TERRESTRIAL COSMOGENIC NUCLIDES: SPANNING THE GEOMORPHOLOGICAL SCALES OF SPACE AND TIME

Since the early 1990’s three technology-driven advances have enabled us to revisit questions of landscape evolution conceived at the beginning of the century and new questions have been spawned. Increased efficacy of repetitive analyses of digital elevation data, fruition of the (U-Th)/He thermochronology method, and development of TCN exposure methods have triggered the current revolution in global geomorphology.

The effective upper limit of the TCN chronometer (millions of years) was attained early in the Dry Valleys of Antarctica by K. Nishiizumi and others. We are now approaching the lower age limit of centuries and decades on sites with sufficient cosmic ray flux. This opens new doors for monitoring human and global change influences on the landscape. Only a decade ago were single and multiple TCN used to estimate rates of erosion on single landforms. Today, larger scale basin-wide average erosion rates may be estimated from modern stream samples to bridge modern geodetic records and long term exhumation histories. Multi-nuclide approaches have provided means of dating stream sediment long abandoned in shielded caves, and documenting which mountains have been buried by non-erosive ice.

Although much work remains to reduce uncertainty in scaling TCN production rates from place to place, improvements in the analytical techniques have allowed the community to begin evaluating smaller random and systematic uncertainties (1% to 5% effects on production rates, such as atmospheric paleodynamics, forest cover, muonic interactions, and shape and size of the landform). Systematic uncertainties can be avoided when using the technique to correlate moraines in neighbouring regions or alluvial fans along adjacent mountain fronts. New isotope-mineral systems will expand applicability, and as chemical and analytical costs decrease and more laboratories open, other questions currently prohibited by expense can be addressed.