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281.
282.
The Olorgesailie Formation (1.2-0.49 Ma) consists of fluvial and lacustrine rift sediments that have yielded abundant Acheulean artifacts and a fossil hominin (Homo cf. erectus). In testing prior understandings of the paleoenvironmental context, we define nine new geochemical zones. A Chemical Index of Alteration suggests increased catchment weathering during deposition of Members 1, 2, 7, 11, and 13. Biophile elements (Br, S) peak in M8-9 and lower M13 possibly reflecting increased input from soil erosion. REE data show that the Magadi Trachytes supplied most siliciclastic grains. Sixteen diatom stages indicate conductivities of 200-16,000 μS cm− 1 and pH of 7.5-9.5 for five deep-water lakes, ten shallow lakes and sixteen wetlands. These results are compared with diatom data from other sections in the basin and show aquatic spatial variability over km-scale distances. Similar floras are traceable over several kilometers for M2, M3 and M9, indicating broadly homogeneous lacustrine conditions during these times, but diatoms in other members imply variable conditions, some related to local tectonic controls. This lateral and temporal variability emphasizes the importance of carrying out stratigraphic sampling at multiple sites within a basin in efforts to define the environmental context relevant to human evolution. 相似文献
283.
Justine J. Owen Ronald Amundson William E. Dietrich Kunihiko Nishiizumi Brad Sutter Guillermo Chong 《地球表面变化过程与地形》2011,36(1):117-135
Decoupling the impacts of climate and tectonics on hillslope erosion rates is a challenging problem. Hillslope erosion rates are well known to respond to changes in hillslope boundary conditions (e.g. channel incision rates) through their dependence on soil thickness, and precipitation is an important control on soil formation. Surprisingly though, compilations of hillslope denudation rates suggest little precipitation sensitivity. To isolate the effects of precipitation and boundary condition, we measured rates of soil production from bedrock and described soils on hillslopes along a semi‐arid to hyperarid precipitation gradient in northern Chile. In each climate zone, hillslopes with contrasting boundary conditions (actively incising channels versus non‐eroding landforms) were studied. Channel incision rates, which ultimately drive hillslope erosion, varied with precipitation rather than tectonic setting throughout the study area. These precipitation‐dependent incision rates are mirrored on the hillslopes, where erosion shifts from relatively fast and biologically‐driven to extremely slow and salt‐driven as precipitation decreases. Contrary to studies in humid regions, bedrock erosion rates increase with precipitation following a power law, from ~1 m Ma?1 in the hyperarid region to ~40 m Ma?1 in the semi‐arid region. The effect of boundary condition on soil thickness was observed in all climate zones (thicker soils on hillslopes with stable boundaries compared to hillslopes bounded by active channels), but the difference in bedrock erosion rates between the hillslopes within a climate region (slower erosion rates on hillslopes with stable boundaries) decreased as precipitation decreased. The biotic‐abiotic threshold also marks the precipitation rate below which bedrock erosion rates are no longer a function of soil thickness. Our work shows that hillslope processes become sensitive to precipitation as life disappears and the ability of the landscape to respond to tectonics decreases. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
284.
Human activities in the karst Ozark Plateaus can impact water quality of springs where surface water is rapidly transferred
to subsurface conduits. Bennett Spring, in southern Missouri, is the fourth largest spring in the state and supports local
tourism activities. Questions regarding poorly functioning on-site wastewater systems (OWS) have raised concerns over the
long-term water quality of the spring. This study reports the results of a surface water quality monitoring program in the
recharge area where monthly samples were collected at base flow to identify potential pollution sources to the spring. Base
flow hydrology of the recharge area was highly variable over the study period, which was drier than normal, causing an incomplete
sampling record due to no flow conditions at some sites. For most of the year, nutrient levels were less than the eutrophic
threshold (ET) of 0.075 mg/l total phosphorus (TP) and 1.5 mg/l total nitrogen (TN). Sites that consistently displayed concentrations
of TP and TN higher than the ET were influenced by wastewater treatment plants (WTP) or OWS. Sites with nutrient concentrations
above the ET were likely influenced by the re-release of nonpoint source related TP and TN delivered to streams during storm
events. Water quality and discharge at the spring outlet remained consistent over the sampling period suggesting diffuse recharge
from a deep aquifer source is able to dilute shallow ground water sources carrying nonpoint pollutants at base flow. Historical
and regional data comparisons show these trends have been consistent over at least the last two decades. 相似文献
285.
The relatively low value of Xe/Kr in the atmospheres of Earth and Mars seems to rule out meteorites as the major carriers of noble gases to the inner planets. Laboratory experiments on the trapping of gases in ice forming at low temperatures suggest that comets may be a better choice. It is then possible to develop a model for the origin of inner planet atmospheres based on volatiles delivered by comets added to volatiles originally trapped in planetary rocks. The model will be tested by results from the Galileo Entry Probe. 相似文献
286.
