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21.
22.
The last glacial-interglacial transition (LGIT; 19–9 ka) was characterized by rapid climate changes and significant ecosystem reorganizations worldwide. In western Colorado, one of the coldest locations in the continental US today, mountain environments during the late-glacial period are poorly known. Yet, archaeological evidence from the Mountaineer site (2625 m elev.) indicates that Folsom-age Paleoindians were over-wintering in the Gunnison Basin during the Younger Dryas Chronozone (YDC; 12.9–11.7 ka). To determine the vegetation and fire history during the LGIT, and possible explanations for occupation during a period thought to be harsher than today, a 17-ka-old sediment core from Lily Pond (3208 m elev.) was analyzed for pollen and charcoal and compared with other high-resolution records from the southern Rocky Mountains. Widespread tundra and Picea parkland and low fire activity in the cold wet late-glacial period transitioned to open subalpine forest and increased fire activity in the Bølling–Allerød period as conditions became warmer and drier. During the YDC, greater winter snowpack than today and prolonged wet springs likely expanded subalpine forest to lower elevations than today, providing construction material and fuel for the early inhabitants. In the early to middle Holocene, arid conditions resulted in xerophytic vegetation and frequent fire. 相似文献
23.
Application of Direct Push Methods to Investigate Uranium Distribution in an Alluvial Aquifer 总被引:2,自引:0,他引:2
Wesley McCall Thomas M. Christy Thomas Christopherson Howard Issacs 《Ground Water Monitoring & Remediation》2009,29(4):65-76
The U.S. EPA 2000 Radionuclide Rule established a maximum contaminant level (MCL) for uranium of 30 µg/L. Many small community water supplies are struggling to comply with this new regulation. At one such community, direct push (DP) methods were applied to obtain hydraulic profiling tool (HPT) logs and install small diameter wells in a section of alluvial deposits located along the Platte River. This work was conducted to evaluate potential sources of elevated uranium in the Clarks, Nebraska drinking water supply. HPT logs were used to understand the hydrostratigraphy of a portion of the aquifer and guide placement of small diameter wells at selected depth intervals. Low-flow sampling of the wells provided water quality parameters and samples for analysis to study the distribution of uranium and variations in aquifer chemistry. Contrary to expectations, the aquifer chemistry revealed that uranium was being mobilized under anoxic and reducing conditions. Review of the test well and new public water supply well construction details revealed that filter packs extended significantly above the screened intervals of the wells. These filter packs were providing a conduit for the movement of groundwater with elevated concentrations of uranium into the supply wells and the community drinking water supply. The methods applied and lessons learned here may be useful for the assessment of unconsolidated aquifers for uranium, arsenic, and many other drinking water supply contaminants. 相似文献
24.
Susan A. Welch Andrew G. Christy Lloyd Isaacson Dirk Kirste 《Geochimica et cosmochimica acta》2009,73(1):44-177
Major, trace and rare earth element concentrations were measured in porewater, surface water and sediments at an acid sulfate soil site. The concentrations of La and Ce in porewater are up to 1-3 ppm. There is a strong correlation between REE concentration and acidity, except that the maximum concentrations were consistently found below the horizon of maximum acidity, associated with an increase in pH (to ca. 4) and change in mineralogy from jarosite-dominated to goethite-dominated mottles. Jarosite replacement by goethite is as expected with the rise in pH, which in turn is due to the occurrence of a fossil shell bed just below. The rare earth element patterns in the porewaters are enriched in the MREE with respect to Post-Archaean Australian Shale (PAAS). Measurements and calculations show that this is in accord with experiments on low-degree partial dissolution of jarosite, even when the jarosite itself is highly enriched in LREE. There is a clear fractionation in the patterns between the clay-rich soil matrix, which is slightly depleted in the LREE when normalized to PAAS (La/YbPAAS ∼0.5), and the secondary mineral phase jarosite, which is enriched in the LREE (La/YbPAAS = 15-50). The REE pattern in the porewater changes with the transition from jarosite- to goethite-rich mottles, becoming relatively more enriched in the LREE compared to the HREE, which is consistent with the incongruent dissolution of jarosite to form goethite and the release of greater amounts of jarosite REE to solution, including proportionately more of the jarosite-compatible LREE.Maximum surface water REE concentrations in acidic water were 100-200 ppb La and Ce. REE patterns in surface water were very similar to the porewater transition zone, enriched in the MREE, but asymmetric, relatively enriched in the LREE compared to the HREE. 相似文献
25.
26.
Karstic aquifers are highly susceptible to rapid infiltration of river water, particularly during periods of high flow. Following
a period of sustained rainfall in the Suwannee River basin, Florida, USA, the stage of the Suwannee River rose from 3.0 to
5.88 m above mean sea level in April 1996 and discharge peaked at 360 m3/s. During these high-flow conditions, water from the Suwannee River migrated directly into the karstic Upper Floridan aquifer,
the main source of water supply for the area. Changes in the chemical composition of groundwater were quantified using naturally
occurring geochemical tracers and mass-balance modeling techniques. Mixing of river water with groundwater was indicated by
a decrease in the concentrations of calcium, silica, and 222Rn; and by an increase in dissolved organic carbon (DOC), tannic acid, and chloride, compared to low-flow conditions in water
from a nearby monitoring well, Wingate Sink, and Little River Springs. The proportion (fraction) of river water in groundwater
ranged from 0.13 to 0.65 at Wingate Sink and from 0.5 to 0.99 at well W-17258, based on binary mixing models using various
tracers. The effectiveness of a natural tracer in quantifying mixing of river water and groundwater was related to differences
in tracer concentration of the two end members and how conservatively the tracer reacted in the mixed water. Solutes with
similar concentrations in the two end-member waters (Na, Mg, K, Cl, SO4, SiO2) were not as effective tracers for quantifying mixing of river water and groundwater as those with larger differences in
end-member concentrations (Ca, tannic acid, DOC, 222Rn, HCO3).
