CO2 Fugacity Research Articles

Satellite-derived CO2 fugacity in surface seawater of the tropical Atlantic Ocean using a feedforward neural network

ABSTRACTA feedforward neural network is used to quantify the fugacity of CO in surface seawater (f (CO)) of the tropical Atlantic Ocean, exclusively from satellite data: sea-surface temperature, sea-surface salinity, and chlorophyll-a (chl-a), at a 4 km 4 km spatial resolution, for the period of spring (March and April). The model was constructed using 7188 in situ data provided by the ‘Surface Ocean CO ATlas’ (SOCAT) products, and the ‘EC-funded project CARBOOCEAN IP program’ products, available for the years 2001, 2002, 2004, 2006, 2007, and 2009. The model was tested using remote sensing data of the Moderate Resolution Imaging Spectroradiometer Aqua. This approach was validated over the area extending from 8°N-61°W to 23°N-20°W. A comparison with multiple linear regression model was established. The neural network has provided better results (root mean square error (RMSE) of 8.7 μatm (0.881 Pa)) than linear regression (RMSE of 9.6 μatm (0.973 Pa)) for f (CO) interpolation using remote sensing data. Since the required input data are available, this approach could be applied to the whole tropical Atlantic Ocean and for the remaining seasons (summer, fall, and winter).

Estimates and Variability of the Air-Sea CO<sub>2</sub> Fluxes in the Gulf of Guinea during the 2005-2007 Period

Measurements of CO2 parameters (i.e. Total Alkalinity (TA) and Dissolved Inorganic Carbon (DIC)) were made from June 2005 to September 2007 in six EGEE (“Etude de la circulation oceanique et de savariabilitedans le Golfe de GuinEE”) cruises to better assess air-sea CO2 fluxes in the Gulf of Guinea (6°N - 10°S, 10°E - 10°W). Two empirical relationships TA-Salinity and DIC-Salinity-Temperature were established. These relationships were then used to estimate the monthly fugacity of CO2 (fCO2) and air-sea CO2 fluxes. The monthly mean flux of CO2 reaches 1.76 ± 0.82 mmol·m-2·d-1 (resp. 2.90 ± 1.45 mmol·m-2·d-1) at the north of the Equator (resp. at the South). The north-south gradient observed as the patterns of the air-sea CO2 fluxes was mainly driven by the oceanic fCO2. This gradient was due to the low values of the CO2 parameters flowing by the Guinea Current (6°N - 0°) from the west to the east while the air-sea CO2 fluxes increased in the south (10°S - 0). In the north, the climatology of Takahashi underestimated the CO2 fluxes in the Gulf of Guinea when comparing to the estimated fluxes. This was due to the north-south gradient, which did not well reproduce by the climatology of Takahashi.

New insights into <i>f</i>CO<sub>2</sub> variability in the tropical eastern Pacific Ocean using SMOS SSS

Abstract. Complex oceanic circulation and air–sea interaction make the eastern tropical Pacific Ocean (ETPO) a highly variable source of CO2 to the atmosphere. Although the scientific community have amassed 70 000 surface fugacities of carbon dioxide (fCO2) data points within the ETPO region over the past 25 years, the spatial and temporal resolution of this data set is insufficient to fully quantify the seasonal to interannual variability of the region, a region where fCO2 has been observed to fluctuate by > 300 μatm. Upwelling and rainfall events dominate the surface physical and chemical characteristics of the ETPO, with both yielding unique signatures in sea surface temperature and salinity. Thus, we explore the potential of using a statistical description of fCO2 within sea-surface salinity–temperature space. These SSS/SST relationships are based on in situ surface ocean CO2 atlas (SOCAT) data collected within the ETPO. This statistical description is then applied to high-resolution (0.25°) Soil Moisture and Ocean Salinity (SMOS) sea surface salinity (SSS) and Operational Sea Surface Temperature and Sea Ice Analysis (OSTIA) sea surface temperature (SST) in order to compute regional fCO2. As a result, we are able to resolve fCO2 at sufficiently high resolution to elucidate the influence that various physical processes have on the fCO2 of the surface ETPO. Normalised (to 2014) oceanic fCO2 between July 2010 and June 2014 within the entire ETPO was 39 (±10.7) μatm supersaturated with respect to 2014 atmospheric partial pressures, and featured a CO2 outgassing of 1.51 (±0.41) mmol m−2 d−1. Values of fCO2 within the ETPO were found to be broadly split between the Gulf of Panama region and the rest of the tropical eastern Pacific Ocean. The northwest, central and offshore regions were supersaturated, with wintertime wind-jet-driven upwelling found to constitute the first-order control on fCO2 values. This contrasts with the southeastern/Gulf of Panama region, where heavy rainfall combined with rapid stratification of the upper water column act to dilute dissolved inorganic carbon, and yield fCO2 values undersaturated with respect to atmospheric fugacities of CO2.

