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

["Dewey M. McLean" <dmclean@vt.edu>]

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