Abstract This paper presents primarily the soundings obtained by the Research Vessel ATLANTIS during the summer field season of 1947. It also utilizes some of the data obtained by later expeditions of the ATLANTIS and by several other vessels. Xhe ATLANTIS is equipped with a continuously recording fathometer with a range up to 4000 fathoms. The outstanding features of the area investigated in 1947 (ATLANTIS cruise #150) are: (1) A conspicuously flat plain at a depth of 2900 fathoms occupies the deeper sections of tlfe North Atlantic basin between, Bermuda and the Azores. More recent data indicate that this horizontal stretch of sea floor extends at least as far south as Lat. 29°N. and possibly as far north as Lat. 40°N., between Long. 50°W. and 56°W., and also that at the-same depth a similar plain is found east of the Ridge in the Northern Canary basin. They are dotted with small sea mounts of variable height whose exact shapes have not yet been determined. The size and number of these appear to increase toward the south, and the southern extremities of the plains consjjgi of smooth horizontal stretches interrupted by pronounced elevations and depressions. (2) The Mid-Atlantic Ridge is characterized by two strongly contrasting types of topography: (a) The central backbone of the Ridge, or Main which is shoaler than 1600 fathoms, consists of a series of parallel ranges trending NE.-SW. Several oPthege rise to less than 800 fathoms. Their trend folloWtpughly tbilgfchf Main Range. The portion of the Main Rang which the most attention wis devoted is between lat 30° and 34°N. In this area its width is of the order of 150 miles, and it is of the order of Several hundred miles in.the area of the Azores. (b) The flanks, between the 1600- and 2500- fathorn isobaths, consist of a succession of smooth shelves, each from 1 to 50 nautical miles or more in width. This zone is 200-300 miles in width. (c) The region between the limit of the 2900- fathom plain and the foot of the first of the series of shelves characterizing the flanks of the Ridge, at 2500 fathoms, in soirte localities stands out as a distinct physiographic province. (3) The folio wing local features are noteworthy: (a) Close to 31°N.Lat. a deep east-west trench extends from about 41° to 43°WLong. and cuts deep irito the Maip Range. Its deepest point is at. 2800 fathoms. Crushed and metamorphosed ultra- basics were broiqfct to the surface by dredging its flanks. (b) At 30°15′W., 34°N., a flat-topped sea mount rises to 180 fathoms, about 100 miles southeast of the Main Range. The dredge brought up calcareous discs of probably Cenozoic age off its top. They were about 15 cm in diameter and about 4,cm thick. (c) Another sea mount was found at 33°43′N., 62°30′W., 150 miles NE. of Bermuda, shoaling to 780 fathoms, having a NW.-SE. elongation, an unknown total length, and a width of 4 miles across its flat top. (d) Another flat suboceanic plain occupies the bottom of the basin separating Beriftuda from the Northeastern United States, at a depth of 2650 fathoms.

Abstrac Seismic-refraction measurements in the western basins of the North Atlantic Ocean show that an average crustal section consists of ½–1kmkm of low-velocity sediments and 4-6 km of oceanic crustal rock in which the seismic velocity is about of 6.5km/sec. There is good evidence from sub-bottom reflections andllshear waves that in many places there is a layer with a velocity between 4.5 and 5.5 km/sec and a thickness of 1-2 km betwleten the Jow-velocity Pediments and-the 6.5 km/sec layer, although it is not usually detected by refracted arrivals. These layers are underlain by the mantle which has an average seismic velocity near 8 km/sec. Measurements in the eastern basins show a similar crustal section, but the velocity below the deep discontinuity appears to be lower (7.7–7.8 km/sec). Measurements in the Mediterranean Sea show only low-velocity sediments underlain by a refracting layer in which the average velocity is about 4.5 km/sec. On the Mid-Atlantic Ridge the sediments are underlain by two refracting layers with velocities averaging 5.6 and 7.4 km/sec respectively. The results indicate that the ridge has been built blithe upwelling of great amounts of basalt magma along a tenslonal fracture zone. Presumably the extcnsional forces and the supply of basalt magma come from convection currents deep in the mantle. Measurements in the Norwegian and Greenland seas show results very similar to those on the Mid-Atlantic Ridge, and, from this and the extension of the belt of active seismicity, ft appears that the ridge structure continues through the Norwegian and Greenland seas into the Arctic Ocean. The results of a few stations on the continental shelf of North America, Britain, and Norway are presented and compared with previously published results in these areas

