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981.
U. Morgenstern R. G. Ditchburn E. G. Vologina M. Sturm 《Journal of Paleolimnology》2013,50(3):345-352
We applied the 32Si dating technique to a sediment core from Lake Baikal to obtain the sediment chronology for the last millennium. The core was recovered about 4 km offshore from the north slope of the South Basin in 1,366 m water depth. The sediment material consisted of continuously accumulated diatom-rich geogenic-terrigenous mud, intercalated with a number of dark olive-grey turbidite layers. The sediment layers containing the turbidites were excluded from 32Si sampling to obtain the chronology that is representative of the continuous sedimentation. The initial 32Si activity of 31.3 dpm kg?1 SiO2, measured in sediment trap samples, confirms the trend of 32Si specific activities of biogenic silica found in other Northern Hemisphere lakes. The four sediment core samples from depth 0–48 cm have 32Si specific activities between 23.5 and 0.5 dpm kg?1 SiO2, with corresponding ages between 60 and 860 years and constant sedimentation rate of 0.036 ± 0.004 cm year?1 over the most recent 800 years. 32Si allowed us for the first time to date the uppermost turbidites in the South Basin of Lake Baikal, to 1030, 1310 and 1670 ad. Given these dates, the last long-distance turbidity current triggered by slope instabilities had occurred 330 years before 2000 ad, and the intervals between the 1310 and 1670 ad event and between the 1030 and 1310 ad event were 360 and 280 years, respectively. The 32Si ages allow unprecedented time resolution for reconstruction of the former environmental and climatic conditions during the past millennium. 相似文献
982.
L. Sedda P.M. Atkinson M.R. Filigheddu G. Cotzia S. Dettori 《International journal of geographical information science》2013,27(7):1083-1096
Cork oak is one of the most valuable natural forest genera in the Mediterranean basin. Modelling cork oak growth has been a challenge for foresters in recent years because of strong site and genetic influences, below-ground competition, management regimes and age effects. Because cork productivity is related to forest height, which is, in turn, related directly with site characteristics, an increase in the accuracy of height prediction implies improved productivity estimation. A Bayesian maximum entropy (BME) geostatistical model was applied to characterize the space–time pattern of height of young cork oak in a forest stand from central Sardinia in the years 2000, 2002, 2003, 2006 and 2008. Cork oak height maps were produced for each of the 5 years. The main goals were to analyse and interpret through time (i) the changes in spatial correlation and (ii) the changes in spatial distribution of cork oak height. The plantation was characterized by an increasing spatial dependence through time, whereas the temporal range was 2 years. Cork oak height was significantly correlated with wind speed (reduced by a neighbouring forest) in all the years implying a single trend. The correlations were larger for 2006 and 2008 than for previous years. Three other environmental variables (shade, elevation and slope) were less significant and their influence restricted to 2 years only. This research has several implications for the management of cork oak in the young phase. 相似文献
983.
Andy Breckenridge Thomas V. Lowell Timothy G. Fisher Shiyong Yu 《Journal of Paleolimnology》2012,47(3):313-326
The evolution of the early Great Lakes was driven by changing ice sheet geometry, meltwater influx, variable climate, and
isostatic rebound. Unfortunately none of these factors are fully understood. Sediment cores from Fenton Lake and other sites
in the Lake Superior basin have been used to document constantly falling water levels in glacial Lake Minong between 9,000
and 10,600 cal (8.1–9.5 ka) BP. Over three meters of previously unrecovered sediment from Fenton Lake detail a more complex
lake level history than formerly realized, and consists of an early regression, transgression, and final regression. The initial
regression is documented by a transition from gray, clayey silt to black sapropelic silt. The transgression is recorded by
an abrupt return to gray sand and silt, and dates between 9,000 and 9,500 cal (8.1–8.6 ka) BP. The transgression could be
the result of increased discharge from Lake Agassiz overflow or the Laurentide Ice Sheet, and hydraulic damming at the Lake
Minong outlet. Alternatively ice advance in northern Ontario may have blocked an unrecognized low level northern outlet to
glacial Lake Ojibway, which switched Lake Minong overflow back to the Lake Huron basin and raised lake levels. Multiple sites
in the Lake Huron and Michigan basins suggest increased meltwater discharges occurred around the time of the transgression
in Lake Minong, suggesting a possible linkage. The final regression in Fenton Lake is documented by a return to black sapropelic
silt, which coincides with varve cessation in the Superior basin when Lake Agassiz overflow and glacial meltwater was diverted
to glacial Lake Ojibway in northern Ontario. 相似文献
984.
