Marek Zreda

Spatial and temporal distribution of secondary cosmic-ray nucleon intensities and applications to in situ cosmogenic dating

Cosmogenic nuclide production rates depend critically on the spatio-temporal distribution of cosmic-ray nucleon fluxes. Since the 1950s, measurements of the altitude, latitude and solar modulation dependencies of secondary cosmic-ray fluxes have been obtained by numerous investigators. However, until recently there has been no attempt to thoroughly evaluate the large body of modern cosmic-ray literature, to explain systematic discrepancies between measurements or to put these data into a rigorous theoretical framework appropriate for cosmogenic dating. The most important parameter to be constrained is the dependence of neutron intensity on atmospheric depth. Our analysis shows that effective nucleon attenuation lengths measured with neutron monitors over altitudes 0^5000 m range from 128 to 142 g cm 32 at effective vertical cutoff rigidities of 0.5 and 14.9 GV, respectively. Effective attenuation lengths derived from thermal neutron data are somewhat higher, ranging from 134 to 155 g cm 32 at the same cutoff rigidities and over the same altitudes. We attribute the difference to a combination of two factors: the neutron monitor is more sensitive to the higher end of the nucleon energy spectrum, and the shape of the nucleon energy spectrum shifts towards lower energies with increasing atmospheric depth. We have derived separate scaling models for thermal neutron reactions and spallation reactions based on a comprehensive analysis of cosmic-ray survey data. By assuming that cosmic-ray intensity depends only on atmospheric depth and effective vertical cutoff rigidity, these models can be used to correct production rates for temporal changes in geomagnetic intensity using paleomagnetic records. 6 2002 Elsevier Science B.V. All rights reserved.

Measuring soil moisture content non‐invasively at intermediate spatial scale using cosmic‐ray neutrons

[3] We present a novel non-invasive technique that utilizes the dependence of the low-energy cosmic-ray neutron intensity above the ground surface on the hydrogen content of soil. The cosmic-ray method is based on slowing down and thermalization of cosmic-ray neutrons by hydrogen atoms present in soil. Soil moisture greatly affects the rate at which fast neutrons are moderated, controlling neutron concentration in soils and prescribing their emission into the air. Dry soils have low moderating power and are therefore highly emissive; wet soils are more moderating and therefore less emissive as highly moderated neutrons are more efficiently removed from the system. The change in soil neutron emission is sufficient to produce a clear signal in the neutron intensity above the surface. For soil moisture content varying from zero to 40% volumetrically, the corresponding decrease in cosmic-ray neutron intensity above the surface is 60%, a hundredth of which can easily be measured using a neutron detector.

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.

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.

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.

Assessment of the SMAP Passive Soil Moisture Product

The National Aeronautics and Space Administration (NASA) Soil Moisture Active Passive (SMAP) satellite mission was launched on January 31, 2015. The observatory was developed to provide global mapping of high-resolution soil moisture and freeze-thaw state every two to three days using an L-band (active) radar and an L-band (passive) radiometer. After an irrecoverable hardware failure of the radar on July 7, 2015, the radiometer-only soil moisture product became the only operational soil moisture product for SMAP. The product provides soil moisture estimates posted on a 36 km Earth-fixed grid produced using brightness temperature observations from descending passes. Within months after the commissioning of the SMAP radiometer, the product was assessed to have attained preliminary (beta) science quality, and data were released to the public for evaluation in September 2015. The product is available from the NASA Distributed Active Archive Center at the National Snow and Ice Data Center. This paper provides a summary of the Level 2 Passive Soil Moisture Product (L2_SM_P) and its validation against in situ ground measurements collected from different data sources. Initial in situ comparisons conducted between March 31, 2015 and October 26, 2015, at a limited number of core validation sites (CVSs) and several hundred sparse network points, indicate that the V-pol Single Channel Algorithm (SCA-V) currently delivers the best performance among algorithms considered for L2_SM_P, based on several metrics. The accuracy of the soil moisture retrievals averaged over the CVSs was 0.038 m3/m3 unbiased root-mean-square difference (ubRMSD), which approaches the SMAP mission requirement of 0.040 m3/m3.

