Glacimarine diamicts are produced by diverse processes, and genetic differentiation is often problematic using macro-sedimentological
criteria alone. Micromorphology offers a potentially helpful tool in such investigations. Macroscopically massive diamict
samples of known glacimarine origin, from the Polar North Atlantic, Antarctica and north Irish Sea, were prepared for micromorphological
analysis to (1) identify microstructures unique to different modes of sedimentation and (2) interpret genetic processes from
those structures. The samples comprised examples of debris-flow, iceberg-turbate and suspension settling deposits from late
Quaternary glacier-influenced marine environments: tidewater glacier, sub- or pro-ice shelf and continental slopes in front
of ice stream termini. Results show two significant features of debris-flow sediments: a bimodal grain fabric of near-horizontal
and -vertical grains, and laminated clay and silt coatings on sand and pebble grains. Coatings are best developed in sediments
with finer grain-size distributions and in debris-flow sediments which have had relatively long run-out distances on trough-mouth
fans, suggesting continuous rotation of grains in a buoyant, turbulent aqueous environment. This is significant because it
precludes debris-flow delivery by plug flow. The micromorphology of iceberg turbate has not been described previously. It
contains structures similar to those described in tills, so that unambiguous identification of these sediments seems unlikely
based on micromorphological criteria alone. Suspension sediments range from fine-grained massive diamicts containing microfossils
to more heterogeneous coarser sediments characterised by abrupt textural variations, from ice-distal and ice-proximal glacimarine
environments respectively. The ice-proximal sediments contain fine vertical lineations marking the trajectories of dropstones
through wet matrix. These dropstone tracks have not been reported in previous studies. 相似文献
The study of the extreme values of a variable such as wave height is very important in flood risk assessment and coastal design. Often values above a sufficiently large threshold can be modelled using the Generalized Pareto Distribution, the parameters of which are estimated using maximum likelihood. There are several popular empirical techniques for choosing a suitable threshold, but these require the subjective interpretation of plots by the user.In this paper we present a pragmatic automated, simple and computationally inexpensive threshold selection method based on the distribution of the difference of parameter estimates when the threshold is changed, and apply it to a published rainfall and a new wave height data set. We assess the effect of the uncertainty associated with our threshold selection technique on return level estimation by using the bootstrap procedure. We illustrate the effectiveness of our methodology by a simulation study and compare it with the approach used in the JOINSEA software. In addition, we present an extension that allows the threshold selected to depend on the value of a covariate such as the cosine of wave direction. 相似文献
The available data on known reserves of extracted resources have not previously been tabulated and graphed. In this article we compile the reserves data in one place for the first time. This serves to (1) show the trends in the reserves series, (2) make the series available to others, and (3) place the reserves data in the context of price data for which longer series have been available, and with which they can now be seen to be consistent in both showing increasing availability rather than increasing scarcity. 相似文献
14C has been measured in three North American and seven Antarctic meteorites with the Chalk River MP tandem accelerator. In most cases cosmogenic14C, which is tightly bound, was separated from absorbed atmospheric radiocarbon by stepwise heating extractions. Terrestrial ages obtained by comparing cosmogenic14C in the meteorite to that in Bruderheim are (7.2 ± 0.6) × 103 years for Yamato 7304, (11.6 ± 0.4) × 103 years for Estacado, and range from (32.7 ± 0.5) × 103 to (41.0 ± 0.8) × 103 years for six meteorites recovered at Allan Hills and its vicinity. The present upper limit to age determination by the accelerator method varies from 50 × 103 to 70 × 103 years depending upon mass and carbon content of the sample. The natural limit caused by cosmic ray production of14C in silicate rocks at 2000 m elevation is estimated to be (55 ± 5) × 103 years. “Weathering ages” were estimated for the Antarctic meteorites from the specific activity of loosely-bound CO2 considered to be absorbed from the terrestrial atmosphere on weathering. The accelerator measurements are in accordance with previous low-level counting measurements but have higher precision and sensitivity. 相似文献
