Surface Water pCO2 Research Articles

Spatial variability and temporal dynamics of surface water pCO 2, ΔO 2/Ar and dimethylsulfide in the Ross Sea, Antarctica

We report results from two surveys of pCO 2, biological O 2 saturation (ΔO 2/Ar) and dimethylsulfide (DMS) in surface waters of the Ross Sea polynya. Measurements were made during early spring (November 2006–December 2006) and mid-summer (December 2005–January 2006) using ship-board membrane inlet mass spectrometry (MIMS) for high spatial resolution ( i.e. sub-km) analysis. During the early spring survey, the polynya was in the initial stages of development and exhibited a rapid increase in open water area and phytoplankton biomass over the course of our ∼3 week occupation. We observed a rapid transition from a net heterotrophic ice-covered system (supersaturated pCO 2 and undersaturated O 2) to a high productivity regime associated with a Phaeocystis-dominated phytoplankton bloom. The timing of the early spring phytoplankton bloom was closely tied to increasing sea surface temperature across the polynya, as well as reduced wind speeds and ice cover, leading to enhanced vertical stratification. There was a strong correlation between pCO 2, ΔO 2/Ar, DMS and chlorophyll a (Chl a) during the spring phytoplankton bloom, indicating a strong biological imprint on gas distributions. Box model calculations suggest that pCO 2 drawdown was largely attributable to net community production, while gas exchange and shoaling mixed layers also exerted a strong control on the re-equilibration of mixed layer Ο 2 with the overlying atmosphere. DMS concentrations were closely coupled to Phaeocystis biomass across the early spring polynya, with maximum concentrations exceeding 100 nM. During the summer cruise, we sampled a large net autotrophic polynya, shortly after the seasonal peak in phytoplankton productivity. Both diatoms and Phaeocystis were abundant in the phytoplankton assemblages during this time. Minimum pCO 2 was less than 100 ppm, while ΔO 2/Ar exceeded 30% in some regions. Mean DMS concentrations were ∼2-fold lower than during the spring, although the range of concentrations was similar between the two surveys. There was a significant correlation between pCO 2, ΔO 2/Ar and Chl a across the summer polynya, but the strength of these correlations and the slope of O 2 vs. CO 2 relationship were significantly lower than during the early spring. Summertime DMS concentrations were not significantly correlated to phytoplankton biomass (Chl a), pCO 2 or ΔO 2/Ar. In contrast to the early spring time, there were no clear temporal trends in summertime gas concentrations. Rather, small-scale spatial variability, likely resulting from mixing and localized sea-ice melt, was clearly evident in surface gas distributions across the polynya. Analysis of length-scale dependent variability demonstrated that much of the spatial variance in surface water gases occurred at scales of <20 km, suggesting that high resolution analysis is needed to fully capture biogeochemical heterogeneity in this system.

Note on the CO 2 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 CO 2 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 CO 2 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 pCO 2 to altered atmospheric pCO 2. In regions where the local oceanic pCO 2 is not primarily controlled by the atmospheric boundary pCO 2 but by other processes such as biological activity, the atmosphere ocean pCO 2 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.

Air—sea flux of CO2 in arctic coastal waters influenced by glacial melt water and sea ice

Annual air–sea exchange of CO2 inYoung Sound,NEGreenlandwas estimated using pCO2 surface-water measurements during summer (2006–2009) and during an ice-covered winter 2008. All surface pCO2 values were below atmospheric levels indicating an uptake of atmospheric CO2. During sea ice formation, dissolved inorganic carbon (DIC) content is reduced causing sea ice to be under saturated in CO2. Approximately 1% of the DIC forced out of growing sea ice was released into the atmosphere while the remaining 99% was exported to the underlying water column. Sea ice covered the fjord 9 months a year and thereby efficiently blocked air–sea CO2 exchange. During sea ice melt, dissolution of CaCO3 combined with primary production and strong stratification of the water column acted to lower surface-water pCO2 levels in the fjord. Also, a large input of glacial melt water containing geochemically reactive carbonate minerals may contribute to the low surface-water pCO2 levels. The average annual uptake of atmospheric CO2 was estimated at 2.7 mol CO2 m−2 yr−1 or 32 g C m−2 yr−1 for the study area, which is lower than estimates from the Greenland Sea. Variability in duration of sea ice cover caused significant year-to-year variation in annual gas exchange.

