Goethite Internal Thermometry - Improvements and Applications

California Institute Of Technology, Pasadena CA

Investigators

Abstract

Earth's climate varies over timescales from tens of years to millions of years in response to factors such as the sun's output, the arrangement of continents, ocean circulation, and the composition of the atmosphere. Much of what we know about past climate comes from analysis of minerals that carry a record of environmental conditions under which they grew. For example, polar ice constitutes an archive of climate extending back hundreds of thousands of years, while the carbonate skeletons of marine organisms carry similar information extending to hundreds of millions of years ago. At present, little is known about environmental conditions on the continents in deep time, primarily because few materials are known that carry a record of paleoclimate and for which an accurate formation age can be determined. Recent work indicates that the mineral goethite, a common iron oxide produced during weathering of continental rocks, can be radiometrically dated and also contains a record of its formation temperature. This award will explore this new method of characterizing continental paleoclimate by developing an efficient and automated technique to assess goethite formation temperature, and, once established, will use that technique to refine the temperature calibration of the method and apply it to suites of dated goethite specimens spanning the last 66 million years from localities in Europe, Brazil, and Australia. The end product will be an improved analytical methodology and amongst the first multimillion year paleotemperature records from the continents. The work will support a graduate student and will also engage students from a local community college in both the science of the undertaking and in the development and programming of the automated analytical system to be developed. Goethite (FeOOH) is formed when Fe-bearing minerals interact with oxygen-bearing ground and surface waters. In weathering environments, this very insoluble phase forms, survives and accumulates, becoming a major constituent of ferricretes, gossans, paleosols and laterites. Two recent advances suggest that goethite can provide a unique record of paleotemperatures in settings, such as continental interiors, where almost no alternative paleothermometer exists. First, using the (U-Th)/He method goethite can be dated with an uncertainty <5%. A compilation of more than 1500 dates reveals that goethite formation spans, almost without interruption, the entire Cenozoic Era (0-66 Ma). Second, the two crystallographically-distinct oxygen sites have a readily-measured temperature-dependent contrast in 18O/16O, allowing single-phase paleothermometry. This project will link these two advances by undertaking three tasks designed to improve and refine the goethite-internal thermometry method, and to apply it in a systematic way for the first time. Task 1 is to design and build a new oxygen extraction line that can process goethite specimens in a completely automated and efficient way. This is a critical step to permit the large number of oxygen isotopic analyses required by this project. Task 2 is to refine the calibration of the goethite internal thermometer by analyzing synthetic goethites grown under a more diverse range of temperature and chemical conditions than previous work. This task will improve the uncertainty on goethite-internal temperatures, and will identify any as-yet unrecognized secondary controls on derived temperatures. Task 3 consists of the first systematic investigation of the goethite internal thermometry archive. Three suites of already-dated goethites will be analyzed for paleotemperature (total ~100 samples): supergene goethites from a deeply weathered sulfide deposit in the Amazon basin, goethite pisoliths from Central Europe, and goethites from the enigmatic Channel Iron Deposits of Western Australia. From each of these localities we will obtain oxygen isotope data that continuously spans many millions of years. These records will allow the researchers to assess internal consistency of the goethite temperature estimates, and to document for the first time paleotemperatures at high temporal resolution over the entire Cenozoic from three different continents. A fourth task is designed to engage members of the community with this project and simultaneously to teach them a useful skill. The principal investigator will lead a 3-day short-course in Labview, a programming language that fundamentally enables the proposed work. Competitively-selected students participating in the course will include individuals from local high schools and/or community colleges. The short-course will introduce the science and the approaches of goethite-internal thermometry, and coding examples and exercises will be drawn from the proposed analytical work. At the end of the short course one of these students will be selected for a ten-week internship to develop/document Labview code for the proposed extraction line. More traditional Broader Impacts of this project include support for a PhD student, refinement and verification of an entirely new paleoclimate tool that can be broadly applied, and creation of new paleoclimate records from previously unstudied areas that will be useful in geoscience and beyond. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.