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VOL. 45, 1959 GEOPHYSICS: EWING, HEEZEN, AND ERICSON 355 Trenches, but it is almost certainly present in the valley bottoms. It is not found on the trench floors, perhaps due to its deep burial. If the white ash layer can be identified with the same time of occurrence in all of the cores, much can be learned of the sedimentation rates in this part of the Pacific and particularly about the variability of these rates. The white ash immediately suggests a volcanic origin and the proximity of the Andes suggests the source. However, the great extent of the ash and its shallow cover would imply such a great amount of recent activity for a short time that it may well be difficult to ascribe it to the Andes. Until its extent, age, and identifica- tion are determined one can only speculate. Perhaps sub-bottom echoes from other areas can also be correlated with this white ash layer. If so, it may be necessary to attribute the layer to a world-wide volcanism or perhaps to the fiery end of bodies of cosmic origin. It will be possible to measure the acoustic properties of the clays above and below the layer and of the layer itself. This area can then become an excellent testing ground for theories of reflectivity for various layer thicknesses and depth of burial and for the effectiveness of various sounding equipments. Maurice Ewing, D. B. Ericson, J. E. Nafe, and B. C. Heezen provided advice and assistance. Thanks are due to the officers and crew of R.V. Vema who assisted in the work necessary to carry out this research. Of especial assistance were Captain H. C. Kohler and Kerry Oxner. Annette Trefzer, Stephanie Budge, and Sandra L. Worzel assisted in the preparation of the manuscript. This work was carried out under contract with the National Science Foundation and the Office of Naval Research and the Bureau of Ships of the Department of the Navy. * Lamont Geological Observatory Contribution No. 337. lHeezen, B. C., M. Tharp, and M. Ewing, Geol. Soc. of Am., Spec. Paper No. 65 (in press). 2 Luskin, B., B. C. Heezen, M. Ewing, and M. Landisman, Deep Sea Res., 1, 131 (1954). 3Luskin, B., and Archie C. Roberts, Lamont Geol. Obs. Tech. Rept No. 6, CU-15-55-N6 onr 27124 Geol. 1955. 4General Bathymetric Chart of the Oceans, published by the International Hydrographic Bureau of Monaco, 1936. 6 Heacock, John G., Jr., and J. Lamar Worzel, Bull. Geol. Soc. Am., 66, 773 (1955). 6 Shumway, George, Deep Sea Re,., 4, 250 (1957). 7Ziegler, J. M., W. D. Athearn, and H. Small, Deep Sea Res., 4, 238 (1957). SIGNIFICANCE OF THE WORZEL DEEP SEA ASH* BY MAURICE EWING, BRUCE C. HEEZEN, AND DAVID B. ERICSON LAMONT GEOLOGICAL OBSERVATORY, COLUMBIA UNIVERSITY, PALISADES, NEW YORK Communicated January 2, 1959 Introduction.-In the preceding communication Worzel has described a bed of white ash which he discovered in deep sea cores and was able to trace over a large area in the Pacific as a sub-bottom sound reflector. While following the sub-bottom echo, he was able to take additional cores to prove the correlation between the ash Downloaded by guest on September 30, 2021 356 GEOPHYSICS: EWING, HEEZEN, AND ERICSON Pioc. N. A. S. layer and the reflector. Worzel shows that the ash extends at least from 110 North to 12° South Latitude within a few hundred miles of the coasts of Central and South America. A single ash layer of 5 to 30 cm thickness over such a wide area must record a notable event in the history of the area. It could hardly be without some recorded consequences of global extent. The following is a prelimi- nary note, based on shipboard descriptions of the cores, calling attention to other data relevant to Worzel's discovery. The writers hope that other investigators will join them in determining the properties and extent of this layer. Sub-bottom Reflectors.ub-bottom reflectors have been observed since the first self-recording echo sounders were put into use. However, sub-bottom echoes were rarely recorded in water depths greater than 300 fm because the compressed scales of most deep water recorders severely limited resolution. The Hughes echo sounder of the Albatross and Galathea was provided an expanded scale suitable for obser- vation of deep water sub-bottom echoes.' Deep water sub-bottom echoes were not observed by American workers until the development of the Precision Depth Recorder in 1953.2 Sub-bottom reflections obtained with sounders operating at 10 or 12 kc/sec have been identified only rarely by sediment coring. Heezen et alV correlated a prominent sub-bottom echo in the Cariaco Trench with the base of the Recent anaerobic layer. In shallow water, Smith,4 Moore,5 and others have identified bed rock, the base of the Recent, and other horizons. However, Worzel has made the first successful identification of a sub-bottom reflector in very deep water. Koczy1 has published the echograms of the round-the-world Albatross expedition. Examination of the photographic reproductions of these echograms reveals the existence of sub-bottom echoes in many of the areas traversed. In certain areas sub-bottom echoes are observed as much as 25 fm below the ocean floor. Heezen, Tharp, and Ewing' have described sub-bottom reflections observed in the North Atlantic. Sub-bottom reflections in shallow water, in restricted basins, from abyssal plains, and in the lee of topographic highs can be related to local causes: turbidity currents, unconformities, and so -forth.