A statistical gap‐filling method to interpolate global monthly surface ocean carbon dioxide data

AbstractWe have developed a statistical gap‐filling method adapted to the specific coverage and properties of observed fugacity of surface ocean CO2 (fCO2). We have used this method to interpolate the Surface Ocean CO2 Atlas (SOCAT) v2 database on a 2.5°×2.5° global grid (south of 70°N) for 1985–2011 at monthly resolution. The method combines a spatial interpolation based on a “radius of influence” to determine nearby similar fCO2 values with temporal harmonic and cubic spline curve‐fitting, and also fits long‐term trends and seasonal cycles. Interannual variability is established using deviations of observations from the fitted trends and seasonal cycles. An uncertainty is computed for all interpolated values based on the spatial and temporal range of the interpolation. Tests of the method using model data show that it performs as well as or better than previous regional interpolation methods, but in addition it provides a near‐global and interannual coverage.

Intraseasonal variability linked to sampling alias in air-sea CO2 fluxes in the Southern Ocean

The Southern Ocean (SO) contributes most of the uncertainty in contemporary estimates of the mean annual flux of carbon dioxide CO2 between the ocean and the atmosphere. Attempts to reduce this uncertainty have aimed at resolving the seasonal cycle of the fugacity of CO2 (fCO2). We use hourly CO2 flux and driver observations collected by the combined deployment of ocean gliders to show that resolving the seasonal cycle is not sufficient to reduce the uncertainty of the flux of CO2 to below the threshold required to reveal climatic trends in CO2 fluxes. This was done by iteratively subsampling the hourly CO2 data set at various time intervals. We show that because of storm-linked intraseasonal variability in the spring-late summer, sampling intervals longer than 2 days alias the seasonal mean flux estimate above the required threshold. Moreover, the regional nature and long-term trends in storm characteristics may be an important influence in the future role of the SO in the carbon-climate system.

Ductile flow in sub-volcanic carbonate basement as the main control for edifice stability: New experimental insights

Limestone in volcanic basements has been identified as a hazard in terms of edifice stability due to the propensity of calcite to decompose into lime and CO2 at high temperatures (>600 °C), causing a decrease in mechanical strength. To date, such hypotheses have been tested by experiments performed at ambient pressure. The present work determines the mechanical strength of limestone under sub-volcanic conditions of pressure and temperature and evaluates the effect of calcite decomposition. To this end, we use Mt. Etna as a case study, deforming sub-Etnean carbonate samples under triaxial compression using a Paterson deformation apparatus. We evaluate the effect of thermal decomposition of calcite on sample strength by comparing closed and open systems and measuring the permeability evolution under static conditions. Mechanical and micro-structural observations at a constant strain rate of 10−5 s−1 and at a confining pressure of 50 MPa indicate that the rocks are brittle up to and including 300 °C. At higher temperatures the deformation becomes macroscopically ductile, i.e., deformation is distributed throughout the sample. The brittle to ductile transition is accompanied by an irreversible permeability decrease from ∼10−17 to ∼10−19 m2 between 200 and 600 °C. We present new evidence that permanent change in permeability is due to ductile processes closing the initial pore space. Samples deformed at temperatures up to 900 °C do not contain any decarbonation products. At these temperatures, permeability is sufficiently low to permit CO2 pore pressures to increase, thereby increasing local CO2 fugacity, which in turn strongly limits the decarbonation reaction. We note that, for non-pure calcite rocks, permeability might be sufficient to allow decarbonation reactions to occur. As such, variability in lithologies may slightly influence the efficiency of decarbonation reactions. We conclude that, in a closed system, the instability of Mt. Etna is related to high temperature induced ductile flow of basement limestone rather than chemical/mineralogical changes. This may have important implication for the stability of volcanoes within carbonate-rich basement, as carbonates become significantly weak at high temperatures, which may increase the risk of sector collapse.

