Photochemical production of hydrogen peroxide in Antarctic Waters

Photochemical production rates of hydrogen peroxide (H₂O₂) were determined in Antarctic waters during two research cruises. The first cruise was from mid-October to mid-November, 1993, in the confluence of the Weddell and Scotia Seas, and the second cruise was in December, 1994, along the coast of the Antarctic Peninsula. During these cruises, midday sea-surface production rates ranged from 2.1 to 9.6 nM h⁻¹, with an average rate of 4.5 nM h⁻¹. Production rates were consistently smaller than rates determined at lower latitudes (>9 nM h⁻¹), primarily due to the colder temperatures and lower ultraviolet irradiances in polar waters. In situ production rates were determined with a free-floating drifter that was deployed for 12–14 h. Production rates, averaged over the deployment time, were highest at or near the surface (ca. 2.4–3.5 nM h⁻¹) and decreased rapidly with depth to 0.1–0.7 nM h⁻¹ at 10–20 m. The decrease in production rates with depth generally paralleled the decrease in ultraviolet irradiance in the water column. Production rates of hydrogen peroxide in Antarctic seawater were largely controlled by the ultraviolet irradiance in the water column, although there was some evidence for production in the blue region of the solar spectrum. A laboratory study was conducted to determine the wavelength dependence of the apparent quantum yield for the photochemical formation of hydrogen peroxide in Antarctic waters. Apparent quantum yields determined at 0°C decreased from 0.74×10⁻³ mol einstein⁻¹ at 290 nm to 1.0×10⁻⁵ mol einstein⁻¹ 410 nm. At 20°C, apparent quantum yields for the photochemical production of hydrogen peroxide were within a factor of two of apparent quantum yields determined in temperate waters at 20–25°C. Sunlight-normalized H₂O₂ production rates were determined as a function of wavelength using noontime irradiance data from Palmer Station, Antarctica. A decrease in stratospheric ozone from 336 to 151 Dobson units resulted in a predicted 19–42% increase in the photoproduction of H₂O₂ at the sea surface in Antarctic waters. The magnitude of this increase depends on the concentration and absorbance characteristics of dissolved organic matter in the photic zone, as well as on other factors such as cloudiness and decreasing solar zenith angle that tend to lower photochemical rates offsetting increases due to stratospheric ozone depletion.     

Photochemical production of the hydroxyl radical in Antarctic waters

Photochemical production rates and steady-state concentrations of the highly reactive OH radical were determined in Antarctic seawater in the Weddell-Scotia Confluence during the austral spring of 1993 and along the Antarctic Peninsula during the austral summer of 1994. OH radical photoproduction rates were 30±2 nM/day and 46±2 nM/day in surface open oceanic and coastal waters, respectively. Corresponding steady-state concentrations were 2.6×10⁻¹⁹ and 4.3×10⁻¹⁹ M which are similar to those found in tropical latitudes. In-situ irradiation experiments (drifter deployments) at different depths in the upper water column indicated that multiple sources for the OH radical existed at three Antarctic stations. Ultrafiltration studies and model calculations based on wavelength-dependent OH radical quantum yields indicated that the main sources were photochemical reactions of low molecular weight dissolved organic matter (DOM), nitrate, and nitrite. Production of the OH radical from nitrate photolysis was almost exclusively UV-B dependent, while OH radical production from nitrite photolysis was mainly UV-A dependent. OH production from DOM photolysis was both UV-A and UV-B dependent. In the upper few meters at open oceanic sites, nitrate and DOM were the dominant OH radical sources, while deeper in the water column DOM and nitrite were important because of the greater importance of UV-A with depth. During non-ozone hole conditions, nitrate contributed about 33%, while DOM plus nitrite contributed about 67% of the predicted OH radical production in open oceanic surface waters. During an ozone hole (151 Dobson units), the corresponding percentages changed to about 40 and 60% for nitrate and DOM due to the higher UV-B irradiance. Model calculations predict that during an ozone hole (151 Dobson units), OH radical production in surface waters will be enhanced by at least 20%, mostly from nitrate photolysis and to a lesser extent from DOM photochemical reactions. This study indicates that ozone hole events significantly increase OH radical production, as well as the photolysis of DOM, in Antarctic waters, and that rates can be as high or higher than those at lower latitudes, especially if differences in temperature and solar irradiance are taken into account.     

