Lake Chicot is an oxbow lake located along the western side of the Mississippi River in southeastern Arkansas. A major flood
in 1927, levee construction, land use changes from bottomland hardwood to agriculture, a large increase in drainage area,
and stream channelization have altered the appearance of contributing watersheds and the lake. The lake often has high suspended
sediment concentrations making it undesirable for recreation and aquatic production. As part of a coordinated study to determine
changes in the lake, sediment accumulation patterns and rates were determined in Lake Chicot using the137Cs technique. Major sediment accumulation is occurring near the major inlet and along the thalweg of the oxbow. Average sediment
accumulation of 1.8 cm/yr since 1963 was measured for twelve samples. Calculations show sediment accumulation is decreasing.
This study has shown that the137Cs technique can provide useful planning information for the environmental geologist or reservoir manager. Results of the
current study will be used to develop better sediment and water management strategies for Lake Chicot.
Contribution of the U.S. Department of Agriculture, Agricultural Research Service, in cooperation with the Vicksburg District
of the U.S. Army Corps of Engineers. 相似文献
A detailed analysis is undertaken of the Atlantic-European climate using data from 500-year-long proxy-based climate reconstructions, a long climate simulation with perpetual 1990 forcing, as well as two global and one regional climate change scenarios. The observed and simulated interannual variability and teleconnectivity are compared and interpreted in order to improve the understanding of natural climate variability on interannual to decadal time scales for the late Holocene. The focus is set on the Atlantic-European and Alpine regions during the winter and summer seasons, using temperature, precipitation, and 500 hPa geopotential height fields. The climate reconstruction shows pronounced interdecadal variations that appear to “lock” the atmospheric circulation in quasi-steady long-term patterns over multi-decadal periods controlling at least part of the temperature and precipitation variability. Different circulation patterns are persistent over several decades for the period 1500 to 1900. The 500-year-long simulation with perpetual 1990 forcing shows some substantial differences, with a more unsteady teleconnectivity behaviour. Two global scenario simulations indicate a transition towards more stable teleconnectivity for the next 100 years. Time series of reconstructed and simulated temperature and precipitation over the Alpine region show comparatively small changes in interannual variability within the time frame considered, with the exception of the summer season, where a substantial increase in interannual variability is simulated by regional climate models. 相似文献
Most studies on the impact of climate change on regional water resources focus on long-term average flows or mean water availability,
and they rarely take the effects of altered human water use into account. When analyzing extreme events such as floods and
droughts, the assessments are typically confined to smaller areas and case studies. At the same time it is acknowledged that
climate change may severely alter the risk of hydrological extremes over large regional scales, and that human water use will
put additional pressure on future water resources. In an attempt to bridge these various aspects, this paper presents a first-time
continental, integrated analysis of possible impacts of global change (here defined as climate and water use change) on future
flood and drought frequencies for the selected study area of Europe. The global integrated water model WaterGAP is evaluated
regarding its capability to simulate high and low-flow regimes and is then applied to calculate relative changes in flood
and drought frequencies. The results indicate large ‘critical regions’ for which significant changes in flood or drought risks
are expected under the proposed global change scenarios. The regions most prone to a rise in flood frequencies are northern
to northeastern Europe, while southern and southeastern Europe show significant increases in drought frequencies. In the critical
regions, events with an intensity of today's 100-year floods and droughts may recur every 10–50 years by the 2070s. Though
interim and preliminary, and despite the inherent uncertainties in the presented approach, the results underpin the importance
of developing mitigation and adaptation strategies for global change impacts on a continental scale. 相似文献
A palynology and organic matter study has been carried out on samples of the Upper Triassic Raibl beds from an Upper Austroalpine
thrust sheet, preserved in the area of the Iberg Klippen (Laucherenst?ckli). The palynological assemblages indicate an early
Carnian (Julian) age. Comparison with well calibrated successions from the Southern Alps suggests a correlation with the Late
Julian ammonoid zone Austrotrachyceras austriacum.
The lithofacies of the studied sections suggests a substantial similarity to Raibl beds of the Upper Austroalpine of the Northern
Calcareous Alps and those of the Silvretta- and the S-charl nappe of the Grisons. The minute Upper Austroalpine thrust sheet
represents the westward extension of series with similar facies, which are still preserved in the Northern Calcareous Alps
of Liechtenstein and western Austria (Vorarlberg).
According to the Thermal Alteration Scale (TAS) of Batten (1996) the observed medium brown colors of the pollen grains correspond
to a value of 4/5, which is equivalent to vitrinite reflectance (VR) values of 0.7–0.9 %Ro. In comparison with other sites in Liechtenstein, Vorarlberg and the Upper Austroalpine of Central Grisons (Silvretta and
S-charl nappe) with measured VR values of > 2%Ro the organic matter of the studied samples shows only minor thermal alteration, indicating that the Upper Austroalpine thrust
sheets of Iberg represent the highest part of the nappe stack in Central Switzerland, formerly covered by a comparatively
thin overburden.
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Explosive eruptions at Mauna Loa summit ejected coarse-grained blocks (free of lava coatings) from Moku'aweoweo caldera. Most are gabbronorites and gabbros that have 0–26 vol.% olivine and 1–29 vol.% oikocrystic orthopyroxene. Some blocks are ferrogabbros and diorites with micrographic matrices, and diorite veins (≤ 2 cm) cross-cut some gabbronorites and gabbros. One block is an open-textured dunite.
