Eine quantitative Analyse der Sedimentakkumulation und Massenbilanz ausgewählter Regionen im Ostatlantik

Um den global zu beobachtenden Anstieg der neogenen Akkumulationsraten zu erklären wurden sechs Gebiete des Ostatlantiks untersucht. Die betrachteten Regionen umfassen den östlichen Teil des Europäischen Nordmeeres, die Biskaya und die Porcupine Tiefsee-Ebenen, das Kanarische Becken, das Kapverdische Becken, das Sierra Leone Becken und das Guineabecken, sowie die dazugehörenden potentiellen Sedimentliefergebiete. Das entwickelte Konzept erlaubt es, eine große Menge verschiedener Datenquellen miteinander zu verknüpfen, um ein komplettes Abbild der Sedimentbedeckung und der Krustenstruktur zu erhalten. Die dazu verwendeten Informationen umfassen unter anderem Auswertungen von über 80 DSDP- und ODP- Bohrungen der Regionen und seismischer Daten der ATLANTIS II, CONRAD und VEMA Ausfahrten vor Westafrika. Durch Vergleiche der Einflüsse unterschiedlicher Faktoren auf die Massenverteilungen können regionale von globalen Ereignissen unterschieden und in ihrer Größenordnung interpretiert werden. Dabei zeigt sich in allen Regionen ein auffälliger Anstieg der Sedimentakkumulation im Neogen. Während dieser im östlichen Europäischen Nordmeer durch tektonische Ereignisse begründet werden kann, zeigen die übrigen Regionen keinen derartigen Einfluß. Sowohl in der Biskaya /Porcupine Region, wie in den westafrikanischen Gebieten, ist der neogene Anstieg der Sedimentakkumulation im wesentlichen auf klimatische Sekundäreffekte zurückzuführen. Dabei spielt die Erosion der Schelf gebiete durch stark wechselnde Meeresspiegelstände und eine verstärkte Wiederaufarbeitung von Sedimenten in der Tiefsee durch Bodenwasserströmungen, eine bedeutende Rolle. Der Einfluß regionaler tektonischer Ereignisse ist demgegenüber als gering einzuschätzen. Die langfristigen Akkumulationssraten zeigen für die westafrikanischen Regionen und die Biskaya einen über das Känozoikum sehr guten Zusammenhang mit der Fläche der Liefergebiete und Schelfe. Dieser Zusammenhang wird in den unterschiedlichen Zeitintervallen verschieden stark von klimatischen Faktoren beeinflußt und überprägt, sodaß es nicht möglich ist, ohne deren genaue Kenntnis Schlüsse auf andere Gebiete zu ziehen. Ein Vergleich der känozoischen Denudationsraten der Liefergebiete, mit heutigen Flußfrachten in diesen Regionen belegt, daß diese starken Schwankungen unterworfen waren. Daher muß es als zweifelhaft angesehen werden, daß diese Daten Rückschlüsse auf globale Zusammenhänge liefern können, ohne deren genaue Variabilität über unterschiedliche Zeitdimensionen zu kennen

Evaporites and the salinity of the ocean during the Phanerozoic: Implications for climate, ocean circulation and life

A compilation of data on volumes and masses of evaporite deposits is used as the basis for reconstruction of the salinity of the ocean in the past. Chloride is tracked as the only ion essentially restricted to the ocean, and past salinities are calculated from reconstructed chlorine content of the ocean. Models for ocean salinity through the Phanerozoic are developed using maximal and minimal estimates of the volumes of existing evaporite deposits, and using constant and declining volumes of ocean water through the Phanerozoic. We conclude that there have been significant changes in the mean salinity of the ocean accompanying a general decline throughout the Phanerozoic. The greatest changes are related to major extractions of salt into the young ocean basins which developed during the Mesozoic as Pangaea broke apart. Unfortunately, the sizes of these salt deposits are also the least well known. The last major extractions of salt from the ocean occurred during the Miocene, shortly after the large scale extraction of water from the ocean to form the ice cap of Antarctica. However, these two modifications of the masses of H2O and salt in the ocean followed in sequence and did not cancel each other out. Accordingly, salinities during the Early Miocene were between 37‰ and 39‰. The Mesozoic was a time of generally declining salinity associated with the deep sea salt extractions of the North Atlantic and Gulf of Mexico (Middle to Late Jurassic) and South Atlantic (Early Cretaceous). The earliest of the major extractions of the Phanerozoic occurred during the Permian. There were few large extractions of salt during the earlier Palaeozoic. The models suggest that this was a time of relatively stable but slowly increasing salinities ranging through the upper 40‰'s into the lower 50‰'s. Higher salinities for the world ocean have profound consequences for the thermohaline circulation of the ocean in the past. In the modern ocean, with an average salinity of about 34.7‰, the density of water is only very slightly affected by cooling as it approaches the freezing point. Consequently, salinization through sea-ice formation or evaporation is usually required to make water dense enough to sink into the ocean interior. At salinities above about 40‰ water continues to become more dense as it approaches the freezing point, and salinization is not required. The energy-consuming phase changes involved in sea-ice formation and evaporation would not be required for vertical circulation in the ocean. The hypothesized major declines in salinity correspond closely to the evolution of both planktonic foraminifera and calcareous nannoplankton. Both groups were restricted to shelf regions in the Jurassic and early Cretaceous, but spread into the open ocean in the mid-Cretaceous. Their availability to inhabit the open ocean may be directly related to the decline in salinity. The Permian extraction may have created stress for marine organisms and may have been a factor contributing to the end-Permian extinction. The modeling also suggests that there was a major salinity decline from the Late Precambrian to the Cambrian, and it is tempting to speculate that this may have been a factor in the Cambrian explosion of life

Alternative global Cretaceous paleogeography

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

Is the initiation of glaciation on Antarctica related to a change in the structure of the ocean?

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 modern oceanic structure, but it remains uncertain whether the ocean changed in response to the development of ice covered polar regions or vice versa