Oxygen Seasonality, Utilization Rate, and Impacts of Vertical Mixing in the Eighteen Degree Water Region of the Sargasso Sea as Observed by Profiling Biogeochemical Floats

Abstract

AbstractSeasonal oxygen structure and utilization in the Sargasso Sea are characterized using nine profiling floats with oxygen 2021 sensors (years 2005–2008), deployed in an Eighteen Degree Water (EDW) experiment (CLIMODE). During autumn‐winter when the mixed layer is deepening, oxygen increases from the surface to the base of the EDW at 400 m. During spring‐summer, oxygen decreases except between the seasonal pycnocline and compensation depth, creating the seasonal shallow oxygen maximum layer (SOMax) with oxygen production of 0.04 μmol kg−1·day−1. In the underlying seasonal oxygen minimum (SOMin), the oxygen utilization rate (OUR) is 0.10 μmol kg−1·day−1, decreasing with depth to 0.04 μmol kg−1·day−1 in the EDW. Remineralization in May to August is double that of August to November. The Sargasso Sea is a net carbon producer; estimated annual export production from the top 100–250 m is 2.9 mol C m−2 and from the top 400 m is 4.2 mol C m−2. Below the EDW, oxygen decreases seasonally at the same time as in the EDW, indicating remineralization down to 700 m. However, on isopycnals in this deeper layer, oxygen increases during May to September, likely due to lateral advection from nonlocal surface outcrops. Summer shoaling of these isopycnals creates this paradox. The complex vertical oxygen structure in the upper 200 m enables important vertical diffusive flux that modifies the OUR calculated from oxygen change. Ignoring mixing underestimates maximum remineralization by 19% and underestimates maximum net production by 88%. However, vertical mixing is negligible in the deeper layers, so the associated total integrated remineralization error is 5%–9%.