Marine dissolved organic matter: can its C : N ratio explain carbon overconsumption?

Carbon overconsumption, i.e. the consumption of inorganic carbon relative to inorganic nitrogen in excess of the Redfield ratio at the sea surface, was examined in relation to the dynamics of dissolved organic carbon and nitrogen (DOC and DON) in the northeast Atlantic. We observed the presence of N-poor dissolved organic matter (DOM) in surface water during summer, requiring the consumption of inorganic carbon and nitrogen in a ratio exceeding the Redfield ratio. The C : N ratio of bulk DOM is not only different from the Redfield ratio but also variable, i.e. no fixed conversion factor of C and N exists where DOM is important in C and N transformations. The existence of N-poor DOM is recognized as a feature typical of oligotrophic systems. At the same time, the C : N ratios of particles conform to Redfield stoichiometry as does deep-ocean chemistry. The implications of this finding are discussed, the conclusion being that, while DOM buildup contributes to CO₂ drawdown seasonally, its impact on long-term carbon and nitrogen balance of the ocean is small.     

Variability of the apparent quantum efficiency of CO photoproduction in the Gulf of Maine and Northwest Atlantic

The photochemical oxidation of colored, dissolved organic matter (CDOM) is important for carbon cycling in the ocean. This oxidation process produces a number of products, including carbon monoxide (CO). While the photochemical production efficiency of CO (apparent quantum yield, AQY, defined in terms of CDOM absorbance) has been reported to be similar for many water types, a full evaluation of the observed natural variability in CO AQY requires additional study. Here we use a polychromatic irradiation system to determine twenty AQY spectra at sea on fresh samples ranging from the near coastal waters of the Gulf of Maine to the offshore waters of the Northwest Atlantic. Despite the geographic variability of these marine samples the AQY of CO production in the Gulf of Maine and Northwest Atlantic exhibited only a small degree of variability, none of which was not correlated with measured environmental parameters. Consequently, a single aggregate AQY spectrum _λ = e^{(−(9.134+0.0425(λ−290)))+e(−(11.316+0.0142(λ−290)))} was found to adequately represent the entire data set. Significantly, the accuracy of an AQY spectrum determined using this multispectral/statistical technique was confirmed with data obtained from a monochromatic irradiation technique on a single open ocean sample. Taken together, the AQY spectra determined in this study were similar in magnitude and shape to those previously published for marine samples and, overall, were somewhat lower than those previously reported for freshwater studies.     

Microbial respiration in the Levantine Sea: evolution of the oxidative processes in relation to the main Mediterranean water masses

First data on microbial respiration in the Levantine Sea are reported with the aim of assessing the distribution of oxidative processes in association with the main Mediterranean water masses and the changing physical structure determined by the Eastern Mediterranean Transient. Respiratory rates, in terms of metabolic carbon dioxide production, were estimated from measured electron transport system activities in the polygonal area of the Levantine Sea (32.5–36.5 N Latitude, 26.0–30.25 E Longitude) and at Station Geo’95, in the Ionian Sea (35°34.88 N; 17°14.99 E). At the Levantine Sea, the mean carbon dioxide production rate decreased from the upper to the deeper layers and varied from 22.0±12.4 μg C h⁻¹ m⁻³ in the euphotic layer to 1.30±0.5 μg C h⁻¹ m⁻³ in the depth range between 1600 and 3000 m. Significant differences were found among upper, intermediate and bottom layers. The euphotic zone supported a daily carbon dioxide production of 96.6 mg C d⁻¹ m⁻² while the aphotic zone (between 200 and 3000 m) sustained a 177.1 mg C d⁻¹ m⁻² carbon dioxide production. In Station Geo’95, the carbon dioxide production rates amounted to 170.4 and 102.2 mg C d⁻¹ m⁻² in the euphotic and aphotic zones, respectively. The rates determined in the identified water masses showed a tight coupling of respiratory processes and Mediterranean circulation patterns. The increasing respiratory rates in the deep layers of the Levantine Sea are explained by the introduction of younger waters recently formed in the Aegean Sea.     

