New Doubts Raised Over What Killed Off Dinosaurs

Deccan Volcanism: Cretaceous-Tertiary Transition Scenario
McLean, D. M., 1982, abstract in Russell, D. A., and Rice, G., eds., K-Tec II Cretaceous-Tertiary Extinctions and Possible Terrestrial and Extraterrestrial Causes: Syllogeus Series 39, National Museums of Canada (Proceedings, May 1981 workshop), p. 143-144.

Deccan Volcanism and the Cretaceous-Tertiary Transition Scenario: A Unifying Causal Mechanism

Dewey M. McLean

The Cretaceous-Tertiary (K-T ) transition Deccan flood basalt volcanism, one of the greatest outpourings of lavas from the Earth in geological history, was synchronous with, and accounts for via fluctuation of the carbon cycle, the K-T transition: (1) marine CaCO3 dissolution and "clay" events, and the apparently linked extinctions of calcareous microplankton, (2) drops in d13C and d18O values, and (3) via "greenhouse" conditions, the extinctions of the dinosaurs by heat-infertility linkage.

Theorized to represent mantle plume activity originating near Earth's core-mantle boundary, this volcanism provides a mechanism for conveying iridium and osmium from the core (where they presumably exist in cosmical proportions) to be distributed via volcanic exhalations. Carbon gases are among the most abundant volcanic emanations, and possibly reflect degassing of Earth's interior of material trapped during Earth's accretion.

Deep origin volcanism spanning the 5 million years of the Deccan volcanism (65 to 60 Myr ago) would have brought vast amounts of carbon gases to Earth's surface. Volcanic CO2 and reduced carbon in igneous rocks have low d13C values (-7‰ and -19 to 28‰, respectively), accounting for the drop in d13C values across the K-T contact.

Build up of atmospheric CO2 would result in "greenhouse" conditions and the drop in d18O values (a warming signal) across the K-T contact. Acidic volcanic gases injected into the oceans would cause CaCO3 dissolution; the holding of CaCO3 components in solution would allow accumulation of a concentrated clay layer (e.g. the Danish fish clay) on the Cretaceous erosional (corrosional) surface.

Lowered marine pH would also trigger calcareous microplankton extinctions; they were most severe in the Tethyan region in proximity to the Deccan volcanism. "Greenhouse" conditions would have affected reptilian reproduction according to size. Larger organisms have relatively small surface-volume ratios; during warming, organisms above a critical size (10 kilograms) would have retained excessive body heat, causing degeneration of internal germinal tissues in males and, in females, disruption of calcium metabolism and hormonal systems.

The abrupt terminations of geological ranges of many taxa at the K-T contact seemingly reflect hiatus control of ranges associated with the shallow marine CaCO3 dissolution event, and terrestrial hiatuses and facies control associated with the terminal Cretaceous regression of epeiric seas off the continents, that have created the illusions of simultaneous global range terminations.

In all likelihood, the K-T extinctions spanned hundreds of thousands, if not millions, of years.

The asteroid impact was just the finishing touch. A number of environmental factors (including Deccan Vulcanism), were stressing the biosphere at that time. Many species had become extinct prior to the impact, but the impact cleaned up the last remaining lines, and cleared the way for a new biosphere with a new climate, full of ice ages and mammals.

See the link below for a general summary:

http://www-geol.unine.ch/MSA/causes.html

a number of geologists have been quietly working on actual extinction curves and , since their work doesnt show much "sex appeal' in print, they get ignored. the dinosaurs were going extinct long before a terminal event , Chixclub occured. The data supporting chixclubs date has been pretty solid, and involves stratigraphy of tsunami debris , terminal strata dates, and lots of geochron data. Ill be skeptical on the 'drillhole data" until its vetted by careful study and comparitive information.. The Mclean paper has been discounted in the last 20 years since it was published because we now know that the various stable isotopes can be secondarily enriched in post diagenesis activity.

the deccans have always been a reasonable explanation before Alvarez came up with the "crater of doom' hypothesis. however the Chixclub event did as rosborne said, have a "last bats' effect

What happened to the hypothesis that the Deccan Traps are a product of the Chicxulub impact? I seem to recall some thinking that they might have been the result of crustal ruptures due to impact induced stress.

Sheth has written extensively on the triple junction of the three flood basalts that cap the Indian himalyan provinces. his stuff, which is a series of good compilations of individusal works by others, makes a good argument that the rifting of the indian subcontinent from its breaking from africa /madagascar/Seyschelles accounts for the flood basalts, and not a "seismic focus" of transverse waves resulting from the chixclub. i know it was a neat explanation that sewed up everything. However there were 3 flood basalt episodes starting at about 100my, then at 80My then 60-65 my. I found some stuff about Sheth but no journals with web sites

http://www.geos.iitb.ac.in/hcs_publications.html

looking around some more, i found an article, a WEB article. (Remember these kinds of articles arent peer reviewed)

http://www.mantleplumes.org/Deccan.html

BBB
Thanks everyone, this is fascinating comming from folks who know of what they speak.

BBB

I understand most of this except the d13c and d180. Please explain. Neil

it should read C13 and O18. these are just stable isotope distributions compared to the standard isotopes of c12 and O16. Its part of a distribution differential equation . Hotter temps favor the higher isotopes. in the environment, which then gets picked up in the Carbonates and Oxides 9and seawater and ice) In normal atmospheres and seas the lightest isotope is most abundant but as temp increases, the heavire ones become more abundant so the ratios favor the O18 and C13. there was an original study in which a swiss geochemist (1960s) compiled O isotope data in the aLPS. his isotopic abundance was heavily dependent on temp. itsjust another tool,