White Sheri N.

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Sheri N.

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Now showing 1 - 8 of 8
  • Article
    Optical tools for ocean monitoring and research
    (Copernicus Publications on behalf of the European Geosciences Union, 2009-12-10) Moore, C. ; Barnard, Andrew H. ; Fietzek, P. ; Lewis, Marlon R. ; Sosik, Heidi M. ; White, Sheri N. ; Zielinski, Oliver
    Requirements for understanding the relationships between ocean color and suspended and dissolved materials within the water column, and a rapidly emerging photonics and materials technology base for performing optical based analytical techniques have generated a diverse offering of commercial sensors and research prototypes that perform optical measurements in water. Through inversion, these tools are now being used to determine a diverse set of related biogeochemical and physical parameters. Techniques engaged include measurement of the solar radiance distribution, absorption, scattering, stimulated fluorescence, flow cytometry, and various spectroscopy methods. Selective membranes and other techniques for material isolation further enhance specificity, leading to sensors for measurement of dissolved oxygen, methane, carbon dioxide, common nutrients and a variety of other parameters. Scientists are using these measurements to infer information related to an increasing set of parameters and wide range of applications over relevant scales in space and time.
  • Article
    Mineral phase analysis of deep-sea hydrothermal particulates by a Raman spectroscopy expert algorithm : toward autonomous in situ experimentation and exploration
    (American Geophysical Union, 2009-05-14) Breier, John A. ; German, Christopher R. ; White, Sheri N.
    This paper demonstrates that a Raman spectroscopy, point-counting technique can be used for phase analysis of minerals commonly found in deep-sea hydrothermal plumes, even for minerals with similar chemical compositions. It also presents our robust autonomous identification algorithm and spectral database, both of which were developed specifically for deep-sea hydrothermal studies. The Raman spectroscopy expert algorithm was developed and tested against multicomponent mixtures of minerals relevant to the deep-sea hydrothermal environment. It is intended for autonomous classification where many spectra must be examined with little or no human involvement to increase analytic precision, accuracy, and data volume or to enable in situ measurements and experimentation.
  • Preprint
    Determination of gas bubble fractionation rates in the deep ocean by laser Raman spectroscopy
    ( 2004-10-29) White, Sheri N. ; Brewer, Peter G. ; Peltzer, Edward T.
    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.
  • Thesis
    An investigation into the characteristics and sources of light emission at deep-sea hydrothermal vents
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2000-06) White, Sheri N.
    A spectral camera (ALISS - Ambient Light Imaging and Spectral System) was used to image ambient light from high-temperature vents at 9°N East Pacific Rise and the Juan de Fuca Ridge during 1997 and 1998 Alvin dive cruises. ALISS is a low-light digital camera with custom-designed optics. A set of nine lenses, each covered by an individual bandpass filter (50 and 100 nm nominal bandwidths), allows vents to be imaged in nine wavelength bands simultaneously spanning the range of 400-1 000 nm. Thus, both spatial and spectral information are obtained. ALISS was used to image three types of vents: black smokers, flange pools, and beehives. The primary source of light is thermal radiation due to the high temperature of the hydrothermal fluid (-350°C). This light is dominant at wavelengths greater than 700 nm. At flange pools, where the fluid is relatively stable, only thermal radiation is present. Black smokers and beehives, however, are subject to mixing with ambient seawater (2°C) leading to mineral precipitation. Data from these types of vents show the existence of non-thermal, temporally varying light in the 400-700 nm region. This light is probably caused by mechanisms related to mixing and precipitation, such as chemiluminescence, crystalloluminescence and triboluminescence.
  • 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
    Laser Raman spectroscopy as a technique for identification of seafloor hydrothermal and cold seep minerals
    ( 2008-11) White, Sheri N.