Iron and mixing affect biological carbon uptake in SOIREE and EisenEx,two Southern Ocean iron fertilisation experiments

This study explores the changes in the surface water fugacity of carbon dioxide (fCO₂) and biological carbon uptake in two Southern Ocean iron fertilisation experiments with different hydrographic regimes. The Southern Ocean Iron Release Experiment (SOIREE) experiment was carried out south of the Antarctic Polar Front (APF) at 61°S, 141°E in February 1999 in a stable hydrographic setting. The EisenEx experiment was conducted in a cyclonic eddy north of the APF at 48°S, 21°E in November 2000 and was characterised by a rapid succession of low to storm-force wind speeds and dynamic hydrographic conditions. The iron additions promoted algal blooms in both studies. They alleviated algal iron limitation during the 13-day SOIREE experiment and probably during the first 12 days of EisenEx. The fCO₂ in surface water decreased at a constant rate of 3.8 μatm day⁻¹ from 4 to 5 days onwards in SOIREE. The fCO₂ reduction was 35 μatm after 13 days. The evolution of surface water fCO₂ in the iron-enriched waters (or ‘patch’) displayed a saw tooth pattern in EisenEx, in response to algal carbon uptake in calm conditions and deep mixing and horizontal dispersion during storms. The maximum fCO₂ reduction was 18–20 μatm after 12 and 21 days with lower values in between. The iron-enriched waters in EisenEx absorbed four times more atmospheric CO₂ than in SOIREE between 5 and 12 days, as a result of stronger winds. The total biological uptake of inorganic carbon across the patch was 1389 ton C (±10%) in SOIREE and 1433 ton C (±27%) in EisenEx after 12 days (1 ton=10⁶ g). This similarity probably reflects the comparable size of the iron additions, as well as algal growth at a similar near-maximum growth rate in these regions. The findings imply that the different mixing regimes had less effect on the overall biological carbon uptake across the iron-enriched waters than suggested by the evolution of fCO₂ in surface water.     

Increase in anthropogenic CO2 in the Atlantic Ocean in the last two decades

Data from the first systematic survey of inorganic carbon parameters on a global scale, the GEOSECS program, are compared with those collected during WOCE/JGOFS to study the changes in carbon and other geochemical properties, and anthropogenic CO₂ increase in the Atlantic Ocean from the 1970s to the early 1990s. This first data-based estimate of CO₂ increase over this period was accomplished by adjusting the GEOSECS data set to be consistent with recent high-quality carbon data. Multiple Linear Regression (MLR) and extended Multiple Linear Regression (eMLR) analyses to these carbon data are applied by regressing DIC with potential temperature, salinity, AOU, silica, and PO₄ in three latitudinal regions for the western and eastern basins in the Atlantic Ocean. The results from MLR (and eMLR provided in parentheses) indicate that the mean anthropogenic CO₂ uptake rate in the western basin is 0.70 (0.53) mol m^?2 yr^?1 for the region north of 15°N; 0.53 (0.36) mol m^?2 yr^?1 for the equatorial region between 15°N and 15°S; and 0.83 (0.35) mol m^?2 yr^?1 in the South Atlantic south of 15°S. For the eastern basin an estimate of 0.57 (0.45) mol m^?2 yr^?1 is obtained for the equatorial region, and 0.28 (0.34) mol m^?2 yr^?1 for the South Atlantic south of 15°S. The results of using eMLR are systematically lower than those from MLR method in the western basin. The anthropogenic CO₂ increase is also estimated in the upper thermocline from salinity normalized DIC after correction for AOU along the isopycnal surfaces. For these depths the results are consistent with the CO₂ uptake rates derived from both MLR and eMLR methods.     

