- CRONUS-Earth Project
Motivation and Rationale
The practicality of measuring rare nuclides produced in materials at the surface of the earth by reactions with cosmic-ray particles was first demonstrated in 1986. In the subsequent 16 years cosmic-ray produced nuclides have found a wide range of applications in fields as diverse as geomorphology, paleoclimatology, tectonics, and glacial geology. Numbers of analyses performed are increasing every year, as is the level of interest in the general earth-science community.
Unfortunately, rigorous understanding of the systematics of cosmic-ray produced nuclides has lagged the rapidity with which the applications have developed. This lag has resulted in considerable part from the interdependence of the various parameters needed to calculate surface exposure ages or erosion rates. Estimates by independent researchers of the production rates of many nuclides vary considerably. One factor that contributes to this situation is the widely differing types of calibration sites that have been used. Another is that measurements made at different geographical positions must be scaled for intercomparison, and the scaling formulations have recently been a matter of controversy. In addition to spatial scaling, the temporal variation of production rates must also be considered, and the magnitude of this effect is also uncertain. The end result of these compounded uncertainties is that although ages or erosion rates determined using in-situ cosmogenic nuclides should in principle be quantifiable to approximately ±5% (1 ), current actual reliability for positions where local calibration is not available is probably no better than 15% (1 ). Different nuclides, or even the same nuclide employed by different investigators, may give markedly differing results. These inconsistencies could ultimately result in a loss of credibility for the technique.
| 1. Random Errors |
2. Systematic Errors |
| 1.1 Sample characteristics |
2.1 Radionuclide half life |
| 1.1.1 Surface geometry correction |
|
| 1.1.2 Shielding correction |
2.2 Production rate |
| 1.1.3 Thickness |
2.2.1 Basic calibration |
| 1.1.4 Meteoric contamination |
2.2.2 Whole rock prod rate estimates |
| 1.1.5 Erosion rate and style |
2.2.3 Nuclear cross sections |
| 1.1.6 Burial |
2.2.4 Attenuation lengths (rock and atmos.) |
| 1.1.7 Prior irradiation (inheritance) |
2.2.5 Altitude scaling, Standard Atmosphere |
| 2.2.6 Latitude and longitude scaling |
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| 1.2 Sample preparation and analyses |
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| 1.2.1 Sample interchange |
2.3 Temporal variations |
| 1.2.2 Contamination from physical processing |
2.3.1 Cosmic ray flux |
| 1.2.3 Weighing of sample |
2.3.2 Solar modulation |
| 1.2.4 Addition of carrier |
2.3.3 Geomagnetic polar wander |
| 1.2.5 Contamination with non-target minerals |
2.3.4 Geomagnetic paleointensity |
| 1.2.6 Analysis of stable isotope |
2.3.5 Non-dipole uncertainties |
| 1.2.7 Other major & trace elemental analyses |
2.3.6 Atmospheric thickness variations |
| 1.3 Mass spectrometric measurement |
2.4 Stable element measurements |
| 1.3.1 Poisson |
|
| 1.3.2 Background subtraction |
2.5 Carrier and standards |
| 1.3.3 Blank correction |
|
| 1.3.4 Sample reproducibility and normalization |
2.6 Fractionation, spectrometry |
| 1.3.5 Precision of standard |
|
| 1.3.6 Correction for non-cosmogenic gases |
2.7 Other assigned constants |
| 2.8 Calculation errors |
*Error magnitudes (1 ) are based on estimates calculated by J. Klein (pers. comm., 2000) for fifteen 10Be exposure ages from quartz in boulders on a broad terminal moraine ridge approximately 22 kyr old, at 2250 m elevation, 43oN latitude, using carrier manufactured from a shielded beryl, with blank less than 5% of the measured ratio for the unknowns. Boulders average 1.8 m high, were probably not covered by significant snow or sediment, and rest on the crest of the moraines so are not shielded. Samples were less than 5 cm thick and were collected with chisel and hammer from the middle of flat-topped boulders. (na) indicates the error does not apply to 10Be TCN dating.