Today, the ocean is characterized by pools of warm tropical-subtropical water bounded poleward and at depth by cold water. In the tropics and subtropics, the warm waters are floored at depth by the thermocline-pycnocline, which crops out on the ocean surface between the subtropical and polar frontal systems that form the poleward boundary. It is along and between the frontal systems that the thermocline waters enter the ocean interior. These frontal systems form beneath the maxima of the zonal component of the westerly winds. Today, the location of the westerly winds is stabilized by the persistent high-pressure systems at the polar regions produced by the ice cover of the Antarctic and sea-ice cover of the Arctic.The paleobiogeographic distribution of plankton fossils indicates that, prior to the Oligocene, the subtropical and polar frontal systems were not persistent features. Recent climate model experiments show that without perennial ice cover in the polar regions a seasonal alternation between high and low atmospheric pressure systems can occur. These seasonal alternations would force major changes in the location and strength of the westerly winds, preventing the development of the well-defined frontal systems that characterize the Earth today. Without the subtropical and polar frontal systems, the thermocline would be less well developed and the pycnocline could be dominated by salinity differences.Evidence from ocean drilling suggests that the glaciation of East Antarctica began at the Eocene-Oligocene boundary, but took time to spread over the entire continent. The presence of calcareous nannoplankton in the Arctic basin prior to the Oligocene and their absence thereafter suggests that the ice cover of the Arctic Ocean also developed at the Eocene-Oligocene boundary. Both events appear to be related to the development of the modem oceanic structure, but it remains uncertain whether the ocean changed in response to the development of ice covered polar regions or vice versa. (c) 2004 Elsevier B.V. All rights reserved.

The paleoclimatology and paleoceanology of the Late Jurassic and Early Cretaceous are of special interest because this was a time when large amounts of marine organic matter were deposited in sediments that have subsequently become petroleum source rocks. However, because of the lack of outcrops, most studies have concentrated on low latitudes, in particular the Tethys and the “Boreal Realm,” where information has been based largely on material from northwest Germany, the North Sea, and England. These areas were all south of 40°N latitude during the Late Jurassic and Early Cretaceous. We have studied sediment samples of Kimmeridgian (∼154 Ma) to Barremian (∼121 Ma) age from cores taken at sites offshore mid‐Norway and in the Barents Sea that lay in a narrow seaway connecting the Tethys with the northern polar ocean. During the Late Jurassic‐Early Cretaceous these sites had paleolatitudes of 42–67°N. The Late Jurassic‐Early Cretaceous sequences at these sites reflect the global sea‐level rise during the Volgian‐Hauterivian and a climatic shift from warm humid conditions in Volgian times to arid cold climates in the early Hauterivian. The sediments indicate orbital control of climate, reflected in fluctuations in the clastic influx and variations in carbonate and organic matter production. Trace element concentrations in the Volgian‐Berriasian sediments suggest that the central part of the Greenland‐Norwegian Seaway might have had suboxic bottom water beneath an oxic water column. Both marine and terrigenous organic matter are present in the seaway sediments. The Volgian‐Berriasian strata have unusually high contents of organic carbon and are the source rocks for petroleum and gas fields in the region. The accumulation of organic carbon is attributed to restricted conditions in the seaway during this time of low sea level. It might be that the Greenland‐Norwegian segment was the deepest part of the transcontinental seaway, bounded at both ends by relatively shallow swells. The decline in organic matter content of the sediments in the Valanginian‐Hauterivian indicates greater ventilation and more active flow through the seaway as the sea level rose. The same benthic foraminifera assemblages are encountered throughout the seaway. Endemic assemblages of arenaceous foraminifera in the Volgian‐Berriasian give way to more diverse and cosmopolitan Valanginian‐Hauterivian benthic communities that include calcareous species. The foraminiferal assemblages also suggest low oxygen content bottom waters during the earlier Cretaceous, changing to more fully oxygenated conditions later. The calcareous nannoplankton, particularlyCrucibiscutum salebrosum, which is rare at low latitudes and abundant in high latitudes, reflect the meridional thermal gradient. They indicate that the Greenland‐Norwegian segment of the seaway was north of a subtropical frontal zone that acted as a barrier between the Tethyan and Boreal Realms. This implies the existence of stable climatic belts during the early Valanginian and Hauterivian, significant meridional temperature gradients, and moderate “ice house” conditions.