Summer and winter air–sea CO2 fluxes in the Southern Ocean

The seasonal variability of the carbon dioxide (CO₂) system in the Southern Ocean, south of 50°S, is analysed from observations obtained in January and August 2000 during OISO cruises conducted in the Indian Antarctic sector. In the seasonal ice zone, SIZ (south of 58°S), surface ocean CO₂ concentrations are well below equilibrium during austral summer. During this season, when sea-ice is not obstructing gas exchange at the air–sea interface, the oceanic CO₂ sink ranges from −2 to −4 mmol/m²/d in the SIZ. In the permanent open ocean zone, POOZ (50–58°S), surface oceanic fugacity fCO₂ increases from summer to winter. The seasonal fCO₂ variations (from 10 to 30 μatm) are relatively low compared to seasonal amplitudes observed in the subtropics or the subantarctic zones. However, these variations in the POOZ are large enough to cross the atmospheric level from summer to winter. Therefore, this region is neither a permanent CO₂ sink nor a permanent CO₂ source. In the POOZ, air–sea CO₂ fluxes calculated from observations are about −1.1 mmol/m²/d in January (a small sink) and 2.5 mmol/m²/d in August (a source). These estimates obtained for only two periods of the year need to be extrapolated on a monthly scale in order to calculate an integrated air–sea CO₂ flux on an annual basis. For doing this, we use a biogeochemical model that creates annual cycles for nitrate, inorganic carbon, total alkalinity and fCO₂. The changing pattern of ocean CO₂ summer sink and winter source is well reproduced by the model. It is controlled mainly by the balance between summer primary production and winter deep vertical mixing. In the POOZ, the annual air–sea CO₂ flux is about −0.5 mol/m²/yr, which is small compared to previous estimates based on oceanic observations but comparable to the small CO₂ sink deduced from atmospheric inverse methods. For reducing the uncertainties attached to the global ocean CO₂ sink south of the Polar Front the regional results presented here should be synthetized with historical and new observations, especially during winter, in other sectors of the Southern Ocean.     

Simulated sequestration of anthropogenic carbon dioxide at a deep-sea site: Effects on nematode abundance and biovolume

One proposal for ameliorating global warming is to sequester large amounts of carbon dioxide in the deep ocean, but the environmental consequences of sequestration for sediment-dwelling animals are poorly known. In a previous publication, we reported that ∼80% of benthic copepods were killed in an experimental release of CO₂ off northern California at 3262 m. The effects of this release on nematodes are reported here. We examined samples of nematodes taken inside two ‘corrals’ into which CO₂ was directly injected (providing an extreme endpoint for CO₂ exposure) and taken near to and far from this CO₂ source. After 30 days, pore-water pH was unchanged (∼7.8) at the sediment–water interface far (∼40 m) from corrals, but pH profiles were reduced by ∼0.75 near (∼2 m) corrals. Corral pH was highly acidic (5.4 in a measurement from a subsequent experiment). Fifty randomly selected nematodes from each of four vertical layers from the 14 cores were photographed. They were assigned to a tail group (based on morphology), and individual biovolume was estimated from measurements of body length and width. Although nematode abundance (expressed as total nematodes and by tail group) was not affected, length, width, and individual biovolume significantly differed between near and far samples. Median nematode biovolume examined across tail group and core layer increased by ∼48% inside and near corrals. Differences between near and corral samples were always less than differences between near and far samples. However, nematode length:width ratio did not differ between near and far, and the shapes of length, width, and biovolume frequency distributions were similar in all samples. We postulate that the nematode community throughout the upper 3 cm suffered a high rate of mortality after exposure to CO₂, and that nematodes were larger because postmortem expansions in body length and width occurred. Decomposition rates were probably low and corpses did not disintegrate in 30 days. The observable effects of a reduction in pH to about 7.0 after 30 days were as great as an extreme pH reduction (5.4), suggesting that ‘moderate’ CO₂ exposure, compared to the range of exposures possible following CO₂ release, causes high mortality rates in the two most abundant sediment-dwelling metazoans (nematodes and copepods).     

CO2 outgassing off central Chile (31–30°S) and northern Chile (24–23°S) during austral summer 1997: the effect of wind intensity on the upwelling and ventilation of CO2-rich waters

The distribution of pH and alkalinity has been used to calculate the distribution of total inorganic carbon (TC) and fugacity of carbon dioxide (fCO₂) in the upper 200 m of the water column in coastal upwelling areas off northern Chile (23–24°S, near Antofagasta) and central Chile (30–31°S, near Coquimbo) during austral summer 1997. In these upwelling areas, colder surface waters were oxygen poor and strongly CO₂ supersaturated (100% near Antofagasta and 200% near Coquimbo), although below the pycnocline the CO₂ supersaturation invariably exceeded 200% in both areas. The larger surface CO₂ supersaturation and outgassing at 30°S were associated with stronger winds that promoted the upwelling of denser water (richer in CO₂) as well as a higher air–sea CO₂ transfer velocity. The consistent decrease in intensity of the southerly winds (as derived from NSCAT scatterometer data) from 30–31°S to 23–24°S suggests a corresponding decline in the intensity of the CO₂ outgassing due to upwelling. Additionally, we suggest here that the intensity of the local upwelling forcing (i.e. alongshore–equatorward winds) plays a role in determining the water mass composition and phytoplankton biomass of the coastal waters. Thus, while deep upwelling of salty and cold water resulted in high fCO₂ (up to 1000 μatm) and very low phytoplankton biomass (chlorophyll a concentration lower than 0.5 mg m⁻³), the shallow upwelling of less salty (e.g. salinity <34.5) and less CO₂-supersaturated water resulted in a higher phytoplankton biomass, which further reduced surface water fCO₂ by photosynthesis.     

