Surface ocean carbon budget in the 2017 south Georgia diatom bloom: Observations and validation of profiling biogeochemical argo floats

Estimates of the partial pressure of CO2 (pCO2) derived from biogeochemical Argo floats have the potential to improve our knowledge of the highly variable and partially observed Southern Ocean carbon sink through sampling at improved temporal and spatial resolution. Here we use the data from six biogeochemical Argo floats to characterise near-surface dissolved inorganic carbon (DIC) concentrations and fluxes at the site of an intense (Chl-a >3 mg m-3) mesoscale diatom bloom situated northwest of South Georgia. Concurrently, we provide independent analysis and validation of the methodology used by the Southern Ocean Carbon and Climate Observational and Modelling (SOCCOM) project for deriving surface pCO2 from float-based pH and oxygen measurements. We compare the float observations with co-located ship data from bottle samples over a monthlong period. When compared to data sampled within 24 h and 25 km of each float profile, we find good agreement with a mean offset of -0.005 +/- 0.018 (1a) between float pH and bottle-derived pH. This translates to comparable pCO2 estimates between ship measurements and floats with a mean difference of 2.6 +/- 12.8 (1a) mu atm, providing support for the use of biogeochemical Argo float data to supplement shipboard pCO2 measurements in the Southern Ocean. Based on float-derived pCO2 we calculate a sizeable local flux of CO2 of 24 +/- 7 mmol C m- 2 d-1 (over a 27-day period) from the atmosphere into the surface mixed layer, driven by a large airsea pCO2 gradient and strong but variable winds. Despite the considerable air-sea flux, the local mixed layer carbon budget appears to be dominated by entrainment and detrainment of carbon-rich waters into and out of the mixed layer. However, given the large uncertainties associated with these fluxes and the significant challenges associated with closing the mixed layer budget, further research is required to refine float-based mixed layer DIC fluxes.

Automated summary

Using biogeochemical Argo floats, we studied the dissolved inorganic carbon (DIC) concentrations and fluxes of a mesoscale diatom bloom in the Southern Ocean. The methodology for deriving surface pCO2 from float-based pH and oxygen measurements was independently analyzed and validated. The results showed good agreement between float and ship measurements, supporting the use of biogeochemical Argo floats to supplement shipboard pCO2 measurements. The study also calculated a significant CO2 flux from the atmosphere into the surface mixed layer, driven by air-sea pCO2 gradient and variable winds.