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Late Jurassic-early Cretaceous oceanography
Helmut Jürg Weissert

Building: Muséum d'Histoire Naturelle de Genève
Room: Amphithéâtre
Date: 2018-12-07 10:50 AM – 11:20 AM
Last modified: 2018-11-23

Abstract


In this study, evolution of low-latitude east-west trending Atlantic and Tethys Oceans is traced from the Late Jurassic into the Early Cretaceous. C-isotope geochemistry serves as a stratigraphic tool and as a proxy of global carbon cycling through geological time.

Late Jurassic Oceans experienced a major change in the carbonate system. Calcareous nannoplankton started to proliferate open ocean settings and it became a rock-forming constituent. The establishment of a global pelagic carbonate factory occurred during the late Jurassic. In the Tethys Ocean pelagic sedimentation changed from radiolarian-dominated facies to a calcareous nannofossil facies. Pelagic nannofossil limestones, forming the Southern Tethyan Maiolica Formation, can be traced from the western North Atlantic through the “alpine Tethys Ocean” into the eastern Tethys. A change from siliceous pelagic sediments (Sidr Chert) to a siliceous white nannofossil limestone is documented from the Hawasina Basin (Oman Mountains). This change in carbonate production resulted in the establishment of a deep Calcite Compensation Depth (CCD) in the modern oceanographic sense. The base of the Maiolica formation and its equivalents are dated as Tithonian and the transition into the Cretaceous occurs within the lower part of the Maiolica Formation. Pelagic nannofossil limestones were formed in low latitude oceans, under oligotrophic conditions across the Jurassic-Cretaceous boundary.

A major change in low-latitude oceanography occurred during the Valanginian, when gradual deepening of the Hispanic corridor reached a critical depth where deep-water exchange between Atlantic and Pacific Oceans became possible. A change from white nannofossil limetones to a limestone-marlstone/claystone succession was established in the Atlantic Ocean and in the Tethys Ocean. A transequatorial east-west trending surface-water current was established, equatorial upwelling was, according to ocean circulation models, intensified. These were the boundary conditions which facilitated increased burial of organic carbon during times of major, volcanically driven C-cycle perturbations. The first of these perturbations occurred during the late Valanginian and it marks the beginning of an oceanography marked by multiple OAE’s. This early Cretaceous oceanography ended at a time when a deep North-South connection was established in the Atlantic Ocean.

For a perspective of low-latitude oceanography there is no evident change occurring near the proposed Jurassic – Cretaceous boundary. Major changes occurred in the Tithonian and later in the Valanginian.