4:00 PM - 5:00 PM
I will present experimental, modeling, and field data sets demonstrating the utility and value of applying a quantitative, process-based approach to sediment transport to a long-standing puzzle in carbonate sedimentology: how ooids form. Ooids are common and enigmatic concentrically coated carbonate sand grains that provide a deep-time archive of changes in climate and seawater chemistry and are arguably the world’s most economically significant petroleum reservoirs. Substantial debate persists concerning the roles of physical, chemical, and microbial processes in their growth, including whether carbonate precipitation on ooid surfaces is driven by seawater chemistry or microbial activity, and what role—if any—sediment transport and abrasion play. To test these ideas, I developed an approach to study ooids in the laboratory—these experiments produced ooid abrasion and precipitation rates four orders of magnitude faster than radiocarbon net growth rates for natural ooids, with the corollary expectation that ooids approach a stable size representing a dynamic equilibrium between precipitation and abrasion. These results also demonstrate that the physical environment is as important as seawater chemistry in controlling ooid growth. The expansive ooid shoals surrounding Little Ambergris Cay in the Turks and Caicos Islands, provide a natural laboratory to further test the dynamic equilibrium hypothesis. Grain size and shape, surface and internal textures, and radiocarbon age data all indicate active ooid growth and abrasion occur during transport in the high energy shoal, but not in lower energy environments that might otherwise be identified as likely “ooid factories” under the assumptions of prevailing ooid formation theories. The upshot of this finding that ooid growth is likely governed by the balance of chemical and physical processes is significant for interpreting ooids in the rock record, suggesting that variations in properties like grain size, shape, and texture can be connected to and predicted by paleoenvironmental conditions including current energy and seawater carbonate chemistry.