Fugacity Of Carbon Dioxide Research Articles

A rapid transition from ice covered CO<sub>2</sub>–rich waters to a biologically mediated CO<sub>2</sub> sink in the eastern Weddell Gyre

Abstract. Circumpolar Deep Water (CDW), locally called Warm Deep Water (WDW), enters the Weddell Gyre in the southeast, roughly at 25° E to 30° E. In December 2002 and January 2003 we studied the effect of entrainment of WDW on the fugacity of carbon dioxide (fCO2) and dissolved inorganic carbon (DIC) in Weddell Sea surface waters. Ultimately the fCO2 difference across the sea surface drives air-sea fluxes of CO2. Deep CTD sections and surface transects of fCO2 were made along the Prime Meridian, a northwest-southeast section, and along 17° E to 23° E during cruise ANT XX/2 on FS Polarstern. Upward movement and entrainment of WDW into the winter mixed layer had significantly increased DIC and fCO2 below the sea ice along 0° W and 17° E to 23° E, notably in the southern Weddell Gyre. Nonetheless, the ice cover largely prevented outgassing of CO2 to the atmosphere. During and upon melting of the ice, biological activity rapidly reduced surface water fCO2 by up to 100 μatm, thus creating a sink for atmospheric CO2. Despite the tendency of the surfacing WDW to cause CO2 supersaturation, the Weddell Gyre may well be a CO2 sink on an annual basis due to this effective mechanism involving ice cover and ensuing biological fCO2 reduction. Dissolution of calcium carbonate (CaCO3) in melting sea ice may play a minor role in this rapid reduction of surface water fCO2.

The island mass effect and biological carbon uptake for the subantarctic Crozet Archipelago

Marine productivity is often higher downstream than upstream of islands. This so-called island mass effect was tested and quantified with respect to biological carbon uptake and air–sea exchange of carbon dioxide (CO2) at the Crozet Plateau between November 2004 and January 2005 during two CROZEX cruises. The remote plateau is situated at 45.5–47.0°S 49.0–53.0°E, south of the Subantarctic Front (SAF) in the Polar Frontal Zone (PFZ). Surface waters upstream (south) of the plateau had high nutrient and low chlorophyll (HNLC) concentrations. The fugacity of carbon dioxide (fCO2) in surface water was just below the atmospheric value and oceanic CO2 uptake was small (0.2±0.1 mol m−2) throughout CROZEX. The mixed-layer concentration of dissolved inorganic carbon (DIC) decreased by 15 μmol kg−1 from November to January in these HNLC waters, indicating significant biological carbon uptake. Extensive phytoplankton blooms occurred downstream (north) of the plateau in austral spring. These reduced surface water fCO2 by 30–70 μatm and DIC by 30–60 μmol kg−1 and created an important oceanic sink for atmospheric CO2 of 0.6–0.8±0.4 mol m−2, corresponding to a total uptake of 1.3±0.8 Tg C (1 Tg=1012 g). The reduction of DIC in the upper 100 m was much larger downstream (2–3 mol m−2) than upstream (1 mol m−2) of the plateau in January, further confirming the existence of the island mass effect for the Crozet Archipelago. An additional finding is the sizeable DIC deficit in the HNLC waters upstream (south) of the plateau, suggesting that some HNLC waters of the PFZ are more productive than commonly thought. Deep mixed layers of 60–90 m may hide such sustained, modest marine productivity from detection by satellite.

