Takashi Ohsumi

The Lake Nyos gas disaster : Chemical and isotopic evidence in waters and dissolved gases from three Cameroonian crater lakes, Nyos, Monoun and Wum.

Abstract To better understand the cause of the Nyos gas disaster of August 21, 1986, we conducted geochemical and limnological surveys in October 1986, of three lakes (Nyos, Monoun and Wum) which are located in the Cameroon volcanic zone that is characterized by a prevalence of young alkaline basalts and basanitoids. Lake Wum was studied as a non-active control: CO2 is dissolved in significant concentrations (about 1 5 of saturation) in gas-active lakes (Nyos and Monoun), but is virtually absent in Lake Wum. Stable isotopic ratios of total dissolved carbon (δ13C= −3% for Nyos and −5.5% for Monoun) and of helium (5.7 Ratm for Nyos and 3.6 Ratm for Monoun) indicate a mantle origin of these gases. However, SO42− and Cl concentrations are found to be very low. Concentrations of dissolved chemical species like Fe2+, Mg2+, Ca2+, and HCO3− are high in the two gas-active lakes, whereas they are very low in the gas-inactive lake. High salinities in the gas-active lakes are probably due to dissolution of indigenous mafic rocks and transported lateritic soil in acidic CO2-rich, warm water. The gas-active lakes are characterized by increasing temperature and salinity with increasing depth, indicating an active influx of heat and dissolved materials at the bottom. Density estimates show that the lake water is stably stratified in spite of the inverse temperature profile of the lakes, on account of dissolved chemical species. The concentrations of dissolved carbonate species (CO2(aq) and HCO3−) are positively correlated with those of ionic dissolved species, indicating their common occurrence in the bottom water. The August 1986 gas bursts from Lake Nyos were most likely caused by rapid exsolution of dissolved CO2 within the lake; an explosive process such as a phreatic eruption or a CO2 gas-jetting from beneath the bottom is unlikely because of low concentrations of Cl− and SO42−, no oxygen isotopic shift, low turbidity, and no reported perturbation of the bottom sediments. Exsolution of CO2 bubbles could occur if CO2-saturated bottom water was displaced upwards by an increased influx of high salinity water from the bottom during the rainy season. Exsolution of CO2 at the upper layers was possibly accelerated by upwelling of a two-phase fluid (CO2 bubbles and solution), a mechanism known as a pneumatic lift pump, resulting in discharge of a large amount of CO2 gas. The H2S concentration in the gas cloud must have been kept far below the lethal level because of a high Fe2+ concentration of the lake water.

Density change of water due to dissolution of carbon dioxide and near-field behavior of CO2 from a source on deep-sea floor

Abstract CO 2 recovery from the flue gases of fossil energy power sources followed by ocean disposal should be carefully examined in terms of engineering and ecological constraints. For detailed assessment of ocean floor disposal options, the prediction of solute CO 2 behavior in the bottom ocean current is important for evaluating the degree of possible damage by the CO 2 to the benthic ecosystem. A water coexisting with liquid CO 2 in a 35-ml cell (at 3 °C, 343 atm) was sampled and introduced into a vibrating-tube type densitometer. After the density measurement the sample was expanded into a buret system, where the CO 2 can be determined volumetrically. Density increase of 0.00284 g/cm 3 per 1 wt.% CO 2 dissolution was observed, which corresponds to the CO 2 partial molar volume of 31.0 cm 3 /mole. A preliminary computer simulation experiment of CO 2 behavior around “CO 2 lake” formed on the deep ocean floor was conducted under the condition as follows: bottom water current; 5 cm/s, thickness of benthic boundary layer; 100 m, vertical eddy diffusivity in the benthic boundary layer; 100 cm 2 /s. The density flow regime was remarkably observed near the “CO 2 lake”.

