Re: extinction

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

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.

Cordially,
Dewey McLean