Calcic hydrosilicate metasomatic rocks at the Gumeshevsk skarn-porphyry copper deposit in the central Urals, Russia

The body of hydroxylellestadite metasomatic rock penetrated by a borehole drilled at the Gumeshevsk deposit at depths of 530–534 m includes a thin interval of younger lower temperature tobermorite-plombierite metasomatic rock with subordinate amounts of Ca-Si gel, tacherenite, cubic lime, and thaumasite. Hydroxylellestadite has never before been found in calc skarns. The hydroxylellestadite metasomatic rock is cut by gypsum and fukalite veinlets, and the tobermorite-plombierite metasomatic rock is intersected by thaumasite veinlets. The pristine rock of the metasomatics was marble, and the metasomatic rock replaced andradite-bearing wollastonite skarn (with wollastonite replaced by foshagite). The ore minerals (chalcopyrite, valleriite, sphalerite, and others) were formed after the hydroxylellestadite metasomatite but most probably before the tobermorite-plombierite metasomatic rock and the veinlets of calcic minerals. The metasomatic rock was produced at significant variations in the oxygen, sulfur, and carbon dioxide fugacities. The composition of the hydroxylellestadite is, according to its microprobe analysis, as follows (wt %): SiO2 17.10, TiO2 0.01, Al2O3 0.02, FeO 0.20, MnO 0.00, MgO 0.04, CaO 55.40, Na2O 0.14, K2O 0.09, P2O5 0.12, CO2 1.90 (chemical analysis), SO3 21.60, F 0.16, Cl 0.14, total 96.92. The plombierite (SiO2 43.8–44.1 wt %, CaO 30.5–31.1 wt %) in the metasomatic rock notably differs from rare plombierite (SiO2 48.18 wt %, CaO 39.19 wt %) contained in the veinlets of thaumasite (SiO2 12.70 wt %, CaO 30.69 wt %, SO3 17.78 wt %).

Simultaneous spectrophotometric flow-through measurements of pH, carbon dioxide fugacity, and total inorganic carbon in seawater

An autonomous multi-parameter flow-through CO 2 system has been developed to simultaneously measure surface seawater pH, carbon dioxide fugacity ( fCO 2), and total dissolved inorganic carbon (DIC). All three measurements are based on spectrophotometric determinations of solution pH at multiple wavelengths using sulfonephthalein indicators. The pH optical cell is machined from a PEEK polymer rod bearing a bore-hole with an optical pathlength of ∼15 cm. The fCO 2 optical cell consists of Teflon AF 2400 (DuPont) capillary tubing sealed within the bore-hole of a PEEK rod. This Teflon AF tubing is filled with a standard indicator solution with a fixed total alkalinity, and forms a liquid core waveguide (LCW). The LCW functions as both a long pathlength (∼15 cm) optical cell and a membrane that equilibrates the internal standard solution with external seawater. fCO 2 is then determined by measuring the pH of the internal solution. DIC is measured by determining the pH of standard internal solutions in equilibrium with seawater that has been acidified to convert all forms of DIC to CO 2. The system runs repetitive measurement cycles with a sampling frequency of ∼7 samples (21 measurements) per hour. The system was used for underway measurements of sea surface pH, fCO 2, and DIC during the CLIVAR/CO 2 A16S cruise in the South Atlantic Ocean in 2005. The field precisions were evaluated to be 0.0008 units for pH, 0.9 μatm for fCO 2, and 2.4 μmol kg −1 for DIC. These field precisions are close to those obtained in the laboratory. Direct comparison of our measurements and measurements obtained using established standard methods revealed that the system achieved field agreements of 0.0012 ± 0.0042 units for pH, 1.0 ± 2.5 μatm for fCO 2, and 2.2 ± 6.0 μmol kg −1 for DIC. This system integrates spectrophotometric measurements of multiple CO 2 parameters into a single package suitable for observations of both seawater and freshwater.

