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The main objective of the work programme of CRONUS-EU (cosmic ray produced nuclide systematics - The European contribution) is to advance TCN techniques in Europe into a robust tool for Earth surface and environmental sciences.

In doing this CRONUS-EU will train a community of future high quality scientists required to develop and apply the technique for the future benefit of European science and society. The CRONUS-EU consortium will develop TCNs into an accurate absolute chronometer for use in solving a wide range of challenges in Earth and environmental sciences. The ultimate goal is the development of an internationally accepted protocol that allows accurate age determinations that are consistent with the mature geochronometers that are currently available. In the future, users of the technique must be able to obtain the same ages or process rates when analysing the same samples, irrespective of their scientific and cultural background. While this goal is in principle achievable it is not achieved to date.

Terrestrial cosmogenic nuclides - a short primer

Terrestrial cosmogenic nuclides (TCNs) are continuously produced in the uppermost layer of the Earthís surface by interactions of cosmic rays with matter. Upon approaching the Earth, the incident primary cosmic ray particles (predominantly protons) induce a nuclear reaction cascade in the upper atmosphere, which results in a particle flux dominated by high-energy neutrons in the lower atmosphere. Underground, the cosmic ray flux decreases rapidly with depth, with more than 99% of the TCNs formed in the uppermost 3 metres. Due to the interaction of primary protons and secondary neutrons with the Earth's magnetic field and the atmosphere, the cosmic ray flux is, to a first approximation, a function of latitude and altitude. Consequently, production rates of TCNs in surface rocks, for instance required to calculate ages of exposed surfaces from measured TCN concentrations, are also dependent on latitude and altitude. At the surface, production rates of TCNs vary between a few atoms and several hundred atoms per gram per year. The analysis of TCNs therefore requires advanced equipment (accelerator mass spectrometry (AMS) or high-sensitivity noble gas mass spectrometry). For TCNs to be useful to the Earth Sciences, their background concentration in geological material not exposed to cosmic rays must be extremely low. This limits the commonly used TCN to 3He, 10Be, 21Ne, 26Al and 36Cl. These are either rare noble gas isotopes (3He, 21Ne) or short-lived radionuclides (10Be, 26Al, 36Cl with half-lives of 1.5, 0.7 and 0.3 Myr, respectively) which usually do not exist in rocks unless they are produced by cosmic rays. So far geological surfaces between 50 yr and more than 10 Myr have been dated successfully with this technique. Few other geological dating tools cover a similar - 6 orders of magnitude - range where they can be applied.

The lack of a theoretical and empirical foundation to consistently calculate TCN production rates to better than 10-20% is the outstanding challenge to the routine application of the technique. Yet, advanced applications currently demand accuracy better than 5%. The danger arising from this situation is that individual investigations of TCN systematics and corresponding protocols provide significantly different pathways to calculate ages (e.g. Lal 1991; Stone 2000; Dunai 2000, 2001a,b; Masarik et al. 2001; Desilets and Zreda 2003). In application of the technique this will inevitably lead to inconsistent results, ultimately lowering the credibility and use of the method. With the work programme CRONUS-EU aims at improving current accuracy of age determinations from 10-20% to better than 5%. The goal will be achieved by:

  1. High quality calibration of TCN production rates at independently dated surfaces
  2. High quality calibration of TCN production rates using artificial targets
  3. Systematic cross calibration of production rates of different TCNs
  4. Refinement of scaling factors that describe the spatial and temporal variation of the cosmic ray flux relevant for TCN production using calibration measurements (objectives 1-3) and numerical modelling from physical principles
  5. Reducing the uncertainty of decay constants
  6. Establishing the use of additional mineral phases in exposure age dating
  7. Improvement and standardization of chemical routines
  8. Laboratory cross calibrations

CRONUS-Earth - A simultaneous US-initiative

The effort necessary to achieve above goal is significant even for the strong network team represented by this proposal. To strengthen our effort and to achieve international evaluation and acceptance, we are seeking close collaboration with CRONUS-Earth , a parallel-running northern American sister initiative obtained funding through NSF. Formal links between the two initiatives are firmly established and each consortium will address complementary aspects to achieve the common goal.