Stephen L. Eittreim

Suspended matter along the continental margin of the North American Basin

Abstract Vertical profiles of light scattering at 31 stations along the western margin of the North American Basin establish the bottom nepheloid layer as a permanent (non-seasonal) feature of the bottom waters of this region in depths greater than 3000 m. Its thickness ranges from 300 to 2400 m, the greatest thickness occurring toward the south off the Blake Plateau and Bahama Islands. Scattering intensities are greatest on the lower continental rise and abyssal plain north of Cape Hatteras. This layer represents sediment in suspension in the southerly bottom current along the western margin. Frequently near the outer ridges, the water is relatively clear adjacent to the bottom under the main nepheloid layer. In the Gulf Stream at about 500 m, the approximate depth of a high gradient in horizontal velocity, a marked increase in light scattering may mark the Sargasso Sea Water-Slope Water boundary.

Sediment distribution in the Indian Ocean

Abstract Seismic data in the Indian Ocean are used to construct an isopach map of unconsolidated sediments. The major thickness of sediments is found in the following places: (a) the Indus and Ganges cones, which comprise about 40% of the sediment of the Indian Ocean; (b) basins adjacent to the continents on the east and west containing largely terrigenous sediments; (c) a zone south of the Polar Front, which is high in biologic productivity and where extensive non-stratified accumulations of sediment occur. Sediment cores indicate that the sediments in this last category are largely siliceous oozes. The crestal area of the mid-oceanic ridge is consistently bare of sediment to a distance of at least 100 km from the axis, except on the southwestern branch where thick sediment pockets have been found within 100 km of the crest. This relative abundance of sediment close to the ridge axis in places on the southestern branch may be related to a much slower spreading rate here than elsewhere, or to the complications introduced by the numerous fracture zones offsetting the ridge crest.

Turbidity distribution in the Atlantic Ocean

Abstract The regional coverage of Lamont nephelometer data in the North and South Atlantic can be used to map seawater turbidity at all depths. At the level of the clearest water, in the mid-depth regions, the turbidity distribution primarily reflects the pattern of productivity in the surface waters. This suggests that the ‘background’ turbidity level in the oceans is largely a function of biogenic fallout. The bottom waters of the western Atlantic generally exhibit large increases in turbidity. The most intense benthic nepheloid layers are in the southwestern Argentine basin and northern North American basin; the lowest bottom water turbidity in the western Atlantic is in the equatorial regions. Both the Argentine and North American basin bottom waters appear to derive their high turbidity largely from local resuspension of terrigenous input in these basins. In contrast to the west, the eastern Atlantic basins show very low turbidities with the exception of three regions: the Mediterranean outflow area, the Cape basin, and the West European basin.

Transition from continental to oceanic crust on the Wilkes‐Adelie margin of Antarctica

The Wilkes-Adelie margin of East Antarctica, a passive margin rifted in the Early Cretaceous, has an unusually reflective Moho which can be traced seismically across the continent-ocean transition. Velocity models and depth sections were constructed from a combined set of U.S. and French multichannel seismic reflection lines to investigate the transition from continental to oceanic crust. These data show that the boundary between oldest oceanic crust and transitional continental crust is marked by a minimum in subsediment crustal thickness and, in places, by a shoaling of Moho. The Moho reflection is continuous across the edge of oceanic crust, and gradually deepens landward under the continental edge

Oceanic crustal thickness and seismic character along a central Pacific transect

Seismic reflection data along a flow line of crustal generation in the central Pacific that spans ages of zero to 85 Ma and spreading half rates of 30 to 100 km m.y.−1 shows a nearly constant travel time of 2 s through igneous crust to reflection Moho. The highest-amplitude and most laterally continuous Moho reflections were recorded over 20–30 Ma crust that was emplaced at “superfast” (75–95 km m.y.−1) spreading rates. The superfast spread portion also records the lowest scatter about the 2-s average travel time to reflection Moho. Seismic images show that lower crustal reflectors dip consistently eastward toward the ridge crest. These dipping reflectors are truncated by the reflection Moho.

Turbidity Distribution in the Deep Waters of the Western Atlantic Trough

The “background” turbidity (clearest water of the water column) has been mapped in the western Atlantic using standardized nephelometer measurements taken on R/V Conrad cruises 15 and 16. This background has then been utilized as a reference turbidity to map features of the bottom-water turbidity using older unstandard-ized relative nephelometer profiles. Variations in background turbidity of a factor of 3 apparently reflect the pattern of biological productivity in the overlying surface waters: high background in an equatorial belt and in latitudes greater than the subtropical convergences, but low background in the temperate latitudes. The resulting distributions of near-bottom turbidity values show a maximum in the southwestern Argentine Basin and a lesser maximum along the continental rise of the North American Basin. Bottom turbidity in the nepheloid layer decreases from both basins toward the equator and appears to be both a function of bottom current velocity and proximity to terrigenous sediment sources.