AXIOM: advanced X-ray imaging of the magnetosphere 总被引:1,自引:0,他引:1
Graziella Branduardi-Raymont Steve F. Sembay Jonathan P. Eastwood David G. Sibeck Tony A. Abbey Patrick Brown Jenny A. Carter Chris M. Carr Colin Forsyth Dhiren Kataria Steve Kemble Steve E. Milan Chris J. Owen Lisa Peacocke Andy M. Read Andrew J. Coates Michael R. Collier Stan W. H. Cowley Andrew N. Fazakerley George W. Fraser Geraint H. Jones Rosine Lallement Mark Lester F. Scott Porter Tim K. Yeoman 《Experimental Astronomy》2012,33(2-3):403-443
Planetary plasma and magnetic field environments can be studied in two complementary ways—by in situ measurements, or by remote sensing. While the former provide precise information about plasma behaviour, instabilities and dynamics on local scales, the latter offers the global view necessary to understand the overall interaction of the magnetospheric plasma with the solar wind. Some parts of the Earth’s magnetosphere have been remotely sensed, but the majority remains unexplored by this type of measurements. Here we propose a novel and more elegant approach employing remote X-ray imaging techniques, which are now possible thanks to the relatively recent discovery of solar wind charge exchange X-ray emissions in the vicinity of the Earth’s magnetosphere. In this article we describe how an appropriately designed and located X-ray telescope, supported by simultaneous in situ measurements of the solar wind, can be used to image the dayside magnetosphere, magnetosheath and bow shock, with a temporal and spatial resolution sufficient to address several key outstanding questions concerning how the solar wind interacts with the Earth’s magnetosphere on a global level. Global images of the dayside magnetospheric boundaries require vantage points well outside the magnetosphere. Our studies have led us to propose ‘AXIOM: Advanced X-ray Imaging of the Magnetosphere’, a concept mission using a Vega launcher with a LISA Pathfinder-type Propulsion Module to place the spacecraft in a Lissajous orbit around the Earth–Moon L1 point. The model payload consists of an X-ray Wide Field Imager, capable of both imaging and spectroscopy, and an in situ plasma and magnetic field measurement package. This package comprises a Proton-Alpha Sensor, designed to measure the bulk properties of the solar wind, an Ion Composition Analyser, to characterise the minor ion populations in the solar wind that cause charge exchange emission, and a Magnetometer, designed to measure the strength and direction of the solar wind magnetic field. We also show simulations that demonstrate how the proposed X-ray telescope design is capable of imaging the predicted emission from the dayside magnetosphere with the sensitivity and cadence required to achieve the science goals of the mission. 相似文献
287.
288.
D. E. Owen 《Geological Journal》1966,5(1):135-148
The highly fossiliferous fore-reef limestones of B2 age from near Castleton, Derbyshire are rich in Polyzoa. Ten species are described, six of them new. 相似文献
289.
290.
Y. V. Bogdanova C. J. Owen G. Siscoe A. N. Fazakerley I. Dandouras O. Marghitu Z. Kaymaz H. Rème E. A. Lucek 《Solar physics》2007,244(1-2):233-261
We present a study of the plasma properties inside and dynamics of the low-latitude boundary layer (LLBL)/cusp during the
ICME event on 7 November 2004 based on data from the four Cluster spacecraft. The interplanetary magnetic field (IMF) is predominantly strongly northward, up to 50 nT, with some short-duration
rotations. The observed LLBL/cusp is very thick (∼6 – 7° invariant latitude (ILAT)) and migrates equatorward with rates of
0.55° and 0.04° ILAT per minute during quick southward IMF rotations and stable northward IMF, respectively. The LLBL/cusp
observed by Cluster 1 and Cluster 4 is in a fast transition between different states and is populated by different types of plasma injection, presumably coming
from multiple reconnection sites. During a period of extremely northward IMF, signatures of pulsed dual reconnection inside
the LLBL/cusp are observed by Cluster 3, suggesting that at least part of the LLBL/cusp is on closed field lines. However, analysis of the ion data implies that
the boundary layer is formed in the dawn sector of the magnetosphere and does not slowly convect from the dayside as has been
suggested previously. A statistical study of the location of the LLBL/cusp equatorward boundary during the ICME events on
28 – 29 October 2003 and 7 – 10 November 2004 is performed. During extreme conditions the LLBL/cusp position is offset by
−7° ILAT from the location under normal conditions, which might be explained by the influence of the high solar wind dynamic
pressure. The LLBL/cusp moves equatorward with increasing southward and northward IMF. However, the LLBL/cusp position under
strong southward IMF is more poleward than expected from previous studies, which could indicate some saturation in the dayside
reconnection process or enhancement of the nightside reconnection rate. The LLBL/cusp position under strong northward IMF
is extremely low and does not agree with the location predicted in previous studies. For the events with solar wind dynamic
pressure >10 nPa, the LLBL/cusp position does not depend on the solar wind dynamic pressure. This might indicate some saturation
in the mechanism of how the LLBL/cusp location depends on the solar wind dynamic pressure. 相似文献