ARTHUR L. DAY MEDAL: WORLDS WITH AND WITHOUT OXYGEN: A CAREER-LONG JOURNEY FROM MODERN EARTH TO ANCIENT EARTH TO EARTH-LIKE PLANETS FAR BEYOND

My interests in biospheric oxygen began fortuitously with opportunities to collect and analyze samples from the famously anoxic modern Black Sea and Cariaco Basin. These samples became touchstones for developing, validating, and calibrating a wide diversity of proxies for ancient biogeochemical cycles and paleoredox, including many newly emerging nontraditional stable-isotope systems. My collaborators and I quickly began to apply these methods to oxygen-lean intervals in Earth history, including classic Mesozoic oceanic anoxic events and a diversity of Paleozoic black shale-hosting intervals often marked by dramatic biotic events. Luckily, my early attention was also turned to first-order redox evolution of the Proterozoic and Archean. And as astrobiology grew as a discipline at NASA so did our realization that the many chapters of Earth history, or alternative Earths, provide wide-ranging templates in the search for life in our galactic neighborhood far beyond our solar system. Any evaluation of potential biosignature gases in the atmospheres of exoplanets requires a tight systems’ grasp of the full gamut of biotic and abiotic controls, and Earth teaches us how to take the first steps. This talk will explore our path to current understandings of co-evolving early environments and life and, importantly, new directions and opportunities here at home and far beyond.