Thematic Articles

Landscapes are sculpted by a variety of processes that weather and erode bedrock, converting it into soils and sediments that are moved downslope. Quantifying erosion rates provides important insights into a wide range of questions in disciplines from tectonics and landscape evolution to the impacts of land use. Cosmogenic nuclides contained in quartz sediment provide a robust tool for determining spatially averaged erosion rates across scales ranging from single hillslopes to continental river basins and are providing fundamental clues to how landscapes evolve. Cosmogenic nuclides in buried sediments contain unique information about paleo–erosion rates up to millions of years in the past. This article explores some of the basic ideas behind various methods used to infer catchment-wide erosion rates and highlights recent examples related to problems in tectonics, climate, and land use.

The conversion of rock to soil prepares Earth’s surface for erosion by wind, water, gravity, and life. Together these agents wear down hills and mountains even as the land rises up under the stress of tectonic forces in the crust. Meanwhile, weathering liberates nutrients from minerals and disaggregates rock into regolith, generating hospitable substrates for life. Over the last two decades, geochemists, geomorphologists, and soil scientists have increasingly used cosmogenic nuclides to quantify how fast soils are made, modified, and finally swept away in hilly and mountainous landscapes around the world. These studies are revolutionizing our understanding of soils and their role in feedbacks that shape Earth’s surface, influence overlying ecosystems, and modulate climate over millions of years.

When the recurrence intervals of large earthquakes span several thousands of years, the dating of fault movements over long time intervals is essential for estimating the next event. Constraining the age of faulting, earthquake recurrence, or toppled rocks is especially important for determining if a fault is likely to break again soon. In recent years, cosmogenic nuclides have provided new insights into the dating of these ground movements. Approaches to gathering this information can be direct, such as dating fault surfaces with 36Cl, or indirect, such as dating fault-offset alluvial fans with 10Be or 26Al. New results from these methods are certain to better define the tectonic and seismic hazards in areas with increasing population density.

Cosmogenic nuclides are remarkably well suited to dating glacial landforms. Exposure dating of boulders on moraines and of glacially sculpted bedrock allows the determination of the ages of former ice margins, from which past glaciations can be temporally constrained. Where moraines are lacking or are poorly preserved, outwash is dated with depth profile dating. Two-nuclide methods can be used to determine the ages of buried till. Multinuclide measurements of bedrock ages also provide insights into periods of non-erosive ice coverage and can be used to identify regions with selective linear erosion. Of particular interest is the use of cosmogenic nuclides to assess rates of glacier retreat and glacial erosion.

Over the last 60 years, our understanding of how cosmic rays produce cosmogenic nuclides has grown from basic physical considerations. We introduce the different types of cosmic ray particles and how their flux varies with altitude, latitude, and time. Accurately describing these variations remains a challenge for some regions when calculating production rates. We describe current and emerging computational methods for calculating production rates that address this challenge. Continuing developments in our understanding of modern and prehistoric cosmic ray fluxes and energy spectra in Earth’s atmosphere and at its surface are bound to contribute in the future to more robust applications.

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.