Holocene and Pleistocene sea-level indicators at the coast of Jericoacoara,Ceará, NE Brazil     

Georg Irion  Jáder Onofre de Morais  Friederike Bungenstock 《Quaternary Research》2012,77(2):251-257

Beach-rock exposures provide a record of Holocene sea-level rise along the 560-km-long northeast-facing coast of Ceará, Brazil, that differs from the record available along the other 4300 km of Brazilian coastline further south. Whereas documentation is available from southern Brazil to show Holocene sea levels as much as 5 m above today's level, our observations along the northeastern coast indicate that sea level here was not above the present-day level during the Holocene. Near Jericoacoara, about 240 km northwest of Fortaleza, characterized by strong surf, Precambrian rocks crop out from under a temporary cover of sand in small protected locations with less surf. Here in this upper tidal zone beach rock is being formed, while it is being dismembered synchronously by erosion at lower tide levels. This shows a rising sea level. Along the entire coast of Ceará west of Ponta Grossa the absence of beach rock higher than spring tide level indicates that sea-level was not above its present-day level during the Holocene.Notches in bedrock situated between 2 m and 6 m above spring-tide high-water level that we formerly described as Holocene, are now believed to be Sangamonian.  相似文献   

Carbonate diagenesis and feldspar alteration in fracture-related bleaching zones (Buntsandstein,central Germany): possible link to CO<Subscript>2</Subscript>-influenced fluid–mineral reactions     

Jens Wendler  Jens Köster  Jens Götze  Norbert Kasch  Norbert Zisser  Jonas Kley  Dieter Pudlo  Georg Nover  Reinhard Gaupp 《International Journal of Earth Sciences》2012,101(1):159-176

Fracture-related bleaching of Lower Triassic Buntsandstein red beds of central Germany was related to significant carbonate diagenesis and feldspar alteration caused by CO₂-rich fluids. Using cathodoluminescence microscopy and spectroscopy combined with electron microprobe analysis and stable carbon isotope study, two major fluid–mineral interactions were detected: (1) zoned, joint-filling calcites and zoned pore-filling calcite cements, the latter replacing an earlier dolomite, were formed during bleaching. During the calcite formation and dolomite–calcite transformation, iron was incorporated into the calcite cement crystal cores due to Fe availability from the coeval bleaching. The dedolomitisation was ultimately associated with a volume increase. The related permeability decrease implies a certain degree of sealing and increasing retention of CO₂, and the volume increase offers a minor CO₂ sink. Carbonate-rich sandstone, therefore, can provide advantages for underground CO₂ storage especially when situated in the fringes of the reservoir. (2) Alkali-feldspar alteration due to the bleaching fluids is reflected in cathodoluminescence spectra predominantly by the modulation of a brown luminescence emission peak (~620 nm). This peak represents a newly discovered effect related to alkali-feldspar alteration not solely associated with bleaching. Its modulation by the bleaching is interpreted to be due to Na depletion or a lattice defect in the Si–O bonds of the SiO₄-tetrahedron. Alteration reflected by this luminescence feature has a destructive effect on the feldspars implying the possibility of diminished rock integrity due to bleaching and, hence, CO₂-rich fluids. Two further CL spectral changes related to bleaching occur, (a) decreased intensity between around 570 nm assigned to Mn-depletion, and (b) increased amplitude and wavelength shift of the red (~680 nm) band. Converging evidence from carbonate and feldspar diagenesis, stable carbon isotope data and analysis of fracture directions suggests that CO₂ fluids contributed to a significant extent to the bleaching phenomena and alteration in the studied Buntsandstein strata.  相似文献   

Origin and extent of fresh groundwater, salty paleowaters and recent saltwater intrusions in Red River flood plain aquifers, Vietnam   总被引：1，自引：0，他引：1  

Luu T. Tran  Flemming Larsen  Nhan Q. Pham  Anders V. Christiansen  Nghi Tran  Hung V. Vu  Long V. Tran  Hoan V. Hoang  Klaus Hinsby 《Hydrogeology Journal》2012,20(7):1295-1313

