Seasonal and interannual variability of the air–sea CO2 flux in the Atlantic sector of the Barents Sea

The seasonal and interannual variability of the air–sea CO₂ flux (F) in the Atlantic sector of the Barents Sea have been investigated. Data for seawater fugacity of CO₂ (fCO₂^sw) acquired during five cruises in the region were used to identify and validate an empirical procedure to compute fCO₂^sw from phosphate (PO₄), seawater temperature (T), and salinity (S). This procedure was then applied to time series data of T, S, and PO₄ collected in the Barents Sea Opening during the period 1990–1999, and the resulting fCO₂^sw estimates were combined with data for the atmospheric mole fraction of CO₂, sea level pressure, and wind speed to evaluate F.The results show that the Atlantic sector of the Barents Sea is an annual sink of atmospheric CO₂. The monthly mean uptake increases nearly monotonically from 0.101 mol C m^− 2 in midwinter to 0.656 mol C m^− 2 in midfall before it gradually decreases to the winter value. Interannual variability in the monthly mean flux was evaluated for the winter, summer, and fall seasons and was found to be ± 0.071 mol C m^− 2 month^− 1. The variability is controlled mainly through combined variation of fCO₂^sw and wind speed. The annual mean uptake of atmospheric CO₂ in the region was estimated to 4.27 ± 0.68 mol C m^− 2.     

fCO2 variability at the CARIACO tropical coastal upwelling time series station

Monthly seawater pH and alkalinity measurements were collected between January 1996 and December 2000 at 10°30′N, 64°40′W as part of the CARIACO (CArbon Retention In A Colored Ocean) oceanographic time series. One key objective of CARIACO is to study temporal variability in Total CO₂ (TCO₂) concentrations and CO₂ fugacity (fCO₂) at this tropical coastal wind-driven upwelling site. Between 1996 and 2000, the difference between atmospheric and surface ocean CO₂ concentrations ranged from about − 64.3 to + 62.3 μatm. Physical and biochemical factors, specifically upwelling, temperature, primary production, and TCO₂ concentrations interacted to control temporal variations in fCO₂. Air–sea CO₂ fluxes were typically depressed (0 to + 10 mmol C m^{− 2} day^{− 1}) in the first few months of the year during upwelling. Fluxes were higher during June–November (+ 10 to 20 mmol C m^{− 2} day^{− 1}). Fluxes were generally independent of the slight changes in salinity normally seen at the station, but low positive flux values were seen in the second half of 1999 during a period of anomalously heavy rains and land-derived runoff. During the 5 years of monthly data examined, only two episodes of negative air–sea CO₂ flux were observed. These occurred during short but intense upwelling events in March 1997 (−10 mmol C m^{− 2} day^{− 1}) and March 1998 (− 50 mmol C m^{− 2} day^{− 1}). Therefore, the Cariaco Basin generally acted as a source of CO₂ to the atmosphere in spite of primary productivity in excess of between 300 and 600 g C m^{− 2} year^{− 1}.     

Short-term variability of fCO2 in seawater and air–sea CO2 fluxes in a coastal upwelling system (Ría de Vigo, NW Spain)

Coastal upwelling systems are regions with highly variable physical processes and very high rates of primary production and very little is known about the effect of these factors on the short-term variations of CO₂ fugacity in seawater (fCO₂^w). This paper presents the effect of short-term variability (<1 week) of upwelling–downwelling events on CO₂ fugacity in seawater (fCO₂^w), oxygen, temperature and salinity fields in the Ría de Vigo (a coastal upwelling ecosystem). The magnitude of fCO₂^w values is physically and biologically modulated and ranges from 285 μatm in July to 615 μatm in October. There is a sharp gradient in fCO₂^w between the inner and the outer zone of the Ría during almost all the sampling dates, with a landward increase in fCO₂^w.CO₂ fluxes calculated from local wind speed and air–sea fCO₂ differences indicate that the inner zone is a sink for atmospheric CO₂ in December only (−0.30 mmol m⁻² day⁻¹). The middle zone absorbs CO₂ in December and July (−0.05 and −0.27 mmol·m⁻² day⁻¹, respectively). The oceanic zone only emits CO₂ in October (0.36 mmol·m⁻² day⁻¹) and absorbs at the highest rate in December (−1.53 mmol·m⁻² day⁻¹).     

