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Deep Water, which causes the uneven distribution of nutrients, alkalinity, and oxygen in the deep sea today, may have been replaced by formation of North Pacific Deep Water during the last deglaciation, reversing the concentration gradients of nutrients, alkalinity, and oxygen.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: Plate tectonic reconstructions for the Cretaceous have assumed that the major continental blocks—Eurasia, Greenland, North America, South America, Africa, India, Australia, and Antarctica—had separated from one another by the end of the Early Cretaceous, and that deep ocean passages connected the Pacific, Tethyan, Atlantic, and Indian Ocean basins. North America, Eurasia, and Africa were crossed by shallow meridional seaways. This classic view of Cretaceous paleogeography may be incorrect. The revised view of the Early Cretaceous is one of three large continental blocks— North America–Eurasia, South America–Antarctica-India-Madagascar-Australia; and Africa—with large contiguous land areas surrounded by shallow epicontinental seas. There was a large open Pacific basin, a wide eastern Tethys, and a circum- African Seaway extending from the western Tethys (“Mediterranean”) region through the North and South Atlantic into the juvenile Indian Ocean between Madagascar-India and Africa. During the Early Cretaceous the deep passage from the Central Atlantic to the Pacific was blocked by blocks of northern Central America and by the Caribbean plate. There were no deep-water passages to the Arctic. Until the Late Cretaceous the Atlantic-Indian Ocean complex was a long, narrow, sinuous ocean basin extending off the Tethys and around Africa. Deep passages connecting the western Tethys with the Central Atlantic, the Central Atlantic with the Pacific, and the South Atlantic with the developing Indian Ocean appeared in the Late Cretaceous. There were many island land areas surrounded by shallow epicontinental seas at high sea-level stands.

Description: The climates of two realistic geographic representations of the Triassic earth, corresponding in age to the Scythian (245 Ma) and the Carnian (225 Ma), are explored using a new atmospheric general circulation model (AGCM) called GENESIS. The GENESIS AGCM is coupled to a slab ocean 50 m thick, with prescribed heat transport; it also incorporates three types of cloud cover and new models for vegetation effects, soil hydrology, snow cover, and sea-ice formation and melting. Boundary conditions prescribed in the separate Scythian and Carnian experiments include realistic paleogeography and estimates of paleotopography, solar insolation, atmospheric CO2 concentration, vegetation and soil types, and oceanic heat flux. Seasonal simulations of Triassic climate were performed using a horizontal spectral resolution of R15 (4.5 degrees latitude by 7.5 degrees longitude) and 12 levels in the vertical for the atmosphere and 2° × 2* for the surface. Results for both time intervals suggest that most of the seasonal precipitation fell on major highland areas of Pangea. Dry continental climates with very large seasonal temperature ranges (〉45°C) were modeled in the dominantly lowland interior of Pangea. Carnian continental climates predicted by the AGCM were wetter than those of the Scythian; however, both time intervals were characterized by strongly monsoonal circulation. Comparison of these results with lithologic and fossil proxy climatic indicators suggests reasonably good correlations. However, the extreme temperature variations predicted for both Scythian and Carnian are somewhat difficult to reconcile with the fossil record, although accurate interpretation of fossil proxy climatic indicators is not a simple matter. Additional AGCM sensitivity studies may be necessary to resolve this problem.

Description: The Campanian age of the Late Cretaceous was warm, with no evidence for permanent or seasonal sea ice at high latitudes. Sea level was high, creating extensive epicontinental and shallow shelf seas. Very low meridional thermal gradients existed in the oceans and on land. Campanian (80 Ma) climate and vegetation have been simulated using GENESIS (Global ENvironmental and Ecological Simulation of Interactive Systems) Version 2.0 and EVE (Equilibrium Vegetation Ecology model), developed by the Climate Change Research section of the Climate and Global Dynamics division at NCAR (National Center for Atmospheric Research). GENESIS is a comprehensive Earth system model, requiring high resolution (2^circ by 2^circ) solid earth boundary condition data as input for paleoclimate simulations. Boundary condition data define certain prescribed global fields such as the distribution of land-sea-ice, topography, orographic roughness, and soil texture, as well as atmospheric chemistry, the solar constant, and orbital parameters that define the latitudinal distribution of solar insolation. A comprehensive, high resolution paleogeography has been reconstructed for the Campanian. The paleogeography, based on a new global plate tectonic model, provides the framework for the solid earth boundary conditions used in the paleoclimate simulation. Because terrestrial ecosystems influence global climate by affecting the exchange of energy, water and momentum between the land surface and the atmosphere, the distribution of global vegetation should be included in pre-Quaternary paleoclimate simulations. However, reconstructing global vegetation distributions from the fossil record is difficult. EVE predicts the equilibrium state of plant community structure as a function of climate and fundamental ecological principles. The model has been modified to reproduce a vegetation distribution based on life forms that existed in the Late Cretaceous. EVE has been applied as a fully interactive component of the Campanian simulation. 1500 ppm CO_2 and a QFACTOR of 4 were sufficient to maintain forest over Antarctica and high northern latitudes. The QFACTOR is the multiplicative of the oceanic heat diffusion coefficient in the slab-mixed layer ocean component of GENESIS. The simulated Campanian oceanic heat transport has maximum values of about 1.7 times 10^{15} W at 25 ^circ north and 2.6 times 10^{15} W at 25^ circ south, similar to present day observed values. Late Cretaceous forests played an important role in the maintenance of low meridional thermal gradients, polar warmth, and equable continental interiors. The Campanian high to polar latitude forests decreased surface albedo (especially in late winter-early spring, prior to snow melt), and increased net radiation and fluxes of sensible and latent heat. This warmed the high latitude troposphere and increased atmospheric moisture. The warmer atmospheric temperatures reduced winter cooling of the high latitude sea surface and aided the advection of warm, moist air from the oceans into the continental interiors.