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The Southern Ocean is a vast net sink for atmospheric carbon dioxide (CO2), with marine phytoplankton playing a crucial role in CO2 fixation. We assessed how changes in the dominant phytoplankton community and net primary productivity (NPP) affected variations in the partial pressure of CO2 in surface water (pCO(2)(sw)) in the Indian sector of the Southern Ocean during austral summer. pCO2sw was negatively correlated with total phytoplankton and diatom abundances, as estimated from pigment signatures, in the zone south of the Antarctic Circumpolar Current; however, pCO(2)(sw) was not correlated with haptophyte abundance. Additionally, a stronger correlation was found between pCO(2)(sw) and total phytoplankton NPP than between chlorophyll a concentration and pCO(2)(sw). We reconstructed pCO(2)(sw) at inter-annual scale using satellite data and assessed the inter-annual variability of air-sea CO2 flux. Over the period from 1997 to 2007, the integrated CO2 fluxes over the study region showed very large variations from a small source to a strong sink. Variations in the integrated CO2 fluxes were also correlated with changes in satellite-derived phytoplankton community in the Indian sector of the Southern Ocean and changes in the dominant phytoplankton community may control CO2 dynamics in the marginal ice zone.

An in situ measurement system was developed for continuous observations of the mole fractions of atmospheric oxygen (defined as δ(O2/N2)) and carbon dioxide (CO2) and the continuous observation was conducted onboard the research vessel SHIRASE 5003 during its voyage between Australia and Syowa Station, Antarctica, in 2017–2018. The CO2 variation was low with respect to that of δ(O2/N2) in the Southern Ocean, suggesting that the land biospheric and fossil-fuel derived CO2 and O2 emissions negligibly influenced the observations. Therefore, the observed significant variations in the atmospheric O2/N2 can be attributed to the atmosphere-ocean gas exchange. During the southbound voyage in December 2017, we observed large spatial variations in δ(O2/N2) due to marine biological production on the western side of the cruise track. Oceanic O2 fluxes based on a simple model and atmospheric δ(O2/N2) variability were consistent with past oceanic observations. No clear longitudinal gradient in δ(O2/N2) was observed at latitudes toward south of 60° S in December 2017 and February–March 2018. However, local δ(O2/N2) maxima were observed in regions with active marine biological production in December 2017. These observations indicate that local O2 fluxes can also modify the spatial distribution of atmospheric δ(O2/N2) in the Southern Ocean.

The ongoing rise in atmospheric CO2 concentration is causing rapid increases in seawater pCO2 levels. However, little is known about the potential impacts of elevated CO2 availability on the phytoplankton assemblages in the Southern Ocean’s oceanic regions. Therefore, we conducted four incubation experiments using surface seawater collected from the subantarctic zone (SAZ) and the subpolar zone (SPZ) in the Australian sector of the Southern Ocean during the austral summer of 2011–2012. For incubations, FeCl3 solutions were added to reduce iron (Fe) limitation for phytoplankton growth. Ambient and high (~750 µatm) CO2 treatments were then prepared with and without addition of CO2-saturated seawater, respectively. Non-Fe-added (control) treatments were also prepared to assess the effects of Fe enrichment (overall, control, Fe-added, and Fe-and-CO2-added treatments). In the initial samples, the dominant phytoplankton taxa shifted with latitude from haptophytes to diatoms, likely reflecting silicate availability in the water. Under Fe-enriched conditions, increased CO2 level significantly reduced the accumulation of biomarker pigments in haptophytes in the SAZ and AZ, whereas a significant decrease in diatom markers was only detected in the SAZ. The CO2-related changes in phytoplankton community composition were greater in the SAZ, most likely due to the decrease in coccolithophore biomass. Our results suggest that an increase in CO2, if it coincides with Fe enrichment, could differentially affect the phytoplankton community composition in different geographical regions of the Southern Ocean, depending on the locally dominant taxa and environmental conditions.