Annual cycle of fCO2 under sea-ice and in open water in Prydz Bay, East Antarctica

The annual cycle of dissolved nutrients and the fugacity of CO₂ (fCO₂), calculated from the concentration of dissolved inorganic carbon (DIC) and pH, was studied over a 14-month long period (December 1993 to February 1995) at a site in Prydz Bay near Davis Station, Vestfold Hills, East Antarctica. Significant spring decreases in fCO₂ began under the sea-ice in mid-October, when both water column and sea-ice algal activity resulted in the removal of nutrients and DIC and increased pH. Minimum fCO₂ (<100 μatm) and lowest nutrient and DIC concentrations occurred in December and January. The low summer fCO₂ values were clearly the result of biological activity. The seasonal depletion of dissolved nitrate reached 85% in mid-summer when chlorophyll-a concentrations exceeded 15 mg m⁻³. Oceanic uptake of carbon dioxide from the atmosphere, calculated from the fugacity difference and daily wind speeds, averaged more than 30 mmol m⁻² day⁻¹ during the summer ice-free period. This exchange replaced approximately half of the DIC consumed by biological activity. Apparent nutrient utilisation ratios (C/N/P) were close to Redfield values. In autumn fCO₂ began to rise, continuing slowly well into winter, and reaching a maximum close to modern atmospheric values between July and September. This increase can be attributed to a combination of local remineralisation of organic carbon in the water column and the steady increase in the mixing depth of the water column. At first glance, this suggests that air–sea equilibration occurred in winter despite the sea-ice cover, perhaps by horizontal circulation from regions outside the pack ice, or through openings in the ice. However, the persistent 15 to 20% undersaturation of dissolved oxygen throughout the winter suggests an alternate explanation. The late winter fCO₂ level may represent a characteristic established by global circulation, so that as a result of increasing atmospheric CO₂ concentrations, these Antarctic waters are in transition from being a winter-time source of CO₂ to the atmosphere to becoming a sink. Our fCO₂ observations emphasize the need to address seasonal variations in assessing Antarctic contributions to the oceanic control of atmospheric CO₂.     

The contribution of atmospheric acid deposition to ocean acidification in the subtropical North Atlantic Ocean

The absorption of anthropogenic CO₂ and atmospheric deposition of acidity can both contribute to the acidification of the global ocean. Rainfall pH measurements and chemical compositions monitored on the island of Bermuda since 1980, and a long-term seawater CO₂ time-series (1983–2005) in the subtropical North Atlantic Ocean near Bermuda were used to evaluate the influence of acidic deposition on the acidification of oligotrophic waters of the North Atlantic Ocean and coastal waters of the coral reef ecosystem of Bermuda. Since the early 1980's, the average annual wet deposition of acidity at Bermuda was 15 ± 14 mmol m^− 2 year^− 1, while surface seawater pH decreased by 0.0017 ± 0.0001 pH units each year. The gradual acidification of subtropical gyre waters was primarily due to uptake of anthropogenic CO₂. We estimate that direct atmospheric acid deposition contributed 2% to the acidification of surface waters in the subtropical North Atlantic Ocean, although this value likely represents an upper limit. Acidifying deposition had negligible influence on seawater CO₂ chemistry of the Bermuda coral reef, with no evident impact on hard coral calcification.     

Uptake of atmospheric carbon dioxide in Arctic shelf seas: evaluation of the relative importance of processes that influence pCO2 in water transported over the Bering–Chukchi Sea shelf

