Diffuse CO2 Flux Research Articles

The seismic sequence of 30th May–9th June 2016 in the geothermal site of Torre Alfina (central Italy) and related variations in soil gas emissions

In the framework of a medium-enthalpy geothermal exploitation project, seismicity and soil gas emissions have been monitored in the area of Castel Giorgio–Torre Alfina (central Italy) since 2014. A dedicated local seismic network, called ReMoTA, allows deepening of the knowledge of the natural local seismicity in terms of source mechanisms, high-quality event localization and magnitude estimation. From November 2014 to May 2016, ReMoTA recorded 846 seismic events with a magnitude range from Md 0.1 to ML 2.8 and with hypocentres between 4 and 8 km depth. Most of these events occurred in six short lasting clusters. On the 30th May 2016 a Mw 4.3 earthquake occurred near Castel Giorgio, followed by almost 1700 aftershocks. The moment tensor solution for the main shock depicts a WNW-ESE oriented normal fault with small right-lateral strike component. An overview of the epicentral distribution of the different clusters recorded since 2014 highlights that the active tectonic structures are orientated principally along the NE–SW and WNW-ESE directions. The relocation of the 1957 Me 4.9 earthquake suggests that this event occurred in the same fault system as of the 2016 seismic sequence. In the study area, there is only one natural emergence (Solfanare) emitting a CO2 dominated gas, having the same chemical and isotopic composition of the gas contained in a cap at the summit of the geothermal reservoir. Apart from small perturbations, no significant compositional variations were recorded during the 2016 seismic sequence in the gas of the Solfanare vents that was analyzed continuously by an automatic gas-chromatographic station. The diffuse soil CO2 flux is monitored since 2013 in six target areas located around the future production and reinjection wells, in order to assess the level of background natural degassing. In all target areas the maximum value of soil CO2 flux has been recorded during the 2016 seismic sequence. However, values remained relatively low (maximum 112 g ⋅m−2 ⋅d−1) and the values of δ13C of the emitted CO2 (−25.25 to −24.22‰ vs. PDB) indicated a shallow biological origin of the gas (by soil respiration). Only at Solfanare high values of diffuse soil CO2 flux were recorded up to a maximum of 20125 g ⋅m−2 ⋅d−1). All the seismicity of the May–June 2016 sequence is located above the ML4.1 main event and is distributed on small distinct faults (such as at San Lorenzo Nuovo and Acquapendente) and triggered by the main shock. The source mechanism provided by the full moment tensor indicates that rupture processes at depth probably deviate from a pure normal fault. The significant contribution of CLVD and isotropic components suggest a possible opening of fluid cracks below the geothermal reservoir hosted in fractured Mesozoic limestones. In spite of the increase of the CO2 flux, no significant changes have been observed in the chemical and isotopic composition of the Solfanare gas. The seismo-tectonic scenario indicates that the Solfanare fault was not activated. Kinematics and orientation of the activated faults suggest a relationship with the Bolsena caldera collapse.

Gas emissions and crustal deformation from the Krýsuvík high temperature geothermal system, Iceland

The Krýsuvík volcanic system is located on the oblique spreading Reykjanes Peninsula, SW Iceland. Since early 2009 the region has been undergoing episodes of localized ground uplift and subsidence. From April–November 2013, we operated near-real time monitoring of gas emissions in Krýsuvík, using a Multi-component Gas Analyzer System (Multi-GAS), collecting data on gas composition from a fumarole (H2O, CO2, SO2, H2S). The dataset in this study, comprises a near-continuous gas composition time series, the quantification of diffuse CO2 gas flux, analytical results for direct samples of dry gas, seismic records, and GPS data. Gas emissions from the Krýsuvík geothermal system were examined and compared with crustal deformation and seismicity.The gas emissions from the Krýsuvík system are H2O-dominated, with CO2 as the most abundant dry gas species, followed by smaller concentrations of H2S. The average subsurface equilibrium temperature was calculated as 278 °C. This is consistent with previous observations made through sporadic spot sampling campaigns. In addition, the semi-continuous Multi-GAS dataset reveals higher variations in gas composition than previously reported by spot sampling.The diffuse soil CO2 flux is found to be variable between the three studied degassing areas in Krýsuvík, ranging from 10.9–70.9 T/day, with the highest flux in Hveradalir where the Multi-GAS station is located. The total flux is estimated as 101 T/day.Comparison between Multi-GAS and geophysical data shows that peaks of H2O-rich emissions appears to follow crustal movements. Coinciding with the H2O-rich peaks, SO2 is detected in minor amounts (~0.6 ppmv), allowing for calculations of H2O/SO2, CO2/SO2 and H2S/SO2 ratios. This is the first time SO2 has been detected in the Krýsuvík area.The large variations in H2O/CO2 and H2O/H2S ratios are considered to reflect variable degassing activity in the fumarole. The activity of the fumarole appears less intense during intervals of low or no recorded seismic events. The H2O/CO2 and H2O/H2S ratios are lower, presumably due to H2O condensation affecting the steam jet before reaching the Multi-GAS inlet tube.

