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Free Content Geology and biological zonation of the reef slope, 50–360 m depth at Enewetak Atoll, Marshall Islands

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Abstract:

Twenty-two submersible dives were made on the outer reef slope of the southern half of Enewetak Atoll, Marshall Islands, to maximum depths of 365 m. No distinct thermocline was encountered. The outer slope of Enewetak is steep, averaging about 60° between depths of 90 m and 365 m on the windward side and slightly steeper on the leeward face. No terraces or shelves were noticed below 30 m. Vertical grooves were found on the windward face below 150 m. Talus accumulated below 250 m, and extensive sediments were encountered seaward of the wide (southern) channel at 200 m. Small-scale scalloping and pitting of the reef face occurred from 150–240 m. Rock surfaces below 250–300 m appeared heavily scoured. A solution unconformity at 360 m and another at 365 m were encountered separately at two locations on the windward side. Branched stony corals were found down to about 60 m, below which only plate-like forms occurred to a maximum depth of about 112 m. At 90 m, less than 1% of the substratum was covered with stony corals. Halimeda spp. algae were the most conspicuous macroalgae on the outer slope, being encountered to depths of 100 m or more. Algal films and small macroalgae were found on rocks at 140 m. Gorgonians and alcyonaceans appeared to dominate the substratum below 100 m. Indentations and caves at 120–160 m were populated with sponges, antipatharians, possible sclerosponges, and other sessile invertebrates. A sand slope below the wide channel had abundant scapens at 220 m. No populations of sclerosponges building reef framework were encountered. Talus blocks on sediment slopes provided attachment points for some sessile invertebrates. Below about 90–100 m, it appears that no significant reef framework is being constructed. In many areas, active erosion is evident. Significant quantities of sediment and talus are being transported downslope from the shallow reefs on the windward side, much of it apparently coming to rest below 365 m depth. The leeward face seemed to have less sediment transport. Pitting on the windward reef face may represent features formed during shallower sea level.

Document Type: Research Article

Publication date: 1986-01-01

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