Recent Advances in Research on Terrestrial and Marine sequences from the mid-Cretaceous Oceanic Anoxic Events (OAEs)

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


Recent Advances in Research on Terrestrial and Marine Sequences from the mid-Cretaceous Oceanic Anoxic Events (OAEs)
By Reishi Takashima, Pitambar Gautam, Hiroshi Nishi The Cretaceous was a time of greenhouse climates characterized by at least 15 ˚C warmer sea surface temperatures than the present one.During the climax of Cretaceous warming, anoxic condition in the oceans globally expanded, resulted in accumulations of organic rich sediments (so-called black shales).
These events are called Oceanic Anoxic Events (OAEs), and occurred at least 8 times during the Cretaceous period (Figure 1).The study of OAEs is important from three different perspectives: (i) the OAEs acted as the thermostat during the greenhouse climate because they eliminated carbon from atmosphere (CO 2 ) to sediments (black shales); (ii) the expansions of anoxic condition in the oceans caused mass extinctions of marine biota; (iii) most of source rocks were formed during Cretaceous time under anoxic oceanic environments.
Discovery of the Cretaceous OAEs (Schlanger and Jenkyns 1976)  Valley Basin, in which the OAEs are devoid of black shales.As regards type (i), results of high-resolution (at a scale of cm~mm order) studies on microfossils and geochemical analyses (both organic and inorganic) of the black shales related to OAEs were discussed.It was demonstrated that the black shales in the continental interior and continental margins were deposited during marine transgressions, each individual lamina corresponding to the increased runoff from the land areas.For type (ii) sections, attention was paid to the integrated analysis of macro and microfossil and carbon isotope stratigraphies for identification of the precise horizons of OAE levels.Although development of conspicuous anoxic environment didn't occur in these sections, clear evidences of remarkable faunal turnover, involving various taxa, were shown at the OAE levels.Studies on the marine sections dealt with core-based data mainly from the Blake Nose situated off Florida, and Demerara Rise off Brazil.Both sections have yielded extremely well preserved calcareous fossils being suitable for isotope geochemical analysis.
Oxygen and carbon isotope data on planktic and benthic foraminifers reveal the development of two types of water-column structure during OAEs.The first type, represented by collapse of the vertical structure of the water column caused by abrupt deep water warming, is recognized in OAE2 and OAE1d.Such OAEs are characterized by the increased primary production, drawing of carbonate platform and world-wide deposition of black shales.The second type corresponds to intensified vertical water column stratification triggered by an increase in the surface water temperature or a decrease in salinity (e.g.Pacquier level of OAE1b).
A question related to OAEs is the identification of factors that are responsible for the increased runoff observed in the land sections and the warming of the as well as deep water observed in marine sections.The emplacements of the large igneous provinces (LIPs) are thought to be most plausible trigger because each OAE occurred contemporarily with them.However, the emplacement positions of LIPS, their depths of eruption, the amount of magma production and the composition of the magma were highly variable and we still lack in adequate data which could allow us to deduce the relationship between the formation of the LIPs and the development of the anoxic water-column in the oceans.
The concluding session of the symposium was devoted to introduction of the activities of IODP and Japan Drilling Earth Science Consortium and discussion on the Cretaceous IODP drilling site for OAE research.Understanding how the OAEs occurred requires knowledge on the thermal structure of the ocean during those greenhouse periods.As reliable data for water temperature for pre-Aptian as well as for the high latitude regions are still inadequate, these aspects need due consideration in future.The Falkland Plateau (Argentina) was chosen as the most appropriate target for the next drilling.
At the end of the symposium, a day-long field excursion to observe the OAE 2 section in Hokkaido was organized.It was explained that there are evidences of prominent positive excursion of δ 13 C organic , weaken bioturbation and occurrence of abundant pyrite, although deposition of typical black shales didn't occur in this section.

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was one of the most important achievements of the Deep Sea Drilling Project, and understanding Cretaceous OAEs is regarded as major scientific objective of "the Extreme Climate" research program outlined in the Integrated Ocean Drilling Program (IODP).To review the recent advances and gain new insights on the study of OAEs, a symposium entitled "Recent Advances in Research on Terrestrial and Marine Sequences from the mid-Cretaceous Oceanic Anoxic Events (OAEs)" was held at Hokkaido University, Sapporo 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.There were 13 talks (eight of which were given by invited scientists from abroad) and 8 posters dealt with land and marine OAE sections shown in Figure 2. Discussion on land sections concentrated on: (i) Umbria-Marche Basin Western Interior NW German Basin and Vocontian Basin, all of which have OAEs accompanied by black shales; and (ii) Anglo-Paris Basin, Yezo Basin and Great

Figure 1 :
Figure 1: OAE horizons, sea level changes and production rate of the oceanic crust.