Rates of vertical mixing, gas exchange and new production: Estimates from seasonal gas cycles in the upper ocean near Bermuda
Abstract:Argon measurements, obtained from one year of monthly detailed vertical profiles near Bermuda (32N 64W), show a maximum in argon supersaturation of about 4% in the seasonal thermocline in late summer. Since the argon supersaturation is 3–4 times smaller than that of oxygen, most of the oxygen supersaturation is not of physical origin and hence must result from biological production.
In the winter mixed layer, air injection produces argon supersaturation in spite of high gas exchange rates. During spring and summer, radiative heating, air injection, and an upward argon flux create an even larger supersaturation in the mixed layer. In the seasonal thermocline, radiative heating maintains argon concentrations above solubility equilibrium in spite of vertical mixing.
The observed seasonal cycles of temperature, argon, helium, and oxygen are simulated with an upper ocean model. We linearized the model's response to variations in vertical diffusivity, air injection, gas exchange rate, and new production and then used an inverse technique (singular value decomposition) to determine the values of these parameters that best fit the data. A vertical turbulent diffusivity of 0.9 ± 0.1 × 10–4 m2 s–1 is consistent with both the thermal history and subsurface argon distribution. The rate of air injection, determined to ±25%, is similar to previous estimates. The seasonally-averaged gas exchange rate is 17 ± 12% lower than predicted by Liss and Merlivat (1986). We estimate a lower limit to depth-integrated new production below the mixed layer of 4.3 ± 1.7 moles O2 m–2 yr–1 during 1985, and obtain an estimate of 5.6 ± 1.5 moles O2 m–1 yr–1 if new production in the mixed layer is fixed at zero.
Document Type: Research Article
Publication date: 1989-02-01
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