Noble Gases in Seawater as Tracers for Physical and Biogeochemical Ocean Processes Rachel H. R. Stanley and William J. Jenkins Abstract Noble gases are biologically and chemically inert, making them excellent tracers for physical processes. There are 5 stable noble gases: He, Ne, Ar, Kr, and Xe, with a range of physicochemical properties; the diffusivities of the noble gases in seawater differ by approximately a factor of 5 and the solubilities of the noble gases in seawater differ by approximately a factor of 10. This broad range in physicochemical characteristics leads to differing response to physical forcing. Thus, measurements of multiple noble gases made concurrently allow quantification of many physical processes. In seawater studies, noble gas measurements have been used to investigate air-sea gas exchange, allowing explicit separation of the bubble component from the diffusive gas exchange component, and to study equilibration during deep water formation. Argon has been used to quantify diapycnal mixing and the heavier noble gases could be useful in such studies as well. Helium, Ne, and Ar have yielded insights on ocean- cryospheric processes such as sea ice formation and basal melting of glaciers. The isotope 3 He has been used extensively in studies of ocean circulation, and also for quantifying ocean-lithospheric interactions. Additionally, noble gases can be combined with biologically active gases, such as O2 or N2, in order to quantify rates of biological production and denitrification. there are five stable noble gases with a range of 1 Introduction solubilities and molecular diffusivities in seawa- Dissolved noble gases are ideal in situ tracers for ter (Fig. 1). The diffusivities in seawater of the physical processes in the ocean because they are noble gases differ by a factor of seven with helium biologically and chemically inert and thus being the most diffusive (Jahne et al. 1987). The respond solely to physical forcing. Additionally, solubilities differ by more than an order of mag- nitude, with xenon being the most soluble (Smith R. H. R. Stanley (&) Á W. J. Jenkins and Kennedy 1983; Wood and Caputi 1966). The Woods Hole Oceanographic Institution, Woods solubilities of the heavier noble gases (Ar, Kr, and Hole, MA 02543, USA Xe) have a strong, non-linear dependence on e-mail: [email protected] P. Burnard (ed.), The Noble Gases as Geochemical Tracers, Advances in Isotope Geochemistry, 55 DOI: 10.1007/978-3-642-28836-4_4, Ó Springer-Verlag Berlin Heidelberg 2013 56 R. H. R. Stanley and W. J. Jenkins (a) Diffusivity (b) Solubility −9 x 10 8 200 Xe He 7 Ne ) Ar 6 150 He −1 Kr atm Xe ) 5 −1 −1 O2 s 2 4 100 Kr N2 D (m 3 Ne Ar , O2 N2 2 50 Kr Bunsen (ml Kg Ar , O2 Xe 1 N2 0 0 Ne , He 10 20 30 10 20 30 Temperature (C) Temperature (C) Fig. 1 a Molecular diffusivities of the noble gases, a function of temperature. Solubility values for He are oxygen, and nitrogen as a function of temperature, as from a modified version of Weiss (1971), Ne, Ar, and N2 calculated from the diffusivity values of Jahne et al. solubilities are from Hamme and Emerson (2004b), Kr (1987) and Wise and Houghton (1966). b Solubilities of solubility is from Weiss and Kyser (1978) and Xe the five noble gases, nitrogen, and oxygen in seawater as solubility is from Wood and Caputi (1966) temperature whereas the solubility of the lighter equilibrium. Additionally, He has two additional ones (He, Ne) are relatively insensitive to tem- sources. 4He is produced by nuclear reactions in perature (Hamme and Emerson 2004b; Smith and rocks,primarilyin the Uranium series. Some ofthis Kennedy 1983; Weiss 1971; Weiss and Kyser 4He enters the ocean through water-rock processes. 1978; Wood and Caputi 1966). Additionally, Additionally 3He is produced by radioactive decay during sea ice formation or melting, the lighter from atmospheric tritium. gases are favorably partitioned into ice whereas Noble gas measurements can be combined the heavier gases remain preferentially in the with dissolved biologically active gases, such as water (e.g. Hood et al. 1998). This broad range in O2 or N2, to yield quantitative insights into physicochemical characteristics leads to signifi- biogeochemical processes. O2 and N2 can be cantly different responses to physical forcing used as geochemical tracers for quantifying rates (Fig. 2). Thus, measurements of multiple noble of important biological processes such as net gases made concurrently in seawater allow one to community production and denitrification. diagnose and quantify physical processes, such as However, physical processes such as air-sea gas air-sea gas exchange, diapycnal mixing, and exchange and thermal forcing affect O2 and N2, subsurface basal glacial melting. making direct interpretation of those gas records The main source of