Surface heat flow and CO2 emissions within the Ohaaki hydrothermal field,Taupo Volcanic Zone,New Zealand

Carbon dioxide emissions and heat flow have been determined from the Ohaaki hydrothermal field, Taupo Volcanic Zone (TVZ), New Zealand following 20 a of production (116 MW_e). Soil CO₂ degassing was quantified with 2663 CO₂ flux measurements using the accumulation chamber method, and 2563 soil temperatures were measured and converted to equivalent heat flow (W m⁻²) using published soil temperature heat flow functions. Both CO₂ flux and heat flow were analysed statistically and then modelled using 500 sequential Gaussian simulations. Forty subsoil CO₂ gas samples were also analysed for stable C isotopes. Following 20 a of production, current CO₂ emissions equated to 111 ± 6.7 T/d. Observed heat flow was 70 ± 6.4 MW, compared with a pre-production value of 122 MW. This 52 MW reduction in surface heat flow is due to production-induced drying up of all alkali–Cl outflows (61.5 MW) and steam-heated pools (8.6 MW) within the Ohaaki West thermal area (OHW). The drying up of all alkali–Cl outflows at Ohaaki means that the soil zone is now the major natural pathway of heat release from the high-temperature reservoir. On the other hand, a net gain in thermal ground heat flow of 18 MW (from 25 MW to 43.3 ± 5 MW) at OHW is associated with permeability increases resulting from surface unit fracturing by production-induced ground subsidence. The Ohaaki East (OHE) thermal area showed no change in distribution of shallow and deep soil temperature contours despite 20 a of production, with an observed heat flow of 26.7 ± 3 MW and a CO₂ emission rate of 39 ± 3 T/d. The negligible change in the thermal status of the OHE thermal area is attributed to the low permeability of the reservoir beneath this area, which has limited production (mass extraction) and sheltered the area from the pressure decline within the main reservoir. Chemistry suggests that although alkali–Cl outflows once contributed significantly to the natural surface heat flow (∼50%) they contributed little (<1%) to pre-production CO₂ emissions due to the loss of >99% of the original CO₂ content due to depressurisation and boiling as the fluids ascended to the surface. Consequently, the soil has persisted as the major (99%) pathway of CO₂ release to the atmosphere from the high temperature reservoir at Ohaaki. The CO₂ flux and heat flow surveys indicate that despite 20 a of production the variability in location, spatial extent and magnitude of CO₂ flux remains consistent with established geochemical and geophysical models of the Ohaaki Field. At both OHW and OHE carbon isotopic analyses of soil gas indicate a two-stage fractionation process for moderate-flux (>60 g m⁻² d⁻¹) sites; boiling during fluid ascent within the underlying reservoir and isotopic enrichment as CO₂ diffuses through porous media of the soil zone. For high-flux sites (>300 g m⁻² d⁻¹), the δ¹³CO₂ signature (−7.4 ± 0.3‰ OHW and −6.5 ± 0.6‰ OHE) is unaffected by near-surface (soil zone) fractionation processes and reflects the composition of the boiled magmatic CO₂ source for each respective upflow. Flux thresholds of <30 g m⁻² d⁻¹ for purely diffusive gas transport, between 30 and 300 g m⁻² d⁻¹ for combined diffusive–advective transport, and ?300 g m⁻² d⁻¹ for purely advective gas transport at Ohaaki were assigned. δ¹³CO₂ values and cumulative probability plots of CO₂ flux data both identified a threshold of ∼15 g m⁻² d⁻¹ by which background (atmospheric and soil respired) CO₂ may be differentiated from hydrothermal CO₂.     

CO and CO2 emissions from spontaneous heating of coal under different ventilation rates

Carbon monoxide (CO) and carbon dioxide (CO₂) emissions during a spontaneous heating event in a coal mine are important gases to monitor for detecting the spontaneous heating at an early stage. However, in underground coal mines, the CO and CO₂ concentrations and their related fire ratios may be affected by mine ventilation. In this study, CO and CO₂ emissions from spontaneous heating of a U.S. coal sample were evaluated in an isothermal oven under different airflow ventilation rates ranging from 100 to 500 cm³/min. Laboratory experiments were conducted at oven temperatures of 70, 90, and 100 °C. The temperature at the center of the coal sample was continually monitored, while the CO, CO₂, and oxygen (O₂) concentrations of the exit gas were continually measured. The results indicate that CO was generated immediately after the airflow passed through the coal, while CO₂ was generated in a late phase. The amounts of CO generated under different airflow rates were approximately the same at the initial temperature of 70 °C, while the amounts of CO generated increased significantly as the airflow rates and initial temperatures increased. The ratio of CO/CO₂ was found to be independent of airflow rate and initial temperature, approaching a constant value of 0.2 quickly if there was no thermal runaway. The value tended to decrease when a thermal runaway took place. The CO/O₂ deficiency ratio was dependent on both airflow rates and the initial temperature. The experimental results are in qualitative agreement with some large-scale test and field monitoring results.     

