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Sediment Transport Controls Ooid Growth

Abstract

The widely-accepted model of ooid formation was developed decades ago – ooid cortices form via precipitation of aragonite or calcite from fluids supersaturated with respect to CaCO3 while ooid nuclei are fully suspended in the water column, with ooid size expected to increase monotonically until grains are buried and/or achieve a maximum size at which they can no longer be suspended. This model, however, does not account for the observation that net growth rates of modern ooids are many orders of magnitude slower than ooid growth rates anticipated from field, experimental, and empirical data. We present modeling, experimental, and field data sets demonstrating that abrasion occurs at a significant rate for carbonate sand transported as bedload and suspended load. Ooid abrasion rates are comparable in magnitude with anticipated rapid precipitation rates, supporting the hypothesis that ooids approach a stable size that represents a dynamic equilibrium between precipitation and abrasion. These results also highlight counterintuitive size dynamics due to tradeoffs in abrasion as a function of transport regime: larger ooids can be produced by increasing current energy without changing precipitation rate. The expansive ooid shoals surrounding Little Ambergris Cay in the Turks and Caicos Islands provide a natural laboratory to examine the effects of transport on ooid size and texture. Currents driven by sustained easterly winds transport ooids westward along the ~7 km shoreline of Little Ambergris Cay and the ~20 km long ooid shoal extending westward from the cay. On the shoal, ooids are actively transported near the threshold of suspension on sand waves, while incipiently-cemented hardgrounds occur in barform troughs and off the edges of the active shoal, indicating rapid and early cementation outside the zone of active transport, as well as substantial sediment bypass. We sampled ooid sand from the intertidal zone along the northern edge of the cay and sand wave crests along the length of shoal. Size, texture, and radiocarbon age data indicate active ooid growth during transport along the shoal, even as abrasion modifies grain shapes. Placed in the context of our experimental and modeling results, the Little Ambergris ooids demonstrate larger stable ooid sizes resulting from increased current energy, highlighting the significant role that physical environment plays in controlling ooid growth.