Inorganic carbon in the Labrador Sea: Estimation of the anthropogenic component

The intermediate and deep waters of the Labrador Sea are dominated by recently ventilated water masses (ventilation ages <20 yr). Atmospheric gases such as CO₂ and chlorofluorocarbons are incorporated into these water masses at the time of formation and subsequently transported via boundary currents into the North Atlantic interior. Recent measurements of total carbonate were used in tandem with total alkalinity and oxygen to estimate the levels of anthropogenic carbon dioxide in the Labrador Sea region. Upper water column anthropogenic CO₂ estimated in this manner showed good agreement with levels calculated from CO₂ increase in the atmosphere. In spring 1997, anthropogenic contributions to total carbonate (CTant) were 40±3 μmol/kg in water penetrated by deep convection the previous winter and slightly lower (37±2 μmol/kg) in the deeper convective layer formed in the winters of 1992–1994. Consistent with the concurrent profiles of CFC-11, levels decrease into the older NEADW (North East Atlantic Deep Water) with levels of 30±3 μmol/kg and then increase near bottom within the layer of DSOW (Denmark Strait Overflow Water). The distribution of CTant shows the flow of new LSW southwards with the western boundary current and also eastwards into the Irminger Sea. We estimate that 0.15–0.35 Gt carbon of anthropogenic origin flow through the Labrador Sea within the Western Boundary Undercurrent per year.     

Sulfate reduction in Black Sea sediments: in situ and laboratory radiotracer measurements from the shelf to 2000 m depth

Sulfate reduction rate measurements by the ³⁵SO₄²⁻ core injection method were carried out in situ with a benthic lander, LUISE, and in parallel by shipboard incubations in sediments of the Black Sea. Eight stations were studied along a transect from the Romanian shelf to the deep western anoxic basin. The highest rates measured on an areal basis for the upper 0–15 cm were 1.97 mmol m⁻² d⁻¹ on the shelf and 1.54 mmol m⁻² d⁻¹ at 181 m water depth just below the chemocline. At all stations sulfate reduction rates decreased to values <3 nmol cm⁻³ d⁻¹ below 15 cm depth in the sediment. The importance of sulfate reduction relative to the total mineralization of organic matter was very low, 6%, on the inner shelf, which was paved with mussels, and increased to 47% on the outer shelf at 100 m depth. Where the oxic–anoxic interface of the water column impinged on the sea floor at around 150 m depth, the contribution of sulfate reduction increased from >50% just above the chemocline to 100% just below. In the deep sea, mean sulfate reduction rates were 0.6 mmol m⁻² d⁻¹ corresponding to an organic carbon oxidation of 1.3 mmol m⁻² d⁻¹. This is close to the mean sedimentation rate of organic carbon over the year in the western basin. A comparison with published data on sulfate reduction in Black Sea sediments showed that the present results tend to be higher in shelf sediments and lower in the deep-sea than most other data. Based on the present water column H₂S inventory and the H₂S flux out of the sediment, the calculated turnover time of H₂S below the chemocline is 2100 years.     

The response of abyssal organisms to low pH conditions during a series of CO2-release experiments simulating deep-sea carbon sequestration

