1Chapter 17 Tritium, Carbon 14 and other "dyes" James Murray 5/15/01 Univ. Washington (note: Figures not included yet) I. Cosmic Ray Production Cosmic ray interactions produce a wide range of nuclides in terrestrial matter, particularly in the atmosphere, and in extraterrestrial material accreted by the earth. These nuclides have a wide range of chemical properties and half lives which enable them to be applied to a wide variety of geochemical problems. A summary of these isotopes, along with relevant information such as their mode of production and decay, half lives and pre-nuclear global inventory are summarized in Table 17-1. Carbon-14 and tritium (3H) are the most well known of these isotopes. The main cosmic ray produced isotopes used on oceanography are listed below with their half-lives and pre-nuclear inventory. Isotope Half-life Global inventory 3H 12.3 yr 3.5 kg 14C 5730 yr 54 ton 10Be 1.5 x 106 yr 430 ton 7Be 54 d 32 g 26Al 7.4 x 105 yr 1.7 ton 32Si 276 yr 1.4 kg Carbon-14 is produced in the upper atmosphere as follows: Cosmic Ray Flux → Fast Neutrons → Slow Neutrons + 14N* → 14C (thermal) Cosmic rays (protons traveling through space with energies in the BEV range) hit the atmosphere and interact with molecules to produce fast-moving neutrons. These are slowed down by collisions so that by the time they reach about 50,000 ft they are slowly moving thermal neutrons. These thermal neutrons react with 14N (the most abundant nuclide in the atmosphere) to produce 14C. A neutron enters a 14N nucleus, knocking out a proton and remaining in its place. The loss of one proton converts the atom of nitrogen into carbon. The atomic weight remains the same at 14. The reaction is written: 14N + n → 14C + p (7n, 7p) (8n, 6p) Since almost all the neutrons produced by cosmic rays are consumed to produce 14C the production rate is 2 atoms cm-2 min-1 and the pre-nuclear inventory (determined from the2balance between production and decay) was 54 tons. The production rate varies with the 11 yr sunspot cycle (Stuiver and Quay, 1981). Tritium is produced from cosmic ray interactions with N and O. After production it exists as tritiated water ( H - O -3H ), thus it is an ideal tracer for water. Tritium concentrations are TU (tritium units) where 1 TU = 1018 (3H / H) Thus tritium has a well defined atmospheric input via rain and H2O vapor exchange. Its residence time in the atmosphere is on the order of months. In the pre-nuclear period the global inventory was only 3.5 kg which means there was very little 3H in the ocean at that time. II. Bomb Fallout Nuclear weapons testing and nuclear reactors (e.g. Chernobyl) have been an extremely important sources of nuclides used as ocean tracers. The artificially produced radionuclides that have been detected in the marine environment are listed in Table 17-2. Some nuclides are conservative (3H) while others are nonconservative (e.g. 95Zr, 95Nb). Others are only partially removed from seawater by adsorption or biological uptake (e.g. 65Zn, 60Co). In addition to 3H and 14C the main bomb produced isotopes have been: Isotope Half Life Decay 90Sr 28 yrs beta 238Pu 86 yrs alpha 239+240Pu 2.44 x 104 yrs alpha 6.6 x 103 yrs alpha 137Cs 30 yrs beta, gamma Nuclear weapons testing has been the overwhelmingly predominant source of 3H, 14C, 90Sr and 137Cs to the ocean. Although nuclear weapons testing peaked in 1961-1962 there were significant deliveries of radionuclides beginning as early as 1954. Fallout nuclides act as "dyes" that are spread over the sea surface, thus they have a well defined surface input. Another group of man-made tracers that fall in this category but are not bomb-produced and are not radioactive are the chlorofluorocarbons (CFCs). A. Tritium Atmospheric testing of nuclear weapons increased the global inventory of 3H from 3.5 kg to over 200 kg. The fusion bombs tested in the 1960's were much dirtier (2.0 kg 3H / megaton) with 3H than the fission bombs (0.07 kg 3H / megaton) that had been tested earlier. Most of this tritium was injected into the stratosphere. The exchange between the3stratosphere and troposphere occurs mostly during "spring leaks" but once in the stratosphere the residence time of tritium is 30-60 days. The time history of 3H in rain from Adak, Alaska (52°N) to Johnston Island (17°N) from 1962 to 1967 is shown in Fig 17-1. The maximum in 1963 is clear as are the annual oscillations in input due to the "spring leak" between the stratosphere and troposphere. Most 3H input occurred in the northern hemisphere where bombs were tested. The short residence time of atmospheric H2O prevented inter-hemisphere transport of 3H. The latitude dependence of 3H input was maximum at about 50°N where the stratosphere/troposphere mixing occurs (Fig 17-2). The key points about 3H are that: 1. 3H is input to the surface ocean 2. there is an input maximum at high latitude in the northern Hemisphere 3. there was a pulse input in 1963. Once in the ocean, tritiated water follows the circulation and mixing patterns of the surface ocean (a man-made "dye" experiment). Surface 3H in the Pacific Ocean determined in the 1974 GEOSECS Program (Fig 17-3) showed: 1. highest 3H in the north Pacific 2. 3H decreases southward. The north-south section of 3H through the Pacific (0-1000m) (Fig 17-4)(GEOSECS) shows how far the 3H had penetrated into the ocean after only about 10 years. 1. 3H was mostly in the northern hemisphere 2. most 3H was within 1000m of the surface 3. there was a subsurface maximum of 3H near the equator. The implications of these distributions are that: 1. The equator is an effective barrier to N-S transport in the upper ocean. 2. Transport into the thermocline region occurs on a decadal time scale. 3. The sub-surface peak in 3H implies rapid horizontal transport from high latitude can dominate over vertical transport. The activity of 3H appears to follow density surfaces (σt). The N-S sections of σt in the Atlantic, Indian and Pacific Oceans are shown in Fig 17-5. The dashed line shows the penetration of 3H to the 3H detection limit. We can test the ideas that transport of 3H is along density or isoplycnal surfaces by plotting 3H versus density rather than depth (Fig 17-6). Note that when we do that we see that the maximum in 3H at 12°N is on the same density as