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101.
Cristina?PulidoEmail author Kaj?Sand-Jensen Esther?C.?H.?E.?T.?Lucassen Jan?G.?M.?Roelofs Klaus?P.?Brodersen Ole?Pedersen 《Aquatic Sciences - Research Across Boundaries》2012,74(2):351-360
Isoetids, as indicators of near-pristine softwater lakes, have a high priority in national and international (European Water
Directive Framework) assessments of ecological lake quality. Our main goal was to identify the most important environmental
factors that influence the composition of plant communities and specifically determine the presence and abundance of the isoetid
Lobelia dortmanna in NW European softwater lakes. Geographical position and composition of surface water, porewater, sediment and plant communities
were examined in 39 lakes in four regions (The Netherlands, Denmark, West Norway and East Norway) distributed over a 1,200-km
long distance. We confirmed that lake location was accompanied by significant changes in environmental variables between NW
European lakes. Lake location was the single most important determinant of vegetation composition and it had significant individual
contributions independent of the coupling to environmental variables. This influence of location was supported by a significant
decline of community similarity with geographical distance between pairs of lakes at regional, inter-regional and international
scales. Combining the geographical position with environmental variables for surface water, porewater and sediment significantly
improved prediction of vegetation composition. Specifically, the combination of latitude, surface water alkalinity, porewater
phosphate and redox potential offered the highest correlation (BIO ENV correlation 0.66) to vegetation composition. This complex
analysis can also account for high sediment variability in the littoral zone of individual lakes, by using site-specific physico-chemical
sediment factors, and offer better predictions of vegetation composition when lake water chemistry is relatively homogeneous
among lakes within regions. 相似文献
102.
Triassic stretching directions in Iberia and North Africa inferred from magnetic fabrics 总被引:1,自引:0,他引:1
Ruth Soto Antonio M. Casas‐Sainz Beln Oliva‐Urcia Cristina García‐Lasanta Esther Izquierdo‐Llavall Bennacer Moussaid Jos Carlos Kullberg Teresa Romn‐Berdiel Yolanda Snchez‐Moya Alfonso Sopea Sara Torres‐Lpez Juan Jos Villalaín Hamidou El‐Ouardi Inmaculada Gil‐Pea Ann M. Hirt 《地学学报》2019,31(5):465-478
During the Triassic, Iberia and western North Africa displayed a unique situation in relation with the Central and North Atlantic opening and westward expansion of the Tethys. Unravelling the stretching direction in Triassic deposits of the studied area can help in our understanding of this scenario. The tectonic setting is characterized by localized basins with strong thickness variations greatly influenced by previous post‐Variscan mechanical discontinuities. In this work, we revise and compile magnetic fabric data from eight Triassic depocentres in terms of defining the stretching direction (i.e. magnetic lineation), resulting from extensional deformation of this period. Data show the importance of the opening of the Atlantic rift as the leading process during the Triassic. Dextral transtension can explain the deflection of the extensional direction observed in most studied depocentres that is caused by the activity of previous major oblique faults. 相似文献
103.
