Specifically, the solubilities of these gases range over an order of magnitude, with Xe being much more soluble in water (and the temperature dependence of that solubility much greater) than He. The noble gases (He, Ne, Ar, Kr, and Xe) have a broad range of physical characteristics. In addition, the back-flux of the tritiugenic 3He to the ocean surface can be used to constrain the rate of supply of nutrients to the surface ocean, another important determinant of biological production. These determinations can in turn lead to regional estimates of primary productivity and illuminate the dynamics of the marine food web. In particular, we can estimate rates of oxygen consumption, and hence bacterial respiration. The reader might consult some of the following papers.īeing able to date the subduction of surface water into the thermocline also allows us to determine the rates of biogeochemical processes occuring in the shallow ocean. It is still possible to obtain useful information, but one must be careful not to interpret tritium-helium ages literally. Thus we could use this a measure of the ventilation time-scale for water masses in the upper few hundred meters of the ocean (see lower figure on this page). As with all "transient tracers" the problem is complicated due to mixing effects. In principle this dating tool is useful on time scales ranging from a month or so up to a few decades. Measuring both the 3He and the tritium allows us to determine the time that the water has been away from the ocean surface. If the water sinks below the ocean surface, however, 3He can accumulate. Seawater at the ocean surface tends to lose this excess by gas-echanging with the atmosphere, driving the seawater 3He/ 4He ratio toward solubility equilibrium with the atmosphere. The tritium has the added benefit that its decay can produce measureable amounts of excess "tritiugenic" 3He in the shallow ocean. We can observe the pentration of this bomb-tritium into the ocean, much like a dye and X-Ray sequence might be used to study a patient in the hospital.
Because it is an isotope of hydrogen, tritium travels with water, and hence is a beautiful tracer of the water cycle on our planet. Tritium and Helium-3 in the Shallow OceanĪtmospheric nuclear weapons testing during the 1950s and 1960s produced copious amounts of the radioisotope tritium, which decays with a 12.4 year half-life to 3He. It is indeed the isotopic "shot heard around the world". This volcanic helium signal extends into the Antarctic Circumpolar Current, and is traceable all the way into the North Atlantic. Notice the structure in the more southerly sections (middle and lower panels of the figure) which shows the imprint of deep gyre circulations in the South Pacific. Thus, once away from the seafloor and volcanic centers, we can use the volcanic signature as a tracer of ocean mixing and circulation. Since helium is an inert gas, its behavior in seawater is simple and conservative. Notice the plume of volcanic helium in the northernmost section (uppermost panel in the figure) that extends all the way across the Pacific. The signature of this volcanic helium is observable over many thousands of km in the abyssal Pacific (see upper figure on this page), and is usually described as a percentage deviation in the observed isotope ratio from the atmospheric ratio. This helium is generally has a higher 3He/ 4He than atmospheric He, often approaching 10 X the atmospheric ratio, and sometimes as high as 30 X or more. The first is He that is released from volcanic or hydrothermal activity on or near the sea floor. Aside from He dissolved from the atmosphere, there are two other sources of non-atmospheric He in the ocean.
Thus 3He is a million times less abundant than it's more prolific cousin. That is, there is only one atom of 3He for every 700,000 atoms of 4He in air. The isotopic ratio 3He/ 4He is only 1.4 X 10 -6 in air. Thus it is present only at a level of only about 2 nano-moles per kg of seawater. Helium is present in the atmosphere as a trace gas (~ 5 parts per million), and is also rather insoluble in seawater. Although chemically identical (their nuclei contain 2 protons each) they each have different numbers of neutrons, and hence atomic mass. Helium consists of two stable isotopes: 3He and 4He. Helium Isotopes and Noble Gases in Seawater Powerful tools for ocean circulation and biogeochemistry Volcanic Helium Isotopes in Seawater
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