Short polarity intervals within the Matuyama: transitional field records from hydraulic piston cored sediments from the North Atlantic     

Brad M. Clement  Dennis V. Kent 《Earth and Planetary Science Letters》1987,81(2-3)

Detailed sampling of two short magnetozones within the Matuyama Chronozone recorded at DSDP Site 609 (49.86°N, 335.77°E) confirms that one, the Cobb Mountain Subchronozone (1.12 Ma), is a very short, full normal polarity interval and that the other, the older interval, is a record of a geomagnetic excursion which occurred at approximately 1.55 Ma. The Cobb Mountain Subchron lasted approximately 25,000 years, one third the duration of the Jaramillo Subchron. The normal polarity interval is bounded by two transition zones which document an antisymmetry in the sequence of directions in the reverse to normal and normal to reverse polarity transitions. We interpret the antisymmetry as reflecting a dependence upon the sense of the reversal, without significant changes in the relative contributions of non-dipole terms. The polarity interval recorded at 1.55 Ma lasted only 8,800 years with what may be regarded as full polarity directions observed across only 3 cm of stratigraphic section. This feature is interpreted as an excursion of the geomagnetic field and appears to be correlative with the Gilsa Subchron. Similarities between the transition bounding these two magnetozones suggest that these features occur as the result of the same process or triggering mechanisms in the earth's outer core.  相似文献   

Measurements of Trace Gases in the Inflow of South China Sea Background Air and Outflow of Regional Pollution at Tai O, Southern China     

T. Wang  H. Guo  D. R. Blake  Y. H. Kwok  I. J. Simpson  Y. S. Li 《Journal of Atmospheric Chemistry》2005,52(3):295-317

We present a 16-month record of ozone (O₃), carbon monoxide (CO), total reactive nitrogen (NO_y), sulphur dioxide (SO₂), methane (CH₄), C₂ – C₈ non-methane hydrocarbons (NMHCs), C₁ – C₂ halocarbons, and dimethyl sulfide (DMS) measured at a southern China coastal site. The study aimed to establish/update seasonal profiles of chemically active trace gases and pollution tracers in subtropical Asia and to characterize the composition of the `background' atmosphere over the South China Sea (SCS) and of pollution outflow from the industrialized Pearl River Delta (PRD) region and southern China. Most of the measured trace gases of anthropogenic origin exhibited a winter maximum and a summer minimum, while O₃ showed a maximum in autumn which is in contrast to the seasonal behavior of O₃ in rural eastern China and in many mid-latitude remote locations in the western Pacific. The data were segregated into two groups representing the SCS background air and the outflow of regional continental pollution (PRD plus southern China), based on CO mixing ratios and meteorological conditions. NMHCs and halocarbon data were further analyzed to examine the relationships between their variability and atmospheric lifetime and to elucidate the extent of atmospheric processing in the sampled air parcels. The trace gas variability (S) versus lifetime (τ) relationship, defined by the power law, S_lnx = Aτ^{− b}, (where X is the trace gas mixing ratio) gives a fit parameter A of 1.39 and exponent b of 0.42 for SCS air, and A of 2.86 and b of 0.31 for the regional continental air masses. An examination of ln[n-butane]/ln[ethane] versus ln[propane]/ln[ethane] indicates that their relative abundance was dominated by mixing as opposed to photochemistry in both SCS and regional outflow air masses. The very low ratios of ethyne/CO, propane/ethane and toluene/benzene suggest that the SCS air mass has undergone intense atmospheric processing since these gases were released into the atmosphere. Compared to the results from other polluted rural sites and from urban areas, the large values of these species in the outflow of PRD/southern China suggest source(s) emitting higher levels of ethyne, benzene, and toluene, relative to light alkanes. These chemical characteristics could be unique indicators of anthropogenic emissions from southern China.  相似文献   

Reducing model complexity for explanation and prediction   总被引：1，自引：1，他引：1  

A. Brad Murray   《Geomorphology》2007,90(3-4):178

Numerical models can be useful for explaining poorly understood phenomena or for reliable quantitative predictions. When modeling a multi-scale system, a ‘top-down’ approach—basing models on emergent variables and interactions, rather than explicitly on the much faster and smaller scale processes that give rise to them—facilitates both goals. Parameterizations representing emergent interactions range from highly simplified and abstracted to more quantitatively accurate. Empirically based large-scale parameterizations lead more reliably to accurate large-scale behavior than do parameterizations of much smaller scale processes. Conversely, purposefully simplified representations of model interactions can enhance a model's utility for explanation, clarifying the key feedbacks leading to an enigmatic behavior. For such potential insights to be relevant, the interactions in the model need to correspond to those in the ‘real’ system in some straightforward way. Such a correspondence usually holds for models constructed for predictive purposes, although this is not a requirement. The goals motivating a modeling endeavor help determine the most appropriate modeling strategies, as well as the most appropriate criteria for judging model usefulness.  相似文献   

Energetic Compound False Positives in Ground Water Profile Samples     

Jay Clausen  Elizabeth Wessling  Brad Chirgwin 《Ground Water Monitoring & Remediation》2007,27(3):90-101

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Why the Sacramento Delta area differs from other parts of the great valley: Numerical modeling of thermal structure and thermal subsidence of forearc basins     

V. O. Mikhailov  T. Parsons  R. W. Simpson  E. P. Timoshkina  C. Williams 《Izvestiya Physics of the Solid Earth》2007,43(1):75-90