    In situ sensors capable of real-time measurements and analyses in the deep ocean are necessary to fulfill the potential created by the development of autonomous, deep-sea platforms such as autonomous and remotely operated vehicles, and cabled observatories. Laser Raman spectroscopy (a type of vibrational spectroscopy) is an optical technique that is capable of in situ molecular identification of minerals in the deep ocean. The goals of this work are to determine the characteristic spectral bands and relative Raman scattering strength of hydrothermally- and cold seep-relevant minerals, and to determine how the quality of the spectra are affected by changes in excitation wavelength and sampling optics. The information learned from this work will lead to the development of new, smaller sea-going Raman instruments that are optimized to analyze minerals in the deep ocean. Many minerals of interest at seafloor hydrothermal and cold seep sites are Raman active, such as elemental sulfur, carbonates, sulfates and sulfides. Elemental S8 sulfur is a strong Raman scatterer with dominant bands at ~219 and 472 Δcm-1. The Raman spectra of carbonates (such as the polymorphs calcite and aragonite) are dominated by vibrations within the carbonate ion with a primary band at ~1085 Δcm-1. The positions of minor Raman bands differentiate these polymorphs. Likewise, the Raman spectra of sulfates (such as anhydrite, gypsum and barite) are dominated by the vibration of the sulfate ion with a primary band around 1000 Δcm-1 (~1017 for anhydrite, ~1008 for gypsum, and ~988 for barite). Sulfides (pyrite, marcasite, chalcopyrite, isocubanite, sphalerite, and wurtzite) are weaker Raman scatters than carbonate and sulfate minerals. They have distinctive Raman bands in the ~300-500 Δcm-1 region. Raman spectra from these mineral species are very consistent in band position and normalized band intensity. High quality Raman spectra are obtained from all of these minerals using both green and red excitation lasers, and using a variety of sampling optics. The highest quality spectra (highest signal to noise) were obtained using green excitation (532 nm Nd:YAG laser) and a sampling optic with a short depth of focus (and thus high power density). Significant fluorescence was not observed for the minerals analyzed using green excitation. Spectra were also collected from pieces of active and inactive hydrothermal chimneys, recovered from the Kilo Moana vent field in 2005 and 11ºN on the East Pacific Rise in 1988, respectively. Profiles of sample J2-137-1-r1-a show the transition from the chalcopyrite-rich “inner” wall to the sphalerite-dominated “outer” wall, and indicate the presence of minor amounts of anhydrite. Spectra collected from sample A2003-7-1a5 identify Cu-S tarnishes present on the surface of the sample.
  • 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.
  • Preprint
    Development and deployment of a precision underwater positioning system for in situ laser Raman spectroscopy in the deep ocean
    ( 2005-10-25) White, Sheri N. ; Kirkwood, William ; Sherman, Alana ; Brown, Mark ; Henthorn, Richard ; Salamy, Karen ; Walz, Peter ; Peltzer, Edward T. ; Brewer, Peter G.
    The field of ocean geochemistry has recently been expanded to include in situ laser Raman spectroscopic measurements in the deep ocean. While this technique has proved to be successful for transparent targets, such as fluids and gases, difficulty exists in using deep submergence vehicle manipulators to position and control the very small laser spot with respect to opaque samples of interest, such as many rocks, minerals, bacterial mats, and seafloor gas hydrates. We have developed, tested, and successfully deployed by remotely operated vehicle (ROV) a precision underwater positioner (PUP) which provides the stability and precision movement required to perform spectroscopic measurements using the Deep Ocean In Situ Spectrometer (DORISS) instrument on opaque targets in the deep ocean for geochemical research. The positioner is also adaptable to other sensors, such as electrodes, which require precise control and positioning on the seafloor. PUP is capable of translating the DORISS optical head with a precision of 0.1 mm in three dimensions over a range of at least 15 cm, at depths up to 4000 m, and under the normal range of oceanic conditions (T, P, current velocity). The positioner is controlled, and spectra are obtained, in real time via Ethernet by scientists aboard the surface vessel. This capability has allowed us to acquire high quality Raman spectra of targets such as rocks, shells, and gas hydrates on the seafloor, including the ability to scan the laser spot across a rock surface in sub-millimeter increments to identify the constituent mineral grains. These developments have greatly enhanced the ability to obtain in situ Raman spectra on the seafloor from an enormous range of specimens.