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RE: Where The New Papers Are



And Shiva is conspicuous by its absence from the database of "confirmed"
impacts.


Jourdan, F., Renne, P.R., and Reimold, W.U. 2009. An appraisal of the ages
of terrestrial impact structures. Earth and Planetary Science Letters
286(1-2):1-13. doi: 10.1016/j.epsl.2009.07.009.

ABSTRACT: There are 174 confirmed impact structures known on Earth (e.g.,
http://www.unb.ca/passc/ImpactDatabase/; late 2008) but a far smaller number
of impact structures has yielded a well-constrained age. Precise and
accurate age constraints are crucial for (1) correlating causes and effects
on the bio- and geosphere of catastrophic processes, (2) better constraining
the impactor flux through geological time and evaluation of potential impact
periodicity, (3) calibrating the absolute chronostratigraphic time scale,
(4) calibrating the age of within-crater continental sedimentary deposits
(e.g., for regional paleo-climatic analysis), and (5) correlating impact
events and distal impact ejecta occurrences.
     Of these 174 listed impact structures only a few have precisely
constrained ages (mostly obtained using radio-isotopic techniques, e.g. U/Pb
and 40Ar/39Ar), with only 25 ages having a stated precision better than ±
2%, and a mere 16 ages with a precision better than ± 1%. Yet, even the
accuracy of some of these ages can be challenged and probably improved based
on more detailed interpretations and statistically more rigorous data
analysis. Although geochronologists are often circumspect and advise caution
in accepting calculated ages, these ages tend to propagate into the
literature without further critical evaluation, are considered ?robust?, and
become widely accepted ages. A review of the age data for the 25
short-listed structures suggests that 11 ages are accurate, 12 are at best
ambiguous and should not be reported with any uncertainty, and 2 are not
well characterized at all. We report detailed examples of misleading ages
and/or age uncertainties (e.g., poor stratigraphic constraints, data
over-interpretations, ambiguity due to inconsistent results), and highlight
the robustness of the 11 well-defined ages. Based on observations and
modeling, suggestions are made on how to obtain better ages by carrying out
adequate sample preparation. We also indicate how to interpret ages for
non-geochronologists. This brief review should be interpreted as a call for
immediate, drastic qualitative and quantitative improvements of the ages of
terrestrial impact structures.