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31.
Many ecosystems have evolved under conditions where ground water is critical to the survival of the unique species contained therein. One example is Nebraska's Sand Hills fens. The Sand Hills region comprises lush interdunal valleys amid grass-stabilized dunes. One valley type, fens, are ground water-fed wetlands that have several meters of saturated, organic soils comprising peat formed under anaerobic conditions where plant growth exceeds decomposition. These fens support unique flora and fauna such as rush aster and northern redbelly dace that are typically found in more boreal regions of North America. This study characterized the hydrogeology of a representative fen in the Jumbo Valley to explore how these relict species survive in a hot, arid climate and to understand present and historical relations between ground water and these unique ecosystems. Results indicate that ground water's present role in sustaining the fens is twofold: (1) wetland plant transpiration of shallow ground water across the valley creates a cooler, humid microclimate that sustains the fens in the region's hot, arid climate and (2) the large volume of ground water pumped through the fens by plant transpiration transports necessary nutrients to the root zone. Results suggest that, in the past, in addition to its sustaining role, ground water also played a vital role in fen development and in fen preservation through nearly 15,000 years of existence. Thus, as with like ecosystems, the role of ground water must be considered in future Sand Hills fen protection, restoration, and management. 相似文献
32.
The consideration of uncertainties plays an increasing role in the design of geotechnical structures. An important procedure in the uncertainty analysis is the reliability assessment whereby the required statistical quantities and distributions are assumed to be known exactly. Due to small sample observations and missing information this is not the case in practical applications and uncertainties in the stochastic parameters themselves have to be considered. In our study we estimate the parameter variation of soil properties by statistical procedures and perform an extended reliability analysis of a shallow foundation considering these uncertainties. We propose a novel approach based on a Taylor series expansion which enables the estimation of the failure probability variation very efficiently. 相似文献
33.
The archaeological site of Qasr Tilah, in the Wadi Araba, Jordan is located on the northern Wadi Araba fault segment of the Dead Sea Transform. The site contains a Roman-period fort, a late Byzantine–Early Umayyad birkeh (water reservoir) and aqueduct, and agricultural fields. The birkeh and aqueduct are left-laterally offset by coseismic slip across the northern Wadi Araba fault. Using paleoseismic and archaeological evidence collected from a trench excavated across the fault zone, we identified evidence for four ground-rupturing earthquakes. Radiocarbon dating from key stratigraphic horizons and relative dating using potsherds constrains the dates of the four earthquakes from the sixth to the nineteenth centuries. Individual earthquakes were dated to the seventh, ninth and eleventh centuries. The fault strand that slipped during the most recent event (MRE) extends to just below the modern ground surface and juxtaposes alluvial-fan sediments that lack in datable material with the modern ground surface, thus preventing us from dating the MRE except to constrain the event to post-eleventh century. These data suggest that the historical earthquakes of 634 or 659/660, 873, 1068, and 1546 probably ruptured this fault segment. 相似文献
34.
Anaerobic oxidation of methane and sulfate reduction along the Chilean continental margin 总被引：3，自引：0，他引：3
Tina Treude Jutta Niggemann Paul Wintersteller Antje Boetius Bo Barker Jørgensen 《Geochimica et cosmochimica acta》2005,69(11):2767-2779
Anaerobic oxidation of methane (AOM) and sulfate reduction (SR) were investigated in sediments of the Chilean upwelling region at three stations between 800 and 3000 m water depth. Major goals of this study were to quantify and evaluate rates of AOM and SR in a coastal marine upwelling system with high organic input, to analyze the impact of AOM on the methane budget, and to determine the contribution of AOM to SR within the sulfate-methane transition zone (SMT). Furthermore, we investigated the formation of authigenic carbonates correlated with AOM. We determined the vertical distribution of AOM and SR activity, methane, sulfate, sulfide, pH, total chlorins, and a variety of other geochemical parameters. Depth-integrated rates of AOM within the SMT were between 7 and 1124 mmol m−2 a−1, effectively removing methane below the sediment-water interface. Single measurements revealed AOM peaks of 2 to 51 nmol cm−3 d−1, with highest rates at the shallowest station (800 m). The methane turnover was higher than in other diffusive systems of similar ocean depth. This higher turnover was most likely due to elevated organic matter input in this upwelling region offering significant amounts of substrates for methanogenesis. SR within the SMT was mostly fuelled by methane. AOM led to the formation of isotopically light DIC (δ13C: −24.6‰ VPDB) and of distinct layers of authigenic carbonates (δ13C: −14.6‰ VPDB). 相似文献
35.