The uptake of atmospheric carbon dioxide in the water transported over the Bering–Chukchi shelves has been assessed from the change in carbon-related chemical constituents. The calculated uptake of atmospheric CO₂ from the time that the water enters the Bering Sea shelf until it reaches the northern Chukchi Sea shelf slope (1 year) was estimated to be 86±22 g C m⁻² in the upper 100 m. Combining the average uptake per m³ with a volume flow of 0.83×10⁶ m³ s⁻¹ through the Bering Strait yields a flux of 22×10¹² g C year⁻¹. We have also estimated the relative contribution from cooling, biology, freshening, CaCO₃ dissolution, and denitrification for the modification of the seawater pCO₂ over the shelf. The latter three had negligible impact on pCO₂ compared to biology and cooling. Biology was found to be almost twice as important as cooling for lowering the pCO₂ in the water on the Bering–Chukchi shelves. Those results were compared with earlier surveys made in the Barents Sea, where the uptake of atmospheric CO₂ was about half that estimated in the Bering–Chukchi Seas. Cooling and biology were of nearly equal significance in the Barents Sea in driving the flux of CO₂ into the ocean. The differences between the two regions are discussed. The loss of inorganic carbon due to primary production was estimated from the change in phosphate concentration in the water column. A larger loss of nitrate relative to phosphate compared to the classical ΔN/ΔP ratio of 16 was found. This excess loss was about 30% of the initial nitrate concentration and could possibly be explained by denitrification in the sediment of the Bering and Chukchi Seas.     

Time series of carbonate system variables off Otaru coast in Hokkaido, Japan

We report several biogeochemical parameters (dissolved inorganic carbon (DIC), total alkalinity (TA), dissolved oxygen (DO), phosphate (PO₄), nitrate + nitrite (NO₃ + NO₂), silicate (Si(OH)₄)) in a region off Otaru coast in Hokkaido, Japan on a “weekly” basis during the period of April 2002–May 2003. To better understand the long-term temporal variations of the main factors affecting CO₂ flux in this coastal region and its role as a sink/source of atmospheric CO₂, we constructed an algorithm of DIC and TA using other hydrographic properties. We estimated the CO₂ flux across the air–sea interface by using the classical bulk method. During 1998–2003 in our study region, the estimated fCO_2sea ranged about 185–335 μatm. The maximum of fCO_2sea in the summer was primarily due to the change of water temperature. The minimum of fCO_2sea in the early spring can be explained not only by the change of water temperature but also the change of nutrients and chlorophyll-a. To clarify the factors affecting fCO_2sea (water temperature, salinity, and biological activity), we carried out a sensitivity analysis of these effects on the variation of fCO_2sea. In spring, the biological effect had the largest effect for the minimum of fCO_2sea (40%). In summer, the water temperature effect had the largest effect for the maximum of fCO_2sea (25%). In fall, the water temperature effect had the largest effect for the minimum of fCO_2sea (53%). In winter, the biological effect had the largest effect for the minimum of fCO_2sea (35%).We found that our study region was a sink region of CO₂ throughout a year (−0.78 mol/m²/yr). Furthermore, we estimated that the increase of fCO_2sea was about 0.56 μatm/yr under equilibrium with the atmospheric CO₂ content for the period 1998–2003, with the temporal changes in the variables (T, S, PO₄) on fCO_2sea, thus as the maximum trend of each variable on fCO_2sea was 0.22 μatm/yr, and the trend of residual fCO₂ including gas exchange was 0.34 μatm/yr. This result suggests that interaction among variables would affect gas exchange between air and sea effects on fCO_2sea. We conclude that this study region as a representative coastal region of marginal seas of the North Pacific is special because it was measured, but there is no particular significance in comparison to any other area.     

Increase of anthropogenic CO2 in the Pacific Ocean over the last two decades

The multiple-parameter linear regression method (Monitoring global ocean carbon inventories. Ocean Observing System Development Panel, Texas A&M University, College Station, TX, 1995, 54pp; Global Biogeochem. Cycles 13 (1999) 179) is used to compare inorganic carbon data from the GEOSECS CO₂ survey in the Pacific Ocean in 1973 to the WOCE/JGOFS global CO₂ survey in the 1990s. A model of total dissolved inorganic carbon (DIC) as a function of five variables (AOU, θ, S, Si, and PO₄) has been developed from the recent CO₂ survey data (namely CGC91 and CGC96) in the Pacific Ocean. After correcting for a systematic DIC offset of −30.3±7 μmol kg⁻¹ from the GEOSECS data, the residual DIC based on this model as computed from GEOSECS data has been used to estimate the anthropogenic CO₂ penetration in the Pacific Ocean. In the Northeast Pacific, we obtained an increase of CO₂ of 21.3±7.9 mol m⁻² over the period from GEOSECS in 1973 to CGC91 in 1991. This gives a mean anthropogenic CO₂ uptake rate of 1.3±0.5 mol m⁻² yr⁻¹ over this 17 year time period. In the South Pacific, north of 50°S between 180° and 120°W region, the integrated anthropogenic CO₂ inventory is estimated to be 19.7±5.7 mol m⁻² over the period from GEOSECS in 1974 to CGC96 in 1996. The equivalent mean CO₂ uptake rate is estimated to be 0.9±0.3 mol m⁻² yr⁻¹ over the 22 years. These results are compared with the isopycnal method (Nature 396 (1998) 560) to estimate the anthropogenic CO₂ signal in the Northeast Pacific (30°N, 152°W) at the crossover region between CGC91 and GEOSECS. The results of the isopycnal method are consistent with those derived from the MLR method. Both methods show an increase in anthropogenic CO₂ inventory in the ocean over two decades that is consistent with the increase expected if the ocean uptake has kept pace with the atmospheric CO₂ increase.     