Received, March 1999 / Revised, July 1999 / Accepted, July 1999 相似文献
27.
The observations leading to, and confirming the discovery of, 1978 P 1 (Charon) are reviewed. The original interpretation of these observations remains valid. The circular orbit with synchronous revolution and rotation is in agreement with both observation and theory. The expectation that eclipse events are imminent is supported by our recent data. 相似文献
28.
The beginnings of hydrous mantle wedge melting 总被引:5,自引:3,他引:2
Christy?B.?TillEmail author Timothy?L.?Grove Anthony?C.?Withers 《Contributions to Mineralogy and Petrology》2012,163(4):669-688
This study presents new phase equilibrium data on primitive mantle peridotite (0.33 wt% Na2O, 0.03 wt% K2O) in the presence of excess H2O (14.5 wt% H2O) from 740 to 1,200°C at 3.2–6 GPa. Based on textural and chemical evidence, we find that the H2O-saturated peridotite solidus remains isothermal between 800 and 820°C at 3–6 GPa. We identify both quenched solute from
the H2O-rich fluid phase and quenched silicate melt in supersolidus experiments. Chlorite is stable on and above the H2O-saturated solidus from 2 to 3.6 GPa, and chlorite peridotite melting experiments (containing ~6 wt% chlorite) show that
melting occurs at the chlorite-out boundary over this pressure range, which is within 20°C of the H2O-saturated melting curve. Chlorite can therefore provide sufficient H2O upon breakdown to trigger dehydration melting in the mantle wedge or perpetuate ongoing H2O-saturated melting. Constraints from recent geodynamic models of hot subduction zones like Cascadia suggest that significantly
more H2O is fluxed from the subducting slab near 100 km depth than can be bound in a layer of chloritized peridotite ~ 1 km thick
at the base of the mantle wedge. Therefore, the dehydration of serpentinized mantle in the subducted lithosphere supplies
free H2O to trigger melting at the H2O-saturated solidus in the lowermost mantle wedge. Alternatively, in cool subduction zones like the Northern Marianas, a layer
of chloritized peridotite up to 1.5 km thick could contain all the H2O fluxed from the slab every million years near 100 km depth, which suggests that the dominant form of melting below arcs
in cool subduction zones is chlorite dehydration melting. Slab P–T paths from recent geodynamic models also allow for melts of subducted sediment, oceanic crust, and/or sediment diapirs to
interact with hydrous mantle melts within the mantle wedge at intermediate to hot subduction zones. 相似文献
29.
Christy B. Till Timothy L. Grove Anthony C. Withers 《Contributions to Mineralogy and Petrology》2012,164(6):1073-1076
The comment of Stalder raises three main concerns regarding the interpretation of the experiments presented by Till et al. (2012): (1) our inability to uniquely distinguish between high-pressure hydrous silicate melt and solute-rich aqueous fluid leads to the incorrect interpretation of phase relations, (2) the temperature interval over which hydrous melting takes places is inordinately large and contrary to expectations, and/or (3) the possibility that the system may be above the second critical end point (SCEP) in this H2O-rich silicate system has been insufficiently discussed. In this reply, we provide clarification on these concerns and argue that with the extent of knowledge available today, the chemical characteristics of our experimental products at 3.2 and 4?GPa evince the presence of a silicate melt at temperatures <1,000?°C and we are below the SCEP in the peridotite–H2O system at the P–T conditions of our experiments. If in fact the quench observed in our experiments does represent that of a supercritical (SC) fluid, then our data suggest Mg and Fe are highly soluble in SC fluids at the P–T conditions of the base of the mantle wedge below arc volcanoes. Therefore, our results would require a significant change in thinking about the chemical compositional characteristics of SC fluids. 相似文献
30.
Christy Collis 《GeoJournal》2010,75(4):387-395
This is an article about the politics of territory in Antarctica. It revolves around what at first seems like a very simple geopolitical question: who owns Antarctica? As this article demonstrates, this seemingly simple question is far from easy to answer: it cannot be answered with a straightforward list of states, nor by conventional geopolitical understandings of territorial possession (Agnew and Corbridge, Mastering space: Hegemony, territory, and international political economy, 1995). Struggles between states for territorial possession has characterised much recent geopolitical history; struggles for Antarctica do not entirely follow this pattern, and revolve instead on the nature and the concept of territorial possession itself. The article focuses in particular on the debates about, and changes to, Antarctic legal and geopolitical territories triggered by the 1957–1958 International Geophysical Year: before the IGY Antarctica was an unstable composite of state claims, unclaimed terra nullius, and terra communis or land unavailable to state claim. By the 1959 Antarctic Treaty, this unstable composite legal and geopolitical geography emerged as a new form of territory, one in which the conventional global mode of territory—state possession—was no longer dominant. Understanding Antarctic legal geographies adds depth to critical geopolitical studies which focus on the ways in which space is actively constructed by specific discourses, understandings, and groups. 相似文献