Saqqar: A 34 km diameter impact structure in Saudi Arabia          下载免费PDF全文

Thomas Kenkmann  Abdulkader M. Afifi  Simon A. Stewart  Michael H. Poelchau  Douglas J. Cook  Allen S. Neville 《Meteoritics & planetary science》2015,50(11):1925-1940

Here we present the first proof of an impact origin for the Saqqar circular structure in northwestern Saudi Arabia (Neville et al. 2014 ), with an apparent diameter of 34 km, centered at 29°35′N, 38°42′E. The structure is formed in Cambrian–Devonian siliciclastics and is unconformably overlain by undeformed Cretaceous and Paleogene sediments. The age of impact is not well constrained and lies somewhere between 410 and 70 Ma. The subsurface structure is constrained by 2‐D reflection seismic profiles and six drilled wells. First‐order structural features are a central uplift that rises approximately 2 km above regional datums, surrounded by a ring syncline. The crater rim is defined by circumferential normal faults. The central uplift and ring syncline correspond to a Bouguer gravity high and an annular ring‐like low, respectively. The wells were drilled within the central uplift, the deepest among them exceed 2 km depth. Sandstone core samples from these wells show abundant indicators of a shock metamorphic overprint. Planar deformation features (PDFs) were measured with orientations along (0001), {103}, and less frequently along {101} and {104}. Planar fractures (PFs) predominantly occur along (0001) and {101}, and are locally associated with feather features (FFs). In addition, some shocked feldspar grains and strongly deformed mica flakes were found. The recorded shock pressure ranges between 5 and 15 GPa. The preserved level of shock and the absence of an allochthonous crater fill suggest that Saqqar was eroded by 1–2 km between the Devonian and Maastrichtian. The documentation of unequivocal shock features proves the formation of the Saqqar structure by a hypervelocity impact event.  相似文献   

A note on the extreme intensity of ground‐based temperature inversions at Canadian micrometeorological towers     

I.M. Stewart 《大气与海洋》2013,51(3):23-28

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A new strategy for global climate protection     

Richard B. Stewart  Michael Oppenheimer  Bryce Rudyk 《Climatic change》2013,120(1-2):1-12

This essay proposes an innovative institutional strategy for global climate protection, quite distinct from but ultimately complementary to the UNFCCC climate treaty negotiations. Our “building block” strategy relies on a variety of smaller-scale transnational cooperative arrangements, involving not only states, but also subnational jurisdictions, firms, and civil society organizations, to undertake activities whose primary goal is not climate mitigation but which will achieve greenhouse gas reductions as a byproduct. This strategy avoids the problems inherent in developing an enforceable, comprehensive treaty regime by mobilizing other incentives—including economic self-interest, energy security, cleaner air, and furtherance of international development— to motivate a range of actors to cooperate on actions that will also produce climate benefits. The strategy uses three specific models of regime formation (club, linkage, and dominant actor models) which emerge from economics, international relations, and organizational behavior, to develop a variety of transnational regimes that are generally self-enforcing and sustainable, avoiding the free rider and compliance problems endemic in collective action to provide public goods. These regimes will contribute to global climate action not only by achieving emissions reductions in the short term, but also by creating global webs of cooperation and trust, and by linking the building block regimes to the UNFCCC system through greenhouse gas monitoring and reporting systems. We argue that the building blocks regimes would thereby help secure eventual agreement on a comprehensive climate treaty.  相似文献   

What can ecological science tell us about opportunities for carbon sequestration on arid rangelands in the United States?     

Kayje Booker  Lynn Huntsinger  James W. Bartolome  Nathan F. Sayre  William Stewart 《Global Environmental Change》2013,23(1):240-251

Scientific interest in carbon sequestration on rangelands is largely driven by their extent, while the interest of ranchers in the United States centers on opportunities to enhance revenue streams. Rangelands cover approximately 30% of the earth's ice-free land surface and hold an equivalent amount of the world's terrestrial carbon. Rangelands are grasslands, shrublands, and savannas and cover 312 million hectares in the United States. On the arid and semi-arid sites typical of rangelands annual fluxes are small and unpredictable over time and space, varying primarily with precipitation, but also with soils and vegetation. There is broad scientific consensus that non-equilibrium ecological models better explain the dynamics of such rangelands than equilibrium models, yet current and proposed carbon sequestration policies and associated grazing management recommendations in the United States often do not incorporate this developing scientific understanding of rangeland dynamics. Carbon uptake on arid and semi-arid rangelands is most often controlled by abiotic factors not easily changed by management of grazing or vegetation. Additionality may be impossible to achieve consistently through management on rangelands near the more xeric end of a rangeland climatic gradient. This point is illustrated by a preliminary examination of efforts to develop voluntary cap and trade markets for carbon credits in the United States, and options including payment for ecosystem services or avoided conversion, and carbon taxation. A preliminary analysis focusing on cap and trade and payment for avoided conversion or ecosystem services illustrates the misalignment between policies targeting vegetation management for enhanced carbon uptake and non-equilibrium carbon dynamics on arid United States rangelands. It is possible that current proposed carbon policy as exemplified by carbon credit exchange or offsets will result in a net increase in emissions, as well as investment in failed management. Rather than focusing on annual fluxes, policy and management initiatives should seek long-term protection of rangelands and rangeland soils to conserve carbon, and a broader range of environmental and social benefits.  相似文献   