A model has been established on the origin and extent of fresh groundwater, salty paleowaters and saltwater from recent seawater intrusions in the Red River flood plain in Vietnam. This was done with geological observations, geophysical borehole logging and transient electromagnetic methods. Salt paleowater is present up to 50–75?km from the coastline, with occurrence controlled by the Holocene transgression. A density-driven leaching of salty porewater has occurred from high-permeability Holocene sediments into underlying Pleistocene deposits, whereas diffusion has dominated in low-permeability layers. In the Pleistocene aquifer, the highest content of dissolved solids is found below two intrinsic valleys with Holocene marine sediments and along the coastline. Recent intrusion of saltwater from the South China Sea is observed in shallow groundwater 35?km inland, probably a result of transport of salty water inland in rivers or leaching of paleowaters from very young near-coast marine sediments. The observed inverted salinity profile, with high saline water overlying fresher groundwater, has been formed due to the global eustatic sea-level changes during the last 8,000–9,000?years. The proposed model may therefore be applicable to other coastal aquifers, with a proper incorporation of the local geological environments.  相似文献   

A 10Be‐based reconstruction of the last deglaciation in southern Sweden     

Johanna Anjar  Nicolaj K. Larsen  Lena Håkansson  Per Möller  Henriette Linge  Derek Fabel  Sheng Xu 《Boreas: An International Journal of Quaternary Research》2014,43(1):132-148

We present 23 cosmogenic surface exposure ages from 10 localities in southern Sweden. The new ¹⁰Be ages allow a direct correlation between the east and west coasts of southern Sweden, based on the same dating technique, and provide new information about the deglaciation of the Fennoscandian Ice Sheet in the circum‐Baltic area. In western Skåne, southernmost Sweden, a single cosmogenic surface exposure sample gave an age of 16.8±1.0 ka, whereas two samples from the central part of Skåne gave ages of 17.0±0.9 and 14.1±0.8 ka. Further northeast, in southern Småland, two localities gave ages ranging from 15.2±0.8 to 16.9±0.9 ka (n=5) indicating a somewhat earlier deglaciation of the area than has previously been suggested. Our third locality, in S Småland, gave ages ranging from 10.2±0.5 to 18.4±1.6 ka (n=3), which are probably not representative of the timing of deglaciation. In central Småland one locality was dated to 14.5±0.8 ka (n=3), whereas our northernmost locality, situated in northern Småland, was dated to 13.8±0.8 ka (n=3). Samples from the island of Gotland suggest deglaciation before 13 ka ago. We combined the new ¹⁰Be ages with previously published deglaciation ages to constrain the deglaciation chronology of southern Sweden. The combined deglaciation chronology suggests a rather steady deglaciation in southern Sweden starting at c. 17.9 cal. ka BP in NW Skåne and reaching northern Småland, ～200 km further north, c. 13.8 ka ago. Overall the new deglaciation ages agree reasonably well with existing deglaciation chronologies, but suggest a somewhat earlier deglaciation in Småland.  相似文献   

Flood disaster subcultures in The Netherlands: the parishes of Borgharen and Itteren     

Karen Engel  Georg Frerks  Lucia Velotti  Jeroen Warner  Bart Weijs 《Natural Hazards》2014,73(2):859-882

The Netherlands knows a persistent threat of flooding. To adapt to this dangerous reality, the Dutch have cultivated what disaster research literature has labeled ‘disaster subcultures’ or a set of cultural (tangible and intangible) tools to deal with the recurrent hazard. While there is abundant attention for the way the Dutch ‘coastal’ and ‘low-lying’ communities deal with the recurrent threat of (coastal) flooding, less is known about the way the Dutch ‘high-lands’ deal with the yearly threat of (fluvial) flooding. This article presents the findings of an explorative research endeavor (2011–2013) aimed at discerning if the disaster subculture concept has contemporary relevance in the Netherlands, particularly with respect to flooding, and if so, whether applying this lens would reveal more about the nature of existing disaster subcultures. Because less is known about the Dutch ‘high-lands,’ we chose to look into the existence and attributes of disaster subcultures in the parishes Borgharen and Itteren, which experience a systematic threat of flooding. Our findings suggest that the disaster subculture lens is valuable as it enables the empirical appreciation of disaster subcultures, even in a small country like the Netherlands, and it unveiled elements of these neighboring parishes’ flood reality that otherwise might have gone unnoticed and that seem central to understanding these two parishes’ levels of vulnerability and resilience. It is our contention that the concept ‘disaster subculture’ makes a greater understanding possible of the cultural context from which vulnerability and resilience to specific and recurrent threats emerge.  相似文献   

