2002-2003 Colloquium Series
NAU Physical Sciences (Bldg 19, Rm 321), Friday, 7 March 2003, 4:00 PM
(Refreshments at 3:45 PM)
ASTROBIOLOGY GOLLOQUIUM
Late Triassic Impacts and Extinctions: Sudden Death or Slow Agony?
Lawrence H. Tanner , Bloomsburg University
Abstract
The loss of species at the end-Triassic now is referred to routinely as one of the "big five" mass extinctions of the Phanerozoic, implying a level of suddenness and severity that distinguishes it in the stratigraphic record. However, analysis of Late Triassic extinctions suggests there are several patterns in diversity change through the latest Triassic. Stepwise, or episodic extinction is well documented in major marine groups, such as ammonoids, bivalves, and conodonts. For some groups, such as tetrapod vertebrates, an incomplete record makes an extinction pattern impossible to determine. The paleontological record, therefore, does not support the interpretation of a single, catastrophic end-Triassic extinction. Rather, many major biotic groups, but not all, suffered significant declines in diversity through the Late Triassic, possibly with episodes of extinction scattered among the Carnian-Norian boundary, during the early Norian, at the Norian-Rhaetian boundary, and spread throughout the Rhaetian, as well as at the system boundary.
The underlying causes for this biotic decline may involve some
combination of both catastrophic and gradualistic mechanisms of
environmental change. Examples of the former include bolide impact,
outgassing during prolonged volcanism, and sudden release of frozen
methane hydrates from the sea floor. Gradualistic mechanisms proposed
to explain biotic change include climate change, specifically
widespread aridification, and sea-level change, which may result in
habitat reduction (from regression) or anoxia (from transgression).
Candidate structures for Upper Triassic bolide impacts include the
100-km diameter Manicouagan structure in northeastern Canada, dated
at 214 +/- 1 Ma, the 80-km Puchezh-Katunki structure in Russia,
tentatively dated as 220 +/- 10 Ma, the 40-km-diameter Saint Martin
structure in northwestern Canada, dated at 208 + 14 Ma, the
25-km-diameter Rochechouart structure in France, dated at 214 +/- 8 Ma,
the 15-km Obolon structure in the Ukraine, dated as 215 +/- 25 Ma, and
the 9-km Red Wing structure in North Dakota, USA, dated at 200 + 25
Ma. The sizes and ages of most of the structures are known only
approximately because they are either buried, eroded, or lack melt
samples suitable for dating.
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