The meteoric input of36Cl due to cosmogenic or nuclear-weapon-produced36Cl cannot contribute significantly to the36Cl present in the saline groundwaters (up to 700 mg l−1 Cl−) from the Stripa granite. The extent of in-situ production of36Cl has been estimated on the basis of the neutron fluxes within the granite and its surrounding leptite. The36Cl present in the groundwaters is attributed to either admixture of labelled Cl− from the leptite with Cl− from the granite or to the total derivation of groundwater chlorinity within the leptite followed by radiochemical ingrowth of36Cl during subsequent groundwater residence within the granite. The chloride derived from the leptite may be either matrix chloride or chloride from an external source which has had a long residence time within the leptite. The implications of36Cl in-situ production for the estimation of groundwater residence times and for the geochemical evolution of groundwater chlorinity are discussed. 相似文献
Continental flood basalts are usually regarded as a single tectonomagmatic entity but frequently quoted examples exhibit a variety of tectonic settings. In one well-studied, classic, flood basalt province, the Mesozoic Karoo province of southern Africa, magmatism occurred in the following tectonic settings: (a) continental rifting leading to ocean-floor spreading in the South Atlantic Ocean (Etendeka suite of Namibia); (b) stretched continental lithosphere and rifting not leading directly to ocean-floor formation (Lebombo suite of southeastern Africa); and (c) an a-tectonic, within-plate, continental setting characterized by an absence of faulting or warping (Lesotho highlands and Karoo dolerites of South Africa). By means of spidergrams of the elements Rb, Ba, Th, Nb, K, La, Ce, Sr, Nd, P, Hf, Zr, Sm, Ti, Tb, Y, V, Ni and Cr, uncontaminated tholeiites from (c) above [i.e. the Lesotho-type continental flood basalts (LTCFB)] are compared with mid-ocean ridge basalts (MORB), ocean-island tholeiites (OIT), and tholeiites and calc-alkali basalts from subduction environments. The comparison reveals the LTCFBs are geochemically distinct. The differences are reflected in relative enrichments or depletions of the more incompatible elements (Rb-Ce) to less incompatible elements (Ce-Y), i.e. the overall slope of the spidergrams, and in anomalous enrichments or depletions of one or more of the elements Th, K, Nb, Sr, Ti, Hf, and Zr. The distinctive geochemical character of the Lesotho LTCFBs is interpreted in terms of a lithospheric mantle source for the basalts. This is supported by isotopic data. There are no major geochemical differences between Lesotho CFBs and basalts of the rift-related Etendeka and Lebombo suites, although the latter are somewhat enriched in Rb, Ba and K. However, unlike the Lesotho basalts, the Lebombo and Etendeka basalts are associated with voluminous silicic volcanics or intrusive centres and late-stage dolerites having MORB/OIT (i.e. asthenospheric) geochemical characteristics. The flood basalt/silicic magmatism/late-stage dyke swarm association is characteristic of several rift or thinned lithosphere environments (e.g., Ethiopia, Skye, eastern Greenland) but in many of these the flood basalts have ocean-island basalt (OIT) geochemical characteristics. The Lesotho-type CFB geochemistry is exhibited by the Grande Ronde Basalt of the Columbia River Group (a possible subduction-related flood basalt province) and the basic rocks associated with Mesozoic rifting in the North and South Atlantic. Basalt geochemistry alone is unhelpful in determining the tectonic setting of CFBs although the rift-related environments may be identified by the petrology and geochemistry of the whole igneous suite. A two-source model is proposed for the mantle-derived basic rocks in rift-related CFB provinces. Early enriched basalts are derived from the lithosphere and, following pronounced lithospheric attenuation or rifting, later MORB-like melts are emplaced from the rising asthenosphere. The presence of both Lesotho- and OIT-type geochemical patterns in rift-related CFBs suggests that the lithosphere exhibits different styles of enrichment. 相似文献
The coast of Honduras, Central America, represents the southern end of the Mesoamerican Barrier Reef System, although its marine resources are less extensive and studied than nearby Belize and Mexico. However, the coastal zone contains mainland reef formations, mangroves, wetlands, seagrass beds and extensive fringing reefs around its offshore islands, and has a key role in the economy of the country. Like most tropical areas, this complex of benthic habitats experiences limited annual variation in climatic and oceanographic conditions but seasonal and occasional conditions, particularly coral bleaching and hurricanes, are important influences. The effects of stochastic factors on the country's coral reefs were clearly demonstrated during 1998 when Honduras experienced a major hurricane and bleaching event. Any natural or anthropogenic impacts on reef health will inevitably affect other countries in Latin America, and vice versa, since the marine resources are linked via currents and the functioning of the system transcends political boundaries. Much further work on, for example, movement of larvae and transfer of pollutants is required to delineate the full extent of these links.
