TESTING THE LINKS BETWEEN CLIMATE AND SEDIMENTATION IN THE ATACAMA DESERT, NORTHERN CHILE: INSIGHTS THROUGH COSMOGENIC 36CL

The near-hyperarid Atacama Desert of northern Chile preserves an extensive surficial record of discrete sedimentation events. This setting provides a unique opportunity in which to understand sedimentary response to precipitation at an extreme of Earth’s climate.

The Chajnantor Plateau (23° S) is a 1.3 Ma ignimbrite dome set within the framework of the subtropical Andes Mountains. Recent cosmogenic exposure ages of glacial features on the plateau suggest local glaciation during the Last Glacial Maximum. Below the glacial limit, the plateau is incised by quebradas and ephemeral bedrock channels, some of which extend up to the glacial margin. Along the western margin of the Chajnantor Plateau, Pleistocene and Holocene alluvial fan deposits grade out to the Salar de Atacama salt flat. These sediments are likely sourced from volcaniclastic deposits from higher elevations on the plateau. The fan deposits previously extended onto the surface of the ignimbrite shield and have since been stripped and reworked along the front of the plateau. Subsequent fluvial activity has incised into these sedimentary packages, wherein numerous, extensive debris flow deposits are hosted. Both the alluvial fans and debris flows represent periods of increased fluvial activity, and by proxy, increased precipitation. A clear relationship between these depositional features and periods of increased precipitation will support an interpretation that similar features and their stratigraphic counterparts reflect climatic excursions.

We seek to enhance our understanding of the nature and timing of these depositional features by establishing absolute and relative chronologies for these sedimentation events. We employ terrestrial cosmogenic nuclide (TCN) exposure dating to determine the ages of these sedimentary deposits, with ³⁶Cl as the targeted radionuclide. In order to constrain potential inheritance in the nuclide signals for the fan deposits, we measure a TCN concentration depth profile. To help validate our exposure ages, we utilize morphometric analysis on debris flow boulders and fan surface cobbles as an alternative method to correlate deposits.