Entrevista a Mike Benton sobre la extinción masiva del límite Permotriásico.
Realizada en Jaén durante el Ciclo de Conferencias Cambios Globales en la Historia de La Tierra, en el que dio la conferencia “The End Permian mass Extinction in the Red Beds of Russia”.
(Matías Reolid) For the end-Permian event, the killing models are controversial. Was the agent the impact of a huge meteorite (Bedout Crater and Winkes Land Crater) or prolonged volcanic eruptions?
(Mike Benton) The best evidence at present is that the end-Permian mass extinction was
triggered by massive volcanic eruptions in Russia - the Siberian traps
represent huge amounts of basalt lava that erupted at just this time. Some
geologists have argued for impact, but the evidence about the Bedout crater
and impact effects are generally seen as quite weak.
(Matías Reolid) What were the phases of the "cycle of death" triggered by the Siberian traps eruptions?
(Mike Benton) The model suggests three effects:
(1) First, the eruptions produced a great deal of carbon dioxide, which is a greenhouse gas, so there was some global warming. This may have warmed the oceans a little, and this released some frozen gas hydrate deposits from the deep oceans. These consist of
decayed plankton, and store methane. Methane is a very potent greenhouse
gas, and as it bubbled up through the ocean, warming increased even more.
Evidence for warming: oxygen isotopes show a 5-7 degree C rise in
temperature at the Permo-Triassic boundary.
(2) A key effect of global warming was ocean stagnation. Surface waters of
the sea were warmed and this slowed the normal circulation of deep cold
water which wells up, picks up oxygen and then goes down to the ocean
floor. If the cycle stops, oxygen is not fed to the sea bed, and anoxia
follows. Evidence for anoxia: black sediments with pyrite in earliest
(3) The volcanic gases (CO2, SO2, NO) mix with atmospheric water to make
acid and so acid rain is a final consequence. Acid rain killed the trees,
and they died, releasing soils which got washed into the oceans. Evidence:
major changes in paleosols, the 'coal gap' of 15 Myr in Early and Middle
Triassic; wash-off of sediment into the sea.
(Matías Reolid) Are comparable the effects of Siberian Traps eruption during end-Permian and the Deccan Traps eruption during the K/T boundary?
(Mike Benton) The Deccan Traps eruptions at the KT boundary must have had many similar effects, even though they were much smaller in scale. Integrating effects
of Deccan eruptions and the Chicxulub impact is still not clear.
(Matías Reolid) Where was more intense the end-Permian mass extinction, in marine or in terrestrial environments?
(Mike Benton) As far as we can see, the loss of life was on a similar scale - maybe 50%
of families and 80%+ of genera and species - it seemed to happen equally
and at the same time on land and in the sea.
(Matías Reolid) What were the features of the survivors of this cataclysm? Is it a
good example for comparing the Red Queen model and the Court Jester model?
(Mike Benton) Survivors were a random selection at first (the killing was probably so
severe that there was not much selectivity), but in the grim initial
post-extinction times, certain species were probably better at surviving
than others - not large, general diet, didn't need much food, wide
environmental tolerance. It seems that bad conditions (warming, anoxia)
continued for 5 Myr all through the Early Triassic, so ecosystems did not
really recover until the Middle Triassic.
I guess this is a pure CJ (not RQ) model where everything is physical
environment, and normal natural selection initially played little role.
(Matías Reolid) How was the recolonisation of terrestrial environments? What are the disaster taxa?
(Mike Benton) _Lystrosaurus_ is the most famous disaster taxon, and it lived worldwide
for 1-2Myr in earliest Triassic, having survived from latest Permian. In
Russia, we find mainly just amphibians, and more teerestrial forms appear
only at the end of the Early Triassic, 3-5 Myr after the extinction. Then
terrestrial ecosystems, with dinosaur ancestors, became more established
about 20 Myr into the Triassic at end of Ladinian. early Carnian, with big
herbivores (1-tonne dicynodonts) and big carnivores (5m-long rauisuchians).
(Matías Reolid) How was the recolonisation in marine environments? Are there any differences between boreal and tropical realms?
(Mike Benton) There is evidence for rapid recovery of ammonoids, but many other marine groups were slower to recover. Most notable was the 'reef gap', with no
corals and no reefs for 15 Myr. So, marine groups came back slowly, and top
levels (carnivorous fishes and reptiles) emerged only 10-12 Myr after the
PT boundary, in the mid to late Anisian (e.g. German Muschelkalk, and
Chinese Panxiang/ Luoping biotas).
MJ Benton, Dept. of Earth Sciences, University of Bristol, Bristol, BS8
FAX -44-117-925 3385
Messages -44-117-954 5400