Effect of copolymer composition, temperature, and carbon dioxide fugacity on pure- and mixed-gas permeability in poly(ethylene glycol)-based materials: Free volume interpretation

Network copolymers, prepared by photopolymerizing poly(ethylene glycol) diacrylate (PEGDA: CH 2 CHCOO(CH 2CH 2O) 14OCCH CH 2) and poly(ethylene glycol) methyl ether acrylate (PEGMEA: CH 2 CHCO(OCH 2CH 2) 8OCH 3), have been studied for mixed-gas CO 2/H 2 and CO 2/CH 4 separations. Gas separation properties in the networks are sensitive to copolymer composition, temperature, and carbon dioxide fugacity in the feed. In conventional approaches, the effect of temperature can be described by the Arrhenius equation, while the influence of copolymer composition and fugacity can only be described empirically; the corresponding models require a large number of adjustable parameters. However, as shown here, the three factors influencing gas transport correlate well with free volume, and a simplified free volume model with only two adjustable parameters can satisfactorily describe the effects of copolymer composition, temperature, and CO 2 fugacity on pure- and mixed-gas transport properties. Copolymer composition and temperature can be directly related to the fractional free volume, while CO 2 fugacity of the feed can be correlated with free volume via the glass transition temperature of the polymer–gas mixture, which is computed using Chow's model.

Carbon distribution and fluxes during the SERIES iron fertilization experiment with special reference to the fugacity of carbon dioxide (fCO2)

Abstract Surface seawater fugacity of carbon dioxide (fCO2) was measured during the Subarctic Ecosystem Response to Iron Enrichment Study (SERIES), July 9–August 5, 2002. Three ships sampled the iron-fertilized waters near Ocean Station P (50°N, 145°W): the Canadian CCGS J.P. Tully (July 9–23, 2002), the chartered Mexican M/V El Puma (July 9–28, 2002), and the Japanese fisheries research ship M/V Kaiyo Maru (July 24–August 5, 2002). Data used here are from the CCGS J.P. Tully and the M/V Kaiyo Maru. From the onset of the experiment to the peak of the iron-induced diatom bloom on day 19, sea-surface fCO2 decreased from 350 to 265 μatm and average DIC concentration in the upper 30 m decreased from 2030 to 1990 μmol kg−1. Changes in fCO2 in and near the iron patch as observed from the CCGS J.P. Tully and later from the M/V Kaiyo Maru were used to estimate CO2 drawdown and air-sea fluxes, and in generating a carbon budget during the growth phase (days 3–19) of the experiment. Without considering patch dilution, sources of dissolved inorganic carbon to the patch (1.6±0.25 mol m−2) were nearly double the sum (0.87±0.34 mol m−2) of the sinks: accumulations of dissolved organic and particulate carbon, and the flux of particulate carbon to sediment traps below the patch. However, the budget is balanced after considerations of the effects of patch expansion on property concentrations within the patch. A comparison with other iron fertilization experiments from 1995 to present was made to assess the CO2 drawdown values.

Surface CO 2 measurements in the English Channel and Southern Bight of North Sea using voluntary observing ships

Ships of opportunity have been used to investigate ocean–atmosphere CO 2 fluxes in the English Channel and Southern Bight of the North Sea. Continuous underway measurements of the fugacity of seawater carbon dioxide ( fCO 2 sw), chlorophyll, temperature and salinity have been performed along 26 transects during the spring and autumn periods. The spatial fCO 2 sw distribution along the Channel and Southern Bight is modulated by the photosynthetic activity, temperature changes and water mixing between inputs from the North Atlantic Ocean and riverine discharges. The seasonal variability of fCO 2 sw is assessed and discussed in terms of the biology and temperature effects, these having similar impacts. The variation of fCO 2 sw shows similar interannual patterns, with lower values in spring. The annual average of air–sea CO 2 fluxes places the English Channel as neutral area of CO 2 uptake. The spring and autumn data allow differentiating between distal and proximal continental areas. The Southern Bight shows a tendency towards net CO 2 uptake on the distal continental shelf, whereas the Scheldt and Thames Plumes show a CO 2 source behaviour on the proximal continental shelves.

