- © 2014 by the Mineralogical Society of America
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.
When Victor Hess took off on his balloon flight from Vienna in 1912 (Fig. 1), he probably did not imagine that his discovery of cosmic rays (see Glossary for this and many other terms) at high altitudes would lead to the 1936 Nobel Prize in Physics. Moreover, he could not possibly have imagined that the same discovery would revolutionize the way geoscientists view the dynamic Earth's surface 100 years later. Cosmic rays produce rare cosmogenic nuclides, both in the atmosphere and in rocks exposed at the surface. In recent years, we have realized that these cosmogenic nuclides enable us to determine the ages of landforms and the rates at which material is removed by erosion or accumulated by sedimentation.
To put this into perspective, this revolution—featured in this issue of Elements—was preceded by the discovery of radiocarbon (14C), another nuclide produced by cosmic rays in the atmosphere, which revolutionized archeology. The radiocarbon method, not dealt with here, was developed by Willard F. Libby at the University of Chicago and earned him the Nobel Prize in Chemistry in 1960. Cosmogenic nuclides are also associated with the Nobel laureate Raymond Davis from Brookhaven National Laboratory. Although Davis earned his award for developing solar neutrino detectors, he and Oliver Schaeffer showed, as early as 1955, that the cosmogenic nuclide chlorine-36 (36Cl) was present in the surface of a mafic rock. However, the scientist who deserves the most credit for making cosmogenic nuclides a quantitative tool for measuring Earth-surface processes was Devendra Lal. At the Tata Institute of Fundamental Research in Bombay, he published, with Bernhard Peters, a classic paper in Handbuch der Physik. In this landmark work, Lal and Peters (1967) laid out the first estimates of production rates for these nuclides and the effects of altitude and latitude on these rates. It wasn't until 1982, however, when accelerator mass spectrometry was becoming available for detecting low levels of cosmogenic nuclides in geologic materials, that the first 10Be measurements were reported. This isotope, produced in the atmosphere and delivered to the oceans through precipitation, was detected in lavas erupted after the 10Be had been subducted in oceanic sediment (Brown et al. 1982).
Four years later, in 1986, cosmogenic nuclides made their debut into the study of the Earth's surface. Fred Phillips and colleagues discovered the first in situ cosmogenic 36Cl in lava flows, Kuni Nishiizumi and colleagues described the first 10Be and 26Al measured in quartz, Jeff Klein and colleagues reported the first 10Be found in desert glass, and both Mark Kurz and Harmon Craig with Robert Poreda published the first measurements of cosmogenic 3He in volcanic rocks [see the review by Gosse and Phillips (2001) for an account of these events]. Another seminal paper by Devendra Lal (Lal 1991) opened the field to geomorphology by elegantly converting the underlying fundamental physics into a framework usable by Earth scientists.
In this issue, we focus on applications of cosmogenic nuclides in terrestrial Earth-surface processes, in particular, dates (the ages of the landforms in which they are produced) and rates (the speed at which such processes occur). Many other applications of cosmogenic nuclides are not addressed here, such as radiocarbon dating, groundwater dating with 36Cl, ocean metal scavenging and oceansediment dating with 10Be, aerosol cycling with 7Be, and reconstructions of magnetic field variations in marine sediments and of solar activity based on 10Be in ice cores or 14C in tree rings. This Elements issue is about the dynamics of the Earth's land surface.
Although it might sound so implausible as to appear impossible, cosmic radiation, comprised mostly of hydrogen atoms (protons), is constantly being propelled toward us from supernova explosions that occurred far away in the galaxy. On the way to Earth, the trajectories of these particles are bent by forces exerted by the Sun, and their paths depend upon their angle of entry along the Earth's magnetic field lines. The intensity of this particle flux is greatest at the poles, where the subparallel particle trajectories and magnetic field lines allow essentially all the charged particles that make up the cosmic radiation to arrive at the Earth's surface. At the equator, on the other hand, the subperpendicular particle trajectories and magnetic field lines block a significant portion of the flux. This flux is also greatest at times when the Earth's magnetic field is weak.