Diffuse gas emissions at the Ukinrek Maars,Alaska: Implications for magmatic degassing and volcanic monitoring

Diffuse CO₂ efflux near the Ukinrek Maars, two small volcanic craters that formed in 1977 in a remote part of the Alaska Peninsula, was investigated using accumulation chamber measurements. High CO₂ efflux, in many places exceeding 1000 g m⁻² d⁻¹, was found in conspicuous zones of plant damage or kill that cover 30,000–50,000 m² in area. Total diffuse CO₂ emission was estimated at 21–44 t d⁻¹. Gas vents 3-km away at The Gas Rocks produce 0.5 t d⁻¹ of CO₂ that probably derives from the Ukinrek Maars basalt based on similar δ¹³C values (∼−6‰), ³He/⁴He ratios (5.9–7.2 R_A), and CO₂/³He ratios (1–2 × 10⁹) in the two areas. A lower ³He/⁴He ratio (2.7 R_A) and much higher CO₂/³He ratio (9 × 10¹⁰) in gas from the nearest arc-front volcanic center (Mount Peulik/Ugashik) provide a useful comparison. The large diffuse CO₂ emission at Ukinrek has important implications for magmatic degassing, subsurface gas transport, and local toxicity hazards. Gas–water–rock interactions play a major role in the location, magnitude and chemistry of the emissions.     

Carbon dioxide degassing and thermal energy release in the Monte Amiata volcanic-geothermal area (Italy)

The quaternary volcanic complex of Mount Amiata is located in southern Tuscany (Italy) and represents the most recent manifestation of the Tuscan Magmatic Province. The region is characterised by a large thermal anomaly and by the presence of numerous CO₂-rich gas emissions and geothermal features, mainly located at the periphery of the volcanic complex. Two geothermal systems are located, at increasing depths, in the carbonate and metamorphic formations beneath the volcanic complex. The shallow volcanic aquifer is separated from the deep geothermal systems by a low permeability unit (Ligurian Unit). A measured CO₂ discharge through soils of 1.8 × 10⁹ mol a⁻¹ shows that large amounts of CO₂ move from the deep reservoir to the surface. A large range in δ¹³C_TDIC (−21.07 to +3.65) characterises the waters circulating in the aquifers of the region and the mass and isotopic balance of TDIC allows distinguishing a discharge of 0.3 × 10⁹ mol a⁻¹ of deeply sourced CO₂ in spring waters. The total natural CO₂ discharge (2.1 × 10⁹ mol a⁻¹) is slightly less than minimum CO₂ output estimated by an indirect method (2.8 × 10⁹ mol a⁻¹), but present-day release of 5.8 × 10⁹ mol a⁻¹ CO₂ from deep geothermal wells may have reduced natural CO₂ discharge. The heat transported by groundwater, computed considering the increase in temperature from the infiltration area to the discharge from springs, is of the same order of magnitude, or higher, than the regional conductive heat flow (>200 mW m⁻²) and reaches extremely high values (up to 2700 mW m⁻²) in the north-eastern part of the study area. Heat transfer occurs mainly by conductive heating in the volcanic aquifer and by uprising gas and vapor along fault zones and in those areas where low permeability cover is lacking. The comparison of CO₂ flux, heat flow and geological setting shows that near surface geology and hydrogeological setting play a central role in determining CO₂ degassing and heat transfer patterns.     

Gas seepage from Tokamachi mud volcanoes, onshore Niigata Basin (Japan): Origin, post-genetic alterations and CH4-CO2 fluxes

Methane and CO₂ emissions from the two most active mud volcanoes in central Japan, Murono and Kamou (Tokamachi City, Niigata Basin), were measured in from both craters or vents (macro-seepage) and invisible exhalation from the soil (mini- and microseepage). Molecular and isotopic compositions of the released gases were also determined. Gas is thermogenic (δ¹³C_CH4 from −32.9‰ to −36.2‰), likely associated with oil, and enrichments of ¹³C in CO₂ (δ¹³C_CO2 up to +28.3‰) and propane (δ¹³C_C3H8 up to −8.6‰) suggest subsurface petroleum biodegradation. Gas source and post-genetic alteration processes did not change from 2004 to 2010. Methane flux ranged within the orders of magnitude of 10¹-10⁴ g m⁻² d⁻¹ in macro-seeps, and up to 446 g m⁻² d⁻¹ from diffuse seepage. Positive CH₄ fluxes from dry soil were widespread throughout the investigated areas. Total CH₄ emission from Murono and Kamou were estimated to be at least 20 and 3.7 ton a⁻¹, respectively, of which more than half was from invisible seepage surrounding the mud volcano vents. At the macro-seeps, CO₂ fluxes were directly proportional to CH₄ fluxes, and the volumetric ratios between CH₄ flux and CO₂ flux were similar to the compositional CH₄/CO₂ volume ratio. Macro-seep flux data, in addition to those of other 13 mud volcanoes, supported the hypothesis that molecular fractionation (increase of the “Bernard ratio” C₁/(C₂ + C₃)) is inversely proportional to gas migration fluxes. The CH₄ “emission factor” (total measured output divided by investigated seepage area) was similar to that derived in other mud volcanoes of the same size and activity. The updated global “emission-factor” data-set, now including 27 mud volcanoes from different countries, suggests that previous estimates of global CH₄ emission from mud volcanoes may be significantly underestimated.     

