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181.
Detachment of the sliver-like Cimmerian terrane from eastern Gondwana in the Early Permian triggered mafic volcanism in many parts of the rift zone. To understand this tectonic episode we have carried out paleomagnetic investigations on mafic volcanic for-mations that were erupted on key terranes that now form part of Tibet. Specifically, we will present data from sections near Lhasa City (central Lhasa block) and Tuotuohe (central Qiangtang Block) as well as near Gyanyima (Paleotethyan sea-mount) that was emplaced onto the floor of Palaeotethys during the Late Permian. Paleomagnetic plots from each location will be used for tectonic calculations. Our new data will be used to evaluate regional scale models con-cerned with how the Cimmerian terranes in southern and SE Asia (from Iran-Tibet-SW China-Myanmar- Thailand-Sumatra) formerly abutted eastern Gond-wana. 相似文献
182.
Hydraulic and flood-loss modeling of levee,floodplain, and river management strategies,Middle Mississippi River,USA 总被引:4,自引:2,他引:2
In this investigation, four scenarios were used to quantify the balance between the benefits of levees for flood protection
and their potential to increase flood risk using Hazards U.S. Multi-Hazard flood-loss software and hydraulic modeling of the
Middle Mississippi River (MMR). The goals of this study were (1) to quantify the flood exposure under different flood-control
configurations and (2) to assess the relative contributions of various engineered structures and flood-loss strategies to
potential flood losses. Removing all the flood-control structures along the MMR, without buyouts or other mitigation, reduced
the average flood stages between 2.3 m (100-year flood) and 2.5 m (500-year), but increased the potential flood losses by
$4.3–6.7 billion. Removing the agricultural levees downstream of St. Louis decreased the flood stages through the metro region
by ~1.0 m for the 100- and 500-year events; flood losses, without buyouts or other mitigation, were increased by $4.3–6.7 billion. Removing the agricultural levees downstream of St. Louis decreased the flood stages through the metro region
by ~1.0 m for the 100- and 500-year events; flood losses, without buyouts or other mitigation, were increased by 155 million
for the 100-year flood, but were decreased by $109 million for the 500-year flood. Thus, agricultural levees along the MMR
protect against small- to medium-size floods (up to the ~100-year flood level) but cause more damage than they prevent during
large floods such as the 500-year flood. Buyout costs for the all the buildings within the 500-year floodplain downstream
of urban flood-control structures near St. Louis are ~40% less than the cost of repairing the buildings damaged by the 500-year
flood. This suggests large-scale buyouts could be the most cost-effective option for flood loss mitigation for properties
currently protected by agricultural levees. 相似文献
183.
Wafa M. Al Lawati Athanasios Rizoulis Elisabeth Eiche Christopher Boothman David A. Polya Jonathan R. Lloyd Michael Berg Patricio Vasquez-Aguilar Bart E. van Dongen 《Applied Geochemistry》2012
High concentrations of geogenic As in the groundwaters of south and SE Asia, which are used as drinking waters, are causing severe health impacts to the exposed human populations. It is widely accepted that As mobilisation from sediments into these shallow reducing groundwaters requires active metal-reducing microbes and electron donors such as organic matter (OM). Although OM in such Holocene aquifers has been characterised, there is a dearth of data on Pleistocene aquifers from the same areas. Reported here are preliminary studies of OM and microbial communities present in two aquifers, one of Pleistocene and one of Holocene age, with contrasting concentrations of As (viz. Pleistocene: low As <10 μg/L; Holocene: high As up to 600 μg/L) from Van Phuc village in the Red River Delta, Vietnam. Results revealed OM inputs from multiple sources, including potential contributions from naturally occurring petroleum seeping into the shallow aquifer sediments from deeper thermally mature source rocks. Although concentrations vary, no noticeable systematic differences in biomarker distribution patterns within the OM were observed between the two sites. Microbial analyses did not show a presence of microbial communities previously associated with As mobilisation. All clone libraries were dominated by α-, β-, and γ-Proteobacteria not known to be able to reduce Fe(III) or sorbed As(V). Furthermore, representatives of the Fe(III)-reducing genus Geobacter could only be detected at very low abundance by PCR, using highly selective 16S rRNA gene primers, supporting the hypothesis that metal reduction is not a dominant in situ process in these sediments. No correlation between As concentration in groundwater and OM composition nor microbial community in the host sediments was found. This suggests that either (i) As is not being significantly mobilised in situ in these sediments, instead As appears to be mobilised elsewhere and transported by groundwater flow to the sites or (ii) sorption/desorption processes, as implicated by geochemical data from the cores, play a critical role in controlling As concentrations at these sites. 相似文献
184.
