Godfrey/EAR-1117496/Rutgers University The 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.
A pilot study was conducted in Northern Chile to address the potential for a large and unusual carbonate units for recording climate and tectonic changes during the late Miocene to Pliocene (10-2 Myrs ago). The area is currently very dry desert (the Atacama) and the frequency of rainfall is primarily a reflection of ENSO activity. This makes it an ideal location to study ENSO-like actiivity durng a recent period of Earth history when conditions were slightly warmer than present. The location is also positioned ideally for studying tectonic changes associated with the uplift of the Andes, a key factor in determining modern hyperaridity. The purpose of the study was to determine the changes in the carbonate fabric across different parts of the Calama Basin and their temporal relationship, and to acquire a small set of geochemical data to address the climate and tectonic informaiton these carbonates might contain. Carbonates formed in wetland conditions between approximately 8 and 3 Myrs ago based on ash horizons over ~800 sqkm in the lower part of the basin. They contain stromatolites, oncolites, gastropods and freshwater sponges. Around the sites of groundwater discharge, finely laminated microterraces and low barrage dams formed. Distributed throughout the basin, at high and low elevation are spring carbonates formed of laminated aragonite, some of which bear the evidence of deformation during tectonic activity associated with mountain building. Our first age measurement directly on the carbonate of one of these spring sites shows it was formed very rapidly around 7 Myrs ago. Dating of other carbonates within the basin is underway. Carbon and oxygen isotope data of the spring carbonate show that there was a large release of CO2 during the uplift phase of the Andes, the source of which will be part of our continued investigation. Strontioum isotopes, used to fingerprint the water sources in which the carbonate formed, indicate two sources of water, the developing Andes to the east and the older Pre-Cordillera to the west. Today, precipitation on the Andes is derived from the Atlantic and only under exceptional circumstances does it reach areas further west. Moisture from the Pacific falls around the coastal areas during El Nino events. For the Pre-cordillera to receive enough rainfall to be recorded in the carbonate units, the climate system during the late Miocene-Pliocene suggests wind fields and ocean circulation were quite different from the modern.