Seasonal cycle of CO2 from the sea ice edge to island blooms in the Scotia Sea, Southern Ocean

The Scotia Sea region contains some of the most productive waters of the Southern Ocean. It is also a dynamic region through the interaction of deep water masses with the atmosphere. We present a first seasonally-resolved time series of the fugacity of CO2 (fCO2) from spring 2006, summer 2008, autumn 2009 and winter (potential temperature minimum) along a 1000km transect from the pack ice to the Polar Front to quantify the effects of biology and temperature on oceanic fCO2. Substantial spring and summer decreases in sea surface fCO2 occurred in phytoplankton blooms that developed in the naturally iron-fertilised waters downstream (north) of South Georgia island (54–55°S, 36–38°W) and following sea ice melt (in the seasonal ice zone). The largest seasonal fCO2 amplitude (ΔfCO2) of −159μatm was found in the South Georgia bloom. In this region, biological carbon uptake dominated the seasonal signal, reducing the winter maxima in oceanic fCO2 by 257μatm by the summer. In the Weddell–Scotia Confluence, the southern fringe of the Scotia Sea, the shift from wintertime CO2-rich conditions in ice covered waters to CO2 undersaturation in the spring blooms during and upon sea ice melt created strong seasonality in oceanic fCO2. Temperature effects on oceanic fCO2 ranged from ΔfCO2 sst of ~55μatm in the seasonal ice zone to almost double that downstream of South Georgia (98μatm). The seasonal cycle of surface water fCO2 in the high-nutrient low-chlorophyll region of the central Scotia Sea had the weakest biological control and lowest seasonality. Basin-wide biological processes dominated the seasonal control on oceanic fCO2 (ΔfCO2 bio of 159μatm), partially compensated (43%) by moderate temperature control (ΔfCO2 sst of 68μatm). The patchwork of productivity across the Scotia Sea creates regions of seasonally strong biological uptake of CO2 in the Southern Ocean.

Latitudinal Trends and Drivers in the CO2–Carbonic Acid System of Papahānaumokuākea Marine National Monument

Abstract Emissions of anthropogenic carbon dioxide (CO2) to the atmosphere and the consequent effects of climate change and ocean acidification on coral reef ecosystems have motivated significant interest in describing and understanding the CO2–carbonic acid system of diverse coral reef environments. Although numerous studies have been successful in monitoring reef metabolism both in the field and in the laboratory, physical and biological forcings produce distinct conditions among environments. Due to the geographic isolation and associated difficulties with measuring marine carbon chemistry in waters of the Papahānaumokuākea Marine National Monument (PMNM), relatively few studies have described the CO2–carbonic acid system and carbonate saturation state gradients of these waters. Yet, PMNM remains one of the largest conservation areas in the world with extensive and diverse coral reef ecosystems that could offer valuable insight into our current and future understanding about regional and global impacts of ocean acidification. In order to provide a broad overview of latitudinal trends and features of the marine carbon system in PMNM waters, continuous measurements for surface seawater fugacity of CO2 (fCO2) and pH were collected during August 2011 and July 2012 cruises of the NOAA Ship Hi’ialakai. These measurements indicate that pH and fCO2 are three times more variable in nearshore monument waters relative to open ocean transect measurements. This variability can be observed up to 50 km away from the nearest reef and is likely the result of the direct and significant impact of coral reef metabolism on marine carbon chemistry around the islands and atolls. The relationship between total alkalinity and dissolved inorganic carbon is consistent with net calcification which creates an alkalinity sink throughout PMNM waters. Additionally, our measurements show clear latitudinal trends in fCO2, pH, and aragonite saturation state that are influenced by environmental forcings, including temperature, wind speed, and residence time of the water. Collectively, our results suggest that coral reefs located at the northernmost atolls of PMNM may be the most susceptible to the adverse impacts of climate change and ocean acidification.