Deformation of dry and wet sandstone targets during hypervelocity impact experiments,as revealed from the MEMIN Program     

Elmar BUHL  Michael H. POELCHAU  Georg DRESEN  Thomas KENKMANN 《Meteoritics & planetary science》2013,48(1):71-86

Abstract– Hypervelocity impact experiments on dry and water‐saturated targets of fine‐grained quartz sandstone, performed within the MEMIN project, have been investigated to determine the effects of porosity and pore space saturation on deformation mechanisms in the crater’s subsurface. A dry sandstone cube and a 90% water‐saturated sandstone cube (Seeberger Sandstein, 20 cm side length, about 23% porosity) were impacted at the Fraunhofer EMI acceleration facilities by 2.5 mm diameter steel spheres at 4.8 and 5.3 km s^?1, respectively. Microstructural postimpact analyses of the bisected craters revealed differences in the subsurface deformation for the dry and the wet target experiments. Enhanced grain comminution and compaction in the dry experiment and a wider extent of localized deformation in the saturated experiment suggest a direct influence of pore water on deformation mechanisms. We suggest that the pore water reduces the shock impedance mismatch between grains and pore space, and thus reduces the peak stresses at grain–grain contacts. This effect inhibits profound grain comminution and effective compaction, but allows for reduced shock wave attenuation and a more effective transport of energy into the target. The reduced shock wave attenuation is supposed to be responsible for the enhanced crater growth and the development of “near surface” fractures in the wet target.  相似文献   

Magnetic and nuclear structure and thermal expansion of orthorhombic and monoclinic polymorphs of CoGeO3 pyroxene     

Günther Josef Redhammer  Anatoliy Senyshyn  Gerold Tippelt  Clemens Pietzonka  Georg Roth  Georg Amthauer 《Physics and Chemistry of Minerals》2010,37(5):311-332

CoGeO₃ was synthesized at 1,273 and 1,448 K using ceramic sintering techniques in the monoclinic and orthorhombic modification, respectively. The two compounds were analysed by magnetic susceptibility measurements and neutron diffraction in order to study magnetic ordering and spin structures at low temperature. The monoclinic form of CoGeO₃ has C2/c symmetry and orders magnetically below 36 K with a small negative paramagnetic Curie temperature θ _P = −4.6 (2) K. The magnetic structure can be described with k = (1, 0, 0) in the magnetic space group C2′/c′ having a ferromagnetic spin arrangement within the chains of M1 sites, but a dominating antiferromagnetic coupling between the chains. At the M1 sites the magnetic spins are aligned within the a–c plane forming an angle of 120° with the +a-axis and they are not parallel to the spins at M2. Here spins are also ferromagnetically coupled within, but antiferromagnetically coupled between the M1/M2 site bands. The orthorhombic phase of CoGeO₃ displays Pbca symmetry and transforms to an antiferromagnetically ordered state [θ _P = −18.6(2) K] below 33 K. The magnetic spin structure can be described with k = (0, 0, 0) in space group Pbca′ and it is similar to the one of the C2/c phase except that it is non-collinear in nature, i.e. there are components of the magnetic moment along all three crystallographic axes. Small magneto-elastic coupling is observed in the orthorhombic phase.  相似文献   

Numerical simulation of subsurface characterization methods: application to a natural aquifer analogue     

Janet Whittaker  Georg Teutsch 《Advances in water resources》1999,22(8):603

Information from an outcrop is used as an analogue of a natural heterogeneous aquifer in order to provide an exhaustive data set of hydraulic properties. Based on this data, two commonly used borehole based investigation methods are simulated numerically. For a scenario of sparse sampling of the aquifer, the process of regionalization of the borehole hydraulic conductivity values is simulated by application of a deterministic interpolation approach and conditioned stochastic simulations. Comparison of the cumulative distributions of particle arrival times illustrates the effects of the sparse sampling, the properties of the individual investigation methods and the regionalization methods on the ability to predict flow and transport behaviour in the real system (i.e. the exhaustive data set).  相似文献   