The effects of low-pH, high-pCO₂ conditions on deep-sea organisms were examined during four deep-sea CO₂ release experiments simulating deep-ocean C sequestration by the direct injection of CO₂ into the deep sea. We examined the survival of common deep-sea, benthic organisms (microbes; macrofauna, dominated by Polychaeta, Nematoda, Crustacea, Mollusca; megafauna, Echinodermata, Mollusca, Pisces) exposed to low-pH waters emanating as a dissolution plume from pools of liquid carbon dioxide released on the seabed during four abyssal CO₂-release experiments. Microbial abundance in deep-sea sediments was unchanged in one experiment, but increased under environmental hypercapnia during another, where the microbial assemblage may have benefited indirectly from the negative impact of low-pH conditions on other taxa. Lower abyssal metazoans exhibited low survival rates near CO₂ pools. No urchins or holothurians survived during 30–42 days of exposure to episodic, but severe environmental hypercapnia during one experiment (E1; pH reduced by as much as ca. 1.4 units). These large pH reductions also caused 75% mortality for the deep-sea amphipod, Haploops lodo, near CO₂ pools. Survival under smaller pH reductions (ΔpH<0.4 units) in other experiments (E2, E3, E5) was higher for all taxa, including echinoderms. Gastropods, cephalopods, and fish were more tolerant than most other taxa. The gastropod Retimohnia sp. and octopus Benthoctopus sp. survived exposure to pH reductions that episodically reached −0.3 pH units. Ninety percent of abyssal zoarcids (Pachycara bulbiceps) survived exposure to pH changes reaching ca. −0.3 pH units during 30–42 day-long experiments.     

Continuous shipboard sampling of gas tension,oxygen and nitrogen

A novel shipboard gas tension device (GTD) that measures total dissolved air pressure in ocean surface waters is described and demonstrated. In addition, an improved method to estimate dissolved N₂ levels from simultaneous measurements of gas tension, dissolved O₂, water temperature, and salinity is described. Other than a flow-through plenum, the shipboard GTD is similar to the previously described moored-mode GTD (McNeil et al., 1995, Deep-Sea Research I 42, 819–826). The plenum has an integrated water-side screen to protect the membrane, and prevent the membrane from flexing in super-saturated near surface waters. The sampling scheme uses a well mixed and thermally insulated 15 L container that is flushed by the ship's seawater intake at a rate of 3–15 L min⁻¹. Dissolved gas sensors are placed inside this container and flushed with a small recirculation pump. Laboratory data that characterize the response of the modified GTD are presented. The modified GTD has a constant, isothermal, characteristic (e-folding) response time of typically 11±2 min at 20 °C. The response time decreases with increasing temperature and varies by ±35% over a temperature range of 5–35 °C. Results of field measurements, collected on the R.V. Brown between New York and Puerto Rico during September 2002, are presented, and provide the first look at co-variability in surface ocean N₂, O₂, and CO₂ levels over horizontal length scales of several kilometers. Dissolved N₂ concentrations decreased by approximately 16% as the ship sailed from the colder northern continental shelf waters, across the Gulf Stream, and into the warmer northwestern Atlantic Ocean. Historical database measurements, buoy time series, and satellite imagery, are used to aid interpretation of the dissolved gas levels.     

The carbonate system in the East China Sea in winter

Measurements of dissolved inorganic carbon (DIC), pH, total alkalinity (TA), and partial pressure of CO₂ (pCO₂) were conducted at a total of 25 stations along four cross shelf transects in the East China Sea (ECS) in January 2008. Results showed that their distributions in the surface water corresponded well to the general circulation pattern in the ECS. Low DIC and pCO₂ and high pH were found in the warm and saline Kuroshio Current water flowing northeastward along the shelf break, whereas high DIC and pCO₂ and low pH were mainly observed in the cold and less saline China Coastal Current water flowing southward along the coast of Mainland China. Difference between surface water and atmospheric pCO₂ (ΔpCO₂), ranging from ~ 0 to ? 111 μatm, indicated that the entire ECS shelf acted as a CO₂ sink during winter with an average flux of CO₂ of ?13.7 ± 5.7 (mmol C m^? 2 day^? 1), and is consistent with previous studies. However, pCO₂ was negatively correlated with temperature for surface waters lower than 20 °C, in contrast to the positive correlation found in the 1990s. Moreover, the wintertime ΔpCO₂ in the inner shelf near the Changjiang River estuary has appreciably decreased since the early 1990s, suggesting a decline of CO₂ sequestration capacity in this region. However, the actual causes for the observed relationship between these decadal changes and the increased eutrophication over recent decades are worth further study.     