Noise source level density due to surf. I. Monterey Bay, CA     

Wilson  O.B. Stewart  M.S. Wilson  J.H. Bourke  R.H. 《Oceanic Engineering, IEEE Journal of》1997,22(3):425-433

Ambient noise measurements made in Monterey Bay, CA, in 1981 were reduced by estimations of wave-breaking noise and the residual noise was combined with modeled transmission loss (TL) to estimate the spectral source level of surf-generated noise. A Hamilton geoacoustic model of the coastal environment was derived and used in a finite-element parabolic equation propagation-loss model to obtain TL values. Estimates of both the continuous, or local, and discrete components of wave-breaking noise intensity were subtracted from the total measured noise field to determine the contribution due to surf only. Surf breaking on a uniform 12.5-km linear section of beach near Ft. Ord was found to be the dominant source of surf-generated noise. Estimated noise source level densities for heavy surf at Ft. Ord beach varied from 138 dB ref. 1 μPa Hz^-1/2 m at 1 m from the source at 50 Hz to 107 dB at 1 kHz, with a slope of about -5 dB per octave. Although these results must be considered as preliminary, since they are based on a small number of measurements, they may he useful for prediction of ambient noise in other littoral regions  相似文献   

THE CORRELATION BETWEEN SUNSPOT AND CORONAL HOLE CYCLES AND A FORECAST OF THE MAXIMUM OF SUNSPOT CYCLE 23     

Bravo  S.  Stewart  G. A. 《Solar physics》1997,173(1):193-198

We have shown in previous papers that a close relationship exists between the evolution of polar coronal hole area, estimated from K-coronameter observations, and the Wolf sunspot number, with a time lag of about half a solar cycle. In this paper we study the same relationship, but with the total coronal hole area at the base of the corona as obtained from a potential field model of the coronal magnetic field, which provides a more complete series of about three solar cycles. We confirm the relationship for the two last cycles and find that the forward time shift in the coronal hole area for the best correlation with sunspot number is almost the same for cycles 21 and 22, and this shift is also the time between peaks in both series. We use this result to make an early prediction of the time and size of the sunspot maximum for solar cycle 23, and find that this should occur early in 2001 and have a magnitude of about 190, similar to that of the two previous sunspot cycles.  相似文献   

A piecewise exponential model for seismic well-logging data     

D. L. Jupp  Ian C. F. Stewart 《Mathematical Geology》1974,6(1):33-45

Conventional methods of analyzing sonic log data do not always yield accurate information on each velocity segment of a well. It is shown here that the velocity-depth parameters and the sections of approximately constant velocity may be more precisely defined by using an exponential spline to model the data.  相似文献   

Non-linear compton and inverse compton effect     

P. Stewart 《Astrophysics and Space Science》1972,18(2):377-386

A charged particle of arbitrary initial motion and position is subjected to an intense circularly polarized electromagnetic wave. The electromagnetic radiation which results is calculated exactly and the results are expressed in terms of the power observed per unit bandwidth per unit solid angle.Several special cases are considered in order to demonstrate the range of possibilities which this type of interaction may have in astrophisics and numerical examples are given for the Crab pulsar.  相似文献   

Casual employment and trade unionism in the UK fishing industry     

N.A. Godman  J.D. Stewart 《Marine Policy》1980,4(3):237-241

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Core 14220 and the lateral continuity of soils at Apollo 14 Station G     

J. Stewart Nagle 《Earth, Moon, and Planets》1980,23(2):165-183

Three soil samples at Apollo 14 Station G could potentially be used for stratigraphic correlation. Recently dissected core 14220, and core 14230 are 4 m apart and form the east leg of a triangle with an apex 7m to the west that contains trench samples 14145, 14156 and 14149, from top to bottom. Core 14220 penetrated approximately as deeply as the trench and showed a similar lithologic succession. At the base of both sections is a basalt-rich soil, overlain by a series of units that contain distinctive clasts of light soil and pebble-sized glass fragments. Fine-grained dark soil rich in vesicular glass is at the surface in both sections. The succession of similar soil types suggests that the trench correlates with 14220; but poor sample recovery makes thickness comparisons uncertain. Core 14230 appears to closely correlate with the middle of core 14220. The second layer from the top of 14220 and the top of 14230 are fine-grained with an unusually high percentage of light-matrix breccia, and the next stratum down in both cores is coarser and rich in light annealed-matrix breccia. The principal lithologic types that do not match, successionally, from core to core, are zones rich in vesicular glass and soil breccia. Such rock types are of regolith origin and probably represent patchy and discontinuous deposits.  相似文献