A FLATTENING OF THE (OXYGEN) CURVE: EXPLORING THE RELATIONSHIP BETWEEN TECTONICS AND PROTEROZOIC OXYGENATION TRENDS

Nearly all models of Earth’s oxygenation converge on the premise that the first notable rise of atmospheric oxygen occurred shortly after the Archean-Proterozoic boundary, with the second notable rise occurring shortly before the Proterozoic-Phanerozoic boundary. The trajectory of atmospheric oxygen and oceanic redox conditions in the intervening Proterozoic Eon, representing almost 2 billion years of geological history, is far less certain, despite decades of research. The emerging picture shows a dynamic oxygenation history with global trends that indicate overall high-low-high oxygen levels throughout the Proterozoic Eon. With ever more data and the development of new proxies, the picture becomes more complicated by the overprint of local redox conditions, which might mask global trends. The mid-Proterozoic (1.8 to 0.8 Ga) represents an even more controversial timeframe, with a majority view of sustained low atmospheric oxygen during H. Holland’s “boring billion”, but that level is widely debated (~1 to 10% PAL, or potentially as low as 0.1% PAL). This contravenes the tenet that major orogenic events (e.g., the Himalaya-scale Trans-Hudson orogen) should yield higher oxygen levels, not lower. Plate tectonics should present a positive feedback mechanism for increasing oxygen levels. These processes promote nutrient delivery, efficient biogeochemical cycling of carbon, leading to the burial of organic carbon, which is thought to be the primary regulator of atmospheric oxygen level in a post evolution-of-photosynthesis world. The contrast of a low-oxygen mid-Proterozoic with a high-oxygen Paleoproterozoic is particularly striking, and mechanisms that might have caused this secular change remain unexplained. These themes and questions will be explored in this 4-minute talk, focused particularly on the lead-up to, and aftermath of, the 1.9 to 1.7 Ga interval as a turning point in tectonic evolution.