Air–sea CO2 fluxes in the north-eastern shelf of the Gulf of Cádiz (southwest Iberian Peninsula)

An intra-annual investigation of the fugacity of CO₂ (fCO₂) has been conducted in surface waters of the north-eastern shelf of the Gulf of Cádiz (SW Iberian Peninsula) in four cruises made in 2006 and 2007. Intra-annual variability of fCO₂ was assessed and is discussed in terms of mixing, temperature and biology. In the study area of the shelf, thermodynamic control over fCO₂ predominates from early May to late November, and this is opposite and similar in magnitude to the net biological effect. However, biological control over fCO₂ predominates during winter. The results suggest that surface waters in the coastal area are under-saturated with respect to atmospheric CO₂ during most of the year; therefore they represent a sink for atmospheric CO₂ between November and May (? 1.0 mmol m^? 2 day^? 1), but a weak source in June (1.3 mmol m^? 2 day^? 1). In contrast, the coastal ecosystems studied (the lower estuary of Guadalquivir Estuary and Bay of Cádiz) acted as a weak sink for atmospheric CO₂ during February (? 1.3 mmol m^? 2 day^? 1) and as a source between May and November (2.6 mmol m^? 2 day^? 1). The resulting mean annual CO₂ flux in the north-eastern shelf of the Gulf of Cádiz was ? 0.07 mol m^? 2 year^? 1 (? 0.2 mmol m^? 2 day^? 1), indicating that the area acts as a net sink on an annual basis.     

Biogeochemical study of a coccolithophore bloom in the northern Bay of Biscay (NE Atlantic Ocean) in June 2004

The present paper synthesizes data obtained during a multidisciplinary cruise carried out in June 2004 at the continental margin of the northern Bay of Biscay. The data-set allows to describe the different stages of a coccolithophore bloom dominated by Emiliania huxleyi. The cruise was carried out after the main spring phytoplankton bloom that started in mid-April and peaked in mid-May. Consequently, low phosphate (PO₄ < 0.2 μM) and silicate (DSi < 2.0 μM) concentrations, low partial pressure of carbon dioxide (pCO₂) and high calcite saturation degree in surface waters combined with thermal stratification, probably favoured the blooming of coccolithophores. During the period of the year our cruise was carried out, internal tides induce enhanced vertical mixing at the continental shelf break leading to the injection of inorganic nutrients to surface waters that probably trigger the bloom. The bloom developed as the water-column stratified and as the water mass was advected over the continental shelf, following the general residual circulation in the area. The most developed phase of the bloom was sampled in a remote sensed high reflectance (HR) patch over the continental shelf that was characterized by low chlorophyll-a (Chl-a) concentration in surface waters (<1.0 μg L^?1), high particulate inorganic carbon (PIC) concentration (～8 μmol L^?1) and coccolithophore abundance up to 57 × 10⁶ cells L^?1. Transparent exopolymer particles (TEP) concentrations ranged between 15 and 75 μg C L^?1 and carbon content of TEP represented up to 26% of the particulate organic carbon (POC; maximum concentration of 15.5 μmol L^?1 in the upper 40 m). Integrated primary production (PP) ranged between 210 and 680 mg C m^?2 d^?1 and integrated calcification (CAL) ranged between 14 and 140 mg C m^?2 d^?1, within the range of PP and CAL values previously reported during coccolithophore blooms in open and shelf waters of the North Atlantic Ocean. Bacterial protein production (BPP) measurements in surface waters (0.3–0.7 μg C L^?1 h^?1) were much higher than those reported during early phases of coccolithophore blooms in natural conditions, but similar to those during peak and declining coocolithophorid blooms reported in mesocosms. Total alkalinity anomalies with respect to conservative mixing (ΔTA) down to ?49 μmol kg^?1 are consistent with the occurrence of biogenic precipitation of calcite, while pCO₂ remained 15–107 μatm lower than atmospheric equilibrium (372 μatm). The correlation between ΔTA and pCO₂ suggested that pCO₂ increased in part due to calcification, but this increase was insufficient to overcome the background under-saturation of CO₂. This is related to the biogeochemical history of the water masses due to net carbon fixation by the successive phytoplankton blooms in the area prior to the cruise, hence, the investigated area remained a sink for atmospheric CO₂ despite calcification.     