--However, sub-bottom reflections over gentle rises isolated from down-slope sediment movements must reflect events of wider importance. Thus, while we must be aware of a variety of other causes of reflections, it is conceivable that we may find that the Worzel ash extends well beyond the area in which it has been mapped. A re-examination of the file of Vema echograms is now in progress. It shows that sub-bottom echoes, similar to those found in the eastern Pacific, have also been recorded in the south Atlantic and Indian Oceans. In general, the reflector was so deep that it was not reached by cores, but in the Gulf of Mexico, correlation with an ash layer in some cases and with a carbonate layer in others (Ewing, Ericson, and Heezen,7 pp. 1019 and 1021) is found. From the cores and echo- grams in our collection and from those to be taken during the present cruise of Vema, we hope to identify the reflectors in the areas traversed. Thus we should learn whether all reflectors in each area result from local causes or if some are of world-wide extent. The intensity of a reflection from a thin layer of material having an impedance different from that of the surrounding sediment will be greatest if constructive interference occurs between the sound returning from the top and bottom of the Downloaded by guest on September 30, 2021 VOL. 45, 1959 GEOPHYSICS: EWING, HEEZEN, AND ERICSON 357 layer. Taking 1.60 gm/cm3 and 1.60 km/sec for the green lutite and 2.00 gm/cm3 and 2.40 km/sec for the ash, and neglecting rigidity, the fraction of the incident energy returned to the water from an ash layer when constructive interference occurs has been calculated by J. E. Nafe as 27 per cent, compared with 6 per cent for the bottom echo. Constructive interference would be expected at odd multiples of one-quarter wave length. However, with variations in layer thicknesses and devi- ations from plane parallel interfaces, the reflected energy would ordinarily be an appropriate average between the maximum and minimum of the order of 10 per cent of the incident energy. However, if the layer thickness falls below one-quarter wave length (about 5 cm at 12 kc) the reflected energy would drop rapidly, becom- ing less than the bottom reflection at thicknesses of 1 to 2 cm. Preservation of Stratification.-The action of deep sea bottom currents, bottom burrowing organisms, and surficial gravity movements produces mechanical mixing of most sediments. The ash deposits observed by Kuenen and Neeb8 and Bram- lette and Bradley9 were mixed through a column of sediments several times the thickness of the original ash bed. In addition to this mechanical mixing, solution may vastly alter the sediment before permanent burial is accomplished. Devitri- fication and alteration, proceeding at rates dependent on the environment, may transform an ash bed into products whose origin is not readily recognized.10 Beds have a greater chance of preservation when high rates of deposition occur. Broecker, Turekian, and Heezen"I have shown that the productivity of fora- minifera varies only slightly with time in the equatorial Atlantic. If this is a general characteristic of foraminiferal production, the low carbonate content of the sediments in the area of the Worzel ash indicates high rates of terrigeneous or organic sedimentation. Ash Layers.-Extensive ash layers are now recognized in continental areas throughout the geological record (e.g., Ross,12 p. 428). Frequently the ash was transported so far that its place of origin is in doubt. Until recently ash falling into the ocean was considered lost to the geological record. Murray and Renard"3 identified volcanic particles in practically all of the Challenger surficial samples of deep sea deposits, demonstrating that volcanic detritus is an important component of modern deep sea deposits throughout the world. They suggested that the abyssal clays are largely the result of alteration of volcanic ash. The recognition of beds of ash was impossible until core samples became available.

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