Determination of Elemental Concentrations in Nine CMEA Rock and Ore Standards,in NBS 98 Plastic Clay and in Fired Clay Gent (FCG) by Instrumental Neutron Activation Analysis*

Data for as many as 31 elements were determined by instrumental thermal neutron activation analysis for nine European Council for Mutual Economic Assistance (CMEA) rock and ore standards. The National Bureau of Standards plastic clay 98 and the University of Gent fired clay FCG were also analyzed. Synthetic, multielement standards were used and USGS rock standards provided reference samples. Correction factors for uranium fission products on cerium and molybdenum, and also for less commonly encountered spectral interferences, such as those due to the 213 ppm tantalum in granitoid 2B, were evaluated.     

Age and radiogenic isotopic composition of a late- to post-tectonic anorthosite in the Grenville Province: the Labrieville massif, Quebec

The Labrieville anorthosite massif (LBV) is found in the Central Granulite Terrain of the Grenville Structural Province, but it displays no evidence of post-emplacement deformation or metamorphism, implying intrusion following peak Grenvillian metamorphic conditions. We report U---Pb zircon dates of 1008±3.4 Ma for border leucogabbro and 1010±5.6 Ma for a cogenetic jotunite dike intruding anorthosite. We interpret these dates as igneous crystallization ages, and regard 1010 Ma as a reasonable estimate of the emplacement age for LBV. LBV is thus the youngest massif anorthosite yet recognized in North America, and its age is consistent with late-tectonic emplacement relative to the 1.1-1.0 Ga Grenville Orogeny. We also report a U---Pb date of 1015±1.8 Ma for metamorphic zircon in a country rock amphibolite. This could reflect the age of Grenvillian regional metamorphism, or perhaps a later heating episode resulting from the intrusion of numerous “late” felsic plutons in this area.

Rb---Sr, Sm---Nd and U---Th---Pb isotopic compositions for four rock types (anorthosite, jotunite, leucogabbro and a plagioclase megacryst) span narrow ranges in each case, consistent with comagmatism among these units. I_Sr (T=1010 Ma) range from 0.7032–0.7034 and are among the lowest yet reported for anorthosite in the Grenville Province. Initial ε_Nd-values are positive (+0.8 to +2.5), like other Grenville anorthosites. Pb-isotopic compositions lie near the model mantle evolution curve of Zartman and Doe (1981), implying no involvement of significantly older crust in the petrogenesis of these rocks. Collectively, these data suggest a source for LBV in the mantle or mafic lower crust. LBV is a compositionally extreme anorthosite characterized by alkalic plagioclase (An₃₂Or₁₂) and high levels of Sr (2000 ppm) and Ba (1000 ppm). These properties cannot be attributed to simple crustal contamination of mantle-derived basalt. We suggest, alternatively, that LBV's compositional features may be linked with its late-tectonic character, perhaps reflecting partial melting of mafic lower crust brought about by crustal thickening during the Grenville Orogeny.     

Variability in estimates of oil contamination in the intertidal zone of a gravel beach

Estimates of the surface cover of stranded oil on a gravel beach ranged between 25 and 51%. This variability is attributed primarily to wet weather conditions that altered the colour of the surface substrate, thus making identification of oil or oiled sediments difficult. Estimates and the observations from three surveys in wet weather ranged between 25 and 46%; whereas on a dry day the range from four surveys was only 42–51%. Within this small study area (7200 m²) the same observer provided estimates of the surface oil cover that ranged from 1800 to 3680 m². This wide variability for a small section of shoreline would probably be magnified during reconnaissance surveys where long sections of coast are surveyed to determine the degree of contamination. Surveys of beach contamination for cleanup decisions or for damage assessment should be conducted with regard to the environmental (weather) conditions and the degree of accuracy that is required from the survey.     

