11:00 〜 13:00
[AAS11-P06] 南大洋における表層海水中のCO2分圧及び大気海洋間CO2フラックスの変動
キーワード：南大洋、大気海洋間CO2フラックス、CO2分圧
The oceans have played an important role in reducing global warming by taking up about 30% of anthropogenic CO2 emissions into the atmosphere since the Industrial Revolution. The Southern Ocean has played a significant role in the CO2 uptake, accounting for about 40% of the anthropogenic CO2 absorbed by the global oceans. However, observations of atmospheric and oceanic CO2 in the Southern Ocean are limited due to remoteness from civilization, severe sea conditions and the presence of sea ice.
To investigate the variability of surface water CO2 in the Southern Ocean, we analyzed the CO2 partial pressure (pCO2,sea) data in the surface sea water obtained by shipboard observations. The shipboard observations were conducted in the Indian Ocean of the Southern Ocean (Fremantle – Syowa Station, and Syowa Station – Sydney) during the eleven cruises of the research vessel (R/V) Shirase during the austral summer (December to March) from 2009 to 2019. The pCO2,sea data were obtained by using an on-board continuous measurement system with a nondispersive infrared (NDIR) gas analyzer and a flow-through system type gas-liquid equilibrator. We investigated long-term pCO2,sea variations at the cruise tracks along the 110°E line from 40°S to 60°S (Area 1) and the spatial variations of pCO2,sea around the Antarctic coastal area (Area 2). In this study, the Area 1 was divided into Sub Antarctic Zone (SAZ), Polar Frontal Zone (PFZ) and Antarctic Zone (AZ) according to the oceanic fronts.
In the Area 1, we compared the latitudinal distribution of the pCO2,sea observed on-board the R/V Shirase (pCO2,sea_obs) with that of the global grid data released by Japan Meteorological Agency (pCO2,sea_gird). In AZ, the pCO2,sea_obs data were systematically larger than the pCO2,sea_grid data by about 20 μatm, which could have a significant impact on the estimate of CO2 uptake in the Southern Ocean. The observed data (pCO2,sea_obs) showed rapid change at the Sub Antarctic Front while pCO2,sea_grid did not, indicating that the grid data are unable to represent fine structures of the pCO2,sea distributions.
We calculated CO2 fluxes along the cruise tracks of R/V Shirase by using pCO2,sea, pCO2,air, sea surface temperature (SST), wind speed and sea surface salinity and investigated long-term trends of the CO2 fluxes in December of each year from 2009–2019. As a result, CO2 flux (positive flux represents CO2 uptake form the atmosphere to the ocean) increased at a rate of 0.08 and 0.14 gC/m2/month/yr in SAZ and PFZ. In contrast, there was no trend of CO2 flux in AZ. The CO2 flux increase was found to be largely determined by increase of pCO2 difference between the atmosphere and surface seawater. In SAZ and PFZ, pCO2,sea increased at a rate of 1.1–1.9 μatm/yr, which were smaller than the pCO2 increase in the atmosphere (pCO2,air) (2.4 μatm/yr). On the other hand, the pCO2,air increase in AZ was about the same rate as the pCO2,sea trend. The component analysis of the pCO2,sea trend showed that the contribution of increased dissolved inorganic carbon (DIC) in the sea water was significant.
In the Area 2, We observed low pCO2,sea ( < 300 μatm) in the ocean around Lutzow-Holm Bay, Cape Danley and Totten Glacier. We examined the relationship between pCO2,sea and SST and between pCO2,sea and SSS around Cape Danley and found no correlation between pCO2,sea and SST, but there was a positive correlation between pCO2,sea and SSS, suggesting that a part of pCO2,sea decrease was due to dilution by meltwater from sea ice. However, since the pCO2,sea normalized by salinity shows still low value around 260 μatm. This suggests that the pCO2,sea is reduced by factors other than dilution by meltwater, such as active biological activity at the ice margin region.
To investigate the variability of surface water CO2 in the Southern Ocean, we analyzed the CO2 partial pressure (pCO2,sea) data in the surface sea water obtained by shipboard observations. The shipboard observations were conducted in the Indian Ocean of the Southern Ocean (Fremantle – Syowa Station, and Syowa Station – Sydney) during the eleven cruises of the research vessel (R/V) Shirase during the austral summer (December to March) from 2009 to 2019. The pCO2,sea data were obtained by using an on-board continuous measurement system with a nondispersive infrared (NDIR) gas analyzer and a flow-through system type gas-liquid equilibrator. We investigated long-term pCO2,sea variations at the cruise tracks along the 110°E line from 40°S to 60°S (Area 1) and the spatial variations of pCO2,sea around the Antarctic coastal area (Area 2). In this study, the Area 1 was divided into Sub Antarctic Zone (SAZ), Polar Frontal Zone (PFZ) and Antarctic Zone (AZ) according to the oceanic fronts.
In the Area 1, we compared the latitudinal distribution of the pCO2,sea observed on-board the R/V Shirase (pCO2,sea_obs) with that of the global grid data released by Japan Meteorological Agency (pCO2,sea_gird). In AZ, the pCO2,sea_obs data were systematically larger than the pCO2,sea_grid data by about 20 μatm, which could have a significant impact on the estimate of CO2 uptake in the Southern Ocean. The observed data (pCO2,sea_obs) showed rapid change at the Sub Antarctic Front while pCO2,sea_grid did not, indicating that the grid data are unable to represent fine structures of the pCO2,sea distributions.
We calculated CO2 fluxes along the cruise tracks of R/V Shirase by using pCO2,sea, pCO2,air, sea surface temperature (SST), wind speed and sea surface salinity and investigated long-term trends of the CO2 fluxes in December of each year from 2009–2019. As a result, CO2 flux (positive flux represents CO2 uptake form the atmosphere to the ocean) increased at a rate of 0.08 and 0.14 gC/m2/month/yr in SAZ and PFZ. In contrast, there was no trend of CO2 flux in AZ. The CO2 flux increase was found to be largely determined by increase of pCO2 difference between the atmosphere and surface seawater. In SAZ and PFZ, pCO2,sea increased at a rate of 1.1–1.9 μatm/yr, which were smaller than the pCO2 increase in the atmosphere (pCO2,air) (2.4 μatm/yr). On the other hand, the pCO2,air increase in AZ was about the same rate as the pCO2,sea trend. The component analysis of the pCO2,sea trend showed that the contribution of increased dissolved inorganic carbon (DIC) in the sea water was significant.
In the Area 2, We observed low pCO2,sea ( < 300 μatm) in the ocean around Lutzow-Holm Bay, Cape Danley and Totten Glacier. We examined the relationship between pCO2,sea and SST and between pCO2,sea and SSS around Cape Danley and found no correlation between pCO2,sea and SST, but there was a positive correlation between pCO2,sea and SSS, suggesting that a part of pCO2,sea decrease was due to dilution by meltwater from sea ice. However, since the pCO2,sea normalized by salinity shows still low value around 260 μatm. This suggests that the pCO2,sea is reduced by factors other than dilution by meltwater, such as active biological activity at the ice margin region.