Analysis of long- and short-term temporal variations of the diffuse CO2 emission from Timanfaya volcano,Lanzarote, Canary Islands

Reported herein are the results of eight soil CO₂ efflux surveys performed from 2006 to 2011 at Timanfaya Volcanic Field (TVF), Lanzarote Island with the aim of evaluating the long- and short-term temporal variations of the diffuse CO₂ emission. Soil CO₂ efflux values ranged from non-detectable up to 34.2 g m⁻² d⁻¹, with the highest values measured in September 2008. Conditional sequential Gaussian simulations (sGs) were applied to construct soil CO₂ efflux distribution maps and to estimate the total CO₂ output from the studied area at the TVF. Soil CO₂ efflux maps showed a high spatial and temporal variability. Total CO₂ emission rates ranged between 41 and 518 t d⁻¹, February 2011 (winter) being the season when maximum diffuse CO₂ emission rates were observed. To investigate the influence of external variables on the soil CO₂ efflux, a geochemical station (LZT01) was installed at TVF to measure continuously the soil CO₂ efflux between July 2010 and March 2012 Since external factors such as barometric pressure, rainfall, soil water content, soil and air temperatures, and wind speed influence strongly the observed soil CO₂ effluxes, multiple regression analysis was applied to the time series recorded by the automatic geochemical station LZT01 to remove the contribution of these external factors. The influence of meteorological variables on soil CO₂ efflux oscillations accounts for 13% of total variance, with barometric pressure, rainfall and/or soil water content having the most influence in the control of the soil CO₂ efflux. These observations along with the results from the eight soil gas surveys performed at TVF indicate that the short and long-term trends in the diffuse CO₂ degassing are mainly controlled by environmental factors.     

Time series analysis of CO concentrations from an Eastern Kentucky coal fire

Coal fires in natural outcrops, in abandoned and active coal mines, and in coal and coal-refuse piles are responsible for the uncontrolled emissions of gases, including CO, CO₂, H₂S, hydrocarbons, and volatile aromatics. Typically, measurements of gases at a mine vent are made over a short time interval, perhaps no more than 10 min, including the time for replicate measurements. Such timing provides little information on longer-term variations in emissions, although comparisons of seasonal measurements suggest such changes do occur. To address this problem, we placed temperature and CO data loggers in coal fire vents to collect time series measurements for a period of up to three weeks. For one experiment, 11 days of data at one-minute intervals indicated that the CO emissions were generally in the 400–550 ppmv range. However near daily depressions in CO concentrations occurred and in some cases fell below 50 ppmv; followed by an increase to ~ 700–800 ppmv; in turn followed by a return to the ambient conditions. Data for a separate 21-day collection period in a different vent of the same fire exhibited similar trends, albeit at a higher CO concentration. The drop in CO concentration may be associated with a meteorologically-driven inhalation cycle of the fire, whereby air diluted the combustion generated CO. We propose this was followed by an increase in the intensity of the fire due to increased O₂ from the inhaled air, producing increased CO concentrations, before settling back to the ambient conditions.     

Chemistry of hot springs along the Eastern Lau Spreading Center

The Eastern Lau Spreading Center (ELSC) is the southernmost part of the back-arc spreading axis in the Lau Basin, west of the Tonga trench and the active Tofua volcanic arc. Over its 397-km length it exhibits large and systematic changes in spreading rate, magmatic/tectonic processes, and proximity to the volcanic arc. In 2005, we collected 81 samples of vent water from six hydrothermal fields along the ELSC. The chemistry of these waters varies both within and between vent fields, in response to changes in substrate composition, temperature and pressure, pH, water/rock ratio, and input from magmatic gases and subducted sediment. Hot-spring temperatures range from 229° to 363 °C at the five northernmost fields, with a general decrease to the south that is reversed at the Mariner field. The southernmost field, Vai Lili, emitted water at up to 334 °C in 1989 but had a maximum venting temperature of only 121 °C in 2005, due to waning activity and admixture of bottom seawater into the subseafloor plumbing system. Chloride varies both within fields and from one field to another, from a low of 528 mmol/kg to a high of 656 mmol/kg, and may be enriched by phase separation and/or leaching of Cl from the rock. Concentrations of the soluble elements K, Rb, Cs, and B likewise increase southward as the volcanic substrate becomes more silica-rich, especially on the Valu Fa Ridge. Iodine and δ⁷Li increase southward, and δ¹¹B decreases as B increases, apparently in response to increased input from subducted sediment as the arc is approached. Species that decrease southward as temperature falls are Si, H₂S, Li, Na/Cl, Fe, Mn, and ⁸⁷Sr/⁸⁶Sr, whereas pH, alkalinity, Ca, and Sr increase. Oxygen isotopes indicate a higher water/rock ratio in the three systems on Valu Fa Ridge, consistent with higher porosity in more felsic volcanic rocks. Vent waters at the Mariner vent field on the Valu Fa Ridge are significantly hotter, more acid and metal-rich, less saline, and richer in dissolved gases and other volatiles, including H₂S, CO₂, and F, than the other vent fields, consistent with input of magmatic gases. The large variations in geologic and geophysical parameters produced by back-arc spreading along the ELSC, which exceed those along mid-ocean ridge spreading axes, produce similar large variations in the composition of vent waters, and thus provide new insights into the processes that control the chemistry of submarine hot springs.     

