Inert Gases as Tracers of Diapycnal Mixing in the Upper Ocean

Steven Emerson, Chuck Stump and David Wilber

School of Oceanography, University of Washington, Seattle, WA 98195

Abstract

Determining the oxygen mass balance in the upper ocean requires evaluating two process that are difficult to quantify: the role of bubble processes in gas supersaturation and the rate of diapycnal mixing at the base of the winter mixed layer. Knowing the latter value is critical to determining the mechanism of nutrient supply to the euphotic zone. We are studying oxygen, argon and nitrogen gas mass balances at the Hawaii Ocean Time-series (HOT) Station ALOHA using highly accurate and precise measurements in a simple model of upper ocean circulation. We outline instrumental calibrations necessary for accurate determinations by gas- ratio mass spectrometry and precautions required to evaluate the saturation state including a redetermination of the nitrogen gas solubility.

The importance of diapycnal mixing in supplying gases to the upper ocean depends on the net annual flux across the air-water interface which is calculated from the heat-induced degree of supersaturation and a gas exchange mass transfer coefficient. To determine the degree of heat-induced supersaturation, one must evaluate the role of bubble processes in creating supersaturation. We present a sensitivity analysis that evaluates the bubble-induced degree of supersaturation using two end member models for bubble processes -- air injection by small bubble collapse and exchange across the surface of larger bubbles. The model results are compared with three annual cycles of argon and nitrogen measurements in the upper ocean at HOT. We conclude that the solublities of nitrogen and argon are not different enough and the degree of saturation is not great enough to accurately determine the mechanism of bubble processes. Hence, only bounds can be placed on the net annual degree of temperature-induced supersaturation and the calculated value of diapycnal mixing. Implications for evaluating the oxygen mass balance are discussed. One should be able to help resolve the role of different bubble mechanisms in the upper ocean with measurements of neon (or helium) along with nitrogen and argon because the lighter noble gases are more insoluble than nitrogen and the saturation state is less temperature dependent.