Low carbon dioxide partial pressure in a productive subtropical lake

The purposes of this study were to assess if Lake Apopka (FL, USA) was autotrophic or heterotrophic based on the partial pressure of dissolved carbon dioxide (pCO2) in the surface water and to evaluate factors that influence the long-term changes in pCO2. Monthly average pH, alkalinity and other limnological variables collected between 1987 and 2006 were used to estimate dissolved inorganic carbon (DIC), pCO2 and CO2 flux between surface water and atmosphere. Results indicated that average pCO2 in the surface water was 196 μatm, well below the atmospheric pCO2. Direct measurements of DIC concentration on three sampling dates in 2009 also supported pCO2 undersaturation in Lake Apopka. Supersaturation in CO2 occurred in this lake in only 13% of the samples from the 20-year record. The surface-water pCO2 was inversely related to Chl a concentrations. Average annual CO2 flux was 28.2 g C m−2 year−1 from the atmosphere to the lake water and correlated significantly with Chl a concentration, indicating that biological carbon sequestration led to the low dissolved CO2 concentration. Low pCO2 and high invasion rates of atmospheric CO2 in Lake Apopka indicated persistent autotrophy. High rates of nutrient loading and primary production, a high buffering capacity, a lack of allochthonous loading of organic matter, and the dominance of a planktivorous–benthivorous fish food web have supported long-term net autotrophy in this shallow subtropical eutrophic lake. Our results also showed that lake restoration by the means of nutrient reduction resulted in significantly lower total phosphorus (TP) and Chl a concentrations, and higher pCO2.

Seasonal variability of dissolved inorganic carbon and surface water pCO 2 in the Scotian Shelf region of the Northwestern Atlantic

The seasonal variability of inorganic carbon in the surface waters of the Scotian Shelf region of the Canadian northwestern Atlantic Ocean was investigated. Seasonal variability was assessed using hourly measurements, covering a full annual cycle, of the partial pressure of CO 2, (pCO 2), and hydrographic variables obtained by an autonomous moored instrument (44.3°N and 63.3°W). These measurements were complemented by seasonal shipboard sampling of dissolved inorganic carbon (DIC), total alkalinity (TA), and pCO 2, at the mooring site, and over the larger spatial scale. Biological processes were found to be the dominant control on mixed-layer DIC, with the delivery of carbon-rich subsurface waters also playing an important role. Annual mixed-layer net community production was 2.4 mol C m − 2 yr − 1 . The air–sea fluxes of CO 2 were computed using observed hourly wind speeds from the Sable Island Meteorological Station (43.9°N and 60.3°W). The region acts as a net source of CO 2 to the atmosphere on the annual scale ( F = − 1.4 mol C m − 2 yr − 1 ), with a reversal of this trend occurring only during the spring phytoplankton bloom, when a pronounced undersaturation of the surface waters is reached for a short period. Outside of the spring bloom period, the competing effects of temperature and biology influence surface pCO 2 in roughly equal magnitude.

Air‐sea flux of CO2 in the Arctic Ocean, 1998–2003

The Arctic Ocean has experienced an unprecedented reduction in sea ice over the last 3 decades, increasing the potential for greater exchange of gases such as carbon dioxide (CO2) between the atmosphere and the upper ocean. The present study utilizes remotely sensed data on distributions of both sea ice and chlorophyll a, together with modeled temperature and salinity fields, to obtain high‐resolution basin‐scale estimates of the air‐sea flux of CO2 (FCO2) in the Arctic Ocean for the years 1998–2003. Concentrations of dissolved inorganic carbon (DIC) were derived from multiple linear regression relationships with sea surface temperature, salinity, and chlorophyll a. The partial pressure of carbon dioxide (pCO2) in surface waters was computed from DIC and alkalinity, the latter of which varied with salinity. FCO2 was calculated from the air‐sea difference in pCO2 and wind speed. Annual FCO2 was highest in the Atlantic‐dominated Greenland and Barents sectors due to their lower sea ice cover, although area‐normalized FCO2 in these sectors was low. Only the Siberian sector exhibited a significant increase in annual FCO2 during the time of our study, due to a corresponding increase in ice‐free water. Overall, the Arctic Ocean was a net atmospheric sink for CO2, with annual FCO2 averaging 118 ± 7 Tg C yr−1 during 1998–2003.