Carbon and nitrogen primary productivity in three North Carolina estuaries

Uptake of inorganic carbon and ammonium by the plankton community of three North Carolina estuaries was measured using ¹⁴C and ¹⁵N isotope methods. At 0% light, C appeared to be lost via respiration, and at increasing light levels uptake of inorganic carbon increased linearly, saturated (mean I_k = 358±30 μEin m⁻² s⁻¹), and frequently showed inhibition at the highest light intensities. At 0% light NH₄⁺ uptake was significantly greater than zero and was frequently equivalent to uptake in the light (light independent); at increasing light levels NH₄⁺ uptake saturated (mean I_k = 172±44 μEin m⁻² s⁻¹) and frequently indicated strong inhibition. Light-saturated uptake rates of inorganic carbon and NH₄⁺ were a function of chlorophyll a (r² = 0·7−0·9); average assimilation numbers were 625 nmol CO₂ (μg chl. a)⁻¹ h⁻¹ and 12·9 nmol NH₄⁺ (μg chl. a)⁻¹ h⁻¹ and were positively correlated with temperature (r² = 0·3−0·7). The ratio of dark to light-saturated NH₄⁺ uptake tended to be near 1·0 for large algal populations at low NH₄⁺ concentrations, indicating near light independence of uptake; whereas the ratio was lower for the opposite conditions. These data are interpreted as indicative of nitrogen stress, and it is suggested that uptake of NH₄⁺ deep in the euphotic zone and at night are mechanisms for balancing the C:N of cellular pools. A 24-h study using summed short-term incubations confirmed this; the cumulative C:N of CO₂ and NH₄⁺ uptake during the daylight period was 10–20, whereas over the 24-h period the ratio was 6 due to dark NH₄⁺ uptake. Annual carbon and nitrogen primary productivity were respectively estimated as 24 and 4·0 mol m⁻² year⁻¹ for the South River estuary, 42 and 7·3 mol m⁻² year⁻¹ for the Neuse River estuary, and 9·6 and 1·6 mol m⁻² year⁻¹ for the Newport River estuary.     

Cadmium and phosphate in coastal Antarctic seawater: Implications for Southern Ocean nutrient cycling

Cadmium is a biologically important trace metal that co-varies with phosphate (PO₄³⁻ or Dissolved Inorganic Phosphate, DIP) in seawater. However, the exact nature of Cd uptake mechanisms and the relationship with phosphate and other nutrients in global oceans remain elusive. Here, we present a time series study of Cd and PO₄³⁻ from coastal Antarctic seawater, showing that Cd co-varies with macronutrients during times of high biological activity even under nutrient and trace metal replete conditions. Our data imply that Cd/PO₄³⁻ in coastal surface Antarctic seawater is higher than open ocean areas. Furthermore, the sinking of some proportion of this high Cd/PO₄³⁻ water into Antarctic Bottom Water, followed by mixing into Circumpolar Deep Water, impacts Southern Ocean preformed nutrient and trace metal composition. A simple model of endmember water mass mixing with a particle fractionation of Cd/P (α_Cd–P) determined by the local environment can be used to account for the Cd/PO₄³⁻ relationship in different parts of the ocean. The high Cd/PO₄³⁻ of the coastal water is a consequence of two factors: the high input from terrestrial and continental shelf sediments and changes in biological fractionation with respect to P during uptake of Cd in regions of high Fe and Zn. This implies that the Cd/PO₄³⁻ ratio of the Southern Ocean will vary on glacial–interglacial timescales as the proportion of deep water originating on the continental shelves of the Weddell Sea is reduced during glaciations because the ice shelf is pinned at the edge of the continental shelf. There could also be variations in biological fractionation of Cd/P in the surface waters of the Southern Ocean on these timescales as a result of changes in atmospheric inputs of trace metals. Further variations in the relationship between Cd and PO₄³⁻ in seawater arise from changes in population structure and community requirements for macro- and micronutrients.     