Experimental investigation of single carbon compounds under hydrothermal conditions

The speciation of carbon in subseafloor hydrothermal systems has direct implications for the maintenance of life in present-day vent ecosystems and possibly the origin of life on early Earth. Carbon monoxide is of particular interest because it represents a key reactant during the abiotic synthesis of reduced carbon compounds via Fischer-Tropsch-type processes. Laboratory experiments were conducted to constrain reactions that regulate the speciation of aqueous single carbon species under hydrothermal conditions and determine kinetic parameters for the oxidation of CO according to the water water-gas shift reaction (CO₂ + H₂ = CO + H₂O). Aqueous fluids containing added CO₂, CO, HCOOH, NaHCO₃, NaHCOO, and H₂ were heated at 150, 200, and 300 °C and 350 bar in flexible-cell hydrothermal apparatus, and the abundances of carbon compounds was monitored as a function of time. Variations in fluid chemistry suggest that the reduction of CO₂ to CH₃OH under aqueous conditions occurs via a stepwise process that involves the formation of HCOOH, CO, and possibly CH₂O, as reaction intermediaries. Kinetic barriers that inhibit the reduction of CH₃OH to CH₄ allow the accumulation of reaction intermediaries in solution at high concentrations regulated by metastable thermodynamic equilibrium. Reaction of CO₂ to CO involves a two-step process in which CO₂ initially undergoes a reduction step to HCOOH which subsequently dehydrates to form CO. Both reactions proceed readily in either direction. A preexponential factor of 1.35 × 10⁶ s⁻¹ and an activation energy of 102 kJ/mol were retrieved from the experimental results for the oxidation of CO to CO₂. Reaction rates amongst single carbon compounds during the experiments suggest that ΣCO₂ (CO₂ + HCO₃⁻ + CO₃²⁻), CO, ΣHCOOH (HCOOH + HCOO⁻), and CH₃OH may reach states of redox-dependent metastable thermodynamic equilibrium in subseafloor and other hydrothermal systems. The abundance of CO under equilibrium conditions is strongly dependent on temperature, the total carbon content of the fluid, and host-rock lithology. If crustal residence times following the mixing of high-temperature hydrothermal fluids with cool seawater are sufficiently long, reequilibration of aqueous carbon can result in the generation of additional reduced carbon species such as HCOOH and CH₃OH, and the consumption of H₂. The present study suggests that abiotic reactions involving aqueous carbon compounds in hydrothermal systems are sufficiently rapid to influence metabolic pathways utilized by organisms that inhabit vent environments.     

Can pelagic net heterotrophy account for carbon fluxes from eastern Canadian lakes?

Lakes worldwide are commonly oversaturated with CO₂, however the source of this CO₂ oversaturation is not well understood. To examine the magnitude of the C flux to the atmosphere and determine if an excess of respiration (R) over gross primary production (GPP) is sufficient to account for this C flux, metabolic parameters and stable isotopes of dissolved O₂ and C were measured in 23 Québec lakes. All of the lakes sampled were oversaturated with CO₂ over the sampling period, on average 221 ± 25%. However, little evidence was found to conclude that this CO₂ oversaturation was the result of an excess of pelagic R over GPP. In lakes Croche and à l’Ours, where CO₂ flux, R and GPP were measured weekly, the annual difference between pelagic GPP and R, or net primary production (NPP), was not sufficient to account for the size of the CO₂ flux to the atmosphere. In Lac Croche average annual NPP was 14.4 mg C m⁻² d⁻¹ while the average annual flux of CO₂ to the atmosphere was 34 mg C m⁻² d⁻¹. In Lac à l’Ours average annual NPP was −9.1 mg C m⁻² d⁻¹ while the average annual flux of CO₂ to the atmosphere was 55 mg C m⁻² d⁻¹. In all of the lakes sampled, O₂ saturation averaged 104.0 ± 1.7% during the ice-free season and the isotopic composition of dissolved O₂ (δ¹⁸O_DO) was 22.9 ± 0.3‰, lower than atmospheric values and indicative of net autotrophy. Carbon evasion was not a function of R, nor did the isotopic signature of dissolved CO₂ in the lakes present evidence of excess R over GPP. External inputs of C must therefore subsidize the lake to explain the continued CO₂ oversaturation. The isotopic composition of dissolved inorganic C (δ¹³C_DIC) indicates that the CO₂ oversaturation cannot be attributed to in situ aerobic respiration. δ¹³C_DIC reveals a source of excess C enriched in ¹³C, which may be accounted for by anaerobic sediment respiration or groundwater inputs followed by kinetic isotope fractionation during degassing under open system conditions.     

