Fred M. Phillips

Terrestrial in situ cosmogenic nuclides: theory and application

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 102 to 107 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 Full-size table Table options View in workspace Download as CSV

Accelerator Mass Spectrometry for Measurement of Long-Lived Radioisotopes

Particle accelerators, such as those built for research in nuclear physics, can also be used together with magnetic and electrostatic mass analyzers to measure rare isotopes at very low abundance ratios. All molecular ions can be eliminated when accelerated to energies of millions of electron volts. Some atomic isobars can be eliminated with the use of negative ions; others can be separated at high energies by measuring their rate of energy loss in a detector. The long-lived radioisotopes 10Be, 14C,26A1, 36Cl, and 1291 can now be measured in small natural samples having isotopic abundances in the range 10-12 to 10- 5 and as few as 105 atoms. In the past few years, research applications of accelerator mass spectrometry have been concentrated in the earth sciences (climatology, cosmochemistry, environmental chemistry, geochronology, glaciology, hydrology, igneous petrogenesis, minerals exploration, sedimentology, and volcanology), in anthropology and archeology (radiocarbon dating), and in physics (searches for exotic particles and measurement of halflives). In addition, accelerator mass spectrometry may become an important tool for the materials and biological sciences.

Near-Synchronous Interhemispheric Termination of the Last Glacial Maximum in Mid-Latitudes

Isotopic records from polar ice cores imply globally asynchronous warming at the end of the last glaciation. However, 10Be exposure dates show that large-scale retreat of mid-latitude Last Glacial Maximum glaciers commenced at about the same time in both hemispheres. The timing of retreat is consistent with the onset of temperature and atmospheric CO2 increases in Antarctic ice cores. We suggest that a global trend of rising summer temperatures at the end of the Last Glacial Maximum was obscured in North Atlantic regions by hypercold winters associated with unusually extensive winter sea ice.

Chronology for fluctuations in late Pleistocene Sierra Nevada glaciers and lakes

Mountain glaciers, because of their small size, are usually close to equilibrium with the local climate and thus should provide a test of whether temperature oscillations in Greenland late in the last glacial period are part of global-scale climate variability or are restricted to the North Atlantic region. Correlation of cosmogenic chlorine-36 dates on Sierra Nevada moraines with a continuous radiocarbon-dated sediment record from nearby Owens Lake shows that Sierra Nevada glacial advances were associated with Heinrich events 5, 3, 2, and 1.

Cosmogenic Chlorine-36 Chronology for Glacial Deposits at Bloody Canyon, Eastern Sierra Nevada

Deposits from mountain glaciers provide an important record of Quaternary climatic fluctuations but have proved difficult to date directly. A chronology has been obtained for glacial deposits at Bloody Canyon, California, by measurement ofthe accumulation of chlorine-36 produced by cosmic rays in boulders exposed on moraine crests. The accumulation ofchlorine-36 indicates that episodes of glaciation occurred at about 21, 24, 65, 115, 145, and 200 ka (thousand years ago). Although the timing of the glaciations correlates well with peaks of global ice volume inferred from the marine oxygen isotope record, the relative magnitudes differ markedly. The lengths of the moraines dating from 115 ka and 65 ka show that the early glacial episodes were more extensive than those during the later Wisconsin and indicate that the transition from interglacial to full glacial conditions was rapid.

ECOHYDROLOGICAL CONTROL OF DEEP DRAINAGE IN ARID AND SEMIARID REGIONS

The amount and spatial distribution of deep drainage (downward movement of water across the bottom of the root zone) and groundwater recharge affect the quantity and quality of increasingly limited groundwater in arid and semiarid regions. We synthesize research from the fields of ecology and hydrology to address the issue of deep drainage in arid and semiarid regions. We start with a recently developed hydrological model that accurately simulates soil water potential and geochemical profiles measured in thick (.50 m), unconsolidated vadose zones. Model results indicate that, since the climate change that marked the onset of the Holocene period 10 000-15 000 years ago, there has been no deep drainage in vegetated interdrainage areas and that continuous, relatively low ( ,21 MPa) soil water potentials have been maintained at depths of 2-3 m. A conceptual model con- sistent with these results proposes that the native, xeric-shrub-dominated, plant communities that gained dominance during the Holocene generated and maintained these conditions. We present three lines of ecological evidence that support the conceptual model. First, xeric shrubs have sufficiently deep rooting systems with low extraction limits to generate the modeled conditions. Second, the characteristic deep-rooted soil-plant systems store suffi- cient water to effectively buffer deep soil from climatic fluctuations in these dry environ- ments, allowing stable conditions to persist for long periods of time. And third, adaptations resulting in deep, low-extraction-limit rooting systems confer significant advantages to xeric shrubs in arid and semiarid environments. We then consider conditions in arid and semiarid regions in which the conceptual model may not apply, leading to the expectation that portions of many arid and semiarid watersheds supply some deep drainage. Further ecohy- drologic research is required to elucidate critical climatic and edaphic thresholds, evaluate the role of important physiological processes (such as hydraulic redistribution), and evaluate the role of deep roots in terms of carbon costs, nutrient uptake, and whole-plant devel-