G. Wilson D. Damaske H.-D. Möller K. Tinto T. Jordan 《Geophysical Journal International》2007,170(1):93-100
Magnetic anomaly data are presented from a new helicopter-borne high-resolution aeromagnetic survey in southern McMurdo Sound. Anomaly data have been acquired at a common 305 m elevation above the McMurdo and southern McMurdo ice shelves and draped over the volcanic islands that pin them. The resulting anomaly patterns provide a significant advance in the understanding of the rift related geology beneath the floating ice shelves. More extensive Erebus Volcanic Province (McMurdo Volcanic Group) rocks are indicated along with a significant blanket of glaci-volcaniclastic sediment on the seafloor between the volcanic islands in southern McMurdo Sound. These glaci-volcaniclastic sediments are inferred to originate from former grounding of the southern McMurdo Ice Shelf as a marine ice sheet. A strong N–S fabric is also observed in the anomaly data suggesting that the rift structure observed in the Victoria Land basin persists to the south beneath the McMurdo and southern McMurdo ice shelves. W–N–W transfer faults identified within the Transantarctic Mountain rift flank to the west are not obvious in the aeromagnetic data set, implying that the 'Discovery Accommodation Zone' may be restricted to the region between a southward extension of the range bounding fault that marks the limit of the Victoria Land Basin and the right lateral offset in the Transantarctic Mountain front in southern Victoria Land. 相似文献
985.
986.
G. Anagnostou 《Rock Mechanics and Rock Engineering》1993,26(4):307-331
Summary In this paper, the phenomenon of swelling in tunnelling will be treated as a hydraulic-mechanical coupled process. This approach allows one to model the observed floor heaves realistically, i. e. without the prediction inevitable in the previous models of movements at the tunnel crown and walls. Furthermore, the development of heave and pressure over the course of time can be studied. The absence of deformations above the floor level is here interpreted as a consequence of the hydraulic boundary conditions. Besides the importance of seepage flow, the influence of rock strength is illustrated. Swelling rock is considered as an elastoplastic material. This allows one to predict the often large haaves of a tunnel floor as observed in situ. According to the numerical results, the area of practically relevant swelling strains extends as far as the plastic zone. 相似文献
987.
Oceanic crust west of North America at the beginning of the Jurassic belonged to the Kula plate. The development of the western margin of North America since the Jurassic reflects interaction with the Kula plate, the Kula-Farallon spreading center and the Farallon plate. The Kula plate ceased to exist in the Paleocene and later developments were caused by interaction of the Farallon plate and, subsequently, collision with the East Pacific Rise.At the beginning of the Jurassic, when spreading between North and South America began, the Kula-Farallon-Pacific triple junction moved to the north relative to North America, and the eastern end of the Kula-Farallon spreading center swept northwards along the continental margin.During the Paleocene, Kula-Pacific spreading ceased and the Kula plate fused to the Pacific plate. Throughout the Mesozoic, subduction of the Kula plate took place along the Alaskan continental margin. When the Kula plate joined the Pacific plate a new subduction zone formed along the line of the present Aleutian chain.Wrangellia and Stikinia, anomalous terrains in Alaska and northwestern Canada respectively, were emplaced by transport on the Kula plate from lower latitudes. Hypotheses which require transport of these plates in the Mesozoic from the “far reaches of the Pacific” ignore the problem of transport across either the Kula-Pacific or Kula-Farallon spreading centers. The interaction of the Kula plate and western North America throughout the Jurassic and the Cretaceous should result in emplacement of these terrains by motion oblique to the continental margin. Tethyan faunas in Stikinia must come from the western end of Tethys between North and South America, not the Indonesian region at the eastern end of Tethys.As the northeastern end of the Kula-Farallon ridge moved northward, the sense of motion changed from right lateral shear between the Kula and North American plates to collision or left lateral shear between the Farallon and North American plates. Left lateral shear along zones analogous to the Mojave-Sonora megashear may have been the means by which anomalous terrains were transported to the southeast into the gap between North and South America forming present day Central America. Such a model overcomes the overlap difficulties suffered in previous attempts to reconstruct the Mesozoic paleogeography of Central America. 相似文献
988.
H.V. Warren S.J. Horsky A. Kruckeberg G.H.N. Towers J.E. Armstrong 《Journal of Geochemical Exploration》1983,18(3):169-173
Summary of biogeochemical prospecting for mercury mineralization in the Pinchi Fault is given. One thousand two hundred and eight plant samples were collected and analyzed for mercury. In mineralized areas the typical mercury content of dried plants was not less than 0.2 and 0.4 μg/g and even concentrations between 0.6 and 1.6 μg/g were observed. In nonmineralized zones at least 90% of the plants contained no more than 0.15 μg/g of mercury. Analytical and sampling procedures are described. 相似文献
989.
Astronomy Reports - A large fraction of the energy released during the gravitational collapse of the core of a massive star is carried by neutrinos. Neutrinos play the main role in explaining... 相似文献
990.
Golionko B. G. Basilyan A. E. Nikolsky P. A. Kostyleva V. V. Malyshev N. A. Verzhbitsky V. E. Obmetko V. V. Borodulin A. A. 《Geotectonics》2019,53(6):675-699
Geotectonics - Detailed lithological, stratigraphic, and structural studies of the fold-thrust structures were conducted on New Siberia Island. We have established that the jointly deformed... 相似文献