2, and N2, and perhaps NO3- will correlate with POM abundance and downward flux estimated by these optical sensors. Deployment is planned in a region expected to have strong mesoscale variability in surface productivity and presumably downward OM flux as observed from satellite imagery. The renewal rate of O2 is a key factor in controlling OMZ intensity. High frequency measurements of temperature and salinity, and oxygen, on the floats will be used to compute the diapycnal diffusivity, an important component of the renewal rate, while the trajectories of the floats may provide insights into the mechanisms of lateral renewal. These physical data will also be used to calculate N-loss rates if variations in N2 and/or NO3- are large enough.


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