Nuclides cosmogenic

An alternative method for inferring accumulation rate relies on assuming that the rain of cosmogenic nuclides such as °Be onto the ice sheet surface is known. Then high accumulation rate dilutes the cosmogenic nuclide so its concentration as measured in the ice core is inversely proportional to accumulation rate. 

The geochemistry of marine sediments is a major source of information about the past environment. Of the many measurements that provide such information, those of the U-series nuclides are unusual in that they inform us about the rate and timescales of processes. Oceanic processes such as sedimentation, productivity, and circulation, typically occur on timescales too short to be assessed using parent-daughter isotope systems such as Rb-Sr or Sm-Nd. So the only radioactive clocks that we can turn to are those provided by cosmogenic nuclides (principally or the U-series nuclides. This makes the U-series nuclides powerful allies in the quest to understand the past ocean-climate system and has led to their widespread application over the last decade. 

Cosmogenic Nuclides Isotopes of elements produced by the action of... 

C is a cosmogenic nuclide produced in the atmosphere. C is unstable and decays into N with a half-life of 5730 years (Z = 0.00012097 yr ). Assume that the concentration (or activity) of C in the atmosphere is a steady-state concentration that did not vary with time. If the activity of C in a plant tissue 13.56 dpm per gram of carbon, calculate the atomic ratio of " C/C. 

The cosmogenic nuclide is unstable and decays into with a half-life of 5730 years (X = 0.00012097 yr ). The initial activity of in a newly formed plant tissue is 1 3.56 dpm per gram of carbon (dpm = decays per minute), and in the year of 2000 you measured activity for a piece of tree tissue from the center of a tree and found that it is 12.8 dpm. 

Energetic particles react with solid matter in a variety of ways. Low-energy particles in the solar wind ( 1 KeV/nucleon) are implanted into solids to depths of 50 nm. Energetic heavy particles penetrate more deeply and disrupt the crystal lattice, leaving behind tracks that can be imaged by or chemically etched and observed in an optical microscope. Particles with energies of several MeV or more may induce a nuclear reaction. The two main modes of production of cosmogenic nuclides are spallation reactions and neutron capture. 

Spallation occurs when a high-energy cosmic ray breaks a target nucleus into two or more pieces. These interactions commonly eject neutrons. The secondary neutrons slow down to thermal energies and eventually react with other nuclei in the target material to generate heavier species. Production of cosmogenic nuclides by secondary neutrons increases with depth to a peak at between 0.5 and 1 m below the surface. Therefore, in order to get an... 

The fraction of incoming cosmic rays that generate nuclear reactions is quite low. In a meteorite traveling in space, about one in 108 of the target atoms undergoes a nuclear reaction in a 10-Myr period. However, the cosmogenic nuclides that they produce can be measured to estimate the time that an object has been exposed to cosmic rays. Table 9.1 shows some of the nuclides that are used to estimate cosmic-ray exposure ages in meteorites and in materials from planetary surfaces. 

Cosmic-ray exposure ages are determined from spallation-produced radioactive nuclides. Cosmic-ray irradiation normally occurs while a meteoroid is in space, but surface rocks unshielded by an atmosphere may also have cosmogenic nuclides. These measurements provide information on orbital lifetimes of meteorites and constrain orbital calculations. Terrestrial ages can be estimated from the relative abundances of radioactive cosmogenic nuclides with different half-lives as they decay from the equilibrium values established in space. These ages provide information on meteorite survival relative to weathering. 

Riihimaki CA, Libarkin JC (2007) Terrestrial cosmogenic nuclides as paleoaltimetric proxies. Rev Mineral... 

Calculating elevation from climate variables requires measuring an atmospheric quantity that varies with altitude. In other chapters, paleo-pressure are inferred by techniques such as basalt vesicularity (Sahagian and Maus 1994) or measuring cosmogenic nuclide concentrations in exposed rocks. For example, Brown et al. (1991) and Brook et al. (1995) used concentrations of 10Be and 26Al to place constraints on the uplift rate and duration of exposure of rocks in the Transantarctic Mountains, Antarctica. Additional methods and further discussions are available in the present volume. 

Table 1. Table of terrestrial cosmogenic nuclides of interest to paleoaltimetry. 

Note After Bierman (1994), Gosse and Phillips (2001), and Lai (1988). See Gosse and Phillips (2001) for details about radionuclide decay constants and production rate uncertainties. Isotopes in italics are those that have been used routinely in cosmogenic nuclide studies. Stable isotopes will resolve paleoaltimetry for Cenozoic and older rocks. High-latitude, sea-level rates from empirical studies only. Production mostly via neutron spallation unless noted. Recent debate on die half-life of 10Be has focused on whether this number is off by >10%. 

The concentration of a stable cosmogenic nuclide for a surface sample that experienced a period of continuous exposure over a known duration t, with no erosion or deposition or change in altitude during exposure, followed by subsequent burial and immediate shielding from further production, can be approximated as... 

The concentration of a stable cosmogenic nuclide in a sample that has experienced steady uplift during continuous exposure without erosion or burial requires integration over the range of elevations the sample experienced during exposure. For uplift rate u, modern elevation ymoderm and total exposure time T,... 

POTENTIAL SOURCES OF ERROR IN COSMOGENIC-NUCLIDE PALEOALTIMETRY... 

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