EP036-0027
Weathering of moraines in the Central Andes is recorded by Landsat-8 imagery

Friday, 11 December 2020
Poster
Mitch Keith D'Arcy, University of British Columbia, Vancouver, BC, Canada, Martin Lang, University of Potsdam, Institute of Geosciences, Potsdam, Germany, Sam Brooke, University of California Santa Barbara, Department of Geography, Santa Barbara, CA, United States and Taylor F. Schildgen, Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences, Potsdam, Germany
Abstract:
Multispectral satellite imagery can be used to detect the mineralogical composition of Earth’s surface. In principle, multispectral images should be sensitive to the production of secondary minerals by weathering. This sensitivity presents an opportunity to investigate weathering kinetics and controls by examining the spectral evolution of landforms over time.

We use Landsat-8 imagery to constrain the spectral properties of dated moraines throughout the Central Andes. Moraines comprise unconsolidated sediment susceptible to weathering, and exhibit a range of ages and therefore degrees of weathering maturity. We measured the visible to short-wavelength infrared reflectance of 75 dated moraines, distributed among 15 study sites. Spanning 17° of latitude from Peru to Argentina, these sites experience mean precipitation rates between 100-1100 mm/yr and mean annual temperatures between 3-8 °C.

At all sites, we observed brightness in short-wavelength infrared bands gradually increasing with moraine age, relative to brightness in visible bands. This signal is compatible with the production of secondary clays and oxides by weathering, and we identify a simple band ratio as a representative weathering index. This weathering index increases non-linearly with moraine age, approaching an equilibrium value after ~60 kyr. However, both the magnitude and rate of change of the weathering index are sensitive to lithology and climate, allowing us to explore these environmental controls on weathering kinetics. Colder sites appear to weather faster, suggesting the importance of frost-cracking. Precipitation emerges as a control on inferred weathering rates through time, which accelerated during episodes of wetter climate.

These data provide new insights into weathering over long (10-100 kyr) timescales and the sensitivity of landform weathering to climatic gradients and lithology. Our observations also raise new opportunities, and potential pitfalls, for the remote prediction of moraine ages in the Central Andes, using calibrated satellite imagery.

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