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Observations on living calcareous algae from San Salvador Island, Bahamas, and correlation with their fossil counterparts

San Salvador Island is a small isolated carbonate platform in the Bahamas archipelago. The island is composed of a thin veneer of Pleistocene and Holocene carbonate rocks, predominantly eolianites overlying thick succession of Mesozoic and Tertiary limestones and dolomites. A large area of the island is covered by mostly hypersaline lakes that formed in depressions between the dune ridges. San Salvador is surrounded by barrier reefs with protected lagoons behind them. Sea-water temperature ranges from 24 to 30 oC. The study focuses on modern calcareous algae, particularly Dasycladales, from the paleontologist’ s perspective, in order to better understand their present contribution to carbonate sediments, as well as to compare these algae with their possible fossil counterparts. Investigated localities are include: Long Lake, Fernardez Bay (Telephone Pole Reef), French Bay, Pigeon Creek, Rice Bay and Graham’ s Harbor (North Point and Dump reef). Calcareous algae have been found in diverse environments. The most common are Halimeda and Penicillus. They are widespread, especially in protected lagoons with soft bottom, in both barren and grass-covered areas. They can be also found on patch reefs, in tidal channels and near beaches just below the low-tide level. Other green algae are also common, as Rhipocephalus, Udotea and Avrainvillea. Dasycladales are much less common then Bryopsidales. Among them the most common species is Batophora occidentalis. Relatively common are Acetabularia crenulata and Dasycladus vermicularis. Calcified species Cymopolia barbata and Neomeris annulata are very rare. All of the observed Dasycladales live attached to hard substrate (rock, corals, pebbles, shell fragments) and occur in bushes composed of several individuals. Batophora has been found in lagoon environment, but mostly near beaches, commonly in small pools between callianassid mounds. It also occurs on mangrove roots above low tide level, implying tolerance to temporary desiccation. Acetabularia has been found in the same environment as Batophora, but not above sea level. Dasycladus has been found in hypersaline lake. Cymopolia has been found near shore, where it swings with waves, firmly attached to the substratum. Calcification types vary among different algae. Some algae (Halimeda, Neomeris, Cymopolia) form distinct calcareous skeletons that they use for protection against predators. Postmortem disintegration of these skeletons leads to formation of sand-to-mud-size carbonate sediments. Other algae (Penicillus, Avrainvillea, Rhipocephalus, Udotea, Padina) precipitate small aragonite crystals that aggregate on the surface of the alga and between filaments. Young algae are therefore green, being whiter as aragonite precipitates. Following death, the aragonite envelope breaks down forming mud-size particles. Acetabularia form very thin aragonite envelope that also disintegrates into mud-sized particles, but genetically is closer to the first type of calcification. Non-calcified dasyclads Batophora and Dasycladus act as bafflers, collecting sand and mud-sized particles. The amount of sediment that they baffle is small compared to sea-grasses, but in the geological past, before the appearance of sea-grasses, the non-calcified algae likely were the main sediment-bafflers, although they haven’ t been preserved as fossils. Contribution of Bryopsidales to modern carbonate sediments is well known. Our investigations support previous data on exceptional importance of calcareous algae in formation of sand-to-mud-size carbonate particles. Since the scarcity of calcified Dasycladales prevents an estimation of their contribution to carbonate sediments, living Halimedaceans can be used as their analogues. In Mesozoic and Paleozoic times, when Dasycladales were much more abundant then Bryopsidales, their role in production of carbonates was probably significant.

Calcareous algae; San Salvador; Bahamas

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