Spatially Variable CO2 Degassing in the Main Ethiopian Rift: Implications for Magma Storage, Volatile Transport, and Rift‐Related Emissions

AbstractDeep carbon emissions from historically inactive volcanoes, hydrothermal, and tectonic structures are among the greatest unknowns in the long‐term (∼Myr) carbon cycle. Recent estimates of diffuse CO2 flux from the Eastern Rift of the East African Rift System (EARS) suggest this could equal emissions from the entire mid‐ocean ridge system. We report new CO2 surveys from the Main Ethiopian Rift (MER, northernmost EARS), and reassess the rift‐related CO2 flux. Since degassing in the MER is concentrated in discrete areas of volcanic and off‐edifice activity, characterization of such areas is important for extrapolation to a rift‐scale budget. Locations of hot springs and fumaroles along the rift show numerous geothermal areas away from volcanic edifices. With these new data, we estimate total CO2 emissions from the central and northern MER as 0.52–4.36 Mt yr−1. Our extrapolated flux from the Eastern Rift is 3.9–32.7 Mt yr−1 CO2, overlapping with lower end of the range presented in recent estimates. By scaling, we suggest that 6–18 Mt yr−1 CO2 flux can be accounted for by magmatic extension, which implies an important role for volatile‐enriched lithosphere, crustal assimilation, and/or additional magmatic intrusion to account for the upper range of flux estimates. Our results also have implications for the nature of volcanism in the MER. Many geothermal areas are found >10 km from the nearest volcanic center, suggesting ongoing hazards associated with regional volcanism.

A reliable and effective methodology to monitor CO2 flux from soil: The case of Lipari Island (Sicily, Italy)

A new unit for continuous monitoring of CO2 flux diffused from soil was developed and preliminary tested in the field at Lipari Island (Sicily, Italy). The developed device (CO2-S-POT), based on the accumulation chamber method, is an assembled unit easy to install and to manage. Both dimensions (104 × 61 × 75 cm) and weight (32 Kg) of CO2-S-POT are lower than the equipment which are commonly used for continuous monitoring of diffused CO2 flux. Besides CO2 flux measurements, it is able to measure several environmental parameters [air temperature, soil temperature, atmospheric pressure, Volumetric Water Content (soil VWC%) and soil conductivity]. Laboratory tests showed a very good performance of the assembled system in terms of detection limit (0.1 mol m−2·day−1), reproducibility (RSD better than 3.5%) and linearity (up to 4 order of magnitude). Field tests (3 months) were performed at Lipari Island (Sicily, Italy), where no continuous monitoring of diffuse CO2 flux using the accumulation chamber method is ongoing. During field tests, the CO2-S-POT performed flux measurements every hour, together with monitoring of the environmental parameters. During field tests, no ordinary or extraordinary maintenance was necessary and no missing data were detected. The soil CO2 flux showed strong influence operated by air pressure, air temperature, soil temperature, soil conductivity, Volumetric Water Content and rain. Statistical methods based on Time Series Fourier analysis (TSFA), Principal Component Analysis (PCA) and also Multiple Linear Regression Analysis (MLRA) are used in order to filter the influence of the environmental parameters on the flux data, and, at the same time, to highlight the contribution which could be due from deep system. Raw flux data (in ppm·sec−1) are used in time series analysis, since these represent the real physical variable measured in the field, without transformations. In fact, the conversion from ppm·sec−1 to another dimension (i.e. mol·m−2·day−1) affects the correlation between the measured parameters (i.e. CO2 flux and environmental parameters).The results of the statistical procedure showed that 1) the oscillation at low period (hours and days) are due to variations of environmental parameters and 2) the soil CO2 flux measured at Lipari is characterized also by long period variations (compared to the previous ones, i.e. weeks), difficult to explain just in terms of environmental parameters. The applied methodology, based on multivariate statistical techniques, has allowed to identify residual anomalies, following a suitable procedure to handle the multicollinearity. In the period from August 20 to August 23, 2014, when there were not both significant air pressure variations and rain events, anomalous positive residuals were identified. These residual anomalies are those interesting from volcanic surveillance point of view. However, to assess whether the residual anomalies are effectively controlled by variations induced by the deep system (i.e. seismic and/or volcanic activity) others parameters, such as wind speed and wind direction, and more time of observation should be taken into consideration.

