Carbon export and the uptake of anthropogenic CO\\(_2\\) in the Southern Ocean using stable carbon isotopes of dissolved inorganic carbon
Creators: name: McNeil, BIB
Publisher: name: University Of Tasmania
Year: 2023

description: The main objective of this research was to investigate and better understand carbon cycling in the Southern Ocean. Measurements of Dissolved Inorganic Carbon (DIC) and its stable carbon isotopes were used to quantify both carbon export and the accumulation and uptake of anthropogenic CO\\(_2\\) in the Australian sector of the Southern Ocean. To determine carbon export, we measured DIC and ˜í¬•\\(^{13}\\)C\\(_{DIC}\\) on five cruises which cover winter and summer conditions in the sub-Antarctic zone (SAZ), South of Australia. Seasonal variations in mixed layer DIC (45˜í¬¿mol/kg) and ˜í¬•\\(^{13}\\)C\\(_{DIC}\\) (0.45‚ÄövÑ‚àû) in the region were large. Biological production contributed to about 80% of the winter to summer mixed layer carbon changes in the SAZ with air-sea exchange (10%) and mixing (10%) having smaller contributions. The integrated biological carbon export from the mixed layer from July to February was largest in the northern SAZ (45¬¨‚àûS-47¬¨‚àûS) at 3400mmol/m\\(^2\\) and decreased to 1100mmol/m\\(^2\\) at the sub-Antarctic Front (SAP). The isotopic composition of exported carbon was estimated to be about 18¬¨¬±2‚ÄövÑ‚àû and suggests larger phytoplankton drive the seasonal carbon export in the SAZ. Extrapolating the derived carbon export to the circumpolar SAZ gives a total carbon export from July to February of 0.65GTC. To quantify anthropogenic CO\\(_2\\) accumulation, we compared DIC measurements taken in 1968 to those taken in 1996 using a multi-parametric linear regression technique. For this period, significant accumulation of anthropogenic CO\\(_2\\) (13¬¨¬±10˜í¬¿mol/kg) was observed in Antarctic Bottom Water (AABW) down to 4500m suggesting AABW is important in transporting anthropogenic CO\\(_2\\) from the surface to the deep ocean. Very low penetration and accumulation was observed in the Antarctic Zone (AZ) while while the highest accumulation (&lt;24˜í¬¿mol/kg) was observed in the sub-Antarctic zone (SAZ). The penetration of anthropogenic CO\\(_2\\) south of Australia was consistent with the penetration of 28 year old water based on CFC age estimates. To determine the uptake of anthropogenic CO\\(_2\\), we compared DIC and ˜í¬•\\(^{13}\\)C\\(_{DIC}\\) measurements taken 20 years apart (˜ívÆDIC and ˜ívÆ˜í¬•\\(^{13}\\)C\\(_{DIC}\\)). ˜ívÆDIC increased by up to 20˜í¬¿mol/kg in the SAZ and 10˜í¬¿mo1/kg in the Antarctic Zone (AZ) over the 20 year period while ˜ívÆ˜í¬•\\(^{13}\\)C\\(_{DIC}\\) decreased by up to -0.3‚ÄövÑ‚àû in the SAZ and -0.1‚ÄövÑ‚àû in the AZ. The maximum penetration depth of ˜ívÆ˜í¬•\\(^{13}\\)C\\(_{DIC}\\) was up to 800m shallower than for ˜ívÆDIC in the SAZ. To compare ˜ívÆDIC to ˜ívÆ˜í¬•\\(^{13}\\)C\\(_{DIC}\\), we used the ratio of the two anomalies (˜ívÆRC = -˜ívÆ˜í¬•\\(^{13}\\)C\\(_{DIC}\\) / ˜ívÆDIC). ˜ívÆRC decreased moving south in the Southern Ocean suggesting ˜í¬•\\(^{13}\\)C\\(_{DIC}\\) is not a good predictor of anthropogenic CO\\(_2\\) in the region. ˜ívÆRC decreased in the Southern Ocean due to a factor of ten difference in air-sea equilibration rates between DIC and ˜í¬•\\(^{13}\\)C\\(_{DIC}\\) and is illustrated using a surface box model. ˜ívÆRC is dependent on the residence time and ventilation rate of source waters and could therefore be useful in tracing the history of recently ventilated water masses and any future changes in the formation rate of these water masses. Using a box model and CFC-11 measurements, we estimated the formation and export of SAMW and quantified the uptake of anthropogenic CO\\(_2\\) in the SAZ to be 0.73- 0.86˜í¬¿mol/kg/yr. Extrapolating this estimate to the circumpolar SAZ gives an anthropogenic CO\\(_2\\) uptake of 0.07-0.08GTC/yr. In contrast, particulate organic carbon (POC) export in the SAZ (~0.65GTC/yr) was much greater and any changes to the magnitude of POC export has the potential to significantly impact future anthropogenic CO\\(_2\\)2 uptake by the Southern Ocean.