Surface ocean CO2 in 1990–2011 modelled using a feed‐forward neural network

This dataset includes the monthly distributions of CO2 fugacity in the world surface oceans reconstructed using a feed‐forward neural network model and the CO2 measurements of the Surface Ocean CO2 Atlas version 2.0. It has a spatial resolution of 1 × 1° and spans a period of 22 years, from January 1990 to December 2011. The dataset also includes necessary parameters for the reconstruction and an estimate of the CO2 fluxes between the ocean and the atmosphere. The aim of this work is to provide a dataset for estimating the oceans' contribution to the global carbon budget.

Internal consistency of marine carbonate system measurements and assessments of aragonite saturation state: Insights from two U.S. coastal cruises

This research assesses the thermodynamic consistency of recent marine CO2 system measurements in United States coastal waters. As one means of assessment, we compared aragonite saturation states calculated using various combinations of measured parameters. We also compared directly measured and calculated values of total alkalinity and CO2 fugacity. The primary data set consists of state-of-the-art measurements of the keystone parameters of the marine CO2 system: dissolved inorganic carbon (DIC), total alkalinity (TA), CO2 fugacity (fCO2), and pH. This study is the first thermodynamic CO2 system intercomparison based on measurements obtained using purified meta cresol purple as a pH indicator. The data are from 1890 water samples collected during NOAA's West Coast Ocean Acidification Cruise of 2011 (WCOA2011) and NOAA's Gulf of Mexico and East Coast Carbon Cruise of 2012 (GOMECC-2).Calculations of in situ aragonite saturation states (ΩA) near the saturation horizon exhibited differences on the order of ±10% between predictions based on the (DIC, TA) pair of measurements vs. the (pH, DIC), (fCO2, DIC), or (fCO2, pH) pairs. Differences of this magnitude, which are largely attributable to the imprecision of ΩA calculated from the (DIC, TA) pair, are roughly equivalent to the magnitude of ΩA change projected to occur over the next several decades due to ocean acidification. These observations highlight the importance of including either pH or fCO2 in saturation state calculations.Calculations of TA from (pH, DIC) and (fCO2, DIC) showed that internal consistency could be achieved if minor subtractions of TA (≤4μmolkg−1) were applied to samples of salinity <35. The extent of thermodynamic consistency is also exemplified by the small offset between TA calculated from (DIC, pH) and that calculated from (DIC, fCO2): ~3μmolkg−1, which is similar to the accuracy of the TA measurements. Systematic trends can be detected in the offsets between measured and calculated parameters, but for this high-quality data set the magnitude of methodological improvements required to achieve exact thermodynamic consistency is quite small.

Response of sea surface fugacity of CO2 to the SAM shift south of Tasmania: Regional differences

AbstractUsing observational data collected south of Tasmania during 14 austral summer cruises during 1993–2011, we examined the response of sea surface fugacity of carbon dioxide (fCO2) to the Southern Annular Mode (SAM) shift, which occurred around 2000. In the southern part of the Southern Ocean (SO) or the Polar Zone (PZ) and the Polar Frontal Zone (PFZ), fCO2 increased faster at the sea surface than in the atmosphere before the SAM shift, but not after the shift. In the northern part of the SO or the Subantarctic Zone (SAZ), however, surface fCO2 increased faster than atmospheric fCO2 both before and after the shift. The SAM shift had an important influence on the surface fCO2 trend in the PZ and PFZ but not in the SAZ, which we attribute to differences in regional oceanographic processes (upwelling versus nonupwelling). The SAM shift may have reversed the negative trend of SO CO2 uptake.

Hydrogeochemical study of an ophiolitic aquifer: a case study of Lago (Southern Italy, Calabria)

A hydrogeochemical survey was carried out in the area of Lago (Calabria region, Southern Italy) and outcropping geological units were classified from the hydrogeological point of view, to explain the evolution of local groundwaters. In addition, reaction path modeling was performed to simulate the evolution of groundwaters upon interaction with local metabasalts, phyllites, and serpentinites, by means of the EQ3/6 software package, version 8.0. Simulations were carried at constant temperature of 14.1 °C, which reproduces the average temperature of local groundwaters. Three different simulations have been performed for the average metabasalt fixing the fugacity of CO2 at either 10−2.4 bar (mean value), 10−2.0 bar (mean value + 1σ), or 10−2.9 bar (mean value − 1σ). The simulation for the average serpentinite and phyllite was carried out at the mean value of the fugacity of CO2 (10−2.4 bar). All evidences acquired in this study, namely the hydrogeological framework, the chemical analysis of local groundwaters and the results of reaction path modeling, consistently point out that groundwater circulation under ordinary hydraulic gradient, primarily occurs in the ophiolitic sub-complex and that dissolution of metabasalts and serpentinites controls the chemical characteristics of the groundwaters from the study area.

