A statistical method to evaluate homogeneity of structural populations

For any specific rock unit in a discontinuous rock mass, the determination of structural domain boundaries is important because geologic and hydrologic properties vary from one domain to another. A domain can be considered as a structural population characterized by a distinct pattern of structure orientations (i.e., fracture, bedding, or foliation orientations).Boundaries are usually determined by comparing samples of structural data, each of which consists of a Schmidt plot that commonly displays at least 150 poles to fractures. When fracture orientations appear dispersed and random on the plots, visual comparisons are not sufficient to determine whether the samples were obtained from the same structural domain. An appropriate x ² test has been adapted to this situation and is capable of evaluating the homogeneity of structural populations. The method uses a contingency table analysis based on the frequencies of fracture poles that occur in corresponding patches on the Schmidt plots being compared.     

Continuous zircon growth during long-lived granulite facies metamorphism: a microtextural,U–Pb,Lu–Hf and trace element study of Caledonian rocks from the Arctic

Microtextural, U–Pb, trace element and Lu–Hf analyses of zircons from gneisses dredged from the Chukchi Borderland indicate a long-lived, Cambrian–Ordovician, granulite facies metamorphism. These results reveal a complete prograde, peak and cooling history of zircon growth during anatexis. Early increasing temperatures caused modification and Pb-loss of Precambrian zircons by recrystallization and dissolution/re-precipitation of existing grains. Small variations in initial ¹⁷⁶Hf/¹⁷⁷Hf results (0.282325–0.282042) and flat HREE patterns of these zircons indicate that they grew by dissolution/re-precipitation in the presence of garnet. Zircons subsequently crystallized from a partial melt during peak to post-peak metamorphism from 530 to 485 Ma. A broad range of initial ¹⁷⁶Hf/¹⁷⁷Hf ratios (0.282693–0.282050) and mineral inclusions within zircons suggest that this phase of growth incorporated Zr and Hf obtained from the breakdown of Zr-enriched phases. Microtextural evidence along with trace element and isotopic data suggests that final growth of metamorphic rims on zircon occurred during slow cooling and crystallization of residual partial melts during the early Ordovician (485–470 Ma). Younger, late Ordovician–Silurian (420–450 Ma) euhedral, oscillatory-zoned, trace element-enriched zircons crystallized within leucocratic veins that intrude the gneisses. Their age corresponds to granitoids dated from this same dredge. The intrusives and veins provide evidence that the Chukchi Borderland rifted from a position near Pearya and northwest Svalbard, which represent the northern continuation of the Caledonian orogen. Evidence for earlier Cambrian metamorphism has not been reported from this region. The age of granulite facies metamorphism reported here represents the earliest phase of deformation in the Arctic Caledonides.     

Holocene environmental changes in the Whitefish Dunes Area, Door Peninsula, Northern Lake Michigan Basin, USA

A two meter sediment core taken from an inter-dune pond in Whitefish Dunes State Park, Door Peninsula, Wisconsin, provides a record of paleoenvironmental changes in the area from approximately 3600 RCYBP to 5100 RCYBP. The hydrology of the Whitefish Dunes pond is now apparently dependent on ground water recharge from local precipitation that infiltrates into the dunes and from Clark Lane, making it very sensitive to low magnitude, short term climate fluctuations.Changes in lithology, molluscan species diversity, oxygen isotopes from gastropod shells, and sediment organic content permit division of the core into several intervals representing different environmental regimes. The basal sediments are older than 5130 RCYBP and resemble those of the Liberty Grove Member of the Pleistocene Horicon Formation. The oldest Holocene sediments recorded in the core are an organic rich layer dated at 5130 RCYBP, which accumulated in a depression on the surface of a sand bar. Continued rise in the local water table during the Nipissing Transgression produced paludal conditions in the area, which were followed by the onset of lacustrine conditions. During this interval, the area supported a diverse molluscan fauna. Minor, short term climate fluctuations are reflected in sediment and oxygen isotope changes which are absent from the record in deep water cores from Lake Michigan. This phase in the history of the area ended with the eutrophication of the pond and the development of dunes. Information from Whitefish Dunes pond and other onshore sites within the Lake Michigan basin are an important source of data for reconstructing the history of water levels and climate changes for this area of the Great Lakes drainage.     

Climate and economic competitiveness: Florida freezes and the global citrus processing industry

