For intervals of global warmth, poleward heat transport by warm, saline water masses may be a critical variable in the global climate equation and is widely discussed as a potential mechanism to reconcile differences between computer models and empirical data. However, ocean structure during greenhouse times is poorly constrained, especially on a global scale. The distribution of fossil organisms provides one way to map deep ocean conditions. Inoceramid bivalves reached the acme of their 200 million year range during the Late Cretaceous and then virtually disappeared ∽ 67 million years ago in an event associated with the deterioration of the Cretaceous greenhouse climate (e.g., MacLeod et al., 1996). In 1448 samples (73 sites) representing bathyal paleodepths across the last ∽ 45 million years of the Cretaceous, inoceramid abundance is remarkably constant in time (extinction interval excluded) but not in space. In general, inoceramids were common to abundant throughout the Atlantic and Indian oceans but relatively rare in the Pacific. Geochemical studies of two Indian Ocean sites revealed changes in the δO values of benthic and deep dwelling planktic foraminifers suggesting that bottom waters became cooler and less saline at the time of the inoceramid extinction (MacLeod and Huber, 1996a); new data from a peri-Tethyan site in the western North Atlantic suggest that surface waters became warmer and/or less saline at approximately the same time.
The first-order climate history of the Phanerozoic eon is now known, and it reveals that globally warm intervals without major icecaps represent approximately 75% of the last 540 million years (Frakes, 1979; Fischer, 1982; Crowley, Chapter 14). Study of these greenhouse climates is taking on increased importance in light of climatic warming predicted to result from the doubling of pre-industrial CO2 concentrations over the next century (Broecker, 1997). In spite of the prevalence of warm climates in earth history and the potential practical significance of understanding them, the fundamental causes, nature, and mechanics of warm climates are still poorly understood. We have assembled this book as a way of reviewing what is known about the causes and consequences of globally warm climates, of demonstrating current directions of research on warm climates, and of outlining the central problems that remain unresolved. In serving these goals the chapters present new research on a number of different warm climate intervals from the early Paleozoic to the early Cenozoic. The chapters also integrate a range of approaches, from paleoclimate simulation with atmospheric and oceanic general circulation models, to paleoclimate reconstruction from paleontological and geochemical data, to refinement of paleogeography, to the study of the effects of climate change on marine and terrestrial organisms.
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