Active Ooid Growth Driven By Sediment Transport in a High-Energy Shoal, Little Ambergris Cay, Turks and Caicos Islands

Abstract

Ooids are a common component of carbonate successions of all ages and present significant 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 field, largely due to the inherent challenges of tracking individual grains over long timescales. This study presents a detailed field test of ooid-growth hypotheses on Little Ambergris Cay in the Turks and Caicos Islands, British Overseas Territories. This field site is characterized by westward net sediment transport from waves driven by persistent easterly trade winds. This configuration 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 (d 13 C, d 18 O, and d 34 S), 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 influenced 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 lm 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 biofilm 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.

The Enigma of Neoproterozoic Giant Ooids—Fingerprints of Extreme Climate?

“…The carbonate mineral saturation states required for giant ooids with D = 10 mm varied substantially depending on mineralogy (aragonite, calcite, or ikaite) and temperature (T = 0°C, 25°C, 40°C) ( Figure 1). Aragonitic or calcitic ooids of this size under conditions similar to those on modern carbonate platforms (i.e., T = 25°C) required Ω aragonite ≅ 19 or Ω calcite ≅ 26 (Figures 1a and 1b), both of which are notably higher than saturation states observed in modern shallow marine settings (e.g., Ω aragonite = 5 in the Turks and Caicos Islands, Trower et al, 2018). In contrast, aragonitic giant ooids at T = 40°C required Ω aragonite ≅ 10 ( Figure 1c), and ikaite giant ooids at T = 0°C required Ω ikaite ≅ 4 ( Figure 1d).…”

“…TROWER 3 of 10 shoal in the Turks and Caicos Islands (Trower et al, 2018)-but this infrequent active transport could be explained by the large heights and wavelengths characteristic of gravel bedforms (Carling, 1999). The value of Ω required for each D eq can be solved for by rearranging the volumetric precipitation rate equation for the carbonate mineral of interest:…”

The Origin of Carbonate Mud

“…Similarly, tumbling barrel experiments have shown that breakdown of skeletal carbonate can be rapid (Ford & Kench, )—the mud‐sized products of which have been identified in modern carbonate muds (Gischler & Zingeler, ). In addition to polishing grain surfaces (Trower et al, , ), this mechanical abrasion of sand‐sized particles produces finer material at a volumetric rate equal to the volumetric rate of sand diminution. In this study, we constructed a model to estimate the rate of production of carbonate mud via abrasion of coarser particles as a function of sand grain size and sediment transport mode by modifying a recent ooid abrasion rate model (Trower et al, ).…”

The enigma of Neoproterozoic giant ooids-Fingerprints of extreme climate?

“…The carbonate mineral saturation states required for giant ooids with D = 10 mm varied substantially depending on mineralogy (aragonite, calcite, or ikaite) and temperature (T = 0°C, 25°C, 40°C) ( Figure 1). Aragonitic or calcitic ooids of this size under conditions similar to those on modern carbonate platforms (i.e., T = 25°C) required Ω aragonite ≅ 19 or Ω calcite ≅ 26 ( Figure 1a-b), both of which are notably higher than saturation states observed in modern shallow marine settings (e.g., Ω aragonite = 5 in the Turks and Caicos Islands, Trower et al, 2018). In contrast, aragonitic giant ooids at T = 40°C required Ω aragonite ≅ 10 ( Figure 1c) and ikaite giant ooids at T = 0°C required Ω ikaite ≅ (Figure 1d).…”

“…Islands (Trower et al, 2018)-but this infrequent active transport could be explained by the large heights and wavelengths characteristic of gravel bedforms (Carling, 1999). The value of Ω required for each D eq can be solved for by rearranging the volumetric precipitation rate equation for the carbonate mineral of interest:…”