Evaluation of the sinks and sources of atmospheric CO2 by artificial upwelling

Artificial upwelling is considered a promising way to reduce the accumulation of anthropogenic carbon dioxide in the atmosphere. This practice could transport nutrient-rich deep water to the euphotic zone, enhance phytoplankton growth and consequently increase organic carbon exportation to the deep ocean via the biological pump. However, only a few studies quantitatively assess changes in oceanic CO2 uptake resulting from artificial upwelling. This article uses a simulation to examine the effect of hypothetical artificial upwelling-induced variations of CO2 fugacity in seawater (fCO2) using observed carbon and nutrient data from 14 stations, ranging from 21 to 43°N, in the West Philippine Sea (WPS), the East China Sea (ECS) and the Sea of Japan. Calculations are based on two basic assumptions: First, a near-field mixing of a nutrient-rich deep-ocean water plume in a stratified ocean environment is assumed to form given the presence of an artificial upwelling devise with appropriate technical parameters. Second, it is assumed that photosynthesis of marine phytoplankton could deplete all available nutrients following the stoichiometry of the modified Redfield ratio C/H/O/N/S/P=103.1/181.7/93.4/11.7/2.1/1. Results suggest artificial upwelling has significant effects on regional changes in sea–air differences (ΔfCO2sea–air) and the carbon sequestration potential (ΔfCO2mixed‐amb). Large variations of ΔfCO2sea–air and ΔfCO2mixed‐amb are shown to be associated with different regions, seasons and technical parameters of the artificial upwelling device. With proper design, it is possible to reverse the contribution of artificial upwelling from a strong CO2 source to sink. Thus, artificial upwelling has the potential to succeed as a geoengineering technique to sequester anthropogenic CO2, with appropriate technical parameters in the right region and season.

Conditions of kimberlite magma generation: experimental constraints

Melting and multiple saturation experiments with systems simulating primary kimberlite magma compositions at 5.5-6.5GPa provide constraints on magma generation conditions. The liquidus of model kimberlitic systems exceeds the hottest temperatures of lithospheric mantle (1400°C) but is 150-200°C lower in systems with lower CO2/(CO2+H2O) ratios. The high melting points require additional heat sources for the generation of kimberlite magmas. Multiple saturation of experimental melts and the stability of individual near-liquidus phases depend on both major-element contents and XCO2 (as the CO2/(CO2+H2O) molar ratio) in the starting composition. Generally, olivine-bearing assemblages are stable at XCO2<0.5, while an increase in MgO/CaO from 1.8 to>4.0 leads to progressive changes in the equilibrium assemblages: Ol+Grt+Cpx ^ Ol+Grt+Opx+Cpx ^ Ol+Grt+Opx. The results of geochemical reconstructions and multiple saturation experiments indicate partial or complete wehrlitization of the kimberlitic source regions. Most of primary magmas with XCO2<0.5 may have been derived from carbonated garnet lherzolite. Some highly calcic (MgO/CaO<2) magmas with Xco2<0.5 likely originated from carbonated garnet wehrlite. A probable scenario is that melts and/or fluids repeatedly metasomatized and oxidized the protolith (caused its carbonation and phlogopitization) and thus provided conditions for buffering CO2 and H2O fugacities in the forming kimberlitic magma, at least early in the melting history. During later magma generation, water was, likely, extracted from nominally anhydrous minerals having hydrated (OH) defects in the structure.

