Terrestrial in situ cosmogenic nuclides: theory and application

Abstract

The cosmogenic nuclide exposure history method is undergoing major developments in analytical, theoretical, and applied areas. The capability to routinely measure low concentrations of stable and radioactive cosmogenic nuclides has led to new methods for addressing long-standing geologic questions and has provided insights into rates and styles of surficial processes. The different physical and chemical properties of the six most widely used nuclides: 3He, 10Be, 14C, 21Ne, 26Al, and 36Cl, make it possible to apply the surface exposure dating methods on rock surfaces of virtually any lithology at any latitude and altitude, for exposures ranging from 10 2 to 10 7 years. The terrestrial in situ cosmogenic nuclide method is beginning to revolutionize the manner in which we study landscape evolution. Single or multiple nuclides can be measured in a single rock surface to obtain erosion rates on boulder and bedrock surfaces, fluvial incision rates, denudation rates of individual landforms or entire drainage basins, burial histories of rock surfaces and sediment, scarp retreat, fault slip rates, paleoseismology, and paleoaltimetry. Ages of climatic variations recorded by moraine and alluvium sediments are being directly determined. Advances in our understanding of how cosmic radiation interacts with the geomagnetic field and atmosphere will improve numerical simulations of cosmic-ray interactions over any exposure duration and complement additional empirical measurements of nuclide production rates. The total uncertainty in the exposure ages is continually improving. This article presents the theory necessary for interpreting cosmogenic nuclide data, reviews estimates of parameters, describes strategies and practical considerations in field applications, and assesses sources of error in interpreting cosmogenic nuclide measurements. TABLE OF CONTENTS 1. Introduction 1.1. Development of the TCN methods 1.2. Applications of TCN Exposure methods 1.3. Previous reviews 2. Glossary 2.1. Terminology 2.2. Notation 3. Principles 3.1. Introduction 3.1.1. Source of the primary radiation 3.1.2. Effects of the geomagnetic field on GCR 3.1.3. Trajectory models and models of secondary nuclide production rates 3.1.4. Recent numerical models of GCR particle production 3.1.5. Nuclide production from primary GCR 3.1.6. TCN production by energetic nucleons 3.1.7. TCN production by low-energy neutron 3.1.8. TCN production by muons 3.1.9. Factors limiting TCN applications 3.2. Numerical simulation of low-energy neutron behavior 3.3. Analytical equations for TCN production 3.3.1. Fast neutron (Spallation) production 3.3.2. Production by epithermal neutrons 3.3.3. Production by thermal neutrons 3.3.4. Production by muons and muon-derived neutrons 3.3.5. Total nuclide production 3.4. Energetic neutron attenuation length 3.5. Temporal variations in production rates 3.5.1. Variations in the primary GCR flux 3.5.2. Variations due to solar modulation of the magnetic field 3.5.3. Effects of the geomagnetic field 3.5.4. Variations in atmospheric shielding 3.5.5. Other sources of temporal variations in production 3.6. Estimation of production rates 3.6.1. Helium-3 3.6.2. Beryllium-10 3.6.3. Carbon-14 3.6.4. Neon-21 3.6.5. Aluminum-26 3.6.6. Chlorine-36 3.7. Scaling and correction factors for production rates 3.7.1. Spatial scaling 3.7.2. Topographic shielding 3.7.3. Surface coverage 3.7.4. Sample thickness 3.7.5. Thermal neutron leakage 3.8. Exposure dating with a single TCN 3.9. Exposure dating with multiple nuclides 3.10. Nuclide-specific considerations 3.10.1. Helium-3 3.10.2. Beryllium-10 3.10.3. Carbon-14 3.10.4. Neon-21 3.10.5. Aluminum-26 3.10.6. Chlorine-36 3.11. TCN dating of sediment 4. Sampling strategies 4.1. Field sampling considerations 4.1.1. Sample description 4.1.2. Sampling methodology 4.2. Other lithological considerations 4.3. How much sample is needed? 4.4. Strategies for concentration-depth profiles 5. Sample preparation and experimental data interpretation 5.1. TCN sample preparation 5.1.1. Preparation time 5.1.2. Physical and chemical pretreatment 5.1.3. Isotopic extraction 5.2. Experimental data interpretation 6. Uncertainty and sources of error 6.1. Sources of error 6.1.1. Sample characteristics 6.1.2. Sample preparation and elemental analyses 6.1.3. Mass spectrometry 6.1.4. Systematic errors 6.2. Reporting the uncertainty 6.2.1. Error propagation 6.2.2. Evaluating accuracy by intercomparison 6.2.3. Multiple sample measurements 6.2.4. Sensitivity analysis 7. Directions of future contributions Acknowledgements References