Changes of the oceanic CO2 sink in the Eastern Indian sector of the Southern Ocean

Changes in the carbon dioxide fugacity (fCO2) and air–sea CO2 flux observed in the Southern Ocean, south of Tasmania were analysed and compared for two different years: 1996/1997 and 2002/2003. The CO2 flux showed large and contrasting interannual changes in the permanent open ocean zone (POOZ, 53–61◦S) between the 2 yr where the oceanic CO2 sink increased from about -0.3 mmol m-2 d-1 in February 1997 to -20.6 mmol m-2 d-1 in February 2003. The strong sink in February 2003 was associated with increased phytoplankton biomass in this High-Nutrient, Low- Chlorophyll (HNLC) region. Three hypotheses that may have influenced the biomass and fCO2 changes in the POOZ were investigated: sea surface temperature (SST) and El Ni˜no/ Southern Oscillation (ENSO) event, total stratospheric ozone column and ultraviolet (UV) radiation, and atmospheric dust inputs. The strong CO2 sink in 2003 in the POOZ cannot be explained by the observed changes in SST or UV, but would be qualitatively consistent with the presence of episodic atmospheric dust inputs.

Nutrient and carbon parameters during the Southern Ocean iron experiment (SOFeX)

During the 2002 austral summer, two in situ mesoscale iron fertilization experiments were conducted in two distinct silicic acid regimes of the Southern Ocean (SOFeX). The iron was added to the low Si(OH) 4 waters of the Subantarctic (<3 μmol kg −1, North Patch) and the high Si(OH) 4 waters poleward of the southern boundary of the Antarctic Circumpolar Current (ACC), (∼63 μmol kg −1, South Patch). The low iron availability in these two regimes of the Southern Ocean limits biological utilization of the ambient nutrients, influences phytoplankton species composition and species-specific nutrient consumption rates. The response of the carbon dioxide system and macronutrients to initial and subsequent iron additions were monitor relative to measurements outside the iron-enriched waters. These iron fertilization experiments demonstrate that iron addition to the High Nutrient Low Chlorophyll (HNLC) regions of the Southern Ocean increases primary productivity and decreases the fugacity of carbon dioxide. Dilution of the iron fertilized sites diminished the observed geochemical signals produced as a result of phytoplankton growth, while the entrainment of silicic acid sustained the increase of diatom biomass in the low Si(OH) 4 waters of the Subantarctic. Empirical observation of the elemental composition of the iron fertilized ecosystems and the observed nutrient-depletion ratios upon the addition of iron were indicative of changes in the community structure and/or changes in the unique physiological nutrient consumption rates of a species within the community. Ultimately, variations in the nutrient-depletion ratios induced by increased iron concentrations influences biogeochemical cycles in the Southern Ocean and the export of carbon. Molar ratios in the north and south iron fertilization sites, respectively, were determined to be ΔN:ΔP=14.1±0.2 and 14.2±0.4; ΔC:ΔN=9.1±1.7 and 7.2±0.6; ΔC:ΔP=106±4 and 99±4; ΔC:ΔSi=7.7±0.8 and 6.2±0.5; ΔC:ΔO 2=−0.75±0.02 and −0.73±0.02. Combining these results, the following stoichiometry is obtained for the growth of phytoplankton during the SOFeX in the north and south iron enrichment sites, 106CO 2+120H 2O+14HNO 3+H 3PO 4+14SiO 2→ ( CH 2 O ) 92 ( CH 2 ) 14 ( NH 3 ) 14 ( H 3 PO 4 ) ( SiO 2 ) 14 +141O 2 and 99CO 2+113H 2O+14HNO 3+H 3PO 4+16SiO 2→ ( CH 2 O ) 81 ( CH 2 ) 18 ( NH 3 ) 14 ( H 3 PO 4 ) ( SiO 2 ) 16 +136O 2, respectively. Variations in the ratios of carbon to macronutrients utilized by the ecosystem under iron replete conditions are important for the interpretation of the effect of iron fluxes on atmospheric CO 2 concentrations in glacial times, highlighting the integral role of Fe and the Southern Ocean regulation of atmospheric carbon dioxide.