Processes governing the carbon chemistry during the SAGE experiment

Measurements of pCO 2, pH and alkalinity in the surface waters of an iron fertilised patch of sub-Antarctic water were made during SAGE (SOLAS SAGE: Surface-Ocean Lower Atmosphere Studies Air–Sea Gas Experiment). The iron addition induced a minor phytoplankton bloom, however the patch dynamics were dominated by physical processes which suppressed and masked the biological effects. The Lagrangian nature of the experiment allowed the carbonate chemistry in the patch to be followed for 15.5 days, and the relative importance of the biological and physical factors influencing the surface water pCO 2 was estimated. The pCO 2 of the surface waters of the patch increased from 327 μatm prior to iron addition to 338 μatm on Day 14, effects of vertical and horizontal mixing offset the 15 μatm drawdown that would have occurred had the induced biological uptake been the sole factor to influence the pCO 2. The air–sea carbon flux calculated using the measured skin temperature and a piston velocity parameterisation determined during SAGE ( Ho et al., 2006) was 98.5% of the flux determined using conventional bulk temperature measurement and the Wanninkhof (1992) piston velocity parameterisation. The skin temperature alone contributed to an 8% increase in the flux compared with that determined using bulk temperature.

Seasonal variability in air‐sea fluxes of CO2 in a river‐influenced coastal margin

Recent studies in the northern Gulf of Mexico and elsewhere have demonstrated that enhanced biological production in large river plumes may contribute to a net surface influx of atmospheric CO2. However, large rivers also deliver significant amounts of terrestrial carbon into continental margin waters; hence, the potential for large and variable signals in carbon flux exist in these regions. Here, we used a combination of satellite and ship‐based observations to examine variability in surface pCO2 and air‐sea flux of carbon dioxide in relation to variations in river discharge and seasonal environmental conditions. Underway surface pCO2 showed large seasonal differences based on observations acquired during cruises in August 2004, October 2005, and April 2006. Strong cross‐shelf gradients in pCO2 were observed during August 2004 and April 2006, influenced by river outflow. Uniformly high values observed during October 2005 likely reflected the disturbed nature of the system after two major storm events (hurricanes Katrina and Rita). Satellite‐derived assessments of pCO2 were used in conjunction with estimates of wind fields to produce regional maps of surface water pCO2 and air‐sea fluxes. The region was a net sink for atmospheric CO2 in August 2004 (−0.96 to −1.2 mmol C m−2 d−1) and net source during October 2005 and April 2006 (1.0 to 5.4 mmol C m−2 d−1). Uncertainties in flux estimates, particularly for low salinity waters in April 2006, highlighted the need for more extensive in situ observations. Our results illustrate the utility of satellite approaches for providing regional assessments of coastal carbon budgets.

INORGANIC CARBON UTILIZATION BY ROSS SEA PHYTOPLANKTON ACROSS NATURAL AND EXPERIMENTAL CO2 GRADIENTS1

We present results from a field study of inorganic carbon (C) acquisition by Ross Sea phytoplankton during Phaeocystis‐dominated early season blooms. Isotope disequilibrium experiments revealed that HCO3− was the primary inorganic C source for photosynthesis in all phytoplankton assemblages. From these experiments, we also derived relative enhancement factors for HCO3−/CO2 interconversion as a measure of extracellular carbonic anhydrase activity (eCA). The enhancement factors ranged from 1.0 (no apparent eCA activity) to 6.4, with an overall mean of 2.9. Additional eCA measurements, made using membrane inlet mass spectrometry (MIMS), yielded activities ranging from 2.4 to 6.9 U · [μg chl a]−1 (mean 4.1). Measurements of short‐term C‐fixation parameters revealed saturation kinetics with respect to external inorganic carbon, with a mean half‐saturation constant for inorganic carbon uptake (K1/2) of ∼380 μM. Comparison of our early springtime results with published data from late‐season Ross Sea assemblages showed that neither HCO3− utilization nor eCA activity was significantly correlated to ambient CO2 levels or phytoplankton taxonomic composition. We did, however, observe a strong negative relationship between surface water pCO2 and short‐term 14C‐fixation rates for the early season survey. Direct incubation experiments showed no statistically significant effects of pCO2 (10 to 80 Pa) on relative HCO3− utilization or eCA activity. Our results provide insight into the seasonal regulation of C uptake by Ross Sea phytoplankton across a range of pCO2 and phytoplankton taxonomic composition.