A reevaluation of cosmogenic 36Cl production rates in terrestrial rocks

We have measured 36Cl in a suite of 33 rock samples having well-constrained exposure histories and ages. The 36Cl production parameters were estimated by minimizing the squared deviations between the 36Cl and independent ages, yielding the following production parameters: spallation and muon production from Ca 2940±200 atoms 36Cl (mole Ca)−1 yr−1, spallation from K 6020±400 atoms 36Cl (mole K)−1 yr−1, and neutron production in air 586±40 fast neutrons (g air)−1 yr−1. The new production constants for spallation on Ca and thermal neutron activation are in good agreement with previous results, but that for spallation on K is about 50% larger. The past decade has seen a rapid growth in the application of terrestrial cosmogenic nuclides produced by cosmic-ray particles within rocks at the earth surface. Nuclear cross sections, particularly for reactions initiated by neutrons and muons, are not known with sufficient accuracy so production rates for these nuclides must be empirically calibrated using independently dated geological samples. Direct production estimates for 36Cl were first obtained by Zreda et al. (1991), who measured 36C1 in mineral separates from granitic boulders on moraine crests at Chiatovitch Creek in the eastern White Mountains of California and Nevada. This data set was the best available at the time, but since then several factors have prompted a reevaluation of the 36Cl production rates. One is that Zreda et al. (1994) have shown that there can be significant scatter of the cosmogenic ages of individual glacial boulders toward ages younger than that of the moraine, due to soil erosion. A second factor is that a much wider range of samples is now available for this purpose. Finally, recent theoretical advances in calculating production by thermal neutron absorption (Liu et al., 1994) now permit more realistic calibration of this reaction.

Measurement depth of the cosmic ray soil moisture probe affected by hydrogen from various sources

[1] We present here a simple and robust framework for quantifying the effective sensor depth of cosmic ray soil moisture neutron probes such that reliable water fluxes may be computed from a time series of cosmic ray soil moisture. In particular, we describe how the neutron signal depends on three near-surface hydrogen sources: surface water, soil moisture, and lattice water (water in minerals present in soil solids) and also their vertical variations. Through a combined modeling study of one-dimensional water flow in soil and neutron transport in the atmosphere and subsurface, we compare average water content between the simulated soil moisture profiles and the universal calibration equation which is used to estimate water content from neutron counts. By using a linear sensitivity weighting function, we find that during evaporation and drainage periods the RMSE of the two average water contents is 0.0070 m 3 m � 3 with a maximum deviation of 0.010 m 3 m � 3 for a range of soil types. During infiltration, the RMSE is 0.011 m 3 m � 3 with a maximum deviation of 0.020 m 3 m � 3 , where piston like flow conditions exists for the homogeneous isotropic media. Because piston flow is unlikely during natural conditions at the horizontal scale of hundreds of meters that is measured by the cosmic ray probe, this modeled deviation of 0.020 m 3 m � 3 represents the worst case scenario for cosmic ray sensing of soil moisture. Comparison of cosmic ray soil moisture data and a distributed sensor soil moisture network in Southern Arizona indicates an RMSE of 0.011 m 3 m � 3 over a

Chronology of Quaternary glaciations in East Africa

Abstract A new glacial chronology for equatorial East Africa is developed using in situ cosmogenic 36 Cl measured in 122 boulders from moraines on Mount Kenya and Kilimanjaro. The oldest deposits sampled on Kilimanjaro yield a limiting 36 Cl age of >360 calendar kyr (all 36 Cl ages are in calendar years, cal. kyr or cal. yr). On Mount Kenya, the oldest moraines give ages of 355–420 kyr (Liki I) and 255–285 kyr (Teleki). Given the uncertainty in our 36 Cl ages, the Liki I moraine may correspond to either marine isotope stage 10 or 12, whereas the Teleki moraine correlates with stage 8. There is no evidence for stage 6 on either mountain. The Liki II moraines on Mt. Kenya and moraines of the Fourth Glaciation on Kilimanjaro give ages of 28±3 kyr and 20±1 kyr, respectively. They represent the last glacial maximum (LGM) and correlate with stage 2 of the marine isotope record. A series of smaller moraines above the LGM deposits record several readvances that occurred during the late glacial. On Mt. Kenya, these deposits date to 14.6±1.2 kyr (Liki IIA), 10.2±0.5 kyr (Liki III), 8.6±0.2 kyr (Liki IIIA) and ∼200 yr (Lewis); the corresponding deposits on Kilimanjaro have mean ages of 17.3±2.9 kyr (Fourth Glaciation–Saddle), 15.8±2.5 kyr (Little Glaciation–Saddle), and 13.8±2.3 kyr (Fourth Glaciation–Kibo). These data indicate that the climate of the tropics was extremely variable at the end of the last glacial cycle.