Fluxes of CO2 between the atmosphere and the ocean during the POMME project in the northeast Atlantic Ocean during 2001

In the eastern North Atlantic, carbon dioxide fugacity (fCO2) in the upper mixed layer and discrete pH and total alkalinity measurements in the upper 2000 m were studied during three cruises (winter, spring, and summer 2001) within the framework of the Programme Océan Multidisciplinaire Méso Echelle (POMME) project. This extensive region is located between 39° and 45°N and 16° and 21°W. The mesoscale variability of fCO2 on the sea surface and in the atmosphere during each season was determined to understand the mechanisms of evolution that control the spatial and temporal variability of fCO2 together with an estimation of the fluxes of CO2 between the atmosphere and the ocean. If we consider the observation to be 22 days per cruise, the region was in‐taking 0.30 Tg C during the winter cruise and 0.36 Tg C during the spring cruise, whereas it was out‐gassing 0.07 Tg C during the summer cruise. These values are clear indications that the area is acting as a sink of CO2 on an annual scale, with an estimated flux value of −1.1 mol m−2 yr−1, which is over twice as much as the mean global flux of −0.5 mol m−2 yr−1 (Takahashi et al., 2002). The changes with time observed in the fCO2 values over the surface layer between the winter and the spring cruises have been described considering thermodynamics, gas exchange, water transport, and biological activity in the area. The estimation of the subduction of inorganic carbon yielded a value of 0.25 Pg C yr−1, which is approximately 10% of the global net oceanic CO2 sink flux.

Biogeochemical evolution of the carbon dioxide system in the waters of long-lived mesoscale eddies in the Northeast Pacific Ocean

Abstract An anticyclonic eddy, named Haida-2000, formed off the southern Queen Charlotte Islands early in the year 2000. The eddy subsequently tracked northwest, transporting fresh, warm, and nutrient-rich water into the Northeast Pacific Ocean. Based on total dissolved inorganic carbon (C T ), total alkalinity (A T ), and nutrients from six seasonal cruises between February 2000 and September 2001, we found that the oceanic carbonate system of the surface waters in the eddy changed significantly. The greatest loss of C T from the surface waters of Haida-2000 occurred during the first year, between February and June 2000. Based on a concomitant loss in nitrate (NO 3 ), the large C T loss we observed was likely due to biological production of organic carbon and probably resulted in a large drawdown of atmospheric CO 2 , as supported by the observed fugacity of carbon dioxide ( f CO 2 ) in the surface waters. During fall, C T , f CO 2 , and NO 3 values increased, probably due to oxidation of organic matter and vertical entrainment of C T and nutrient-rich water from below with fall mixing. As a result, the surface waters of the eddy were highly oversaturated in CO 2 relative to the atmosphere. Another Haida eddy (Haida-2001) had a significantly smaller nitrate drawdown than Haida-2000 during its first spring, but the C T losses were similar in the two eddies. In early summer (mid-June) of the second year, the surface and sub-surface waters in Haida-2000 gained C T , A T , and NO 3 , partly caused by a merging with another 2001 Haida eddy, showing that exchange with surrounding waters took place and was significant at times. The high nitrate/low chlorophyll (HNLC) surface waters at Ocean Station Papa (OSP), showed less f CO 2 undersaturation during summer and less total nitrate and C T loss than Haida-2000, indicating that both eddies were larger sinks for carbon than HNLC waters. The waters surrounding the eddies generally had concentrations of C T , A T , and NO 3 that were similar to or slightly higher than the waters of both Haida eddies throughout the study. Also, the sub-surface waters in these eddies showed no significant change in total carbon content, suggesting that at these depths eddies are recycling, not exporting systems.

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 ( fCO 2) 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 fCO 2 in surface water decreased at a constant rate of 3.8 μatm day −1 from 4 to 5 days onwards in SOIREE. The fCO 2 reduction was 35 μatm after 13 days. The evolution of surface water fCO 2 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 fCO 2 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 CO 2 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=10 6 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 fCO 2 in surface water.