Priyadarsi D. Roy M.P. Jonathan Ma. Consuelo Macias José Luis Sánchez Rufino Lozano S. Srinivasalu 《Chemie der Erde / Geochemistry》2012,72(1):91-95
The tsunami of 11th March 2011 was originated at the east coast of Japan and deposited ca.1 cm thick sediment layer along the coast of southwestern Mexico up to a maximum distance of 320 m from the beach. The sedimentological, mineralogical and geochemical characteristics of the sediments deposited during the tsunami (JT) are compared with the pre-tsunami sediments (PRT). JT sediments consist of dominant coarser fractions (>54% of medium to coarse sand), whereas PRT deposits comprise abundant finer fractions (>58% of fine sand). Assemblage of mafic and heavy minerals suggests similar provenance for both. The higher abundance and variation of heavy minerals along with higher concentrations of bromine (Br) and sodium (Na) in the JT deposits reveal the influence of high energy sea waves in transportation of heavy mineral rich coarse sediments onto the coastal lowlands. 相似文献
185.
Jonathan D. Radley 《Proceedings of the Geologists' Association. Geologists' Association》2012,123(2):233-234
This paper summarises the author's research association with Percival (‘Perce’) Allen FRS (1917–2008), whose wide-ranging and seminal contributions to Wealden (non-marine Lower Cretaceous) sedimentological and palaeoenvironmental interpretation spanned seven decades. The Geological Conservation Review (GCR) Special Issue on the Wealden was initiated as a collaborative research programme initiated during the late 1990s. Stemming from this, the GCR accounts are seen very much as the summation of Perce Allen's lifelong Wealden studies, as demonstrated by GCR sites throughout southern England. 相似文献
186.
Jonathan D. Radley Percival Allen 《Proceedings of the Geologists' Association. Geologists' Association》2012,123(2):374-381
Non-marine Lower Cretaceous beds of Wealden aspect have long been known from the northern margin of the Wessex–Weald Basin, between Wiltshire and the south Midlands. Termed the Whitchurch Sands Formation, these badly exposed and generally poorly fossiliferous strata appear to represent interdigitating alluvial and brackish-marine units separated by significant sedimentary breaks. Geological Conservation Review sites within the Whitchurch Sands are described and interpreted for their chronostratigraphic, palaeoenvironmental and palaeoclimatic significance. 相似文献
187.
Recently published work estimates that global sea level rise (SLR) approaching or exceeding 1 m by 2100 is plausible, thus significantly updating projections by the Fourth Assessment of the Intergovernmental Panel on Climate Change. Furthermore, global greenhouse gas (GHG) emissions over the 21st century will not only influence SLR in the next ??90 years, but will also commit Earth to several meters of additional SLR over subsequent centuries. In this context of worsening prospects for substantial SLR, we apply a new geospatial dataset to calculate low-elevation areas in coastal cities of the conterminous U.S.A. potentially impacted by SLR in this and following centuries. In total, 20 municipalities with populations greater than 300,000 and 160 municipalities with populations between 50,000 and 300,000 have land area with elevations at or below 6 m and connectivity to the sea, as based on the 1 arc-second National Elevation Dataset. On average, approximately 9% of the area in these coastal municipalities lies at or below 1 m. This figure rises to 36% when considering area at or below 6 m. Areal percentages of municipalities with elevations at or below 1?C6 m are greater than the national average along the Gulf and southern Atlantic coasts. In contrast to the national and international dimensions of and associated efforts to curb GHG emissions, our comparison of low-elevation areas in coastal cities of the conterminous U.S.A. clearly shows that SLR will potentially have very local, and disproportionate, impacts. 相似文献
188.
