Paleoceanographic events—Recognition, resolution, and reconsideration

Studies of the last 125 million years of oceanographic and climatic history have benefited greatly from the impetus provided by the Deep Sea Drilling Project. Knowledge of the sedimentary and paleontologic record of the major ocean basins, in conjunction with study of pelagic marine sections exposed on land, has permitted both the testing of old and the development of new hypotheses to explain local and global ocean chemical, sedimentologic and biotic events. Some of the more striking and topical problems in paleoceanography are the oceanic “anoxic events” of early to middle Cretaceous age, the biotic crisis at the Cretaceous/Tertiary boundary, the Eocene/Oligocene extinctions and climatic and circulation events, the Messinian “salinity crisis” (late Miocene) and its effects on the world ocean, and Pleistocene glacial cycles and paleoceanography. Possible explanations of these events, which have been proposed over the last five years, are reviewed in this paper. Application of new concepts and techniques, especially in micropaleontology and geochemistry, has led to refinements in stratigraphic resolution and in recognition of paleoenvironmental signals. Among the most powerful tools now in use is stable isotope geochemistry. Paleomagnetic studies and statistical techniques for processing micropaleontological data have contributed greatly to stratigraphic resolution. Although we are beginning to unravel the complex interactions of global sea level changes, climate and ocean chemistry and their influence on life, there are innumerable challenging problems still remaining. As stratigraphic resolution and coverage improve so does our perception of the ocean system.