Multi-proxy reconstruction of surface water pCO2 in the northern Arabian Sea since 29 ka

We report here the results of a multi-proxy study to reconstruct surface water pCO2 concentrations in the northern Arabian Sea. Our results show that δ11B and Mg/Ca measurements of the planktonic foraminifer Globigerinoides sacculifer yield consistent pCO2 values with those reconstructed from the δ13C of alkenones when used in conjunction with foraminifera δ13C and Cd/Ca values. They reveal that this area of the oceans has been a constant source of CO2 to the atmosphere during the interval 5–29ka, and that the intensity of this source was greatest between 11 and 17ka, when atmospheric CO2 levels were rising rapidly. We interpret our data as reflecting variation in the strength of the Asian Summer Monsoon (ASM), thus indicating that the strength of the ASM varied in phase with summer insolation over the Tibetan plateau between 5 and 29ka. In contrast to a previous study (Clemens and Prell, 2003), we observe no significant lag between the rise in insolation and the response of the ASM. Rather, our data support a recent study by Rohling et al. (2009) that northern hemisphere climatic forcing factors play a greater role in controlling the intensity of the ASM during times of intense monsoon activity, and that the southern hemisphere forcing is more important during times of weak monsoons.

Evidence of O2 consumption in underway seawater lines: Implications for air‐sea O2 and CO2 fluxes

We observed O2 deficits of 0.5 to 2.0% (1 to 4 μmol/kg) in the underway seawater lines of three different ships. Deficits in O2/Ar and isotopic enrichments in dissolved O2 observed in underway seawater lines indicate a respiratory removal process. A 1% respiratory bias in underway lines would lead to a 2.5–5 μatm (2.5–5 μbar) enhancement in surface water pCO2. If an underway pCO2 bias of this magnitude affected all measurements, the global oceanic carbon uptake based on pCO2 climatologies would be 0.5–0.8 Pg/yr higher than the present estimate of 1.6 Pg/yr. Treatment of underway lines with bleach for several hours and thorough flushing appeared to minimize O2 loss. Given the increasing interest in underway seawater measurements for the determination of surface CO2 and O2 fluxes, respiration in underway seawater lines must be identified and eliminated on all observing ships to ensure unbiased data.

Inorganic carbon and biological oceanography above a shallow oxygen minimum in the entrance to the Gulf of California in the Mexican Pacific

The eastern Pacific intermediate oxygen minimum layer (OML) is particularly well‐developed and shoals close to the Mexican coast. We obtained hydrographic profiles including oxygen concentration [O], measured dissolved inorganic carbon (DIC), continuous surface pCO2 in water and air and took biological data south of the Gulf of California in March 2005. The core of the OML, with close to zero [O] was centered around 475 m with a thickness of the core varying between 80 m and 700 m and the upper limit of the core ranging from 500 m to 80 m. At the surface [O] was close to air‐saturation and showed no relation with the depth of the OML contrary to DIC and pCO2 concentrations. Below 50‐m depth the changes in DIC and Apparent Oxygen Utilization (AOU) yielding a molar ratio of δDIC = 0.79 x δAOU. When the OML shoaled, surface temperature, chlorophyll concentration [CHL], and depth‐integrated zooplankton increased, resulting in positive correlations among plankton biomass, pCO2, and DIC in surface waters. When [CHL] is recalculated as particulate organic carbon (POC), a linear relationship with the sum of DIC and POC is observed as expected for concomitant transport of DIC and inorganic nutrients supporting POC formation. Neglecting ventilation of carbon into the atmosphere and the production of dissolved organic carbon, the results suggested that ~16% of the carbon transported up from the OML was present in surface waters in the form of POC and the rest in inorganic form.

Large CO2 disequilibria in tropical lakes

On the basis of a broad compilation of data on pCO2 in surface waters, we show tropical lakes to be, on average, far more supersaturated and variable in CO2 (geometric mean ± SE pCO2 = 1804 ± 35 μatm) than temperate lakes (1070 ± 6 μatm). There was a significant negative relationship between pCO2 and latitude, resulting in an average decrease of pCO2 by 2.8 ± 0.5% per degree latitude. In addition, we found a general positive relationship between pCO2 and water temperature across lakes involving an average increase (±SE) in 6.7 ± 0.8% per °C. A conservative annual efflux from global lakes to the atmosphere was reestimated to 0.44 Gt C. Our results show tropical lakes maintain large CO2 disequilibria with the atmosphere, playing a disproportionate and variable role in the flux of CO2 between lakes and the atmosphere, thereby being a significant component of the global C cycle.