CO2 flux geothermometer for geothermal exploration

A new geothermometer (TCO2 Flux) is proposed based on soil diffuse CO2 flux and shallow temperature measurements made on areas of steam heated, thermally altered ground above active geothermal systems. This CO2 flux geothermometer is based on a previously reported CO2 geothermometer that was designed for use with fumarole analysis. The new geothermometer provides a valuable additional exploration tool for estimating subsurface temperatures in high-temperature geothermal systems. Mean TCO2 Flux estimates fall within the range of deep drill hole temperatures at Wairakei (New Zealand), Tauhara (New Zealand), Rotokawa (New Zealand), Ohaaki (New Zealand), Reykjanes (Iceland) and Copahue (Argentina). The spatial distribution of geothermometry estimates is consistent with the location of major upflow zones previously reported at the Wairakei and Rotokawa geothermal systems. TCO2 Flux was also evaluated at White Island (New Zealand) and Reporoa (New Zealand), where limited sub-surface data exists. Mode TCO2 Flux at White Island is high (320°C), the highest of the systems considered in this study. However, the geothermometer relies on mineral–water equilibrium in neutral pH reservoir fluids, and would not be reliable in such an active and acidic environment. Mean TCO2 Flux at Reporoa (310°C) is high, which indicates Reporoa has a separate upflow from the nearby Waiotapu geothermal system; an outflow from Waiotapu would not be expected to have such high temperature.

Estimates of GHG emissions by hydroelectric reservoirs: The Brazilian case

This article reports and discusses the application of an analysis of reservoir greenhouse gas (GHG) emission using the net emission approach (difference between post-impoundment and pre-impoundment emissions) to assessments of GHG emissions from hydropower reservoirs compared with GHG emissions from thermal plants producing electrical energy. Reservoirs were chosen from a representative set of different Brazilian biomes and climatic regimes. A field campaign program was performed between 2011 and 2013 to estimate net GHGs emissions from 8 Brazilian representative hydropower plants. Four field campaigns to each hydropower plant were scheduled within two-month intervals to measure CH4, CO2 and N2O diffusive fluxes on air-water interfaces (reservoir surface area and downstream river, CH4 and CO2 ebullitive fluxes on the air-water interfaces of the reservoir and degassing rates at powerhouse turbines). Permanent carbon burial rates were also measured in the reservoirs. Statistical analyses of the data were representative of post-impoundment annual values for GHG emissions for each pathway and for permanent carbon burial rates. Values for pre-impoundment representative annual GHG emission were calculated taking into account previous land cover mapping data from the period of the creation of the reservoir, and reference GHG flux values established by the literature for each land cover class. To compare with thermopower generation, annual values obtained were expressed as GHG intensity in g of CO2eq.kWh−1 considering a Global Warming Potential value for 100 years; the factor 44/12 was considered as the permanent carbon burial rate necessary to transform carbon into carbon dioxide; and production of plant firm energy was also considered. Annual net GHG emissions estimates were expressed as GHG intensity in g of CO2eq kWh−1.In all cases analyzed, hydroelectric plants emit less greenhouse gases than natural gas and coal-fired thermoelectric plants. The exception was the Balbina hydroelectric dam, which has a low annual power generation and a very large reservoir: it emits more gas per unit of energy than the two thermoelectric plants abovementioned.

