Active ooid growth driven by sediment transport in a high-energy shoal, Little Ambergris Cay, Turks and Caicos Islands

Elizabeth J. Trower, Marjorie D. Cantine, Maya L. Gomes, John P. Grotzinger, Andrew H. Knoll, Michael P. Lamb, Usha Lingappa, Shane S. O'Reilly, Theodore M. Present, Nathan Stein, Justin V. Strauss, Woodward W. Fischer

Research output: Contribution to journalArticlepeer-review

Abstract

Ooids are a common component of carbonate successions of all ages and present signicant potential as paleoenvironmental proxies, if the mechanisms that control their formation and growth can be understood quantitatively. There are a number of hypotheses about the controls on ooid growth, each offering different ideas on where and how ooids accrete and what role, if any, sediment transport and abrasion might play. These hypotheses have not been well tested in the eld, largely due to the inherent challenges of tracking individual grains over long timescales. This study presents a detailed eld test of ooid-growth hypotheses on Little Ambergris Cay in the Turks and Caicos Islands, British Overseas Territories. This eld site is characterized by westward net sediment transport from waves driven by persistent easterly trade winds. This conguration makes it possible to track changes in ooid properties along their transport path as a proxy for changes in time. Ooid size, shape, and radiocarbon age were compared along this path to determine in which environments ooids are growing or abrading. Ooid surface textures, petrographic fabrics, stable-isotope compositions (δ13C, δ18O, and δ34S), lipid geochemistry, and genetic data were compared to characterize mechanisms of precipitation and degradation and to determine the relative contributions of abiotic (e.g., abiotic precipitation, physical abrasion) and biologically inuenced processes (e.g., biologically mediated precipitation, fabric destruction through microbial microboring and micritization) to grain size and character. A convergence of evidence shows that active ooid growth occurs along the transport path in a high-energy shoal environment characterized by frequent suspended-load transport: median ooid size increases by more than 100 μm and bulk radiocarbon ages decrease by 360 yr westward along the ∼ 20 km length of the shoal crest. Lipid and 16S rRNA data highlight a spatial disconnect between the environments with the most extensive biolm colonization and environments with active ooid growth. Stable-isotope compositions are indistinguishable among samples, and are consistent with abiotic precipitation of aragonite from seawater. Westward increases in ooid sphericity and the abundance of well-polished ooids illustrate that ooids experience subequal amounts of growth and abrasion-in favor of net growth-as they are transported along the shoal crest. Overall, these results demonstrate that, in the Ambergris system, the mechanism of ooid growth is dominantly abiotic and the loci of ooid growth is determined by both carbonate saturation and sediment transport mode. Microbes play a largely destructive, rather than constructive, role in ooid size and fabric.

Original languageEnglish
Pages (from-to)1132-1151
Number of pages20
JournalJournal of Sedimentary Research
Volume88
Issue number9
DOIs
Publication statusPublished - Sep 2018

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