Description: The central problem of paleoceanography is the history of the circulation of the ocean. Although speculation about ancient oceanic circulation goes back to the past century, the field of paleoceanography was founded in the 1950s as oxygen-isotope studies suggested that oceanic deep waters were warmer in the past than they are today. Extensive coring of deep-sea sediments by numerous expeditions after World War II was followed by the ocean drilling programs, providing a rich data base. Paleoceanographic interpretations have tried to explain the most obvious changes in sea-floor sediments and their contained fossils: changing paleotemperatures indicated by oxygen isotopes, fluctuations in the calcium carbonate compensation depth, accumulations of organic carbon-rich sediments, and the unexpected abundance of hiatuses in a setting which had been thought to be the ultimate sedimentary sink. The result has been the intriguing discovery that although the positions and circulation of the major surface gyres is generally stable, the deep circulation of the ocean may reverse on a variety of time scales. It has been suggested that formation of North Atlantic Deep Water, which causes the uneven distribution of nutrients, alkalinity, and oxygen in the deep sea today, may have been replaced by formation of North Pacific Deep Water during the last deglaciation, reversing the concentration gradients of nutrients, alkalinity, and oxygen. On a longer time scale, the present general circulation, which is dominated by production of oxygen-rich cold deep water in the subpolar regions today, may have replaced a pre-Oligocene general circulation in which warm, saline, oxygen-poor deep waters were formed in warm seas in the arid zones. Paleoceanography is still in its infancy; many new clues to the history of the ocean are being discovered, and many new ideas about conditions in the past are being developed. The beginning of the next century should see continuing rapid growth and maturation in this exciting new field.

Description: The drilling vesselSubmarex of Global Marine was used to drill and core sediments to a depth of 56.4 m on the Nicaragua Rise, between Walton Bank and Jamaica, in 610 m of water. Seismic reflection profiles revealed thick accumulations of layered sediments with some fossil reefs. The sediments consist of undisturbed layers rich in planktonic microfossils alternating with turbidite layers. Absence of older coccoliths indicates that the redeposited material was not appreciably older than the time of redeposition, and oxygen isotopic analysis of benthonic elements shows that this material was derived from a depth not much shallower. The lower portion of the cored section correlates with the Manchioneal Formation of Jamaica. Taxonornic analysis of the calcareous nannoplankton indicates that the level at 2354 cm correlates with the midportions of the eastern equatorial Pacific cores 58 and 62; with the “Nebraskan-Aftonian” boundary of the Gulf Coast; and with the appearance of Hyalinea baltica at Le Castella, southern Italy. This level, therefore, represents the Plio-Pleistocene boundary as officially designated, and an age of about 700,000 years is estimated for the bou ndary. Oxygen isotopic analysis shows important oscillations, with a full glacial-interglacial amplitude, occurring both above and below the Plio-Pleistocene boundary.

Description: The ultimate cause of the onset of glaciations remains elusive, but in the case of northem hemisphere glaciation it is probable that several factors acted in combination. General global cooling resulted from reduction of atmospheric C02 by weathering of silicate rocks exposed by erosion of late Cenozoic uplifts. Uplifts in south Asia, southwestern North America and Scandinavia occurred at distances appropriate for the generation of quasi-permanent Rossby waves in the atmosphere. The resulting winds, given suitable moisture sources, were favourable for causing large-scale precipitation at mid-latitudes on the northern continents. Moisture sources were provided by the closure of the Central American isthmus. Gulf Stream flow increased, carrying warm subtropical waters to high latitudes. The Denmark Strait deepened permitting greater outflow of deep water from the Norwegian-Greenland Sea. The relative importance of each of these factors should be investigated by additional atmospheric and ocean climate model sensitivity studies.