Management effects on greenhouse gas dynamics in fen ditches

Globally, large areas of peatland have been drained through the digging of ditches, generally to increase agricultural production. By lowering the water table it is often assumed that drainage reduces landscape-scale emissions of methane (CH4) into the atmosphere to negligible levels. However, drainage ditches themselves are known to be sources of CH4 and other greenhouse gases (GHGs), but emissions data are scarce, particularly for carbon dioxide (CO2) and nitrous oxide (N2O), and show high spatial and temporal variability. Here, we report dissolved GHGs and diffusive fluxes of CH4 and CO2 from ditches at three UK lowland fens under different management; semi-natural fen, cropland, and cropland restored to low-intensity grassland. Ditches at all three fens emitted GHGs to the atmosphere, but both fluxes and dissolved GHGs showed extensive variation both seasonally and within-site. CH4 fluxes were particularly large, with medians peaking at all three sites in August at 120–230mgm−2d−1. Significant between site differences were detected between the cropland and the other two sites for CO2 flux and all three dissolved GHGs, suggesting that intensive agriculture has major effects on ditch biogeochemistry. Multiple regression models using environmental and water chemistry data were able to explain 29–59% of observed variation in dissolved GHGs. Annual CH4 fluxes from the ditches were 37.8, 18.3 and 27.2gCH4m−2yr−1 for the semi-natural, grassland and cropland, and annual CO2 fluxes were similar (1100 to 1440gCO2m−2yr−1) among sites. We suggest that fen ditches are important contributors to landscape-scale GHG emissions, particularly for CH4. Ditch emissions should be included in GHG budgets of human modified fens, particularly where drainage has removed the original terrestrial CH4 source, e.g. agricultural peatlands.

Estimation of the CO2 flux from Furnas volcanic Lake (São Miguel, Azores)

A study on diffuse CO2 degassing was undertaken at Furnas lake (São Miguel island, Azores) in order to estimate the total diffuse CO2 output and identify anomalous degassing areas over the lake. Furnas lake is located in Furnas Volcano, the easternmost of the three active central volcanoes of the São Miguel island. The lake has an area of 1.87km2 and a maximum length and width equal to 2025 and 1600m, respectively. The maximum depth of the water column is 15m and the estimated water storage is 14×106m3. Lake water temperature is cold, with temperature values between 13°C and 15°C in the winter period and 18.9°C to 19.3°C in early autumn, and the variation along the water column suggests a monomictic character. The major-ion relative composition is in decreasing order Na+>K+>Ca2+>Mg2+ for cations and HCO3−>Cl−>SO42− for anions, and conductivity and pH measurements, respectively in the range of 152 to 165μScm−1 and 5.3 to 8.7, suggests that Furnas has neutral-diluted waters and can be classified as a non-active lake. Diffuse CO2 flux measurements were made using the accumulation chamber method with a total of 1537 and 2577 measurements performed in two different sampling campaigns. The total amount of diffuse CO2 emitted to the atmosphere was estimated between 28 and 321tkm−2d−1, respectively, in the second and first sampling campaigns, corresponding to ~52 and ~600td−1. The main anomalous degassing area identified over the Furnas lake during both surveys is probably associated to a WNW–ESE trending tectonic structure. Other secondary areas are also suggested to be tectonically influenced. Identified anomalous areas showed similarities to the ones observed during previous soil CO2 degassing studies.

Application of diffuse gas flux measurements and soil gas analysis to geothermal exploration and environmental monitoring: Example from the Reykjanes geothermal field, SW Iceland

Soil gas geochemistry and diffuse CO2 flux from soil, together with gas concentrations in 7 production wells and 10 steam vents in the Reykjanes geothermal field (Iceland), were studied with the aim of: 1) studying the potential use of the CO2/He ratio in soil gas to constrain CO2 retention in calcite in the bedrock; and 2) quantifying the effect of 100 MWe power production from the Reykjanes geothermal reservoir on surface activity. The presence of biogenic CO2 in the soil gas prevented the use of the CO2/He ratio for the quantification of CO2 retention in calcite. The CO2/He ratio in about 20 soil gas samples was, however, low and consistent with CO2 retention. Our results suggest that detailed soil gas and soil temperature mapping may provide excellent information for defining drilling targets. The surface activity in the Reykjanes geothermal field increased noticeably after the commissioning of the 100 MWe Reykjanes Power Plant in 2006. When increased steam flow from steam vents is taken into account the total increase in surface activity in Reykjanes between 2004 and 2007 is of the order of 50%.