Christopher J. Stevenson Peter J. Talling Esther J. Sumner Douglas G. Masson Micheal Frenz Russell B. Wynn 《Sedimentology》2014,61(7):1982-2019
Submarine gravity currents, especially long run‐out flows that reach the deep ocean, are exceptionally difficult to monitor in action, hence there is a need to reconstruct how these flows behave from their deposits. This study mapped five individual flow deposits (beds) across the Agadir Basin, offshore north‐west Africa. This is the only data set where bed shape, internal distribution of lithofacies, changes in grain size and sea floor gradient, bed volumes, flow thickness and depth of erosion into underlying hemipelagic mud are known for individual beds. Some flows were 30 to 120 m thick. However, flows with the highest fraction of sand were less than 5 to 14 m thick. Sand was most likely to be carried in the lower 5 to 7 m of these flows. Despite being relatively thin, one flow was capable of transporting very large volumes of sediment (ca 200 km3) for large distances across very flat sea floor. These observations show that these relatively thin flows could travel quickly enough on very low gradients (0·02° to 0·05°) to suspend sand several metres to tens of metres above the sea floor, and maintain those speeds for up to 250 km across the basin. Near uniform hemipelagic mud interval thickness between beds, and coccolith assemblages in the mud caps of beds, suggest that the flows did not erode significantly into the underlying sea floor mud. Simple calculations imply that some flows, especially in the proximal part of the basin, were powerful enough to have eroded hemipelagic mud if it was exposed to the flow. This suggests that the flows were depositional from the moment they arrived at a basin plain location, and that deposition shielded the underlying hemipelagic mud from erosion. Reproducing the field observations outlined in this exceptionally detailed field data set is a challenge for future experimental and numerical models. 相似文献
104.
Jürgen Gose Esther Schmädicke Margit Markowitz Anton Beran 《Mineralogy and Petrology》2010,99(1-2):105-111
The infrared (IR) spectra of gem-quality olivine crystals from Pakistan, formed in serpentinised dunitic rocks, are characterised by strongly pleochroic absorption bands at 3,613, 3,597, 3,580 and 3,566 cm?1. These bands are assigned to O-H stretching vibrations of OH point defects corresponding to H2O concentrations of about 35 wt ppm. Unlike other olivine spectra, the dominating bands are strongly polarised parallel to the b-axis. The unusual spectra type, excludes the presence of planar defects. This finding is supported by transmission electron microscopy. The 3,613 cm?1 band is related to vacant Si sites, the slightly lower energetic bands preferentially to vacant M2 sites. The exclusive presence of these bands is not only a characteristic feature of olivines treated under high P,T conditions equivalent to mantle environment, the presence of these bands in untreated natural olivine also indicates formation conditions equivalent to crustal rocks. 相似文献
105.
L. Gordon Medaris Brad S. Singer Brian R. Jicha David H. Malone Joshua J. Schwartz Esther K. Stewart Amanda Van Lankvelt Michael L. Williams Peter W. Reiners 《地学前缘(英文版)》2021,12(5):101174
New geochronologic data from midcontinental Laurentia demonstrate that emplacement of the 1476-1470 Ma Wolf River granitic batholith was not an isolated igneous event,but was accompanied by regional metamorphism,deformation,and sedimentation.Evidence for such metamorphism and deformation is best seen in siliciclastic sedimentary rocks of the Baraboo Interval,which were deposited closely following the1.65-1.63 Ga Mazatzal orogeny.In Baraboo Interval strata,muscovite parallel to slatey cleavage,in hydrothermal veins,in quartzite breccia,and in metamorphosed paleosol yielded ~(40)Ar/~(39)Ar plateau age s of 1493-1465 Ma.In addition,U-Th-total Pb dating of neoblastic overgrowths on detrital monazite gave an age of 1488±20 Ma,and recrystallized hematite in folded metapelite gave a mean U/Th-He age of 1411± 39 Ma.Post-Baraboo,arkosic polymictic conglomerate,which contains detrital zircon with a minimum peak age of 1493 Ma,was intruded by a 1470 Ma granite porphyry at the northeastern margin of the Wolf River batholith.This episode of magmatism,regional deformation and metamorphism,and sedimentation,which is designated herein as the Baraboo orogeny,provides a midcontinental link between the Picuris orogeny to the southwe st and the Pinware orogeny to the northeast,completing the extent of early Mesoproterozoic(Calymmian) orogenesis for 5000 km along the southern margin of Laurentia.This transcontinental orogen is unique among Precambrian orogenies for its great width(~1600 km),the predominance of ferroan granites derived from partial melting of lower continental crust,and the prevalence of re gional high T-P metamorphism related to advective heating by granitic magmas emplaced in the middle to upper crust. 相似文献