Anne-Sofie Ahm, Ph.D.

Education: University of Copenhagen, Ph.D., Geology-Geoscience
Institution: Princeton University (laboratory of John Higgins)

SCOL Project: Do Carbon Isotopes in Carbonate Tell Us When Life Became a Major Player in the Global Carbon and Oxygen Cycles?

Throughout the history of our planet, the biogeochemical cycles on Earth have gone through critical transitions. Perhaps most profound is the evolution of photosynthesis and the rise of atmospheric oxygen approximately 2.3 billion years ago (the Great Oxidation Event). These evolutionary events are expected to cause major changes in the global carbon cycle, recorded by the composition of stable carbon isotopes in carbonate rocks. However, ancient carbonate sediments deposited before the Great Oxidation Event have baseline carbon isotope values of approximately 0%, broadly similar to values recorded in Phanerozoic sediments deposited more than 2 billion years later. This observation challenges our understanding of how sediments record the global carbon cycle on early Earth. It is likely that the carbon cycle on early Earth was significantly different from the modern and included fluxes that are not relevant on Earth today. Moreover, only a fraction of ancient carbonate sediments is preserved in the geologic record due to the continuous recycling of ocean crust through the forces of plate tectonics. The chemistry of sediment may also be altered during the transformation of primary sediment into rocks (diagenesis). As a result, it is possible that the chemistry of the ancient carbonate sediments that we can study today is not telling us a true story of the ancient carbon cycle. In this proposal, we will use new calcium and magnesium isotope measurements from ancient carbonate rocks (approximately 2.8–2.5 billion years old) to constrain the carbon cycle on early Earth leading up to the Great Oxidation Event. The calcium and magnesium isotopic composition of carbonate sediments records information about what fraction of carbonate sediments is preserved in the geologic record and whether this fraction is isotopically representative of the global carbonate sink. In addition to new measurements, we plan to use numerical models to constrain the degree of diagenetic alteration and identify the main carbonate sink. Armed with this knowledge, we will be able to infer, or perhaps directly identify, the pieces missing in our understanding of the ancient carbon cycle.