Cahill Kevin L.

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Cahill
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Kevin L.
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  • Article
    Gas fluxes and steady state saturation anomalies at very high wind speeds
    (American Geophysical Union, 2022-09-25) Stanley, Rachel H. R. ; Kinjo, Lumi ; Smith, Andrew W. ; Aldrett, Danielle ; Alt, Helene ; Kopp, Emily ; Krevanko, Callan ; Cahill, Kevin ; Haus, Brian K.
    Gas exchange at high wind speeds is not well understood—few studies have been conducted at wind speeds above 20 ms−1 and significant disagreement exists between gas exchange models at high wind speeds. In this study, noble gases (He, Ne, Ar, Kr, and Xe) were measured in 35 experiments in the SUSTAIN wind‐wave tank where the wind speeds ranged from 20 to 50 m s−1 and mechanical waves were generated as monochromatic or with a short‐crested JONSWAP frequency spectrum. Bubble size spectra were determined using shadowgraph imagery and wave statistics were measured using a wave wire array. The steady state saturation anomalies and gas fluxes initially increased as wind speeds increased but then leveled off, similar to prior studies of heat and momentum flux coefficients. Noble gas fluxes and steady state saturation anomalies are correlated most strongly with bubble volumes for the less soluble noble gases and with wind speed and wave Reynolds number for the more soluble noble gases. In the JONSWAP experiments, significant wave height was the most important predictor for gas steady state saturation anomalies with correlation coefficients of greater than 0.92 for He, Ne, and Ar (P < 0.05). Furthermore, invasion fluxes were larger than evasion fluxes when other conditions were similar. Taken together, these lab‐based experiments suggest more attention should be paid to parameterizations based on wave characteristics and bubbles and that current wind‐speed based gas exchange parameterizations should not be applied to conditions with very high wind speeds.
  • Preprint
    The deep distributions of helium isotopes, radiocarbon, and noble gases along the U.S. GEOTRACES East Pacific Zonal Transect (GP16)
    ( 2017-03) Jenkins, William J. ; Lott, Dempsey E. ; German, Christopher R. ; Cahill, Kevin L. ; Goudreau, Joanne ; Longworth, Brett E.
    We report the deep distributions of noble gases, helium isotopes, and radiocarbon measured during the U.S. GEOTRACES GP16 East Pacific Zonal Transect between 152 and 77°W at 12- 15°S in the South Pacific. The dominant feature is an intense tongue of hydrothermal effluent that extends more than 4,000 km westward from the East Pacific Rise (EPR) at ~2500m depth. The patterns reveal significant “downstream” variations in water mass structure, advection, and mixing that belie the simple perception of a continuous plume extending westward from the EPR. For example, one feature observed at 120°W, 14°S has tracer signatures that are consistent with a water mass originating from an area as much as 2,000 km south of this section, suggesting a quasi-permanent northward flow on the western flank of the EPR. Helium isotope variations in the plume show a uniquely high 3He/4He source in the tongue compared with typical mid-ocean ridge basalts (MORB), consistent with the anomalously high ratios observed in MORB glasses from the EPR segment just south of this transect. The water column data also reveal that the background 3He/4He east of the EPR is significantly lower than values characteristic of MORB, suggesting an additional, more geographically distributed radiogenic 4He flux of order 107 mol/y into the deep Pacific. In the western end of the section, incoming bottom waters have relatively less hydrothermal hydrothermal helium, more radiocarbon, and more oxygen, as well as negative saturation anomalies for the heavy noble gases (Ar, Kr, and Xe). During the basin-scale upwelling of this water, diapycnal mixing serves to erase these negative anomalies. The relative magnitudes of the increases for the heavy noble gases (Ar, Kr, and Xe) are quantitatively consistent with this process. This leads us to estimate the relatively smaller effects on He and Ne saturations, which range from near zero to 0.2% and 0.3% respectively. With this information, we are able to refine our estimates of the magnitude of 3He and 4He excesses and the absolute 3He/4He ratio of non-atmospheric helium introduced into deep Pacific waters.
  • Article
    Using noble gases to compare parameterizations of air‐water gas exchange and to constrain oxygen losses by ebullition in a shallow aquatic environment
    (John Wiley & Sons, 2018-09-07) Howard, Evan M. ; Forbrich, Inke ; Giblin, Anne E. ; Lott, Dempsey E. ; Cahill, Kevin L. ; Stanley, Rachel H. R.
    Accurate determination of air‐water gas exchange fluxes is critically important for calculating ecosystem metabolism rates from dissolved oxygen in shallow aquatic environments. We present a unique data set of the noble gases neon, argon, krypton, and xenon in a salt marsh pond to demonstrate how the dissolved noble gases can be used to quantify gas transfer processes and evaluate gas exchange parameterizations in shallow, near‐shore environments. These noble gases are sensitive to a variety of physical processes, including bubbling. We thus additionally use this data set to demonstrate how dissolved noble gases can be used to assess the contribution of bubbling from the sediments (ebullition) to gas fluxes. We find that while literature gas exchange parameterizations do well in modeling more soluble gases, ebullition must be accounted for in order to correctly calculate fluxes of the lighter noble gases. In particular, for neon and argon, the ebullition flux is larger than the differences in the diffusive gas exchange flux estimated by four different wind speed‐based parameterizations for gas exchange. We present an application of noble gas derived ebullition rates to improve estimates of oxygen metabolic fluxes in this shallow pond environment. Up to 21% of daily net oxygen production by photosynthesis may be lost from the pond via ebullition during some periods of biologically and physically produced supersaturation. Ebullition could be an important flux of oxygen and other gases that is measurable with noble gases in other shallow aquatic environments.