models that assume stepwse mutaton of the STRP ~m p y that the varlaton of allele sizes Increases Inearly w~th tme (48) [D. 5. Godstein, A. Ruz Lnares, L. L. Cavali-Sforza, M. W. Feldman, Genetics 139, 463 (1995jl. Thus, we can est~mate R as the ratio of varances of allele szes (in base pa~rs) for alleles less than 110 bp (22 5/0.75 = 30.0), glvng a maxlmum age of 167,000 Y.B.P. Because we have considered only a slnge locus, the standard error on this varance-ratlo estmate IS high. However, the apparent lower boundary of 85 bp to the STRP allele slze results In an underestmate of the t~me of orlgn of the Au(-) chromosome in Afrca, so agaln ths estlmate of R is conservatve. 42. Despte a smaller effect~ve populat~on sze of A h -) chromosomes (because of a lower frequency), the mean varlance of STRP alleles on Alu(-j chromsomes among the 10 sub-Saharan Afr~can popua-t~ons is still substantial [64% of the mean varance of STRP alleles on Alu(+) chromosomes], suggest~ng that whle the orig~n of the Alu(-) chromosome IS more recent than the orig~n of the Alu(+j chromosome, it is st11 quite ancent. 43 C B. Str~nger, In The Origin and Evolution ofHumans and Humanness, D. T. Rasmussen Ed. (Jones and Bartett, Boston, 19931, pp. 75-94. 44 M. Slatk~n, Mol Blol. El/ol 12, 473 (1995). 45 Both the STRP and the Alu deletion polymorphism are located In noncodng regions of the CD4 gene and are unlkey to have any functional s~gn~f~cance. Except In the unlikely case of strong posltlve CISactng epstass of functorial varants flankng exons 1 and 2 of the 90-bp Au(-) chromosomes, seecton cannot expla~n the mantenance of the l~nkage d~sequlhbrum seen In non-Afr~can populatons because there would be nothing to prevent format~on of "non-90 bp" Alu(--) chromosomes by recombna-t~on between the markers or mutaton at the STRP, In addtion, the very low frequency of the Alu(-) chromosomes In Asa, the Pacifc islands, and the New World, as well as the very h~gh frequency of 85-bp Alu(+j and 11 0-bp Alu(+) chromosomes In all non-African populat~ons, argues aga~nst strong positive selecton for the Alu(-) chromosome. S A.Orbital histories of ejecta from the terrestrial planets were numerically integrated to study their transfer to Earth. The properties of the lunar and martian meteorites are consistent with a recurrent ejection of small meteoroids as a result of impacts on their parent bodies. Long-range gravitational effects, especially secular resonances, strongly influence the orbits of many meteoroids, increasing their collision rates with other planets and the sun. These effects and collisional destruction in the asteroid belt result in shortened time scales and higher fluxes than previously believed, especially for martian meteorites. A small flux of mercurian ejecta appears possible; recovery of meteorites from the Earth and Venus is less likely.
Abstract-The Haughton impact structure has been the focus of systematic, multi-disciplinary field and laboratory research activities over the past several years. Regional geological mapping has refined the sedimentary target stratigraphy and constrained the thickness of the sedimentary sequence at the time of impact to ∼1880 m. New 40 Ar-39 Ar dates place the impact event at ∼39 Ma, in the late Eocene. Haughton has an apparent crater diameter of ∼23 km, with an estimated rim (final crater) diameter of ∼16 km. The structure lacks a central topographic peak or peak ring, which is unusual for craters of this size. Geological mapping and sampling reveals that a series of different impactites are present at Haughton. The volumetrically dominant crater-fill impact melt breccias contain a calciteanhydrite-silicate glass groundmass, all of which have been shown to represent impact-generated melt phases. These impactites are, therefore, stratigraphically and genetically equivalent to coherent impact melt rocks present in craters developed in crystalline targets. The crater-fill impactites provided a heat source that drove a post-impact hydrothermal system. During this time, Haughton would have represented a transient, warm, wet microbial oasis. A subsequent episode of erosion, during which time substantial amounts of impactites were removed, was followed by the deposition of intra-crater lacustrine sediments of the Haughton Formation during the Miocene. Present-day intracrater lakes and ponds preserve a detailed paleoenvironmental record dating back to the last glaciation in the High Arctic. Modern modification of the landscape is dominated by seasonal regional glacial and niveal melting, and local periglacial processes. The impact processing of target materials improved the opportunities for colonization and has provided several present-day habitats suitable for microbial life that otherwise do not exist in the surrounding terrain.
Abstract-Contrary to the previous interpretation of a single allochthonous impactite lithology, combined field, optical, and analytical scanning electron microscopy (SEM) studies have revealed the presence of a series of impactites at the Haughton impact structure. In the crater interior, there is a consistent upward sequence from parautochthonous target rocks overlain by parautochthonous lithic (monomict) breccias, through allochthonous lithic (polymict) breccia, into pale grey allochthonous impact melt breccias. The groundmass of the pale grey impact melt breccias consists of microcrystalline calcite, silicate impact melt glass, and anhydrite. Analytical data and microtextures indicate that these phases represent a series of impact-generated melts that were molten at the time of, and following, deposition. Impact melt glass clasts are present in approximately half of the samples studied. Consideration of the groundmass phases and impact glass clasts reveal that impactites of the crater interior contain shock-melted sedimentary material from depths of >920 to <1880 m in the preimpact target sequence. Two principal impactites have been recognized in the near-surface crater rim region of Haughton. Pale yellow-brown allochthonous impact melt breccias and megablocks are overlain by pale grey allochthonous impact melt breccias. The former are derived from depths of >200 to <760 m and are interpreted as remnants of the continuous ejecta blanket. The pale grey impact melt breccias, although similar to the impact melt breccias of the crater interior, are more carbonate-rich and do not appear to have incorporated clasts from the crystalline basement. Thus, the spatial distribution of the crater-fill impactites at Haughton, the stratigraphic succession from target rocks to allochthonous impactites, the recognition of large volumes of impact melt breccias, and their probable original volume are all analogous to characteristics of coherent impact melt layers in comparatively sized structures formed in crystalline targets.
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