Description: Four mud extrusions were investigated along the erosive subduction zone off Costa Rica. Active fluid seepage from these structures is indicated by chemosynthetic communities, authigenic carbonates and methane plumes in the water column. We estimate the methane output from the individual mud extrusions using two independent approaches. The first is based on the amount of CH4 that becomes anaerobically oxidized in the sediment beneath areas covered by chemosynthetic communities, which ranges from 104 to 105 mol yr− 1. The remaining portion of CH4, which is released into the ocean, has been estimated to be 102–104 mol yr− 1 per mud extrusion. The second approach estimates the amount of CH4 discharging into the water column based on measurements of the near-bottom methane distribution and current velocities. This approach yields estimates between 104–105 mol yr−1. The discrepancy of the amount of CH4 emitted into the bottom water derived from the two approaches hints to methane seepage that cannot be accounted for by faunal growth, e.g. focused fluid emission through channels in sediments and fractures in carbonates. Extrapolated over the 48 mud extrusions discovered off Costa Rica, we estimate a CH4 output of 20·106 mol yr− 1 from mud extrusions along this 350 km long section of the continental margin. These estimates of methane emissions at an erosional continental margin are considerably lower than those reported from mud extrusion at accretionary and passive margins. Almost half of the continental margins are described as non-accretionary. Assuming that the moderate emission of methane at the mud extrusions off Costa Rica are typical for this kind of setting, then global estimates of methane emissions from submarine mud extrusions, which are based on data of mud extrusions located at accretionary and passive continental margins, appear to be significantly too high.

Description: The geologic evidence for worldwide uplift of mountain ranges in the Neogene is ambiguous. Estimates of paleoelevation vary, according to whether they are based on the characteristics of fossil floras, on the masses and grain sizes of eroded sediments, or on calculations of increased thickness of the lithosphere as a result of faulting. Detrital erosion rates can be increased both by increased relief in the drainage basin and by a change to more seasonal rainfall patterns. The geologic record provides no clear answer to the question whether uplift caused the climatic deterioration of the Neogene or whether the changing climate affected the erosion system in such a way as to create an illusion of uplift. We suggest that the spread of C4 plants in the Late Miocene may have altered both the erosion and climate systems. These changes are responsible for the apparent contradictions between data supporting uplift and those supporting high elevations in the past.

Description: 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.

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.