Prediction of solute carbon dioxide behavior around a liquid carbon dioxide pool on deep ocean basins

Abstract The geochemical consideratino on the source and sink terms of a prediction model describing the behavior of the solute CO 2 within the benthic boundary layer is given for the liquid CO 2 injection into ocean basins greater than 3 km in depth. The solute CO 2 source term into the benthic boundary water mass should be understood in view of the interfacial phenomena over the “liquid CO 2 pool”. The sink of the solute CO 2 exists at the top of the sediment, where carbonaceous ooze dissolves as CaCO 3 + CO 2 + H 2 O -> Ca 2+ + 2HCO 3 − , so to acts as a neutralizer bed at the bottom of benthic boundary layer. The existing laboratory data of the carbonate dissolution experiments revealed that under low pH and high pCO 2 conditions the following reaction mechanisms also prevail to enhance the dissolution rate: CaCO 3 + H + -> Ca 2+ HCO 3 − , and CaCO 3 + H 2 CO 3 -> Ca 2+ + 2HCO 3 − . Thus, the near-field (high concentration of solute CO 2 ) enhancement of the carbonate dissolution might play an important role to mitigate the hazardous impact on the benthic biota around the “liquid CO 2 pool”.

Dispersion of CO2 injected into the ocean at the intemediate depth

Abstract The biological impact on the ambient marine environment should be assessed in the CO 2 injection at the intermediate depth. The long term dispersion (several decades) will also have a potential impact on the marine ecosystem in the wide area. The velocity field and dispersion of the substance in the North Pacific ocean at the intermediate depth were computed using an OGCM, and discussed with the comparison with the observed temperature, salinity and CFCs profiles. A model was applied to estimate the diffusion of the artificially injected CO 2 .

LABORATORY MEASUREMENTS OF SEISMIC WAVE VELOCITY BY CO2 INJECTION IN TWO POROUS SANDSTONES

Publisher Summary This chapter presents a preliminary result of measurements on velocity changes while injecting CO2 into water-saturated Shirahama and Tako sandstone. Wave velocity and attenuation in porous sandstone are widely studied in fields of reservoir engineering and geo-engineering. Seismic survey provides substantial information concerning positions for new wells and modification of the existing depletion strategy. Cross-well seismic tomography is considered as a promising monitoring method to map the movement of CO2 in the subsurface. The formation water, which existed in pore spaces within reservoir rocks, will be partially displaced by the injected CO2. This process will affect the propagation characteristics of the seismic waves. Seismic properties depend on the mineralogical composition of the rock as well as factors such as porosity, fluid content, and in situ stress. Previous works on effects of CO2 flooding on seismic wave velocity clearly show that CO2 flooding caused compressional wave (P-wave) velocities to substantially decrease. Interpretation of seismic monitoring of CO2 flooding requires an understanding of the effects of pore pressure buildup caused by the CO2 injection and CO2 saturation. Experimental studies, such as converting field measurements of wave velocities and attenuations to CO2 saturation, support the interpretation of the survey results. A series of seismic tomography experiments on porous sandstone samples to demonstrate the use of cross-well seismic profiling for monitoring the migration of CO2 in geological sequestration projects have been conducted.

A numerical study with an eddy-resolving model to evaluate chronic impacts in CO2 ocean sequestration

Abstract To evaluate chronic impacts of CO 2 ocean sequestration, we simulated the distribution of injected CO 2 using an oceanic general circulation model (OGCM) with a horizontal resolution of 0.1°. The model can explicitly express transport and dispersion of dissolved CO 2 by mesoscale eddies. The CO 2 which is continuously injected by a moving ship dissolves and accumulates within the first several to 10 years, but the CO 2 concentration has an upper limit after its initial increase as a result of the dilution effect of mesoscale eddies which counterbalances the accumulation effect of injection. We can estimate the CO 2 injection flux with the CO 2 maximum concentration below the “Predicted No Effect Concentration” (PNEC), an index to estimate concentration causing no effects on biota.

