Pleistocene mass-flow deposits of basaltic hyaloclastite on a shallow submarine shelf,South Iceland     

Steffen G Bergh  Gudmundur E Sigvaldason 《Bulletin of Volcanology》1991,53(8):597-611

A Pleistocene subaqueous, volcanic sequence in South Iceland consists of flows of basaltic hyaloclastite and lava with interbedded sedimentary diamictite units. Emplacement occurred on a distal submarine shelf in drowned valleys along the southern coast of Iceland. The higher sea level was caused by eustatic sea-level change, probably towards the end of a glaciation. This sequence, nearly 700 m thick, rests unconformably on eroded flatlying lavas and sedimentary rocks of likely Tertiary age. A Standard Depositional Unit, describing the flows of hyaloclastite, starts with compact columnar-jointed basalt overlain by cubejointed basalt, and/or pillow lava. This in turn is overlain by thick unstructured hyaloclastite containing aligned basalt lobes, and bedded hyaloclastite at the top. A similar lithofacies succession is valid for proximal to distal locations. The flows were produced by repeated voluminous extrusions of basaltic lava from subaquatic fissures on the Eastern Rift Zone of Iceland. The fissures are assumed to lie in the same general area as the 1783 Laki fissure which produced 12 km³ of basaltic lava. Due to very high extrusion rates, the effective water/melt ratio was low, preventing optimal fragmentation of the melt. The result was a heterogeneous mass of hyaloclastite and fluid melt which moved en masse downslope with the melt at the bottom of the flow and increasingly vesicular hyaloclastite fragments above. The upper and distal parts of the flow moved as low-concentration turbulent suspensions that deposited bedded hyaloclastite.  相似文献   

The polarization of X-ray emission of some solar flares in July 1974     

I. P. Tindo  A. I. Shuryghin  W. Steffen 《Solar physics》1976,46(1):219-227

Preliminary results of polarization measurements at three solar flares of July 5 and 6, 1974, are given. The measurements were performed at h 15 keV with Thomson-scattering polarimeter on the Intercosmos 11 satellite.At the decay phase of the flare on July 5 we obtained P = 3.4–5.0%, which did not exclude some contribution from non-thermal processes. At the flares of July 6 we found P =1.0–1.9% and P = 0.1–2.0%, respectively: neither value exceeds the level of statistical fluctuations.Presented at XVIII COSPAR Meeting (VARNA, May–June 1975), contribution III.B.2.9.  相似文献   

Common-Reflection-Surface Stack for Converted Waves     

Bergler  Steffen  Duveneck  Eric  Höcht  German  Zhang  Yonghai  Hubral  Peter 《Studia Geophysica et Geodaetica》2002,46(2):165-175

The finite-offset (FO) common-reflection-surface (CRS) stack has been shown to be able to handle not only P-P or S-S but also arbitrarily converted reflections. It can provide different stack sections such as common-offset (CO), common-midpoint (CMP) and common-shot (CS) sections with significantly increased signal-to-noise ratio from the multi-coverage pre-stack seismic data in a data-driven way. It is our purpose in this paper to demonstrate the performance of the FO CRS stack on data involving converted waves in inhomogeneous layered media. In order to do this we apply the FO CRS stack for common-offset to a synthetic seismic data set involving P-P as well as P-S converted primary reflections. We show that the FO CRS stack yields convincing improvement of the image quality in the presence of noisy data and successfully extracts kinematic wavefield attributes useful for further analyses. The extracted emergence angle information is used to achieve a complete separation of the wavefield into its P-P and P-S wave components, given the FO CRS stacked horizontal and vertical component sections.  相似文献   

Holocene peat and carbon accumulation rates in the southern taiga of western Siberia     

Wiebe Borren  Wladimir Bleuten 《Quaternary Research》2004,61(1):42-51

Although recent studies have recognized peatlands as a sink for atmospheric CO₂, little is known about the role of Siberian peatlands in the global carbon cycle. We have estimated the Holocene peat and carbon accumulation rate in the peatlands of the southern taiga and subtaiga zones of western Siberia. We explain the accumulation rates by calculating the average peat accumulation rate and the long-term apparent rate of carbon accumulation (LORCA) and by using the model of Clymo (1984, Philosophical Transactions of the Royal Society of London Series B 303, 605-654). At three key areas in the southern taiga and subtaiga zones we studied eight sites, at which the dry bulk density, ash content, and carbon content were measured every 10 cm. Age was established by radiocarbon dating. The average peat accumulation rate at the eight sites varied from 0.35 ± 0.03 to 1.13 ± 0.02 mm yr⁻¹ and the LORCA values of bogs and fens varied from 19.0 ± 1.1 to 69.0 ± 4.4 g C m⁻² yr⁻¹. The accumulation rates had different trends especially during the early Holocene, caused by variations in vegetation succession resulting in differences in peat and carbon accumulation rates. The indirect effects of climate change through local hydrology appeared to be more important than direct influences of changes in precipitation and temperature. River valley fens were more drained during wetter periods as a result of deeper river incision, while bogs became wetter. From our dry bulk density results and our age-depth profiles we conclude that compaction is negligible and decay was not a relevant factor for undrained peatlands. These results contribute to our understanding of the influence of peatlands on the global carbon cycle and their potential impact on global change.  相似文献   

