TERRETRIAL COSMOGENIC NUCLIDE METHODS: TAMING ELEPHANTS IN THE ROOM

Theoretical and experimental developments in the TCN method over the past decade have helped the technique improve its chronometric capacity. In particular, the CRONUS-Earth and CRONUS-EU programs provided an opportunity to overcome challenges to the method, characterised and improved random and systematic error, and established methodological protocols. Over a dozen new calibration sites have helped to improve our knowledge of TCN production rates over the past 20 ka, tests of previous and new (altitudinal and latitudinal) spatial scaling of production rates have been completed, and an empirical correction to previous estimates of muonic interactions is being realized. This year the CRONUS-Earth program will introduce calculators for every nuclide that are based on interactions over the full energy spectrum of neutrons supported by improved nuclear cross sections. The calculators couple erosion with time-varying production rates which benefit from recent improvements in geomagnetic paleointensity records. External error in production rate and an estimate of random error resulting from interlaboratory comparisons significantly improve our capacity to evaluate uncertainty of routine measurements. Sub-percent 1σ precisions for high quality samples have been repeatedly achieved. New approaches to previously intangible questions have been realised, particularly in burial dating and the establishing of ¹⁴C as a viable short-lived TCN method. However, we still struggle with fundamental obstacles that preclude the ubiquitous application of the method. Erosion rates, inheritance, moisture content, and sediment bulk densities are usually not time-constant, yet calculators require this assumption. Complex exposures including episodic stripping and aggradation events are not treated by the calculators. Improved production rate control is needed at high elevations, low latitudes, and for exposure durations beyond 20 ka. Uncertainty in snow, moisture, and geometric effects significantly reduces the accuracy of ³⁶Cl dating of Cl-rich mineral phases. The rate of deep, >6000 g cm^-2 (>20 m rock) muonic production of TCN remains uncertain, precluding the accurate radionuclide burial dating of deep samples. Quantification of environmental controls will remain a priority.