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Foraminifera and Geochemistry to Solve Stratigraphic Problems, Sea Level Change, and Water Mass History: Mesozoic Case Studies

Abstract

Foraminifera continue to play a pivotal role in applied biostratigraphy and paleoenvironmental analysis in industry, and these diverse and widely dispersed protists are central to paleoceanographic research for relative age determination and the geochemistry their shells provide. By combining planktic and benthic biostratigraphic and paleoenvironmental analyses with stable isotope analyses, much more can be learned about conditions in the uppermost water column and at the seafloor. Several Mesozoic examples are highlighted here.

Multiple studies of the Cenomanian/Turonian boundary interval and Oceanic Anoxic Event 2 (OAE2), from Texas (Eagle Ford Shale) into the U.S. Western Interior Basin (e.g., Mancos Shale and Greenhorn Formation), as well as the organic-rich Coniacian-Santonian Niobrara interval provide new insights into how sea level rise, water mass origins, and climate contribute to the development of organic-rich facies. Foram assemblages combined with high resolution stratigraphy, and C- and Nd-isotopes have demonstrated the key role that the Western Interior Seaway played in the development of OAE2 globally, as well as how Boreal and Tethyan water masses, and water mass stratification controlled the distribution of organic-rich facies across the basin.

ODP Site 763 off Exmouth Plateau, NW Australia, recovered a spectacular uppermost Aptian-lower Cenomanian section recording several possible anoxic/dysoxic events, in addition to OAE 1b across the Aptian/Albian boundary. Foram assemblages, P/B and planktic/rad ratios, and planktic, benthic and bulk carbonate stable isotopes reveal latest Aptian cooling, latest Albian warming, and a dynamic history of sea level and organic matter production during the Albian.

The Middle Jurassic to mid-Cretaceous witnessed the continued break-up of Pangea, creation of new ocean basins, changes in climate, sea level, and ocean circulation, and the evolution and diversification of phytoplankton and planktic forams. An explosion in the production of organic matter resulted in the expansion of oxygen minimum zones. Benthic forams experienced significant evolutionary change, driven in part by depth stratification, as young continental margins subsided and the world ocean became increasingly stratified by water masses of varying sources and properties. Benthic forams, in combination with C- and O-isotopes are a powerful tool for understanding ocean-climate change through this less-well-known time interval.