Constraining bubble dynamics and mixing with dissolved gases: Implications for productivity measurements by oxygen mass balance

Abstract

We used a dynamic mixed layer model to determine carbon export by the oxygen mass balance method from a time series of O 2 /Ar, N 2 /Ar and Ne measurements collected at station ALOHA near Hawaii from July 2000 to June 2001. The inert gas measurements constrain the flux of oxygen into the mixed layer from small, collapsing bubbles (injection) to be greater than or equal to the flux from larger bubbles (exchange), with mean estimates of the ratio in the range of 1-2. We also show that monthly observations of temperature and inert gases cannot constrain the rate of diapycnal mixing at this location, because of uncertainties in air-sea heat flux estimates and bubble dynamics. Organic carbon export from the mixed layer calculated from our dataset was 1.1 ± 0.5 mol C m −2 yr −1 , with most of the error deriving from uncertainties in the parameterization of diffusive gas exchange with wind speed. Our estimates of carbon export from the zone beneath the mixed layer but still in the euphotic zone ranged from 0 to 0.6 mol C m −2 yr −1 as the rate of background diapycnal mixing was increased from 0.1 to 1.0 cm 2 s −1 . We conclude that the oxygen mass balance method has errors of about a factor of two in areas similar to the subtropical North Pacific, with the main uncertainties deriving from mixing rates and the parameterization of diffusive gas exchange.

A determination of air-sea gas exchange and upper ocean biological production from five noble gases and tritiugenic helium-3

“…Additionally, the air injection flux can change dramatically with the wind speed (given the cubic dependence) and thus comparing estimates of air injection at different locations and times is difficult. The O 2 bubble flux estimated by Hamme and Emerson (2006) The partitioning between complete and partial trapping we calculate here is within the range of that predicted by Hamme and Emerson (2006). We found that in the winter, completely and partially trapped bubbles lead to significant contributions in the saturation anomalies of even soluble gases, suggesting that air injection may be more important than previously thought for soluble gases, such as CO 2 .…”

“…In previous seawater studies, Ar has been used in conjunction with O 2 to estimate biological production (Craig and Hayward, 1987;Spitzer and Jenkins, 1989). Time-series of He, Ne and Ar measurements in seawater have been used to investigate air-sea gas exchange (Spitzer and Jenkins, 1989) as have time-series of Ne, Ar, and N 2 (Hamme and Emerson, 2006). Helium measurements in seawater have been used to investigate escape mechanisms for the exosphere (Bieri et al, 1967).…”

“…In seawater studies, noble gas measurements have been used to investigate air-sea gas exchange (Spitzer and Jenkins, 1989;Emerson et al, 1995;Hamme and Emerson, 2006), biological production (Jenkins and Goldman, 1985;Craig and Hayward, 1987), diapycnal mixing (Henning et al, 2006;Ito and Deutsch, 2006), and escape mechanisms for the exosphere (Bieri et al, 1967). Extending the measurements beyond the three noble gases (He, Ne, and Ar) used in previous work allows one to constrain air-sea gas exchange parameters to levels significantly better than can be obtained when only three of the noble gases are used (Stanley et al, 2006).…”

“…It is difficult to estimate the total air injection flux from theoretical treatment of bubbles (Woolf and Thorpe, 1991;Woolf, 1993;Keeling, 1993) because of uncertainties in bubble size spectra, bubble dynamics, and depth distribution. A few parameterizations of bubble processes have been formulated (Asher and Wanninkhof, 1998;Zhang et al, 2006;Hamme and Emerson, 2006), allowing estimation of bubble flux given fraction of white-cap coverage. These formulations have large uncertainties and in many cases do not distinguish between bubbles that have been completely dissolved, and therefore inject atmospheric abundances, versus bubbles that only partially dissolve before rising to the surface, thus fractionating the gases (Fuchs et al, 1987;Jenkins, 1988a).…”

“…Spitzer and Jenkins (1989) used a timeseries of He, Ne, and Ar in order to estimate the rate of diffusive gas exchange and air injection. More recently, Hamme and Emerson (2006) used a time series of Ne, Ar, and N 2 to investigate the partitioning of completely trapped vs. partially dissolved bubbles. The study presented here expands on previous work by including measurements of Kr and Xe.…”