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Mesozoic Palaeoceanography and Black Shales |
Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112-1183, USA
The biogeography and preservation of calcareous nannofossil assemblages allow mapping of surface water fertility patterns and put constraints on carbonate dissolution in the global oceans of the middle Cretaceous. Cyclic changes in carbonate dissolution and surface water biologic productivity in the Cretaceous oceans were linked to oceanographic and climatic changes that had frequencies close to the Milankovitch cycles (tens of thousands to hundreds of thousand years). Evolution of calcareous nannoplankton during the Jurassic and Cretaceous periods occurred in two primary cycles of about 75 Ma duration that were punctuated by major diversification events, occurring every 15–30 Ma, that marked the beginning of periods of increased marine organic matter production and preservation (stagnation events). Calcareous nannoplankton evolved largely in shallow seas during the Rhaetian to Kimmeridgian. In the latest Jurassic, calcareous nannoplankton conquered the oceanic realm, became major producers of pelagic carbonates, and are thus responsible for the shift of carbonate deposition from shallow seas to the deep ocean. In the late Cretaceous chalk seas, nannofossils replaced shallow water benthos as major carbonate producers. Warm climates, due to high atmospheric CO2 concentrations and increased albedo during high sea-level stands caused by global tectonic processes, made these tiny creatures flourish during the late Mesozoic.