CO<Subscript>2</Subscript> Flux Measurements over the Sea Surface by Eddy Correlation and Aerodynamic Techniques

The measurements of the vertical transport of CO₂ were carried out over the Sea of Japan using the specially designed pier of Kyoto University on September 20 to 22, 2000. CO₂ fluxes were measured by the eddy correlation and aerodynamic techniques. Both techniques showed comparable CO₂ fluxes during sea breeze conditions: −0.001 to −0.08 mg m⁻²s⁻¹ with the mean of −0.05 mg m⁻²s⁻¹. This means that the measuring site satisfies the fetch requirement for meteorological observations under sea breeze conditions. Moreover, the eddy diffusivity coefficient used in the aerodynamic technique is found to be consistent with the coefficient used in the eddy correlation technique. The present result leads us to conclude that the aerodynamic technique may be applicable to underway CO₂ flux measurements over the ocean and may be used in place of the bulk technique. The important point is the need to maintain a measuring accuracy of CO₂ concentration difference of the order of 0.1 ppmv on the research vessels or the buoys.     

A seasonal carbon budget for the sub-Antarctic Ocean, South of Australia

Changes from winter (July) to summer (February) in mixed layer carbon tracers and nutrients measured in the sub-Antarctic zone (SAZ), south of Australia, were used to derive a seasonal carbon budget. The region showed a strong winter to summer decrease in dissolved inorganic carbon (DIC;  45 µmol/kg) and fugacity of carbon dioxide (fCO₂;  25 µatm), and an increase in stable carbon isotopic composition of DIC (δ¹³C_DIC;  0.5‰), based on data collected between November 1997 and July 1999.The observed mixed layer changes are due to a combination of ocean mixing, air–sea exchange of CO₂, and biological carbon production and export. After correction for mixing, we find that DIC decreases by up to 42 ± 3 µmol/kg from winter (July) to summer (February), with δ¹³C_DIC enriched by up to 0.45 ± 0.05‰ for the same period. The enrichment of δ¹³C_DIC between winter and summer is due to the preferential uptake of ¹²CO₂ by marine phytoplankton during photosynthesis. Biological processes dominate the seasonal carbon budget (≈ 80%), while air–sea exchange of CO₂ (≈ 10%) and mixing (≈ 10%) have smaller effects. We found the seasonal amplitude of fCO₂ to be about half that of a study undertaken during 1991–1995 [Metzl, N., Tilbrook, B. and Poisson, A., 1999. The annual fCO₂ cycle and the air–sea CO₂ flux in the sub-Antarctic Ocean. Tellus Series B—Chemical and Physical Meteorology, 51(4): 849–861.] for the same region, indicating that SAZ may undergo significant inter-annual variations in surface fCO₂. The seasonal DIC depletion implies a minimum biological carbon export of 3400 mmol C/ m² from July to February. A comparison with nutrient changes indicates that organic carbon export occurs close to Redfield values (ΔP:ΔN:ΔC = 1:16:119). Extrapolating our estimates to the circumpolar sub-Antarctic Ocean implies a minimum organic carbon export of 0.65 GtC from the July to February period, about 5–7% of estimates of global export flux. Our estimate for biological carbon export is an order of magnitude greater than anthropogenic CO₂ uptake in the same region and suggests that changes in biological export in the region may have large implications for future CO₂ uptake by the ocean.     