On the physical and biogeochemical processes driving the high frequency variability of CO2 fugacity at 6°S, 10°W: Potential role of the internal waves

AbstractThe availability of nutrients in the mixed layer is the main limitation to organic carbon biological production in the tropical regions. In this paper, we investigate the potential role of internal waves at promoting the development of biological activity on a PIRATA mooring at 6°S, 10°W. This mooring is located above the Mid‐Atlantic Ridge where we observe strong internal waves. Using a one‐dimensional physical and biogeochemical coupled model, we simulate dissolved inorganic carbon (DIC). Providing the influence of vertical advection and turbulent diapycnal diffusivity are (it is vertical advection + turbulent dipycnal mixing) accounted for, we find that this model provides a good fit with observed in situ CO2 fugacity (fCO2). Main effect of internal waves is to rapidly increase the DIC, thus the fCO2 and the nutrients in the mixed layer. The latter induce progressive development of biological activity leading to gradual DIC decrease. Our study highlights the importance of correctly taking into account the effect of internal waves in tropical regions.

Spring carbonate chemistry dynamics of surface waters in the northern East China Sea: Water mixing, biological uptake of CO2, and chemical buffering capacity

AbstractWe investigated sea surface total alkalinity (TAlk), dissolved inorganic carbon (DIC), dissolved oxygen (DO), and satellite‐derived chlorophyll‐a in the connection between the Yellow Sea and the East China Sea (ECS) during April to early May 2007. In spring, Changjiang dilution water (CDW), ECS offshore water, and together with Yellow Sea water (YSW) occupied the northern ECS. Using 16 day composite satellite‐derived chlorophyll‐a images, several algal blooms were identified in the CDW and ECS offshore water. Correspondingly, biological DIC drawdown of 73 ± 20 μmol kg−1, oversaturated DO of 10–110 μmol O2 kg−1, and low fugacity of CO2 of 181–304 μatm were revealed in these two waters. YSW also showed CO2 uptake in spring, due to the very low temperature. However, its intrusion virtually counteracted CO2 uptake in the northern ECS. In the CDW and the ECS offshore water, Revelle factor was 9.3–11.7 and 8.9–10.6, respectively, while relatively high Revelle factor values of 11.4–13.0 were revealed in YSW. In the ECS offshore water, the observed relationship between DIC drawdown and oversaturated DO departed from the Redfield ratio, indicating an effect of chemical buffering capacity on the carbonate system during air‐sea reequilibration. Given the fact that the chemical buffering capacity slows down the air‐sea reequilibration of CO2, the early spring DIC drawdown may have durative effects on the sea surface carbonate system until early summer. Although our study is subject to limited temporal and spatial coverage of sampling, these insights are fundamental to understanding sea surface carbonate chemistry dynamics in this important ocean margin.

A Global Surface Ocean fCO2 Climatology Based on a Feed-Forward Neural Network

Abstract A feed-forward neural network is used to create a monthly climatology of the sea surface fugacity of CO2 (fCO2) on a 1° × 1° spatial resolution. Using 127 880 data points from 1990 to 2011 in the track-gridded database of the Surface Ocean CO2 Atlas version 2.0 (Bakker et al.), the model yields a global mean fCO2 increase rate of 1.50 μatm yr−1. The rate was used to normalize multiple years’ fCO2 observations to the reference year of 2000. A total of 73 265 data points from the normalized data were used to model the global fCO2 climatology. The model simulates monthly fCO2 distributions that agree well with observations and yields an anthropogenic CO2 update of −1.9 to −2.3 PgC yr−1. The range reflects the uncertainty related to using different wind products for the flux calculation. This estimate is in good agreement with the recently derived best estimate by Wanninkhof et al. The model product benefits from a finer spatial resolution compared to the product of Lamont–Doherty Earth Observatory (Takahashi et al.), which is currently the most frequently used product. It therefore has the potential to improve estimates of the global ocean CO2 uptake. The method’s benefits include but are not limited to the following: (i) a fixed structure is not required to model fCO2 as a nonlinear function of biogeochemical variables, (ii) only one neural network configuration is sufficient to model global fCO2 in all seasons, and (iii) the model can be extended to produce global fCO2 maps at a higher resolution in time and space as long as the required data for input variables are available.

