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Re: extinction

Dear Colleagues,

The key to the K-T--and dinosaurian--extinctions is stratigraphy. For us to ever understand the physicochemical-biological interactions that contributed to the K-T extinctions, we must understand the old strata that contain the key. And, we must understand their critical time-rock relationships, for those are often a trap for the unwary, casting illusions of abrupt extinctions where, in fact, the geobiological record demonstrates that none actually occurred. There was no vast, general, catastrophic extinction of life at the K-T boundary.

My last two posts cited Peter Vail, the father of sequence stratigraphy, whose works have allowed us to better understand the history of the K-T transition. Geologists who ignore Peter Vail are like physicists who ignore Albert Einstein, or biologists who would dismiss Charles Darwin, or geneticists who would flip off Watson and Crick.

As pertains to the topic of single, or multiple, iridium enrichment in the K-T transition, the paper "Sequence stratigraphic setting of the Cretaceous-Tertiary boundary in central Alabama" that I have cited several times has three iridium spikes. That paper provides the best key to understanding the distribution of iridium in the marine record that I know of. Peter Vail was one of the authors of that paper. Following are some quotations from the paper:

"Eustatic falls and rises produce major changes in basin configuration, stratal patterns, and sedimentation rates, as well in oceanic geochemistry. The major eustatic fall in the latest Maastrichtian, marked by the withdrawal of epicontinental seas throughout the globe [refs], and the subsequent eustatic rise had a tremendous effect on global depositional patterns, as well as on the biotic and geochemical conditions in the world's oceans. Globally, the K-T boundary is marked by a period of marine terrigenous-sediment starvation."

"The faunal K-T boundary occurs with this transgressive systems tract, which is dated from the latest Maastrichtian through the earliest Danian."

"Of the three distinct iridium anomalies at Bragg (Fig. 8), the lowest occurs in the late Maastrichtian, the middle near the K-T boundary, and the upper within faunal zone NP1. These anomalies coincide with marine-flooding surfaces interpreted as parasequence boundaries."

"The presence of iridium at these flooding surfaces suggests that iridium was present in the open ocean from the latest Maastrichtian through earliest Danian and was concentrated on the Alabama paleoshelf during periods of terrigenous-sediment starvation caused by rapid sea-level rises."

"Thus, it appears that iridium was not introduced into the atmosphere during a unique event occurring at the K-T boundary, but was present in the atmosphere for a much longer period of time."

"Whatever the cause for the increase in iridium concentrations in the water column during this period, the decrease in terrigenous sedimentation associated with a global rise in sea level was a fundamental part of the process that concentrated the iridium and other cosmogenic debris in the sediments."

"Although Alvarez and others (1982) have generally discounted the effects of sea-level changes on the deeper marine record across the K-T boundary, we believe this assumption is incorrect."

As time permits, I will discuss in future posts the implications for the so-called postulated "fireball" layer(s), the Rocky Mountain K-T record that some impactors would have you believe reflects direct air fall from an impact, and other interesting controversial topics.

Dewey McLean