Dunk R. M.

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Dunk
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R. M.
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  • Article
    In situ Raman analyses of deep-sea hydrothermal and cold seep systems (Gorda Ridge and Hydrate Ridge)
    (American Geophysical Union, 2006-05-25) White, Sheri N. ; Dunk, R. M. ; Peltzer, Edward T. ; Freeman, J. J. ; Brewer, Peter G.
    The Deep Ocean Raman In Situ Spectrometer (DORISS) instrument was deployed at the Sea Cliff Hydrothermal Field and Hydrate Ridge in July 2004. The first in situ Raman spectra of hydrothermal minerals, fluids, and bacterial mats were obtained. These spectra were analyzed and compared to laboratory Raman measurements of standards and samples collected from the site. Spectra of vent fluid (∼294°C at the orifice) at ∼2700 m depth were collected with noncontact and immersion sampling optics. Compared to spectra of ambient (∼2°C) seawater, the vent fluid spectra show changes in the intensity and positions of the water O-H stretch bands due to the elevated temperature. The sulfate band observed in seawater spectra is reduced in vent fluid spectra as sulfate is removed from vent fluid in the subseafloor. Additional components of hydrothermal fluid are present in concentrations too low to be detected with the current Raman system. A precision underwater positioner (PUP) was used to focus the laser spot on opaque samples such as minerals and bacterial mats. Spectra were obtained of anhydrite from actively venting chimneys, and of barite deposits in hydrothermal crusts. Laboratory analysis of rock samples collected in the vent field also detected the presence of gypsum. Spectra of bacterial mats revealed the presence of elemental sulfur (S8) and the carotenoid beta-carotene. Challenges encountered include strong fluorescence from minerals and organics and insufficient sensitivity of the instrument. The next generation DORISS instrument addresses some of these challenges and holds great potential for use in deep-sea vent environments.
  • Preprint
    Gas hydrate measurements at Hydrate Ridge using Raman spectroscopy
    ( 2006-11-14) Hester, K. C. ; Dunk, R. M. ; White, Sheri N. ; Brewer, Peter G. ; Peltzer, Edward T. ; Sloan, E. D.
    Oceanic gas hydrates have been measured near the seafloor for the first time using a seagoing Raman spectrometer at Hydrate Ridge, Oregon, where extensive layers of hydrates have been found to occur near the seafloor. All of the hydrates analyzed were liberated from the upper meter of the sediment column near active gas venting sites in water depths of 770-780 m. Hydrate properties, such as structure and composition, were measured with significantly less disturbance to the sample than would be realized with core recovery. The natural hydrates measured were sI, with methane as the predominant guest component, and minor/trace amounts of hydrogen sulfide present in three of the twelve samples measured. Methane large-to-small cage occupancy ratios of the hydrates varied from 1.01 to 1.30, in good agreement with measurements of laboratory synthesized and recovered natural hydrates. Although the samples visually appeared to be solid, varying quantities of free methane gas were detected, indicating the presence of occluded gas a hydrate bubble fabric and/or partial hydrate dissociation in the under-saturated seawater.