Tina Svan Hansen 《Singapore journal of tropical geography》2005,26(2):170-190
This paper focuses on the large‐scale land use and land cover changes that have taken place in Sarawak state, East Malaysia over the three decades of 1972‐2002. Results are presented from a detailed land use and cover change (LUCC) study in the Niah River catchment using satellite imagery, questionnaire surveys and interviews. Successive waves of land cover changes have taken place. Large forest areas have been logged and gradually replaced by oil palm plantations, which now occupy more than 40 per cent of the total land area in the catchment. Concurrently, small‐scale farming systems have also changed. Formerly dominant Iban shifting cultivation practices are increasingly being replaced by cash crop production on permanent fields and impacted by off‐farm activities involving many ethnic groups. It is argued that land cover changes are continuous and complex processes involving a large number of variables which can be analysed for different time periods at various scales. 相似文献
36.
Late Pleistocene and Holocene slip rate of the Northern Wadi Araba fault, Dead Sea Transform, Jordan
Tina M. Niemi Hongwei Zhang Mohammad Atallah J. Bruce J. Harrison 《Journal of Seismology》2001,5(3):449-474
The Wadi Araba Valley is a morphotectonic depression along part of theDead Sea Transform (DST) plate boundary that separates the Arabian plateon the east from the Sinai subplate on the west. The Wadi Araba fault(WAF) is the main strike-slip faults one of between the Gulf of Aqaba and the E-Wtrending Khunayzira (Amatzayahu) fault that bounds the southern end ofthe Dead Sea. Just south of the Dead Sea, the WAF cuts across severalgenerations of alluvial fans that formed on tributaries to the Wadi Dahalafter the regression of Late Pleistocene Lake Lisan ca. 15 ka. Geomorphicand stratigraphic evidence of active faulting, including left-laterally offsetstream channels and alluvial-fan surfaces, yielded fault slip-rate data for thenorthern segment of WAF. Typical cumulative displacements of 54 m,39 m, and 22.5 m of stream channels and alluvial-fan surfaces acrossthe fault were measured from detailed geologic and topographic mapping.The 54 m offset of the oldest alluvial-fan surface (Q
f1
) occurredafter the final lowering of Lake Lisan (16–15 ka) and before 11 ka yieldinga slip-rate range of 3.4 mm/yr to 4.9 mm/yr. Based on radiocarbonages of charcoal and landsnail shell samples from the buried Q
f2
alluvial-fan deposits exposed in trenches excavated across the fault, the39 m and 22.5 m offsets occurred after 9 ka and 5.8 ka, respectively. These data yield a slip-rate range between 3.9 mm/yr and 6.0 mm/yr.The small variability in these slip-rate estimates for different time periodssuggests that the northern Wadi Araba fault has maintained a relativelyconstant slip rate in the past 15 ka. We calculate an average slip rate of 4.7± 1.3 mm/yr since 15 ka based on the three separate displacementsand age estimates. Five separate offsets of 3 m were measured from gullybends and the offset of small fault-scarp alluvial fans. These displacementdata suggest a coseismic slip of 3 m in the last earthquake, or acumulative slip of 3 m in the past few earthquakes. A maximum slip of3 m correspond to a Mw 7 earthquake that ruptures about 49 km offault length. Using an average slip rate of 4.7 ± 1.3 mm/yr togetherwith a 3-m slip-per-event suggests a maximum earthquake recurrence intervalof this fault segment of 500 to 885 years. 相似文献
37.
Alessandro Amorosi Giulia Barbieri Luigi Bruno Bruno Campo Tina M. Drexler Wan Hong Veronica Rossi Irene Sammartino Daniele Scarponi Stefano Claudio Vaiani Kevin M. Bohacs 《Sedimentology》2019,66(7):3029-3052
Although general trends in transgressive to highstand sedimentary evolution of river‐mouth coastlines are well‐known, the details of the turnaround from retrogradational (typically estuarine) to aggradational–progradational (typically coastal/deltaic) stacking patterns are not fully resolved. This paper examines the middle to late Holocene eustatic highstand succession of the Po Delta: its stratigraphic architecture records a complex pattern of delta outbuilding and coastal progradation that followed eustatic stabilization, since around 7·7 cal kyr bp . Sedimentological, palaeoecological (benthic foraminifera, ostracods and molluscs) and compositional criteria were used to characterize depositional conditions and sediment‐dispersal pathways within a radiocarbon‐dated chronological framework. A three‐stage progradation history was reconstructed. First, as soon as eustasy stabilized (7·7 to 7·0 cal kyr bp ), rapid bay‐head delta progradation (ca 5 m year?1), fed mostly by the Po River, took place in a mixed, freshwater and brackish estuarine environment. Second, a dominantly aggradational parasequence set of beach‐barrier deposits in the lower highstand systems tract (7·0 to 2·0 cal kyr bp ) records the development of a shallow, wave‐dominated coastal system fed alongshore, with elongated, modestly crescent beaches (ca 2·5 m year?1). Third, in the last 2000 years, the development of faster accreting and more rapidly prograding (up to ca 15 m year?1) Po delta lobes occurred into 30 m deep waters (upper highstand systems tract). This study documents the close correspondence of sediment character with stratal distribution patterns within the highstand systems tract. Remarkable changes in sediment characteristics, palaeoenvironments and direction of sediment transport occur across a surface named the ‘A–P surface’. This surface demarcates a major shift from dominantly aggradational (lower highstand systems tract) to fully progradational (upper highstand systems tract) parasequence stacking. In the Po system, this surface also reflects evolution from a wave‐dominated to river‐dominated deltaic system. Identifying the A–P surface through detailed palaeoecological and compositional data can help guide interpretation of highstand systems tracts in the rock record, especially where facies assemblages and their characteristic geometries are difficult to discern from physical sedimentary structures alone. 相似文献
38.