Towards the detection and attribution of an anthropogenic effect on climate

It has been hypothesized recently that regional-scale cooling caused by anthropogenic sulfate aerosols may be partially obscuring a warming signal associated with changes in greenhouse gas concentrations. Here we use results from model experiments in which sulfate and carbon dioxide have been varied individually and in combination in order to test this hypothesis. We use centered [R (t)] and uncentered [C (t)] pattern similarity statistics to compare observed time-evolving surface temperature change patterns with the model-predicted equilibrium signal patterns. We show that in most cases, the C (t) statistic reduces to a measure of observed global-mean temperature changes, and is of limited use in attributing observed climate changes to a specific causal mechanism. We therefore focus on R (t), which is a more useful statistic for discriminating between forcing mechanisms with different pattern signatures but similar rates of global mean change. Our results indicate that over the last 50 years, the summer (JJA) and fall (SON) observed patterns of near-surface temperature change show increasing similarity to the model-simulated response to combined sulfate aerosol/CO₂ forcing. At least some of this increasing spatial congruence occurs in areas where the real world has cooled. To assess the significance of the most recent trends in R (t) and C (t), we use data from multi-century control integrations performed with two different coupled atmosphere-ocean models, which provide information on the statistical behavior of 'unforced' trends in the pattern correlation statistics. For the combined sulfate aerosol/CO₂ experiment, the 50-year R (t) trends for the JJA and SON signals are highly significant. Results are robust in that they do not depend on the choice of control run used to estimate natural variability noise properties. The R (t) trends for the CO₂-only signal are not significant in any season. C (t) trends for signals from both the CO₂-only and combined forcing experiments are highly significant in all seasons and for all trend lengths (except for trends over the last 10 years), indicating large global-mean changes relative to the two natural variability estimates used here. The caveats regarding the signals and natural variability noise which form the basis of this study are numerous. Nevertheless, we have provided first evidence that both the largest-scale (global-mean) and smaller-scale (spatial anomalies about the global mean) components of a combined CO₂/anthropogenic sulfate aerosol signal are identifiable in the observed near-surface air temperature data. If the coupled-model noise estimates used here are realistic, we can be highly confident that the anthropogenic signal that we have identified is distinctly different from internally generated natural variability noise. The fact that we have been able to detect the detailed spatial signature in response to combined CO₂ and sulfate aerosol forcing, but not in response to CO₂ forcing alone, suggests that some of the regional-scale background noise (against which we were trying to detect a CO₂-only signal) is in fact part of the signal of a sulfate aerosol effect on climate. The large effect of sulfate aerosols found in this study demonstrates the importance of their inclusion in experiments designed to simulate past and future climate change. Received: 10 November 1994 / Accepted: 19 July 1995     

Comparison of amino acid racemization geochronometry with lithostratigraphy, biostratigraphy, uranium-series coral dating, and magnetostratigraphy in the Atlantic Coastal Plain of the southeastern United States

The results of an integrated study comprising litho- and biostratigraphic investigations, uranium-series coral dating, amino acid racemization in molluscs, and paleomagnetic measurements are compared to ascertain relative and absolute ages of Pleistocene deposits of the Atlantic Coastal Plain in North and South Carolina. Four depositional events are inferred for South Carolina and two for North Carolina by all methods. The data suggest that there are four Pleistocene units containing corals that have been dated at about 100,000 yr, 200,000 yr, 450,000 yr, and over 1,000,000 yr. Some conflicts exist between the different methods regarding the correlation of the younger of these depositional events between Charleston and Myrtle Beach. Lack of good uranium-series dates for the younger material at Myrtle Beach makes the correlation with the deposits at Charleston more difficult.     

Normal faulting and flexure in an elastic-perfectly plastic plate

An elastic-perfectly plastic plate model has been developed to analyze the flexure associated with normal faulting. The model consists of a thin layer, which is completely cut by a normal fault, overlying a fluid substratum. For a given applied bending moment at the fault, the relationship between the amount of displacement on the fault and the extent of the failure zone can be calculated. The model is applied to the Wasatch Front region in the eastern Basin and Range Province, USA to determine the correlation of its parameters with geological and geophysical data in the vicinity of a major normal fault, the Wasatch fault, along which there has been 3–4 km of Late Cenozoic uplift. In this region, most seismic activity occurs away from the Wasatch fault in a zone 30 km wide, roughly centered 30 km east of the fault. This activity occurs at depths of 15 km or less. In order to match the observations, the lithospheric layer must have a flexural rigidity of 0.5 to 1.1 · 10²² n-m and a yield stress of 1–2 kb and must have zero applied bending moment at the fault. The effective mechanical thickness of the lithosphere in this region is 20–25 km. These results indicate that the lithosphere in long-term mechanical studies in the eastern Basin and Range is thin and weak. Evaluating these results as compared to the seismic lithospheric thickness and temperature regime of the region produces some interesting correlations with studies in oceanic regions.     

