Disequilibrium response of permafrost in boreal continental western Canada to climate change

In the boreal forest of continental western Canada, permafrost is restricted toSphagnum-dominated peatlands on which air photo interpretation reveals the occurrence of five types of surface physiography. Concentrated in the northern part of the boreal forest, permafrost is present in peat plateaus with and without collapse scars. In the southern part of the boreal forest, continental bogs dominate, representing ombrotrophic peatlands that have never contained permafrost. In the midboreal zone, internal lawns are present in bogs and in fens. These internal lawns do not presently contain permafrost but did in the recent past, representing degradation of permafrost since the Little Ice Age. Evaluation of the distribution of these peat landforms indicates that today 30% of bogs contain permafrost at the –0.4 °C isotherm and 50% of bogs contain permafrost at the –1.2 °C isotherm, whereas in the past, 30% of bogs contained permafrost at the –1.4 °C isotherm and 50% of bogs contained permafrost at the –2.3 °C isotherm. Although spatial degradation has occurred with a shifting of permafrost northwards in response to warming since the Little Ice Age, permafrost cover has increased in any given area where present-day temperatures are between 0.5 and –3.5 °C.     

Thermal and petrological structure of the lithosphere beneath Hannuoba, Sino-Korean Craton, China: evidence from xenoliths

Deep-seated xenoliths entrained in the Hannuoba basalts of the northern Sino-Korean Craton include mafic and felsic granulites, mantle wall-rock from spinel– and garnet–spinel peridotite facies, and basaltic crystallisation products from the spinel-pyroxenite and garnet-pyroxenite stability fields. The mineral compositions of the xenoliths have been used to estimate temperatures and, where possible, pressures of equilibration, and to construct a geothermal framework to interpret the upper mantle and lower crustal rock-type sequences for the region. The xenolith-derived paleogeotherm is constrained in the depth interval of 45–65 km and like others from areas of young basalt magmatism, is elevated and strongly convex toward the temperature axis. Two-pyroxene granulites give the lowest temperatures and garnet pyroxenites the highest, while the spinel lherzolites fall between these two groups. The present-day Moho beneath the Hannuoba area is defined at 42 km by seismic data, and coincides with the deepest occurrence of granulite. Above this boundary, there is a lower crust–upper mantle transition zone about 10-km thick, in which spinel lherzolites and mafic granulites (with variable plagioclase contents) are intermixed. It is inferred that this underplating has resulted in a lowering of the original pre-Cenozoic Moho (then coinciding with the crust–mantle boundary, CMB) from about 30 km to its present-day position and was due to intrusions of basaltic magmas that displaced peridotite mantle wall-rock and equilibrated to mafic granulites. Trace element patterns of the diopsides (analysed by laser ablation-ICPMS) from the Cr-diopside series spinel lherzolites and associated layered xenoliths (spinel lherzolites and pyroxenites) indicate a fertile uppermost mantle with moderate depletion by low degrees of partial melting and little evidence of metasomatic activity. The similarity in major and trace element compositions of the minerals in both rock types suggests that the layered ultramafic xenoliths formed by mantle deformation processes (metamorphic segregation), rather than by melt veining or metasomatism.     

Variations in trapping temperatures and trace elements in peridotite-suite inclusions from African diamonds: evidence for two inclusion suites,and implications for lithosphere stratigraphy

The proton microprobe has been used to measure the trace element contents of Cr-pyrope, olivine and orthopyroxene inclusion (DI) in>60 diamonds from southern Africa, and in concentrate garnets from the host kimberlites. Olivine inclusions show a negative correlation of Ca with Fo content, but olivines coexisting with very subcalcic garnets are anomously depleted in Ca relative to Fo. The maximum and median values of Ti, Zn, Ga, Zr and Y in DI garnets decrease as Ca decreases relative to Cr, consistent with increasing depletion by removal of silicate melts. However, the most subcalcic garnets are anomously enriched in Sr (and LREE). The contrasting depletion in Zr, Y, Ti etc. and enrichment in Sr is not consistent with a single-stage depletion event. The extreme Ca depletion of the most subcalcic garnets and their coexisting olivines, and the Sr enrichment, are interpreted as the result of metasomatism by a carbonatitic fluid following depletion. Trapping temperatures of garnet DI, estimated by nickel thermometry range from 950 to > 1500°C. Most DI with T>1200°C are lherzolitic, rather than subcalcic. In most pipes studied, the trapping T of DI garnets is higher than the T range of equivalent garnets from concentrates. The high Ts are interpreted as reflecting the formation of diamonds during short-lived thermal pulses, followed by cooling toward a conductive geotherm. The T distribution of calcic and subcalcic garnets in concentrates from kimberlites suggests that lherzolite and harzburgite are intimately intermixed over the depth range 150–180 km beneath the Kalahari craton. The abundance of calcic garnets with T>1200°C suggests the presence of significant amounts of lherzolite at greater depths. This deeper lherzolite may be associated with eclogite, and may be the source region for some high-T lherzolitic DI garnets.     