39.
Tatiana Goded Andrew Beaupre Michael DeMarco Tina Dutra Andro Gogichaishvili Daniel Haley Alex Hyman Nicholas Kepka Calvetti John Potter Maureen Coomer Kim Wright Andrew King 《Natural Hazards》2017,87(1):185-212
The Canterbury (New Zealand) earthquake sequence of 2010–2012 caused unexpectedly extreme levels of damage and disruption, being an unparalleled event in New Zealand in terms of the damage extent. Christchurch’s heritage buildings were seriously damaged during these events, with churches especially affected in 22 February 2011 M w 6.2 earthquake. During this earthquake, a total of 84% of the heritage unreinforced stone and 81% of the clay brick masonry churches in the Canterbury region were either considered unsafe (receiving red placards) or with restricted access (yellow placards). Following the earthquakes, authorities across New Zealand are reassessing the capacity of older buildings to resist earthquakes. Current legislation requires that a building judged as earthquake prone either be strengthened by retrofitting or be demolished within a legislated number of years. Many building owners are facing the problems of owning earthquake-prone buildings and lacking the funding to upgrade. This affects both community and heritage buildings, resulting in the likely abandonment or demolition of some buildings. To address the problem of the balance between life safety and preservation in the Wellington Region, this project gathered and compared the perspectives of the general public, church communities, heritage specialists, professional engineers, and local authorities to assist in balancing the interests of these stakeholders. As a result of the findings, several recommendations have been provided that include standardizing structural assessment processes and training, feasibility of additional public funding to upgrade buildings, new signage to increase public awareness of earthquake-prone buildings, and regular communication among stakeholders to understand and resolve differences. 相似文献
40.
The Campanian Cliff House Formation represents a series of individually progradational shoreface tongues preserved in an overall landward-stepping system. In the Mancos Canyon area, the formation consists of four, 50- to 55-m-thick and 10- to 20-km-wide sandstone tongues, which pinch out landwards into lower coastal plain and lagoonal deposits of the Upper Menefee Formation and seawards into offshore shales of the Lewis Shale Formation. Photogrammetric mapping of lithofacies along the steep and well-exposed canyon walls was combined with sedimentary facies analysis and mapping of the detailed facies architecture. Two major facies associations have been identified, one comprising the mostly muddy and organic-rich facies of lagoonal and lower coastal plain origin and one comprising the sandstone-dominated facies of shoreface origin. Key stratigraphic surfaces were identified by combining the mapped geometry of the lithofacies units with the interpretation of depositional processes. The stratigraphic surfaces (master ravinement surface, shoreface/coastal plain contact, transgressive surface, maximum flooding surface and the sequence boundary) allow each major sandstone tongue to be divided into a simple sequence, consisting of a basal transgressive system tract (TST) overlain by a highstand system tract (HST). Within each sandstone tongue, a higher frequency cyclicity is evident. The high-frequency cycles show a complex stacking pattern development and are commonly truncated in the downdip direction by surfaces of regressive marine erosion. The complexities of the Cliff House sandstone tongues are believed to reflect changes in the rate of sea-level rise combined with the responses of the depositional system to these changes. Synsedimentary compaction, causing a thickness increase in the sandstone tongues above intervals of previously uncompacted lagoonal/coastal plain sediments, also played a role. This study of the facies architecture, geometry and sequence stratigraphy of the Cliff House Formation highlights the fact that there may be some problems in applying conventional sequence stratigraphical methods to landward-stepping systems in general. These difficulties stem from the fact that no single stratigraphic surface can easily be identified and followed from the non-marine to the fully marine realm (i.e. from the landward to the basinward pinch-out of the sandstone tongues). In addition, the effects of synsedimentary compaction and changes in the shoreface dynamics are not easily recognized in limited data sets such as from the subsurface. 相似文献