Summary Casual observers of the impacts associated with four recent freezes in Florida's citrus producing areas might be inclined to agree with an assessment by Miami Herald reporters that these freezes had caused the king of citrus to be toppled from its throne, enabling Brazil to take its place. Research on the citrus industry, however, reveals that the impacts of these recent freezes only explain part of the story of the interaction between climate variability and the relationship between the citrus industries of Florida and Brazil. Climate characteristics and their variability have directly as well as indirectly affected the economic competitiveness of citrus producers whose output is in large measure climate-dependent. Climate variability has had direct impacts on Florida's citrus industry by adversely affecting the productivity of citrus groves in some areas, by altering growers' perceptions of freeze probabilities and, occasionally, by suddenly reducing output, thus elevating the price that consumers must pay for that commodity. Indirectly, competition can be affected by climate as a potential producer identifies a weakness in the supply system of an existing industry and seeks to fill the gap.Brazil's involvement in the toppling of King Citrus began not in the early 1980s (as a result of the four freezes in the past six years), but in 1962 as a result of a major freeze in that year, one that sharply increased FCOJ prices by reducing Florida's output. It was then that the climate had an impact on the economic competitiveness of the citrus industry. The records document the steady, almost meteoric, rise in Brazilian FCOJ production and exports. Subsequent freezes only served to abet a process that had been well underway two decades before the recent devastating freezes.As for Florida's ability to continue and perhaps expand its key role in the global citrus economy, the recent freezes do not appear to have fatally damaged that. Rather, those freezes have reawakened Florida's citrus producers to the fact that they are involved in a climate-sensitive industry and have reminded them that the potential for freeze-related problems is never far away. That reawakening has sparked interest in developing hardier citrus varieties, more effective freeze protection methods, and better ways to hedge economically against freeze impacts to the industry.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Copper(II) carbonate complexation in seawater

Cupric carbonate and cupric bicarbonate complexation constants were determined in natural seawater and in a variety of synthetic media. The formation constants of CuHCO₃⁺, CuCO₃⁰ and Cu(CO₃)₂^2? at 25°C and zero ionic strength are: log β_H⁰ = 1.8, log β₁⁰ = 6.82 and log β⁰₂ = 10.6. Formation constants of these species appropriate to 0.7 molar ionic strength and 25°C are log β_H ～- 1, log β₁ = 5.73, log β₂ = 9.3. Our results indicate that the inorganic speciation scheme of Cu(II) in seawater is dominated by CuCO₃⁰ and that the ternary species, CuCO₃OH^?, is of substantial importance.     

Geochemistry of the Sweetwater Wash Pluton,California: Implications for “anomalous” trace element behavior during differentiation of felsic magmas

Field relations, mineralogy and major and trace element data for the very felsic, peraluminous Sweetwater Wash pluton establish a differentiation sequence dominantly controlled by fractional crystallization processes. Elements Ba and Sr show depletion by factors of 50–60X from the earliest granite unit analyzed to the late-stage pegmatites and aplites. The strong Ba depletion is largely due to the partitioning behavior of this element in K-feldspar, while the Sr depletion is due to the combined effects of the two feldspars. The 4-fold increase in Rb during crystallization is also predictable from mineral/ melt partition coefficients.Coefficients for the light rare-earth elements (LREE) in major mineral species predict that these elements should behave incompatibly during crystallization and increase with fractionation. In fact, the LREE abundances decrease by a factor of 10–20X during crystallization. This anomalous behavior is commonly observed in felsic plutonic and volcanic sequences. In the Sweetwater Wash pluton monazite occurs in minute quantities, but it is sufficiently abundant to govern the partitioning of LREE and Th during crystallization. Petrographic observations indicate that monazite was on the liquidus throughout most of the crystallization. Analyses of silicate mineral separates suggest that the monazite contains more than 75% of the LREE in the whole rocks.Fractionation of REE-rich accessories (in particular monazite) from felsic magmas may be the general cause of REE depletion during differentiation of these melts. Monazite can easily be mistaken for zircon and, because it typically contains 50% LREE, extremely minute and easily overlooked quantities of monazite can control LREE abundances.     

First Results from Viper: Detection of Small-scale Anisotropy at 40 GHz

Using all available major samples of Seyfert galaxies and their corresponding closely matched control samples of nonactive galaxies, we find that the bar ellipticities (or axial ratios) in Seyfert galaxies are systematically different from those in nonactive galaxies. Overall, there is a deficiency of bars with large ellipticities (i.e., "thin" or "strong" bars) in Seyfert galaxies compared to nonactive galaxies. Accompanied with a large dispersion due to small number statistics, this effect is strictly speaking at the 2 sigma level. To obtain this result, the active galaxy samples of near-infrared surface photometry were matched to those of normal galaxies in type, host galaxy ellipticity, absolute magnitude, and, to some extent, redshift. We discuss possible theoretical explanations of this phenomenon within the framework of galactic evolution, and, in particular, of radial gas redistribution in barred galaxies. Our conclusions provide further evidence that Seyfert hosts differ systematically from their nonactive counterparts on scales of a few kiloparsecs.     

Selective Depletions and the Abundances of Molecules Used to Study Star Formation

We have investigated the dependences of the fractional abundances of a number of species on the selective depletions of elemental carbon, nitrogen, oxygen and sulphur, as well as metals. Of the most commonly observed species, HCO₊ is the one having the fractional abundance that is least sensitive to the selective elemental depletions and for this reason is a particularly appropriate species to study in efforts to diagnose collapse dynamics in star formation from the analysis of infall signatures in spectral line profiles. We suggest that a combination of molecules could be the focus of observations aimed at the investigation of collapse dynamics. Comparisons of the profiles of spectral features of these species would also give insight into the behaviours of the selective depletions of the different elements in the infalling gas.