probable. Development of an integrated real-time scale for the Cenomanian-Turonian extinction interval, blending new 40Ar-39Ar ages from volcanic ashes (bentonites), with 100,000- and 41,000-yr Milankovitch climate cycle deposits across the boundary, allows a precise timetable for environmental perturbation and C-T mass extinction to be developed at a resolution comparable to Quaternary studies of global change. The Cenomanian-Turonian mass extinction may thus serve as a model for the rates, patterns, causes, and consequences of a global biodiversity crisis, leading to mass extinction, in a greenhouse world.
The modern Earth is undergoing a geologically "instantaneous" transformation, or global change, characterized by increasing concentrations of atmospheric greenhouse gases, ozone depletion, global warming, environmental deterioration, habitat destruction, and the mass extinction of species, resulting in a global biodiversity crisis (Wilson, 1988). These extraordinarily rapid global perturbations are related largely to the overpopulation of a single species, Homo sapiens, whose numbers may already exceed the resource-carrying capacity of the Earth. Of the estimated 30 million species of plants and animals (Wilson, 1988) that probably existed on Earth prior to man's recent population explosion, more than half live within complex, easily perturbed, tropical ecosystems (e.g., rain forests and reefs) that are currently threatened by human activity. The predicted destruction of these ecosystems may result in the loss of more than half of global biodiversity. We have thus entered an early phase of a global mass extinction, but at a rate that exceeds that for nearly all well-documented ancient mass extinction events. It is imperative that we develop integrated physical, chemical, and biological data that will help us understand the processes and consequences of global change and biodiversity decline, not just from the familiar but geologically atypical icehouse world of the Quaternary, but also from past greenhouse worlds lacking permanent polar ice and cold climates. Greenhouse worlds are characterized by higher sea-level; warmer, more equable, maritime-influenced climates; expanded tropics; and a largely stenotopic global biota delicately perched on the verge of extinction. This is a world that we may soon be entering through accelerated global warming.
The focus of global change research on Quaternary history is built on three main premises: (1) there is an urgent need to understand the natural evolution of Quaternary Earth systems as a baseline for assessing the staggering impact of the human species on global environments and ecosystems during the past 9000 to 15,000 yr, but especially during the past 3000 yr (the agrarian and industrial "revolutions"); (2) the physical, chemical, and biological processes characterizing Quaternary Earth history can largely be observed, interpreted, and modeled from modern observations—the Uniformitarian approach; and (3) the preservation and resolution of Quaternary physical, chemical, and biological data relevant to understanding the dynamics of global change are unparalleled in the geologic record. These are justifiable approaches to an urgent problem. Yet, when considering the current rate of global warming associated with modern environmental changes and the expanding biodiversity crisis, it is necessary to refocus some of our global change research to understanding the greenhouse intervals that characterized so much of Earth history and to study in detail not only the processes, but also the ecological and genetic consequences, of global mass extinction. Research focused on ancient global change characteristically depends on achieving a resolution among physical, geochemical, and paleobiological data that is adequate for interpretation and predictive modeling of modern global change phenomena.
In searching for a geological test case for the study of ancient global change with relevance to the modern Earth, the Cretaceous Period emerges as one of the best candidates. In particular, the middle Cretaceous presents a unique opportunity to document and model dynamic changes in ocean-climate systems associated with a global mass extinction (Cenomanian-Turonian boundary bioevent) in a greenhouse world, and then to compare these with the environmental and ecological crisis on the modern Earth as it potentially moves from an icehouse to a greenhouse state. This chapter demonstrates that (1) resolution of middle Cretaceous physical, chemical, and biological data are sufficient to make comparisons with major trends in Quaternary ocean-climate systems, and biodiversity decline; (2) systems of chronology and regional correlation of environmental and biological changes in the Cretaceous are comparable to those used in Quaternary global change studies; (3) significant similarity in patterns of environmental change and biological response exists between Cretaceous and Quaternary extinction intervals to allow construction of predictive models from Cretaceous observations that are applicable to our understanding of the potential long-term consequences of the modern biodiversity crisis.