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Geological Sciences Seminar: Ardiansyah Ibnu, Stanford University

Tuesday, Apr 20, 2021 12:00 PM
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Faculty/Staff, Students, Alumni/Friends
Department of Geological Science

Testing the sensitivity of secular variation in ooid size as a proxy for Phanerozoic changes in seawater chemistry and climate

Marine ooids are spherical to sub-spherical, concentrically coated carbonate grains typically 100 to 2000 µm in diameter but occasionally reaching >1 cm. Ooid size in natural systems can be predicted based on the expected equilibrium between precipitation and abrasion rates. Therefore, ooid size distributions can be used to make quantitative inferences about past ocean conditions. Indeed, exceptionally large ooids from the Lower Triassic have stimulated speculation that this interval was characterized by unusually high long-term average carbonate saturation state (Ω). To date, however, ooid sizes have not been compiled systematically across the Phanerozoic and equilibrium models have not been explored comprehensively to determine the extent to which ooid sizes present constraints on conditions of formation. Here, a new compilation of ooid sizes spanning the Phanerozoic is combined with a model for the equilibrium ooid size by coupling convolutional neural networks and Monte Carlo simulations. Across the Phanerozoic, the ooid size distributions were centered between 250 and 800 µm. Unlike normal-sized ooids, giant ooids (>>2000 µm) are useful for constraining past geological parameters important for ooid formation, indicating high Ω and temperature and being favored by the more rapid precipitation kinetics of the aragonite over calcite. These model predictions are consistent with the occurrence of giant ooids primarily during intervals of aragonite seas and greenhouse or transitional climate. When combined with other constraints on local conditions from sedimentary and geochemical data, even ooid size distributions within the more typical range may be useful in further constraining ancient Earth surface conditions.

Ardi Ibnu is currently a postdoctoral research fellow in the Geological Sciences department, Stanford University. He received his Ph.D. from the University of Manchester, UK, M.S. from King Fahd University of Petroleum and Minerals, Saudi Arabia, and B.S. from Bandung Institute of Technology, Indonesia. His current research interests include artificial intelligence for visual recognition in geosciences and quantitative carbonate sedimentology and diagenesis.

Email Kelly Wells,, for Zoom link and password.

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