Enhanced productivity led to increased organic carbon burial in the euxinic North Atlantic basin during the late Cenomanian oceanic anoxic event
Tóm tắt
Three Cenomanian/Turonian (C/T, ∼93.5 Ma) black shale sections along a northeast‐southwest transect in the southern part of the proto‐North Atlantic Ocean were correlated by stable carbon isotope stratigraphy using the characteristic excursion in δ13C values of both bulk organic matter (OM) and molecular fossils of algal chlorophyll and steroids. All three sites show an increase in marine organic carbon (OC) accumulation rates during the C/T Oceanic Anoxic Event (OAE). The occurrence of molecular fossils of anoxygenic photosynthetic green sulfur bacteria, lack of bioturbation, and high abundance of redox sensitive trace metals indicate sulfidic conditions, periodically reaching up into the photic zone before as well as during the C/T OAE. During the C/T OAE, there was a significant rise of the chemocline as indicated by the increase in concentrations of molecular fossils of green sulfur bacteria and Mo/Al ratios. The presence of molecular fossils of the green strain of green sulfur bacteria indicates that euxinic conditions periodically even occurred at very shallow water depths of 15 m or less during the C/T OAE. However, bottom water conditions did not dramatically change as indicated by more or less constant V/Al and Zn/Al ratios at site 367. This suggests that the increase in OC burial rates resulted from enhanced primary productivity rather than increased anoxia, which is supported by stable carbon isotopic evidence and a large increase in Ba/Al ratios during the C/T OAE. The occurrence of the productivity event during a period of globally enhanced organic carbon burial rates (i.e., the C/T OAE) points to a common cause possibly related to the formation of a deep water connection between North and South Atlantic basins.
Từ khóa
Tài liệu tham khảo
Arthur M. A., 1984, in Fine‐Grained Sediments, 527
Berger W. H., 1972, Cretaceous and Cenozoic sediments from the Atlantic Ocean, Initial Rep. Deep Sea Drill. Project, 14, 787
Brumsack H. J., 1986, The geochemical facies of black shales from the Cenomanian/Turonian boundary event (CTBE), Mitt. Geol. Palaeont. Inst. Univ. Hamburg, 60, 247
Caron M., 1999, Estimation de la durée de l'événement anoxique global au passage Cénomanien/Turonien, Approche cyclostratigraphique dans la formation Bahloul en Tunisie centrale, Bull. Soc. Geol. Fr., 170, 144
Gradstein F., 1995, A Triassic, Jurassic, and Cretaceous time scale, Soc. Sediment. Geol., SEPM Spec. Publ., 54, 95
Hayes D. E., 1972, Sites 143 and 144, Initial Rep. Deep Sea Drill. Project, 14, 283
Imhoff J. F., 1995, in Anoxygenic Photosynthetic Bacteria, 1
Kuhnt W., 1990, Distribution of Cenomanian‐Turonian organic facies in the western Mediterranean and along the Adjacent Atlantic Margin, AAPG Stud. Geol., 30, 133
Parrish J. T., 1995, in Paleogeography, Paleoclimate and Source Rocks, 1
Poulsen C. J. The mid‐Cretaceous ocean circulation and its impact on Greenhouse Climate dynamics Ph.D. thesis Pa. State Univ. University Park 1999.
Ricken W., 1993, Sedimentation as a three‐component system, Lect. Notes Earth Sci., 51, 1
Schlanger S. O., 1976, Cretaceous oceanoc anoxic events: Causes and consequences, Geol. Mijnbouw, 55, 179
Scholle P. A., 1980, Carbon‐isotope fluctuations in Cretaceous pelagic limestones: Potential stratigraphic and petroleum exploration tool, AAPG Bull., 64, 67
C. R. Scotese J. Golonka 1992 Univ. Tex. Arlington
Shipboard Scientific Party, 1977, Site 367: Cape Verde Basin, Initial Rep. Deep Sea Drill. Project, 41, 163
Thurow J. W., 1988, The Cenomanian‐Turonian Boundary Event (CTBE) at Leg 103/Hole 641A, Proc. Ocean Drill. Program Sci. Results, 103, 587
Tucholke B. E., 1979, Western North Atlantic: Sedimentary evolution and aspects of tectonic history, Initial Rep. Deep Sea Drill. Project, 43, 791