Diurnal variability in the oceanic carbon dioxide system and oxygen in the Southern Ocean surface water

Abstract During the SWEDARP cruise to the Atlantic sector of the Southern Ocean 1997/1998 six 24-hour stations were occupied in the areas of the Spring Ice Edge (SIE1, SIE2 and SIE3), the Winter Ice Edge (WIE), and the Antarctic Polar Front (APF1 and APF2). Samples were taken at the surface (2 m) every second hour and analyzed for total dissolved inorganic carbon, total alkalinity, pH and dissolved oxygen. By the use of wind speed measurements, calculated fugacity of carbon dioxide, f CO 2 , and oxygen concentrations in the surface water, sea–air fluxes of carbon dioxide (CO 2 ) and oxygen were calculated. These fluxes and the diurnal change in the chemical properties are discussed in relation to changes in biological activity. The fluctuations in wind speed showed a larger impact on the variability of the calculated fluxes than the fluctuations in surface water f CO 2 or oxygen saturation. The calculated fluxes and the variability also showed large differences depending on how the wind speed was used, instantaneously or averaged over 24 hours. The calculated sea–air CO 2 fluxes using instantaneous wind speed varied between −0.012 and −0.11 mmol m −2 h −1 in the SIE1, −0.0073 and –0.30 mmol m −2 h −1 in the WIE and –0.043 and –1.65 mmol m −2 h −1 in the APF2. The mean values of sea–air CO 2 fluxes were calculated to −0.046±0.044, −0.10±0.094 and −0.52±0.64 mmol m −2 h −1 for the SIE1, WIE and the APF2, respectively. The mean values of sea–air oxygen fluxes were calculated to 0.072±0.073, −0.12±0.54 and 1.4±1.3 mmol m −2 h −1 for the corresponding regions.

Evaluation of the National Oceanic and Atmospheric Administration/Coupled‐Ocean Atmospheric Response Experiment (NOAA/COARE) air‐sea gas transfer parameterization using GasEx data

During the two recent GasEx field experiments, direct covariance measurements of air‐sea carbon dioxide fluxes were obtained over the open ocean. Concurrently, the National Oceanic and Atmospheric Administration/Coupled‐Ocean Atmospheric Response Experiment air‐sea gas transfer parameterization was developed to predict gas transfer velocities from measurements of the bulk state of the sea surface and atmosphere. The model output is combined with measurements of the mean air and sea surface carbon dioxide fugacities to provide estimates of the air‐sea CO2 flux, and the model is then tuned to the GasEx‐1998 data set. Because of differences in the local environment and possibly because of weaknesses in the model, some discrepancies are observed between the predicted fluxes from the GasEx‐1998 and GasEx‐2001 cases. To provide an estimate of the contribution to the air‐sea flux of gas due to wave‐breaking processes, the whitecap and bubble parameterizations are removed from the model output. These results show that moderate (approximately 15 m s−1) wind speed breaking wave gas transfer processes account for a fourfold increase in the flux over the modeled interfacial processes.

Interannual controls on Weddell Sea surface water fCO 2 during the autumn–winter transition phase