Chemical and isotopic constraints on water/rock interactions at the Lost City hydrothermal field, 30°N Mid-Atlantic Ridge

Low temperature vent fluids (<91 °C) issuing from the ultramafic-hosted hydrothermal system at Lost City, 30°N Mid-Atlantic Ridge, are enriched in dissolved volatiles (H₂,CH₄) while attaining elevated pH values, indicative of the serpentization processes that govern water/rock interactions deep in the oceanic crust. Here, we present a series of theoretical models to evaluate the extent of hydrothermal alteration and assess the effect of cooling on the systematics of pH-controlled B aqueous species. Peridotite-seawater equilibria calculations indicate that the mineral assemblage composed of diopside, brucite and chrysotile likely dictates fluid pH at moderate temperature serpentinization processes (<300 °C), by imposing constraints on the aCa⁺⁺/a²H⁺ ratios and the activity of dissolved SiO₂. Based on Sr abundances and the ⁸⁷Sr/⁸⁶Sr isotope ratios of vent fluids reported from Lost City, estimated water/rock mass ratios (w/r = 2-4) are consistent with published models involving dissolved CO₂ and alkane concentrations. Combining the reported δ¹⁸O values of vent fluids (0.7‰) with such w/r mass ratios, allows us to bracket subseafloor reaction temperatures in the vicinity of 250 °C. These estimates are in agreement with previous theoretical studies supporting extensive conductive heat loss within the upflow zones. Experimental studies on peridotite-seawater alteration suggest that fluid pH increases during cooling which then rapidly enhances boron removal from solution and incorporation into secondary phases, providing an explanation for the highly depleted dissolved boron concentrations measured in the low temperature but alkaline Lost City vent fluids. Finally, to account for the depleted ¹¹B composition (δ¹¹B ∼25-30‰) of vent fluids relative to seawater, isotopic fractionation between tetrahedrally coordinated aqueous boron species with BO₃-bearing mineral sites (e.g. in calcite, brucite) is proposed.     

The noble gas geochemistry of natural CO2 gas reservoirs from the Colorado Plateau and Rocky Mountain provinces, USA

Identification of the source of CO₂ in natural reservoirs and development of physical models to account for the migration and interaction of this CO₂ with the groundwater is essential for developing a quantitative understanding of the long term storage potential of CO₂ in the subsurface. We present the results of 57 noble gas determinations in CO₂ rich fields (>82%) from three natural reservoirs to the east of the Colorado Plateau uplift province, USA (Bravo Dome, NM., Sheep Mountain, CO. and McCallum Dome, CO.), and from two reservoirs from within the uplift area (St. John’s Dome, AZ., and McElmo Dome, CO.). We demonstrate that all fields have CO₂/³He ratios consistent with a dominantly magmatic source. The most recent volcanics in the province date from 8 to 10 ka and are associated with the Bravo Dome field. The oldest magmatic activity dates from 42 to 70 Ma and is associated with the McElmo Dome field, located in the tectonically stable centre of the Colorado Plateau: CO₂ can be stored within the subsurface on a millennia timescale.The manner and extent of contact of the CO₂ phase with the groundwater system is a critical parameter in using these systems as natural analogues for geological storage of anthropogenic CO₂. We show that coherent fractionation of groundwater ²⁰Ne/³⁶Ar with crustal radiogenic noble gases (⁴He, ²¹Ne, ⁴⁰Ar) is explained by a two stage re-dissolution model: Stage 1: Magmatic CO₂ injection into the groundwater system strips dissolved air-derived noble gases (ASW) and accumulated crustal/radiogenic noble gas by CO₂/water phase partitioning. The CO₂ containing the groundwater stripped gases provides the first reservoir fluid charge. Subsequent charges of CO₂ provide no more ASW or crustal noble gases, and serve only to dilute the original ASW and crustal noble gas rich CO₂. Reservoir scale preservation of concentration gradients in ASW-derived noble gases thus provide CO₂ filling direction. This is seen in the Bravo Dome and St. John’s Dome fields. Stage 2: The noble gases re-dissolve into any available gas stripped groundwater. This is modeled as a Rayleigh distillation process and enables us to quantify for each sample: (1) the volume of groundwater originally ‘stripped’ on reservoir filling; and (2) the volume of groundwater involved in subsequent interaction. The original water volume that is gas stripped varies from as low as 0.0005 cm³ groundwater/cm³ gas (STP) in one Bravo Dome sample, to 2.56 cm³ groundwater/cm³ gas (STP) in a St. John’s Dome sample. Subsequent gas/groundwater equilibration varies within all fields, each showing a similar range, from zero to ∼100 cm³ water/cm³ gas (at reservoir pressure and temperature).     

