Keith C. Hester

In situ Raman‐based measurements of high dissolved methane concentrations in hydrate‐rich ocean sediments

Ocean sediment dissolved CH4 concentrations are of interest for possible climate-driven venting from sea floor hydrate decomposition, for supporting the large-scale microbial anaerobic oxidation of CH4 that holds the oceanic CH4 budget in balance, and for environmental issues of the oil and gas industry. Analyses of CH4 from recovered cores near vent locations typically show a maximum of similar to 1 mM, close to the 1 atmosphere equilibrium value. We show from novel in situ measurement with a Raman-based probe that geochemically coherent profiles of dissolved CH4 occur rising to 30 mM (pCH(4) = 3 MPa) or an excess pressure similar to 3x greater than CO2 in a bottle of champagne. Normalization of the CH4 Raman nu(1) peak to the ubiquitous water nu(2) bending peak provides a fundamental internal calibration. Very large losses of CH4 and fractions of other gases (CO2, H2S) must typically occur from recovered cores at gas rich sites. The new data are consistent with observations of microbial biomass and observed CH4 oxidation rates at hydrate rich sites and support estimates of a greatly expanded near surface oceanic pore water CH4 reservoir. Citation: Zhang, X., K. C. Hester, W. Ussler, P. M. Walz, E. T. Peltzer, and P. G. Brewer (2011), In situ Raman-based measurements of high dissolved methane concentrations in hydrate-rich ocean sediments, Geophys. Res. Lett., 38, L08605, doi: 10.1029/2011GL047141.

Creating Controlled CO 2 Perturbation Experiments on the Seafloor - Development of FOCE Techniques

Experimental recent progress on the design and testing of systems for carrying out controlled CO₂ perturbation experiments on the sea floor with the goal of simulating the conditions of a future high CO₂ world. Controlled CO₂ enrichment (FACE) experiments have long been carried out on land to investigate the effects of elevated atmospheric CO₂ levels on vegetation, but only limited work on CO₂ enrichment on enclosed systems has yet been carried out in the ocean. With rising concern over the impacts of ocean acidification on marine life there is a need for greatly improved techniques for carrying out in situ experiments, which can create a DeltapH of 0.3 to 0.5 by addition of CO₂, on natural ecosystems such as coral reefs, cold water corals, and other sensitive benthic habitats. This is no easy task. Unlike land based experiments where simple mixing in air is all that is required, CO₂ has complex chemistry in seawater with significantly slow reaction kinetics. Scientists must design systems to take this into account. The net result of adding a small quantity of CO₂ to sea water is to reduce the concentration of dissolved carbonate ion, and increase bicarbonate ion through the following reaction:CO₂+H₂O+CO₃^2- -> 2HCO₃ In practice the reaction between CO₂ and H₂O is slow and is a complex function of temperature, pH, and TCO₂, with the reaction proceeding more rapidly at lower pH and higher temperatures. Marine animals in the natural ocean will typically experience only small and temporary shifts from environmental CO₂ equilibrium. Valid perturbation experiments must try to expose an experimental region to a stable lower pH condition, and avoid large and rapid pH variability. The most common sensor used for experimental control is the pH electrode, and this detects only H+ ion, not any of the dissolved CO₂ species. We first explored the reaction kinetics of a CO₂ perturbation in a series of closed loop pH cell experiments carried out at various depths under ROV control. These were found to be well matched to the Zeebe & Wolf-Gladrow [1] model. From these results, functions for the delay time required for equilibrium were devised and a design for a delay loop to achieve at least 2 e-folding times between CO₂ injection and animal exposure was developed. We tested this prototype system in October 2007 in a series of ROV controlled experiments at a depth of 1000 meters. The working fluid used for enrichment was surface sea water saturated at one atmosphere with pure CO₂ gas to create a solution of about pH 4.8 and 56 mM total CO₂. This was carried to depth in a 56 liter piston accumulator, and dispensed as needed into a flexible polyethylene bag for subsequent addition into the experimental unit. The design consisted of a 4 meter delay loop leading to a control volume (square box, 25 cm per side) outfitted with three pH electrodes and a CTD. To determine the uniformity of the pH, two pH electrodes were positioned in the control volume and a third electrode was positioned just beyond the control volume in the flow stream. Ambient seawater, pumped at a desired rate with a modified thruster, was mixed at the beginning of the delay loop with controlled continuous injection of the CO₂-rich working fluid in a ratio typically of about 2001 depending on the pH perturbation desired. For these initial tests, a feed-forward system was used where flow rates of both the ambient seawater and CO₂-rich seawater were varied to produce a desired pH change. Future designs will incorporate a feedback loop to allow for automated precision pH control. These field tests were successful in showing that a plume of lower pH seawater could be accurately created and maintained in the deep ocean. The pH was reduced by up to 0.9 pH units from the ambient value of 7.8 covering well beyond the range of projected ocean pH scenarios for the next century. Near future goals will involve use of the MARS undersea cable recently deployed in Monterey Bay, California for power, communication and control, and a long-term experiment will be performed to demonstrate the operational feasibility of this technology for ocean acidification studies worldwide.pH electrodes and a CTD. To determine the uniformity of the pH, two pH electrodes were positioned in the control volume and a third electrode was positioned just beyond the control volume in the flow stream. Ambient seawater, pumped at a desired rate with a modified thruster, was mixed at the beginning of the delay loop with controlled continuous injection of the CO2-rich working fluid in a ratio typically of about 200:1 depending on the pH perturbation desired. For these initial tests, a feed-forward system was used where flow rates of both the ambient seawater and CO2-rich seawater were varied to produce a desired pH change. Future designs will incorporate a feedback loop to allow for automated precision pH control. These field tests were successful in showing that a plume of lower pH seawater could be accurately created and maintained in the deep ocean. The pH was reduced by up to 0.9 pH units from the ambient value of 7.8 covering well beyond the range of projected ocean pH scenarios for the next century. Near future goals will involve use of the MARS undersea cable recently deployed in Monterey Bay, California for power, communication and control, and a long-term experiment will be performed to demonstrate the operational feasibility of this technology for ocean acidification studies worldwide.

HYDRATE NUCLEATION MEASUREMENTS USING HIGH PRESSURE DIFFERENTIAL SCANNING CALORIMETRY

Understanding when hydrates will nucleate has notable importance in the area of flow assurance. Attempts to model hydrate formation in subsea pipelines currently requires an arbitrary assignment of a nucleation subcooling. Previous studies showed that sII hydrate containing a model water-soluble former, tetrahydrofuran, would nucleate over a narrow temperature range of a few degrees with constant cooling. It is desirable to know if gas phase hydrate formers, which are typically more hydrophobic and hence have a very low solubility in water, also exhibit this nucleation behavior. In this study, differential scanning calorimetry has been applied to determine the hydrate nucleation point for gas phase hydrate formers. Constant cooling ramps and isothermal approaches were combined to explore the probability of hydrate nucleation. In the temperature ramping experiments, methane and xenon were used at various pressures and cooling rates. In both systems, hydrate nucleation occurred over a narrow temperature range (2-3°C). Using methane at lower pressures, ice nucleated before hydrate; whereas at higher pressures, hydrate formed first. A subcooling driving force of around 30°C was necessary for hydrate nucleation from both guest molecules. The cooling rates (0.5-3°C/min) did not show any statistically significant effect on the nucleation temperature for a given pressure. The isothermal method was used for a methane system with pure water and a water-in-West African crude emulsion. Two isotherms (-5 and -10°C) were used to determine nucleation time. In both systems, the time required for nucleation decreased with increased subcooling.

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.