The seasonal cycle of ocean‐atmosphere CO2 flux in Ryder Bay, west Antarctic Peninsula

Oliver J. Legge,Martin T. Johnson,Peter J. Brown,Hugh J. Venables,Michael P. Meredith,Gareth A. Lee,Dorothee C. E. Bakker

doi:10.1002/2015gl063796

Abstract

AbstractApproximately 15 million km2 of the Southern Ocean is seasonally ice covered, yet the processes affecting carbon cycling and gas exchange in this climatically important region remain inadequately understood. Here, 3 years of dissolved inorganic carbon (DIC) measurements and carbon dioxide (CO2) fluxes from Ryder Bay on the west Antarctic Peninsula (WAP) are presented. During spring and summer, primary production in the surface ocean promotes atmospheric CO2 uptake. In winter, higher DIC, caused by net heterotrophy and vertical mixing with Circumpolar Deep Water, results in outgassing of CO2 from the ocean. Ryder Bay is found to be a net sink of atmospheric CO2 of 0.90–1.39 mol Cm−2 yr−1 (average of 3 years). Seasonal sea ice cover modifies the net annual CO2 uptake, but its effect on gas exchange remains poorly constrained. A reduction in sea ice on the WAP shelf may reduce the strength of the oceanic CO2 sink in this region.

Highlights

The Southern Ocean south of 44∘S is responsible for approximately 25–30% of the global ocean uptake of anthropogenic carbon [Fletcher et al, 2006; Lenton et al, 2013], but accurately quantifying this sink and understanding the processes behind it remains challenging
dissolved inorganic carbon (DIC) was measured by coulometry [Johnson et al, 1985] following standard operating procedure (SOP) 2 of Dickson et al [2007], and total alkalinity (TA) was measured by potentiometric titration [Mintrop et al, 2000] following SOP 3b of Dickson et al [2007]
There is a gradual increase from around April to maximum DIC concentrations of ∼2200 μmol kg−1 in September. This increase during autumn and winter is caused by net heterotrophy and mixing with relatively old, carbon-rich Circumpolar Deep Water (CDW) as the mixed layer deepens

Summary

Introduction

The Southern Ocean south of 44∘S is responsible for approximately 25–30% of the global ocean uptake of anthropogenic carbon [Fletcher et al, 2006; Lenton et al, 2013], but accurately quantifying this sink and understanding the processes behind it remains challenging. Seasonal sea ice cover increases the net annual CO2 uptake, but its effect on gas exchange remains poorly constrained. The net ocean-atmosphere CO2 flux of the high-latitude, seasonally ice-covered Southern Ocean is especially difficult to quantify due to a scarcity in observational data, during the ice-covered winter months [Bakker et al, 2014].

Results

Conclusion