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91.
Thomas F. Strasser Curtis Runnels Karl Wegmann Eleni Panagopoulou Floyd Mccoy Chad Digregorio Panagiotis Karkanas Nick Thompson 《第四纪科学杂志》2011,26(5):553-560
Discussions of dispersals of early hominins from Africa assume that Southwest Asia and the Arabian Peninsula were the primary passageways for migrations to Eurasia. The Mediterranean is usually viewed as a barrier to early hominin movements because pre‐sapiens hominins were thought to lack the technical means or the cognitive skills to construct boats. The discovery of early Palaeolithic artefacts in an archaeological survey on the Greek island of Crete challenges this view. Here we show that Palaeolithic artefacts in the Plakias region in southwestern Crete are associated with geological contexts that can be dated to the late Middle or early Late Pleistocene. Because Crete has been separated from the mainland throughout the Pleistocene, the presence of Pleistocene age artefacts there suggests that early hominins were able to cross open water. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
92.
Chad Schell Stephen Paul Linder 《Oceanic Engineering, IEEE Journal of》2006,31(3):672-684
The fine-scale swimming behavior of fish can now be studied because of the development of sophisticated measurement devices such as multibeam sonar and stereo video systems. However, even with these sensors, improved methods are still required to generate quality estimates of swimming speeds and turn rates. Biologists have commonly relied on pointwise differentiation of noisy position measurements while engineers have focused on Bayesian algorithms to track underwater vehicles. A comparative evaluation of the performance of these tracking algorithms for the analysis of fine-scale behavior of fish was performed using a data set of 100 fish tracks recorded simultaneously with a multibeam sonar and a stereo video camera system. The segmenting track identifier, a non-Bayesian curve fitting and segmenting tracker, is shown to be most effective for tracking the unpredictable and complex horizontal motion of fish while a Kalman smoother using a constant-velocity model is shown to be most effective for tracking the more predictable and piecewise linear vertical motion of fish. Both are shown to be more effective than pointwise differentiation. Criteria for selecting an appropriate algorithm for a given motion study are provided 相似文献
93.
Christopher R. Scotese Rob Van der Voo Chad McCabe 《Physics of the Earth and Planetary Interiors》1982,30(4):385-395
Paleomagnetic directions for the Upper Silurian and Lower Devonian carbonates of the Helderberg escarpment (New York State) differ from expected Late Silurian and Early Devonian directions for cratonic North America. The mean direction (D = 165°, I = −10°; paleopole at 50°N 129°E) is similar to Late Carboniferous and Early Permian results. Negative fold tests, and a lack of reversals, suggest that the magnetization is secondary. However, low coercivities, low blocking temperatures, the thermomagnetic curves (TC near 570°C) and the acquisition of isothermal remanent magnetizations all suggest that the remanence is carried by magnetite. If a detrital origin of these magnetites is assumed, the secondary nature of the remanence would argue for thermal resetting as a result of deep burial of the rocks. However, no evidence for such thermal resetting is seen in the alteration of conodonts. More likely perhaps is a chemical or thermochemical origin of the remanence; this would require the magnetites to be authigenic. 相似文献
94.
Countries can use both mitigation and adaptation strategies to protect their citizens from catastrophic risk posed by climate
change (e.g., shift in the jet stream). A nation can mitigate by reducing CO2 emissions, which reduces the probability of a catastrophic event; it can adapt by altering the infrastructure so that damages
can be reduced in the event a catastrophe is realized. Herein we add to the current literature by extending the endogenous
risk framework into a dynamic framework permitting analysis of both mitigation and adaptation while allowing for the dynamic
process of global climate change. Our results suggest adaptation to catastrophe is a small fraction of the national climate
protection budget relative to mitigation when nations cooperate fully, when damages are both continuous and catastrophic,
and when nations have a short planning horizon. Adaptation becomes more important relative to mitigation when nations are
unlikely to cooperate, when damages are mainly catastrophic, or when the nation’s planning horizon increases. 相似文献
95.
Estimation of fault-zone conductance by calibration of a regional groundwater flow model: Desert Hot Springs,California 总被引:1,自引:2,他引:1
The hydraulic conductance of a large fault zone has been estimated by calibrating a regional groundwater flow model. Drops in groundwater elevations of over 80 m have been observed along a 15-km length of the Mission Creek fault, California, USA. The large drops in elevation are attributed to the reduced hydraulic conductivity of the fault materials. A conceptual and numerical model of the two hydrologic subbasins in Desert Hot Springs, separated by the Mission Creek fault, was developed. The model was used to estimate the hydraulic conductance along the fault. The parameter estimation involved calibrating the model with observed groundwater elevations from over 40 locations over a 60-year period. The fault hydraulic conductances were estimated assuming a linear trend in the fault length, yielding variations in the fault hydraulic conductance of about an order of magnitude along the fault length (2?×?10?11–4?×?10?10 1/s). When an average fault thickness of 35 m is assumed, the fault hydraulic conductivity values are estimated to be from three to five orders of magnitude lower than the surrounding materials. A sensitivity analysis indicated that assumptions made in the conceptual model do not significantly affect estimated fault hydraulic conductances. 相似文献
96.
