Determination of gas bubble fractionation rates in the deep ocean by laser Raman spectroscopy

Author Posting. © The Authors, 2004. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Marine Chemistry 99 (2006): 12-23, doi:10.1016/j.marchem.2004.10.006.A new deep-sea laser Raman spectrometer (DORISS – Deep Ocean Raman In Situ Spectrometer) is used to observe the preferential dissolution of CO2 into seawater from a 50%-50% CO2-N2 gas mixture in a set of experiments that test a proposed method of CO2 sequestration in the deep ocean. In a first set of experiments performed at 300 m depth, an open-bottomed 1000 cm3 cube was used to contain the gas mixture; and in a second set of experiments a 2.5 cm3 funnel was used to hold a bubble of the gas mixture in front of the sampling optic. By observing the changing ratios of the CO2 and N2 Raman bands we were able to determine the gas flux and the mass transfer coefficient at 300 m depth and compare them to theoretical calculations for air-sea gas exchange. Although each experiment had a different configuration, comparable results were obtained. As expected, the ratio of CO2 to N2 drops off at an exponential rate as CO2 is preferentially dissolved in seawater. In fitting the data with theoretical gas flux calculations, the boundary layer thickness was determined to be ~42 μm for the gas cube, and ~165 μm for the gas funnel reflecting different boundary layer turbulence. The mass transfer coefficients for CO2 are kL = 2.82 x 10- 5 m/s for the gas cube experiment, and kL = 7.98 x 10- 6 m/s for the gas funnel experiment.Funding was provided by a grant to MBARI from the David and Lucile Packard Foundation, and by the U.S. Dept. of Energy Ocean Carbon Sequestration Program (Grants No. DE-FC26-00NT40929 and DE-FC03-01ER6305)

CAN HYDRATE DISSOLUTION EXPERIMENTS PREDICT THE FATE OF A NATURAL HYDRATE SYSTEM?

Here, we present a dissolution study of exposed hydrate from outcrops at Barkley Canyon. Previously, a field experiment on synthetic methane hydrate samples showed that mass transfer controlled dissolution in under-saturated seawater. However, seafloor hydrate outcrops have been shown to have significant longevity compared to expected dissolution rates based upon convective boundary layer diffusion calculations. To help resolve this apparent disconnect between the dissolution rates of synthetic and natural hydrate, an in situ dissolution experiment was performed on two distinct natural hydrate fabrics. A hydrate mound at Barkley Canyon was observed to contain a “yellow” hydrate fabric overlying a “white” hydrate fabric. The yellow hydrate fabric was associated with a light condensate phase and was hard to core. The white hydrate fabric was more porous and relatively easier to core. Cores from both fabrics were inserted to a mesh chamber within a few meters of the hydrate mound. Time-lapse photography monitored the dissolution of the hydrate cores over a two day period. The diameter shrinkage rate for the yellow hydrate was 45.5 nm/s corresponding to a retreat rate of 0.7 m/yr for an exposed surface. The white hydrate dissolved faster at 67.7 nm/s yielding a retreat rate of 1.1 m/yr. It is possible these hydrate mounds were exposed due to the fishing trawler incident in 2001. If these dissolution experiments give a correct simulation, then the exposed faces should have retreated ~ 3.5 m and 5.5 m, respectively, from 2001 to this expedition in August 2006. While the appearance of the hydrate mounds appeared quite similar to photographs taken in 2002, these dissolution experiments show natural hydrate dissolves rapidly in ambient seawater. The natural hydrate dissolution rate is on the same order as the synthetic dissolution experiment strongly implying another control for the dissolution rates of natural hydrate outcrops. Several factors could contribute to the apparent longevity of these exposed mounds from upward flux of methane-rich fluid to protective bacterial coatings.Non UBCUnreviewe

The coral proto free ocean carbon enrichment system (CP-FOCE): Engineering and development

Ocean acidification is driven by increasing atmospheric CO and represents a key threat to the Great Barrier Reef (GBR) and other coral reefs globally. Previous investigations have depended on studies in aquaria that are compromised by reduced ecological complexity and buffering capacity, and problems associated with containment. These aquaria studies also include artifacts such as artificial flow, light, temperature, and water quality conditions. In order to avoid these issues a new technology was needed for in situ science. This need was the driver behind development of the Free Ocean Carbon Enrichment (FOCE) approach. FOCE is similar in approach to the Free Air Carbon Enrichment (FACE) experiments pursued on land for almost two decades. FOCE as a systems concept was developed at the Monterey Bay Aquarium Research Institute (MBARI) to perform controlled in situ studies on the effects of increased carbon dioxide on ocean environments. FOCE systems inject carbon dioxide enriched water into the desired control volume to lower the environmental pH to a specified value. The challenge of maintaining reef conditions while manipulating the carbonate chemistry further advanced the FOCE concept. A shallow water reef version of FOCE was needed to perform this research at the University of Queensland. Working with MBARI the University of Queensland developed the Coral Proto - Free Ocean Carbon Dioxide Enrichment (CP - FOCE) system. Although the CP-FOCE does not differ conceptually from the original FOCE it is different in a couple of respects. First, it requires that a region of the coral flat be semi-enclosed in the chamber section of CP-FOCE. This allows the required amount of CO to be optimised. Second, by closing the enclosure off fully for a short time, the oxygen levels and carbonate chemistry can be accurately measured to determine net production/respiration as well as the calcification/dissolution rates of the organisms living within the chamber. In this paper we present the engineering details of the CP-FOCE system design. This paper details the unique engineering design and challenges of the CP-FOCE system The paper briefly outlines the chemical and biological requirements that provided the technical specifications for CP-FOCE to successfully study the impacts of the changing water chemistry on the physiology of calcareous reef organisms including corals and calcareous algae. We have also a brief outline of the methods used to perform measurements of calcification and dissolution rates. Additionally, we include discussion on production and respiration rates in CP-FOCE systems when maintained at ambient and two different increased pCO scenarios. We present technical results of this first deployment and address future plans for modifications and deployments of CP-FOCE. Forthcoming peer reviewed papers will describe the biological, chemical, and geochemical responses