Late Quaternary biotic and abiotic controls on long-term sediment flux in a northern Australian tropical river system     

Annegret Larsen  Jan-Hendrik May  Xavier Carah 《地球表面变化过程与地形》2019,44(12):2494-2509

The modern distribution of monsoonal rainforest in the Australian tropics is patchy and is mainly associated with river corridors and groundwater springs, which indicates a strong dependence on hydrologic and geomorphic conditions. While their present distribution is well known, very little data exists on past spatial and temporal dynamics of these ecosystems, or their medium- to longer-term controls. Factors such as (i) fire frequency and type, and/or (ii) hydroclimatic conditions (e.g. droughts) have been proposed to control riverine corridor rainforest extent. Recent observations, however, also suggest an additional (iii) geomorphic control induced by alluvial knickpoint migration. Sediment sequences provide valuable archives for the reconstruction of longer-term (a) floodplain sedimentary dynamics, (b) local vegetation history, and (c) catchment-wide fire histories. This study investigates such a sediment sequence at Wangi Creek, and shows that a phase of aggradation, lasting ~4000 years, was recently disrupted by channel incision and floodplain erosion. The aggradational phase is characterized by sand deposition with average vertical floodplain accretion rates of 0.8 cm/yr and includes phases of soil development. The recent incisional phase has changed hydro-geomorphic conditions and caused widespread degradation of vegetation, erosion and lowering of the macro-channel surface. While there is no evidence in our data for an erosional event of similar magnitude since the onset of late Holocene floodplain aggradation, Wangi Creek experienced significant erosion and incision immediately before ~4000 years, providing the first evidence for a tropical cut-and-fill river system. We hence argue that phases of aggradation mainly controlled by biotic processes alternate and depend on feedbacks with incision phases controlled mainly by abiotic processes. The results show that eco-hydro-geomorphic feedbacks may play a crucial role in the medium- to longer-term history of tropical fluvial systems and need to be considered when interpreting fluvial archives with regards to climate, fire or human induced change. © 2019 John Wiley & Sons, Ltd.  相似文献   

Upper Cretaceous bottom current deposits,north-east Greenland     

Jussi Hovikoski  Alfred Uchman  Rikke Weibel  Henrik Nøhr-Hansen  Emma Sheldon  Jon Ineson  Morten Bjerager  Jens Therkelsen  Mette Olivarius  Michael Larsen  Peter Alsen  Jørgen Bojesen-Koefoed 《Sedimentology》2020,67(7):3619-3654

Reported ancient bottom current deposits in deep marine settings are scarce and most of them remain contentious. This study describes sedimentological, ichnological and petrographical characteristics of a drill core that covers ca 10 Myr of Upper Cretaceous stratigraphy at Hold with Hope, north-east Greenland. The core is divided into four facies associations, which are interpreted to reflect deposition from bottom currents, turbidity flows and hemipelagic settling in slope and/or near slope environments. The evidence for bottom current influence is three-fold. Firstly, pervasive indications of winnowing such as marine bioclast-rich lags and outsized clasts on ‘mud on mud’ contacts are suggestive of low-sediment concentration flows capable of transporting up to pebble-sized clasts. Common Mn–Fe–Mg rich carbonate matrix cements and various types of hiatal chemogenic lag deposits showing glauconite, apatite and carbonate clasts also point to condensation, prolonged exposure at the sediment–water interface and recurrent phases of sea-floor erosion. Secondly, such deposits can show indicators for tidal processes such as double mud-drapes, tangential bottom sets in dune-scale cross-bedding and cyclic rhythmites. Thirdly, inverse to normal grading at various scales is common in fully marine, commonly seafloor-derived sediments. Ichnological data indicate considerable taxonomic variability in the bottom current deposits, but recurrent fabrics are characteristically dominated by morphologically simple burrows such as Thalassinoides and Planolites, with secondary Phycosiphon, Nereites, Zoophycos and/or Chondrites. In general, opportunistic taxa are common whereas mature composite ichnofabrics are rare. The omission surfaces are locally burrowed with stiffground to firmground trace fossil suites. The results contribute to establishing sedimentological, ichnological and mineralogical criteria for recognition of bottom current deposits as well as to the understanding of the Late Cretaceous palaeoenvironmental evolution of the Arctic region.  相似文献