Bemis Karen G.

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Bemis
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Karen G.
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  • Article
    The relative effect of particles and turbulence on acoustic scattering from deep sea hydrothermal vent plumes revisited
    (Acoustical Society of America, 2017-03-03) Xu, Guangyu ; Jackson, Darrell R. ; Bemis, Karen G.
    The relative importance of suspended particles and turbulence as backscattering mechanisms within a hydrothermal plume located on the Endeavour Segment of the Juan de Fuca Ridge is determined by comparing acoustic backscatter measured by the Cabled Observatory Vent Imaging Sonar (COVIS) with model calculations based on in situ samples of particles suspended within the plume. Analysis of plume samples yields estimates of the mass concentration and size distribution of particles, which are used to quantify their contribution to acoustic backscatter. The result shows negligible effects of plume particles on acoustic backscatter within the initial 10-m rise of the plume. This suggests turbulence-induced temperature fluctuations are the dominant backscattering mechanism within lower levels of the plume. Furthermore, inversion of the observed acoustic backscatter for the standard deviation of temperature within the plume yields a reasonable match with the in situ temperature measurements made by a conductivity-temperature-depth instrument. This finding shows that turbulence-induced temperature fluctuations are the dominant backscattering mechanism and demonstrates the potential of using acoustic backscatter as a remote-sensing tool to measure the temperature variability within a hydrothermal plume.
  • Article
    A preliminary 1-D model investigation of tidal variations of temperature and chlorinity at the Grotto mound, Endeavour Segment, Juan de Fuca Ridge
    (John Wiley & Sons, 2017-01-18) Xu, Guangyu ; Larson, Benjamin I. ; Bemis, Karen G. ; Lilley, Marvin D.
    Tidal oscillations of venting temperature and chlorinity have been observed in the long-term time series data recorded by the Benthic and Resistivity Sensors (BARS) at the Grotto mound on the Juan de Fuca Ridge. In this study, we use a one-dimensional two-layer poroelastic model to conduct a preliminary investigation of three hypothetical scenarios in which seafloor tidal loading can modulate the venting temperature and chlorinity at Grotto through the mechanisms of subsurface tidal mixing and/or subsurface tidal pumping. For the first scenario, our results demonstrate that it is unlikely for subsurface tidal mixing to cause coupled tidal oscillations in venting temperature and chlorinity of the observed amplitudes. For the second scenario, the model results suggest that it is plausible that the tidal oscillations in venting temperature and chlorinity are decoupled with the former caused by subsurface tidal pumping and the latter caused by subsurface tidal mixing, although the mixing depth is not well constrained. For the third scenario, our results suggest that it is plausible for subsurface tidal pumping to cause coupled tidal oscillations in venting temperature and chlorinity. In this case, the observed tidal phase lag between venting temperature and chlorinity is close to the poroelastic model prediction if brine storage occurs throughout the upflow zone under the premise that layers 2A and 2B have similar crustal permeabilities. However, the predicted phase lag is poorly constrained if brine storage is limited to layer 2B as would be expected when its crustal permeability is much smaller than that of layer 2A.
  • Thesis
    Geothermal heat flux from hydrothermal plumes on the Juan de Fuca ridge
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1990-05) Bemis, Karen G.
    Estimates of the heat output of hydrothermal vents, identified along the Endeavor and Southern Segments of the Juan de Fuca Ridge, are used to evaluate the total heat flux associated with hydrothermal circulation for the ridge segment. An array carried by D/V ALVIN sampled the temperature and velocity structure of hydrothermal plumes from individual vents . The maximum heat flux calculated for a single vent is 50 MW, but the average vent output is only 13 MW per vent for 31 vents. The estimates for any given vent may vary over an order of magnitude. This uncertainty is due mainly to the difficulty of locating the centerline of the plume relative to the point of measurement, although the uncertainty in determining the constants from the appropriate equations based on laboratory experiments contributes a significant share to the net error. For the Endeavor Segment, the minimum total geothermal heat flux due to hydrothermal circulation exceeds 70 MW. The minimum estimate for the Southern Segment is 16 MW. The maximum estimate is probably closer to the total heat flux (236 MW and 66 MW respectively) . The estimated heat flux density is 3300 W/m2 for the Endeavor vent field and 39 W/m2 for the Southern vent field. Focused hydrothermal venting accounts for only a small fraction of the heat available according to steady-state predictions of conductive heat flux; however, other hydrothermal phenomena (e.g., diffuse flow) account for the greater share of the total hydrothermal heat flux.