The fugacity of carbon dioxide ( fCO 2) of the surface waters of the Weddell Sea along the prime meridian has been described for the austral autumn in 1996 and 1998. For individual years, fCO 2 has a strong linear relationship with sea surface temperature, although the relationships cannot be reconciled to provide an interannually consistent algorithm for remotely sensed assessment of fCO 2. However, from the assumption that Weddell Sea surface water has a single end member (upwelled Warm Deep Water) we have determined the relative contributions of heating, ice-melt, and biological activity on fCO 2. A breakdown of the controls shows that the measured annual fCO 2 distributions can be recreated for both transects by adjusting solely for thermodynamic forcing, and model adjustments for salinity are small except in regions of significant upwelling during 1998. The incorporation of nitrate utilisation into the model results in a general and significant underestimation of fCO 2. This runs contrary to the earlier findings of Sabine and Key (Mar. Chem. 60 (1998) 95) in the Southern Ocean although it is consistent with models in the area (Louanchi et al., Deep-Sea Res. I 48 (2001) 1581). A major caveat to these findings is the significant departure of the thermodynamic model and a tightening of the nitrate-adjusted model in 1998 in areas with deeper mixing in the southern Weddell Sea. We propose that there are two reasons for the discrepancies in our model: the source waters are not as homogenous as the model assumes; and there are geographical and seasonal variations of CO 2 exchange with the atmosphere and the input of inorganic carbon and nitrate from below the mixed layer resulting in imbalances in the mixed layer concentration ratios.

Experimental determination of siderite stability and application to Martian Meteorite ALH84001

The pressure-temperature equilibrium curve of the reaction siderite + hematite = magnetite + CO 2 was determined in the range 5-12 kbar and 480-650 °C by piston-cylinder experiments, with NaCl as a pressure medium. Silver oxalate was used as a CO 2 source and samples were buffered at hematitemagnetite oxygen fugacity. Reaction progress was monitored by extent of CO 2 gas loss and by X-ray diffraction (XRD) analysis. The data define a univariant curve, which is described by P = -14.599 + 0.025T + 0.000027 T 2 with P in kbar and T in °C. Calculations based on these data give ΔH 0 f (298K) siderite = -760.6 ± 0.9 kJ (kilojoules) from the oxides. The formation of siderite requires a specific range of ambient oxygen and carbon dioxide fugacities, dependent upon temperature and pressure. The stable assemblage of siderite and magnetite, at a given temperature and pressure, implies more restrictive ranges of oxygen and carbon dioxide fugacities, defined by reactions among siderite, magnetite, graphite, and hematite. Experimental and thermodynamic investigation of the Fe-C-O system indicates that the formation of magnetite along with Ca-Fe-Mg carbonate globules by inorganic processes is possible and may be relevant to Martian meteorite ALH84001. Decarbonation of the siderite component of the carbonate, either by a transient heating event or by a change in oxygen fugacity of a coexisting fluid, may have formed the observed grains, although this study does not address the size or morphology of magnetite grains formed by this mechanism.

Sea–air flux of CO2 in the Caribbean Sea estimated using in situ and remote sensing data

Empirical relationships between sea surface carbon dioxide fugacity (fCO2sw) and sea surface temperature (SST) were applied to datasets of remotely sensed SST to create fCO2sw fields in the Caribbean Sea. SST datasets from different sensors were used, as well as the SST fields created by optimum interpolation of bias corrected AVHRR data. Empirical relationships were derived using shipboard fCO2 sw data, in situ SST data, and SST data from the remote sensing platforms. The results show that the application of a relationship based on shipboard SST data, on fields of remotely sensed SST yields biased fCO 2sw values. This bias is reduced if the fCO 2sw-SST relationships are derived using the same SST data that are used to create the SST fields. The fCO2sw fields found to best reproduce observed fCO2sw are used in combination with wind speed data from QuikSCAT to create weekly maps of the sea-air CO2 flux in the Caribbean Sea in 2002. The region to the SW of Cuba was a source of CO2 to the atmosphere throughout 2002, and the region to the NE was a sink during winter and spring and a source during summer and fall. The net uptake of CO2 in the region was doubled when potential skin layer effects on fCO2sw were taken into account. © 2003 Elsevier Inc. All rights reserved.