High-resolution measurement of Southern Ocean CO2 and O2/Ar by membrane inlet mass spectrometry

This paper evaluates the simultaneous measurement of dissolved gases (CO₂ and O₂/Ar ratios) by membrane inlet mass spectrometry (MIMS) along the 180° meridian in the Southern Ocean. The calibration of pCO₂ measurements by MIMS is reported for the first time using two independent methods of temperature correction. Multiple calibrations and method comparison exercises conducted in the Southern Ocean between New Zealand and the Ross Sea showed that the MIMS method provides pCO₂ measurements that are consistent with those obtained by standard techniques (i.e. headspace equilibrator equipped with a Li–Cor NDIR analyser). The overall MIMS accuracy compared to Li–Cor measurements was 0.8 μatm. The O₂/Ar ratio measurements were calibrated with air-equilibrated seawater standards stored at constant temperature (0 ± 1 °C). The reproducibility of the O₂/Ar standards was better than 0.07% during the 9 days of transect between New Zealand and the Ross Sea.The high frequency, real-time measurements of dissolved gases with MIMS revealed significant small-scale heterogeneity in the distribution of pCO₂ and biologically-induced O₂ supersaturation (ΔO₂/Ar). North of 65°S several prominent thermal fronts influenced CO₂ concentrations, with biological factors also contributing to local variability. In contrast, the spatial variation of pCO₂ in the Ross Sea gyre was almost entirely attributed to the biological utilization of CO_2, with only small temperature effects. This high productivity region showed a strong inverse relationship between pCO₂ and biologically-induced O₂ disequilibria (r² = 0.93). The daily sea air CO₂ flux ranged from − 0.2 mmol/m² in the Northern Sub-Antarctic Front to − 6.4 mmol/m² on the Ross Sea shelves where the maximum CO₂ influx reached values up to − 13.9 mmol/m². This suggests that the Southern Ocean water (south of 58°S) acts as a seasonal sink for atmospheric CO₂ at the time of our field study.     

Modelling the monthly sea surface fCO2 fields in the Indian Ocean

In order to construct monthly fields of sea surface fugacity of carbon dioxide (f_CO2) on a large scale in the Indian Ocean, we use a one-dimensional model which takes into account the main physical and biogeochemical processes controlling f_CO2 variations in the ocean. Physical and biogeochemical processes are constrained by the monthly variations of sea surface temperature, salinity, chlorophyll concentration, wind speed and mixed-layer depth. The model is applied to four locations in the Indian Ocean and it well predicts observed temporal variations in f_CO2 at these locations. Regarding to monthly f_CO2 observations, the model also well simulates the f_CO2 distribution and its temporal variations along a track located between 20 ° and 50 °S with a maximal error of + 10 μatm. The model is also used to predict f_CO2 for 2 ° × 2 ° grids over the entire Indian Ocean and simulates seasonal cycles that are consistent with observations. The monthly f_CO2 fields derived from the model are used to estimate a global air-sea CO₂ flux over the Indian Ocean basin. We estimate a net sink of 0.5 Gt/yr C for the Indian Ocean (20 °N-50 °S), with the main sink located between 20 ° and 50 °S.     

Deep ocean fluxes and their link to surface ocean processes and the biological pump

Intense studies of upper and deep ocean processes were carried out in the Northwestern Indian Ocean (Arabian Sea) within the framework of JGOFS and related projects in order to improve our understanding of the marine carbon cycle and the ocean’s role as a reservoir for atmospheric CO₂. The results show a pronounced monsoon-driven seasonality with enhanced organic carbon fluxes into the deep-sea during the SW Monsoon and during the early and late NE Monsoon north of 10°N. The productivity is mainly regulated by inputs of nutrients from subsurface waters into the euphotic zone via upwelling and mixed layer-deepening. Deep mixing introduces light limitation by carrying photoautotrophic organisms below the euphotic zone during the peak of the NE Monsoon. Nevertheless, deep mixing and strong upwelling during the SW Monsoon provide an ecological advantage for diatoms over other photoautotrophic organisms by increasing the silica concentrations in the euphotic zone. When silica concentrations fall below 2 μmol l⁻¹, diatoms lose their dominance in the plankton community. During diatom-dominated blooms, the biological pathway of uptake of CO₂ (the biological pump) appears to be more efficient than during blooms of other organisms, as indicated by organic carbon to carbonate carbon (rain) ratios. Due to the seasonal alternation of diatom and non-diatom dominated exports, spatial variations of the annual mean rain ratios are hardly discernible along the main JGOFS transect.Data-based estimates of the annual mean impact of the biological pump on the fCO₂ in the surface water suggest that the biological pump reduces the increase of fCO₂ in the surface water caused by intrusion of CO₂-enriched subsurface water by 50–70%. The remaining 30 to 50% are attributed to CO₂ emissions into the atmosphere. Rain ratios up to 60% higher in river-influenced areas off Pakistan and in the Bay of Bengal than in the open Arabian Sea imply that riverine silica inputs can further enhance the impact of the biological pump on the fCO₂ in the surface water by supporting diatom blooms. Consequently, it is assumed that reduced river discharges caused by the damming of major rivers increase CO₂ emission by lowering silica inputs to the Arabian Sea; this mechanism probably operates in other regions of the world ocean also.     