Clam community composition and prey shell size impacts moon snail (Gastropod: Naticidae) drilling frequencies in South Carolina,USA     

Dimitar B. Zlatev  Michael C. Lordon  Emma A. Schremp  Andrew C. Steffen  Allison E. Grecco  Jason S. Bystriansky  Jalene M. LaMontagne 《Marine Ecology》2019,40(3)

Moon snail predation on clams is a common model system of predator–prey interactions. In this system, the predator bores through the shell of its prey, leaving a distinct and identifiable hole. Some paleoecological and behavioral research on moon snails suggests a trend in predation preference directed toward clams with small shells. Rarely, however, have studies tested relative drilling frequencies across species and size ranges in natural assemblages of clam communities. We examined the clam community composition at two beaches in South Carolina, USA, and we then tested moon snail predator preferences for (a) clam prey species and (b) whether their selection is related to prey shell size. We collected a total of 1,879 clam shells, identified each shell to species and recorded their anteroposterior length. The species composition of clams differed significantly between the two beaches; Anadara ovalis was dominant at both sites, but three of ten total species were only collected at one beach. Folly Beach had nearly a 60% higher the overall drilling frequency (34.6%) versus Edisto Beach (21.8%), and this may be linked to the differences in clam community compositions at the sites. For A. ovalis and Mulinia lateralis, shells with larger lengths have lower probabilities of being bored by a moon snail. Anadara brasiliana, which generally is a thinner‐shelled clam species, had the highest total drilling frequency (77.2%), and Noetia ponderosa, a thicker‐shelled clam, had a considerably lower drilling frequency (12.0%). We conclude that both community level factors (species composition) and population characteristics (shell size distributions) may influence the local drilling frequency by moon snails.  相似文献   

Assessing quality of volunteer crowdsourcing contributions: lessons from the Cropland Capture game   总被引：3，自引：0，他引：3  

Carl F. Salk  Tobias Sturn  Linda See  Steffen Fritz  Christoph Perger 《International Journal of Digital Earth》2016,9(4):410-426

Volunteered geographic information (VGI) is the assembly of spatial information based on public input. While VGI has proliferated in recent years, assessing the quality of volunteer-contributed data has proven challenging, leading some to question the efficiency of such programs. In this paper, we compare several quality metrics for individual volunteers’ contributions. The data were the product of the ‘Cropland Capture’ game, in which several thousand volunteers assessed 165,000 images for the presence of cropland over the course of 6 months. We compared agreement between volunteer ratings and an image's majority classification with volunteer self-agreement on repeated images and expert evaluations. We also examined the impact of experience and learning on performance. Volunteer self-agreement was nearly always higher than agreement with majority classifications, and much greater than agreement with expert validations although these metrics were all positively correlated. Volunteer quality showed a broad trend toward improvement with experience, but the highest accuracies were achieved by a handful of moderately active contributors, not the most active volunteers. Our results emphasize the importance of a universal set of expert-validated tasks as a gold standard for evaluating VGI quality.  相似文献   

Depositional setting,provenance, and tectonic-volcanic setting of Eocene–Recent deep-sea sediments of the oceanic Izu–Bonin forearc,northwest Pacific (IODP Expedition 352)     

Alastair H. F. Robertson  Steffen Kutterolf  Aaron Avery  Alan T. Baxter  Katerina Petronotis  Gary D. Acton 《International Geology Review》2018,60(15):1816-1854

New biostratigraphical, geochemical, and magnetic evidence is synthesized with IODP Expedition 352 shipboard results to understand the sedimentary and tectono-magmatic development of the Izu–Bonin outer forearc region. The oceanic basement of the Izu–Bonin forearc was created by supra-subduction zone seafloor spreading during early Eocene (c. 50–51 Ma). Seafloor spreading created an irregular seafloor topography on which talus locally accumulated. Oxide-rich sediments accumulated above the igneous basement by mixing of hydrothermal and pelagic sediment. Basaltic volcanism was followed by a hiatus of up to 15 million years as a result of topographic isolation or sediment bypassing. Variably tuffaceous deep-sea sediments were deposited during Oligocene to early Miocene and from mid-Miocene to Pleistocene. The sediments ponded into extensional fault-controlled basins, whereas condensed sediments accumulated on a local basement high. Oligocene nannofossil ooze accumulated together with felsic tuff that was mainly derived from the nearby Izu–Bonin arc. Accumulation of radiolarian-bearing mud, silty clay, and hydrogenous metal oxides beneath the carbonate compensation depth (CCD) characterized the early Miocene, followed by middle Miocene–Pleistocene increased carbonate preservation, deepened CCD and tephra input from both the oceanic Izu–Bonin arc and the continental margin Honshu arc. The Izu–Bonin forearc basement formed in a near-equatorial setting, with late Mesozoic arc remnants to the west. Subduction-initiation magmatism is likely to have taken place near a pre-existing continent–oceanic crust boundary. The Izu–Bonin arc migrated northward and clockwise to collide with Honshu by early Miocene, strongly influencing regional sedimentation.  相似文献   