Field experimental data in the atmospheric surface layer are analyzed using toolsfrom statistical geometry. The data consist of velocity measurements from sonicanemometer arrays. In the context of large eddy simulations (LES), these arrayspermit the spatial filtering needed to separate large from small scales. Time seriesof various quantities relevant to LES are evaluated from the data. Results show thatthe preferred filtered fluid deformation is axisymmetric extension and the preferredsubgrid stress state is axisymmetric contraction. The filtered fluctuating vorticityshows preferred alignments with the mean vorticity, with the streamwise direction,and with the intermediate strain-rate eigenvector. The alignment between eigenvectorsof the subgrid-scale stress and filtered strain rate is used to test eddy viscosity andmixed model formulations. In qualitative agreement with prior laboratory measurements at much lower Reynolds numbers, a bimodal distribution is observed, which can be reduced to good alignment with eddy viscosity closure using the mixed model. 相似文献
97.
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99.
The influence of surface heterogeneities extends vertically within the atmospheric surface layer to the so-called blending height, causing changes in the fluxes of momentum and scalars. Inside this region the turbulence structure cannot be treated as horizontally homogeneous; it is highly dependent on the local surface roughness, the buoyancy and the horizontal scale of heterogeneity. The present study analyzes the change in scalar flux induced by the presence of a large wind farm installed across a heterogeneously rough surface. The change in the internal atmospheric boundary-layer structure due to the large wind farm is decomposed and the change in the overall surface scalar flux is assessed. The equilibrium length scale characteristic of surface roughness transitions is found to be determined by the relative position of the smooth-to-rough transition and the wind turbines. It is shown that the change induced by large wind farms on the scalar flux is of the same order of magnitude as the adjustment they naturally undergo due to surface patchiness. 相似文献
100.
Evolution of silicic magmas in the Kos-Nisyros volcanic center,Greece: a petrological cycle associated with caldera collapse 总被引:2,自引:1,他引:1
Olivier?BachmannEmail author Chad?D.?Deering Janina?S.?Ruprecht Christian?Huber Alexandra?Skopelitis Cedric?Schnyder 《Contributions to Mineralogy and Petrology》2012,163(1):151-166
Multiple eruptions of silicic magma (dacite and rhyolites) occurred over the last ~3 My in the Kos-Nisyros volcanic center
(eastern Aegean sea). During this period, magmas have changed from hornblende-biotite-rich units with low eruption temperatures
(≤750–800°C; Kefalos and Kos dacites and rhyolites) to hotter, pyroxene-bearing units (>800–850°C; Nisyros rhyodacites) and
are transitioning back to cooler magmas (Yali rhyolites). New whole-rock compositions, mineral chemistry, and zircon Hf isotopes
show that these three types of silicic magmas followed the same differentiation trend: they all evolved by crystal fractionation
and minor crustal assimilation (AFC) from parents with intermediate compositions characterized by high Sr/Y and low Nb content,
following a wet, high oxygen fugacity liquid line of descent typical of subduction zones. As the transition between the Kos-Kefalos
and Nisyros-type magmas occurred immediately and abruptly after the major caldera collapse in the area (the 161 ka Kos Plateau
Tuff; KPT), we suggest that the efficient emptying of the magma chamber during the KPT drew out most of the eruptible, volatile-charged
magma and partly solidified the unerupted mush zone in the upper crust due to rapid unloading, decompression, and coincident
crystallization. Subsequently, the system reestablished a shallow silicic production zone from more mafic parents, recharged
from the mid to lower crust. The first silicic eruptions evolving from these parents after the caldera collapse (Nisyros units)
were hotter (up to >100°C) than the caldera-forming event and erupted from reservoirs characterized by different mineral proportions
(more plagioclase and less amphibole). We interpret such a change as a reflection of slightly drier conditions in the magmatic
column after the caldera collapse due to the decompression event. With time, the upper crustal intermediate mush progressively
transitioned into the cold-wet state that prevailed during the Kefalos-Kos stage. The recent eruptions of the high-SiO2 rhyolite on Yali Island, which are low temperature and hydrous phases (sanidine, quartz, biotite), suggest that another large,
potentially explosive magma chamber is presently building under the Kos-Nisyros volcanic center. 相似文献