Abstract

Hogsty Reef (21 degrees 40'N, 74 degrees 20'W) is a small (5x9 km) Bahamian atoll, with a distinct peripheral reef, a shallow (6 to 8 m) lagoon, and a pronounced leeward pass. Wind-driven currents move rapidly through the lagoon, so that the residence period of lagoon water is too short for any noticeable change in salinity or temperature. There are three types of sediments at Hogsty Reef: sand and gravel, derived from the peripheral reef; cay sand, also derived from the surrounding reefs; and lagoonal sediment. The latter, the most abundant sediment type at Hogsty Reef, is partly composed of peripheral reef debris and some lagoonal skeletal material; but the major component is "non-skeletal" fragments, which increase westward in the lagoon, as currents become more rapid, skeletal components become fewer, and the rate of sediment accumulation (supposedly) decreases. The non-skeletal components, which contain high (10 per thousand ) Sr aragonite, are hypothesized to be primarily inorganically precipitated. It is suggested that in tropical waters, high current energy (combined, perhaps, with low skeletal productivity) may be the critical condition(s) necessary for inorganic precipitation of calcium carbonate, and may explain why such sediments have not been found on other atolls.

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