Investigation of the triple oxygen isotope systematics of shales in the Precambrian

University Of Oregon Eugene, Eugene OR

Investigators

Abstract

Shale is a major sedimentary rock type that records the composition of the Earth's crust and its weathering. As the process of weathering proceeds with a great excess of water, the oxygen isotope value in shales reflects the isotopic value of water interacting with the rock and the temperature of weathering. Using triple oxygen isotopes (17O/16O versus 18O/16O) systematics in the main sedimentary rock type, shale and associated diagenetic silica, the investigator proposes to gain insight into paleoenvironmental and weathering conditions during early Earth's history. His collection of shales covers 3.5 billion years and spans the major snowball Earth glacial and greenhouse episodes, the Great Oxidation Event, the Neoproterozoic appearance of multicellular life in oceans, and the appearance of life on land. As part of the educational component, this project will support a graduate student. As an outreach of this effort, the investigator will lead a one-day workshop at the American Geophysical meeting in San Francisco in 2020, to which he will invite colleagues working on this topic, as well as those interested in the advent of precise microanalytical and rare isotope methods for the Precambrian. He will additionally develop a set of standards for 17O that span a large range in delta 18O and delta 17O values for cross-laboratory calibration purposes and make it available to the community for analysis of sedimentary and other materials. The project will: 1) make measurements, using the investigator's newly constructed high-precision 17O extraction line, of a collection of shales across geologic history that has been previously characterized for delta 18O, Titanium isotopes, and chemical composition; 2) describe long-term and geological interval-specific, triple oxygen trends of maturing lithosphere, evolving hydrosphere and/or increasing continental subaerial extent; 3) examine triple oxygen signatures of quartz within shales and zircons through time, and 4) build a geochemical box model of coupled hydrosphere-crustal evolution with additional 17O input parameters that will permit resolution of previous controversies related to hotter environments in the Archean versus low delta 18O hydrosphere based on their measurements and those generated by other laboratories. 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.