Data on present-day heat flow, subsidence history, and paleotemperature for the Sacramento Delta region, California, have been employed to constrain a numerical model of tectonic subsidence and thermal evolution of forearc basins. The model assumes an oceanic basement with an initial thermal profile dependent on its age subjected to refrigeration caused by a subducting slab. Subsidence in the Sacramento Delta region appears to be close to that expected for a forearc basin underlain by normal oceanic lithosphere of age 150 Ma, demonstrating that effects from both the initial thermal profile and the subduction process are necessary and sufficient. Subsidence at the eastern and northern borders of the Sacramento Valley is considerably less, approximating subsidence expected from the dynamics of the subduction zone alone. These results, together with other geophysical data, show that Sacramento Delta lithosphere, being thinner and having undergone deeper subsidence, must differ from lithosphere of the transitional type under other parts of the Sacramento Valley. Thermal modeling allows evaluation of the rheological properties of the lithosphere. Strength diagrams based on our thermal model show that, even under relatively slow deformation (10⁻¹⁷ s⁻¹), the upper part of the delta crystalline crust (down to 20–22 km) can fail in brittle fashion, which is in agreement with deeper earthquake occurrence. Hypocentral depths of earthquakes under the Sacramento Delta region extend to nearly 20 km, whereas, in the Coast Ranges to the west, depths are typically less than 12–15 km. The greater width of the seismogenic zone in this area raises the possibility that, for fault segments of comparable length, earthquakes of somewhat greater magnitude might occur than in the Coast Ranges to the west. The text was submitted by the authors in English.  相似文献   

Seismotectonic deformation during the filling of Toktogul Reservoir,Kirghizia     

S. Yunga  D. Simpson  A. Kondratenko 《Pure and Applied Geophysics》1996,147(2):419-431

Natural seismicity and induced seismic events are discussed in the context of the seismotectonic deformation method through the example of earthquakes in the Toktogul Reservoir region in the Central Tien Shan, Kirghizia. The parameters of seismotectonic deformation of various sites in the Toktogul Reservoir region are described, based on a statistical study of focal mechanisms. The relationship of induced seismicity to changes of water level in the reservoir is reviewed. The temporal stress-strain characteristics are investigated. Average focal mechanisms for the entire region, as well as areas in the immediate vicinity of Toktogul Dam are analyzed. The vertical component of seismotectonic deformation varies in time from compression to extension-opposite to what is expected from the influence of the reservoir load; strike-slip motions become oblique thrusts. Changes in the orientation of focal mechanisms coincide with the time of maximum rate of the filling of the reservoir.  相似文献   

Editor’s preface     

Eugene S. Simpson 《Hydrogeology Journal》1992,1(1):1-1

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The tidal intrusion front: An estuarine convergence zone     

J.H. Simpson  R.A. Nunes 《Estuarine, Coastal and Shelf Science》1981,13(3):257-IN1

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Defending community? Indigeneity, self-determination and institutional ambivalence in the restoration of Lake Whakaki     

Brad Coombes 《Geoforum》2007,38(1):60-72

Conservation practitioners have scrutinized the credibility and effectiveness of community-based natural resource management, noting its romantic misconceptions about communities and their capacities. Early approaches failed to acknowledge the heterogeneity of collective agents, the synergy between decentralization and neoliberalism, or the need to affirm rural peoples’ entitlements to resources. A Maori community’s attempt to restore Lake Whakaki on New Zealand’s east coast confirms many of these critiques. The restoration confronts institutional ambivalence, obstructive forces from beyond the zone of Maori influence and non-correspondence between community and catchment dynamics. Fulfilment of the project requires exogenous resources and authority, but state conservation agencies are ambivalent towards local demands for self-determined development. Nonetheless, an uncommon degree of agency which is grounded within community aspirations for sovereignty suggests that the motivational characteristics of community retain their importance in debates about integrated conservation and development.  相似文献   

Controls on spatial and temporal distribution of Precambrian eolianites     

Kenneth A. Eriksson  Edward L. Simpson 《Sedimentary Geology》1998,120(1-4):275-294

Inversely graded stratification, generated by the migration of wind ripples, and adhesion structures permit unequivocal identification of Precambrian eolianites. These criteria, in combination with scale of cross-beds, angle of inclination of foresets, geometry of depositional units, and associated non-eolian facies, are used to discriminate between Precambrian dune/draa, dune-plinth, sand-sheet, and interdune deposits that formed in inland and coastal settings. Based on an analysis of published literature, fundamental conclusions can be drawn on the spatial and temporal distribution of Precambrian eolianites. The oldest reported eolianites are from the ca. 2.1 Ga Deweras Group in Zimbabwe and Hurwitz Group in Canada and numerous examples of eolianites are reported from the 1.8 Ga and younger rock record. Lack of Archean and early Paleoproterozoic eolianites and their widespread development after 1.8 Ga are examined with respect to: absence of vegetation, crustal growth and tectonic setting, relative sea-level fluctuations, unfavorable atmospheric and/or climatic change, and non-recognition. The lack of pre-2.2 Ga eolianites may be related to reworking by braided rivers combing across non-vegetated floodplains, reworking of coastal eolianites during transgression or their non-recognition in the Early Precambrian record. The temporal concentration of eolianites at 1.8 Ga may best be related to the early stages of breakup and the assembly phases of supercontinents.  相似文献