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471.
Scaling up point-in-space heat tracing of seepage flux using bed temperatures as a quantitative proxy 总被引:1,自引:1,他引:0
It is challenging to quantify reach-scale surface-water–groundwater interactions, while maintaining the fine-scale spatial resolution required in hyporheic studies. One-dimensional heat-transport modeling was used to simulate streambed fluxes at discrete points using time-series temperature records. A predictive relationship was then developed between point-in-time streambed temperature and modeled flux rates. Flux was mapped at high spatial resolution by applying the predictive relationship to mapped streambed temperatures, which allowed for high-resolution quantification of flux by proxy. Inferred patterns of flux are consistent with morphology and yielded a net flux to a 30-m stream reach of 1.0?L s–1. Discharge of saline groundwater (5.7?g L–1 Cl–) allowed for comparison between the temperature proxy method and geochemical variability. Maximum upwelling locations (>35 cm day–1) were spatially coincident with areas of high conductance at the bed interface (5–25?mS cm–1). Differences between gross flux estimates from heat and geochemical methods are attributed to differences in the spatial extent over which estimates were derived and limited sensitivity of the temperature-as-proxy method. When bed temperatures are near their inherent limits (groundwater and stream-water temperatures) the flux magnitude can be underestimated. Caution must be used when determining gross, reach-scale fluxes from temperature-as-proxy methods when flux rates are outside the sensitivity limits. 相似文献
472.
This paper examines how neoliberal development discourse contributes to the production and maintenance of problematic gendered hierarchies and spaces. By interrogating the basic assumptions undergirding this discourse, this paper explores how neoliberalism produces spaces which normalise certain identities—especially those associated with individualism and economic rationality, and makes errant values such as communalism and altruism. Drawing on perspectives from feminist geographies, we argue that by normalising and privileging certain masculine identities, neoliberalism reinscribes and legitimizes gendered power relations that are counterproductive to addressing HIV/AIDS. The ‘ideal’ person fighting HIV/AIDS in the neoliberal framework is rational, competitive and self-interested, but these characteristics are complicit in worsening HIV prevalence and mobilize problematic gender roles and identities. Given the pervasiveness of this ideology in Malawi, we propose ways in which families, communities and institutions can challenge and reshape gender identities and potential solutions to HIV within this context. 相似文献
473.
Alan L. Mayo Rachel M. Henderson David Tingey William Webber 《Hydrogeology Journal》2010,18(3):725-747
The 1,750-km2 endorheic Honey Lake basin (California–Nevada, USA) was part of the 22,000-km2 Pleistocene Lake Lahontan pluvial lake system which existed between 5,000 and 40,000 years BP. The basin consists of two subbasins separated by a low elevation divide. Groundwater in the western subbasin has a maximum total dissolved solids (TDS) content of only ~1,300 mg/L; however eastern subbasin groundwater has a maximum TDS of ~46,000 mg/L. This TDS distribution is unexpected because 94% of surface water TDS loading is to the western subbasin. In situ reactions and upwelling thermal groundwater contributing to groundwater chemistry were modeled using NETPATH. The TDS difference between the subbasins is attributed to post-Lake Lahontan isostatic rebound about 13,000 years ago. Prior to rebound the subbasins did not exist and the low point of the basin was in the eastern area where hydraulic isolation from the larger Lake Lahontan and frequent desiccation of the basin surface water resulted in evaporite mineral deposition in accumulating sediments. After rebound, the terminal sink for most surface water shifted to the western subbasin. Although most closed basins have not been impacted by isostatic rebound, results of this investigation demonstrate how tectonic evolution can impact the distribution of soluble minerals accumulating in shallow basins. 相似文献
474.
475.
