Carbon cycling dynamics in the seasonal sea-ice zone of East Antarctica

The Southern Ocean plays a critical role in the global carbon cycle, accounting for over 40% of the global ocean uptake of anthropogenic carbon dioxide (CO\\(_2\\)). Observations are often localized and widely separated in both space and time, resulting in a heavy dependence on models to characterize carbon fluxes at regional scales in this area. Subsequently, notable discrepancies exist between model and observational results within the seasonal sea-ice zone (SIZ) around Antarctica. Given the important role of atmospheric CO\\(_2\\) in the climate system and its influence on changing ocean chemistry (ocean acidification), there is a need to accurately attribute the causes of change and develop a regional understanding of the CO\\(_2\\) sink/source nature of the Southern Ocean. In this thesis, the carbon cycle in the seasonal sea-ice zone of East Antarctica was investigated across a variety of spatial and temporal scales. In this region a large variability in the drivers and timing of carbon cycling dynamics was observed. Analysis of data from an oceanographic survey carried out during the austral summer (January ‚Äö- March 2006), between 30¬¨‚àû-80¬¨‚àûE and 60¬¨‚àû-69¬¨‚àûS, showed the SIZ to be a weak net source of CO\\(_2\\) to the atmosphere of 0.07 ¬¨¬± 0.13 mol C m\\(^{-2}\\) during the spring/summer ice-free period. Narrow bands of CO\\(_2\\) uptake were observed near the continental margin and north of the Southern Antarctic Circumpolar Current Front. Continuous surface measurements of dissolved oxygen and the fugacity of CO\\(_2\\) \\(f\\)(CO\\(_2\\)) were combined with net community production estimates from oxygen/argon ratios to show that surface heat gain and photosynthesis were responsible for the majority of surface water biogeochemical variability during the survey. On seasonal timescales, winter sea-ice cover acted to reduce the flux of CO\\(_2\\) to the atmosphere in the study area, followed by biologically driven drawdown of CO\\(_2\\) as the ice retreated in spring-summer. This highlights the import role that sea-ice formation and retreat has on the biogeochemical dynamics of the region. The influence of sea-ice formation and retreat was observed in greater detail at a coastal site in Prydz Bay near Davis station (66.5766vÄv¿S, 77.9674vÄv¿E), where the annual cycles of dissolved CO\\(_2\\) system parameters were determined using samples collected from May 2010 to February 2011. These observations show the seasonal influence of ice formation and melt, biological production, and sea-air CO\\(_2\\) flux on changes in total dissolved inorganic carbon (DIC), pH\\(_{sws}\\) and the saturation state of aragonite (˜í¬©\\(_{ar}\\)). Net community production of 1.8 ¬¨¬± 0.4 mol C m\\(^{-2}\\) in the productive summer months (November-February) caused large seasonal decreases in DIC. The decrease in DIC caused a change in surface water partial pressure of CO\\(_2\\) from values over-saturated with respect to the atmosphere in the ice-covered winter period, to undersaturated waters in the summer months. In contrast to the offshore SIZ, the coastal study site was estimated to be an annual net sink for CO\\(_2\\) of 0.54 ¬¨¬± 0.11 mol C m\\(^{-2}\\) year\\(^{-1}\\). The calculated pH\\(_{sws}\\) and ˜í¬©\\(_{ar}\\) values varied seasonally from 7.99 to 8.20 and 1.19 to 1.92, respectively. The observed variability was compared to similar measurements carried out in 1993-95 at the same location, and this revealed that natural variability in carbon cycle dynamics caused changes in pH\\(_{sws}\\) that were nearly twice as large as those expected from changes estimated due to ocean acidification over this time. In addition to the analysis of carbon cycle dynamics in offshore and coastal East Antarctica, an experiment was designed to assess the impact of ocean acidification on benthic communities near Casey station (66.2818vÄv¿S, 110.5276vÄv¿E) in East Antarctica from December 2014 to February 2015. Changes in dissolved CO\\(_2\\) system parameters within this first Antarctic free ocean CO\\(_2\\) enrichment (antFOCE) experiment showed how the system successfully manipulated seawater carbonate chemistry to maintain a mean pH offset from ambient values of 0.38 ¬¨¬± 0.07 pH units for approximately 6 weeks of the 8-week experimental period. Diel and seasonal fluctuations in ambient pH were duplicated in experimental chambers, located on the seafloor under sea ice, where the seawater pH was manipulated to match values expected by the end of this century under the Intergovernmental Panel on Climate Change Representative Concentration Pathway 8.5 greenhouse gas concentration trajectory. The mean pH\\(_{sws}\\), ˜í¬©\\(_{ar}\\) and \\(f\\)CO\\(_2\\) values in the experimental chambers were 7.680 ¬¨¬± 0.085, 0.62 ¬¨¬± 0.15 and 914 ¬¨¬± 160 ˜í¬¿atm, respectively. The experiment demonstrates the feasibility of FOCE systems, even under extreme conditions experienced in the Antarctic. The dynamic nature of the SIZ in general and the observed variability in dissolved CO\\(_2\\) system parameters in the broader region, demonstrates the need for continued monitoring of the marine carbon cycle so that regional models can accurately attribute causes of change and predict impacts of future ocean acidification.