Ages of Impact Craters
Tuesday, 1. September 2009, 09:32:57
At present there are 174 confirmed impact structures known on Earth (e.g. http://www.unb.ca/passc/ImpactDatabase/ ) but in most cases the age is rather uncertain. Precise and accurate dating is non the less crucial for correlating causes and effects on the bio- and geosphere of catastrophic processes - can some of them for instance be effectively linked to mass extinctions, like the one that killed the dinosaurs about 65 million years ago?
The known impact structures on Earth range from the largest — the 2-billion-year-old Vredefort that stretches 300 km across South Africa — to about 20 or so structures smaller than the 1-km-diameter Barringer crater in Arizona; most of these are less than 1 million years old. These structures represent only the preserved fraction of the overall series of asteroid impacts on Earth.
Of the 174 listed impact structures only a few are precisely dated (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. Nevertheless the calculated ages tend to pop up in the literature without further critical evaluation, 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. In a new paper Jourdan et al. 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. They also indicate how to interpret ages for non-geochronologists.
It is difficult to infer much from the current crater data about periodicities or changes in impacts through time. The new brief review is meant as a call for immediate, drastic qualitative and quantitative improvements of the ages of terrestrial impact structures.
Reference:
An appraisal of the ages of terrestrial impact structures
by Jourdan et al.
in Earth and Planetary Science Letters (in press at time of writing)
doi:10.1016/j.epsl.2009.07.009
The known impact structures on Earth range from the largest — the 2-billion-year-old Vredefort that stretches 300 km across South Africa — to about 20 or so structures smaller than the 1-km-diameter Barringer crater in Arizona; most of these are less than 1 million years old. These structures represent only the preserved fraction of the overall series of asteroid impacts on Earth.
Of the 174 listed impact structures only a few are precisely dated (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. Nevertheless the calculated ages tend to pop up in the literature without further critical evaluation, 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. In a new paper Jourdan et al. 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. They also indicate how to interpret ages for non-geochronologists.
It is difficult to infer much from the current crater data about periodicities or changes in impacts through time. The new brief review is meant as a call for immediate, drastic qualitative and quantitative improvements of the ages of terrestrial impact structures.
Reference:
An appraisal of the ages of terrestrial impact structures
by Jourdan et al.
in Earth and Planetary Science Letters (in press at time of writing)
doi:10.1016/j.epsl.2009.07.009
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http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V61-4WWFN84-4&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=996839772&_rerunOrigin=google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=90881a187161101a92c5a4d97195accb
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http://www.unb.ca/passc/ImpactDatabase/
WIDGETIZE
GOZooM # 6. September 2009, 18:10
http://ottawa-rasc.ca/wiki/index.php?title=Odale-Articles
nielsol # 6. September 2009, 20:12
My impression is that they don't suggest better methods, but rather emphasize the importance of extremely careful sample preparation. But as I said - ask the authors.
And thank you for your link, my first look gave me a good impression.
I hope you keep on "doing your best" - and get sufficient resources to do so.