Tracers: helium, tritium and noble gases
Helium, tritium and noble gases may be used for identifying the origins of a water mass. They help elucidate mixing and dilution rates, circulation patterns, ocean ventilation and the changes in the water mass characteristics over time. As part of the ANDREX tracers team our primary role is to collect samples of helium isotopes (3He and 4He) and tritium (3H, an isotope of hydrogen) for analysis back in the UK. We were also interested in the entire suite of dissolved noble gases including isotopes of Argon (40Ar and 36Ar), Ar/Kr/Xe ratios.
Water samples are retrieved using a CTD rosette of Niskin bottles. Seawater samples are taken at different intervals starting at the seafloor and finishing at the sea surface. Each Niskin bottle is closed remotely, trapping seawater at that location inside. On ANDREX we are particularly interested in the import and export routes of Circumpolar Deep Water (CDW) and Antarctic Bottom Water (AABW) so it is important that we capture CDW and AABW entering or escaping the Weddell Basin through the deep passages. We will sample to within 10m of the seafloor down to ~6000m. Once the CTD has arrived back on deck seawater samples are taken from the Niskin bottles. Noble gases are sampled once CFCs have been sampled and tritium is sampled once all the dissolved gases are taken. Noble gases are collected in copper tubes that are fitted with Tygon tubing and clamps at both ends. Once the Noble gas sample has been taken it is brought into the wet lab where the tube is sealed using a crimping device that forms a cold weld at each end of the copper tube containing the sample. Noble gas samples are collected in copper tubing as it is impervious to Helium which is a very small atom and can pass through most metals, plastic and glass. Tritium is collected in Argon back filled bottles. The bottles are back filled with argon to isolate the water sample from the atmosphere and prevent any re-equilibrium with atmospheric concentrations of helium. Both Noble gas and tritium samples are labelled and carefully wrapped in bubble wrap and placed in boxes for transport back to the UK for analysis.
Helium is a trace gas in the atmosphere with an atmospheric concentration of ~5 parts per million (ppm). It has very low solubility in sea water (2 nmol Kg−1). Helium in the atmosphere is a mixture of two stable isotopes 3He and 4He. The isotopic ratio of 3He/4He is 1.4 × 10−6 in air with 4He being one million times more prolific than 3He. Helium in the surface waters of the world ocean is in solubility equilibrium with the atmosphere. Volcanic and hydrothermal activity on the sea floor are a source of helium to intermediate depth waters in the ocean. The ratio of 3He/4He in helium that originates from mantle out gassing is between ten and thirty times greater than the atmospheric 3He/4He ratio. As helium is a member of the Noble gases it does not take part in any chemical or biological processes and is said to be conservative in its nature. Any changes in concentration are caused solely by physical processes. This is what makes conservative tracers so useful in tracking the mixing history and circulation pattern of water bodies. It is possible to identify the source of helium input to the oceans, those from atmosphere at the ocean surface and those and those of volcanic or hydrothermal sources at intermediate depths, by measuring the 3He/4He ratio.
Tritium was first detected in the environment in the late 1940s. As tritium is an isotope of hydrogen it is oxidised to HTO (tritiated water) and so is the perfect tracer for studies of the natural water cycle. Before scientists were able to exploit tritium and its applications to the natural water cycle anthropogenic tritium was released in large amounts during the mid to late 1950s and early 1960 as a result of nuclear bomb testing swamping natural background levels. Fortunately bomb tritium also allowed us a new tool with which to measure the movement of water. As we know both the time scale and the amount of bomb tritium that was introduced to the atmosphere it is possible to figure out the rate of introduction. Water that was in contact with the surface ocean is marked with the concentration of bomb tritium in the surface of the ocean at that time. Once the water mass moves away from the surface of the ocean the concentration of bomb tritium within it may only be changed by physical processes such as mixing. The applications of bomb tritium are limited as an aging tool in waters that have been in contact with the surface oceans after the 1970s and 1980s as concentrations decreased below those which allow the age of water mass to be determined. However, if we measure tritium and its radioactive decay product, tritugenic 3He, simultaneously we can calculate the tritium/3He age of the water mass (the amount of time the water parcel has been isolated from the surface of the ocean). When a parcel of water is in contact with the surface of the ocean helium is lost to the atmosphere and tritium is taken up from the surface water. Once the water parcel moves away from the surface of the ocean helium can no longer be lost to the atmosphere and tritium can no longer be taken up from ocean surface sources, i.e., the tritium/3He clock is set to zero (Clarke et al., 1976). The length of time that the parcel of water has been isolated from the surface ocean is calculated from the concentrations of tritugenic 3He, tritium and the half-life of tritium. In order to make this calculation the concentration of 3He added to the water mass from the decay of tritium also needs to be calculated.
Dissolved noble gases
Noble gases are attractive tracer to sample because they are only effected by physical processes and they show a wide range of physical characteristics that cause them to respond differently to the various physical phenomena that we are interested in. Noble gases have may be used in detecting and quantifying the relative contributions of air-sea exchange, sea-ice formation, sub-glacial melting, and sub-glacial marine ice production in the production of Ice Shelf Water (ISW) and Weddell Sea Bottom Water (WSBW) (Jenkins, 2008).
The overall objective of the tracer group is provide information that will allow help in providing an optimal, self-consistent estimate of the rates of ventilation and AABW formation in, and AABW export from, the Weddell gyre. Transient tracers, including tritium, CFC-11. CFC-12, CFC-113 and SF6 and their ratios CFC-11/CFC-12, CFC-11/tritium and SF6/CFC-12 will be used to quantify the ventilation age and transit time distribution of water masses on the outer rim of the Weddell gyre and allow a second estimation of dilution times that will compliment the physical data collected. The variety of tracers used will allow estimates of ventilation ages and transit time distributions to be optimised through constraining the mixing history of the water parcels. The end result of the model investigation of parameter space provided by both the physical and chemical tracer data collected on ANDREX will allow a comprehensive evaluation of ventilation timescales of the water masses flowing into and out of the Weddell gyre.
Clarke, W.B., W.J. Jenkins and Z. Top. (1976) Determination of tritium by mass spectrometric measurement of 3He. The International Journal of Applied Radiation and Isotopes 27: 515–522. Jenkins, 2008. Characterizing the formation, nature and export of Weddell Sea Bottom Water using noble gases and transient tracers. NFS funding proposal.