Testing the attenuation of light in evaporite-dominated sediments from the Atacama Desert.

The northern Atacama Desert displays a unique set of geomorphological features, similar to those detected on Mars. One of these features are surface polygonal patterned grounds associated with calcium-sulphate wedges in the subsurface. Vertically laminated subsurface-wedge structures occur in evaporite-bearing deposits of the Aroma alluvial fan situated in the Central Depression of Atacama Desert. The high content of calcium-sulphate phases in the wedges is assumed to lead to haloturbation processes resulting in subsurface wedge growth and polygonal patterned ground formation on the surface. To resolve wedge-growth phases and episodes of moisture supply in this generally extremely water-limited environment, geochronological data of the calcium-sulphate wedges is crucial to potentially use calcium-sulphate wedges as palaeoclimate archives in the Atacama Desert. First tests of coarse-grain feldspar luminescence dating on two subsamples extracted from the inner part of a calcium-sulphate wedge from the Aroma-fan site showed widespread and clustered equivalent dose (D_e) distributions ranging from ~20 Gy to saturation [1]. The broad post-IR IRSL₂₂₅ D_e distribution was interpreted to reflect different wedge-growth phases. However, these two wedge subsamples are not sufficient to enable an interpretation of wedge stratigraphy. To further investigate the drivers behind D_e distributions measured and to enable linking luminescence results with geomorphological interpretations, we sampled a second calcium-sulphate wedge in higher resolution. Post-IR IRSL₂₂₅ D_e distributions obtained for five subsamples from the inner part of the second wedge from the Aroma-fan site revealed similar pattern, thus letting us question the validity of the D_e distributions measured. To explore potential sources of the broad and clustered De distributions, we conduct light transmission experiments based on the study of Ou et al. [2]. Using calcium-sulphate wedge slices of different thickness we determine the light attenuation of a Xe-lamp and record transmitted wavelengths using a spectrometer. We combine these light transmission experiments with D_e measurements of feldspar grains sampled from a sediment-core-like transect into the wedge. Testing the light attenuation of calcium sulphate-bearing deposits from the Atacama Desert will help in interpreting the widespread D_e distributions recorded in different calcium-sulphate wedges and it will also aid in assessing the applicability of luminescence dating on such evaporite-dominated deposits from the Atacama Desert. [ABSTRACT FROM AUTHOR]