[meteorite-list] Papers on astro-impact 12.8kya ending Clovis culture

From: Robin Whittle <rw_at_meteoritecentral.com>
Date: Sun, 18 Aug 2013 12:39:14 +1000
Message-ID: <521033D2.1020004_at_firstpr.com.au>

There is a site: The Cosmic Tusk - Abrupt climate change induced by
comets and asteroids during human history:

  http://cosmictusk.com

which has some quotes from and discussion:


http://cosmictusk.com/mahaney-new-evidence-from-a-black-mat-site-in-the-northern-andes-supporting-a-cosmic-impact-12800-years-ago/

of a recent paper:

  New Evidence from a Black Mat Site in the Northern Andes Supporting a
  Cosmic Impact 12,800 Years Ago
  William C. Mahaney, Leslie Keiser, Dave Krinsley, Volli Kalm, Roelf
  Beukens, and Allen West
  The Journal of Geology
  Vol. 121, No. 4 (July 2013), pp. 309-325
  http://www.jstor.org/stable/10.1086/670652 (Behind paywall.)

Abstract:

    Previous work has ascribed a cosmic impact origin to black,
    high-temperature, carbon-encrusted beds (2?3 cm thick),
    associated with the Younger Dryas readvance of ice at 12.8 ka
    during the Late Glacial in the northern Andes of Venezuela.
    The evidence for this includes carbon spherules,
    aluminosilicate melt rocks, melted coatings of glass-like
    amorphous carbon, and Fe-Mn on sands and clasts derived from
    local felsic gneiss and granite. These sediments have been
    subjected to renewed investigation using high-resolution
    scanning electron microscopy and energy-dispersive
    spectrometry, and new data show that spherules at site MUM7B
    exhibit unique morphologies and compositions. Molar oxide
    weight percentages prove the spherules are not volcanic and
    show little overlap with cosmic materials. Spherule
    microstructures display quench melting and, thus, could not
    have formed from slow geological authigenic, diagenetic, or
    metamorphic processes. Instead, geochemical values for the
    Venezuelan samples plot within the limits of impact-related
    materials, including tektites, ejecta, and impact spherules
    from a number of craters and strewnfields (cf. Chicxulub
    Crater, Chesapeake Bay Crater, Tunguska, Australasian
    tektite field, Lake Bosumtwi Crater, Ries Crater, and others).
    These results are identical to previously reported spherules
    from the Younger Dryas boundary layer (YDB) on three
    continents, North America, Europe, and Asia, and the most
    likely origin is from a cosmic impact/airburst 12.8 ka, as
    previously proposed. The MUM7B site is one of the two
    southernmost sites (Venezuela and Peru) in South America,
    thus extending the evidence supporting the YDB impact event
    into a new hemisphere on a new continent.

A comment by Hermann Burchard includes some quotes from the article.
Other comments concern how something resembling today's technologies and
civilization might have developed (probably earlier, but I think perhaps
later or maybe not at all) "If we?ve gotten whacked hard enough to kill
the mammoths, sabre-toothed tigers and Clovis man . . ."

A still more recent paper which cites this first one:

  Cosmogenic nuclide enhancement via deposition from long-period comets
  as a test of the Younger Dryas impact hypothesis
  Andrew C. Overholt, Adrian L. Melott
  2013-07-24
  http://arxiv.org/abs/1307.6557 (Full text available.)

Abstract [with my notes]:

    We explore the idea that detectable excursions in 26Al
    [a radioactive isotope of Aluminium with a half-life of
    717,000 years] may arise from direct deposition by any
    bolide, and excursions in 14C and 10Be [radioactive
    isotopes of carbon and beryllium] abundances in the
    atmosphere may result from long-period comet impacts.
    This is very different from the usual processes of
    production by cosmic rays within Earths atmosphere.
    Long-period comets experience greatly increased cosmic
    ray flux beyond the protection of the suns magnetic field.
    We report the computed amount of 14C, 10Be, and 26Al
    present on long-period comets as a function of comet mass.
    We find that the amount of nuclide mass on large
    long-period comets entering the Earths atmosphere may be
    sufficient for creating anomalies in the records of 14C
    and 10Be from past impacts. In particular, the
    estimated mass of the proposed Younger Dryas comet is
    consistent with its having deposited sufficient isotopes
    to account for recorded 14C and 10Be increases at that
    time. The 26Al/10Be ratio is much larger in
    extraterrestrial objects than in the atmosphere, and so,
    we note that measuring this ratio in ice cores is a
    suitable definitive test for the Younger Dryas impact
    hypothesis, even if the hypothetical bolide is not a
    long-period comet and/or did not contribute to the 14C
    and 10Be increases.


The paper ends with a graph of Carbon 14 and Beryllium 10 levels in the
period 14,500 to 9,500 years before present, derived from (C14) tree
rings, coral and marine sediment and (B10) ice cores. There is a sharp
jump around 12,900 years ago, close to the beginning of the Younger
Dryas event.


I am writing this to three mailing lists, one of which is not ordinarily
much connected to the other two:

The Anthropology mailing list:
  http://listserv.buffalo.edu/archives/anthro-l.html

Evolutionary Psychology:
  http://tech.groups.yahoo.com/group/evolutionary-psychology/

The Meteorite List:
  http://six.pairlist.net/mailman/listinfo/meteorite-list


  - Robin
Received on Sat 17 Aug 2013 10:39:14 PM PDT