Anthropogenic impacts, largely driven by the increasing population and proportion of people living in coastal areas, are numerous and include key factors such as agricultural run-off, over-fishing, urban and industrial pollution (particularly sewage) and infrastructure development. Many of these threats act synergistically and, for example, poor watershed management via shifting cultivation, increases sedimentation and pesticide run-off onto coral reefs, which increases stress to corals already affected by decreasing water quality and coral bleaching. Threats from agriculture and fishing are particularly significant because of the size of both industries. The desire to generate urgently required revenue within Honduras has also led to increased tourism which provides an over-arching stress to marine resources since most tourists spend time in the coastal zone. Hence the last decade has seen a dramatic increase in coastal development, a greater requirement for sewage treatment and more demand for freshwater, particularly in the Bay Islands.
Although coastal zone management is relatively recent in Honduras, it is gaining momentum from both large-scale initiatives, such as the Ministry of Tourism's ‘Bay Islands Environmental Management Project', and national and international NGO projects. For example, a series of marine protected areas and legislative regulations have been established, but management capacity, enforcement and monitoring are limited by funding, expertise and training. Existing and future initiatives, supported by increased political will and environmental awareness of stakeholders, are vital for the long-term economic development of the country. 相似文献
Several strike–slip faults at Crackington Haven, UK show evidence of right-lateral movement with tip cracks and dilatational jogs, which have been reactivated by left-lateral strike–slip movement. Evidence for reactivation includes two slickenside striae on a single fault surface, two groups of tip cracks with different orientations and very low displacement gradients or negative (left-lateral) displacements at fault tips.
Evidence for the relative age of the two strike–slip movements is (1) the first formed tip cracks associated with right-lateral slip are deformed, whereas the tip cracks formed during left-lateral slip show no deformation; (2) some of the tip cracks associated with right-lateral movement show left-lateral reactivation; and (3) left-lateral displacement is commonly recorded at the tips of dominantly right-lateral faults.
The orientation of the tip cracks to the main fault is 30–70° clockwise for right-lateral slip, and 20–40° counter-clockwise for left-lateral slip. The structure formed by this process of strike–slip reactivation is termed a “tree structure” because it is similar to a tree with branches. The angular difference between these two groups of tip cracks could be interpreted as due to different stress distribution (e.g., transtensional/transpressional, near-field or far-field stress), different fracture modes or fractures utilizing pre-existing planes of weakness.
Most of the d–x profiles have similar patterns, which show low or negative displacement at the segment fault tips. Although the d–x profiles are complicated by fault segments and reactivation, they provide clear evidence for reactivation. Profiles that experienced two opposite slip movements show various shapes depending on the amount of displacement and the slip sequence. For a larger slip followed by a smaller slip with opposite sense, the profile would be expected to record very low or reverse displacement at fault tips due to late-stage tip propagation. Whereas for a smaller slip followed by larger slip with opposite sense, the d–x profile would be flatter with no reverse displacement at the tips. Reactivation also decreases the ratio of dmax/L since for an original right-lateral fault, left lateral reactivation will reduce the net displacement (dmax) along a fault and increase the fault length (L).
Finally we compare Crackington Haven faults with these in the Atacama system of northern Chile. The Salar Grande Fault (SGF) formed as a left-lateral fault with large displacement in its central region. Later right-lateral reactivation is preserved at the fault tips and at the smaller sub-parallel Cerro Chuculay Fault. These faults resemble those seen at Crackington Haven. 相似文献
