Paleosols and prehistoric populations in the high plains

Excavations in 1980–1981 at sites (32MZ319 and 32MZ380) along Cinnamon Creek Ridge in the Little Missouri River Badlands of McKenzie County, North Dakota, unexpectedly revealed at each locality the presence of one or more deeply buried paleosols with associated cultural materials. At both sites, 32MZ319 and 32MZ380, multiple paleosols are present, each of which is terminated by an episode of aeolian deposition. Fifteen radiocarbon dates on these paleosols establish an incipient chronology for paleosol development in the study area and permit correlation with other High Plains archaeological complexes. Geological and archaeological data for the two multiple paleosol sites are summarized.     

Ground motion at canyons of arbitrary shape under incident sh waves

A method for calculating the two-dimensional scattering of incident SH waves by canyons of arbitrary shape is presented. The problem is formulated in terms of a Fredholm integral equation of the first kind with the integration path outside the boundary. Point-source discretization and a least-squares scheme are used. Numerical results are compared with the known analytic solution for a semi-cylindrical canyon. Spatial variations of surface amplitudes are computed for triangular and half-cycle sinusoidal canyons as well.     

Wave Generation from Explosions in Rock Cavities

—?We have developed a measurement method to monitor P- and S-waves generated from laboratory-scale explosions in meter-sized rock samples at a series of stations, as well as invented a device to drill spherical cavities in rock, with diameters up to 10?centimeters. We applied these to experiments in Bedford limestone in which spherical/cylindrical explosives (0.2 to 1.9?g) were centrally placed in 1.2- to 3-cm diameter cavities. Stress waves generated by the explosions were recorded within a radius of 25?cm. The radial stress wave records and post-explosion studies demonstrate that S-waves are generated from explosions in cavities as a result of both wave mode-conversion from the cavity wall and crack propagation in rocks. The experimental results of wave generation from the explosions in spherical and cylindrical cavities demonstrate the cavity geometrical effect on the resulting wave pattern. The P- and S-waves generated by explosions and crack propagation in rocks are analyzed. A simple analytic model for P-wave generation is proposed to explain the differences of P-wave-induced displacement histories between the observed waveforms and those predicted by a step-pressure source. Generally, the qualitative predictions of this model fit the observations. The present results demonstrate the importance of rock cracking and cavities in P- and S-wave generation.     

Uranium and thorium series nuclides in oriented ferromanganese nodules: growth rates,turnover times and nuclide behavior

Three ferromanganese nodules handpicked from the tops of 2500 cm² area box cores taken from the north equatorial Pacific have been analysed for their U-Th series nuclides.²³⁰Th_exc concentrations in the surface 1–2 mm of the top side of the nodules indicate growth rates of 1.8–4.6 mm/10⁶ yr. In two of the nodules a significant discontinuity in the²³⁰Th_exc depth profile has been observed at ～0.3 m.y. ago, suggesting that the nodule growth has been episodic. The concentration profiles of²³¹Pa_exc (measured via²²⁷Th) yield growth rates similar to the²³⁰Th_exc data. The bottom sides of the nodules display exponential decrease of²³⁰Th_exc/²³²Th activity ratio with depth, yielding growth rates of 1.5–3.3 mm/10⁶ yr.The²³⁰Th_exc and²³¹Pa_exc concentrations in the outermost layer of the bottom face are significantly lower than in the outermost layer of the top face. Comparison of the extrapolated²³⁰Th_exc/²³²Th and²³⁰Th_exc/²³¹Pa_exc activity ratios for the top and bottom surfaces yields an “age” of (5?15) × 10⁴ yr for the bottom relative to the top. This “age” most probably represents the time elapsed since the nodules have attained the present orientation.The²¹⁰Pb concentration in the surface ～0.1 mm of the top side is in large excess over its parent²²⁶Ra. Elsewhere in the nodule, up to ～1 mm depth in both top and bottom sides,²¹⁰Pb is deficient relative to²²⁶Ra, probably due to²²²Rn loss. The absence of²¹⁰Pb_exc below the outermost layer of the top face rules out the possibility of a sampling artifact as the cause of the observed exponentially decreasing²³⁰Th_exc and²³¹Pa_exc concentration profiles. The flux of²¹⁰Pb_exc to the nodules ranges between 0.31 and 0.58 dpm/cm² yr. The exhalation rate of²²²Rn, estimated from the²²⁶Ra-²¹⁰Pb disequilibrium is ～570 dpm/cm² yr from the top side and >2000 dpm/cm² yr from the bottom side.²²⁶Ra is deficient in the top side relative to²³⁰Th up to ～0.5–1 mm and is in large excess throughout the bottom. The data indicate a net gain of²²⁶Ra into the nodule, corresponding to a flux of (24?46) × 10^?3 dpm/cm² yr. On a total area basis the gain of²²⁶Ra into the nodules is <20% of the²²⁶Ra escaping from the sediments. A similar gain of²²⁸Ra into the bottom side of the nodules is reflected by the high²²⁸Th/²³²Th activity ratios observed in the outermost layer in contact with sediments.     

Paleomagnetism of early Aphebian diabase dykes from the slave structural province, Canada

Early Aphebian dykes (lowermost Proterozoic) intrude the Archean terrain of the Slave Structural Province of the Canadian Shield and paleomagnetic results from them are reported. The Dogrib dykes, with an Rb/Sr age of 2692 ± 80 mm.y., have directions of magnetization directed toward the NW without reversals (16 sites; 309, + 37; α₉₅ = 4°; pole 35S, 050W). The Indin dykes, with an Rb/Sr age of 2093 ± 86 mm.y., have magnetization directed toward the SE with reversals (13 sites; 131, + 58; α₉₅ = 8°; pole 19N, 076W). Other, less well-documented data from a third dyke swarm (the “X” dykes) and a basic sill (the Duck Lake Sill), are also presented, and a very tentative polar path for the Slave Province in the earlier Proterozoic is given. This path is not greatly different from a similar very tentative early Aphebian polar path from the Archean Superior Province, considering the uncertainties in the paleomagnetic and age determinations. We interpret this to mean either that the intervening Hudsonian Structural Province (−1850 m.y.) was not the site of a wide plate-style opening and closing ocean, or if it was, the two bounding Archean cratons returned approximately to their original relative position.     

The visualization of turbulence data using a wavelet‐based method

This technical communication presents some MATLAB^® routines for visualizing the structure in turbulent signals based on a standard conditional averaging and thresholding approach. Up to three velocity components may be analysed and the resulting output highlights the time at which important flow events occur as well as the frequency levels that contribute the greatest energy to this particular event. The latter information is derived from a wavelet decomposition of the signal and may assist in providing a process‐based explanation of observed flow features. Copyright © 2006 John Wiley & Sons, Ltd.