Mineral and chemical compositions of sedimentary and volcano-sedimentary rocks drilled in DSDP Legs 43 and 44

Sedimentary cover on the bottom of the Northwest Atlantic Ocean is underlain by Late Jurassic - Cretaceous tholeiite-basalt formation. It consists of come sedimentary formations with different lithologic features and age. Their composition, stratigraphic position and, distribution are described on materials of deep-sea drilling. Mineralogical and geochemical studies of DSDP Leg 43 and Leg 44 holes lead to new ideas about composition and genesis of some sediment types of and their associations. High metal contents in the chalk formation of black clays on the Bermuda Rise probably result from exhalations. Connection of red-colored and speckled deposits with hiatuses in sedimentation is shown. Main stages of geological history of the North American Basin are reflected in accumulation of the followed formations: ancient carbonate formation (Late Jurassic - Early Cretaceous), formation of black clays rich in organic matter (Cretaceous), formation of speckled clays (Late Cretaceous), siliceous-clayey turbidite formation (Eocene), hemipelagic and pelagic clayey formation (Neogene), and terrigenous turbidite formation (Pleistocene).

Stable oxygen isotope record and benthic foraminiferal abundances in Cretaceous sediments of Demerara Rise

During the mid-Cretaceous period, the global subsurface oceans were relatively warm, but the origins of the high temperatures are debated. One hypothesis suggests that high sea levels and the continental configuration allowed high-salinity waters in low-latitude epicontinental shelf seas to sink and form deep-water masses (Brass et al., 1982, doi:10.1038/296620a0; Arthur and Natland, 1979; Chamberlin, 1906). In another scenario, surface waters in high-latitude regions, the modern area of deep-water formation, were warmed through greenhouse forcing (Bice and Marotzke, 2001, doi:10.1029/2000JC000561), which then propagated through deep-water circulation. Here, we use oxygen isotopes and Mg/Ca ratios from benthic foraminifera to reconstruct intermediate-water conditions in the tropical proto-Atlantic Ocean from 97 to 92 Myr ago. According to our reconstruction, intermediate-water temperatures ranged between 20 and 25 °C, the warmest ever documented for depths of 500-1,000 m. Our record also reveals intervals of high-salinity conditions, which we suggest reflect an influx of saline water derived from epicontinental seas around the tropical proto-North Atlantic Ocean. Although derived from only one site, our data indicate the existence of warm, saline intermediate waters in this silled basin. This combination of warm saline intermediate waters and restricted palaeogeography probably acted as preconditioning factors for the prolonged period of anoxia and black-shale formation in the equatorial proto-North Atlantic Ocean during the Cretaceous period.