Assessing impacts of climate change on forests: The state of biological modeling

Models that address the impacts of climate change on forests are reviewed at four levels of biological organization: global, regional or landscape, community, and tree. The models are compared for their ability to assess changes in fluxes of biogenic greenhouse gases, land use, patterns of forest type or species composition, forest resource productivity, forest health, biodiversity, and wildlife habitat. No one model can address all of these impacts, but landscape transition models and regional vegetation and land-use models have been used to consider more impacts than the other models. The development of landscape vegetation dynamics models of functional groups is suggested as a means to integrate the theory of both landscape ecology and individual tree responses to climate change. Risk assessment methodologies can be adapted to deal with the impacts of climate change at various spatial and temporal scales. Four areas of research needing additional effort are identified: (1) linking socioeconomic and ecologic models; (2) interfacing forest models at different scales; (3) obtaining data on susceptibility of trees and forest to changes in climate and disturbance regimes; and (4) relating information from different scales.The U.S. Government right to retain a non-exclusive, royalty-free license in and to any copyright is acknowledged.Managed by Martin Marietta Energy Systems, Inc., for the U.S. Department of Energy under contract DE-AC05-84OR21400.     

Interactions Between An Alluvial-Aquifer Wellfield And The Scioto River,Ohio, USA

Data from wellfields in the Scioto River valley aquifer near Piketon, Ohio, USA, were used to evaluate groundwater quantity and quality and to formulate wellfield-management concepts for future use of the alluvial aquifer. The percentage of induced river infiltration pumped from wells next to the river was estimated using a numerical groundwater-flow model and chemical mass-balance techniques. These analyses indicate that 50 to 88 percent of the wellfield water is derived from induced infiltration of river water. Model simulations correlate the percentage of well water from induced infiltration to wellfield pumpage, river stage, proximity of wells to the river, riverbed hydraulic conductivity, aquifer hydraulic conductivity, and river-stage level. Induced infiltration of river water has influenced the quality of groundwater, making the wellfield water more similar to the chemical character of the river than to that of the alluvial aquifer.

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Resumen: Los datos procedentes de un conjunto de pozos en el acuífero de valle del Río Scioto, cerca de Pinkerton, Ohio; se usaron para evaluar la cantidad y calidad de las aguas subterráneas, así como para planificar la gestión de los pozos cercanos al mismo, fue estimado mediante un modelo numérico de flujo y técnicas de balance químico de masa. Los análisis indican que entre el 50 y 88 porciento del agua bombeada procede de infiltración inducida. Simulaciones realizadas con el modelo permitieron correlacionar el porcentaje del agua bombeada que proviene de infiltración inducida, con distintas variables: caudal de bombeo, tramo del río, proximidad de los pozos al río, conductividad hidráulica del lecho del río, conductividad hidráulica del acuífero y nivel del tramo de río. La infiltración inducida ha afectado a la calidad del agua de los pozos, cuyo carácter químico es más parecido al del agua del río que al del acuífero aluvial.

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Résumé: Les données provenant de champs captants dans l'aquifère de la valée de la Scioto, près de Piketon (Ohio), ont été utilisées pour évaluer la quantité et la qualité de l'eau souterraine et pour définir les conditions de gestion de l'exploitation future de cet aquifère alluvial. La part de la réalimentation par la rivière, sous l'effet de pompages, dans les eaux des puits voisins du cours d'eau, a été estimée au moyen d'un modèle numérique d'écoulement souterrain et par des méthodes chimiques du bilan de matière. Ces analyses indiquent que 50 à 88 pourcent de l'eau pompée provient de la réalimentation par la rivière. Les simultations corrèlent la part pompée provenant de la réalimentation avec les pompages, le niveau de la rivière. La réalimentation provoquée à partir de la rivière a influencé la qualité de l'eau souterraine en conférant à l'eau des puits des caractères plus proches de ceux de la rivière que de ceux de l'aquifère alluvial.

     

Temporal and genetic relationships between the Kidston gold-bearing Breccia Pipe and the Lochaber Ring Dyke Complex, North Queensland, Australia: insights from in situ U–Pb and Hf-isotope analysis of zircon

An existing model for the temporal and genetic relationships between the Kidston gold-bearing Breccia Pipe and the nearby Lochaber Ring Dyke Complex has been evaluated using in situ U–Pb and Hf-isotope analyses of zircon grains. The Oak River Granodiorite, the host rock to the Kidston Breccia Pipe, has 1,551?±?6 Ma old zircon cores overgrown by 417.7?±?2.2 Ma rims. The Black Cap Diorite and Lochaber Granite within the Lochaber Ring Dyke Complex have crystallisation ages of 350.7?±?1.3 and 337.9?±?2.6 Ma respectively. The gold-rich Median Dyke within the Kidston Breccia Pipe has a crystallisation age of 335.7?±?4.2 Ma, and thus is temporally related to the Lochaber Granite. However, zircon grains from the Median Dyke have less radiogenic Hf-isotope compositions (? _Hf from ?7.8 to ?15.8) than those from the Black Cap Diorite ?_Hf?=?0.4 to ?7.2) and the Lochaber Granite (? _Hf?=??1.0 to ?7.5), but within the range defined by zircons from the Oak River Granodiorite ? _Hf?=??8.0 to ?29.2). The Hf-isotope data thus rule out the proposed fractional crystallisation relationship between the Kidston gold-bearing rocks and the Lochaber Ring Dyke Complex. The Kidston Median Dyke may have been produced by mixing between Lochaber Granite magmas and magmas derived by remelting of the Oak River Granodiorite, which was itself derived from Proterozoic crust. There is no evidence for a juvenile component in the Lochaber Ring Dyke Complex or the Median Dyke. The gold enrichment in the Kidston rocks thus may reflect the multi-stage reworking of the Proterozoic crust, which ultimately produced the Carboniferous felsic magmas.