Geochemistry of hydrothermal fluids from the PACMANUS, Northeast Pual and Vienna Woods hydrothermal fields, Manus Basin, Papua New Guinea

Processes controlling the composition of seafloor hydrothermal fluids in silicic back-arc or near-arc crustal settings remain poorly constrained despite growing evidence for extensive magmatic-hydrothermal activity in such environments. We conducted a survey of vent fluid compositions from two contrasting sites in the Manus back-arc basin, Papua New Guinea, to examine the influence of variations in host rock composition and magmatic inputs (both a function of arc proximity) on hydrothermal fluid chemistry. Fluid samples were collected from felsic-hosted hydrothermal vent fields located on Pual Ridge (PACMANUS and Northeast (NE) Pual) near the active New Britain Arc and a basalt-hosted vent field (Vienna Woods) located farther from the arc on the Manus Spreading Center. Vienna Woods fluids were characterized by relatively uniform endmember temperatures (273-285 °C) and major element compositions, low dissolved CO₂ concentrations (4.4 mmol/kg) and high measured pH (4.2-4.9 at 25 °C). Temperatures and compositions were highly variable at PACMANUS/NE Pual and a large, newly discovered vent area (Fenway) was observed to be vigorously venting boiling (358 °C) fluid. All PACMANUS fluids are characterized by negative δD_H2O values, in contrast to positive values at Vienna Woods, suggesting substantial magmatic water input to circulating fluids at Pual Ridge. Low measured pH (25 °C) values (∼2.6-2.7), high endmember CO₂ (up to 274 mmol/kg) and negative δ³⁴S_H2S values (down to −2.7‰) in some vent fluids are also consistent with degassing of acid-volatile species from evolved magma. Dissolved CO₂ at PACMANUS is more enriched in ¹³C (−4.1‰ to −2.3‰) than Vienna Woods (−5.2‰ to −5.7‰), suggesting a contribution of slab-derived carbon. The mobile elements (e.g. Li, K, Rb, Cs and B) are also greatly enriched in PACMANUS fluids reflecting increased abundances in the crust there relative to the Manus Spreading Center. Variations in alkali and dissolved gas abundances with Cl at PACMANUS and NE Pual suggest that phase separation has affected fluid chemistry despite the low temperatures of many vents. In further contrast to Vienna Woods, substantial modification of PACMANUS/NE Pual fluids has taken place as a result of seawater ingress into the upflow zone. Consistently high measured Mg concentrations as well as trends of increasingly non-conservative SO₄ behavior, decreasing endmember Ca/Cl and Sr/Cl ratios with increased Mg indicate extensive subsurface anhydrite deposition is occurring as a result of subsurface seawater entrainment. Decreased pH and endmember Fe/Mn ratios in higher Mg fluids indicate that the associated mixing/cooling gives rise to sulfide deposition and secondary acidity production. Several low temperature (?80 °C) fluids at PACMANUS/NE Pual also show evidence for anhydrite dissolution and water-rock interaction (fixation of B) subsequent to seawater entrainment. Hence, the evolution of fluid compositions at Pual Ridge reflects the cumulative effects of water/rock interaction, admixing and reaction of fluids exsolved from silicic magma, phase separation/segregation and seawater ingress into upflow zones.     

Silicate weathering in anoxic marine sediments

Two sediment cores retrieved at the northern slope of Sakhalin Island, Sea of Okhotsk, were analyzed for biogenic opal, organic carbon, carbonate, sulfur, major element concentrations, mineral contents, and dissolved substances including nutrients, sulfate, methane, major cations, humic substances, and total alkalinity. Down-core trends in mineral abundance suggest that plagioclase feldspars and other reactive silicate phases (olivine, pyroxene, volcanic ash) are transformed into smectite in the methanogenic sediment sections. The element ratios Na/Al, Mg/Al, and Ca/Al in the solid phase decrease with sediment depth indicating a loss of mobile cations with depth and producing a significant down-core increase in the chemical index of alteration. Pore waters separated from the sediment cores are highly enriched in dissolved magnesium, total alkalinity, humic substances, and boron. The high contents of dissolved organic carbon in the deeper methanogenic sediment sections (50-150 mg dm⁻³) may promote the dissolution of silicate phases through complexation of Al³⁺ and other structure-building cations. A non-steady state transport-reaction model was developed and applied to evaluate the down-core trends observed in the solid and dissolved phases. Dissolved Mg and total alkalinity were used to track the in-situ rates of marine silicate weathering since thermodynamic equilibrium calculations showed that these tracers are not affected by ion exchange processes with sediment surfaces. The modeling showed that silicate weathering is limited to the deeper methanogenic sediment section whereas reverse weathering was the dominant process in the overlying surface sediments. Depth-integrated rates of marine silicate weathering in methanogenic sediments derived from the model (81.4-99.2 mmol CO₂ m⁻² year⁻¹) are lower than the marine weathering rates calculated from the solid phase data (198-245 mmol CO₂ m⁻² year⁻¹) suggesting a decrease in marine weathering over time. The production of CO₂ through reverse weathering in surface sediments (4.22-15.0 mmol CO₂ m⁻² year⁻¹) is about one order of magnitude smaller than the weathering-induced CO₂ consumption in the underlying sediments. The evaluation of pore water data from other continental margin sites shows that silicate weathering is a common process in methanogenic sediments. The global rate of CO₂ consumption through marine silicate weathering estimated here as 5-20 Tmol CO₂ year⁻¹ is as high as the global rate of continental silicate weathering.     

