Geoscience Reference
In-Depth Information
The largest of the Big Five extinctions occurred at the boundary between the
Permian and Triassic Periods—the P-Tr—which also marks the end of the Paleo-
zoic Era. It saw the demise of 96 percent of all marine species and 70 percent of
land-based vertebrates, as well as many insect species. Try to imagine how many
individuals must die for 96 percent of marine species to disappear. The number is
truly incomprehensible.
At least a dozen different causes have been proposed for the P-Tr extinction,
alone or in some combination. After the proposal of the Alvarez theory, specu-
lation naturally arose that impact might also explain the P-Tr, but no conclusive
impact markers were found, nor was a putative crater. As in the case of the K-T
extinction, a huge volcanic eruption, in this case, the Siberian traps, occurred at
about the same time as the P-Tr boundary. The most recent age determination of
the boundary is 252.28 ± 0.06 million years, 22 whereas the traps date to 251.2 ±
0.3 million years. 23
The search for a crater of P-Tr age and of large enough size to account for the
extinction failed to produce a suitable candidate. One possibility, the Araguainha
structure in Brazil, was not recognized as an impact crater until Robert Dietz and
Bevan French found that it contained impact breccias and shocked quartz. But at
only forty kilometers in diameter, compared with 180 kilometers for Chicxulub,
Araguainha has appeared to be too small to have caused the greatest extinction.
Moreover, age measurements had dated it to ten million years younger than the
boundary. But recent discoveries are causing some to reconsider. The latest age
measurement dates Araguainha at 254.7 ± 2.5 million years, overlapping both the
P-Tr boundary and the Siberian trap eruptions. 24 A new paper with the intriguing
title “Shaking a Methane Fizz” focuses on the target rock at Araguainha: an oil
shale. 25 The authors calculate that the impact itself and the postimpact collapse of
the initial crater would have triggered thousands of large earthquakes and tsunami.
Liquefaction of the oil shale would have produced the world's greatest fracking
operation, releasing 1.6trillion tons ofmethane, oneofthe most potent greenhouse
gases. The methane in turn might have caused the P-Tr mass extinction.
Scientists have not yet solved the mysteries of the Clovis and P-Tr extinctions,
but they have shown how quickly new evidence can rejuvenate a theory declared
dead. If these two extinctions, one small but near to us in time, the other colossal
and distant, should be caused by impact, it would surely show just how difficult it
is to rule out impact as the cause of mass extinction.
Search WWH ::




Custom Search