Noriko Nakayama

Deep ocean experiments with fossil fuel carbon dioxide: Creation and sensing of a controlled plume at 4 km depth

The rapidly rising levels of atmospheric and oceanic CO2 from the burning of fossil fuels has lead to well-established international concerns over dangerous anthropogenic interference with climate. Disposal of captured fossil fuel CO2 either underground, or in the deep ocean, has been suggested as one means of ameliorating this problem. While the basic thermodynamic properties of both CO2 and seawater are well known, the problem of interaction of the two fluids in motion to create a plume of high CO2/low pH seawater has been modeled, but not tested. We describe here a novel experiment designed to initiate study of this problem. We constructed a small flume, which was deployed on the sea floor at 4 km depth by a remotely operated vehicle, and filled with liquid CO2. Seawater flow was forced across the surface by means of a controllable thruster. Obtaining quantitative data on the plume created proved to be challenging. We observed and sensed the interface and boundary layers, the formation of a solid hydrate, and the low pH/high CO2 plume created, with both pH and conductivity sensors placed downstream. Local disequilibrium in the CO2 system components was observed due to the finite hydration reaction rate, so that the pH sensors closest to the source only detected a fraction of the CO2 emitted. The free CO2 molecules were detected through the decrease in conductivity observed, and the disequilibrium was confirmed through trapping a sample in a flow cell and observing an unusually rapid drop in pH to an equilibrium value.

Nitrogen, oxygen and argon dissolved in the northern north Pacific in early summer

Major gases dissolved in seawater were accurately determined with a shipboard gas chromatographic method. The standard deviations were 0.28, 0.34 and 0.36% for N2, O2 and Ar, respectively. The method was applied to water from the northwestern North Pacific Ocean collected in May to June 2000. We got 127 duplicate seawater samples from the surface 200 m layer at 11 stations. The O2 concentrations obtained by this method agreed with those given by the Winkler method. All the seawater samples from the surface 200 m, especially those from the upper 30 m, were supersaturated with respect to atmospheric N2 and Ar concentrations. In the topmost 30 m layer, the degrees of supersaturation in the inventory were 2.7–4.3% for N2 (ΔN2) and 1.7–2.6% for Ar (ΔAr), and their ratios, ΔN2/ΔAr, ranged from 1.53 to 1.81. This supersaturation seems to be chiefly due to air bubbles injected into the water and dissolved due to the water pressure, because the N2/Ar ratio of the air is around 2. The amounts of air bubbles dissolved in the upper 30 m water were relatively large, with mean value of 0.41 ml/kg or 18.4 μmol/kg. The ΔN2, ΔAr and ΔN2/ΔAr values were all positively well correlated with the wind velocities averaged for the last 24 hours prior to sampling, allowing the conclusion to be drawn that the weaker the wind velocity, the dissolved gas composition approaches in equilibrium with the air; while the stronger the wind velocity, it approaches in the air composition.

Difference in O2 and CO2 gas transfer velocities in Funka Bay

At a station in Funka Bay (92 m deep), regular observations were made almost once a month for 16 months; and the water column concentrations of dissolved oxygen (DO), phosphate, total carbonate and the δ 13 C of CO 2 , as well as some auxiliary components, were measured. Funka Bay has a characteristic water exchange which occurs twice a year and is otherwise limited. This enabled us to apply a closed-system model for the water, except during the flushing periods, to obtain gas transfer velocities from the changes in concentrations. The TC budget corrected for biological production and decomposition showed an increase in water column content, due to atmospheric CO 2 absorbed into the bay water at the rate of 7 mol m -2 /year or more. This result was in agreement with the observation that the fugacity, f(CO 2 ). of surface seawater was always lower than the atmospheric f(CO 2 ), by about 200 ppm, especially in April. By applying the closed-system model, we obtained a mean gas transfer velocity for O 2 of 3.3 ± 1.6 m/day in summer and 7.4 ± 4.3 m/day in winter. The gas transfer velocity of CO 2 from carbonate was larger than that of CO 2 converted from O 2 . The mean ratio of the gas transfer velocity of CO 2 obtained from carbonate to that of CO 2 from O 2 was 1.76 ± 0.34. The transfer velocity of CO 2 estimated from the δ 13 C budget was even larger; its ratio to that of CO 2 from O 2 was 2.8 ± 1.3, although the velocity was obtained for only one period in winter. This order of increase in the gas transfer velocities coincides with the increase in the times necessary for gas exchange equilibration at the surface. This suggests the importance of bubbles or air masses being taken into deeper water.