Processes regulating oxygen and carbon dioxide in surface waters west of the Antarctic Peninsula

Geographic surveys of oxygen (O 2) and carbon dioxide (CO 2) concentrations in surface waters west of the Antarctic Peninsula show the large temporal and spatial variability predicted for polar regions with winter ice cover and vernal phytoplankton blooms. The saturation states of surface seawater fugacity of carbon dioxide (fCO 2{Sat}) ranged from 27% to 112% relative to the atmosphere and oxygen saturation states (O 2{Sat}) ranged from 81% to 157% of the corresponding air-saturated values. Areas of O 2 supersaturation and fCO 2 undersaturation occurred mostly in coastal waters and were correlated with increases in chlorophyll a indicating a biological influence. Net community production (NCP) showed offshore to onshore gradients with O 2 production as high as 23.8 mmol O 2 m −3 day −1 and CO 2 consumption as high as 19.2 mmol C m −3 day −1 within coastal areas. Areas of surface water O 2 and fCO 2 supersaturation were found further offshore, suggesting heating as a locally significant process. Areas of fCO 2 supersaturation and O 2 undersaturation were also found further offshore suggesting upwelling as a source of the CO 2-enriched waters. Finally, within selected coastal areas, O 2 and fCO 2 were undersaturated suggesting cooling as a locally significant process. The seasonal phasing of biological, chemical and physical processes can help explain the temporal and regional variability that was observed in the saturation states of O 2 and fCO 2.

Continued CO2 outgassing in an upwelling area off northern Chile during the development phase of El Niño 1997–1998 (July 1997)

Carbonate system parameters were measured in the upper 200 m of the water column during July 1997 in an upwelling area off northern Chile (22.6°–24°S), and the CO2 fluxes were estimated. At this time (during the onset of El Niño 1997–1998), the water column that feeds the coastal upwelling was less dense, warmer, and saltier than in non‐El Niño winters. Nevertheless, the major vertical gradients in pH, total inorganic carbon (CT), carbon dioxide fugacity (fCO2), and apparent oxygen utilization (AOU) remained confined to the upper 100 m of the water column, so that the active upwelling forced by southerly winds caused the upwelling of CO2‐rich water leading a CO2 flux from the ocean to the atmosphere. However, these fluxes were found to be highly variable. Grid surveys 2 weeks apart show a change in CO2 flux from +3.9 mol C m−2 yr−1 to +0.4 mol C m−2 yr−1: the change is thought to be associated with a pulsed upwelling forcing in combination with an active biological uptake of CO2. This high short‐term variability of CO2 fluxes makes it difficult to assess the interannual variability of CO2 outgassing in this area based on low‐frequency direct CO2 observations. The fact that the oxycline, whose location usually coincides with the carboncline, also remained within the upper 100 m during the remarkably warm 1972 and 1983 El Niño winters seems to imply that the CO2 outgassing during those warm periods can be as strong as we report for 1997 under similar upwelling favorable winds.

Interannual variability in the wintertime air–sea flux of carbon dioxide in the northern North Atlantic, 1981–2001

Gridded fields of sea surface temperature (SST), sea level pressure (SLP), and wind speed were used in combination with data for the atmospheric mole fraction of CO 2 and an empirical relationship between measured values of the fugacity of carbon dioxide in surface water and SST, to calculate the air–sea CO 2 flux in the northern North Atlantic. The flux was calculated for each of the months October–March, in the time period 1981 until 2001, allowing for an assessment of the interannual variations in the region. Locally and on a monthly time scale, the interannual variability of the flux could be as high as ±100% in regions seasonally covered by sea ice. However, in open-ocean areas the variability was normally between ±20% and ±40%. The interannual variability was found to be approximately halved when fluxes averaged over each winter season were compared. Summarised over the whole northern North Atlantic, the air to sea carbon flux over winter totalled 0.08 Gton, with an interannual variability of about ±7%. On a monthly basis the interannual variations were slightly higher, about ±8% to ±13%. Changes in wind speed and atmospheric fCO 2 (the latter directly related to SLP variations) accounted for most of the interannual variations of the computed air–sea CO 2 fluxes. A tendency for increasing CO 2 flux into the ocean with increasing values of the NAO index was identified.