Topographic controls on the depth distribution of soil CO2 in a small temperate watershed

Accurate measurements of soil CO2 concentrations (pCO2) are important for understanding carbonic acid reaction pathways for continental weathering and the global carbon (C) cycle. While there have been many studies of soil pCO2, most sample or model only one, or at most a few, landscape positions and therefore do not account for complex topography. Here, we test the hypothesis that soil pCO2 distribution can predictably vary with topographic position. We measured soil pCO2 at the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO), Pennsylvania, where controls on soil pCO2 (e.g., depth, texture, porosity, and moisture) vary from ridge tops down to the valley floor, between planar slopes and slopes with convergent flow (i.e., swales), and between north and south-facing aspects. We quantified pCO2 generally at 0.1–0.2 m depth intervals down to bedrock from 2008 to 2010 and in 2013. Of the variables tested, topographic position along catenas was the best predictor of soil pCO2 because it controls soil depth, texture, porosity, and moisture, which govern soil CO2 diffusive fluxes. The highest pCO2 values were observed in the valley floor and swales where soils are deep (≥0.7 m) and wet, resulting in low CO2 diffusion through soil profiles. In contrast, the ridge top and planar slope soils have lower pCO2 because they are shallower (≤0.6 m) and drier, resulting in high CO2 diffusion through soil profiles. Aspect was a minor predictor of soil pCO2: the north (i.e., south-facing) swale generally had lower soil moisture content and pCO2 than its south (i.e., north-facing) counterpart. Seasonally, we observed that while the timing of peak soil pCO2 was similar across the watershed, the amplitude of the pCO2 peak was higher in the deep soils due to more variable moisture content. The high pCO2 observed in the deeper, wetter topographic positions could lower soil porewater pH by up to 1 pH unit compared to porewaters equilibrated with atmospheric CO2 alone. CO2 is generally the dominant acid driving weathering in soils: based on our observations, models of chemical weathering and CO2 dynamics would be improved by including landscape controls on soil pCO2.

Diffusive emission of methane and carbon dioxide from two hydropower reservoirs in Brazil.

The role of greenhouse gas emissions from freshwater reservoirs and their contribution to increase greenhouse gas concentrations in the atmosphere is currently under discussion in many parts of the world. We studied CO2 and CH4 diffusive fluxes from two large neotropical hydropower reservoirs with different climate conditions. We used floating closed-chambers to estimate diffusive fluxes of these gaseous species. Sampling campaigns showed that the reservoirs studied were sources of greenhouse gases to the atmosphere. In the Serra da Mesa Reservoir, the CH4 emissions ranged from 0.530 to 396.96 mg.m(-2).d(-1) and CO2 emissions ranged from -1,738.33 to 11,166.61 mg.m(-2).d(-1) and in Três Marias Reservoir the CH4 fluxes ranged 0.720 to 2,578.03 mg.m(-2).d(-1) and CO2 emission ranged from -3,037.80 to 11,516.64 to mg.m(-2).d(-1). There were no statistically significant differences of CH4 fluxes between the reservoirs, but CO2 fluxes from the two reservoirs studied were significantly different. The CO2 emissions measured over the periods studied in Serra da Mesa showed some seasonality with distinctions between the wet and dry transition season. In Três Marias Reservoir the CO2 fluxes showed no seasonal variability. In both reservoirs, CH4 emissions showed a tendency to increase during the study periods but this was not statistically significant. These results contributed to increase knowledge about the magnitude of CO2 and CH4 emission in hydroelectric reservoirs, however due to natural variability of the data future sampling campaigns will be needed to better elucidate the seasonal influences on the fluxes of greenhouse gases.

Diffuse volcanic gas emission and thermal energy release from the summit crater of Pico do Fogo, Cape Verde

We report the first detailed study of diffuse emission of carbon dioxide (CO2), hydrogen sulfide (H2S), helium (He), and hydrogen (H2) from the summit crater of Pico do Fogo volcano, Cape Verde. Diffuse CO2, H2S, He, and H2 gas fluxes were measured at 57 sampling sites and ranged up to 12,800, 13, 1, and 6 g m−2 day−1, respectively. Soil temperature measurements at each sampling site were used to evaluate the heat flux. Most of the summit crater shows relatively high CO2 efflux, with highest values close to the fumarolic area, suggesting a structural control of the degassing process. In contrast, H2S effluxes were negligible or very low at the summit crater, except close to the fumarolic area where anomalously high CO2 efflux and soil temperatures were also measured. We estimate total CO2, H2S, He, and H2 diffuse gas fluxes of 219 t day−1, 25, 4, and 33 kg day−1, respectively. Based on a H2O/CO2 mass ratio of 1.52 measured at the fumaroles, we estimate a diffuse steam flux from the summit crater of approximately 330 t day−1. The enthalpy of this steam is equivalent to a heat flux of about 10.3 MW. The diffuse gas emission and thermal energy released from the summit crater of Pico do Fogo volcano are comparable to those observed at other volcanoes. Sustained surveillance of Pico do Fogo using these methods will be valuable for monitoring the activity of one of the most active volcanoes in the Atlantic Ocean.

