Isotope cosmogeneous

All three isotopes of Ne are produced at almost equal rates by cosmogenic particles at the Earth s surface. As Ne is the least abundant isotope, cosmogenic production of Ne is more likely to be a significant portion of the crustal Ne budget than for Nc or Ne. Similarly, Ar is the least abundant isotope of Ar as well as the most rapidly produced (for typical crustal compositions), therefore the effects of cosmogenic Ar accumulation on the crustal noble gas budget will be examined. 

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... 

The most famous cosmogenic radionuclide is 14C (t1/2 = 5730 a), which is produced by the interaction of cosmic ray neutrons via an (n,p) reaction with nitrogen [14N(n, p)14C], whereas the radioactive decay of 14C takes place by (3 decay to form the stable 14N isotope. 14C is the most important cosmogenic radionuclide for dating (see Section 9.7.5) in archaeology and can be analyzed using isotope sensitive accelerator mass spectrometry. Extremely small isotope ratios 14C/12C = 12 in nature can be measured by means of AMS.28... 

The atmospheric concentration of natural and bomb-produced radionuclides has been measured at ground level for several years at three locations throughout the world. The manner in which the concentration decreased suggested a half-residence time for stratospheric aerosols of 11.8 months at 46°N latitude. The annual spring concentration maximum occurred one to four months earlier at 71°N than at 46°N. Cosmogenic 7Be attained a maximum concentration before the bomb-produced radionuclides at 71° N and later than the bomb-produced isotopes at 46°N. The rate of increase toward the annual peak concentration for most radionuclides could be approximated by an exponential in which the concentration doubled every 60 days likewise, the rate of decrease from the maximum concentration could be approximated by an exponential with a half-time of about 40 days for most radionuclides except 7Be at 46°N, which shows a half-time of about 60 days. 

Another method uses the decay of cosmogenic isotopes that are produced in the atmosphere and then incorporated into terrestrial reservoirs. Examples of this approach include standard 14C and l0Bc dating. 

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%. 

Bierman PR (1994) Using in situ produced cosmogenic isotopes to estimate rates of landscape evolution A review from the geomorphic perspective. J Geophys Res 99 13885-13896... 

Kouwenberg LLR, Kiirschner WM, McElwain JC (2007) Stomatal frequency change over altitudinal gradients prospects for paleoaltimetry. Rev Mineral Geochem 66 215-242 Lai D (1988) In situ-produced cosmogenic isotopes in terrestrial rocks. Ann Rev Earth Planet Sci 16 355-388 Lai D (1991) Cosmic ray labeling of erosion surfaces in situ nuclide production rates and erosion models. Earth Planet Sci Lett 104 424-439... 

Stone JO (2000) Air pressure and cosmogenic isotope production. J Geophys Res 105 23753-23759 Stone JOH, Evans JM, Fifield LK, Allan GL, Cresswell RG (1998) Cosmogenic chlorine-36 production in calcite by muons. Geochim Cosmochim Acta 63 433-454... 

Radiogenic and cosmogenic nuclides. These 2 chapters cover low-temperature thermochronologic approaches (Reiners) and cosmogenic isotopes (Riihimaki and Libarkin). 

Trumbore, S. E. (1996). Measurement of cosmogenic isotopes by accelerator mass spectrometry Applications to soil science. In Applications of Accelerator Mass Spectrometry, Boutton, T., and Yamasaki, S. eds., Marcel Dekker, New York, pp. 311-340. 

Figure 2.15 Ne three-isotope plot for a grain-size suite of plagioclase separates from lunar high land soil that were treated by the CSSE treatment (see text). The best fitted line through the data from all etched samples (line p) passes close to the data point GCR (galactic cosmic ray) of cosmogenic Ne. On the left side, the path of mass fractionation of SWC (solar wind composition)-Ne intersects line p at a 20Ne/22Ne ratio of -11.3, which is interpreted to represent SEP (solar energetic particle) Ne (cf. Section 2.8). Open symbols unetched sample. Solid symbols etched samples. SF Solar flare Ne. Reproduced from Signer et al. (1993).

Lai, D. (1988) In situ-produced cosmogenic isotopes in terrestrial rocks. Ann. Rev. Earth Planet. Sci., 16, 355-88. 

Another isotopic manifestation is the so-called Cosmogenic Radioactivity which is produced in meteorites owing to reactions of their stable atoms with the cosmic rays that bombard the meteorites. Owing to this, meteorites have many live short-lived radioactivities within them when they fall to ground, and measuring their amounts determines when the meteorite fell and how deep within it was the location of the sample prior to the collision that released it into space (see Extinct radioactivity and Meteorites). 

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