EAGER: Early-mid Pliocene Climate Change in Northern Chile

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Godfrey/EAR-1117496/Rutgers UniversityThe aim of this early-concept proposal for exploratory research is to determine the climate significance of the Opache formation, a carbonate rock unit deposited in the Atacama Desert of northern Chile between ~7.8 and 3.37 million years ago. Rainfall in the Atacama, both in its central area and in the Andes to its east, is strongly influenced by El Niño, the Pacific Decadal Oscillation, and on longer timescales by the South American monsoon. The Opache is made up of two carbonate facies, a laminated tufa which formed from basin over-spill and a palustrine unit. The aim of this study is to reconstruct climate (hydrologic balance) from sedimentological features such as lamina thickness. The investigators believe that two bands were deposited annually with the seasonal cycle strengthening roughly every seventeen years, but they want to demonstrate that two laminae form annually and are directly related to climate. The palustrine carbonate unit deposited in the Calama Basin will be used to infer the long-term climate of the region during a time of climate re-organization.This proposal will focus on modern analogs to aid in interpreting the rock record. The Opache is unique in the central Andes in regard to its size rather than formation mechanism. Within 30 km, active carbonate deposition occurs in and close to the Rio Loa from groundwater springs as well as in high altitude lakes and both can provide important insight to the formation of both Opache facies. In addition to study of sediment fabrics from thin section, the investigators will use stable isotopes to constrain kinetic effects on CO2 such as degassing. Dating for these modern analog studies will utilize Ra-226 (half life 1599 years), combined with U-Th dating at Rutgers. Laminae cyclicity can be determined by counting between dated horizon from active growth surface and compared with recorded climate data. This EAGER proposal includes addition of dates of the laminated unit by U-Pb, bringing the technique to Rutgers.Successful testing of the hypothesis that a decadal climate record is possible will result in a submission of a full proposal which will yield a rare continental decadal climate record from the early-mid Pliocene in a region influenced by ENSO. This project will include undergraduate involvement in international research and collaboration with Rutgers University which broadens their research experience. The investigators have high expectations that a recent climate record will be extracted from the modern analog studies.