Gas saturation in the oceans

Results of all the dual tracer experiments to date (Fig. 10.7) suggest a dependence on wind speed that falls between the two relations presented in Fig. 10.6. Using this compromise curve and global wind speed data to determine the mean ocean transfer velocity, one derives a global Ggoo value of 4.1m d (Nightingale et al, 2000), which is certainly within the error of the values determined by both natural and bomb uptake (Ggoo = 4.4 1.2 m d and 3.5 1.2 moU, respectively). 

Gas transfer velocity, G oo. a function of v ind speed, L/ o, for all dual tracer experiments. N is from the North Sea, G is from the Georges Bank and F from the Florida Shelf. Lines are the same as those described in the caption to Fig. 10.6, v ith the addition of the one in the middle, which is the best fit through the dual tracer data (Gtoo = 0.23 xU +0.1 xU (Sc/600) where U is in m s and G is in cm h ). Redrawn from Nightingale et al. (2000). 

Measured oxygen concentrations in ocean surface waters as a function of temperature from the WOCE data base. The line marks the saturation value with atmospheric oxygen at a salinity of 35. 

Most readers who have seen the surface of the ocean during a moderate wind have noticed that breaking waves introduce lots of small air bubbles into the water (Fig. 10.9). The hydrostatic pressure only 1 m below the ocean surface is already equal to about 10% of the entire atmospheric pressure, so the pressure inside a bubble is c.110% of that at the ocean surface. Thus, there is a strong tendency for bubbles entrained in the downwelling limb of a breaking wave to lose some or all of their gas to the surrounding fluid by diffusion across the bubble surface. This causes the surface waters to be supersaturated with respect to saturation equilibrium. We define the degree of supersaturation, A %), as 

To account for the process of bubble-induced gas exchange we modify the gas transfer equation to include both the transfer at the air-water interface, Fawi (Eq. (10.1)), and the flux caused by bubbles, Fb. The total flux, Fj, is now 

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Craig, H., Weiss, R. F. (1971) Dissolved gas saturation anomalies and excess helium in the ocean. Earth Planet. Sci. Lett., 10, 289-96. 

The temperature dependences of the Henry s Law coefficients of the different gases listed in Table 3.6 are quite variable (Fig. 3.11). Helium, the least soluble noble gas, has very little solubility temperature dependence between 0 and 30 °C. On the other hand, Kr, the second most soluble of the non-radioactive noble gases, is much less soluble at higher temperatures. More details about gas solubilities are presented in the chapter on air-sea gas exchange (Chapter 10). Another notable aspect of the temperature dependence of the gas solubilities is that they are not linear. Thus, mixing between parcels of water of different temperatures at saturation equilibrium with the atmosphere results in a mixture that is supersaturated. This effect has been observed for noble gases in the ocean and may ultimately have a utility as a tracer of mixing across density horizons. 

The first term on the right side represents the interface exchange that is the product of a gas transfer velocity, G, and the concentration difference between that measured in the ocean s surface, [A ], and that expected at saturation equihhrium,... 

Craig H, Lupton JE (1981) Helium-3 and mantle volatiles in the ocean and the oceanic cmst. In The Sea, Vol. 7. C Emiliani (ed) John Wiley Sons, New York, p 391-428 Craig H, Weiss RF (1971) Dissolved gas saturation anomalies and excess helium in the ocean. Earth Planet Sci Lett 10 289-296... 

Gas hydrate forms wherever appropriate physical conditions exist—moderately low temperature and moderately high pressure—and the materials are present—gas near saturation and water. These conditions are found in the deep sea commonly at water depths greater than about 500 m or somewhat shallower depths (about 300 m) in the Arctic, where bottom-water temperature is colder. Gas hydrate also occurs beneath permafrost on land in arctic conditions, but, by far, most natural gas hydrate is stored in ocean floor deposits. A simplified phase diagram is shown in Fig. 2A, in which pressure has been converted to water depth in the ocean (thus, pressure increases downward in the diagram). The heavy line in Fig. 2A is the phase boundary, separating conditions in the temperature/pressure field where methane hydrate is stable to the left of the curve (hatched area) from conditions where it is not. In Fig. 2B, some typical conditions of pressure and temperature in the deep ocean were chosen to define the region where methane hydrate is stable. The phase boundary indicated is the same as in Fig. 2A, so methane hydrate is stable... 

The dawn of the modern era of terrestrial noble gas studies can be traced to a suggestion by Suess and Wanke (1965) that the ocean floor should be characterized by a steady-state loss of helium equal to its production in the mantle. Although they measured a 6% excess helium saturation anomaly in Pacific deep water, doubts were... 

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