Morphometric Controls and Basin Response in The Cascade Mountains

Morphometric variables associated with 36 debris torrent, 78 snow avalanche, 45 composite debris torrent and snow avalanche and 14 streamflow basins in the Cascade Mountains of southwestern British Columbia, Canada are examined. The results show significant statistical differences in top and bottom elevations, relief, channel length and gradient, basin area, fan gradient and area, and basin ruggedness between snow avalanche basins and the two basin types affected by debris torrents, reflecting the very different nature of these processes. Only top and bottom elevations and fan area differ significantly between debris torrent and debris torrent-snow avalanche basins, implying that the latter are probably debris torrent basins in origin. As many as six morphometric variables are significantly different between streamflow basins and the other basin types, allowing the former to be differentiated despite their small, steep character. Discriminant analysis indicates that bottom elevation and channel or path gradient are the best variables for classifying the four basin types by process. Generally strong correlations exist between basin area on the one hand and relief, channel length and channel gradient on the other in debris torrent, debris torrent-snow avalanche, and streamflow basins. Fan gradient and area are, however, weakly or modestly correlated with basin area or ruggedness. No such morphometric relations are present in snow avalanche basins. The results of this study also indicate that in debris torrent-prone basins the fan gradient and Melton's R have identifiable lower thresholds while basin area has an upper threshold, but use of these thresholds for identification of debris torrent hazard is complicated by overlapping thresholds for streamflow basins.     

Chemical assembly of Archean anorthosites from amphibolite- and granulite-facies terranes, SW Greenland

We report whole-rock, major- and trace-element compositions (obtained by XRF and INA methods) for the amphibolite-facies Buksefjorden and granulite-facies Nordland anorthosites, SW Greenland. In a previous petrologic study on the same sample suite, we documented differences in texture, mineralogy, and mineral compositions between these two anorthosite bodies. Chemical analyses confirm differences in composition between the two bodies, but these differences cannot be explained by variations in metamorphic conditions, and point towards differences in the nature of their protoliths. Analyzed Nordland samples are anorthosites and leucogabbros with 88-98% normative plagioclase, whereas those from Buksefjorden include anorthosite, leucogabbro, and gabbro with ~55-95% normative plagioclase. Two or more compositional groupings can be recognized at each site, which correspond to differences in color and mineralogy of the hand samples. Samples from Buksefjorden are mainly quartz-normative, whereas those from Nordland are olivine (- nepheline) normative. Other differences include higher Ni/Co ratio and REE contents in the granulite-facies anorthosites from Nordland. REE pattern shapes are similar, however, being moderately fractionated at ~0.5-102 chondrites with positive Eu-anomalies. Calculated equilibrium melt patterns are similar for both anorthosites, being relatively flat at ~50-1502 chondrites, suggesting unfractionated (but evolved) parental magmas. Olivine must have been present in the protoliths of the Nordland rocks compared with Buksefjorden. Otherwise, the protoliths contained plagioclase with variable An-content (~An₆₂-An₉₂) and a mafic component with variable Fe/Mg (mg ~0.3-0.8). This mafic component was either hornblende or a combination of ortho- and clinopyroxene in fixed proportions, plus a small amount of magnetite. Mixing calculations demonstrate that some Buksefjorden anorthosites contain two varieties of plagioclase: a calcic type that may correspond to cumulus crystals, and a sodic-type that may correspond to a trapped-melt component. On plots of normative whole-rock An versus mg, compositions of the Buksefjorden and Nordland anorthosites form crude negative arrays that differ from the generally positive trends of mafic layered intrusions (Kiglapait, Skaergaard) and from the generally flats trends of plagioclase-rich cumulate rocks (St. Urbain and Stillwater anorthosites). This difference provides further evidence for the distinctive nature of Archean calcic-anorthosite complexes compared with other types of mafic intrusions. Moreover, this distribution of data points is consistent with the assembly of the protolith of the SW Greenland anorthosites mainly as mixtures of plagioclase and hornblende. Finally, the field for the Buksefjorden and Nordland anorthosites overlaps only slightly with that for the Fiskenaesset Complex, thus extending the known range of compositions for Archean anorthosites in West Greenland.