RATES OF OCEAN MIXING and DEEP WATER VENTILATION:
CONSTRAINTS FROM ATMOSPHERIC GAS CONCENTRATIONS
Steven Emerson, School of Oceanography, University of Washington, Seattle,
WA, USA
The concentrations of nitrogen (N2) and noble gases (He, Ne, Ar, Kr, and
Xe) are (nearly) chemically and biologically inert in the ocean; however, recent
accurate measurements indicate that they are not equal to the values predicted
by thermodynamic equilibrium with the atmosphere. Because of the temperature
dependence of atmospheric equilibrium, relatively slow air-water gas exchange
rates, and the nonlinearity of atmospheric equilibrium temperature dependence,
concentrations in the interior ocean depart from atmospheric saturation values.
These properties can be exploited to constrain rates of diapycnal (across density
surface) mixing in the ocean thermocline and deep water formation outcrop area.
One of the major uncertainties in oceanography is the extent to which the
thermocline is maintained by mixing along and across density surfaces. There
has long been a dichotomy between the rate of nutrient supply to the sun-lit
subtropical ocean required to maintain measured biological productivity and that
which can be supplied from depth by mixing rates derived from tracer release
experiments. For the same reason that turbulent mixing in the atmosphere
causes rain, gas supersaturation in the thermocline is created by mixing of
waters of different temperature. In the thermocline the degree of supersaturation
depends on the relative rates of diapycnal mixing and transport along density
surfaces. I use argon data from ocean transects in the North and South Pacific
in a simple model to constrain the rates of diapycnal mixing.
Another important uncertainty in oceanography is the degree to which interaction
between the atmosphere and ocean at high latitudes affects atmospheric pCO2
over geological time. One of the key unknowns in models of this process is the
effective area of the high latitude outcrop. Inert gas concentrations in the deep
ocean range from supersaturation to undersaturation with respect to the
atmosphere, depending on the physical chemistry of the gas. The reasons for
this have to do with rates of cooling, and atmosphere-ocean gas exchange
during deep water formation. With a suite of inert gas tracers one can sort out
the importance of these processes and evaluate the deep-water formation
outcrop area which constrains the sensitivity of atmospheric pCO2 to marine
high-latitude processes.