CONFOUNDINGLY COMPLEX EXPOSURE HISTORIES REVEALED BY 21NE AND 10BE ABUNDANCES IN ALLUVIAL DEPOSITS OF THE ATACAMA DESERT, NORTHERN CHILE (Invited Presentation)

The climate history of northern Chile’s Atacama Desert is intimately linked to geologic interests ranging from the uplift history of the Andes to the formation and preservation of rich supergene copper deposits. As a result, there have been many recent efforts to constrain the age of relict landforms in the Atacama Desert using cosmogenic nuclides in the hope that these dates will help clarify the climate history of the region. These studies report some of the oldest dates and slowest erosion rates on Earth quantified using cosmogenic nuclides. To this growing body of work, our study adds a suite of ¹⁰Be and ²¹Ne abundances in alluvial sediments comprising both surface samples and depth profiles from sites centered at approximately 24°S. The concentration-depth profiles are some of the few available for the Atacama, and the majority present a story of confoundingly complex exposure histories rather than simple surface exposure dates or process rates.

We evaluate the magnitude of inheritance at each of our sample sites, and offer a first-order attempt to quantify how many cycles of burial and re-exposure each sample may have undergone. High ²¹Ne/¹⁰Be ratios require burial, shielding, or complex exposure, so ¹⁰Be abundances are not used to infer apparent exposure ages or erosion rates. However, we can use ¹⁰Be to determine a baseline for how much of each nuclide is produced in one exposure cycle, and calculate what amount of measured ²¹Ne is in ‘excess’ to one round of exposure. Commonly, ²¹Ne abundances suggest at least one previous cycle of exposure, although some samples record upwards of four exposure/shielding cycles. To link these results back to geomorphic processes, we present simple models for what ratios of burial and exposure durations may explain our samples’ nuclide ratios. A key finding of our work is the caveat that studies of landscape evolution in the Atacama Desert – or any slowly eroding, arid landscape – should consider using a multiple nuclide approach and explicitly account for inherited cosmogenic nuclide abundances if calculating surface exposure dates. If we were to take our ²¹Ne data at face value with no additional context provided by ¹⁰Be, we would incorrectly infer apparent exposure ages for some of our dated surfaces suggesting stability since the mid-to-late Miocene.