Abstract Studies of lithology, particle-size distributions, and micropaleontology and chemical analyses o f 221 Atlantic and Caribbean deep-sea cores lead to new conceptions of processes of sedimentation, rates of sediment accumulation, Pleistocene chronology, and pre-Pleistocene history of the Atlantic Basin. Anomalous layers of sand, silt, and lutite occur widely in the deep basins of the Atlantic. Evidence for deposition of these layers by turbidity currents is as follows: (1) the layers occur in submarine canyons, in deltalike features at the terminal ends of canyons, in basins and depressions, never on isolated rises; (2) they are interbedded with late Pleistocene sediments of abyssal facies; (3) they are well-sorted and commonly graded; and (4) they commonly contain organic remains of.shallow-water origin. Late Pleistocene slumping of compacted Neogene sediments along the banks of the Hudson submarine Canyon at depths exceeding 3000m indicates deepening of the canyon by erosion by turbidity currents Variatiohs in the planktonic Foraminifera in 108 of the cores and extrapolation of rates of sediment accumulation determined by 37° radiocarbon dates-in 10 cores show that the last period of climate comparable with the present ended about 60,000 years, ago. A faunal flange indicating climatic amelioration probably corresponding to the beginning of postglacial time, occurred about 11,000 years ago. Cross-correlations by microipaleontological methods establish the ontinuity of the climatic record deduced from the planktonic Foraminifera. Study of variation in the Planktonic Foraminifera kads to a different, Pleistocene chronology from that proposed by Emiliani (1955). Cross-correlations of faunal zones and radiocarbon dates show that rates of continuous sediment accumulation as opposed to turbidity current deposition range from 0.5cm to 27.4cm in 1000years, depending upon bottom configuration. Cross correlations by means of changes in coiling direction of planktonic Foraminifera give relative rates of sediment accumulation beyond the range of the radiocarbon method of dating. Forty one of the cores contain pre-Pleistocene sediments. The oldest sediment is Upper Cretaceous. Foraminifera and discoasters indicate the ages. Absence of sediment older than Late Cretaceous and thickness, 800-1000m, of sediment in the Atlantic Basin, as determined by seismic methods-suggest that a large-scale reorganization of the Atlantic Basin took place in the Mesozoic.

Abstract Three crossings of the Mid-Atlantic Ridge with the seismic profiler between Buenos Aires and Capetown: Buenos Aires and Dakar; and Dakar and HaTTfax have shown several important features ot the sediment distribution. The total accumulation is remarkably small, averaging 100–200 m. On the northern and middle crossings, the sediments are mainly in pockets, and intervening areas fire almost or entirely bare. A large percentage ot the pockets have almost level surtaces. These facts suggest that the sediments deposited on the ridge flow easily after reaching bottom here. Where impounded, the ridge sediments apparently develop cohesiveness and will not allow easily it subsequently tilted. The sediments are unstratified and remarkably transparent acoustically. Certain areas, particularly on the lower flanks ot the ridge, contain distorted sedimentary bodies that apparently indicate postdepositional tectonic activity. The basement surface on which the sediments rest is unitormly rough trom the crust of the ridge out to the lower flanks and continues so underneath the basin sediments. It is the upper surface of the intermediate layer (seismic velocity about 5 km/sec) that constitutes the upper 1-3 km of the oceanic crust. On the southern crossing the sediment layer tends toward uniform thickness across most of the section. This is-evidence that the ridge sediments here are mainly pelagic, and that the amount of sediment seen on the; record represents the total deposited. Sediment tores and ocean-bottom photographs provide additional information about sediment composition and distribution where the sediment cover is too thin to be measured by the profiler. The photographs also prov ide information about the presence ot currents capable of altering sediment distribution. The results suggest that the total accumulation in the oceans is small compared to that which would be inferred from any of the currently accepted estimates of-Cenozoic rates of deposition, but that the relative amounts of carbonate and red clay conform to the accepted ratio of their respective rates.

A distinct nepheloid layer with a vertical gradient of light scattering is observed on the continental rise of western North America. Throughout the deep North Pacific Basin the nepheloid layer extends from the bottom to the temperature minimum. The intensity of light scattering in the nepheloid layer of the North Pacific is substantially less than that of the Argentine Basin, except on the Alaskan and Tufts Abyssal Plains where it is comparable. Light scattering measured in this nepheloid layer with a photographic nephelometer is due to particulate matter; a model is proposed according to which this matter is derived principally from continents bordering the North Pacific and transported to the sea largely by rivers. Suspended particles are carried from the continental margin to the deep basin by lateral mixing and transport within the nepheloid layer. They are distributed throughout the large basins of the North Pacific by counterclockwise gyres and replace particles lost by sedimentation. Marginal trenches contain a strong nepheloid layer of trapped particles which ultimately settle to the trench floor. The particulate matter in the nepheloid layer enters the water in the North Pacific because the homogeneous bottom water moving northward through narrow equatorial channels along about 175° W. has only a weak nepheloid layer. Rapid increase in temperature and maintenance of an adiabatic gradient during transit of the channels from about 17° S. to 15° N. are attributed to strong vertical turbulence associated with vigorous flow and admixture of warmer water from above the deep temperature minimum. The vertical gradient of light scattering in the nepheloid layer is the result of a balance between particle settling and turbulent mixing in the homogeneous water below the temperature minimum. When data on settling rate of the particulate matter become available, this gradient may be used to estimate the intensity of vertical turbulence in the homogeneous water. Evidence supports the hypothesis that the nepheloid layer is a steady-state phenomenon.