The Pyrenees: Subduction and collision?     

Gilbert Boillot  Raymond Capdevila 《Earth and Planetary Science Letters》1977,35(1):151-160

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Origin of Ross Island basanitoids and limitations upon the heterogeneity of mantle sources for alkali basalts and nephelinites     

Shine Soon Sun  Gilbert N. Hanson 《Contributions to Mineralogy and Petrology》1975,52(2):77-106

Primary basanitoids from Ross Island, Antarctica have REE patterns and Pb isotope ratios similar to those for primary alkali basalts and nephelinites on ocean islands. The lead data from all volcanics on Ross Island have a spread of 4% in the 206/204 ratio and give a two-stage model lead age of 1500 m.y. The age is interpreted to be the time since the development of the chemical heterogeneity of the mantle source, presumably during an earlier melting process. Comparison of REE, K, Rb, Sr, Ba and P₂O₅ concentrations for alkali basalts and nephelinites shows that the chondrite normalized mantle source is enriched in light REE with average La/Sm=3.4, Ce/Sm=2.6, Nd/Sm=1.6. Assuming a mantle source with heavy REE abundances of three times chondrites, nephelinites require 3 to 7% partial melting of the mantle source and alkali basalts require 7 to 15% partial melting. The patterns of K, Cu, V and Ti abundances suggest that phlogopite is a residual mineral for most nephelinite, but not alkali basalt mantle sources, and that a sulfide phase and a titanium-rich mineral are in the residual mantle source for both alkali basalts and nephelinites. Small positive Eu anomalies (2–5%) in near primary alkali basalts and nephelinites suggest that the xxx of the mantle sources is 10^?6 to 10^?9 atm. The progressive enrichment of light REE and incompatible elements in the mantle sources for nephelinites and alkali basalts is proposed to result by intrusion of veins of basaltic melt due to very low percentages of melting 1 000 to 3 000 m.y. ago when this part of the deeper mantle was previously involved in convection and partial melting.  相似文献   

⁴⁰Ar/³⁹Ar spectrum ages for biotite,hornblende and muscovite in a contact metamorphic zone     

Gilbert N. Hanson  Kathleen R. Simmons  A.E. Bence 《Geochimica et cosmochimica acta》1975,39(9):1269-1278

Biotite, hornblende and muscovite from 2700 m.y. old rocks in northeastern Minnesota near the contact of the 1150 m.y. Duluth Complex have been analyzed by ⁴⁰Ar/³⁹Ar technique to determine whether spectrum ages can be used to distinguish partial loss of radiogenic argon due to a reheating event. Biotite and hornblende give plateau ages comparable to the ordinary K-Ar ages for all samples including those with intermediate ages. Muscovite gives plateau ages for the samples with less than 11% argon loss. An intermediate muscovite with a conventional K-Ar age of 1850 m.y. gives progressively older ⁴⁰Ar/³⁹Ar ages for higher temperature fractions.Microprobe analysis reveals no systematic correlation between biotite chemistry and loss of argon in the contact zone. This suggests that the rate-controlling process for the loss of argon from biotite in the contact zone may be volume diffusion or recrystallization without a measurable change in major element composition. Biotites with intermediate ages give plateaus because the rate-controlling processes in the vacuum furnace are related to dehydroxylation and delamination and are unrelated to the process causing loss of argon in the contact zone.The data for the muscovites are not easily interpreted, in part because of the limited number of samples. The hornblende data show a correlation between argon loss and change in major element composition suggesting that recrystallization may be a rate-controlling process for the loss of argon from hornblende in the contact zone. The small number of samples precludes a definitive statement.  相似文献   

Buying environmental insurance: Prospects for trading of global climate-protection services     

Joel N. Swisher  Gilbert M. Masters 《Climatic change》1991,19(1-2):233-240

In order to alleviate the threat of global climate change, coordinated international action is needed. This cooperation should include multilateral agreements and new economic initiatives to help implement measures that will slow the accumulation of carbon dioxide in the Earth's atmosphere due to tropical deforestation and fossil-fuel use. An international market in environmental services can help to safeguard the Earth's climate and foster economic development through a North-South transfer of financial resources.We suggest international carbon-emission offsets (ICEOs) as a means by which international markets, under a policy umbrella such as a multilateral climate-protection treaty, could trade carbon-saving services. Such a market would provide a currency for rewarding actions that reduce global carbon emissions, allowing carbon emitters to seek the least expensive ways to reduce emissions. This currency would transfer cash and/or debt relief from industrialized nations to developing nations, allowing the developing nations to profit from the use of clean energy technologies and the protection, rather than depletion, of tropical forests.  相似文献   

Are topographic data sets fractal?     

