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Re: extinction (Deccan Traps and the K-T extinctions)



Dear Colleagues,

Here is the latest on timing of the Deccan Traps flood basalt volcanism relative to the gradual Late Cretaceous-Early Tertiary biological turnover.

In 1993, Asish Basu, Paul Renne, Deb DasGupta, Friedrich Teichmann, and Robert Poreda published a paper "Early and Late Alkali Igneous Pulses and a High He3 Plume Origin for the Deccan Flood basalts." Their work showed that the main bulk of the Deccan Traps (90% of the vast lava pile) began erupting 65 million years ago, right at K-T boundary time.

In July 2002, Asish told me, "Clearly the bulk of the Deccan is 65--there is no doubt about that." However, his work identified an earlier pulse of Deccan Traps that preceded K-T boundary time by several million years. He also stated, "But the question is whether it is 90 percent or 70 percent at the K-T boundary."

In a January 2004 communication, Asish noted that I must have seen the paper by Ravizza and Peucker-Ehrenbrink in the Nov 21 issue of Science concerning their Os-isotopic evidence for the early eruption of the Deccan Traps--well before the K/T boundary. He also noted that he is not fully convinced that they are right about the timing of the Os-isotopic shift from the marine sediments, and that he still believes that bulk of Deccan erupted at the K/T, because of the Ar-Ar age distribution and the paleomagnetic evidence.

Now, please compare the timing of the Deccan Traps eruptions with the Conclusions of the MacLeod et al. paper "The Cretaceous-Tertiary Biotic Transition" (1997):

"First, global events at the K-T boundary occurred within a longer period of sustained biotic change. This longer episode affected different groups at different times, but most often manifested itself as a progressive reduction in biotic diversity throughout the Maastrichtian."

"Second, a much shorter global biotic event appears to have taken place close to the K-T boundary. This event is most prominent among a few groups of marine microfossils (e.g. calcareous nannoplankton, planktonic foraminifera), which seem to have remained relatively unaffected by the long-term Masstrichtian decline....The extent to which this short-term, near-boundary event was influenced (or precipitated) by a bolide impact is uncertain. In most microfossil lineages, with the possible exception of calcareous nannoplankton, decline in species numbers begins prior to the occurrence of impact debris in various K-T boundary successions...."

For the first point, early phases of the Deccan Traps were coeval with the progressive reduction in biotic diversity throughout the Maastrichtian.

For the second point, eruptions of the vast bulk of the Deccan Traps lavas 65 million years ago coincided with the short-duration K-T boundary extinctions of calcareous marine plankton.

The Deccan Traps volcanism released prodigious amounts of mantle CO2 onto earth's surface, overwhelming surficial systems. The deep oceans were too warm to serve as a sink for the mantle CO2. Thus, CO2 released by the many pulses of Deccan Traps volcanism could only build up in the atmosphere and mixed layer of the oceans, triggering greenhouse warming and acidification of the upper oceans, and likely the terrestrial environment to some degree.

Eruption of the bulk of the Deccan Traps 65 million years ago coincided with shifts in the C13 and O18 stable isotope records, and "Strangelove" conditions in the oceans (low surface-water productivity, organisms indicative of stressed conditions, and dwarfing) in the mixed layer of the oceans. It also explains the long cutoff of carbonate sedimentation in the oceans, a boundary clay, perturbation of floras, and even accounts for the Gregory Retallack et al. (1987) erosion of carbonates below the K-T record of Montana (I like Gregory's work).

Integrate a K-T boundary impact with the Deccan Traps record, and we have a plausible methodology by which to initiate serious investigation into what really triggered the K-T transition extinctions. Much (most) work remains to be done. If we do it in a spirit of cooperation in which impactors and volcanists work together, we stand to gain much understanding of the processes that influence evolution of, and extinctions in, earth's biosphere.

Cordially,
Dewey McLean