Atlantis II (Ship : 1963-) Cruise AII93-7

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  • Technical Report
    Abyssal sediment waves in the Amirante Passage, western Indian Ocean
    (Woods Hole Oceanographic Institution, 1977-02) Johnson, David A. ; Bunce, Elizabeth T.
    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.
  • Dataset
    Magnetics Data from R/V Atlantis II, (WHOI, 1963-1996), April 9-May 5, 1976
    ( 2016-08-15)
    Magnetic Data recordings, hand-written transcribed measurements from recorded tape, date stamps, time, and measurement
  • Technical Report
    Cruise data report, R/V Atlantis II 93 Leg 7
    (Woods Hole Oceanographic Institution, 1978-04) Groman, Robert C. ; Dunworth, Jane A.
    The R/V ATLANTIS II Cruise 93 Leg 7 left Port Louis, Mauritius, 8 April 1976 on a 29 day geophysical and geological survey in the Mascarene and Somali Basins in the Western Indian Ocean. Table 1 lists the members of the scientific party. Seventeen piston cores were successfully recovered in the Mascarene Basin, Amirante Trench and Somali Basin regions. Table 2 summarizes these coring stations. Single channel continuous seismic profiles were made on 2920 km of ship's track in the Somali Basin. Echo soundings, total geomagnetic field and gravity field data were collected throughout the entire leg. The R/V ATLANTIS II arrived in Mombasa, Kenya, on 6 May 1976. For analysis of the data collected during this cruise see Bunce and Molnar (1977) and Johnson and Bunce (1977). We present here summary charts of the underway data collected during this cruise.