When high-energy cosmic rays collide with atoms in the upper atmosphere, they initiate a cascade of nuclear reactions, producing particles that proceed all the way to the surface of the Earth. Upon hitting target atoms in the atmosphere or on the surface, these particles—mostly neutrons—produce unique cosmogenic nuclides. In the atmosphere, this nuclear reaction, called spallation, produces atmospheric (meteoric) cosmogenic nuclides, such as 7Be, 10Be, 14C, and 36Cl (Fig. 2). Some of these secondary particles, including very-high-energy neutrons and low-mass muons, survive the atmospheric cascade of collisions and eventually reach the Earth's surface. There, they can even penetrate a few meters of rock. Within the minerals of this surface layer, they produce in situ cosmogenic nuclides. These nuclides remain inside the minerals in rocks and soil particles until they decay, in the case of the radioactive nuclides such as 10Be, 14C, 26Al, 36Cl, and 53Mn; until they diffuse out of the minerals, in the case of the noble gases such as 3He and 21Ne; or until Earth scientists dissolve the rocks and capture, measure, and interpret the nuclides in their tiny numbers.
We have seen from the above description that the arrival of cosmic rays into the atmosphere and their interactions with atoms in minerals at the Earth's surface are not a straightforward affair. The sites and distribution of meteoric cosmogenic nuclides depend on atmospheric pressure and circulation (Willenbring and von Blanckenburg 2010). The in situ production of cosmogenic nuclides at the Earth's surface depends on the total mass of atmosphere in the overlying air column (Stone 2000), and hence depends on altitude. Production is further modified by the geomagnetic field strength, and therefore depends on latitude. Thus, the intensity of production of both meteoric and in situ cosmogenic nuclides is a function of variations in the Earth's magnetic field. The chemistry of the target minerals, attenuation laws, and the depth of the material govern the production of these nuclides in minerals. One might imagine that the myriad physical laws to consider would impair the use of these tools. However, cosmogenic nuclide scientists have been very insightful in developing the principles that we now apply. In their article in this issue, Dunai and Lifton (2014) provide an account of these methods.
TYPES OF NUCLIDES AND THEIR DETECTION
Atmospheric cosmogenic nuclides are produced at much greater rates than the nuclides formed in situ in minerals at the Earth's surface. The materials in which they are detected are also different: meteoric nuclides fall down, via rain or dry aerosol deposition, onto the Earth's surface, where they stick firmly onto fine-grained soil particles (Willenbring and von Blanckenburg 2010). The flux of these nuclides is approximately one million atoms per square centimeter per year. In contrast, the production rates of in situ nuclides are incredibly low, only a few atoms per gram of mineral per year (Table 1). Their measurement thus requires very sensitive detection systems, and the mineral grains must be easily separated and have simple chemical compositions. Heroic efforts in chemical isolation procedures and developments in particle physics instrumentation now allow us to measure these few thousand atoms in a single sample. Now, routine measurements by large accelerator mass spectrometers provide concentrations that are only on the order of 105 atoms of the radioactive nuclides produced in situ in a handful of sand (see the Toolkit article by Christl et al. 2014). Geochemists can measure meteoric 10Be in as little as a few milligrams of clay. A very different technique is required to measure the in situ–produced stable noble gas nuclides, such as 3He or 21Ne. These noble gas mass spectrometers are incredibly sensitive, sometimes detecting only a few atoms of gas in less than a gram of mineral, without chemical pretreatment.
