Freethought & Rationalism Archive

MrDarwin · 05-13-2003, 06:32 AM

Haven't there been claims by creationists that there are no fossil meteorites?

Meteorites Rained On Earth After Massive Asteroid Breakup; Geologists Find Meteorites 100 Times More Common In Wake Of Ancient Asteroid Collision

Quote:

Using fossil meteorites and ancient limestone unearthed throughout southern Sweden, marine geologists at Rice University have discovered that a colossal collision in the asteroid belt some 500 million years ago led to intense meteorite strikes over the Earth's surface.

The research, which appears in this week's issue of Science magazine, is based upon an analysis of fossil meteorites and limestone samples from five Swedish quarries located as much as 310 miles (500 km.) apart. The limestone formed from sea bottom sediments during a 2 million-year span about 480 million years ago, sealing the intact meteorites, as well as trace minerals from disintegrated meteorites, in a lithographic time capsule.

ps418 · 05-13-2003, 06:59 AM

There's also a new paper in Geology on the Acraman impact, whose 90km diameter crater is located in S. Australia, arguing that the impact at 580Ma is associated with an extinction event, a carbon isotope excursion (i.e. a change in carbon isotope ratios), and the subsequent diversification of life in the latest Precambrian. There is rapid proliferation of new species immediately above the ejecta horizon. This is a pattern shared by other major mass extinctions.

Quote:

Biostratigraphic and chemostratigraphic studies of Australian late Neoproterozoic (Ediacarian) fossil plankton (acritarch) successions reveal a striking relationship between a radical palynofloral change, a short-lived negative excursion in the carbon isotope composition of kerogen, and a debris layer from the ca. 580 Ma Acraman bolide impact event. Palynomorphs changed from an assemblage dominated by long-ranging, simple spheroids to a much more diverse assemblage characterized by short-ranging, large, complex, process-bearing (acanthomorph) acritarchs, with the first appearance of 57 species. A marked negative carbon isotope excursion was followed by a steady rise coinciding with acanthomorph radiation. There are no apparent sedimentological controls on this radiation. Although the snowball Earth hypothesis predicts postglacial biotic change, radiation did not happen until long after the Marinoan glaciation and not until a second postglacial transgression. We propose that a global extinction and recovery event may have been associated with the Acraman bolide impact. Indications are that the Acraman event could rank with similar Phanerozoic major impact events.

Grey et al, 2003. Neoproterozoic biotic diversification: Snowball Earth or aftermath of the Acraman impact? Geology: Vol. 31, No. 5, pp. 459�462.

Here's the abstract and ref for the first article referenced by MrD:

Quote:

Abundant extraterrestrial chromite grains from decomposed meteorites occur in middle Ordovician (480 million years ago) marine limestone over an area of 250,000 square kilometers in southern Sweden. The chromite anomaly gives support for an increase of two orders of magnitude in the influx of meteorites to Earth during the mid-Ordovician, as previously indicated by fossil meteorites. Extraterrestrial chromite grains in mid-Ordovician limestone can be used to constrain in detail the temporal variations in flux of extraterrestrial matter after one of the largest asteroid disruption events in the asteroid belt in late solar-system history.

Schimtz et al, 2003. Sediment-Dispersed Extraterrestrial Chromite Traces a Major Asteroid Disruption Event. Science 300, pp. 961-964.

This is similar to the evidence for the late Eocene 'dust shower.' Using 3He as a tracer, Farley et al (1998) showed that there was a period of 2.5Ma in the late Eocene (~36Ma) during which the infall of interplanetary dust to earth was greatly enhanced. There were also two major impacts during this time. Farley et al (1998) intepret this in terms of a perturbation of the Oort Cloud.

K. A. Farley, A. Montanari, E. M. Shoemaker, and C. S. Shoemaker. Geochemical Evidence for a Comet Shower in the Late Eocene. Science 1998 May 22; 280: 1250-1253.

Coragyps · 05-13-2003, 07:05 AM

Too cool!

Quote:

Haven't there been claims by creationists that there are no fossil meteorites?

Full-of-crap claims based on not even a web search: see here for a list with maybe 50 buried craters....

lpetrich · 05-13-2003, 11:10 AM

However, fossil meteorites may be difficult to identify, because they may be difficult to distinguish from other stray rocks unless one knows what to look for.

Kosh · 05-13-2003, 03:07 PM

Quote:

Originally posted by lpetrich
However, fossil meteorites may be difficult to identify, because they may be difficult to distinguish from other stray rocks unless one knows what to look for.

Nah, that's easy. You just have to look for the ones that have turned to stone....

Undercurrent · 05-13-2003, 08:29 PM

Quote:

Originally posted by lpetrich
However, fossil meteorites may be difficult to identify, because they may be difficult to distinguish from other stray rocks unless one knows what to look for.

A fact that pretty much guarentees that for every one found, there are hundreds that lie undiscovered.

ps418 · 05-14-2003, 06:09 AM

Though they are indeed rare as you'd expect, there are some examples aside from those mentioned in the OP. For instance, a small 2.5mm piece of the K-T impactor was recovered from DSDP Site 576 in the Pacific Ocean, about 9000 km west of the Chicxulub crater (Kyte, F.T. 1998). Much easier to identify are impact products, such as tectites, shocked minerals (e.g. quartz) with multi-planar deformation, and geochemical anamolies, such as enriched Ir, or chondritic Cr and 187 Os/188 Os ratios (see Kyte, 2002, for a review). Though a crater is hard to find and an impactor even harder, impact products may be distributed over thousands of kms. For instance, the first evidence of a K-T impact was the Ir anamoly at the K-T boundary section at Gubbio, Italy, and the crater was found only ~decade later.

Kyte, F.T. 1998. A meteorite from the Cretaceous/ Tertiary boundary. Nature 396: 237-239

Kyte, 2002. Tracers of the extraterrestrial component in sediments and inferences for Earth's accretion history, in: Koeberl
and MacLeod, Catastrophic Events and Mass Extinctions: Impacts and Beyond, GSA Special Paper 356, 21-37.

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05-13-2003, 11:10 AM	#4
lpetrich Contributor Join Date: Jul 2000 Location: Lebanon, OR, USA Posts: 16,829	However, fossil meteorites may be difficult to identify, because they may be difficult to distinguish from other stray rocks unless one knows what to look for.

05-14-2003, 06:09 AM	#7
ps418 Veteran Member Join Date: Mar 2001 Location: Louisville, KY, USA Posts: 1,840	Though they are indeed rare as you'd expect, there are some examples aside from those mentioned in the OP. For instance, a small 2.5mm piece of the K-T impactor was recovered from DSDP Site 576 in the Pacific Ocean, about 9000 km west of the Chicxulub crater (Kyte, F.T. 1998). Much easier to identify are impact products, such as tectites, shocked minerals (e.g. quartz) with multi-planar deformation, and geochemical anamolies, such as enriched Ir, or chondritic Cr and 187 Os/188 Os ratios (see Kyte, 2002, for a review). Though a crater is hard to find and an impactor even harder, impact products may be distributed over thousands of kms. For instance, the first evidence of a K-T impact was the Ir anamoly at the K-T boundary section at Gubbio, Italy, and the crater was found only ~decade later. Kyte, F.T. 1998. A meteorite from the Cretaceous/ Tertiary boundary. Nature 396: 237-239 Kyte, 2002. Tracers of the extraterrestrial component in sediments and inferences for Earth's accretion history, in: Koeberl and MacLeod, Catastrophic Events and Mass Extinctions: Impacts and Beyond, GSA Special Paper 356, 21-37.

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