Knowledge of the rates of geological processes is an important aspect of basin modeling. Much of the surficial geology of the Earth is the result of erosion and deposition of sediment, Inspection of the inventory of sediments and sedimentary rocks existing today indicates that the global rates of these processes have changed markedly during the Phanerozoic.The mass-age distribution of Phanerozoic sediments and sedimentary rocks existing on Earth today has the general form of an exponential decay curve, reflecting the fact that new sediments are formed mostly from the erosion of older sediments. The dissolved salt in the ocean and in pore waters constitutes a special reservoir of the sedimentary system, representing part of the soluble matter derived from weathering. Additions to the total sedimentary system--sediments plus dissolved salts--come from the weathering of igneous and metamorphic rocks and from extraterrestrial sources but are relatively small in comparison with the overall rate of sediment cycling. Losses to the sedimentary system result from metamorphism and subduction but again are small in comparison with the overall rate of cycling of sedimentary materials. About a fourth of the total mass of sedimentary material has been subducted and replaced by new sedimentary material produced from the weathering of igneous and high-grade metamorphic rocks during the Phanerozoic. Sedimentary flux rates have changed by a factor of five through the Phanerozoic. It is most likely that these variations reflect changes in continental relief, but the high rates in the Early Paleozoic also were a function of the lack of plant cover to bind the soil and retard erosion. Because sedimentary strata accumulate in thin, widespread layers, erosion of sedimentary materials must proceed in such a way that young unconsolidated sediments arc more likely to be eroded than older rocks. Although some soluble rocks may be dissolved as they enter the active near-surface groundwater system, the amount of selective recycling of different lithologies is slight. There is little evidence for evolution of sedimentary materials on the continental blocks. Quartz sand has become less abundant in the later Phanerozoic, and since the mid Cretaceous the site of deposition of carbonates has shifted from the continental blocks to the deep sea. From the inventory of existing evaporites it is possible to use principles of sedimentary cycling to reconstruct the salinity of the ocean during the past. We conclude that average ocean salinities were almost 50parts per thousand in the Palcozoic and declined during the Mesozoic and Cenozoic to finally reach its modern value of 34.7parts per thousand.

The opening of the Equatorial Atlantic Gateway (EAG) during the Cretaceous was accompanied by the disruption of the sedimentary basins that had developed on the conjugate margins of Africa and South America. Drilling along the Côte d'Ivoire–Ghana Transform Margin (ODP Leg 159) provided a transect across the northern rim of this gateway. The interplay of tectonic and oceanic processes along the gateway created a complex continental margin that evolved in three stages interrupted by dramatic changes in sedimentary facies, waterdepths, and subsidence rates. The earliest stage records the formation of small basins with restricted connection to the world ocean and rapid infill with siliciclastic deposits in an Early Cretaceous intracratonic rift or wrench tectonic setting. This stage ended with an uplift event and the formation of a regional unconformity. During the late Albian to middle Coniacian, the oceanward side of the margin subsided below the calcite compensation depth (CCD) and a deepwater connection between Central and South Atlantic became established. Deepening of the basement ridge and its landward slope, in contrast, were delayed and detrital limestones intercalated with carbonaceous shales accumulated at shelf to slope depths. During the ensuing, latest Cretaceous to present stage, passive margin subsidence led to continuous deepening of the basement ridge and on its landward slope. Condensation and gradually decreasing organic contents point to an intensified exposure to deepwater circulation. The replacement of the zonal circulation system through the Mesozoic Tethys and Central Atlantic with a modern, oxidizing meridional circulation system through the EAG appears to be intimately related to the changing depositional conditions over large parts of the Cretaceous Atlantic.