THE COSMOGENIC NUCLIDE CLOCK
The simple starting point for understanding how cosmogenic nuclides are used to provide dates and rates of Earth-surface processes is to recognize that none of these nuclides could exist in minerals before exposure to cosmic radiation or before meteoric nuclides accumulate on the surface. This underlying concept is indeed the case for most of the radioactive nuclides (Table 1), whose half-lives are much shorter than the age of most geologic materials. This prerequisite is also mostly fulfilled even for the stable rare gases, as these are not typically built into the crystal structure when new minerals are forming. However, a few atoms of initial 3He or 21Ne are always present, and they impair one's ability to date the very youngest surfaces. Of the radioactive nuclides, only 36Cl is produced in the lithosphere by exposure of minerals to low-energy neutrons, and a correction for this minor initial amount is required.
The central, elegant idea behind all cosmogenic nuclide methods is this: the longer something has been exposed to cosmic radiation, the greater are the concentrations of cosmogenic nuclides in the minerals within rock and soil. One clock starts here. The clock ticks through accumulation of nuclides. When materials are covered through burial by sediment and no longer exposed to cosmogenic nuclides, the radioactive nuclides decay. Hence, a second decay clock begins to run. These two types of change in nuclide abundances allow us to ask a number of critical questions, such as: When did this landslide or glacier retreat occur? How much material has been eroded? When was this site buried (Fig. 3)?
Because cosmogenic nuclides accumulate over time, they provide ages of surface exposure (Fig. 4a). In climate science, they can record the timing of boulder deposition in moraines that build up on the edges of glaciers or the exposure history of glacially polished surfaces (Fig. 3b). One can also determine the date of disappearing ice, as explained by Ivy-Ochs and Briner (2014 this issue). In fluvial geomorphology, cosmogenic nuclides can reveal when rivers incised a mountain range or abandoned a terrace, or when the ocean retreated from a marine terrace. In the field of tectonics, cosmogenic nuclides in fault scarps can be used to date earthquakes (Fig. 3a), as Benedetti and Van der Woerd (2014 this issue) show in their article. When sediment is buried deeply enough to provide shielding from new nuclide production (for example in caves or deep in river terraces), the second clock of radioactive decay becomes a player. A sample recovered from such a deep deposit can be dated by decay (Fig. 4b), but often, the initial nuclide concentration of the buried material is not known. However, the ratios of radioactive 26Al (half-life 0.7 My) to 10Be (half-life 1.4 My) or to stable 21Ne of material eroded from the Earth surface are known. These ratios decrease due to the radioactive decay of these coupled nuclides (Fig. 4c). In this case, measurement of the ratio gives the amount of time the material has been buried.
The speed at which a soil is formed and eroded (Fig. 3c) and the average erosion rate of an entire river catchment (Fig. 3d) impose an additional control on the concentration of cosmogenic nuclides in rock and soil. Because the accumulation of cosmogenic nuclides is slower in places with fast erosion, the measured concentration scales inversely with the rate of surface removal (Fig. 4d). Hence, the cosmogenic nuclide clock slows with this additional apparent “decay” process. Both meteoric (Willenbring and von Blanckenburg 2010; von Blanckenburg et al. 2012) and in situ–produced (Lal 1991; Bierman and Nichols 2004) cosmogenic nuclides thus indirectly measure the rate of change of a landscape (Fig. 4d).
In most landscapes untouched by humans, erosion rates are typically only a few tens of millimeters per thousand years. They are so slow that they are invisible to the human eye. When measured with cosmogenic nuclides, the power of these rates is that they are insensitive to man-made perturbations and hence detect the prevalent natural erosion processes, either measured in a single soil column or in an entire river catchment. Granger and Schaller (2014 this issue) show how river sediment can be used to determine the rate at which entire mountains are eroded. When compared to short-term erosion monitors, such as modern river loads, we are able to identify how much or how little humans have affected erosion. Moreover, when sediment is buried in caves or terrace deposits, the same measurements can be used to give paleoerosion rates. Earth scientists now have an absolute method to reconstruct the pace of landscape erosion through time (Fig. 4c).
