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

Abstract 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 (NO 2 − ) 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 NO 2 − and thiosulfate (S 2 O 3 2− ) 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 NO 2 − 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 NO 2 − and S 2 O 3 2− 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 NO 2 − and S 2 O 3 2− to support a better understanding of the biogeochemical transformations happening inside these already‐expanding deoxygenated regions.