Evaluating the gas content of coals and isolated maceral concentrates from the Paleocene Guasare Coalfield,Venezuela

This work presents the results from evaluating the gases sorbed by coal samples extracted from the Paleocene Guasare Coalfield (Marcelina Formation, northwestern Venezuela), as well as by their distinct maceral concentrates. The aim of this work has been to obtain an initial experimental main value of the gas content per unit weight of high volatile bituminous A coal samples from the open-pit Paso Diablo mine. An additional goal was to study differences in the CH₄ storage ability of the distinct maceral groups forming part of the coal matrix. Both the coal samples and the maceral concentrates were studied by thermogravimetric analysis (TGA) in order to determine the temperature to be used in subsequent experiments. On-line analyses of hydrocarbons (C₁, C₂, C₃) and CO₂ yielded gas concentrations, plus δ¹³C values. Thermogenic gas is prevalent in the Guasare coals with vitrinite reflectance (%R_o) values from 0.65% to 0.88%. The amount of gas retained in the coals and maceral concentrates was measured with a special device that allows determination of the volume of gas sorbed by a solid sample subjected to controlled thermal treatment. The average coalbed gas concentration obtained was 0.51 cm³/g. The following list of maceral concentrates shows the relative capacity for the volume of sorbed gas per unit weight: inertinite > low-density vitrinite > liptinite ≈ high-density vitrinite. It is concluded that the gas volumes retained in the distinct maceral concentrates are not controlled by porosity but rather by their microscopic morphology.     

Raman spectroscopic measurements of CO2 density: Experimental calibration with high-pressure optical cell (HPOC) and fused silica capillary capsule (FSCC) with application to fluid inclusion observations

Raman spectroscopy is a powerful method for the determination of CO₂ densities in fluid inclusions, especially for those with small size and/or low fluid density. The relationship between CO₂ Fermi diad split (Δ, cm⁻¹) and CO₂ density (ρ, g/cm³) has been documented by several previous studies. However, significant discrepancies exist among these studies mainly because of inconsistent calibration procedures and lack of measurements for CO₂ fluids having densities between 0.21 and 0.75 g/cm³, where liquid and vapor phases coexist near room temperature.In this study, a high-pressure optical cell and fused silica capillary capsules were used to prepare pure CO₂ samples with densities between 0.0472 and 1.0060 g/cm³. The measured CO₂ Fermi diad splits were calibrated with two well established Raman bands of benzonitrile at 1192.6 and 1598.9 cm⁻¹. The relationship between the CO₂ Fermi diad split and density can be represented by: ρ = 47513.64243 − 1374.824414 × Δ + 13.25586152 × Δ² − 0.04258891551 × Δ³ (r² = 0.99835, σ = 0.0253 g/cm³), and this relationship was tested by synthetic fluid inclusions and natural CO₂-rich fluid inclusions. The effects of temperature and the presence of H₂O and CH₄ on this relationship were also examined.     

Calcite dissolution kinetics in the system CaCO3-H2O-CO2 at high undersaturation

Dissolution rates of limestone covered by a water film open to a CO₂-containing atmosphere are controlled by the chemical composition of the CaCO₃-H₂O-CO₂ solution at the water-mineral interface. This composition is determined by the Ca²⁺-concentration at this boundary, conversion of CO₂ into H⁺ and in the solution, and by diffusional mass transport of the dissolved species from and towards the water-limestone interface. A system of coupled diffusion-reaction equations for Ca²⁺, , and CO₂ is derived. The Ca²⁺ flux rates at the surface of the mineral are defined by the PWP-empirical rate law. These flux rates by the rules of stoichiometry must be equal to the flux rates of CO₂ across the air-water interface. In the solution, CO₂ is converted into H⁺ and . At low water-film thickness this reaction becomes rate limiting. The time dependent diffusion-reaction equations are solved for free drift dissolution by a finite-difference scheme, to obtain the dissolution rate of calcite as a function of the average calcium concentration in the water film. Dissolution rates are obtained for high undersaturation. The results reveal two regimes of linear dissolution kinetics, which can be described by a rate law F = α_i(m_ic_eq − c), where c is the calcium concentration in the water film, c_eq the equilibrium concentration with respect to calcite. For index i = 0, a fast rate law, which here is reported for the first time, is found with α₀ = 3 × 10⁻⁶ m s⁻¹ and m₀ = 0.3. For c > m₀c_eq, a slow rate law is valid with α₁ = 3 × 10⁻⁷ m  s⁻¹ and m₁ = 1, which confirms earlier work. The numbers given above are valid for film thickness of several tenths of a millimetre and at 20 °C. These rates are proven experimentally, using a flat inclined limestone plate covered by a laminar flowing water film injected at an input point with known flow rate Q and calcium concentration. From the concentration measured after flow distance x the dissolution rates are determined. These experiments have been performed at a carbon-dioxide pressure of 0.00035 atm and also of 0.01 atm. The results are in good agreement to the theoretical predictions.     

