Petrology and carbon and oxygen stable isotopic composition of macrofossils and sediments from the Blake-Bahama Formation, DSDP Hole 93-603B

Strata that record the evolutionary history of the North American continental margin in a region that serves as the basin margin interface between allochthonous sedimentation from the continent and pelagic sedimentation from the oceanic realm were recovered at Deep Sea Drilling Project Site 603, on the lower continental rise. The lowermost unit recovered at this site is composed of upper Berriasian-Aptian interbedded laminated limestone and bioturbated limestone with sandstone to claystone turbidites. This unit can be correlated with the Blake-Bahama Formation in the western North Atlantic. Studies of the laminated and bioturbated limestones were used to determine the depositional environment. Geochemical and petrographic studies suggest that the laminated limestones were deposited from the suspended particulate loads of the nepheloid layer associated with weak bottom-current activity as well as moderate to poorly oxygenated bottom-water conditions. Fragments of macrofossils are also found in the Blake-Bahama Formation drilled at Site 603. Twelve specimens and their host sediment were analyzed for their carbon and oxygen isotopic composition. The macrofossil samples chosen for analysis consist of nine samples of Inoceramus, two ammonite aptychi, and one belemnite sample. Depletion in 18O is observed in recrystallized specimens. The ammonite aptychi have been diagenetically altered and/or exhibit evidence of isotopic fractionation by the organism. Oxygen isotope paleotemperatures obtained from five well-preserved specimens - four of Inoceramus and one of a belemnite - suggest that bottom-water temperatures in the North Atlantic Basin during the Early Cretaceous were very warm, at least 11°C.

Paleotemperature reconstructions from high Late Jurassic paleolatitudes

Oxygen and carbon isotopes have been determined from Late Jurassic (Oxfordian-Tithonian) belemnites and inoceramid bivalves from two Deep Sea Drilling Project (DSDP) sites located on the Falkland Plateau. Mean belemnite delta18O values, derived from well preserved skeletal material, were -1.29? from DSDP site 330 and -1.45? from DSDP site 511. Assuming a seawater SMOW value of -1.0?, mean palaeotemperatures calculated from the oxygen isotopic composition are 17.2°C and 17.9°C, respectively. The inoceramid bivalves yielded much lighter delta18O values (mean -3.58?). Petrographic and geochemical evidence points to the inoceramid bivalves being altered by diagenesis which accordingly accounts for the observed differences in isotopic values. "Vital effects" or the importation of belemnites or inocerarnids from another area, are considered not to account for the observed isotopic trends. The palaeotemperatures interpreted from the belemnites are significantly warmer than other recent estimates of Late Jurassic temperature (from oxygen isotope studies and climate model predictions) from similar southern palaeolatitudes. We suspect our apparent warmer temperatures are because of a combination of increased freshwater runoff depleting surface waters with respect to delta18O and related to the semi-enclosed nature of the depositional basin retaining warmth, relative to the open ocean of similar latitudes.