Physical, Chemical, and Microbial Controls on Growth and Degradation of Ooids on Ambergris Shoal, Little Ambergris Cay, Turks and Caicos Islands, British Overseas Territories

Abstract

A major challenge in understanding ooid formation lies in untangling the effects of sediment transport and abrasion on ooid size, shape, and texture from those of chemical and microbial processes. Little Ambergris Cay—situated near the southern margin of the Caicos Platform in the Turks and Caicos Islands—is surrounded by expansive modern ooid shoals and provides a natural laboratory to examine the effects of these processes on ooid size and fabric. Currents driven by sustained easterly winds transport ooids westward along the ~7 km shoreline of Little Ambergris Cay (E-W elongation) and the ~20 km long ooid shoal extending westward from the western tip of the island. Westward-elongated tidal bars along the shoreline and the chevron pattern of asymmetric sand waves on the shoal indicate a strong component of westward sediment transport. 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 in July 2016 and August 2017. Ooids steadily increase in size and sphericity along the shoal, indicating that ooids are growing during transport even as abrasion modifies grain shapes; this grain size trend is reproducible from year to year. Petrographic textures and whole ooid radiocarbon ages corroborate this conclusion that ooids are actively growing as they are transported along the shoal. Microscopy, lipid geochemistry, and 16S rRNA data from in situ samples and incubation experiments demonstrate that microbial biofilms play a destructive rather than constructive role—degrading ooid cortices via microboring by endolithic cyanobacteria. δ¹³C_carb, δ¹⁸O_carb, and δ³⁴S_CAS values of ooids from the shoal indicate precipitation near equilibrium with local seawater. Carbonate saturation state (Ω_aragonite) is constant along the length of the shoal, indicating that ooid growth is not being driven by a change in carbonate chemistry. Placed in the context of recent 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.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018