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Sedimentology and Geochemistry of Ooid Sands Buried Beneath Microbial Mats, Little Ambergris Cay, Turks and Caicos Islands


Microbial boundstones and ooid grainstones may be closely associated in commercially viable oil and gas fields (Grotzinger & Al Rawahi, 2014), motivating study of the chemical and physical conditions that result in preservation of microbial textures in the ancient record. Little Ambergris Cay (21.3°N, 71.7°W; ~6 km by ~1.6 km) is at the windward end of a 25 km-long linear ooid shoal developed by strong easterly trade winds that drive sediment transport across the Caicos Platform. Microbial mats are widespread within an interior tidal marsh. In August 2017, 8 vibracores were collected across Little Ambergris Cay to investigate the sedimentary history of the cay, the preservation potential of any microbial mats, and early diagenesis and lithification of the cay sediment.

Cores were 0.86-2.40 m long and had up to 12 cm of microbial mat overlying massive ooid sands with fossiliferous layers and ooid grainstone intraclast and/or nodule layers. Fossils indicate an upper shoreface or high-energy shoal assemblage and include high-spired gastropods, coralline algae, bivalves, and Halimeda. Some intraclasts/nodules preserved ripple cross lamination, burrows, and algal holdfasts.

We sampled the sediments to analyze trends in grain size, DNA content, and stable isotope and elemental composition. Using Rhizons, we sampled interstitial waters to analyze trends in carbonate and sulfur chemistry, salinity, and elemental composition. We also sampled groundwater from piezometers installed across the largest tidal delta on the cay.

Evidence of mat preservation down-core is very rare to absent. In one core, we identified a 1 cm organic-rich, laminated, and pigmented horizon that appeared to be a decayed relict mat or ripped up mat fragment. There was no detectible DNA down-core. Interstitial fluids through the mat layer record increasing total carbon and alkalinity and an active sulfur cycle. Fluids below the mat freshen slightly (~42 ppt salinity) and have low δ13C reflecting remineralized organic matter. These mix with underlying highly saline (~48 ppt), sulfidic water with high total carbon and δ13C that may reflect carbonate dissolution.

Sedimentologic and geochemical data provide evidence for a rapid transition from ooid shoal or shoreface environments to stable tidal marshes colonized by microbial mats. The geochemical data further inform subsurface cementation and dissolution processes controlling early diagenesis and lithification of sediments and microbial mats.