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Publication Title | Trace gas disequilibria during deep-water formation

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Deep-Sea Research I (2007) 54(6) p.939–950. ⃝c 2007 Elsevier B.V. All rights reserved.

Trace gas disequilibria during deep-water formation

Roberta C. Hamme∗

School of Earth and Ocean Sciences, University of Victoria,

P.O. Box 3055 STN CSC, Victoria, BC V8W3P6, Canada

Jeffrey P. Severinghaus

Scripps Institution of Oceanography, University of California San Diego, 9500 Gilman Drive, Dept 0244, La Jolla, CA 92093-0244, USA

Abstract

We present high-precision measurements by a new iso- tope dilution technique of a suite of inert gases in the North Pacific. Remarkably smooth gradients in Ar, Kr and Xe from near equilibrium in intermediate waters to several percent undersaturated in deep waters were ob- served. The general pattern in the deepest waters was that Ar, Kr and Xe were undersaturated (Ar least and Xe most), while N2 was close to equilibrium, and Ne was su- persaturated. We propose that this pattern was produced by the interaction between the different physical proper- ties of the gases (solubility and the temperature depen- dence of solubility) with the rapid cooling and high wind speeds that characterize deep-water formation regions. In a simple model of deep-water formation by convection, the saturations of the more temperature-sensitive gases were quickly driven down by rapid cooling and could not reequilibrate with the atmosphere before the end of the winter. In contrast, the gas exchange rate of the more bubble-sensitive gases (Ne and N2) was able to meet or exceed the drawdown by cooling. Our simple convection model demonstrates that the heavier noble gases (Ar, Kr and Xe) are sensitive on seasonal timescales to the com- peting effects of cooling and air-sea gas exchange that are also important to setting the concentration of CO2 in newly formed waters.

1 Introduction

In this paper, we present full-depth profiles of five in- ert, dissolved gases measured in the subtropical North Pacific. Argon, Kr and Xe have been measured by a new isotope dilution method and demonstrate previously un- known trends with depth. Our focus here is to explain the pattern in gas saturation observed in the deepest waters, where some gases are undersaturated and others supersat- urated. (Saturation refers here to the percent difference between the actual concentration of a gas and the concen- tration at equilibrium with the atmosphere for the poten- tial temperature and salinity of the water.)

One motivation for measuring inert gases is that they may provide a constraint over the efficiency of the carbon solubility pump (Archer, 2003), in which cooling waters create undersaturations that drive CO2 uptake. These un- dersaturations occur because CO2 and most other gases are more soluble at colder temperatures, within the range 0–40oC. The non-anthropogenic CO2 sink observed in the North Atlantic has been partially attributed to this solu- bility pump (Takahashi et al., 1995), and modeling stud- ies have shown that rapidly cooling waters do not fully equilibrate with the atmosphere because of the slow ki- netics of air-sea CO2 exchange (Sarmiento et al., 1995; Toggweiler et al., 2003). Considerable differences ex- ist among models in their treatment of the solubility pump (Archer et al., 2000), which adds uncertainty to their predictive capabilities, particularly in the response

∗ Corresponding author. Tel.: 6200; E-mail: rhamme@uvic.ca

+1-250-472-4014; Fax:

+1-250-721-

1

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