Interannual variability in atmospheric CO2 uptake on the northeast U.S. continental shelf

Continental shelf systems are thought to play an important role in the exchange of carbon dioxide (CO2) between the atmosphere and ocean. Currently, our ability to quantify the air‐sea flux of CO2 on continental shelves is limited due to large spatial and temporal variability coupled with historically sparse oceanographic measurements (e.g., of surface water pCO2). Here we use the Regional Ocean Modeling System (ROMS) to quantify the air‐sea flux of CO2 and its interannual variability on the northeast U.S. continental shelf, which includes the Middle Atlantic Bight (MAB) and Gulf of Maine (GOM). Two years marked by opposite phases of the North Atlantic Oscillation (NAO) are considered in the study. A novel analysis method, second‐order Taylor series decomposition, is used to identify the important processes responsible for producing NAO‐related changes in the CO2 air‐sea flux. On the northeast U.S. shelf, atmospheric CO2 uptake as simulated by ROMS decreases from 2.4 Mt C yr−1 in 1985 (low NAO) to 1.8 Mt C yr−1 in 1990 (high NAO), with most of this decrease (0.5 Mt C yr−1) occurring in the MAB. In the MAB the difference in annual air‐sea flux of CO2 is due mainly to changes in near‐surface wind speed, while the flux difference in the GOM is controlled primarily by surface water pCO2 (CO2 partial pressure) changes resulting from changes in sea surface temperature and new production. The large magnitude of interannual variability in the air‐sea flux of CO2 simulated here suggests the potential for even more significant flux changes in the future as climate change accelerates.

Short-term dynamics of nutrients influenced by upwelling in a small oligotrophic coastal ecosystem, Gan Bay, in the northwest Philippines

We present a time-series analysis of nutrient and pCO 2 (partial pressure of CO 2) levels in an oligotrophic coastal ecosystem (Gan Bay), which was likely to be influenced by upwelled subsurface water. Gan Bay is off Currimao Harbor, in the northwest Philippines and is located at the boundary of the South China Sea (SCS). This 42-h time-series observation was conducted in December 2006. In addition to continuous observations of dissolved oxygen (DO) and pCO 2, discrete samples were collected at a depth of 5 m every 3 h for measurements of nutrients, including soluble reactive phosphorus (SRP) and inorganic nitrogen (NO 3 − + NO 2 −) in order to examine their dynamics and possible physical and biological controls. We observed remarkably large short-term variations in the surface water, spanning a 10-fold change for SRP (32–330 nM) and from <0.3 μM to 4.3 μM for NO 3 − + NO 2 −. DO also varied substantially from a lower end of 171 to 205 μM O 2. Surface water pCO 2 changed from an equilibrium stage with the atmosphere (∼386 μatm) to a stage where it was a significant source for the atmospheric CO 2 (seawater pCO 2 ∼469 μatm). We found that the variation of nutrients was driven neither by tidal mixing nor by biological activities, as was suggested by the variations in the total bacterial abundance and chlorophyll a. Instead, our inverse T– S relationship suggested a two end-member mixing process during the observation period. The N:P ratio throughout the observation period was ∼13.2, which is characteristic of SCS subsurface and deep waters. Moreover, pCO 2 was correlated inversely with the sea surface temperature. It is likely, therefore, that an upwelled subsurface cold water with high nutrients, low-temperature and high- pCO 2 existed. It should be noted that this upwelled cold water did not appear to impact the entire observation period (approximately 35 h of 42 h), which might suggest an extremely dynamic nature for this upwelled cold water mass.

