Measuring erosion rates using rare isotopes in rocks

Mauricio Haag joined Prof. Lindsay Schoenbohm’s tectonic geomorphology group at the University of Toronto Mississauga as a PhD student in 2021 after receiving a Master’s degree at Universidade Federal do Rio Grande do Sul in Brazil. Mauricio’s PhD project aims to investigate erosion rates along the continental escarpment in southeast Brazil, focusing on how lithology, climate, and tectonics influence the spatial pattern of these erosion rates. 

But one may wonder, how are erosion rates measured? When rocks are exposed to the surface, they are impacted by cosmic rays. This process produces rare isotopes (called terrestrial cosmogenic isotopes) that accumulate in the rocks, similar to a sunburn: the longer the rocks are exposed, the more isotopes they accumulate. In a slow-eroding landscape, rocks remain exposed for longer periods, allowing a larger number of cosmogenic isotopes to accumulate. In contrast, a fast-eroding landscape continuously sheds its surface layers, leaving little time for isotopes to build up, resulting in much lower concentrations. So, by measuring the concentration of these rare cosmogenic isotopes in sediments derived from rocks, we can estimate how long the rock has been exposed. Eroded material from hilltops eventually ends up in rivers, deposited as sand and coarser particles. By analyzing cosmogenic isotope concentrations in fluvial sand, we can determine how fast the landscape is eroding.

Thanks to the funding from the CPS Research Visit Fellowship Program, supported by the Vice Dean, Graduate’s Office, and the Department of CPS, Mauricio had the opportunity to visit the NSF Community Cosmogenic Facility in the summer of 2023 to process samples for erosion rates. During this visit, he focused on cleaning and isolating quartz from sand samples to extract Be isotopes. This involved sieving, cleaning, and purifying the samples through a series of physical and chemical processes aimed at removing all minerals except quartz. This step is necessary because the rate of isotope production in quartz is well-established, unlike many other minerals that may also be present in the sample but have less constrained production rates.

This was the first time Mauricio had completed the full spectrum of lab work required to prepare samples for analysis, including quartz purification and chemical treatments. This involved training and acquiring essential experience in working with hazardous acids such as hydrofluoric (HF) and nitric acid (HNO₃), crucial for the sample purification process. Experiencing the entire process - from planning fieldwork and collecting samples to processing them in the lab - has given him a hands-on experience and a deeper understanding of the complexities involved in this technique. Mauricio is grateful for the opportunity to develop these new skills, which allowed him to obtain samples and collaborate closely and learn from the researchers at the NSF Community Cosmogenic Facility.