Note on the CO2 air–sea gas exchange at high temperatures

The temperature dependency of ocean–atmosphere gas transfer velocities is commonly estimated in terms of Schmidt numbers, i.e. the ratio of kinematic viscosity to diffusivity. In numerical models least square regressions are used to fit the limited number of experimentally derived Schmidt numbers to a function of temperature. For CO₂ a well established fit can be found in the literature. This fit constitutes an integral part in standardized carbon cycle simulation projects (e.g. C4MIP, OC4MIP, Friedlingstein et al., 2006). However, the fit is valid only in the range where diffusivity measurements exist, i.e., from 0 to about 30 °C. In many climate warming simulations like e.g. the MPI contribution to the fourth Intergovernmental Panel on Climate Change Assessment Report (IPCC AR 4), sea surface temperatures largely exceed the validated range and approach or even reach the range, where the standard fits leave the physically meaningful range. Thus, this paper underlines the demand for new measurements of seawater diffusivities for CO₂ and other trace gases especially for the temperature range >30 °C.In this paper we provide improved fits for the temperature dependence of the Schmidt number. For carbon dioxide our fit is compared to the established fit under identical climate change simulations carried out with the 3D-carbon cycle model HAMOCC. We find that in many tropical and subtropical high temperature regions the established fit leads to unrealistically short adaption times of the surface water pCO₂ to altered atmospheric pCO₂. In regions where the local oceanic pCO₂ is not primarily controlled by the atmospheric boundary pCO₂ but by other processes such as biological activity, the atmosphere ocean pCO₂ gradient is clearly underestimated when using the established fit. The effect on global oceanic carbon uptake in a greenhouse world is rather small and the potential climate feedback introduced by this bias seems to be negligible. However, the bias will clearly gain in significance the more regions warm up to over 30 °C. On a regional scale, especially in coastal regions at low latitudes, the effect is not negligible and a different steady state is approached.     

Changes in deep-water CO2 concentrations over the last several decades determined from discrete pCO2measurements

Detection and attribution of hydrographic and biogeochemical changes in the deep ocean are challenging due to the small magnitude of their signals and to limitations in the accuracy of available data. However, there are indications that anthropogenic and climate change signals are starting to manifest at depth. The deep ocean below 2000 m comprises about 50% of the total ocean volume, and changes in the deep ocean should be followed over time to accurately assess the partitioning of anthropogenic carbon dioxide (CO₂) between the ocean, terrestrial biosphere, and atmosphere. Here we determine the changes in the interior deep-water inorganic carbon content by a novel means that uses the partial pressure of CO₂ measured at 20 °C, pCO₂(20), along three meridional transects in the Atlantic and Pacific oceans. These changes are measured on decadal time scales using observations from the World Ocean Circulation Experiment (WOCE)/World Hydrographic Program (WHP) of the 1980s and 1990s and the CLIVAR/CO₂ Repeat Hydrography Program of the past decade. The pCO₂(20) values show a consistent increase in deep water over the time period. Changes in total dissolved inorganic carbon (DIC) content in the deep interior are not significant or consistent, as most of the signal is below the level of analytical uncertainty. Using an approximate relationship between pCO₂(20) and DIC change, we infer DIC changes that are at the margin of detectability. However, when integrated on the basin scale, the increases range from 8–40% of the total specific water column changes over the past several decades. Patterns in chlorofluorocarbons (CFCs), along with output from an ocean model, suggest that the changes in pCO₂(20) and DIC are of anthropogenic origin.     

Modelling the carbon export and air–sea flux of CO2 in the Greenland Sea

We have developed a 3D model for the carbon cycle and air–sea flux of CO₂ in the Greenland Sea that consists of three submodels for hydrodynamics, carbon chemistry and plankton ecology. The hydrodynamical model, based on the primitive Navier–Stokes equations, simulates the physical environment that is used for the chemical and biological models. The chemical model calculates the pCO₂ as a function of the total inorganic carbon, alkalinity, temperature and salinity. The ecological model has eight state variables and simulates the transformation of CO₂ into organic carbon, vertical transport, and the respiration processes that convert the organic carbon back into inorganic form. The model gives an average annual primary production of 68 g C m⁻² y⁻¹, of which 44.7 g C m⁻² y⁻¹ is new production. In the eastern part of the Greenland Sea, the average annual new production is above 50 g C m⁻² y⁻¹. Simulated, annual flux of CO₂ from the atmosphere is 53 g C m⁻² y⁻¹, which sums up to 0.026 Gt for the whole Greenland Sea. Of this, 9 g C m⁻² y⁻¹ is exported by sinking particles, 6 g C m⁻² y⁻¹ by migrating zooplankton (mainly Calanus hyperboreus), and 38 g C m⁻² y⁻¹ by advection.