Satellite-derived surface water pCO 2 and air–sea CO 2 fluxes in the northern South China Sea in summer

An empirical approach is presented for the estimation of the partial pressure of carbon dioxide ( pCO 2) and air–sea CO 2 fluxes in the northern South China Sea in summer using satellite-derived sea surface temperatures (SSTs), chlorophyll-a (Chl a) concentrations, and wind fields. Two algorithms were tested. The first used an SST-dependent equation, and the other involved the introduction of Chl a. Regression equations were developed for summer based on in situ data obtained in July, 2004. Using the monthly average SST and Chl a fields derived from the advanced very high resolution radiometer (AVHRR) and the SeaWiFS (sea-viewing wide field of view sensor), respectively, the monthly pCO 2 fields were computed. The derived pCO 2 was compared with the shipboard pCO 2 observations conducted in July, 2000. This resulted in a root-mean-square error of 4.6 μatm, suggesting that the satellite-derived pCO 2 was in general agreement with the in situ observations. The air–sea CO 2 flux was further computed with the aid of the monthly mean QuikSCAT wind speed. We contend that more shipboard data are necessary for refining the empirical algorithms and reducing the uncertainty in the results.

Climatological mean and decadal change in surface ocean pCO2, and net sea–air CO2 flux over the global oceans

A climatological mean distribution for the surface water pCO2 over the global oceans in non-El Nino conditions has been constructed with spatial resolution of 4° (latitude) ×5° (longitude) for a reference year 2000 based upon about 3 million measurements of surface water pCO2 obtained from 1970 to 2007. The database used for this study is about 3 times larger than the 0.94 million used for our earlier paper [Takahashi et al., 2002. Global sea–air CO2 flux based on climatological surface ocean pCO2, and seasonal biological and temperature effects. Deep-Sea Res. II, 49, 1601–1622]. A time-trend analysis using deseasonalized surface water pCO2 data in portions of the North Atlantic, North and South Pacific and Southern Oceans (which cover about 27% of the global ocean areas) indicates that the surface water pCO2 over these oceanic areas has increased on average at a mean rate of 1.5 μatm y−1 with basin-specific rates varying between 1.2±0.5 and 2.1±0.4 μatm y−1. A global ocean database for a single reference year 2000 is assembled using this mean rate for correcting observations made in different years to the reference year. The observations made during El Nino periods in the equatorial Pacific and those made in coastal zones are excluded from the database. Seasonal changes in the surface water pCO2 and the sea-air pCO2 difference over four climatic zones in the Atlantic, Pacific, Indian and Southern Oceans are presented. Over the Southern Ocean seasonal ice zone, the seasonality is complex. Although it cannot be thoroughly documented due to the limited extent of observations, seasonal changes in pCO2 are approximated by using the data for under-ice waters during austral winter and those for the marginal ice and ice-free zones. The net air–sea CO2 flux is estimated using the sea–air pCO2 difference and the air–sea gas transfer rate that is parameterized as a function of (wind speed)2 with a scaling factor of 0.26. This is estimated by inverting the bomb 14C data using Ocean General Circulation models and the 1979–2005 NCEP-DOE AMIP-II Reanalysis (R-2) wind speed data. The equatorial Pacific (14°N–14°S) is the major source for atmospheric CO2, emitting about +0.48 Pg-C y−1, and the temperate oceans between 14° and 50° in the both hemispheres are the major sink zones with an uptake flux of −0.70 Pg-C y−1 for the northern and −1.05 Pg-C y−1 for the southern zone. The high-latitude North Atlantic, including the Nordic Seas and portion of the Arctic Sea, is the most intense CO2 sink area on the basis of per unit area, with a mean of −2.5 tons-C month−1 km−2. This is due to the combination of the low pCO2 in seawater and high gas exchange rates. In the ice-free zone of the Southern Ocean (50°–62°S), the mean annual flux is small (−0.06 Pg-C y−1) because of a cancellation of the summer uptake CO2 flux with the winter release of CO2 caused by deepwater upwelling. The annual mean for the contemporary net CO2 uptake flux over the global oceans is estimated to be −1.6±0.9 Pg-C y−1, which includes an undersampling correction to the direct estimate of −1.4±0.7 Pg-C y−1. Taking the pre-industrial steady-state ocean source of 0.4±0.2 Pg-C y−1 into account, the total ocean uptake flux including the anthropogenic CO2 is estimated to be −2.0±1.0 Pg-C y−1 in 2000.