M1, M2 site populations and distortion parameters in synthetic Mg-Fe orthopyroxenes from Mössbauer spectra and X-ray structure refinements     

M. Chiara Domeneghetti  Gabriele Steffen 《Physics and Chemistry of Minerals》1992,19(5):298-306

Synthetic orthopyroxenes free of trivalent cations Mg _x Fe _2-x Si₂O₆ (x=0; 1; 1.6) were investigated using both Mössbauer spectroscopy (MS) and single-crystal X-ray diffraction (XRD) in order to compare the results obtained on the same samples by the two techniques. The Mg-Fe distribution over M1 and M2 octahedral sites was determined on compounds with x = 1 and x=1.6, unheated and annealed at 650, 850 and 1040° C for 29 to 42 hours to obtain different degrees of order. The data obtained by the two techniques are in very good agreement, thus suggesting, for these compositions, the absence of systematic errors. The investigation of a synthetic compound with x=0 (ferrosilite) confirmed that the accuracy of both methods is very good. The observed small discrepancies between the results are not significant if compared with the relevant standard deviations which, for both methods, were derived by leastsquares calculations. The best fit is obtained for Fe(M1)/ Fe(tot) parameters that, in both methods, are directly calculated without using data from chemical analysis. For the unheated samples the values by MS and XRD are respectively: 0.185(9) and 0.180(4) for x = 1.6; 0.282(8) and 0.285(2) for x=1; 0.503(7) and 0.502(1) for x=0. For all the investigated samples the correlation between the geometrical distortion parameters, calculated by X-ray refinement, and the quadrupole splitting values, measured by Mössbauer spectroscopy, agrees with the theoretical model of Ingalls (1964).  相似文献   

青藏高原东缘晚新生代成都盆地物源分析与水系演化   总被引：15，自引：0，他引：15  

李勇  黎兵  Steffen D  Densmore A L  Richardson N J  周荣军  Ellis M A  张毅 《沉积学报》2006,24(3):309-320

成都盆地位于青藏高原东缘，夹于龙门山与龙泉山之间，盆地中充填了3.6Ma以来的大邑砾岩、雅安砾石层和晚更新世—全新世砾石层，其物源均来源于盆地西侧的龙门山，具横向水系和单向充填的特征。本次以物源区分析作为切入点，以岷江和青衣江水系为重点，采用砾岩成分分析、砂岩岩屑成分分析、重矿物分析和砾石的地球化学分析等基本方法，开展青藏高原东缘晚新生代以来的古水系重建工作，研究结果表明，成都盆地主要有两个物源区，其中成都盆地北部的都江堰街子场、崇州白塔山、大邑白岩沟、大邑氮肥厂、彭州丁家湾、彭州葛仙山等剖面中的砾石层在碎屑成分、重矿物和花岗岩砾石的地球化学成分等方面相似，应为古岷江的产物，而其与现代岷江在砾岩成分和重矿物特征等方面的差异性则表明古岷江可能存在改道的现象；成都盆地南部的庙坡剖面和熊坡东剖面中的砾石层在碎屑成分、重矿物和花岗岩砾石的地球化学成分等方面相似，应为古青衣江的产物，但其流向却与现代青衣江的流向不同，表明熊坡背斜是在大邑砾岩沉积之后隆起的，它的隆起迫使古青衣江改道。  相似文献   

Affine distortion of small GPS networks with large height differences     

Steffen Schön 《GPS Solutions》2007,11(2):107-117

GPS is a promising tool for real-time monitoring of deformations of slopes or large structures. However, remaining systematic effects in GPS phase observations after double differencing and application of a priori models affect the resulting coordinates. They complicate the proper separation of the actual deformations from pseudo-deformations induced by the systematic effects. This paper shows that for small monitoring networks (baseline lengths <5 km) only affine distortions of the network geometry are generated by the remaining distance dependent systematic effects, e.g. unmodelled tropospheric and ionospheric propagation effects, or satellite orbit errors. Hence, a generic correction model is given by a three-dimensional affine transformation involving a maximum of 12 transformation parameters. For the determination of these parameters, four high quality GPS stations are necessary which are not affected by the actual deformations to be monitored. Based on the analysis of network geometries of synthetic GPS networks with large height differences and considering the physics of the GPS observations it is shown, however, that less than 12 parameters are sufficient for the computation of the corrections. The proposed 8 parameter model was applied to the GPS monitoring network of the Gradenbach landslide. For this small network with large height differences, it was shown that the distortions can be reduced by about 75%.  相似文献