Showing papers by "Paul R. Renne published in 2002"

Remains of Homo erectus from Bouri, Middle Awash, Ethiopia

Abstract: The genesis, evolution and fate of Homo erectus have been explored palaeontologically since the taxon's recognition in the late nineteenth century. Current debate1 is focused on whether early representatives from Kenya and Georgia should be classified as a separate ancestral species (‘H. ergaster’)2,3,4, and whether H. erectus was an exclusively Asian species lineage that went extinct5,6. Lack of resolution of these issues has obscured the place of H. erectus in human evolution. A hominid calvaria and postcranial remains recently recovered from the Dakanihylo Member of the Bouri Formation, Middle Awash, Ethiopia, bear directly on these issues. These ∼1.0-million-year (Myr)-old Pleistocene sediments contain abundant early Acheulean stone tools and a diverse vertebrate fauna that indicates a predominantly savannah environment. Here we report that the ‘Daka’ calvaria's metric and morphological attributes centre it firmly within H. erectus. Daka's resemblance to Asian counterparts indicates that the early African and Eurasian fossil hominids represent demes of a widespread palaeospecies. Daka's anatomical intermediacy between earlier and later African fossils provides evidence of evolutionary change. Its temporal and geographic position indicates that African H. erectus was the ancestor of Homo sapiens.

Statistical Methods for Jointly Estimating the Decay Constant of 40K and the Age of a Dating Standard

Abstract: To simultaneously evaluate the decay constant of 40K (λ) and the age of a standard (t std) using isotopic data from geologic materials, we applied a series of statistical methods. The problem of estimating the most probable intercept of many nonlinear curves in λ and t std space is formulated by an errors-in-variables nonlinear regression model. Then a maximum likelihood method is applied to the model for a point estimate, which is equivalent to the nonlinear least square method when measurement error distributions are Gaussian. Uncertainties and confidence regions of the estimates can be approximated using three methods: the asymptotic normal approximation, the parametric bootstrap method and Bonferroni confidence regions. Five pairs of published data for samples with ages from 2 ka to 4.5 Ga were used to estimate λ and the age of Fish Canyon sanidine (t FCs). The statistical procedure yields most probable estimates of λ (5.4755 ± 0.0170 × 10−10 (1σ)/year) and t FCs (28.269 ± 0.0661 (1σ) Ma) which are in between previously published values. These results indicate the power of our approach to provide improved constraints on these parameters, although the preliminary nature of some of the input data require further review before the values can be adopted.

Oriented inclusions of magnetite in clinopyroxene: Source of stable remanent magnetization in gabbros of the Messum Complex, Namibia

Abstract: [1] Crystallographically oriented and highly elongate magnetite inclusions in clinopyroxene are the dominant source of highly stable remanent magnetization in gabbros of the Early Cretaceous Messum Complex, Namibia Rock magnetic properties determined for individual pyroxene crystals indicate a high proportion of single-domain magnetite, consistent with the observed sizes and shape anisotropy of the magnetite inclusions As in previous studies of similar inclusions, these are inferred to have formed by exsolution Two arrays of inclusions are regularly present in the Messum clinopyroxenes, inclined at about 74°, consistent with formation at about 800°C deduced from optimization of phase boundary orientations Virtual geomagnetic poles from these rocks are consistent with reference data, confirming that the magnetization is of thermoremanent origin Bipolar magnetizations are recorded at one site as well in individual clinopyroxene crystals, suggesting that remanence acquisition upon initial cooling of the gabbro spanned a geomagnetic polarity reversal

Paleomagnetism of the early Triassic Semeitau igneous series, eastern Kazakstan

Abstract: [1] Paleomagnetic results obtained from the early Triassic Semeitau igneous series (SIS) yield a high-quality pole for eastern Kazakstan that is concordant with reference poles for Siberia and Europe, strongly supporting the hypothesis that Kazakstan was already joined with Eurasia at this time. Of the 25 sites that were measured, data were straightforward for 20 sites, 2 of which recorded normal polarity and 18 of which recorded reverse; 5 sites were discarded because of severe overprinting, unstable demagnetizations, or extremely scattered within-site directions. Combining sites judged to be the same cooling unit and taking the average of 15 cooling units of equal weight, we calculated a pole located at 56°N, 139°E (N = 15, K = 24.6, α95 = 7.9) for the SIS. We consider this pole to be reliable. The sampling of temporally spaced extrusives and more slowly cooled intrusives and the presence of both polarities strongly suggest that this pole averages secular variation. The 40Ar/39Ar determinations on sanidine crystals from two extrusive units yielded highly consistent ages of 248.2 ± 0.5 and 248.8 ± 0.5 Ma. Comparison of our pole for the SIS with the Siberian reference pole shows relative translation (−8° ± 10°) and rotation (1°±16°) that are indistinguishable from zero at 95% confidence. This result fails to confirm hypothesized post-Permian rotation of the Siberian craton relative to its margins and provides no evidence for left-lateral motion on the east-west shear zones in which the Semeitau is located. The similarity of ages between the SIS and the Siberian Traps supports speculation of a genetic relationship between the two.

Coastal landsliding and catastrophic sedimentation triggered by Cretaceous-Tertiary bolide impact: A Pacific margin example?

Abstract: We report here the first-recognized Pacific margin stratigraphic sequence containing evidence for catastrophic landsliding attributed to bolide impact–related seismic shocking at the Cretaceous-Tertiary (K-T) boundary. The K-T boundary is not commonly preserved in stratigraphic sequences of the Pacific margin, but we have discovered it within a coastal paleovalley in Baja California, Mexico (near El Rosario). This 5-km-wide, 15-km-long, and 200-m-deep coastal paleovalley formed by massive gravitational collapses and rapidly filled with coastal (shallow marine and lesser fluvial) gravels and sands, as well as slide sheets of marine mudstone that range from meters to kilometers in length. We infer that seismic shocking caused liquefaction and extremely rapid sedimentation of the gravels and sands, simultaneous with unleashing of slide sheets. Laser-heating 40Ar/39Ar data for biotite, hornblende, and plagioclase (single crystal and bulk step heating) on a 20-m-thick pumice lapilli tuff in the middle of the valley fill give an age of 65.5 ± 0.6 Ma; this is indistinguishable from the age of Haitian tektites dated by the same laboratory. Our new Pacific margin sequence, like many K-T boundary sequences in the Gulf of Mexico–Caribbean region, provides evidence of giant landslides and catastrophic sedimentation 1800 km from the bolide impact site.

Flood Basalts--Bigger and Badder

Abstract: During episodes of flood volcanism, vast amounts of mass and energy are transferred from Earth's interior to the surface within relatively short periods of time. Episodes of major flood volcanism coincided with severe mass extinctions, and perhaps also with large meteor impacts. In his Perspective, [Renne][1] discusses recent studies into the causes of these events and the extent of the flood basalt provinces created by them. He highlights the report by [ Reichow et al .][2], who show that the Siberian Traps are much larger than previously thought, making it the largest (by volume) of all known flood provinces. [1]: http://www.sciencemag.org/cgi/content/full/296/5574/1812 [2]: http://www.sciencemag.org/cgi/content/short/296/5574/1846