Depth profiles of volatile iodine and bromine-containing halocarbons in coastal Antarctic waters

Measurements of bromoform (CHBr₃), diiodomethane (CH₂I₂), chloroiodomethane (CH₂ICl) and bromoiodomethane (CH₂IBr) were made in the water column (5–100 m depth) of the Southern Ocean within 0–40 km of the Antarctic sea ice during the ANTXX1/2 transect of the German R/V Polarstern, at five locations between 70–72°S and 9–11°W in the Antarctic spring/summer of 2003–2004. Some of the profiles exhibited a very pronounced layer of surface sea-ice meltwater, as evidenced by salinity minima and temperature maxima, along with surface maxima in concentrations of CHBr₃, CH₂I₂, CH₂ICl and CH₂IBr. These results are consistent with in situ surface halocarbon production by ice algae liberated from the sea ice, although production within the sea ice followed by transport cannot be entirely ruled out. Additional sub-surface maxima in halocarbons occurred between 20 and 80 m. At a station further from shore and not affected by surface sea-ice meltwater, surface concentrations of CH₂I₂ were decreased whereas CH₂ICl concentrations were increased compared to the stations influenced by meltwater, consistent with photochemical conversion of CH₂I₂ to CH₂ICl, perhaps during upward mixing from a layer at  70 m enhanced in iodocarbons. Mean surface (5–10 m) water concentrations of halocarbons in these coastal Antarctic waters were 57 pmol l^− 1 CHBr₃ (range 44–78 pmol l^− 1), 4.2 pmol l^− 1 CH₂I₂ (range 1.7–8.2 pmol l^− 1), 0.8 pmol l^− 1 CH₂IBr (range 0.2–1.4 pmol l^− 1), and 0.7 pmol l^− 1 CH₂ICl (range 0.2–2.4 pmol l^− 1). Concurrent measurements in air suggested a sea-air flux of bromoform near the Antarctic coast of between 1 and 100 (mean 32.3, median 10.4) nmol m^− 2 day^− 1 and saturation anomalies of 557–1082% (mean 783%, median 733%), similar in magnitude to global shelf values. In surface samples affected by meltwater, CH₂I₂ fluxes ranged from 0.02 to 6.1 nmol m^− 2 day^− 1, with mean and median values of 1.9 and 1.1 nmol m^− 2 day^− 1, respectively.     

The spatial distribution of surface <Emphasis Type="Italic">f</Emphasis>CO<Subscript>2</Subscript> in the Southwestern East Sea/Japan sea during summer 2005

Fugacity of CO₂ (fCO₂), temperature, salinity, nutrients, and chlorophyll-a were measured in the surface waters of southwestern East Sea/Japan Sea in July 2005. Surface waters were divided into three waters based on hydrographic characteristics: the water with moderate sea surface temperature (SST) and high sea surface salinity (SSS) located east of the front (East water); the water with high SST and moderate SSS located west of the front (West water); and the water with low SST and SSS located in the middle part of the study area (Middle water). High fCO₂ larger than 420 μatm were found in the West water. In the Middle water, CO₂ was undersaturated with respect to the atmosphere, with values between 246 and 380 μatm. Moderate fCO₂ values ranging from 370 to 420 μatm were observed in the East water. For the East and West waters, estimates of temperature dependency of fCO₂ (12.6 and 15.1 μatm °C⁻¹, respectively) were rather similar to a theoretical value, indicating that SST is likely to be a major factor controlling the surface fCO₂ distribution in these two regions. In the Middle water, however, the estimated temperature dependence was somewhat lower than the theoretical value, and relatively high concentrations of surface chlorophyll-a coincided with the low surface fCO₂, implying that biological uptake may considerably affect the fCO₂ distribution. The net sea-to-air CO₂ flux of the study area was estimated to be 0.30±4.81 mmol m⁻² day⁻¹ in summer, 2005.     