Small rivers commonly discharge into coastal settings with topographic complexities - such as headlands and islands - but these settings are underrepresented in river plume studies compared to more simplified, straight coasts. The Elwha River provides a unique opportunity to study the effects of coastal topography on a buoyant plume, because it discharges into the Strait of Juan de Fuca on the western side of its deltaic headland. Here we show that this headland induces flow separation and transient eddies in the tidally dominated currents (O(100 cm/s)), consistent with other headlands in oscillatory flow. These flow conditions are observed to strongly influence the buoyant river plume, as predicted by the “small-scale” or “narrow” dynamical classification using Garvine's (1995) system. Because of the transient eddies and the location of the river mouth on the headland, flow immediately offshore of the river mouth is directed eastward twice as frequently as it is westward. This results in a buoyant plume that is much more frequently “bent over” toward the east than the west. During bent over plume conditions, the plume was attached to the eastern shoreline while having a distinct, cuspate front along its westernmost boundary. The location of the front was found to be related to the magnitude and direction of local flow during the preceding O(1 h), and increases in alongshore flow resulted in deeper freshwater mixing, stronger baroclinic anomalies, and stronger hugging of the coast. During bent over plume conditions, we observed significant convergence of river plume water toward the frontal boundary within 1 km of the river mouth. These results show how coastal topography can strongly influence buoyant plume behavior, and they should assist with understanding of initial coastal sediment dispersal pathways from the Elwha River during a pending dam removal project. 相似文献
189.
Holocene monsoon climate documented by oxygen and carbon isotopes from lake sediments and peat bogs in China: a review and synthesis 总被引:1,自引:0,他引:1
Jiawu Zhang Fahu Chen Jonathan A. Holmes Hui Li Xiaoyan Guo Junlan Wang Shuang Li Yanbin Lü Yan Zhao Mingrui Qiang 《Quaternary Science Reviews》2011,30(15-16):1973-1987
There has been much recent debate about Holocene climate variation in the monsoon region of China, especially the temporal pattern of variations in precipitation, the time-transgressive nature of the Holocene precipitation maximum, and the extent to which variations in regions influenced by the Indian Summer Monsoon (ISM) and the East Asian Summer Monsoon (EASM) have been synchronous. We summarize and compare carbonate oxygen-isotope records (δ18Ocarb) from ten lakes within the present-day ISM region. We discuss their paleoclimate significance considering the present-day moisture source, isotopic composition of precipitation and the hydrological setting. The δ18Ocarb records are controlled mainly by the isotopic composition of lake water, which in turn is a function of regional Precipitation/Evaporation (P/E) balance and the proportion of precipitation that is monsoon-derived. We normalized the δ18Ocarb data and used these records to generate an integrated moisture index. This index, along with oxygen-isotope records from speleothems and carbon-isotope records (δ13Corg) from peats within the monsoon region, suggests that Holocene climate was broadly synchronous across the monsoon region and, within the limits of accuracy of the existing age models, provides no strong evidence for previously-proposed anti-phasing of the ISM and the EASM. Stable-isotope records from lake sediments and peat bogs have excellent potential for providing high-quality paleoclimate data for monsoon Asia, and complement high-resolution speleothem sequences, which are only found in certain localities. 相似文献
190.
Simon J. Haward Mark M. Smits Kristín Vala Ragnarsdóttir Jonathan R. Leake Terence J. McMaster 《Geochimica et cosmochimica acta》2011,75(22):6870-6881
We have used a direct imaging technique, in situ atomic force microscopy (AFM), to observe the dissolution of the basal biotite surface by oxalic acid over a range of temperatures close to ambient conditions, using a specially designed AFM liquid cell and non-invasive intermittent contact mode of operation. From the 3-dimensional nanometre-resolution data sets, we observe a process characterised by the slow formation of shallow etch pits in the (0 0 1) surface and fast growth of etch pits from the resulting steps, which represent proxies for the {h k 0} surface. Measurements of dissolution rates as a function of temperature allow a determination of an apparent activation energy (Ea,app) for the process, via mass-loss calculations from image analysis. We obtain a value of Ea,app = 49 ± 2 kJ mol−1, which is consistent with separate calculations based on planar area etch pit growth, and measurements of etch pit perimeters, indicating that this value of Ea,app is representative of {h k 0} surface dissolution. The measurement of etch pit perimeters also enables an estimation of apparent activation energy as a function of step density indicating substantially higher apparent activation energy, up to Ea,app = 140 kJ mol−1, on extrapolation towards a pristine surface with no defects. We suggest that this higher value of Ea,app represents the slow formation of etch pits into the (0 0 1) surface. 相似文献