A CLIMATE CAROL: ANCIENT GREENHOUSE MASS EXTINCTIONS AND IMPLICATIONS FOR A FUTURE GREENHOUSE WORLD

In the next 100 years it is projected that the Earth will move from an icehouse to a greenhouse climate state. Numerous environmental changes will include increases in ocean acidification and extent of the oxygen minimum zone. With apologies to Dickens, in order to respond to these potentially dramatic environmental changes, we need to go to the past to understand the range of outcomes that have occurred during similar periods of environmental change. The most extreme example in Earth history is the end-Permian mass extinction and the ensuing Early Triassic, which experienced significantly increased warming due to the eruption of the Siberian Traps, with intrusion and eruption through coal and other organically-enriched deposits that enhanced the volcanic contribution of greenhouse gases to the atmosphere, particularly carbon dioxide. The ocean then was characterized by widespread anoxia, which impinged from deep into shallow environments, and typically was euxinic. The effects of this mass extinction on land led to a significant increase of terrigenous runoff to shallow-marine systems. The prevalence of microbial structures implies an ocean more dominated by microbes than is typical of the Phanerozoic. These stressful environmental conditions, including acidification, also led to an Early Triassic metazoan reef gap followed by a reef eclipse interval. Level bottom benthic environments were characterized by low biodiversity, cosmopolitan taxa, and reduced depth and extent of bioturbation. A similar environmental trigger led to the end-Triassic mass extinction, which was caused by an increase in environmental stress due to development of the Central Atlantic Magmatic Province (CAMP), and is the most severe mass extinction experienced by the Modern Fauna. This mass extinction probably included episodes of ocean acidification and includes a short coral gap and a longer metazoan reef eclipse in the Early Jurassic. Ecosystems were commonly dominated by disaster taxa but there is relatively little evidence for increased microbial presence. Just as study of the end-Cretaceous impact mass extinction has led to increased awareness of the possibility of modern asteroid impacts, early Mesozoic greenhouse mass extinction intervals have much to teach us about the future greenhouse world.