Recent Advances in Research on Terrestrial and Marine Sequences From the Mid-Cretaceous Oceanic Anoxic Events (OAEs)

Abstract

The Cretaceous period was a time of greenhouse climates characterized by sea-surface temperatures of at least 15°C warmer than today. During the climax of Cretaceous warming, anoxic conditions in the oceans expanded globally, resulting in accumulations of organic-rich sediments (socalled black shales). These events, called Oceanic Anoxic Events (OAEs), occurred at least eight times during the Cretaceous period (Fig. 1). Three important aspects make the study of OAEs so interesting: (a) the OAEs acted as a global thermostat because carbon to a large extent was sequestered in the sediments (black shales) instead of adding to the CO2 content of the atmosphere, (b) the expansion of anoxic conditions in the oceans caused mass extinctions of marine biota, and (c) most hydrocarbon source rocks from the Cretaceous formed under anoxic oceanic environments. The discovery of the Cretaceous OAEs (Schlanger and Jenkyns, 1976) was an important achievement of the Deep Sea Drilling Project (DSDP), and understanding Cretaceous OAEs is one of the major scientifi c objectives of the extreme climate initiative outlined in the Integrated Ocean Drilling Program (IODP) Initial Science Plan. To support this initiative a symposium entitled “Recent Advances in Research on Terrestrial and Marine Sequences from the midCretaceous Oceanic Anoxic Events (OAEs)” was held at Hokkaido University, Sapporo, Japan during 21–23 September 2005. The symposium was hosted by the 21 st Century Center of Excellence (COE) Program for the Neo-Science of Natural History, and was organized by Hisatake Okada (COE program leader) and Hiroshi Nishi. Thirteen talks were given, eight by invited speakers, and eight posters dealt with land and marine OAE sections shown in Figure 2. Discussion of land sections concentrated on two types of regions: (a) those with black shale, such as Umbria-Marche Basin, Western Interior, Northwest German Basin and Vocontian Basin; and (b) those devoid of black shale, such as the AngloParis Basin, Yezo Basin and Great Valley Basin. For regions of the fi rst type, the group discussed results of high-resolution studies on a millimeter to centimeter scale of microfossils and organic and inorganic geochemical analyses. It was demonstrated that the black shales in continental interiors and on the continental margins were deposited during marine transgressions, with individual laminae corresponding to the increased runoff from the land areas. For regions of the second type, attention was paid to the integrated analysis of macro- and microfossil and carbon isotope stratigraphies for identifi cation of the precise horizons of OAE levels. Although fully anoxic environments apparently did not develop in these basins, clear evidence of remarkable faunal turnover, involving various taxa, was shown at the OAE levels. Studies of the marine sections dealt with core data mainly from Blake Nose (ODP Leg 171B) and the Demerara Rise (ODP Leg 207). Both sections have yielded extremely well-preserved calcareous fossils suitable for isotopic geochemical analysis. Oxygen and carbon isotope data on planktic and benthic foraminifers revealed the development of two types of water-column structures during