Formation of dense shelf water (DSW) in coastal polynyas (open water or thin sea-ice cover) in the sea-ice zone around Antarctica supplies Antarctic Bottom Water (AABW) through overflow down the continental slope. In coastal polynyas, atmospheric carbon dioxide (CO2) is absorbed by the ocean in the early spring because of active primary production, and DSW formation is an important process for transporting this carbon from the sea surface to the deep ocean. However, there have been few quantitative evaluations of carbon consumption by active primary production and transport in coastal polynyas. Here, we examined the carbon dynamics in the Cape Darnley polynya (CDP), East Antarctica during austral summer 2009, by using carbonate system parameters combined with oceanographic mooring data. The partial pressure of CO2 in the CDP surface water was lower than that of the atmosphere and the mean and standard deviation of sea−air CO2 flux was estimated as −6.5 ± 6.9 mmol C m−2 d−1 (a negative value indicates absorption of atmospheric CO2 by the ocean). Vertical profiles of dissolved inorganic carbon (DIC) concentration showed that concentrations in the bottom layer near the ocean floor were lower (by about 20 μmol kg−1) than those in the ambient water (e.g., modified Circumpolar Deep Water, mCDW). The low-DIC in the shelf water was maintained by the strong biological uptake of carbon imported from high-DIC mCDW within the water column. Therefore, low-DIC DSW overflowed down the continental slope, and low-DIC concentrations were maintained through an export pathway to the continental shelf. The annual production of dissolved organic carbon and particulate organic carbon on the shelf was estimated as 0.7 × 1011–1.5 × 1011 mol C using the data for the DIC of DSW and current velocity data from a mooring in the CDP. Our results provide quantitative estimates for the potential role of carbon consumption by the active primary production and carbon transport by dense water formation in Antarctic coastal polynyas.

著者人数: 13名, Accepted: 2018-09-04, 資料番号: SA1180168000

Sampling of atmospheric aerosol particles was carried out at Syowa station, Antarctica (39.58°E, 69.00°S) in 1998. For a better understanding of sea-salt chemistry in the coastal Antarctic regions, individual sea-salt particles were analysed using a scanning electron microscope equipped with energy dispersive X-ray spectrometer (SEM-EDX). Individual particle analysis indicates that more sea-salt particles were modified in fine particles (0.2–2 μm in diameter) through heterogeneous reactions mainly with gaseous sulfur species in the summer and reactive nitrogen oxides in the winter–spring. In particular, sea-salt particles in the coastal Antarctic atmosphere may be modified by heterogeneous reactions with not only SO 2 and H 2 SO 4 but also volatile sulfur species (e.g. CH 3 SO 3 H, DMS and DMSO) derived from bioactivity on the ocean surface during the summer. Also, low air temperature and a larger extent of sea ice offshore Syowa probably enhanced release of fractionated sea-salt particles (S-rich, Mg-rich, K-rich and Ca-rich) from the surface of snow and sea ice, particularly in September–October 1998. In addition, we attempt to estimate the scavenging rate of atmospheric sulfur species and reactive nitrogen oxides by dry deposition of sea-salt particles. Our estimation suggests that the upper limit of the scavenging rate of atmospheric sulfur species by sea-salt particles could rise to approximately 0.5 nmol m −2 day −1 at Syowa station during the summer. This value corresponded to about 30% of the concentration of particulate sulfur species such as non-sea-salt (nss)-SO 2− 4 and CH 3 SO − 3 and ∼10% of total atmospheric sulfur species (nss-SO 2− 4 , CH 3 SO − 3 and SO 2 ). In contrast, the estimated NO − 3 scavenging rate by sea-salt particles was ∼0.2 nmol m −2 day −1 , which is similar to the dry deposition rate of HNO 3 +N 2 O 5 (approximately 0.2–0.3 nmol m −2 day −1 ). Hence, sea-salt particles probably play an important role as scavengers of acidic species in the coastal Antarctic regions. DOI: 10.1111/j.1600-0889.2005.00142.x

Systematic collections of stratospheric air samples have been conducted over Japan since 1985 using a balloon-borne cryogenic sampler. The collection of stratospheric air samples was also carried out twice over Scandinavia and once over Antarctica. Vertical profiles of CO 2 concentration thus obtained over these locations were quite similar to each other; CO 2 concentration decreased with increasing altitude in the lower stratosphere and reached an almost constant value in the mid-stratosphere. δ 13 C of stratospheric CO 2 observed over these locations enriched with increasing altitude. A negative correlation between δ 13 C and CO 2 concentration with Δδ 13 C/ΔCO 2 of −0.02‰ ppmv −1 was found in the lower stratosphere. Although CO 2 concentration was almost constant in the mid-stratosphere, the δ 13 C enrichment was observed in succession. δ 18 O of stratospheric CO 2 also enriched with increasing altitude. The enrichment was significant; δ 18 O was almost 0‰ at the tropopause and reached a maximum value of about 11‰ at a layer with N 2 O concentration of about 10 ppbv. A compact relation between δ 18 O and N 2 O concentration was consistently observed for these locations. Stratosperic CO 2 over Japan showed a secular increase with an average rate of 1.4 ppmv yr −1 for the period 1985–2000. The secular increase was not constant with time, and temporal stagnation of the CO 2 increase was observed in 1997. DOI: 10.1034/j.1600-0889.2003.00059.x

