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Michael M. McGlue Aguinaldo Silva Hiran Zani Fabrício A. Corradini Mauro Parolin Erin J. Abel Andrew S. Cohen Mario L. Assine Geoffrey S. Ellis Mark A. Trees Sidney Kuerten Frederico dos Santos Gradella Giliane Gessica Rasbold 《Quaternary Research》2012,78(2):285-294
The Pantanal is the world's largest tropical wetland and a biodiversity hotspot, yet its response to Quaternary environmental change is unclear. To address this problem, sediment cores from shallow lakes connected to the Upper Paraguay River (PR) were analyzed and radiocarbon dated to track changes in sedimentary environments. Stratal relations, detrital particle size, multiple biogeochemical indicators, and sponge spicules suggest fluctuating lake-level lowstand conditions between ~ 11,000 and 5300 cal yr BP, punctuated by sporadic and in some cases erosive flood flows. A hiatus has been recorded from ~ 5300 to 2600 cal yr BP, spurred by confinement of the PR within its channel during an episode of profound regional drought. Sustained PR flooding caused a transgression after ~ 2600 cal yr BP, with lake-level highstand conditions appearing during the Little Ice Age. Holocene PR flood pulse dynamics are best explained by variability in effective precipitation, likely driven by insolation and tropical sea-surface temperature gradients. Our results provide novel support for hypotheses on: (1) stratigraphic discontinuity of floodplain sedimentary archives; (2) late Holocene methane flux from Southern Hemisphere wetlands; and (3) pre-colonial indigenous ceramics traditions in western Brazil. 相似文献
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The Amazonian lowlands include large patches of open vegetation which contrast sharply with the rainforest, and the origin of these patches has been debated. This study focuses on a large area of open vegetation in northern Brazil, where δ13C and, in some instances, C/N analyses of the organic matter preserved in late Quaternary sediments were used to achieve floristic reconstructions over time. The main goal was to determine when the modern open vegetation started to develop in this area. The variability in δ13C data derived from nine cores ranges from ?32.2 to ?19.6‰, but with nearly 60% of data above ?26.5‰. The most enriched values were detected only in ecotone and open vegetated areas. The development of open vegetation communities was asynchronous, varying between estimated ages of 6400 and 3000 cal a BP. This suggests that the origin of the studied patches of open vegetation might be linked to sedimentary dynamics of a late Quaternary megafan system. As sedimentation ended, this vegetation type became established over the megafan surface. In addition, the data presented here show that the presence of C4 plants must be used carefully as a proxy to interpret dry paleoclimatic episodes in Amazonian areas. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
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This paper investigates the accuracy of pushover-based methods in predicting the seismic response of slender masonry towers, through comparison with the results from a large number of nonlinear time-history dynamic analyses. In particular, conventional pushover analyses, in both their force- and displacement-based variants, are considered, and seismic assessment through the well-established N2 method is also addressed. The study is conducted by applying a simple non-linear elastic model recently developed and implemented in the computational code MADY to represent slender masonry structures. The model enables both pushover analyses and non-linear dynamic analyses to be performed with a minimum of effort. A multi-record incremental dynamic analysis carried out for a quite large number of structural cases, each of which is subjected to a comprehensive set of dynamic nonlinear analyses, is used to evaluate the accuracy of pushover methods in predicting the global structural response, as represented by the usual capacity curve together with a damage curve, both of which are compared with dynamic envelopes. Local responses, in terms of lateral displacements and the distribution of damage along the tower height are also compared. The results reveal that the key issue in the accuracy of pushover methods is the nature of the lateral load applied, that is, whether it is a force or a displacement. Different ranges of expected deformation are suggested for adopting each type of lateral load to better represent the actual behaviour of masonry towers and their damage under seismic events through pushover methods. 相似文献
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Michael M. McGlue Aguinaldo Silva Fabrício A. Corradini Hiran Zani Mark A. Trees Geoffrey S. Ellis Mauro Parolin Peter W. Swarzenski Andrew S. Cohen Mario L. Assine 《Journal of Paleolimnology》2011,46(2):273-289
Sediment records from floodplain lakes have a large and commonly untapped potential for inferring wetland response to global
change. The Brazilian Pantanal is a vast, seasonally inundated savanna floodplain system controlled by the flood pulse of
the Upper Paraguay River. Little is known, however, about how floodplain lakes within the Pantanal act as sedimentary basins,
or what influence hydroclimatic variables exert on limnogeological processes. This knowledge gap was addressed through an
actualistic analysis of three large, shallow (<5 m) floodplain lakes in the western Pantanal: Lagoa Gaíva, Lagoa Mandioré
and Baia Vermelha. The lakes are dilute (CO3
2− > Si4+ > Ca2+), mildly alkaline, freshwater systems, the chemistries and morphometrics of which evolve with seasonal flooding. Lake sills
are bathymetric shoals marked by siliciclastic fans and marsh vegetation. Flows at the sills likely undergo seasonal reversals
with the changing stage of the Upper Paraguay River. Deposition in deeper waters, typically encountered in proximity to margin-coincident
topography, is dominated by reduced silty-clays with abundant siliceous microfossils and organic matter. Stable isotopes of
carbon and nitrogen, plus hydrogen index measured on bulk organic matter, suggest that contributions from algae (including
cyanobacteria) and other C3-vegetation dominate in these environments. The presence of lotic sponge spicules, together with patterns of terrigenous sand
deposition and geochemical indicators of productivity, points to the importance of the flood pulse for sediment and nutrient
delivery to the lakes. Flood-pulse plumes, waves and bioturbation likewise affect the continuity of sedimentation. Short-lived
radioisotopes indicate rates of 0.11–0.24 cm year−1 at sites of uninterrupted deposition. A conceptual facies model, developed from insights gained from modern seasonal processes,
can be used to predict limnogeological change when the lakes become isolated on the floodplain or during intervals associated
with a strengthened flood pulse. Amplification of the seasonal cycle over longer time scales suggests carbonate, sandy lowstand
fan and terrestrial organic matter deposition during arid periods, whereas deposition of lotic sponges, mixed aquatic organic
matter, and highstand deltas characterizes wet intervals. The results hold substantial value for interpreting paleolimnological
records from floodplain lakes linked to large tropical rivers with annual flooding cycles. 相似文献
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R. A. Stephen D. E. Koelsch H. Berteaux A. Bocconcelli S. Bolmer J. Cretin N. Etourmy A. Fabre R. Goldsborough M. Gould S. Kery J. Laurent G. Omnes K. Peal S. Swift R. Turpening C. Zani 《Marine Geophysical Researches》1994,16(4):243-286
The Seafloor Borehole Array Seismic System (SEABASS) has been developed to measure the pressure and threedimensional particle velocity of the VLF sound field (2–50 Hz) below the seafloor in the deep ocean. The system consists of four three-component borehole seismometers (with an optional hydrophone). a borehole digitizing unit, and a seafloor control and recording package. The system can be deployed using a wireline re-entry capability from a conventional research vessel in Deep Sea Drilling Project (DSDP) and Ocean Drilling Project (ODP) boreholes. Data from below the seafloor are acquired either onboard the research vessel via coaxial tether or remotely on the seafloor in a self-contained package. If necessary the data module from the seafloor package can be released independently and recovered on the surface. This paper describes the engineering specifications of SEABASS, the tests that were carried out, and preliminary results from an actual deep sea deployment. VLF ambient noise levels beneath the seafloor acquired on the Low Frequency Acoustic-Seismic Experiment (LFASE) are within 20 dB of levels from previous seafloor borehole seismic experiments and from land borehole measurements. The ambient noise observed on LFASE decreases by up to 12 dB in the upper 100 m of the seafloor in a sedimentary environment. 相似文献
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