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Origin and Evolution of Earth: Research Questions for a Changing Planet
FIGURE 3.15 Number of marine animal genera through time, showing the five major times of “mass depletion” in biological diversity. Only three drops—end-Ordovician, end-Permian, and Cretaceous-Tertiary—are driven primarily by increases in extinction rates, rather than declines in rate of origin. SOURCE: Bambach et al. (2004). Copyright 2004 by the Paleontological Society, Inc. Reproduced with permission.
Problems as diverse as the influence of rainforests on Earth’s hydrological cycle, the role of vegetation in stabilizing the land surface, the relationship between nutrient availability and diversity, and the oceanwide biogeochemical consequences of deep-water anoxia engage a wide range of Earth scientists because they have both deep-time evolutionary components—how did the diversification of woody plants change Earth’s surface?—and topical applications—what will be the consequences for the Earth system of rainforest clear-cutting, increased soil erosion, and seafloor anoxia linked to fertilizer-spiked nutrient flows from agricultural lands to the ocean? Earth scientists have almost limitless opportunities to join with biologists to fashion both a new picture of our planet’s history and a clearer picture of our future.
What Caused Mass Extinctions?
Nothing illustrates how heavily life depends on a favorable surface environment as clearly as a sharp change in that environment—which has occurred several times during the past 500 million years, causing the mass extinction of species (Figure 3.15). In particular, the great extinctions at the end of the Permian (252 Ma) and Cretaceous (65 Ma) periods influenced the course of biological evolution as much as all the accumulated genetic changes during the 187 million years between them. But what specific events or environmental changes precipitated the great mass extinctions, and what aspects of biology influenced the patterns of survival and recovery, are not known. Most Earth scientists agree that a meteorite impact caused the end-Cretaceous extinction of dinosaurs, ammonites, and myriad other plant, animal, and microscopic species (Alvarez, 1997), but the actual kill mechanisms unleashed by this trigger remain poorly understood. The relative importance of coincident environmental perturbations, including an interval of oceanographically driven global change, extensive extrusion of flood basalts, and the particular location of the impact on a tropical continental platform, are simply not known.
Although a single plausible event may account for the end-Cretaceous extinctions, the cause of the end-Permian mass extinction, which may have erased as many as 90 percent of marine species and many terrestrial species (Erwin, 2006), is still debated. Support for an extraterrestrial cause is limited, with growing interest in direct and indirect effects of massive volcanism in what is now Siberia. An emerging view is that massive flood basalts, intruded through thick carbonates and extruded onto thick peat deposits, produced unusually high emissions of carbon dioxide and thermogenic methane, resulting in global warming, acidification of the oceans, depletion of oxygen in ocean waters below the mixed layer, and enhanced production of hydrogen