Morphology, water characteristics and sedimentation in the silled Sulu Sea, southeast Asia

Abstract

During 1977 RV “Valdivia” recovered 46 cores, occupied two hydrographic stations, and ran eleven bathymetric profiles in the roughly rectangular Sulu Sea, which trends northeast and covers about 250,000 km 2. The Sulu Sea Deep in the southeast is separated from the shallower Palawan Depression in the northwest by the northeast-trending Cagayan Ridge. The maximum sill depth between the sea and the open ocean is about 400 m, and the water column is layered, cutting off the deep water, which is homogeneous, warm (10° C), and oxygen deficient (< 1.6 ml/ml O 2), from normal oceanic water. The shelves are generally broad and covered with calcareous sand and gravel, except in the east and southeast, where they are narrow and covered with muddy sand and gravel. The shelf carbonates give way to bathyal oozes, and the muddy sands grade into varied silts dominated by volcanic and sedimentary debris. The smooth abyssal plain of the Sulu Sea Deep is at a depth of 4800–5000 m, and northwest of it is an plain at 4000–4200 m. Turbidites characterize both plains. The aragonite and calcite compensation depths are at about 1400 m and 4500 m, respectively. The turbidite beds average 5–10 cm thick, with all Bouma's divisions recognizable, apart from “a”. The southern turbidites (station 14224) are of two types: calcareous and terrigenous. The calcareous turbidites, characterized by shell debris including shallow-water foraminifera, were derived from the upper slopes flanking the broad shelves. The terrigenous turbidites, characterized by sedimentary and volcanic rock fragments, apparently originated from the slopes off Mindanao. The eastern turbidites (station 14233) are finer grained, characterized by sedimentary rock fragments, and came from the narrow muddy shelves further east, off Panay, Negros and Mindanao. Radiocarbon dating suggests that depositional rates for the turbidite sequences (ca. 100 cm/1000 yr.) are an order of magnitude greater than the rates for bathyal oozes. Organic carbon contents in the surface sediments are highest between 500 m and 1200 m water depth, exceeding 2% off Mindanao (station 14226), and reaching 1.82% on the Palawan slope (station 14206). The lowest contents are at the Palawan Shelf edge (0.11% at station 14204) where the proportion of carbonate sand is very high, and in the bathyal oozes of the central Sulu Sea (< 1%) where sedimentation rates are low and mineralisation of organic matter is effective. Contents average 1% below 4500 m in the Sulu Sea Deep. The colour of the surface sediment appears to be related to the organic carbon content: olive colours above 1200 m and below 4500 m, and brown colours between. The surficial sediments are characterized by living shelly faunas and intense bioturbation above 150 m, and by intense bioturbation and absence of a shelly fauna down to 3000 m. Bioturbation decreases below 3000 m, and in the Sulu Sea Deep is confined to a few Zoophycos, Granulites and Cosmorhaphe burrows. The faunal changes are apparently related to bottom water oxygen concentrations.