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Although most marine ooids have concentric structure and aragonitic composition, radial-structural aragonite ooids occur in Lizard Island. Radial ooids are confined to the lagoonal sediment south of Lizard Island in water depths of 0-7 m (0-23 ft). Primary bed forms vary from minor current ripples to nondescript. Ooids comprise up to 18% of the subarkosic sands and are found in an environment protected from open-ocean energy by patch and fringing reefs. Ooids are dark gray and are frequently bored by endolithic algae. Compound and asymmetric forms are common. Individual aragonite crystals may range up to 12 µm in length.
To assess the environment of formation, in-situ diurnal chemical measurements were made in the ooid and interstitial and ambient waters. Critical parameters examined in a 28-hr period were temperature, pH, PCO2, Cl ^pmil, ACa2+, and ACO32-. Cyclic diurnal changes occur in both water systems, with the ambient water experiencing the greatest changes. Measured extremes for the ambient water are day = 25.3°C, PCO2 of 10-5.08, and Alkc of 2.52 meq; night = 22.3°C, PCO2 of 10-4.98, and Alkc of 4.31 × 10-3 meq. The ACa2+ (2.6 × 10-3) and Cl ^pmil (19.24 ^pmil) are es entially time invariant. The PCO2 ranges from atmospheric saturation in the day to supersaturation during the night. Both ambient and pore waters are supersaturated with respect to aragonite (^ohmARAG-ambient=2.8 to 4.4; ^ohmARAG-porewater=3.1 to 4.1). Our findings suggest alkalinity consumption by carbonate precipitation occurs during the day and is greatest in the upper 10 cm (4 in.) of ooid sands.
This physical/chemical environment differs from other monitored classic ooid localities and is diagnostic of marine radial-structured aragonite ooid growth.
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