The sealing capacity of the cap rock above the Torre Alfina geothermal reservoir (Central Italy) revealed by soil CO2 flux investigations

Torre Alfina is a medium enthalpy (T=140°C) geothermal field in Central Italy, hosted in buried fractured Mesozoic limestones, extensively explored in the 1970s and 1980s, but which so far has not been exploited. A detailed diffuse soil CO2 flux investigation (1336 measurements over a surface of 12.6km2) and the periodic monitoring of soil CO2 flux from target areas indicate that in most of the area, even above a pressurized gas cap existing at the reservoir top, the soil CO2 flux is low and mostly within the background threshold (48gm−2 day−1) and is likely generated by biological soil respiration. Anomalous values (up to 30.250gm−2 day−1) are found only in the proximity of a small zone with gas vents whose composition is identical to that of the gas produced by the geothermal wells tapping the reservoir. This is the only zone where a fault connecting the deep geothermal reservoir with the surface does exist. These data are compared with those of the near Latera high enthalpy geothermal field (T=210°C), where anomalous soil CO2 flux is recorded above the productive reservoir. The difference of soil CO2 release in the two fields is attributed to the differences in the impervious cover (allochthonous flysch deposits) above the carbonate reservoir, which is thin and locally lacking at Latera, and continuous and thick (>400m) at Torre Alfina. Results demonstrate that soil CO2 flux investigations are useful in geothermal exploration, but only high flux values likely indicate the presence of a geothermal reservoir at depth, whereas low flux values can indicate either the lack of an active geothermal reservoir at depth or the presence of a very effective impervious cover above the reservoir.

Identifying blind geothermal systems with soil CO2 surveys

Diffuse soil CO2 flux surveys are a widely applied approach for delineating zones of elevated heat and mass transfer in areas with geothermal surface features including hot-springs, mud pools, and geysers. However, many geothermal systems are capped by relatively impermeable layers that diminish the surface expression of potential resources present at depth. Here, we report diffuse soil CO2 flux survey results with complementary δ13CO2 values and shallow soil temperatures for the Ngatamariki geothermal system (Taupo Volcanic Zone, New Zealand), a largely surface blind system, in an effort to explore the utility of such data as indicators of magmatic carbon emission in geothermal systems with low CO2 flux. Our results include that: (1) the majority (54%) of the soil CO2 flux measurements are below background levels (15gCO2m−2day−1); (2) only 5.6% of the shallow (10cm) soil temperatures exceed 30°C; and (3) no correlation is present between soil temperature and CO2 flux at Ngatamariki. These results belie the fact that a recently developed geothermal resource is present beneath Ngatamariki. Yet, δ13CO2 values when interpreted in the context of soil gas CO2 concentrations, demonstrate that the magmatic-hydrothermal system beneath Ngatamariki can be distinguished from soil-zone carbon sources using diffuse soil gas geochemical tracers. Modeling the Ngatamariki CO2 system as a simple mixture of geothermal biogenic and atmospheric end-members allows for the apportionment of CO2 flux at different locations across the field. Based on these findings, we suggest that delineating potential geothermal sites and assessing the relative contributions of mixed fluid end-members is possible even with low diffuse CO2 flux and limited surface expression of geothermal systems at depth.

Diffuse soil CO2 flux to assess the reliability of CO2 storage in the Mazarrón–Gañuelas Tertiary Basin (Spain)

In the framework of a global investigation of the Spanish natural analogues of CO2 storage and leakage, four selected sites from the Mazarrón–Gañuelas Tertiary Basin (Murcia, Spain) were studied for computing the diffuse soil CO2 flux, by using the accumulation chamber method. The Basin is characterized by the presence of a deep, saline, thermal (∼47°C) CO2-rich aquifer intersected by two deep geothermal exploration wells named “El Saladillo” (535m) and “El Reventón” (710m).The CO2 flux data were processed by means of a graphical–statistical method, kriging estimation and sequential Gaussian simulation algorithms. The results have allowed concluding that the Tertiary marly cap-rock of this CO2-rich aquifer acts as a very effective sealing, preventing any CO2 leak from this natural CO2 storage site, being therefore an excellent scenario to guarantee, by analogy, the safety of a CO2 storage.

