Chapter 4 : Noble Gases in Seawater as Tracers for Physical and Biogeochemical Ocean Processes

by R. Stanley and W. Jenkins

Dissolved noble gases are ideal in situ tracers for physical processes in the ocean because they are biologically and chemically inert and thus respond solely to physical forcing. Additionally, there are five stable noble gases with a range of solubilities and molecular diffusivities in seawater (Fig. 1). The diffusivities in seawater of the noble gases differ by a factor of seven with helium being the most diffusive (Jahne, et al. 1987). The solubilities differ by more than an order of magnitude, with xenon being the most soluble (Smith and Kennedy 1983 ; Wood and Caputi 1966). The solubilities of the heavier noble gases (Ar, Kr, and Xe) have a strong, non-linear dependence on temperature whereas the solubility of the lighter ones (He, Ne) are relatively insensitive to temperature (Hamme and Emerson 2004b ; Smith and Kennedy 1983 ; Weiss 1971 ; Weiss and Kyser 1978 ; Wood and Caputi 1966). Additionally, during sea ice formation or melting, the lighter gases are favorably partitioned into ice whereas the heavier gases remain preferentially in the water (e.g. Hood, et al. 1998). This broad range in physicochemical characteristics leads to significantly different responses to physical forcing (Fig. 2). Thus, measurements of multiple noble gases made concurrently in seawater allow one to diagnose and quantify physical processes, such as air-sea gas exchange, diapycnal mixing, and subsurface basal glacial melting. The main source of noble gases to the ocean is from the atmosphere through the process of air-sea gas exchange. The noble gases are usually close to being in equilibrium with the atmosphere, according to Henry Laws constants, although rapid warming or cooling, ice formation or melting, and bubble injection can lead to departures from equilibrium. Additionally, He has two additional sources. 4He is produced by nuclear reactions in rocks, primarily in the Uranium series. Some of this 4He enters the ocean through water-rock processes. Additionally 3He is produced by radioactive decay from atmospheric tritium. Noble gas measurements can be combined with dissolved biologically active gases, such as O2 or N2, to yield quantitative insights into biogeochemical processes. O2 and N2 can be used as geochemical tracers for quantifying rates of important biological processes such as net community production and denitrification. However, physical processes such as air-sea gas exchange and thermal forcing affect O2 and N2, making direct interpretation of those gas records difficult. Argon has a very similar solubility and diffusivity to O2, and thus can serve as an abiotic analogue to O2. Thus, the difference between O2 and Ar can serve as a tracer for biological productivity. Additionally, Ar can be used in conjunction with nitrogen to construct basin-scale estimates of denitrification.

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Chapter 4

10 juin 2012
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