Abundance of mass 47 CO2 in urban air, car exhaust, and human breath

Atmospheric carbon dioxide is widely studied using records of CO₂ mixing ratio, δ¹³C and δ¹⁸O. However, the number and variability of sources and sinks prevents these alone from uniquely defining the budget. Carbon dioxide having a mass of 47 u (principally ¹³C¹⁸O¹⁶O) provides an additional constraint. In particular, the mass 47 anomaly (Δ₄₇) can distinguish between CO₂ produced by high temperature combustion processes vs. low temperature respiratory processes. Δ₄₇ is defined as the abundance of mass 47 isotopologues in excess of that expected for a random distribution of isotopes, where random distribution means that the abundance of an isotopologue is the product of abundances of the isotopes it is composed of and is calculated based on the measured ¹³C and ¹⁸O values. In this study, we estimate the δ¹³C (vs. VPDB), δ¹⁸O (vs. VSMOW), δ47, and Δ₄₇ values of CO₂ from car exhaust and from human breath, by constructing ‘Keeling plots’ using samples that are mixtures of ambient air and CO₂ from these sources. δ47 is defined as , where is the R⁴⁷ value for a hypothetical CO₂ whose δ¹³C_VPDB = 0, δ¹⁸O_VSMOW = 0, and Δ₄₇ = 0. Ambient air in Pasadena, CA, where this study was conducted, varied in [CO₂] from 383 to 404 μmol mol⁻¹, in δ¹³C and δ¹⁸O from −9.2 to −10.2‰ and from 40.6 to 41.9‰, respectively, in δ47 from 32.5 to 33.9‰, and in Δ₄₇ from 0.73 to 0.96‰. Air sampled at varying distances from a car exhaust pipe was enriched in a combustion source having a composition, as determined by a ‘Keeling plot’ intercept, of −24.4 ± 0.2‰ for δ¹³C (similar to the δ¹³C of local gasoline), δ¹⁸O of 29.9 ± 0.4‰, δ47 of 6.6 ± 0.6‰, and Δ₄₇ of 0.41 ± 0.03‰. Both δ¹⁸O and Δ₄₇ values of the car exhaust end-member are consistent with that expected for thermodynamic equilibrium at∼200 °C between CO₂ and water generated by combustion of gasoline-air mixtures. Samples of CO₂ from human breath were found to have δ¹³C and δ¹⁸O values broadly similar to those of car exhaust-air mixtures, −22.3 ± 0.2 and 34.3 ± 0.3‰, respectively, and δ47 of 13.4 ± 0.4‰. Δ₄₇ in human breath was 0.76  ± 0.03‰, similar to that of ambient Pasadena air and higher than that of the car exhaust signature.     

Stable isotopic evidence in support of active microbial methane cycling in low-temperature diffuse flow vents at 9°50′N East Pacific Rise

A unique dataset from paired low- and high-temperature vents at 9°50′N East Pacific Rise provides insight into the microbiological activity in low-temperature diffuse fluids. The stable carbon isotopic composition of CH₄ and CO₂ in 9°50′N hydrothermal fluids indicates microbial methane production, perhaps coupled with microbial methane consumption. Diffuse fluids are depleted in ¹³C by ∼10‰ in values of δ¹³C of CH₄, and by ∼0.55‰ in values of δ¹³C of CO₂, relative to the values of the high-temperature source fluid (δ¹³C of CH₄ =−20.1 ± 1.2‰, δ¹³C of CO₂ =−4.08 ± 0.15‰). Mixing of seawater or thermogenic sources cannot account for the depletions in ¹³C of both CH₄ and CO₂ at diffuse vents relative to adjacent high-temperature vents. The substrate utilization and ¹³C fractionation associated with the microbiological processes of methanogenesis and methane oxidation can explain observed steady-state CH₄ and CO₂ concentrations and carbon isotopic compositions. A mass-isotope numerical box model of these paired vent systems is consistent with the hypothesis that microbial methane cycling is active at diffuse vents at 9°50′N. The detectable ¹³C modification of fluid geochemistry by microbial metabolisms may provide a useful tool for detecting active methanogenesis.