Devendra Lal

Cosmic ray labeling of erosion surfaces: in situ nuclide production rates and erosion models

A number of in situ cosmogenic radionuclides and stable nuclides have been measured in natural exposed rock surfaces with a view to study their in situ production and rock erosion rates [1]. The in situ radionuclides can be used for a high-resolution tomography of the erosional history of an exposed surface; two stable nuclides (3He, 21Ne) and five radionuclides (10Be, 26Al, 36Cl, 14C, 39Ar) having half-lives in the range of ∼ 300-1.5 × 106 yr half-life are measurable in many rock types. A prerequisite for the application of the in situ nuclides for the study of erosional histories of surfaces is a knowledge of their production rates under different irradiation conditions; altitude, latitude, irradiation geometry and shielding. Relative nuclide production rates can be determined fairly accurately using the extensive available data on cosmic ray neutrons [2]. Absolute nuclide production rates cannot generally be predicted with any accuracy because of lack of data on excitation functions of nuclides unless some normalization is possible, as was done in the case of several cosmic ray produced isotopes in the atmosphere [3]. Based on a recent natural calibration experiment in which erosion free surfaces exposed to cosmic radiation for ∼ 11,000 yrs were sampled, the absolute production rates of 10Be and 26Al in quartz have been accurately estimated for mountain altitudes in Sierra Nevada [4]. The absolute production rates of 10Be and 26Al in quartz can therefore be estimated fairly accurately for any given latitude and altitude. Some measurements of 14C in rocks of low erosion rate [5] similarly allow an estimate of its production rate. Attempts made to measure the in situ production rates of 3He in rocks have not yet led to a convergent production rate. In view of the importance of knowing the production rates of isotopes of He, Ne and Ar, I present here theoretical estimates of their production rates based on available cross-section data. I discuss the information that can be extracted from the study of the in situ nuclides in rocks. Useful parameters characterizing the exposure history of a rock surface are: (1) the effective surface exposure age; and (2) the time-averaged erosion rate. The implications of these parameters for single and multiple nuclide studies are discussed in terms of the erosion models considered.

Production of selected cosmogenic radionuclides by muons: 2. Capture of negative muons

We have determined the production yields for radionuclides in Al2O3, SiO2, S, Ar, K2SO4, CaCO3, Fe, Ni and Cu targets, which were irradiated with slow negative muons at the Paul Scherrer Institute in Villigen (Switzerland). The fluences of the stopped negative muons were determined by measuring the muonic X-rays. The concentrations of the long-lived and short-lived radionuclides were measured with accelerator mass spectrometry (AMS) and γ-spectroscopy, respectively. Special emphasis was put on the radionuclides 10Be, 14C and 26Al produced in quartz targets, 26Al in Al2O3 and S targets, 36Cl in K2SO4 and CaCO3 targets, and 53Mn in Fe2O3 targets. These targets were selected because they are also the naturally occurring target minerals for cosmic ray interactions in typical rocks. We also present results of calculations for depth-dependent production rates of radionuclides produced after cosmic ray μ− capture, as well as cosmic ray-induced production rates of geologically relevant radionuclides produced by the nucleonic component, by μ− capture, by fast muons and by neutron capture.

Production of selected cosmogenic radionuclides by muons 1. Fast muons

To investigate muon-induced nuclear reactions leading to the production of radionuclides, targets made of C9H12, SiO2 ,A l 2O3, Al, S, CaCO3, Fe, Ni, Cu, Gd, Yb and Tl were irradiated with 100 and 190 GeV muons in the NA54 experimental setup at CERN. The radionuclide concentrations were measured with accelerator mass spectrometry and Q-spectroscopy. Results are presented for the corresponding partial formation cross-sections. Several of the long-lived and short-lived radionuclides studied are also produced by fast cosmic ray muons in the atmosphere and at depths underground. Because of their importance to earth sciences investigations, calculations of the depth dependence of production rates by fast cosmic ray muons have been made. : 2002 Elsevier Science B.V. All rights reserved.

Tracing quartz through the environment

Quartz, SiO2, a pure mineral with tight crystal structure, is widespread in rocks and soil. Cosmic rays produce10Be (t1/2=1·5×106, yr) and26Al (t1/2=7·05×105 yr) in quartz exposed at or near the earth’s surface. The use of accelerator mass spectrometry permits measurement of these nuclides in samples exposed at sea level for typical periods.In situ production makes interpretation relatively straightforward. Potential applications include age determination, measurement of erosion and deposition rates, and use as a tracer for continental weathering processes.

Five million year10Be record in Chinese loess and red-clay: climate and weathering relationships

10Be measurements in bulk samples, in different size fractions, in selective chemically leached fractions of several representative loess-paleosol sequences and in the underlying red-clay sediments from the Chinese Loess Plateau, back to ca. 5 Ma, were made using the AMS (accelerator mass spectrometer). Our data support but considerably extend the earlier studies of Shen and Beer and their colleagues in the upper 0.75 Ma Luochuan loess-paleosol sequences. The effect of climatic change is dramatically expressed by10Be concentrations: (1) in the red-clay sediments, for which we estimate model accumulation rates of 5.8 and 8.7 m Ma−1 for the upper sequences in Lingtai and Baoji, respectively; (2) in the paleosols where soil maturation processes are manifest in the10Be distribution within different size fractions, accompanied by downward movement to zones of high authigenic mineral concentration; and (3) in loess samples which, at deposition, had an appreciable endowment of10Be from the dust source regions. On the basis of leaching experiments, we estimate that, on average, the paleosol layers lost about 3±3% of their initial10Be content to the underlying layers by transport with water. Finally, we estimate that the relative average zonal precipitation rate during glacial periods was ∼(2-3)×the average interglacial value. The estimates of the initial10Be concentrations and dust composition are consistent with the measured values for the recent atmospheric dust falls sampled from the Loess Plateau and the Shandong Peninsula.

