Seasonality in Oxygen Structure, Utilization Rates, Export Production, and Impacts of Vertical Mixing in the Eighteen Degree Water Region of the Sargasso Sea as Observed by Profiling Floats

Abstract:

The seasonal oxygen structure and utilization in the Sargasso Sea’s Subtropical Mode Water (Eighteen Degree Water) region is carried out using nine CLIVAR Mode Water Dynamics Experiment (CLIMODE) profiling floats with oxygen sensors (years 2005-2008). During autumn-winter when the mixed layer is deepening, oxygen increases from the surface to the base of the Eighteen Degree Water (EDW) at ~400 m. During spring – summer, oxygen decreases in this layer except between the seasonal pycnocline and the compensation depth, creating the seasonal shallow oxygen maximum layer (SOMax) with oxygen production of ~0.04 μmol/kg/day. In the seasonal oxygen minimum (SOMin) just below the SOMax, the oxygen utilization rate (OUR) is 0.10 μmol/kg/day, decreasing with depth to 0.04 μmol/kg/day in the EDW. Remineralization in May-August is double that of August-November. The Sargasso Sea is a net producer of fixed carbon; estimated annual export production from the top 400 m is ~4.2 mol C/m². 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-September, likely due to advection from the northeast where these isopycnals outcrop. Summertime upward heave of these deeper isopycnals creates this paradox. The complex vertical structure of oxygen in the upper 200 m, composed of multiple upward and downward gradients, suggests that vertical diffusive flux could modify the oxygen distribution. We show that vertical mixing, quantified from float-observed restratification of the remnant winter mixed layer (Eighteen Degree water), modifies the OUR calculated from oxygen change; ignoring mixing leads to a 19% underestimate of maximum remineralization and an 88% underestimate of maximum net production rates. However, vertical mixing is negligible in the deeper layers, so the error in total integrated remineralization is 5 to 9%.