Elementary, My Dear: Al & Be Give Evidence of Past Climate Change

Editors’ Highlights are summaries of recent papers by AGU’s journal editors.

Source: Journal of Geophysical Research: Earth Surface

Terrestrial Cosmogenic Nuclides (TCN) are rare elements formed when cosmic rays interact with atoms in rocks near the surface of the Earth. They are routinely used to estimate erosion rates across landscapes. If erosion is slow, rocks spend a lot of time near the surface of the Earth and accumulate a lot of TCN: river sands (made from the breaking down of rocks) will therefore be rich in TCNs, and vice-versa. However, a fundamental assumption in most published studies is that erosion rates are constant through time.

Recent progress has shown that TCN-based methods have a great potential to identify and quantify changes in erosion rates by using paired TCNs that are produced and disintegrate by radioactive decay at different rates. For example, ¹⁰Be and ²⁶Al have a half-life of 1.4 and 0.7 million years ago (Ma), respectively, where the half-life represents the time over which half of the atoms of a given element have disintegrated within a system (e.g., a mineral). If the erosion rate calculated from the concentrations of ¹⁰Be is the same as the erosion rate calculated from the concentrations of ²⁶Al, then erosion rates have been constant. If the results are different, then there has been a change in erosion rate over the time period considered.

Godard et al. [2024] test this theory and apply it to the Mid-Pleistocene Transition (MPT), a major climatic shift that occurred around 1 Ma and is argued to have led to a global increase in erosion rates. Importantly, they select a study area in the highlands of the Brazilian Southeast that has a series of benefits for this purpose:

Erosion rates are very low, on the order of a few mm per thousand years, meaning that rocks have spent a very long time near the surface of the Earth and the TCN concentrations will reflect erosion rates over the past few millions of years (compared to a few thousands of years, typically, in fast eroding landscapes).
The sampling locations have been chosen to demonstrate that there has been no recent tectonic change (which could also lead to changes in erosion rates): any change in erosion rates would be climatically driven.
The area has experienced no glaciation, so any change could be interpreted in terms of climatic influence on erosion rates through changes in precipitation, temperature, and other related factors such as vegetation, rather than associated with the growth or decay of glaciers.

The authors show a clear disequilibrium between the erosion rates derived from ¹⁰Be and ²⁶Al. With their new model, they demonstrate that the difference could be explained by a significant increase in erosion rates at the MPT, supporting the hypothesis that climate change has increased erosion rates even in areas that are far away from glacial influences. This work demonstrates the potential for paired TCNs to not only quantify erosion rates but also changes in erosion rates over a range of timescales, providing a powerful tool to reconstruct the tectonic and erosional histories of landscapes, and therefore to elucidate the feedbacks between tectonics, climate, and erosion.

Citation: Godard, V., Siame, L. L., Salgado, A. A. R., & ASTER Team (2024). Erosional response to Pleistocene climate changes in the Brazilian highlands. Journal of Geophysical Research: Earth Surface, 129, e2024JF007671. https://doi.org/10.1029/2024JF007671

—Mikaël Attal, Editor, JGR: Earth Surface