Re-equilibration of natural H2O–CO2–salt-rich fluid inclusions in quartz—Part 1: experiments in pure water at constant pressures and differential pressures at 600 °C

Re-equilibration processes of natural H2O–CO2–NaCl-rich fluid inclusions quartz are experimentally studied by exposing the samples to a pure H2O external fluid at 600 °C. Experimental conditions are selected at nearly constant pressure conditions (309 MPa) between fluid inclusions and pore fluid, with only fugacity gradients in H2O and CO2, and at differential pressure conditions (394–398 MPa, corresponding to an internal under-pressure) in addition to similar CO2 fugacity gradients and larger H2O fugacity gradients. Modifications of fluid inclusion composition and density are monitored with changes in ice dissolution temperature, clathrate dissolution temperature and volume fraction of the vapour phase at room temperature. Specific modification of these parameters can be assigned to specific processes, such as preferential loss/gain of H2O and CO2, or changes in total volume. A combination of these parameters can clearly distinguish between modifications according to bulk diffusion or deformation processes. Bulk diffusion of CO2 according to fugacity gradients is demonstrated at constant pressure conditions. The estimated preferential loss of H2O is not in accordance with those gradients in both constant pressure and differential pressure experiments. The development of deformation halos in quartz around fluid inclusions that are either under-pressurized or over-pressurized promotes absorption of H2O from the inclusions and inhibits bulk diffusion according to the applied fugacity gradients.

Long-term trends in surface ocean pH in the North Atlantic

Presently available direct pH measurements do not have a sufficient data density in space or time in order to determine long-term trends across wider geographic regions, limiting our ability to assess the magnitude and impacts of ocean acidification. We overcome this limitation by using the much more frequently measured fugacity of CO2 (fCO2), as synthesized in the SOCAT data product, from which we calculate pH using algorithms for alkalinity based on temperature and salinity. The estimated pH at 25°C, i.e., pHsws25°C has a calculation error of 0.0033±0.0003, and evaluation using co-located pH observations yields a RMSE of 0.010 and a non-significant bias of 0.004. The estimated pHsws25°C is rather sensitive to uncertainties and biases in fCO2, while uncertainties in alkalinity, temperature, and salinity matter much less. The high precision and low bias of the computed pH permit us to apply this method to data from the North Atlantic Subpolar Gyre, for which we find a statistically significant trend in surface ocean pHswsinsitu of −0.0022±0.0004yr−1 over the period 1981 to 2007. This long-term trend in pH is nearly entirely driven by the long-term trend in surface ocean fCO2, while the impact of temperature is negligible. This pH trend is very close to that expected based on the assumption of thermodynamic equilibrium of CO2 between the atmosphere and the surface ocean.

Subsurface pH and carbonate saturation state of aragonite on the Chinese side of the North Yellow Sea: seasonal variations and controls

Abstract. Based upon eight field surveys conducted between May 2011 and May 2012, we investigated seasonal variations in pH, carbonate saturation state of aragonite (Ωarag), and ancillary data on the Chinese side of the North Yellow Sea, a western North Pacific continental margin of major economic importance. Subsurface waters were CO2-undersaturated in May and nearly in equilibrium with atmospheric CO2 in June. From July to October, the fugacity of CO2 (fCO2) of bottom water gradually increased from 438 ± 44 μatm to 630 ± 84 μatm, and pHT decreased from 8.02 ± 0.04 to 7.88 ± 0.06 due to local aerobic remineralization of primary-production-induced biogenic particles. The subsurface community respiration rates in summer and autumn were estimated to be from 0.80 to 1.08 μmol-O2 kg−1 d−1 within a relatively high salinity range of 31.63 to 32.25. From November to May in the next year, however, subsurface fCO2 gradually decreased and pH increased due to cooling and water column ventilation. The corresponding bottom water Ωarag was 1.85 ± 0.21 (May), 1.79 ± 0.24 (June), 1.75 ± 0.27 (July), 1.76 ± 0.29 (August), 1.45 ± 0.31 (October), 1.52 ± 0.25 (November), and 1.41 ± 0.12 (January). Extremely low Ωarag values (from 1.13 to 1.40) were observed mainly in subsurface waters within the high salinity range of 31.63 to 32.25, which covered a major fraction of the study area in October and November. Of the China seas, the North Yellow Sea represents one of the systems most vulnerable to the potential negative effects of ocean acidification.