Abyssal sediment waves in the Amirante Passage, western Indian Ocean

Abstract

The deep western boundary current (DWBC) of the Indian Ocean flows northward along the western margin of the Mascarene Basin, and is funneled through a narrow passage (near 09°S, 52°E) prior to entering the Somali Basin to the north. Recently completed geological and geophysical operations within this passage reveal the presence of fields of well-developed abyssal sediment waves, with amplitudes of 10 to 50 meters and wavelengths of hundreds of meters to ~1 km. The two principal fields of sediment waves are confined to the flanks of the passages, and are restricted to a narrow depth interval (~3950 to 4150 meters). Individual waves appear to be elongated parallel to the passage axis and the presumed flow direction of the DWBC. The waves are relatively transparent to low-frequency echo sounding (3.5 kHz) and seismic profiling (17-70Hz), although weak internal reflectors are present within individual waves. These reflectors are conformable with the overlying sea floor, thereby suggesting neither upslope nor downslope migration of the waves. The sediment comprising the waves is a well-sorted calcareous ooze dominated by fine silt components (principally coccoliths), in marked contrast to the heterogeneous biogenic ooze on the adjacent channel floor. The lithology and local distribution pattern of the waves suggest that they are constructional bedforms associated with the flow of the DWBC, and not slump structures. A deep hydrocast at the sill of the channel indicates a near-bottom layer of Antarctic Bottom Water (Θ <0.9°C) approximately 200 meters thick, whose depth range corresponds with that of the sediment waves. Interpretations of the origin of the sediment waves in terms of the local flow regime will require (1) a more precise description of the physiography and structure of individual sediment waves; and (2) closely-spaced measurements of the nearbottom velocity structure of the DWBC within regions of the passage where sediment waves are apparently forming.