Pillow basalts from the early Archean (3.7 to 3.8 Ga) Isua greenstone belt, West Greenland, are characterized by well-preserved rims and concentric core structures. The pillow rims and cores have different mineral assemblages, and chemical and isotopic compositions. The rims have systematically higher contents of Fe2O3, MgO, MnO, K2O, Rb, Ba, Ga, Y, and transition metals than the cores. In contrast, the cores possess higher concentrations of SiO2, Na2O, P2O5, Sr, Pb, U, Nb, and the light rare earth elements (REEs than the rims). These compositional variations in the rims and cores are likely to reflect the mobility of these elements during posteruption alteration. Variations of many major and trace element concentrations between the rims and cores of the Isua pillow basalts are comparable to those of modern pillow basalts undergoing seafloor hydrothermal alteration. Al2O3, TiO2, Th, Zr, and the heavy REEs display similar values in both rims and cores, suggesting that these elements were relatively immobile during postemplacement alteration.In addition, the rims and cores have distinctive Sm-Nd and Rb-Sr isotopic compositions in that the rims are characterized by higher 143Nd/144Nd and 87Sr/86Sr ratios than the cores. The pillow basalts yield 2569 ± 170 Ma and 1604 ± 170 Ma errorchron ages on 143Nd/144Nd vs. 147Sm/144Nd and 87Sr/86Sr vs. 87Rb/86Sr diagrams, respectively. The Sm-Nd errorchron age may correspond, within errors, to a late Archean tectonothermal metamorphic event recorded in the region. The Sm-Nd errorchron may have resulted from a combination of isotopic homogenization and preferential loss of Nd, relative to Sm, during late Archean metamorphism. Although the Rb-Sr errorchron age overlaps with the timing of an early to mid-Proterozoic tectonothermal metamorphic event recorded in the region, because of a considerably large mean square of weighted deviates value and scatter in 86Sr/87Sr and 87Rb/86Sr ratios, this age may not have a precise geological significance. The 1.6 Ga Rb-Sr errorchron is likely to have resulted from the loss of radiogenic 87Sr. Collectively, the Sm-Nd and Rb-Sr data obtained from the 3.7-3.8 Ga Isua pillow basalt rims and cores are consistent with disturbances of the Sm-Nd and Rb-Sr systems by tectonothermal metamorphic events long after their eruption.In contrast to the Sm-Nd and Rb-Sr systems, the Lu-Hf system appears to be largely undisturbed by metamorphism. Five core samples and three rim samples yield a 3935 ± 350 Ma age, within error of the approximate age of eruption (3.7 to 3.8 Ga). Two rim samples that have gained Lu give an age of 1707 ± 140 Ma, within error of the Rb-Sr errorchron age. Initial 176Hf/177Hf ratios of the undisturbed samples at 3.75 Ga lie within ±1 ε-unit of the chondritic value, suggesting no long-term depletion in the mantle source of the basalts. 相似文献
476.
Parampreet Kaur Naveen Chaudhri Albrecht W. Hofmann Ingrid Raczek Martin Okrusch Susanne Skora Jürgen Koepke 《International Journal of Earth Sciences》2014,103(4):1083-1112
The late Palaeoproterozoic (1.72–1.70 Ga) ferroan granites of the Khetri complex, northern Aravalli orogen, NW India, were extensively metasomatised ~900 Ma after their emplacement, at around 850–830 Ma by low-temperature (ca. 400 °C) meteoric fluids that attained metamorphic character after exchanging oxygen with the surrounding metamorphic rocks. Albitisation is the dominant metasomatic process that was accompanied by Mg and Ca metasomatism. A two-stage metasomatic model is applicable to all the altered ferroan intrusives. The stage I is represented by a metasomatic reaction interface that developed as a result of transformation of the original microcline–oligoclase (An12–14) granite to microcline–albite (An1–3) granite, and this stage is rarely preserved. In contrast, the stage II metasomatic reaction front, where the microcline-bearing albite granite has been transformed to microcline-free albite granite, is readily recognisable in the field and present in most of the intrusives. Some of them lack an obvious reaction interface due to the presence of stage II albite granites only. When studied in isolation, these intrusives were incorrectly classified and their tectonic setting was misinterpreted. Furthermore, our results show that the mafic mineralogy of metasomatised granites has a significant impact on the characterisation of such rocks in the magmatic classification and discrimination diagrams. Nevertheless, the stage I metasomatised granites can be appropriately characterised in these diagrams, whereas the characterisation of the stage II granites will lead to erroneous interpretations. The close spatial association of these high heat producing ferroan granites with iron oxide–copper–gold (IOCG), U and REE mineralisation in the region indicates a genetic link between the metasomatism and the mineralisation. World-class IOCG, U and REE deposits are associated with metasomatised ferroan granites, suggesting that such a relationship may act as a critical first-order exploration target for undiscovered mineral deposits. 相似文献