Weathering histories of Chinese loess deposits based on uranium and thorium series nuclides and cosmogenic 10Be

The long, continuous deposition of dust in the Chinese loess plateau (Liu et al., 1985; Kukla and An, 1989; Ding et al., 1991a) offers an unique opportunity to study the nature of soil weathering in a wide range of climatic conditions. In this paper we report on measurements of concentrations of U- and Th-series nuclides and of major cations in 150 loess and paleosol samples from five sites, going back 2.5 Ma. Using the results for 10Be concentrations in these soils (Gu et al., 1996), we determined the absolute amounts of water added to several soil units and obtained: (1) first-order leaching constants for U and several cations and (2) the compositions of the soils contributing to the dust-source regions and of the dust at deposition. Further, based on analyses of 230Th in soils deposited in the past ca. 140 ka, we determined when the soils weathered in the source regions. We conclude that most of the weathering in the dust-source regions may have occurred during the interglacials.

Cosmic-Ray Record in Solar System Matter

The energetic nuclei in cosmic rays interact with meteoroids, the moon, planets, and other solar system matter. The nuclides and heavy nuclei tracks produced by the cosmic-ray particles in these targets contain a wealth of information about the history of the objects and temporal and spatial variations in the particle fluxes. Most lunar samples and meny meteorites have complex histories of cosmicray exposure from erosion, gardening, fragmentation, orbital changes, and other processes. There appear to be variations in the past fluxes of solar particles, and possibly also galactic cosmic rays, on time scales of 104 to 107 years.

Erosion history of the Tibetan Plateau since the last Interglacial: constraints from the first studies of cosmogenic 10Be from Tibetan bedrock

Abstract The cosmogenic 10Be exposure histories of in situ bedrock surfaces from the Tibetan Plateau indicate low erosion rates of

Low Background Thin‐Wall Flow Counters for Measuring Beta Activity of Solids

The construction and performance of disk‐shaped thin‐walled flow counters suitable for measuring very small beta activities of solids is described. The circular faces, which serve as the cathode, consist of conducting films. Either a single pear‐shaped loop of wire whose plane is parallel to that of the faces, or a group of straight wires stretched across the midplane serves as the anode. The counters are operated in the Geiger region. Samples can be counted against both faces of the counter. Performance data are given for counters having active volumes ranging from 0.2 to 7 cc, and having active areas from 1.6 to 23 cm2. Counting efficiencies for natural potassium (mounted on Lucite) vary from 30% to 47%, depending on the relationship of sample area to counter dimensions. Inside a 20‐cm steel shield and an anticoincidence ring the background counting rates varied approximately as 1 count/hr/cm2 of counting area. Backgrounds as low as 1 count/hr were obtained.The disk geometry permits the use of a smaller...

Beryllium geochemistry in soils: evaluation of 10Be/9Be ratios in authigenic minerals as a basis for age models

Soils contain a diverse and complex set of chemicals and minerals. Being an ‘open system’, both in the chemical and nuclear sense, soils have defied quantitative nuclear dating. However, based on the published studies of the cosmogenic atmospheric 10Be in soils, its relatively long half-life (1.5 Ma), and the fact that 10Be gets quickly incorporated in most soil minerals, this radionuclide appears to be potentially the most useful for soil dating. We therefore studied the natural variations in the specific activities of 10Be with respect to the isotope 9Be in mineral phases in eight profiles of diverse soils from temperate to tropical climatic regimes and evaluated the implications of the data for determining the time of formation of soil minerals, following an earlier suggestion [Lal et al., 1991. Development of cosmogenic nuclear methods for the study of soil erosion and formation rates. Current Sci. 61, 636–639.]. We find that the 10Be/9Be ratios in both bulk soils and in the authigenic mineral phases are confined within a narrower range than in 10Be concentrations. Also, the highest 10Be/9Be ratios in authigenic minerals are observed at the soil-rock interface as predicted by the model. We present model 10Be/9Be ages of the B-horizon and the corresponding soil formation rates for several soil profiles. The present study demonstrates that the 10Be/9Be ratios in the authigenic phases, e.g. clay and Fe-hydroxides, can indeed be used for obtaining useful model ages for soils younger than 10–15 Ma. However, the present work has to be pushed considerably further, to take into account more realistic age models in which, for instance, downward transport 10Be and clays, and in-situ dissolution of clay minerals at depths, altering the 10Be/9Be ratios of the acidic solutions, are included. We show that in the case of younger soils (< 1 Ma) studied here, their 10Be inventories and 10Be/9Be ratios have been significantly disturbed possibly by mixing with transported soils.