An improved approach to calculating low-energy cosmic-ray neutron fluxes near the land/atmosphere interface

Abstract Low-energy cosmic-ray neutrons play an important role in the production of the cosmogenic nuclides 36Cl and 41Ca. Previous approaches to modeling the distribution of low-energy neutrons beneath the surface of the earth have derived the thermal neutrons directly from the high-energy neutron flux. We have improved on this model by deriving the thermal neutrons from the moderation of the epithermal neutron flux, and the epithermal neutrons from the fast neutron flux. Predictions from the improved model agree well with experimental measurements of thermal and epithermal neutron fluxes both above and below the land/atmosphere interface. Recalibration of the 36Cl surface production parameters of Phillips et al. [Phillips, F.M., Zreda, M.G., Flinsch, M.R., Elmore, D., Sharma, P., 1996. A reevaluation of cosmogenic 36Cl production rates in terrestrial rocks. Geophys. Res. Lett., 23, pp. 949–952), incorporating the new approach to simulating the low-energy neutron fluxes, yielded the following values: Ps,Ca 66.8 atoms (g Ca)−1 year−1, Ps,K 137 atoms (g K)−1 year−1, and Pf(0) 626 neutrons (g air)−1 year−1 (this updated calibration also includes mugenic 36Cl production, based on independent work). Comparison of ages of three groups of samples from sites not included in the calibration data set with independently determined ages gave an average absolute error of 6.6% for all three data sets and coefficients of variation among the samples in the groups ranging from 5% to 14%.

Cosmogenic 36Cl and 10Be ages of Quaternary glacial and fluvial deposits of the Wind River Range, Wyoming

We measured cosmogenic 36 Cl in 56 samples from boulders on moraines and fluvial terraces in the vicinity of the Wind River Range, Wyoming. We also measured 10 Be in 10 of the same samples. Most of the 10 Be ages were in good agreement with the 36 Cl ages, indicating that rock-surface erosion rates were very low. The oldest moraine investigated, the type Sacagewea Ridge site, yielded only a limiting minimum age of >232 ka. The oldest moraines in the type Bull Lake complex also could be constrained only to >130 ka. The main sequence of type Bull Lake moraines yielded age distributions indicating deposition within the intervals 130 to 100 ka and 120 to 100 ka; the best estimates are closer to the upper limits of these ranges, and associated uncertainties are in the range of 10% to 15%. These uncertainties could permit deposition in either marine isotope stage 6 or stage 5d. We found no evidence of glacial deposits dating to marine isotope stage 4. Both Bull Lake–age moraines from Fremont Lake, on the opposite side of the Wind River Range, and boulders on a fluvial terrace above the Wind River, gave age distributions very similar to that of the second oldest Bull Lake advance (ca. 130 to 100 ka). The distribution of boulder ages for Pinedale moraines at Bull Lake indicated deposition between 23 and 16 ka, nearly identical to the distribution of 10 Be ages previously reported for the type Pinedale moraines at Fremont Lake.

Climatic and Hydrologic Oscillations in the Owens Lake Basin and Adjacent Sierra Nevada, California

Oxygen isotope and total inorganic carbon values of cored sediments from the Owens Lake basin, California, indicate that Owens Lake overflowed most of the time between 52,500 and 12,500 carbon-14 (14C) years before present (B.P.). Owens Lake desiccated during or after Heinrich event H1 and was hydrologically closed during Heinrich event H2. The magnetic susceptibility and organic carbon content of cored sediments indicate that about 19 Sierra Nevada glaciations occurred between 52,500 and 23,500 14C years B.P. Most of the glacial advances were accompanied by decreases in the amount of discharge reaching Owens Lake. Comparison of the timing of glaciation with the lithic record of North Atlantic core V23-81 indicates that the number of mountain glacial cycles and the number of North Atlantic lithic events were about equal between 39,000 and 23,500 14C years B.P.

Cosmogenic chlorine-36 production rates in terrestrial rocks

Abstract Chlorine-36 is produced in rocks exposed to cosmic rays at the earth surface through thermal neutron activation of 35Cl, spallation of 39K and 40Ca, and slow negative moun capture by 40Ca. We have measured the 36Cl content of 14C-dated glacial boulders from the White Mountains in eastern California and in a 14C-dated basalt flow from Utah. Effective, time-intergrated production parameters were calculated by simultaneous solution of the 36Cl production equations. The production rates due to spallation are 4160 ± 310 and 3050 ± 210 atoms 36Cl yr−1 mol−139K and 40Ca, respectively. The thermal neutron capture rate was calculated to be (3.07 ± 0.24) × 105 neutrons (kg of rock)−1 yr−1. The reported values are normalized to sea level and high geomagnetic latitudes. Production of 36Cl at different altitudes and latitudes can be estimated by appropriate scaling of the sea level rates. Chlorine-36 dating was performed on carbonate ejecta from Meteor Crater, Arizona, and late Pleistocene morainal boulders from the Sierra Nevada, California. Calculated 36Cl ages are in good agreement with previously reported ages obtained using independent methods.