Wednesday, November 14, 2007

Deccan Volcanism and Mass Extinctions

Science and the scientific method is about eternal vigilance, the need to question even well "proven" theory, to prevent theory from turning into dogma, to make sure that "mainstream thinking" does not fossilize into inertia and a reluctance to question "established facts". This post is about how one such well established theory may turn out to be wrong.

During the Paleozoic period the earth has seen five mass extinctions, great dyings, where a significant proportions of the earth's biota has gone extinct within a short period of time. These five extinctions took place in the Ordovician-Silurian, Late Devonian, Permian-Triassic, Triassic-Jurassic, and Cretaceous-Tertiary. Of these, the Permian-Triassic was the mother of all extinctions in which about 95% of species perished. The most famous however is the Cretaceous-Tertiary event in which about 50% of all species including the non-avian dinosaurs went extinct. The reason for a high public profile has been the dramatic explanation for this extinction and also some very famous victims. In the early 1980's physicist Luis Alvarez, his son geologist Walter Alvarez, and chemists Frank Asaro and Helen Michels discovered a high concentration of iridium in rocks at the Cretaceous-Tertiary boundary (K-T). Since iridium is rare in the earth's crust but is common in asteroids, the scientists suggested that the earth was hit by a large asteroid around 65 million years ago, causing environmental stresses and the late Cretaceous extinction. In 1991 in the Yucatan peninsula in Mexico, a large impact crater named the Chicxulub Crater was discovered. This was dated to around 65 million years. The cause was assumed to be found and it became accepted wisdom that the Cretaceous dying was caused by the Chicxulub impact event. The theory quickly took hold over public imagination. The dinosaur story was retold through countless documentaries, popular articles and books, one of the most famous being T rex and the crater of doom by Walter Alvarez. There was just something satisfying about these fearsome creatures who dominated terrestrial ecosystems for over 150 million years being wiped out suddenly by a weapon of mass destruction from outer space.

But nature is never that simple. The facts as always are messy and the Cretaceous extinction story has gotten messier. Mounting evidence reported in various media outlets in the past few months to week here, here and here suggest that the late Cretaceous extinction was not caused by the Chicxulub impact event but had multiple causes including the gigantic Deccan volcanic eruption at the end of the Cretaceous. I live in the Deccan volcanic province and so this a exciting news to me. All those basalt layers that I see everyday may have played a role in changing earth history. The champion of this hypothesis in palaeontologist Gerta Keller who over the last few years has assembled some impressive evidence in support of her theory. First is the debunking of the conventional wisdom that the Chicxulub impact event caused the mass extinction. Keller and her research team drilled and recovered cores from the impact site in Mexico and from another site along Brazos river in Texas, some 1700 km from the impact site. Their detailed analysis of the sedimentological, geochemical and palaeontological aspects of the cores revealed that a) the impact took place almost 300,000 years before the K-T mass extinction b) the impact is recorded by characteristic sedimentary rocks known as breccia (rock containing pulverized and fragmented pieces) and as layers rich in glass spherules which form by the heat and shock of the impact. An analysis of the biota in the sediment below and above this impact layer shows no evidence of accelerated extinction or stressed ecosystems. It appears that the Chicxulub event had little impact on the earth's biota let alone trigger a mass extinction.

Second, Keller has now managed to come up with evidence tying the Deccan volcanics to this mass extinction and this evidence comes from Rajamundhry, Andhra Pradesh, India. A small digression at this point. Travelling to Rajamundhry is a fantastic experience because of the wide bridge across the Godavari River as you approach Rajamundhry from Hyderabad. I crossed it by train at sunset, and the Godavari is an awesome sight. Image below shows the railway bridge.

Back to volcanics. There always have been suggestions that the Deccan volcanics played a role in the late Cretaceous mass extinctions, but due to large uncertainties in establishing the time span of the main eruptive phase it was difficult to establish a link between the volcanism, environmental stresses and extinction. Recent dating of the Deccan basalts indicate that 80% of the volcanism took place in what is known as C29R magnetic polarity zone, which spans the K-T boundary and the mass extinction event (R means that the earth's magnetic field had reversed polarity at that time).

Figure to the left shows the magnetic stratigraphy of the Deccan basalts. Several thousand feet of basalt were erupted in C29R confirming that the volume of basalt erupting at the very end of the Cretaceous over a short period of time must have been enormous, resulting in the release of carbon dioxide and sulphur dioxide in amounts large enough to alter the earth's climate. In Rajamundhry Keller and her team found layers of sediment sandwiched between two Deccan basalt flows. The lower flow was found to have erupted in C29R magnetic polarity, probably coinciding with the K-T event. The sediment above this flow provides a more direct evidence for this connection between the timing of basalt eruption and extinction. They contain early Tertiary post-extinction foraminifera. This biota marks the initial evolution of the foraminifera after the K-T event indicating that the mass extinction must have taken place during the eruption of the main phase of the Deccan volcanics. In detail, the sediments contained foraminifera fossils of early Danian age, which showed that these sediments were deposited in the earliest Tertiary period, and thus were a window into conditions just after the mass extinction and offered to provide a link between volcanism and environmental stresses. Unlike the sequence of fossils spanning the Chicxulub impact layer in Mexico and Texas, the fossils here at Rajamundhry show evidence of stressed ecosystems. The early post-extinction Danian foramainfera are less diverse, tiny, without any ornamentation and show deformities unlike pre-extinction late Cretaceous (Maastrician) foraminifera which are diverse, larger and ornate. This compares well with foraminifera of Danian age elsewhere, suggesting that the conditions recorded in the intertrappean sediments at Rajamundhry are representative of global post-extinction stress. The biota in these sediments suggest that post-extinction recovery of ecosystems was slow. Volcanism continued into the early Tertiary as indicated by the lava flow overlying these sediments. This flow falls in C29N (N means normal polarity) magnetic polarity and occurred about 280,000 years after the K-T event. This continued volcanism Keller suggests may be the reason why ecosystems took so long to recover from the mass extinction. Another small digression. In the figure above of the magnetic stratigraphy, a large part of C29R is made up of the Wai and Lonavala subgroups and the bulk of the C29N by the Mahabaleshwar formation. These are names which must be familiar to readers from Maharashtra. The basalts comprising these subgroups make up the precipitous western ghat escarpment. So, the next time you are standing at Arthur's Seat in Mahabaleshwar (image below), contemplate on the possibility that you are looking at the volcanic pile which altered the course of evolution.

So this is where things stand today. Did I cover everything? Readers may notice a nagging discrepancy. If the Chicxulub impact occured 300,000 years before the K-T boundary, then what caused the irridium anomaly at the boundary? It is no doubt of extraterrestrial origin and so there must have been another asteroid impact that coincided with the peak basalt volcanism. The impact crater has not been found, but expect the story to get even more complicated.

And yes, the Indian media screwed up on this one too. The Times of India (Nov. 8 2007) copied and pasted an article by John Noble Wilford from the New York Times. The article was about some fossil discoveries from post-extinction sediments in New Jersey which showed that some species of Cretaceous ammonoids survived the mass extinction for at least several hundred years. But the connection between the Cretaceous extinction and dinosaurs has become so ingrained that The Times of India in a bombastic subtitle ignored the evidence presented in the article and announced that it was the dinosaurs who lived hundreds of years after the extinction event , for which there is no evidence..... yet.

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