Generation and evolution of hydrothermal fluids at Yellowstone: Insights from the Heart Lake Geyser Basin

We sampled fumaroles and hot springs from the Heart Lake Geyser Basin (HLGB), measured water and gas discharge, and estimated heat and mass flux from this geothermal area in 2009. The combined data set reveals that diverse fluids share an origin by mixing of deep solute‐rich parent water with dilute heated meteoric water, accompanied by subsequent boiling. A variety of chemical and isotopic geothermometers are consistent with a parent water that equilibrates with rocks at 205°C ± 10°C and then undergoes 21% ± 2% adiabatic boiling. Measured diffuse CO2 flux and fumarole compositions are consistent with an initial dissolved CO2 concentration of 21 ± 7 mmol upon arrival at the caldera boundary and prior to southeast flow, boiling, and discharge along the Witch Creek drainage. The calculated advective flow from the basin is 78 ± 16 L s−1of parent thermal water, corresponding to 68 ± 14 MW, or ∼1% of the estimated thermal flux from Yellowstone. Helium and carbon isotopes reveal minor addition of locally derived crustal, biogenic, and meteoric gases as this fluid boils and degasses, reducing the He isotope ratio (Rc/Ra) from 2.91 to 1.09. The HLGB is one of the few thermal areas at Yellowstone that approaches a closed system, where a series of progressively boiled waters can be sampled along with related steam and noncondensable gas. At other Yellowstone locations, steam and gas are found without associated neutral Cl waters (e.g., Hot Spring Basin) or Cl‐rich waters emerge without significant associated steam and gas (Upper Geyser Basin).

Gross CO2 and CH4 emissions from the Nam Ngum and Nam Leuk sub-tropical reservoirs in Lao PDR

Gross CO2 and CH4 emissions (degassing and diffusion from the reservoir) and the carbon balance were assessed in 2009–2010 in two Southeast Asian sub-tropical reservoirs: the Nam Ngum and Nam Leuk Reservoirs (Lao PDR). These two reservoirs are within the same climatic area but differ mainly in age, size, residence time and initial biomass stock. The Nam Leuk Reservoir was impounded in 1999 after partial vegetation clearance and burning. However, GHG emissions are still significant 10years after impoundment. CH4 diffusive flux ranged from 0.8 (January 2010) to 11.9mmolm−2d−1 (April 2009) and CO2 diffusive flux ranged from – 10.6 (October 2009) to 38.2mmolm−2d−1 (April 2009). These values are comparable to other tropical reservoirs. Moreover, degassing fluxes at the outlet of the powerhouse downstream of the turbines were very low. The tentative annual carbon balance calculation indicates that this reservoir was a carbon source with an annual carbon export (atmosphere+downstream river) of about 2.2±1.0GgCyr−1.The Nam Ngum Reservoir was impounded in 1971 without any significant biomass removal. Diffusive and degassing CO2 and CH4 fluxes were lower than for other tropical reservoirs. Particularly, CO2 diffusive fluxes were always negative with values ranging from – 21.2 (April 2009) to – 2.7mmolm−2d−1 (January 2010). CH4 diffusive flux ranged from 0.1 (October 2009) to 0.6mmolm−2d−1 (April 2009) and no degassing downstream of the turbines was measured. As a consequence of these low values, the reservoir was a carbon sink with an estimated annual uptake of – 53±35GgCyr−1.

Eddy covariance imaging of diffuse volcanic CO2 emissions at Mammoth Mountain, CA, USA

Use of eddy covariance (EC) techniques to map the spatial distribution of diffuse volcanic CO2 fluxes and quantify CO2 emission rate was tested at the Horseshoe Lake tree-kill area on Mammoth Mountain, California, USA. EC measurements of CO2 flux were made during September–October 2010 and ranged from 85 to 1,766 g m−2 day−1. Comparative maps of soil CO2 flux were simulated and CO2 emission rates estimated from three accumulation chamber (AC) CO2 flux surveys. Least-squares inversion of measured eddy covariance CO2 fluxes and corresponding modeled source weight functions recovered 58–77% of the CO2 emission rates estimated based on simulated AC soil CO2 fluxes. Spatial distributions of modeled surface CO2 fluxes based on EC and AC observations showed moderate to good correspondence (R2 = 0.36 to 0.70). Results provide a framework for automated monitoring of volcanic CO2 emissions over relatively large areas.

