Cosmogenic Nuclides: Dates and Rates of Earth-Surface Change

Abstract

Cosmogenic nuclides are very rare isotopes that are produced when particles generated in supernovas in our galaxy hit the atmosphere and then the Earth's surface. When the rocks and soils in this thin, ever-changing surface layer are bombarded by such cosmic radiation, the nuclide clock begins to tick, thus providing dates and rates of Earth-surface processes. The measurement of cosmogenic nuclides tells us when earthquakes created topography at faults, when changing climate led to the growth of glaciers, how fast rivers grind mountains down, and how fast rocks weather to soil and withdraw atmospheric CO2. The use of cosmogenic nuclides is currently revolutionizing our understanding of Earth-surface processes and has significant implications for many Earth science disciplines. * Accelerator mass spectrometer (AMS) : Detection system that first accelerates ions to MeV-level energy and then separates them by mass. The technique measures the extremely small number of rare cosmogenic nuclides relative to a stable reference nuclide present in known amounts. Cosmic ray attenuation mean free path and attenuation depth scale : The depth, Λ, at which the intensity of cosmic rays is reduced by a factor of 1/e by interaction with material (units: g cm-2). 150 g cm-2 corresponds to an attenuation depth, z* = Λ/ρ, of 600 mm in silicate rock whose density (ρ) is 2.6 g cm-3. Cosmic rays, primary : High-energy (0.1 to 1020 GeV) galactic particles that are composed primarily of protons (83%), α-particles (13%), and heavier nuclei (1%) Cosmic rays, secondary : Nucleons (neutrons, protons) and muons of 0.1 to 500 MeV energy that are produced by interactions between primary cosmic rays and molecules in the Earth's atmosphere. Secondary cosmic rays form a cascade of particles whose flux decreases with increasing atmospheric pressure. Cosmogenic nuclides, in situ : Nuclides that are produced by interaction of secondary cosmic rays with solids (spallation, negative muon capture) at the Earth's surface. Other acronyms frequently used are TCN (terrestrial cosmogenic nuclides) and CRN (cosmogenic radioactive nuclides). Cosmogenic nuclides, meteoric : Cosmogenic nuclides that are produced in the atmosphere, the flux of some of which (e.g. meteoric 10Be) is ca 103 times greater than the production rate of in situ cosmogenic nuclides. Cosmogenic nuclides, radioactive : Cosmogenic nuclides that decay, and are therefore usually absent in eroding Earth materials prior to exposure (e.g. 10Be, 14C, 26Al, 36Cl) Cosmogenic nuclides, stable : Cosmogenic nuclides that are stable, and therefore might be present in eroding surface material from previous exposure episodes. These cosmogenic nuclides are the rare gases (e.g. 3He, 21Ne, 22Ne). Denudation rate : The total rate of removal of mass from the Earth's surface. It is the combined effect of physical (erosion rate) and chemical (weathering rate) processes. Electron volt (eV) : Energy of the charge of a single electron moved across an electric potential difference of one volt. MeV = mega–electron volt, one million eV. Erosion rate : The rate of removal of material from the Earth's surface by mechanical processes Fault : A planar fracture or discontinuity in a volume of rock, across which there has been significant displacement as a result of Earth movement Geomagnetic latitude : Analogous to geographic latitude, except that bearing is with respect to the magnetic pole, which changes through time, as opposed to the geographic pole Moraine : Debris that forms at the margins of a glacier Muon : A low-mass particle from cosmic radiation that is able to penetrate deeper into the Earth's surface than neutrons due to the low probability that it will interact with target atoms Nucleons : the particles that make up atomic nuclei: neutrons and protons Production rate : The rate at which in situ cosmogenic nuclides are produced in a given mass of chemically defined target material in a given time [units: atoms g-1 (mineral) y-1]. For meteoric cosmogenic nuclides a flux is used [units: atoms cm-2 y-1]. Regolith : The mantle of weathered material overlying bedrock Soil : A mixture of regolith and weathered material from below with organic matter, dust, and chemical precipitates from above Spallation : The ejection of nucleons due to impact causing production of a different nuclide without fission of the product Weathering rate : Partial dissolution of bedrock by surficial fluids, and removal of soluble ions in solution