Abstract
The carbon cycle in seasonally sea-ice covered waters remains poorly understood due to both a lack of observational data and the complexity of the system. Here we present three consecutive seasonal cycles of upper ocean dissolved inorganic carbon (DIC) and total alkalinity measurements from Ryder Bay on the West Antarctic Peninsula. We attribute the observed changes in DIC to four processes: mixing of water masses, air–sea CO2 flux, calcium carbonate precipitation/dissolution and photosynthesis/respiration. This approach enables us to resolve the main drivers of the seasonal DIC cycle and also investigate the mechanisms behind interannual variability in the carbonate system. We observe a strong, asymmetric seasonal cycle in the carbonate system, driven by physical processes and primary production. In summer, melting glacial ice and sea ice and a reduction in mixing with deeper water reduce the concentration of DIC in surface waters. The dominant process affecting the carbonate system is net photosynthesis which reduces DIC and the fugacity of CO2, making the ocean a net sink of atmospheric CO2. In winter, mixing with deeper, carbon-rich water and net heterotrophy increase surface DIC concentrations, resulting in pH as low as 7.95 and aragonite saturation states close to 1. We observe no clear seasonal cycle of calcium carbonate precipitation/dissolution but some short-lived features of the carbonate time series strongly suggest that significant precipitation of calcium carbonate does occur in the Bay. The variability observed in this study demonstrates that changes in mixing and sea-ice cover significantly affect carbon cycling in this dynamic environment. Maintaining this unique time series will allow the carbonate system in seasonally sea-ice covered waters to be better understood.
Highlights
The Southern Ocean plays an important role in the global carbon cycle through the uptake and storage of atmospheric CO2 (Sabine et al, 2004) and the formation of deep water masses (Sallée et al, 2012)
Samples for dissolved inorganic carbon (DIC) and total alkalinity (TA) were collected between December 2010 and February 2014 at the Rothera Time Series (RaTS), about 4 km offshore, in Ryder Bay, on the West Antarctic Peninsula (Fig. 1)
DIC was measured by coulometry (Johnson et al, 1985) following Standard Operating Procedure (SOP) 2 of Dickson et al (2007) and TA was measured by potentiometric titration (Mintrop et al, 2000) following SOP 3b of Dickson et al (2007)
Summary
The Southern Ocean plays an important role in the global carbon cycle through the uptake and storage of atmospheric CO2 (Sabine et al, 2004) and the formation of deep water masses (Sallée et al, 2012). The role of the seasonally sea-ice covered coastal Southern Ocean in the global carbon cycle remains poorly constrained (Lenton et al, 2013) due to a scarcity of observations and the complexity and variability of this environment. The waters of the West Antarctic Peninsula (WAP) exhibit strong seasonal and interannual variability and are experiencing significant climate change. This variability makes the region an ideal location to study carbon system processes and their interactions on various timescales.
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