Who Shot the Sheriff?

Mass extinctions are among the most intriguing events in earth history and their cause a matter of heated and passionate debate. An article in EOS by Keller et al. reports on a recent workshop at the Natural History Museum in London.

Since the seminal paper by Alvarez et al. on the nature of the K-T boundary it had always been the argument mainly between the proponents of the impact hypothesis vs. the flood volcanism hypothesis. And both sides had good arguments to support their cause. At this recent workshop 150 scientists from different disciplines got together to discuss the cause of Phaenerozoic mass-extinctions in a multi-disciplinary approach.

It will be interesting to read the proceedings from this workshop. In preparation, Keller et al. have published an extensive summary of their view on the subject in Geoscientist.

We are still looking for the smoking gun.

Coral reefs in a thermal crisis

Coral reefs were among the systems hardest hit by the end-Triassic events. Volcanism associated with the breakup of Pangaea released enormous amounts of CO2 into the atmosphere. The increased greenhouse effect and the acidification of the oceans caused a widespread extinction. The large and highly diversified reefs of the Triassic disappeared. One of the few survivors was a reef situated in what today is the south of France.

Wolfgang Kiessling of the Museum für Naturkunde Berlin (Berlin Museum of Natural History) and co-workers went at answering the question why this particular reef survived the extinction event and what lesson can be learnt for estimating the effects of the current environmental change through elevated atmospheric CO2 levels. The surviving European reefs later formed the nucleus for the re-radiation of corals. The recovery, however, took a very long time: 15 million years.

In their work, published in the latest issue of Palaios, they come to the conclusion that the reef was situated in deep shelf waters at high latitudes in a region that had been a hotspot of coral reefs already through the Triassic. Their findings also highlight the importance of deep shelf regions for the survival of marine species. It is not always necessarily remote ocean islands.

Kiessling W., Roniewicz E., Villier L., Léonide P., Struck U. 2009. An early Hettangian coral reef in southern France: Implications for the end-Triassic reef crisis. Palaios, 24: 657-671. doi:10.2110/palo.2009.p09-030r

Acid rain responsible for mass extinction at T/J boundary

Today I read an advance online publication of van de Schootbrugge et al. in Nature Geoscience (doi:10.1038/ngeo577) on their interpretation of terrestrial mass extinction at the Triassic/Jurassic boundary. The role of wide spread volcanism at this time has been widedly acknowledged and its effect mainly attributed to CO2 emissions and consequent global warming. While this model explains the marine extinctions quite well, it does not fully explain the observed terrestrial extinction.

Van de Schoortbrugge et al. argue that their field observations suggest widespread acid rain and low sunlight caused by volcanic emmissions. They also point out the role of polycyclic aromatic hydrocarbons, which are toxic and are produced by heating organic compounds, e.g. coal seams.

Their findings strengthen the case that tectonics and resulting volcanism are major forcing mechanisms that trigger global mass extinctions. The parallels to the Permo-Triassic boundary are quite striking where some researchers suggest that the extrusion of the Siberian trap basalts throgh a major coal field would have also shown widespread ecological effects.