The MgO of the gabbronorites and gabbros ranges 7–21 wt.%. Those with MgO > 10 wt.% have some incompatible-element abundances (Zr, Y, REE; positive Eu anomalies) lower than those in Mauna Loa lavas of comparable MgO; gabbros (MgO < 10 wt.%) generally overlap lava compositions. Olivines range Fo83–58, clinopyroxenes have Mg#s 83–62, and orthopyroxene Mg#s are 84–63 — all evolved beyond the mineral-Mg#s of Mauna Loa lavas. Plagioclase is An75–50. Ferrogabbro and diorite blocks have 3–5 wt.% MgO (TiO2 3.2–5.4%; K2O 0.8–1.3%; La 16–27 ppm), and a diorite vein is the most evolved (SiO2 59%, K2O 1.5%, La 38 ppm). They have clinopyroxene Mg#s 67–46, and plagioclase An57–40. The open-textured dunite has olivine Fo83.5. Seven isotope ratios are 87Sr/86Sr 0.70394–0.70374 and 143Nd/144Nd 0.51293–0.51286, and identify the suite as belonging to the Mauna Loa system.
Gabbronorites and gabbros originated in solidification zones of Moku'aweoweo lava lakes where they acquired orthocumulate textures and incompatible-element depletions. These features suggest deeper and slower cooling lakes than the lava lake paradigm, Kilauea Iki, which is basalt and picrite. Clinopyroxene geobarometry suggests crystallization at < 1 kbar P. Highly evolved mineral Mg#s, < 75, are largely explained by cumulus phases exposed to evolving intercumulus liquids causing compositional ‘shifts.’ Ferrogabbro and diorite represent segregation veins from differentiated intercumulus liquids filter pressed into rigid zones of cooling lakes. Clinopyroxene geobarometry suggests < 300 bar P. Open-textured dunite represents olivine-melt mush, precursor to vertical olivine-rich bodies (as in Kilauea Iki). Its Fo83.5 identifies the most primitive lake magma as 8.3 wt.% MgO. Mass balancing and MELTS show that such a magma could have yielded both ferrogabbro and diorite by ≥ 50% fractional crystallization, but under different fO2: < FMQ (250 bar) led to diorite, and FMQ (250 bar) yielded ferrogabbro. These segregation veins, documented as similar to those of Kilauea, testify to appreciable volumes of ‘rhyolitic’ liquid forming in oceanic environments. Namely, SiO2-rich veins are intrinsic to all shields that reached caldera stage to accommodate various-sized cooling, differentiating lava lakes. 相似文献
The non-marine bivalve species Trigonioides (Kumamotoa) quadratus Gu and Ma is described from the Fourth Member of the Hekou Formation in western Fujian, southeastern China. This species previously was regarded to belong to the subgenus Trigonioides (s.s.), which is restricted to the Early Cretaceous. Herein, it is included into the subgenus Kumamotoa. It is compared to Cenomanian (early Late Cretaceous) species from Japan and Korea such as Trigonioides (Kumamotoa) mifunensis Tamura, Trigonioides (Kumamotoa) matsumotoi Kobayashi and Suzuki and Trigonioides (Kumamotoa) paucisulcatus Suzuki. The age of the Fourth Member of the Hekou Formation is discussed. 相似文献
The Bashikaogong-Shimierbulake granitoid complex is about 30 km long and 2―6 km wide, with an area of 140 km2, located at the north margin of the Bashikaogong Basin in the north Altun terrain. It intruded into schist, metapelite and metatuff of Precambrian ages. This granitoid complex consists of darkish quartz diorite, grey granite, pink granite and pegmatite. Geochemically, the quartz diorite has I-type granite affinity and belongs to Calc-alkaline sereies, and the other gran- ites have S-type affinity and to high-K calc-alkaline series. Zircon SHRIMP U-Pb dating shows that the quartz diorite has a bigger age than those of other granites, which is 481.6±5.6 Ma for quartz diorite, 437.0±3.0 Ma―433.1±3.4 Ma for grey granite and 443±11 Ma―434.6±1.6 Ma for pink granite, re- spectively. Combined with regional geology, we think that the quartz diorite formed in tectonic envi- ronment related to oceanic crust subduction and the granites in post-collision. 相似文献
The Bardoc Tectonic Zone (BTZ) of the late Archaean Eastern Goldfields Province, Yilgarn Craton, Western Australia, is physically
linked along strike to the Boulder-Lefroy Shear Zone (BLSZ), one of the richest orogenic gold shear systems in the world.
However, gold production in the BTZ has only been one order of magnitude smaller than that of the BLSZ (∼100 t Au vs >1,500 t
Au). The reasons for this difference can be found in the relative timing, distribution and style(s) of deformation that controlled
gold deposition in the two shear systems. Deformation within the BTZ was relatively simple and is associated with tight to
iso-clinal folding and reverse to transpressive shear zones over a <12-km-wide area of high straining, where lithological
contacts have been rotated towards the plane of maximum shortening. These structures control gold mineralisation and also
correspond to the second major shortening phase of the province (D2). In contrast, shearing within the BLSZ is concentrated to narrow shear zones (<2 km wide) cutting through rocks at a range
of orientations that underwent more complex dip- and strike-slip deformation, possibly developed throughout the different
deformation phases recorded in the region (D1–D4). Independent of other physico-chemical factors, these differences provided for effective fluid localisation to host units
with greater competency contrasts during a prolonged mineralisation process in the BLSZ as compared to the more simple structural
history of the BTZ. 相似文献