We must bear in mind, however, that in eroding settings, in situ–produced nuclides detect the rate at which material is removed, regardless of whether this is through chemical weathering or physical erosion. The sum of both processes is called denudation. Dixon and Riebe (2014 this issue) describe how the production of soil and its simultaneous denudation follow a law governed by the soil's thickness. When combined with indices of chemical weathering, cosmogenic nuclides provide the rate of the formation of soil from the chemical breakdown of the underlying rock, a process important to geochemists and Earth system scientists because of the role of weathering in the atmospheric carbon dioxide sequestration cycle.
Another form of rate determination is the use of radioactive decay to determine sedimentation rates (Fig. 4e). The principle is that soils and other unconsolidated Earth-surface materials incorporate meteoric 10Be with a certain initial concentration that depends on their exposure to this nuclide before sedimentation. The 10Be then decays following the radioactive decay law. When logarithmic concentration is correlated with depth, a sedimentation rate can be calculated (Fig. 4e). Such rates have diverse applications, including reconstructing variations in magnetic field strength (Horiuchi et al. 1999) and determining the paleoenvironmental conditions during hominid evolution (Lebatard et al. 2010).
Scientists also use cosmogenic nuclides to obtain rates of episodic Earth-surface processes using a series of exposure ages. By measuring isotopes over a depth interval for an exposed surface (Fig. 4f), they can determine the slip rate of faulted surfaces and hence estimate the recurrence interval of large earthquakes (Benedetti et al. 2002). Such data are invaluable for seismic risk assessment. These methods can also determine the rate of uplift of an ancient wave-cut platform along a modern, rising coastline. A series of river-terrace exposure or burial ages measured over a depth interval provides river incision rates. Scientists can hence measure how fast a mountain range has risen above the rivers cutting down through them (Granger and Schaller 2014).
NEW AND FUTURE DEVELOPMENTS
Insights from cosmogenic nuclides have begun to shed an entirely new light onto the dynamic Earth surface, and this tool is becoming more accessible to practitioners. The US and European CRONUS (cosmic ray–produced nuclide systematics on Earth) projects have advanced our understanding and synchronized nuclide production rates, half-lives, and scaling laws. The physical principles are accessible in the form of comprehensive review articles (Gosse and Phillips 2001), books written for the newcomer (Dunai 2010), and online code to simplify the myriad calculations necessary to interpret a concentration (Balco et al. 2008).
At the same time, new analytical and methodological developments are emerging that will pave the way for further discoveries. Radiocarbon (14C) is produced not only in the atmosphere but also in situ in minerals. Its short half-life will reveal the stability of sediment in large floodplains, before it is moved along as a river changes its course (Hippe et al. 2012). Also of interest is 38Ar, a rare gas cosmogenic nuclide whose abundance in minerals can be investigated in a manner similar to conventional 39Ar–40Ar dating (Niedermann et al. 2007). Meteoric 10Be can be combined with the stable nuclide 9Be to provide simultaneous erosion and weathering rates for soils and fine-grained river particles (von Blanckenburg et al. 2012). Uncommon nuclides such as 53Mn will allow the use of as-yet-unexplored older target materials, but this system will require accelerator mass spectrometers with higher energies than those currently available (Schaefer et al. 2006). Finally, simply lowering the detection limit of existing techniques will drive scientific development. For example, moraines as young as the Little Ice Age (ca 200 years ago) can now be dated with great precision, enabling us to reconstruct the terrestrial imprint of historic climate variations with unprecedented detail (Schaefer et al. 2009).
This issue of Elements is dedicated to Devendra Lal (1929–2012), a pioneer in the field of cosmogenic nuclides and a scientist who inspired practitioners in the field to do creative, careful science and to learn things every day. We thank Fred Phillips and Nat Lifton for their careful reviews of this article. We are grateful to Trish Dove, principal editor, for her careful and constructive edits of the articles in this issue, and to Pierrette Tremblay for converting them so expertly into the journal's format.
- 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
- 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
- Debris that forms at the margins of a glacier
- 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
- 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].
- The mantle of weathered material overlying bedrock
- A mixture of regolith and weathered material from below with organic matter, dust, and chemical precipitates from above
- 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