Lewis E. Gilbert 《Pure and Applied Geophysics》1989,131(1-2):241-254

The scale invariant properties of fractal sets make them attractive models for topographic profiles because those profiles are the end product of a complex system of physical processes operating over many spatial scales. If topographic data sets are fractal, their power spectra will be well represented by lines in log-log space with slopess such that –3s<–1. The power spectra from a Digital Elevation Model (30 meter sample spacing) of the Sierra Nevada Batholith and from Seabeam center beam depths (425 meter sample spacing) along a flowline in the South Atlantic are curved. Straight sections in the spectra can be identified but the slopes of those sections are strongly dependent upon the particulars of the data analysis. Fractal geometry must be used with caution in the discussion of topographic data sets.  相似文献   

Respiration rates and hypoxia on the Louisiana shelf     

Quay Dortch  Nancy N. Rabalais  R. Eugene Turner  Gilbert T. Rowe 《Estuaries and Coasts》1994,17(4):862-872

The spatial and temporal variation in water-column respiration, estimated from enzymatic respiratory electron-transport-system activity, was measured monthly on a cross-shelf transect on the Louisiana shelf from May through October 1991. In July 1991, water-column respiration was also determined on an alongshore transect, and in situ benthic respiration and photosynthesis rates were determined at jour stations on the cross-shelf transect. Bottom waters were persistently hypoxic (O₂<2 mg 1^?1) at most stations in July and August and sporadically hypoxic at other times. Water-column respiration rates were in the same range as earlier, less extensive studies and not unusually high for coastal and estuarine waters. They were highest in summer, decreased with distance offshore and depth, and increased with temperature. Their variation with pigment and oxygen concentrations were complex functions of season and depth. Oxygen depletion below the oxycline could occur within days to months, depending on the season and location. In July, benthic respiration rates were also not unusually high in comparison with other shallow sediments, although the ratio of benthic: total (water column+benthic) respiration was high. Combined water-column and benthic respiration could deplete the bottom water oxygen in approximately 1 mo. Because the system rarely goes anoxic (defined as observing sulfide), some mechanism(s) must exist to reaerate bottom waters. Most physical mechanisms are unlikely to provide significant reaeration at this time of year. Measured benthic and conservatively estimated bottom-water photosynthesis could resupply 23% of the oxygen lost daily by respiration. Although this is too limited a dataset from which to draw conclusions about the relative importance of bottom-water and benthic respiration and photosynthesis in determining bottom-water oxygen concentrations, it does suggest that all these processes must be considered.  相似文献   

Application of sedimentary and chronological analyses to refine the depositional context of a Late Pleistocene vertebrate deposit,Naracoorte, South Australia     

Amy C. Macken  Nathan R. Jankowski  Gilbert J. Price  Erick A. Bestland  Elizabeth H. Reed  Gavin J. Prideaux  Richard G. Roberts 《Quaternary Science Reviews》2011,30(19-20):2690-2702

Cave deposits of infill sediments and associated vertebrate fossils provide a valuable source of information on terrestrial palaeoenvironments, climatic conditions and palaeocommunities. In the deposits of the Naracoorte Caves World Heritage Area, such records span the last 500 ka and are renowned for their rich, diverse vertebrate assemblages. Previous research into the Grant Hall deposit of Victoria Fossil Cave suggested that it may preserve the only peak last interglacial (ca. 125 ka) faunal community within the World Heritage Area. The current work tested this existing model for the age of faunal remains from Grant Hall using multiple techniques. Physical and geochemical properties of the visually homogeneous sediments were analysed at regular intervals through the sequence to establish meaningful stratigraphic divisions and sediment provenance. Optically stimulated luminescence dating of individual quartz grains indicates that sediments accumulated in Grant Hall from 93 ± 8 to 70 ± 5 ka. Minimum ages provided by U/Th dating of fossil teeth (72.3 ± 2.2 to 38.2 ± 0.8 ka) are consistent with the luminescence chronology, and show that the deposit represents a more recent faunal accumulation than previously modelled for the site. U/Th ages on calcite straws within the deposit are significantly older than the sediments and fossil teeth (>500 to 186.4 ± 1 ka). As such they provide no further constraint on the chronology of the deposit but do indicate that speleothem deposition was active over much of the Middle Pleistocene. Sedimentary analyses resulted in the identification of five depositional units, contrasting with previous divisions which were based only on visual observation of the sedimentary sequence. Sediments within each unit are broadly classified as sandy silts with soil structures and may be indirectly derived from the lunettes of nearby Bool Lagoon, although their ultimate provenance is unknown. As a result of this work, palaeoenvironmental reconstruction based on fossil remains in the deposit may be more accurately related to prevailing climatic and environmental conditions at the time of accumulation. It also contributes to an understanding of the temporal occurrence of regional vertebrate faunas through the Late Pleistocene, reinforcing the value of developing stratigraphically constrained chronologies for cave deposits based on multiple techniques.  相似文献   

