[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: Dinosaur Extinction: Impact vs Volcanoes
Frank Stroik schrieb:
> I have been asked to provide references by a few of you on this
> topic. Here is a reference that shows the biologic debate of K/T
> boundary. It's a good summary, and provides further references to
> pursue.
> David Jablonski, "Progress at the K-T boundary" Nature< 22 May, 1997
> Vol 387 pp. 354-355.
Hello Frank, hello William, hello List!
I unearthed another interesting and controversial K/T abstract in
Meteoritics 25-4, 1990, A412-413:
M e t e o r i t i c versus v o l c a n i c events at the Cretaceous /
Tertiary boundary - An Australian perspective. F.L. Sutherland. Division
of Earth Sciences, The Australian Museum, 6-8 College Street, Sydney,
NSW, 2000, Australia.
A review of the end Cretaceous extinction debate (Sutherland, 1988)
concluded that the cause 'was a coincidence of both impact and volcanic
cycles coming together, but not necessarily the first directly causing
the other.' New literature expands such views on dual, but uncoupled
causes.
Arguments for cosmic body impact are reinforced by:
(1) organic molecular results on soot from global fires
(2) extra-terrestrial amino acids at the boundary
(3) ages of 65-67 Ma for Manson Crater, USA, a suitable source for
shocked quartz
(4) iridium anomalies at older impacts, eg., Lake Acraman, South
Australia.
Arguments for volcanic fallout are supported by:
(1) Ir enrichment in volcanic dust bands in Antarctic blue ice
(2) multiple Ir anomalies across the Bavarian boundary
(3) similar smectite clays at the Danish Ir anomaly and at higher levels
which lack Ir
(4) basaltic 'feldspars' at that boundary
(5) stronger confirmation for rapid Deccan eruptions at the boundary
(6) sharp Ir anomalies related to volcanism at other American extinction
events
(7) differences in platinum group element patterns between northern and
southern hemispheres, weakening an exclusive global impact.
Sufficient eruptives to give observed Ir levels at the boundary are
critical for volcanic arguments. Deccan basalts give a large hot spot
source, particularly as early eruptives seem more mafic. Another is
proposed for NE Australia. The 65 Ma Coral Sea spreading rift of
1500-2000 km diameter approaches plume sizes for flood basalts. Its link
to east Australian hot spots was criticised on absolute motion
modelling, but several southern hemisphere hot spot tracks show similar
discrepancies. The great coincidence in size and shape of the structure
to hot spot distribution is preferred for correlation.
Deccan, Coral Sea, Cameroon Line and possibly Louisville hot spots
provide major southern 65 Ma volcanism to give a 'basaltic' geochemical
pattern in the New Zealand boundary layer. Greenland (Kap Washington
Voleanics) and Hawaii probably contribute to 65 Ma northern volcanism.
Global correlations in hot spot distributions and lower mantle features
suggest plume sources enriched in Ir. Thus, 65 Ma volcanism seems
adequate for boundary Ir anomalies, allowing for secondary enhancements.
Northern hemisphere impact for some boundary contribution seems
sustained, but only a few smallish potential craters are recognised. The
multiple Ir anomalies require spaced impacts. Suggestions that large
hot spots are triggered by impacts require multiple strikes in the
southern hemisphere. A claim for a shock origin for lamellar quartz
below Deccan basalts is generally considered unproven. Coincidental, but
independent, northern impacts and largely southern hot spot outbursts
seem to fit present data. Recent ideas stress the improbable in Earth's
evolution. Simultaneous smaller cratering impacts, larger hot spot
inducing impacts and massive volcanism, however, may represent an
overkill for the extinction boundary.
Whatever the causes, the biologic record for Cretaceous extinctions
seems to be stepped, with decimations increasing amongst terrestrial
animals, land plants and marine organisms respectively. Dinosaur
extinction may stagger beyond the boundary event. A demise due to
decrease in atmospheric oxygen is not confirmed by further studies of
air bubbles in Mesozoic and Tertiary ambers.
Reference: Sutherland F.L. (1988) J. Proc. Roy. Soc. (NSW 121, 123-164).
Best wishes,
Bernd
----------
Archives located at:
http://www.meteoritecentral.com/list_best.html
For help, FAQ's and sub. info. visit:
http://www.meteoritecentral.com/mailing_list.html
----------
Follow-Ups:
References: