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51.
The lipids in a sediment core from Lake Valencia, a hypereutrophic freshwater lake in Venezuela, are examined to understand
environmental changes over the last ∼13,000 years. From the latest Pleistocene to the earliest Holocene, total organic carbon
(TOC) substantially increased from 2.2 to 10%, while total organic carbon over total nitrogen (TOC/TN) decreased from as high
as 34 to as low as 10. Correspondingly, the concentration of terrestrially derived triterpenoids markedly decreased, and the
dominant n-alkane shifted from C31 to C23 or C25. During the same period, algal biomarkers such as botryococcenes, dinosterol, isoarborinol, C20 HBIs and 1,15C32 keto-ol markedly increased in abundance. These changes suggested a greater contribution of algal organic matter at the onset
of the Holocene, which was concurrent with increasing rainfall and the formation of a permanent lake (Lake Valencia) in the
Aragua Valley, Venezuela. The age profile of Paq, a n-alkane based proxy, showed large oscillations (0.20–0.81), reflecting historical variations in source strength of submerged/floating
vs. terrestrial/emergent OM inputs. An abrupt increase in tetrahymanol abundance at ∼7,260 cal years BP suggests the establishment
of an oxic–anoxic boundary in the lake’s water column. After reaching its maximum abundance at ∼2,100 cal year BP, botryococcenes,
a biomarker of Botryococcus braunii, gradually decreased to below the detection limit in the uppermost sediments, while different algal/microbial biomarkers
such as diploptene, dinosterol and isoarborinol substantially increased. These different historical profiles of algal/microbial
biomarkers reflect different responses of source organisms to environmental changes throughout this period. The δ13C determinations presented exceptionally enriched values for botryococcene isomers (−7.7 to −15.1‰), indicating the utilization
of bicarbonate as carbon sources in an extremely productive ecosystem. 相似文献
52.
Nazli Yonca Aydin H. Sebnem Duzgun Friedemann Wenzel Hans Rudolf Heinimann 《Natural Hazards》2018,91(1):37-68
Transportation networks daily provide accessibility and crucial services to societies. However, they must also maintain an acceptable level of service to critical infrastructures in the case of disruptions, especially during natural disasters. We have developed a method for assessing the resilience of transportation network topology when exposed to environmental hazards. This approach integrates graph theory with stress testing methodology and involves five basic steps: (1) establishment of a scenario set that covers a range of seismic damage potential in the network, (2) assessment of resilience using various graph-based metrics, (3) topology-based simulations, (4) evaluation of changes in graph-based metrics, and (5) examination of resilience in terms of spatial distribution of critical nodes and the entire network topology. Our case study was from the city of Kathmandu in Nepal, where the earthquake on April 25, 2015, followed by a major aftershock on May 12, 2015, led to numerous casualties and caused significant damage. Therefore, it is a good example for demonstrating and validating the developed methodology. The results presented here indicate that the proposed approach is quite efficient and accurate in assisting stakeholders when evaluating the resilience of transportation networks based on their topology. 相似文献
53.
Fabian Boesl Max Engel Rodrigo C. Eco JaN B. Galang Lia A. Gonzalo Francesca Llanes Eva Quix Helmut Brückner 《Sedimentology》2020,67(3):1393-1410
Coastal boulder fields provide clues to long-term frequency-magnitude patterns of coastal flooding events and have the potential to play an important role in coastal hazard assessment. Mapping boulders in the field is time and labour-intensive, and work on intertidal reef platforms, as in the present study, is physically challenging. By addressing coastal scientists who are not specialists in remote sensing, this contribution reports on the possibilities and limitations of digital applications in boulder mapping in Eastern Samar, Philippines, where recent supertyphoons Haiyan and Hagupit induced high waves, coastal flooding and boulder transport. It is demonstrated how satellite imagery of sub-metre resolution (from Pléiades and WorldView-3 imagery) enables efficient analysis of transport vectors and distances of larger boulders, reflecting variation in latitudes of both typhoon tracks and approaching angles of typhoon-generated waves. During the investigated events, boulders with a-axes of up to 8 m were clearly identified to have been shifted for up to 32 m, mostly along the seaward margin of the boulder field. It is, however, hard to keep track of smaller boulders, and the length of a-axes and b-axes including their orientation is often impossible to map with sufficient accuracy. Orthophotographs and digital surface models created through the application of an unmanned aerial vehicle and the ‘Structure from Motion’ technique provide ultra-high-resolution data, and have the potential to not only improve the results of satellite image analysis, but also those from field mapping and may significantly reduce overall time in the field. Orthophotographs permit unequivocal mapping of a-axes and b-axes including their orientation, while precise values for c-axes can be derived from the respective digital surface models. Volume of boulders is best inferred from boulder-specific Structure from Motion-based three-dimensional models. Battery power, flight speed and altitude determine the limits of the area covered, while patches shielded by the boulders are difficult to resolve. For some tasks, field mapping remains mandatory and cannot be replaced by currently available remote sensing tools: for example, sampling for rock type, density and age dating, recording of lithological separation of boulders from the underlying geological unit and of geomorphic features on a millimetre to decimetre-scale, or documentation of fine-grained sediment transport in between the boulders in supratidal settings. In terms of future events, the digital products presented here will provide a valuable reference to track boulder transport on a centimetre to decimetre-scale and to better understand the hydrodynamics of extreme-wave events on a fringing reef coastline. 相似文献
54.
Kaelin M. Cawley Youhei Yamashita Nagamitsu Maie Rudolf Jaffé 《Estuaries and Coasts》2014,37(2):399-410
Dissolved organic carbon (DOC) concentration and dissolved organic matter (DOM) optical properties were analyzed along two estuarine river transects during the wet and dry seasons to better understand DOM dynamics and quantify mangrove inputs. A tidal study was performed to assess the impacts of tidal pumping on DOM transport. DOM in the estuaries showed non-conservative mixing indicative of mangrove-derived inputs. Similarly, fluorescence data suggest that some terrestrial humic-like components showed non-conservative behavior. An Everglades freshwater-derived fluorescent component, which is associated with soil inputs from the Northern Everglades, behaved conservatively. During the dry season, a protein-like component behaved conservatively until the mid-salinity range when non-conservative behavior due to degradation and/or loss was observed. The tidal study data suggests mangrove porewater inputs to the rivers following low tide. The differences in quantity of DOM exported by the Shark and Harney Rivers imply that geomorphology and tidal hydrology may be a dominant factor controlling the amount of DOM exported from the mangrove ecotone, where up to 21 % of the DOC is mangrove-derived. Additionally, nutrient concentrations and other temporal factors may control DOM export from the mangroves, particularly for the microbially derived fluorescent components, contributing to the seasonal differences. The wet and dry season fluxes of mangrove DOM from the Shark River is estimated as 0.27?×?109 mg C d?1 and 0.075?×?109 mg C d?1, respectively, and the Harney River is estimated as 1.9?×?109 mg C d?1 and 0.20?×?109 mg C d?1. 相似文献
55.
56.
J. Krause G. E. Brügmann E. V. Pushkarev 《Contributions to Mineralogy and Petrology》2011,161(2):255-273
Uralian-Alaskan-type mafic–ultramafic complexes are recognized as a distinct class of intrusions regarding lithologic assemblage,
mineral chemistry and petrogenetic setting. In the present study, we discuss new data on the distribution of major elements
in minerals of the spinel group in rocks from Uralian-Alaskan-type complexes in the Ural Mountains, Russia. Cr-rich spinel
(Cr2O3 = 20–53 wt%) in dunite with interstitial clinopyroxene and in wehrlite cumulates indicate that it reacted with interstitial
liquid resulting in the progressive substitution of Al2O2 and Cr2O3 by Fe2O3 and TiO2. A distinct change in the spinel chemistry in dunite (Cr2O3 = 47–53 wt%), towards Al2O3- and Cr2O3-poor but Fe2O3-rich compositions monitors the onset of clinopyroxene fractionation in wehrlite (Cr2O3 = 15–35 wt%, Al2O3 = 1–8 wt%, Fe2O3 = 25–55 wt%). In more fractionated mafic rocks, the calculated initial composition of exsolved spinel traces the sustained
crystallization of clinopyroxene by decreasing Cr2O3 and increasing FeO, Fe2O3 and fO2. Finally, the initiation of feldspar crystallization buffers the Al2O3 content in most of the spinels in mafic rocks at very low Cr2O3 contents (<5 wt%). The fractionation path all along and the reaction with interstitial liquid are accompanied by increasing
Fe2O3 contents in the spinel. This likely is caused by a significant increase in the oxygen fugacity, which suggests closed system
fractionation processes. Spinel with Cr2O3 < 27 wt% is exsolved into a Fe2O3-rich and an Al2O3-rich phase forming a variety of textures. Remarkably, exsolved spinel in different lithologies from complexes 200 km apart
follows one distinct solvus line defining a temperature of ca. 600°C. This indicates that the parental magmas were emplaced
and eventually cooled at similar levels in the lithosphere, likely near the crust–mantle boundary. Eventually, these 600°C
hot bodies were rapidly transported into colder regions of the upper crust during a regional tectonic event, probably during
the major active phase of the Main Uralian Fault. 相似文献
57.
Brückner Florian Bahls Rebecca Alqadi Mohammad Lindenmaier Falk Hamdan Ibraheem Alhiyari Mohammad Atieh Ala’a 《Hydrogeology Journal》2021,29(8):2789-2802
Hydrogeology Journal - In 2017, a comprehensive review of groundwater resources in Jordan was carried out for the first time since 1995. The change in groundwater levels between 1995 and 2017 was... 相似文献
58.
Peter Bauer-Gottwein Bibi R. N. Gondwe Guillaume Charvet Luis E. Mar��n Mario Rebolledo-Vieyra Gonzalo Merediz-Alonso 《Hydrogeology Journal》2011,19(3):507-524
The Yucatán Peninsula karst aquifer is one of the most extensive and spectacular karst aquifer systems on the planet. This transboundary aquifer system extends over an area of approximately 165,000? km2 in México, Guatemala and Belize. The Triassic to Holocene Yucatán limestone platform is located in the vicinity of the North American/Caribbean plate boundary and has been reshaped by a series of tectonic events over its long geologic history. At the end of the Cretaceous period, the Yucatán Peninsula was hit by a large asteroid, which formed the Chicxulub impact crater. The Yucatán Peninsula karst aquifer hosts large amounts of groundwater resources which maintain highly diverse groundwater-dependent ecosystems. Large parts of the aquifer are affected by seawater intrusion. Anthropogenic pollution of the aquifer has been increasing over the past few decades, owing to relentless economic development and population growth on the Peninsula. This review summarizes the state of knowledge on the Yucatán Peninsula karst aquifer and outlines the main challenges for hydrologic research and practical groundwater-resources management on the Peninsula. 相似文献
59.
??????????????????(VTEC)??????????????????????????????????4?????????????????????Э????????????????С???????÷????????????????????????г????????????????????????????????????????????????????????(CODE)????????????????????????0.812???????????ο???в???4 TECu????????????????????????????????????????????γ20°?????????????????????????????????????1??7 TECu?????????????????2.4 TECu?????CODE????????20.14%????????????????3.5 TECu?????CODE???????????????????3.0 TECu?????CODE????????8.25%?? 相似文献
60.
POISEL Rainer BEDNARIK Martin HOLZER Rudolf LISCAK Pavel 《山地科学学报》2005,2(3):211-217
A catalogue of possible landslide initial failure mechanisms, taking into account the geological setting and the geometry of the slope, the joint structure, the habitus of the rock blocks, as well as the mechanical behaviour of the rocks and of the rock mass (deformation and strength parameters), is presented. Its aim is to give geologists as well as engineers the opportunity to compare phenomena in the field and phenomena belonging to particular mechanisms and to find the mechanism occurring. The presented catalogue of initial landslide mechanisms only comprises the mechanisms having a clearly defined mechanical model that can be divided into empirical relations and into mechanical models, as well as an overview of run out models, which can be divided into empirical relations and into mechanical models. 相似文献