A relative water-depth model for the Normandy Chalk (Cenomanian–Middle Coniacian,Paris Basin,France) based on facies patterns of metre-scale cycles     

Eric Lasseur  François Guillocheau  Cécile Robin  Franck Hanot  Denis Vaslet  Renaud Coueffe  Didier Neraudeau 《Sedimentary Geology》2009,213(1-2):1-26

A relative water-depth model for the Chalk of the Paris Basin is proposed, based on the lateral variations of the high-frequency metre-scale cycles, which are characteristic features easily identified in the field. The studied outcrops are the Cenomanian–Middle Coniacian cliffs of Normandy. The main result of this study is to highlight the importance of storm activity in the deposition of the Chalk. The relative water-depth model is based on storm-induced shell concentrations observed within the two components of the metre-thick cycles: the depositional interval itself and the top hiatal surface.Six types of shell concentrations are defined, along with seven types of depositional facies making up the depositional units, as well as eight types of hiatal surface. Three cycle associations, differing in their thickness and the amount and type of non-carbonate constituents, can be identified in the Lower to Upper Cenomanian, the Upper Cenomanian to Lower Turonian and the Middle Turonian to Middle Coniacian.A relative water-depth profile model for all these cycles is based on the shell concentrations and a “water-depth equivalence” is proposed between the three cycle associations (lateral “facies” substitution diagram). This model is tested using palaeocological data (irregular echinoids) and by correlating field sections in terms of stacking patterns. Most of the studied deposits accumulated above the storm wave base (upper offshore zone or mid ramp).  相似文献   

Wavelet Analysis and Filtering to Identify Dominant Orientations of Permeability Anisotropy     

Loring Watkins  Roseanna M. Neupauer  Gilbert P. Compo 《Mathematical Geosciences》2009,41(6):643-659

An accurate representation of permeability anisotropy is needed to model the rate and direction of groundwater flow correctly. We develop a wavelet analysis technique that can be used to characterize principal directions of anisotropy in both stationary and non-stationary permeability fields. Wavelet analysis involves the integral transform of a field using a wavelet as a kernel. The wavelet is shifted, scaled, and rotated to analyze different locations, sizes, and orientations of the field. The wavelet variance is used to identify scales and orientations that dominate the field. If the field is non-stationary, such that different zones of the field are characterized by different dominant scales or orientations, the wavelet variance can identify all dominant scales and orientations if they are distinct. If the dominant scales and orientations of different zones are similar, the wavelet variance identifies only the dominant scale and orientation of the primary zone. In this paper, we present a combined wavelet analysis and filtering approach to identify all dominant scales and orientations in a non-stationary permeability field. We apply the method to permeability data obtained in the laboratory from the Massillon sandstone.  相似文献   

New U/Th ages for Pleistocene megafauna deposits of southeastern Queensland,Australia     

Gilbert J. Price  Jian-xin Zhao  Yue-xing Feng  Scott A. Hocknull 《Journal of Asian Earth Sciences》2009,34(2):190-197

Arguments over the extinction of Pleistocene megafauna have become particularly polarised in recent years. Causes for the extinctions are widely debated with climate change, human hunting and/or habitat modification, or a combination of those factors, being the dominant hypotheses. However, a lack of a spatially constrained chronology for many megafauna renders most hypotheses difficult to test. Here, we present several new U/Th dates for a series of previously undated, megafauna-bearing localities from southeastern Queensland, Australia. The sites were previously used to argue for or against various megafauna extinction hypotheses, and are the type localities for two now-extinct Pleistocene marsupials (including the giant koala, Phascolarctos stirtoni). The new dating allows the deposits to be placed in a spatially- and temporally constrained context relevant to the understanding of Australian megafaunal extinctions. The results indicate that The Joint (Texas Caves) megafaunal assemblage is middle Pleistocene or older (>292 ky); the Cement Mills (Gore) megafaunal assemblage is late Pleistocene or older (>53 ky); and the Russenden Cave Bone Chamber (Texas Caves) megafaunal assemblage is late Pleistocene (～55 ky). Importantly, the new results broadly show that the sites date prior to the hypothesised megafaunal extinction ‘window’ (i.e., ～30–50 ky), and therefore, cannot be used to argue exclusively for or against human/climate change extinction models, without first exploring their palaeoecological significance on wider temporal and spatial scales.  相似文献