A lagrangian method combined with high-resolution ocean general circulation model to evaluate CO2 ocean sequestration

Publisher Summary A high-resolution model with a Lagrangian method is used to simulate the distribution and concentration of CO 2 injected into the mid-depth ocean over timescales ranging from a week to a few years. A high-resolution model with a Lagrangian tracer enables effects of eddy activities on dispersion of particles to be well represented. Comparison of Lagrangian with Eulerian tracers shows that the Lagrangian tracer avoids artificial diffusion and enables CO 2 maximum concentration at specific sites to be predicted, which helps in assessing CO 2 impact on the marine ecosystem. Ensemble experiments using high-resolution models explicitly deal with dispersion by advection due to mesoscale eddies which cannot be achieved by coarse-resolution models with implicit eddy diffusion, and enable the distribution of CO 2 maximum concentrations to be predicted. A high resolution model is demonstrated with this method is a powerful tool for assessing the effects of CO 2 injection on the marine environment.

The Second Phase of Japanese R&D Program for CO2 Ocean Sequestration

Publisher Summary The CO2 Ocean Sequestration Project in Japan was started as a national project in 1997. The implementation of this project has been entrusted to two organizations by the New Energy and Industrial Technology Development Organization (NEDO). The Research Institute of Innovative Technology for the Earth (RITE) was appointed to one of these key roles and has been working on RD the lethal CO2 concentration limit value will be identified for the mid-depth plankton of the site by the method developed in the first phase of the NEDO R&D program, and will then used in the food-web based assessment model of the site. The opportunity of the ocean experiment is open to the international partners, and the verified dilution factor attainable by moving-ship concept, coupled with the established assessment methodology, will be submitted to the international forum to obtain the legal acceptance of this technology as a measure of CO2 emission reduction.

Development of Environmental Assessment Technique for CO2 Ocean Sequestration

Increasing atmospheric concentrations of greenhouse gases are suspected of causing a gradual warming of the Earth’s surface and potentially disastrous changes to global climate. Because CO2 is a major greenhouse gas, CO2 ocean sequestration is being explored as one possible option to limit the accumulation of greenhouse gases in the atmosphere. In the case of CO2 ocean sequestration, an assessment of environmental impacts to the ocean is the most important issue. Sequestration research requires a development of new cost-effective observation techniques to monitor dilution and diffusion of the sequestrated CO2 . We developed an in-situ pH/pCO2 sensor, a tracking neutral buoy system and a towing multi-layer monitoring system to observe the pH change, water movement and diffusion at a mid-depth of the ocean as an environmental assessment technique for CO2 ocean sequestration.Copyright

Field study on CO2 fixation by serpentinite rock-bed

Publisher Summary The sequestration of atmospheric CO2 could be an effective means of mitigating global warming. The groundwater and surface water associated with serpentinite shows alkaline conditions and is associated with natural carbonate minerals. A technology to sequester atmospheric CO2 could be based on the reactions forming these carbonate minerals. Ca-carbonate and Mg-hydrocarbonate associated with the serpentinite in the Kamuikotan metamorphic belt, Hokkaido, northern Japan, precipitate around alkaline springs on landslide deposits, on the surface and in fractures in weathered serpentinite. The carbonates yield 14C ages from 22,700 yBP to modern. These carbonates sometimes co-exist with serpentine mousse that is white to pale brown in color. The relationship between δ 18O and δ D values of the serpentine is similar to that of deweylite, which is a serpentine formed at low temperature. The temperature of oxygen isotopic equilibrium between co-existing deweylite and water is approximately 16°C corresponding to the annual average atmospheric temperature in Hokkaido. The relationships of δ 18O among deweylite, water, and carbonate suggest low-temperature formation of the carbonate. From these facts, it can be concluded that the alkaline water associated with serpentinite reacts with atmospheric CO2 under surface conditions and has continued from at least 22,700 yBP to the present. In addition, the system of serpentinite and groundwater could continue to keep itself alkaline and carbonate stable safely for a long period.