BGC-Argo Floats Reveal Nitrite and Thiosulfate Dynamics in the Oceans With High Spatiotemporal Resolution

Marine oxygen deficient zones (ODZs) play a major role in the Earth's biogeochemical cycles and are responsible for nitrogen and sulfur removal from the oceans. Microbial-reducing reaction processes generate nitrite (NO₂⁻) and sulfur compounds as intermediaries that may accumulate in these zones. Current assessments on microbial transformations inside ODZs are based on shipboard measurements, and there are no well-resolved seasonal to annual observations or high-resolution vertical sampling that would characterize variability. Here, we propose an alternative statistical approach to analyze the raw output of the nitrate sensor from BGC-Argo floats with the ability to detect NO₂⁻ and thiosulfate (S₂O₃²⁻) concentrations in addition to nitrate. The new approach provides data with great vertical and spatiotemporal resolution. The method can be applied to UV-spectrometer output data from SUNAs and ISUS nitrate sensors commonly deployed on various observing platforms. We validated the technique in the field by matching shipboard NO₂⁻ bottle data with float data from the Eastern Tropical North Pacific (ETNP) and Eastern Tropical South Pacific (ETSP) ODZs. We then show a complete time series of three floats as study cases. The ability to detect NO₂⁻ and S₂O₃²⁻ concomitantly with other key chemical variables (i.e., oxygen, pH, and bio-optics) at such fine scale allows for novel insights into the nitrogen and sulfur cycling of ODZs and processes driving these cycles. This new approach will enable fine-scale remote quantification of NO₂⁻ and S₂O₃²⁻ to support a better understanding of the biogeochemical transformations happening inside these already-expanding deoxygenated regions.