[Postponed] Seminar: Xiaoming Liu, Dept of Geological Sciences, UNC-CH

“Elemental ratios in marine carbonates as tracers of Earth’s O₂ evolution?” Earth’s O₂–containing atmosphere, unique among known planets, has played an essential role in evolving feedbacks between life and environment. Atmospheric O₂ was extremely low in the Archean Eon (>2.5 billion years ago), and while multiple lines of evidence suggest that Earth’s oxygenation was protracted and dynamic, pO₂ may have risen abruptly at two different points in time: first during the “Great Oxygenation Event” (GOE) at ~2.4 billion years (Ga), when atmospheric O₂ rose from <0.001% to an intermediate value commonly estimated as 1 to 10% of the current level, and again during a “Neoproterozoic Oxygenation Event” (NOE) at ~800 to 542 million years ago. The latter transition may well have continued into the Phanerozoic Eon, eventually resulting in near-present O₂.

However, most early Earth studies have focused on deeper marine facies (shales) as windows to whole-ocean conditions dominated by deep-ocean anoxia and have ironically suffered from a lack of convincing paleoredox data specific to shallow settings often dominated by carbonate facies. Our best proxies available today were developed for the organic-rich shales with an emphasis on constraining atmospheric and deeper ocean conditions. So, this study seeks to remedy that situation by developing and refining paleoredox evaluations of carbonate rocks through (1) validation and calibration of techniques in modern analog settings with a particular emphases on potential diagenetic overprints that might confound our goal of extracting primary information and (2) application to key intervals in the key middle chapters of Earth history and the fascinating but still vexing transitions that book-ended this period of seeming biotic, climatic, and biogeochemical stasis.