noble gases to the ocean is difficult. Argon has a very similar solubility and from the atmosphere through the process of air-sea diffusivity to O2, and thus can serve as an abiotic gas exchange. The noble gases are usually close to analogue to O2. Thus, the difference between O2 being in equilibrium with the atmosphere, and Ar can serve as a tracer for biological pro- according to Henry Laws constants, although rapid ductivity. Additionally, Ar can be used in con- warming or cooling, ice formation or melting, and junction with nitrogen to construct basin-scale bubble injection can lead to departures from estimates of denitrification. Noble Gases in Seawater as Tracers 57 2 1 bubble injection bubble exchange glacial melting glacial increasingincreasing atmospheric salinity pressure warming 0 cooling He (%) Δ sea ice formation −1 decreasing salinity decreasing atmospheric pressure −2 −2 −1 0 1 2 ΔXe (%) Fig. 2 Schematic depicting the effect of different physical gases, different physical processes have differing effects on processes on saturation anomalies of He and Xe. Because of the gases, allowing the gases to be powerful tools for the differences in solubilities and diffusivities of the noble separating and quantifying physical processes sample is drawn, keeping the temperature of 2 Analytical Methods sample containers similar to temperature of the water (i.e. avoid letting the containers sit in the 2.1 Sample Collection sun before collecting samples since the gases in the seawater may exsolve due to warming on The standard protocol for measuring noble gases contact with the container), and carefully in seawater is to collect a sample of seawater, watching for any bubbles in tubing, neck of extract the gases from the water, and measure bottle, etc. the gases on a mass spectrometer. Water sam- The lighter noble gases are more diffusive ples are generally drawn on the deck of the ship than the heavier ones and are not well contained from Niskin bottles mounted on a CTD rosette by o-rings. If samples are collected in stainless system. It is important to draw the samples soon steel cylinders, which have o-rings in the plug after the rosette is retrieved, and immediately valves at the end of the cylinders, it is best to after the Niskin is vented for sampling, as extract the gases from the samples into alumi- exchange with the head-space created from nosilicate glass bulbs within 24–48 h of collec- drawing water from the Niskin will compromise tion (Lott 2001). If samples are stored in the integrity of the dissolved gases (Takahashi custom-made glass bottles, which contain et al. 1988). Samples of seawater can be trans- Louers-Houpert valves that have o-rings, it is ferred to copper tubes (Weiss 1968; Young and best to use valves with two o-rings, to fill the Lupton 1983), stainless steel cylinders (Lott and necks with CO2 to decrease the loss of gas Jenkins 1998), or pre-evacuated custom-made across the valves, and to analyze the samples glass bottles (Emerson et al. 1999) for storage within at most two months of sample collection (although the last is not suitable for helium (Hamme and Emerson 2004a). In contrast, studies). In all cases, great care must be taken samples are well preserved in copper tubes for when sampling to avoid bubbles entering the indefinitely long periods. sample containers. This can be achieved by In the case of stainless steel cylinders, sub- presoaking any transfer tubing that is used, sequent to sample collection, usually at-sea or in tapping on walls of sample containers as the an onshore laboratory, gases are extracted from 58 R. H. R. Stanley and W. J. Jenkins the water by essentially boiling the water in a N2 temperature (77 K) and the other at dry ice/ high-vacuum system and transferring the acetone temperature (196 K) (Sano and Takahata released gas to *25 cc aluminosilicate glass 2005), or onto a trap at liquid He temperature ampoules that can then be flame-sealed and (4 K) (Hamme and Severinghaus 2007; Sever- stored. In the laboratory, these ampoules are inghaus et al. 2003) Methods that measure all five attached to the mass spectrometer sample pro- gases from a single sample have precision of 0.3– cessing system using o-ring seals and the sample 1.0 % using a magnetic sector instrument is introduced by mechanically snapping the end (Beyerle et al. 2000) to 0.1–0.2 % using a series of the glass flame-seal (Lott and Jenkins 1998). of two automated cryogenic traps and a QMS In the case of crimped or clamped copper tube (Stanley et al. 2009a) and to 0.15–0.17 % for Ar, samples, a similar extraction procedure can be Kr and Xe only using an isotope ratio mass used on-shore, but after introduction to the spectrometer (Hamme and Severinghaus 2007; evacuated sample line, samples are stirred at Severinghaus et al.