CAREER: Linking the stable isotope record of Earth's surface and interior across the Great Oxidation Event

California Institute Of Technology, Pasadena CA

Investigators

Abstract

Oxygen levels in Earth’s atmosphere have risen throughout its ~4.5 billion-year history in two major transitions. The first, known as the Great Oxidation Event (GOE), occurred circa 2.3-2.4 billion years ago when atmospheric oxygen levels increased to 1-10% of present day levels. This rise in oxygen levels left an indelible imprint on the sedimentary rock record through the onset of oxidative weathering and shifts in stable isotope compositions. During this same time interval, the igneous (or magmatic) rock record underwent significant chemical and isotopic changes. The proposed research will investigate whether the changes in the sedimentary and igneous rock records are linked through studying the stable isotopes of a unique suite of igneous rocks known as strongly peraluminous granites (SPGs). SPGs form when sedimentary rocks are metamorphosed and partially melted to form granitic melts. SPGs therefore represent an endmember igneous archive of a pure sedimentary contribution (through partial melting and assimilation) to the igneous rock record. Through analyzing the isotopes of SPGs on either side of the GOE, this study will determine if and how changing sedimentary compositions affected the igneous rock record across this critical transition in Earth’s history. The PI will lead an integrated research and education plan designed to encourage participation and retention of under-represented minorities in OFW-based research through a multi-tiered mentoring system. This project supports one female or minority graduate student at Caltech, 3 undergraduate summer fellows, 3 high-school student and teacher interns from the Pasadena Unified School District (PUSD), and annual classroom lessons and field trips for PUSD middle schoolers to the local San Gabriel Mountains. The proposed research in this CAREER project will explore how the GOE influenced the O, S, and Fe isotope record of igneous rocks through partial melting or assimilation of sediments. The research examines Archean and Proterozoic strongly peraluminous granites, which represent pure partial melts of metasedimentary rocks. This research has three main objectives. First, it will shed light on the underlying causes for shifts observed in the stable isotope record of igneous rocks across the GOE. For example, the increase in O isotope ratios in magmatic zircon has been variably ascribed to the onset of plate tectonics and recycling of sedimentary rocks or to the rise of oxidative, subaerial weathering generating heavier O isotope ratios in sediments. If the latter is true, a shift in the oxygen isotope ratios of zircon from Archean and Proterozoic SPGs should be observed. Second, this research will explore in detail how Fe and S isotopes are transferred from sediments to igneous rocks through detailed field studies of two “focus” localities which preserved representative source metasedimentary rocks, as well as, the derivative SPG. Third, the SPGs provide an alternative archive to the sedimentary record in that they homogenize large volumes of sedimentary rocks and thus can yield information on average sedimentary compositions. Importantly, this work will establish a “reference collection” of SPGs, which will be curated at Caltech and made available to external researchers to investigate other aspects of their chemistry. 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.