Tracking the emergence oxygenic photosynthesis with metal isotopes: A case study from the Pongola Supergroup

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Dr. Noah Planavsky has been awarded an NSF Earth Sciences Postdoctoral Fellowship in order to pinpoint the emergence of oxygenic photosynthesis, hosted by the California Institute of Technology. The chemical composition of the ocean has changed dramatically with the gradual oxidation of the Earth's surface. Undoubtedly the single most important event in Earth's progressive oxygenation was the development of oxygenic photosynthesis, which relies only on the ubiquitous molecules carbon dioxide and water, and allowed organisms to pioneer essentially every part of the Earth reached by sunlight. The success of this metabolism is reflected in the abundance of its waste product - molecular oxygen. Despite detailed investigations over the past 50 years, there is still intense debate about the timing of the evolution of oxygenic photosynthesis, with common estimates spanning over one billion years. The proposed work will use emerging heavy metal isotope systems to track the rise of oxygenic photosynthesis. In this study, unique properties of the molybdenum (Mo) and chromium (Cr) isotope systems will be utilized to shed light on manganese (Mn) cycling in the Earth's early oceans. Investigation of Mn oxidation is essential since other redox transformations (e.g., iron oxidation) may take place in oxygen-free conditions. The study will focus on rocks deposited from 3.2 to 2.5 billion years ago, spanning the range of time during which biological oxygen production is most typically predicted to have evolved. In addition, Dr. Planavsky will carry out educational activities by engaging undergraduate students in the investigation. Pinpointing the onset of oxygen production is central to understanding the tempo of biological evolution, and will provide important insights into planetary evolution in general. Further, given the importance of oxygenic photosynthesizers in controlling Earth's redox state and the carbon cycle, addressing the question of whether or not there was oxygen production in the mid-Archean will serve as a grounding point for understanding all major biogeochemical cycles on the early Earth.