To determine annual layers for reconstructing the past environment at annual resolution from ice cores, we employed snow-stake data back to 1972, tritium content, solid electrical conductivity measurements (ECM) and stratigraphic properties for the 73m ice core at the H72 site, east Dronning Maud Land, Antarctica. the average annual surface mass balance at H72 is 307 mma–1w.e. during the last 27 years from continuous accumulation data, 317 mma–1 w.e. according to the densification model and 311 mma–1 w.e. according to the average surface mass balance for 167 years based on annual-layer counting. the ECM age is closely coincident with tritium age, and corresponds with the snow-stake record back to AD 1972 from the surface to 15 m depth. the H72 ice core is dated as AD 1831by ECMat 73.16 mdepth.The time series of yearly surface mass balance at H72 shows an almost constant 311 mm a–1 w.e. for the last 167 years. the oxygen-isotope records indicate a significant trend to lower values, with negative gradient of 1.7% (100 years)–1.

Precise and continuous measurements of the atmospheric CO 2 concentration have been made at Syowa Station, Antarctica since February 1984. The diurnal CO 2 variation was hardly observable throughout the year. The secular CO 2 trend was variable with time, showing slow increase in 1984, 1986 and 1988 and rapid increase in 1985 and 1987. The annual CO 2 increase was especially large in 1987, which may be related to the 1986/1987 ENSO event. The average rate of annual CO 2 increase over the last 5 years was about 1.6 ppmv yr -1 . The average seasonal CO 2 cycle showed minimum and maximum concentrations in mid-April and early in October, respectively, and its peak-to-peak amplitude was about 1.1 ppmv. The measured seasonal cycle was variable from year to year, but there was no indication of a long-term increase of the amplitude. It was found that irregular CO 2 variations with amplitudes of 0.2 ppmv at most and periods of a few weeks show high correlation with airmass exchange by synoptic scale weather disturbances. The results from Syowa are compared with those from the South Pole and Cape Grim, Tasmania. DOI: 10.1034/j.1600-0889.1991.00007.x

In order to elucidate the seasonal and interannual variations of oceanic CO 2 uptake in the Greenland Sea and the Barents Sea, the partial pressure of CO 2 in the surface ocean ( p CO 2 sea ) was measured in all seasons between 1992 and 2001. We derived monthly varying relationships between p CO 2 sea and sea surface temperature (SST) and combined them with the SST data from the NCEP/NCAR reanalysis to determine p CO 2 sea and air–sea CO 2 flux in these seas. The p CO 2 sea values were normalized to the year 1995 by assuming that p CO 2 sea increased at the same growth rate (1.5 μatm yr −1 ) of the p CO 2 in the air ( p CO 2 air ) between 1992 and 2001. In 1995, the annual net air–sea CO 2 fluxes were evaluated to be 52 ± 20 gC m −2 yr −1 in the Greenland Sea and 46 ± 18 gC m −2 yr −1 in the Barents Sea. The CO 2 flux into the ocean reached its maximum in winter and minimum in summer. The wind speed and Δ p CO 2 (= p CO 2 air – p CO 2 sea ) exerted a greater influence on the seasonal variation than the sea ice coverage. The annual CO 2 uptake examined in this study (70°–80°N, 20°W–40°E) was estimated to be 0.050 ± 0.020 GtCyr −1 in 1995. The interannual variation in the annual CO 2 uptake was found to be positively correlated with the North Atlantic Oscillation Index (NAOI) via wind strength but negatively correlated with Δ p CO 2 and the sea ice coverage. The present results indicate that the variability in wind speed and sea ice coverage play a major role, while that in Δ p CO 2 plays a minor role, in determining the interannual variation of CO 2 uptake in this area. DOI: 10.1111/j.1600-0889.2006.00178.x

Systematic and continuous measurements of the atmospheric CO 2 concentration have been carried out at Syowa Station, Antarctica since February 1984. The measurement system was renewed in 1995, but the continuity of the data from the two systems was confirmed by operating them simultaneously. The CO 2 data taken for 17 years from 1984 to 2000 showed clear evidence for a seasonal cycle, a secular trend and interannual variations. The seasonal cycle was variable from year to year, with especially larger amplitudes in 1992 and 1998 and a large phase delay in 1993. A rapid increase in the CO 2 concentration was observed in 1987, 1994 and 1998 in association with ENSO events. The average rate of the secular CO 2 increase for the last 17 years was calculated to be 1.49 ppmv yr −1 . Short-term CO 2 variations with amplitudes of around 1.0 ppmv were found in the austral summer season of several years after 1990, probably due to an intrusion of CO 2 -depleted air mass into the Antarctic region. DOI: 10.1034/j.1600-0889.2003.01471.x