Recommendations for autonomous underway pCO2 measuring systems and data-reduction routines

In order to facilitate the collection of high quality and uniform surface water pCO2 data, an underway pCO2 instrument has been designed based on community input and is now commercially available. Along with instrumentation, agreements were reached on data reduction and quality control that can be easily applied to data from these systems by using custom-made freeware. This new automated underway pCO2 measuring system is designed to be accurate to within 0.1 μatm for atmospheric pCO2 measurements and to within 2 μatm for seawater pCO2, targeted by the scientific community to constrain the regional air–sea CO2 fluxes to 0.2 Pg C year−1. The procedure to properly reduce the underway pCO2 data and perform the steps necessary for calculation of the fugacity of CO2 from the measurements is described. This system is now widely used by the scientific community on many different types of ships. Combined with the recommended data-reduction procedures, it will facilitate producing data sets that will significantly decrease the uncertainty currently present in estimates of air–sea CO2 fluxes.

Tolerance of allogromiid Foraminifera to severely elevated carbon dioxide concentrations: Implications to future ecosystem functioning and paleoceanographic interpretations

Increases in the partial pressure of carbon dioxide (pCO 2) in the atmosphere will significantly affect a wide variety of terrestrial fauna and flora. Because of tight atmospheric–oceanic coupling, shallow-water marine species are also expected to be affected by increases in atmospheric carbon dioxide concentrations. One proposed way to slow increases in atmospheric pCO 2 is to sequester CO 2 in the deep sea. Thus, over the next few centuries marine species will be exposed to changing seawater chemistry caused by ocean–atmospheric exchange and/or deep-ocean sequestration. This initial case study on one allogromiid foraminiferal species ( Allogromia laticollaris) was conducted to begin to ascertain the effect of elevated pCO 2 on benthic Foraminifera, which are a major meiofaunal constituent of shallow- and deep-water marine communities. Cultures of this thecate foraminiferan protist were used for 10–14-day experiments. Experimental treatments were executed in an incubator that controlled CO 2 (15 000; 30 000; 60 000; 90 000; 200 000 ppm), temperature and humidity; atmospheric controls (i.e., ~ 375 ppm CO 2) were executed simultaneously. Although the experimental elevated pCO 2 values are far above foreseeable surface water pCO 2, they were selected to represent the spectrum of conditions expected for the benthos if deep-sea CO 2 sequestration becomes a reality. Survival was assessed in two independent ways: pseudopodial presence/absence and measurement of adenosine triphosphate (ATP), which is an indicator of cellular energy. Substantial proportions of A. laticollaris populations survived 200 000 ppm CO 2 although the mean of the median [ATP] of survivors was statistically lower for this treatment than for that of atmospheric control specimens. After individuals that had been incubated in 200 000 ppm CO 2 for 12 days were transferred to atmospheric conditions for ~ 24 h, the [ATP] of live specimens (survivors) approximated those of the comparable atmospheric control treatment. Incubation in 200 000 ppm CO 2 also resulted in reproduction by some individuals. Results suggest that certain Foraminifera are able to tolerate deep-sea CO 2 sequestration and perhaps thrive as a result of elevated pCO 2 that is predicted for the next few centuries, in a high-pCO 2 world. Thus, allogromiid foraminiferal “blooms” may result from climate change. Furthermore, because allogromiids consume a variety of prey, it is likely that they will be major players in ecosystem dynamics of future coastal sedimentary environments.

CO2 emissions from saline lakes: A global estimate of a surprisingly large flux

The role of saline lakes in CO2 exchange with the atmosphere was evaluated on the basis of calculated partial pressure (pCO2) and CO2 exchange rates with the atmosphere derived from a compilation of published data for 196 saline lakes around the world. The average surface water pCO2 exceeded atmospheric pCO2 by a factor of 5–8 times, indicative of a tendency for saline lakes to emit CO2 to the atmosphere. Chemically enhanced emission, calculated from solute chemistry, pH, and wind speed, increased gas exchange an average of 2.3 times over that of freshwater lakes having equivalent pCO2. The globally distributed lakes emitted CO2 at rates in excess of 80 mmol m−2 d−1. The Caspian Sea was calculated to support alone a total CO2 emission of 0.02 to 0.04 Gt C a−1, with the total CO2 emissions to the atmosphere from saline lakes calculated to be 0.11–0.15 Gt C a−1. Consideration of CO2 emissions from saline lakes raises the total CO2 emissions to the atmosphere from all lakes to 0.28–0.32 Gt CO2. These results point to a significant role of saline lakes in global C cycling.