『Abstract
The study of a sample of river sediment enables the determination
of spatially averaged denudation rates that provide an exceptional
perspective on erosion and weathering processes that have taken
place within a landscape. These measurements are done with in-situ-produced
cosmogenic nuclides (e.g., 10Be, 26Al),
mostly in quartz from alluvial sediment. Cosmogenic nuclides are
produced when secondary cosmic rays interact with the very uppermost
layer of the Earth's surface. They are produced within a characteristic
depth scale of about 1 m, which means that the measured concentrations
record an integrated denudation history while material passed
through this depth interval. Depending on the denudation rate
the resulting integration time scales are 103 to 105
years, and one obtains a robust long-term estimate of natural
denudation that is relatively insensitive to short-term changes.
The last 10 years have seen significant research activity using
these methods, and an array of fascinating tectono-geomorphologic
and geochemical insights are emerging. Amongst these is the ability
to identify the physical and chemical processes with which a landscape
responds to tectonic activity or climate change. A compilation
of world-wide denudation rates in non-glaciated areas, that however,
does not yet include some of the world's most active mountain
belts, has resulted in the following findings, some of which have
been unexpected: (1) No obvious relationship between precipitation
or mean annual temperature and total denudation is apparent. (2)
Topographic relief alone does not result in high rates of denudation.
(3) Denudation rates are high in areas of landscape rejuvenation;
that is triggered and controlled by tectonic activity (faulting,
escarpment formation and retreat, rifting, surface uplift). (4)
Rates of weathering (using a combination of cosmogenic nuclides
and zirconium-normalised cation loss balances) co-vary primarily
with physical erosion rates and much less with temperature or
precipitation. (5) In some areas of high land use short-term severity
of geomorphic change caused by human action. In the future, the
control mechanisms over denudation will be determined on all spatial
scales, ranging from the single soil section to entire river basins.
The same analysis can be done back through time on well-dated
terraces, lake records, and marine sediment cores, which is possible
with 10Be for the past 1-2 My. The rates obtained will
be used to develop a quantitative understanding of tectonic, geomorphologic,
and geochemical landscape processes, which in turn is a prerequisite
to design and calibrate models of the response of landscapes to
tectonic, climate, and anthropogenic forcing.
Keywords: cosmogenic nuclides; erosion; weathering; geomorphology』
1. Introduction
2. Methodological principles
2.1. Cosmic rays, nuclear reactions, and the production of
cosmogenic nuclides
2.2. Scaling laws. surface production rates, absorption, and
analytical techniques
2.2.1. Scaling of cosmic ray intensities
2.2.2. Correction for shielding
2.2.3. Production rates in minerals
2.2.4. Absorption of cosmic rays in rocks and soils
2.2.5. Measurement of cosmogenic nuclides
2.3. Denudation rates
2.4. Let nature do the averaging
2.5. Averaging time scales
3. Inter-method comparison: why time scale matters
4. Topography and rates of geomorphic processes
5. Climate, erosion and rates of chemical weathering
6. Paleo-denudation rates
6.1. Climate time scales
6.2. Tectonic time scales
7. Mountains will erode in the future too
Acknowledgements
References
(Author)
| Cosmic rays, primary | High-energy (0.1 to 1020 GeV) galactic particles, composed primarily of protons (83％), α-particles (13％), and heavier nuclei (1％). When these particles reach the upper atmosphere they cause nuclear reactions that lead to secondary cosmic rays. A 3％ electron contribution is not relevant for cosmogenic nuclide production. |
| Cosmic rays, secondary | Nucleons (neutrons, protons) and muons of 0.1 to 500 MeV energy that have been produced by interaction 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 | Radioactive cosmogenic nuclides decay, and are therefore usually absent in eroding earth materials prior to exposure (e.g., 10Be, 14C, 26Al, 36Cl). Stable cosmogenic nuclides might be present in eroding surface material from previous exposure episodes. These cosmogenic isotopes are the rare gases (e.g., 3He, 21Ne, 22Ne). |
| Cosmogenic nuclides, in situ-produced | 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 “Terrestrial Cosmogenic Nuclides (TCN)” or “Cosmogenic Radio Nuclides (CNR)”. In situ-produced cosmogenic nuclides are distinct from meteoric cosmogenic nuclides that are produced in the atmosphere, the flux of some of which (e.g. meteoric 10Be) is 103 times greater. |
| Denudation rate | The total rate of removal of mass from, and in depths near the Earth's surface. It is the combined effect of physical (→Erosion rate) and chemical (→Weathering rate) processes. Since cosmogenic nuclides accumulate as material moves towards the surface by the removal of material above, they always measure the total (erosion and weathering) rate, and hence the denudation rate [units: e.g., m My-1 = mm ky-1 = μm year-1 = t km-2 year-1/ρ, where ρ is the bedrocks density in t m-3]. |
| Erosion rate | The rate of lowering of the Earth's surface due to mechanical processes. In the literature, it is a common inaccuracy to call cosmogenic nuclide-derived denudation rates “erosion rates” or even “sediment generation rates”. This terminological simplification is a valid approximation only in areas in which the weathering rate is negligible when compared to the (physical) erosion rate. Note that “Soil erosion” as used by geographers and agronomists describes erosion caused by human action. |
| Time scale of denudation rates | The mean time a cosmogenic nuclide-derived denudation rate integrates over. The rate corresponds to the time denuding material resides in one absorption depth scale z*. |
| Weathering rate | Partial dissolution of bedrock by surfacial fluids, and removal of soluble ions in solution. The weathering rates is included in cosmogenic nuclide-derived denudation rates. |