Heterotrophic denitrification vs. autotrophic anammox – quantifying collateral effects on the oceanic carbon cycle

Abstract. The conversion of fixed nitrogen to N2 in suboxic waters is estimated to contribute roughly a third to total oceanic losses of fixed nitrogen and is hence understood to be of major importance to global oceanic production and, therefore, to the role of the ocean as a sink of atmospheric CO2. At present heterotrophic denitrification and autotrophic anammox are considered the dominant sinks of fixed nitrogen. Recently, it has been suggested that the trophic nature of pelagic N2-production may have additional, "collateral" effects on the carbon cycle, where heterotrophic denitrification provides a shallow source of CO2 and autotrophic anammox a shallow sink. Here, we analyse the stoichiometries of nitrogen and associated carbon conversions in marine oxygen minimum zones (OMZ) focusing on heterotrophic denitrification, autotrophic anammox, and dissimilatory nitrate reduction to nitrite and ammonium in order to test this hypothesis quantitatively. For open ocean OMZs the combined effects of these processes turn out to be clearly heterotrophic, even with high shares of the autotrophic anammox reaction in total N2-production and including various combinations of dissimilatory processes which provide the substrates to anammox. In such systems, the degree of heterotrophy (ΔCO2:ΔN2), varying between 1.7 and 6.5, is a function of the efficiency of nitrogen conversion. On the contrary, in systems like the Black Sea, where suboxic N-conversions are supported by diffusive fluxes of NH4+ originating from neighbouring waters with sulphate reduction, much lower values of ΔCO2:ΔN2 can be found. However, accounting for concomitant diffusive fluxes of CO2, the ratio approaches higher values similar to those computed for open ocean OMZs. Based on this analysis, we question the significance of collateral effects concerning the trophic nature of suboxic N-conversions on the marine carbon cycle.

An inverse analysis reveals limitations of the soil-CO<sub>2</sub> profile method to calculate CO<sub>2</sub> production and efflux for well-structured soils

Abstract. Soil respiration is the second largest flux in the global carbon cycle, yet the underlying below-ground process, carbon dioxide (CO2) production, is not well understood because it can not be measured in the field. CO2 production has frequently been calculated from the vertical CO2 diffusive flux divergence, known as "soil-CO2 profile method". This relatively simple model requires knowledge of soil CO2 concentration profiles and soil diffusive properties. Application of the method for a tropical lowland forest soil in Panama gave inconsistent results when using diffusion coefficients (D) calculated based on relationships with soil porosity and moisture ("physically modeled" D). Our objective was to investigate whether these inconsistencies were related to (1) the applied interpolation and solution methods and/or (2) uncertainties in the physically modeled profile of D. First, we show that the calculated CO2 production strongly depends on the function used to interpolate between measured CO2 concentrations. Secondly, using an inverse analysis of the soil-CO2 profile method, we deduce which D would be required to explain the observed CO2 concentrations, assuming the model perception is valid. In the top soil, this inversely modeled D closely resembled the physically modeled D. In the deep soil, however, the inversely modeled D increased sharply while the physically modeled D did not. When imposing a constraint during the fit parameter optimization, a solution could be found where this deviation between the physically and inversely modeled D disappeared. A radon (Rn) mass balance model, in which diffusion was calculated based on the physically modeled or constrained inversely modeled D, simulated observed Rn profiles reasonably well. However, the CO2 concentrations which corresponded to the constrained inversely modeled D were too small compared to the measurements. We suggest that, in well-structured soils, a missing description of steady state CO2 exchange fluxes across water-filled pores causes the soil-CO2 profile method to fail. These fluxes are driven by the different diffusivities in inter- vs. intra-aggregate pores which create permanent CO2 gradients if separated by a "diffusive water barrier". These results corroborate other studies which have shown that the theory to treat gas diffusion as homogeneous process, a precondition for use of the soil-CO2 profile method, is inaccurate for pore networks which exhibit spatial separation between CO2 production and diffusion out of the soil.