Reconstruction of sea surface dimethylsulfide in the North Pacific during 1970s to 2000s

We proposed an empirical equation of sea surface dimethylsulfide (DMS, nM) using sea surface temperature (SST, K), sea surface nitrate (SSN, μM) and latitude (L, °N) to reconstruct the sea surface flux of DMS over the North Pacific between 25°N and 55°N: ln DMS = 0.06346 · SST − 0.1210 · SSN − 14.11 · cos(L) − 6.278 (R² = 0.63, p < 0.0001). Applying our algorithm to climatological hydrographic data in the North Pacific, we reconstructed the climatological distributions of DMS and its flux between 25 °N and 55 °N. DMS generally increased eastward and northward, and DMS in the northeastern region became to 2–5 times as large as that in the southwestern region. DMS in the later half of the year was 2–4 times as large as that in the first half of the year. Moreover, applying our algorithm to hydrographic time series datasets in the western North Pacific from 1971 to 2000, we found that DMS in the last three decades has shown linear increasing trends of 0.03 ± 0.01 nM year^− 1 in the subpolar region, and 0.01 ± 0.001 nM year^− 1 in the subtropical region, indicating that the annual flux of DMS from sea to air has increased by 1.9–4.8 μmol m^− 2 year^− 1. The linear increase was consistent with the annual rate of increase of 1% of the climatological averaged flux in the western North Pacific in the last three decades.     

The influence of island generated eddies on the carbon dioxide system, south of the Canary Islands

Measurements of surface partial pressure of CO₂ and water column alkalinity, pH_T, nutrients, oxygen, fluorescence and hydrography were carried out, south of the Canary Islands during September 1998. Cyclonic and anticyclonic eddies were alternatively observed from the northwestern area to the central area of the Canary Islands. Nutrient pumping and vertical uplifting of the deep chlorophyll maximum by cyclonic eddies were also ascertained by upward displacement of dissolved inorganic carbon. A model was applied to determine the net inorganic carbon balance in the cyclonic eddy. The fluxes were determined considering both the diffusive and convective contributions from the upward pumping and the corresponding horizontal transport of water outside the area. An increase in the total inorganic carbon concentration in the upper layers inside the eddy field of 133 mmol C m^− 2 d^− 1 was determined. The upward flux of inorganic carbon decreased the effect of the increased primary production on the carbon dioxide chemistry. The reduced fCO₂ inside the cyclonic eddy, 15 μatm lower than that observed in non-affected surface water, was explained by thermodynamic aspects, biological activity, eddy upward pumping and diffusion and air–sea water exchange effects.     

Spatial and Temporal Variation of Surface xCO<Subscript>2</Subscript> Providing Net Biological Productivities in the Western North Pacific in June

More than 14,000 measurements of surface water xCO₂ were obtained during two cruises, 3 weeks apart in June 2000, along 155°E between 34 and 44°N in the western North Pacific Ocean. Based on the distributions of salinity and sea surface temperature (SST), the region has been divided into 6 subregions; Oyashio, Oyashio front, Transition, Kuroshio front, and Kuroshio extension I and II zones, from north to south. The surface waters were always undersaturated with respect to atmospheric CO₂. The Oyashio water was the least undersaturated: its xCO₂ decreased slightly by 7 ppm, while SST increased by 2°C. The xCO₂ normalized to a constant temperature decreased considerably. In the two frontal zones, a large drawdown of 30–40 ppm was observed after 18–19 days. In the Kuroshio extension zones, the xCO₂ increased, but the normalized xCO₂ decreased considerably. The Transition zone water may be somewhat affected by mixing with the subsurface water, as indicated by the smallest SST rise, an undecreased PO₄ concentration, and a colder and less stable surface layer than the Oyashio front water. As the uncertainty derived from the air-sea CO₂ flux was not large, the xCO₂ data allowed us to calculate the net biological productivity. The productivities around 60 mmol C m⁻²d⁻¹ outside the Transition zone indicate that the northwestern North Pacific, especially the two frontal zones, can be regarded as one of the most productive oceans in the world.