Ocean abyssal carbon experiments at 0.7 and 4 KM depth

Publisher Summary The chapter reviews and compares observations from the two experiments performed in 2003. The main focus is on the way in which CO 2 behaves and is transported away from the site in realistic deep-sea conditions where hydrates form and where CO 2 is exposed to sediments. The synthesis of observations is then used as a basis for discussing and presenting best estimates of the fate of larger quantities of CO 2 placed on the seafloor. The chapter also discusses observations from small-scale CO 2 experiments conducted off the coast of California at 684 m depth and at 3942 m depth. In both experiments, when the seawater velocity was sufficiently strong, parcels of liquid CO 2 were torn off and transported away as discrete units by the turbulent water current. In the deep experiment, newly formed frazil hydrate was observed at the interface, occasionally including sediment particles. Hydrate furthermore collected and created a floating consolidated solid in the downstream end of the trough, dissolving slowly from one day to the next. These observations have important implications for understanding and modeling of larger scale disposal at the seafloor. When CO 2 is released by the interfacial instability mechanism driven by strong currents, the seawater density increase due to dissolution of CO 2 may not have time to act and stabilize the water column before the discrete parcels of liquid phase CO 2 are advected away from the disposal site. The floating solid that formed at the interface is hypothesized to consist of hydrate and additional trapped seawater. Its appearance was not expected in advance and its role in delaying dissolution cannot be determined from the present experimental set-up. A capability for long-term seafloor perturbation experiments is deemed to be crucial both for direct ocean-storage research and for studying effects of invasion of anthropogenic CO 2 from the atmosphere.

Chemical Weapons on the Sea Floor: A Plea for Complete Information

Ocean scientists have so far paid scant attention to the numerous sites and large quantities of chemical weapons (CW) disposed of on the sea floor, but the time for this inattention may now be past. In recent work we have shown that these sites are not well mapped, that there are no standards for marking or identification, and that unplanned scientific encroachment is beginning to occur. The time is ripe for a new scientific assessment [1] to accommodate safely the changing and intensifying modes of scientific exploration and uses of the ocean, and to assess the effects on marine life. In the years following World War II very large quantities of CW were disposed of in the ocean at numerous sites in the Atlantic, Pacific and Arctic Oceans, and in the Baltic and Adriatic Seas. Navigation inaccuracy from the dead-reckoning or celestial procedures available at that time, poor record keeping, and unauthorized disposal activities all contributed to confusion surrounding locations, quantities, and types of material. Thus navigation charts today do not present an adequate description of the affected areas. Offshore California some 12,000 km2 of sea floor are designated on US charts as CW disposal sites - yet an area of perhaps only 10% of this is likely of concern. In stark contrast US charts of waters offshore Japan show no CW sites - yet there are numerous known locations, often in shallow waters. There simply are no international standards or data bases available on which ocean scientists can plan expeditions. One example of an unplanned interaction of a scientific expedition with a suspected CW site came in 1992. The US Ocean Drilling Program drilled site 889 on the Cascadia Margin to examine gas hydrates, with site selection based on seismic surveys. Only at a late date was it discovered that this location was within 10 nm of a weapons disposal site, and a prohibition on sampling the uppermost 20 m of sediment was quickly put in place. Ocean hydrographic sampling lines often inadvertently transect CW site locations. The principal mode of decomposition of the primary agents is through hydrolysis, but the rates are usually estimated at normal oxygenated sea water pH of about 8. The lower pH (about 7) of typically anoxic sediment pore waters, encountered as materiel becomes buried, will greatly slow the reaction. These types of geochemical interactions should be considered in estimating the contaminant halo and lifetime. Here we highlight newly developed tools, such as in situ Raman spectroscopy, which can be used to remotely measure these materials and their breakdown products.