Deep-sea corals evidence periodic reduced ventilation in the North Atlantic during the LGM/Holocene transition     

Andrea Schröder-Ritzrau  Augusto Mangini 《Earth and Planetary Science Letters》2003,216(3):399-410

Nine new intermediate water ages estimated with coupled uranium-series and radiocarbon measurements on deep-sea corals from the northeastern Atlantic are presented. Together with six intermediate and deep-water ages from the western North Atlantic [Adkins et al., Science 280 (1998) 725-728] and one from the equatorial Atlantic [Mangini et al., Nature 392 (1998) 347-348] they span the time period from 15?400 to 560 yr ago and show abrupt variations of intermediate and deep-water ages from 270 up to 2320 yr. Both major pulses of meltwater discharge MWP 1A, following Heinrich Event 1 and MWP 1B after the Younger Dryas cold event are followed by enhanced supply of southern source water at intermediate depths in the North Atlantic, together with reduced formation of well-ventilated glacial North Atlantic intermediate water (1000-2000 m). This result gives strong support to numerical models, predicting fast and sharp increase of bottom water age in the Atlantic triggered by pulses of freshwater discharge into the North Atlantic [Rahmstorf, Nature 372 (1994) 82-85; Stocker and Wright, Radiocarbon 40 (1998) 359-366].  相似文献   

The influence of bedrock‐derived acidity in the development of surface and underground karst: evidence from the Precambrian carbonates of semi‐arid northeastern Brazil     

Augusto S. Auler  Peter L. Smart 《地球表面变化过程与地形》2003,28(2):157-168

Very extensive cave systems are developed in Precambrian Una Group carbonates in the Campo Formoso area, eastern Brazil. In contrast, the area is largely devoid of significant surface karst landforms, as would be expected given its semi‐arid climate. The caves in the area display many morphological features characteristic of deep‐seated hypogenic caves, such as lack of relationship with the surface, ramiform/network pattern, abrupt variations of passage cross‐sections and absence of fluvial sediments, but do not show evidence of vertical passages marking the ascending path of acidic water nor present extensive gypsum or acid clay mineral deposits. Hydrochemical analyses of present‐day ground water indicate that oxidation of bedrock sulphide is an active process, and sulphuric acid may be the main agent driving carbonate dissolution in the area. A shallow mode of speleogenesis is thus proposed, in which sulphuric acid produced through the oxidation of sulphide beds within the carbonates controls cave initiation and development. Moreover, the geological situation of the area in an ancient stable passive margin precludes the possibility of deep‐seated sources of acidity. Under dry climate, due to the absence of recharge, solutional landforms will be largely subdued in the surface. Hypogenic processes, if present, are likely to predominate, producing a landscape characterized by a marked disparity in the comparative degree of development between surface and underground landforms. Rates of karst landform development have traditionally been analysed through a climatic perspective, runoff being the main controlling factor in promoting karst development. This view needs to be reassessed in the light of the growing awareness of the importance of climate‐independent processes related to hypogenic sources of acidity. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

How precise are U-series coral ages?     

Denis Scholz  Augusto Mangini 《Geochimica et cosmochimica acta》2007,71(8):1935-1948

U-series dating of fossil reef corals is a well established and widely applied technique in paleoclimate research. Many fossil corals, however, show evidence for post-depositional diagenetic alteration, and it is generally accepted that the accuracy of U-series coral ages is more limited due to coral diagenesis than analytical precision. In recent years, three models have been published that try to correct the effects of diagenesis and allow the calculation of model ages [Thompson W. G., Spiegelmann M. W., Goldstein S. L., and Speed R. C. (2003) An open-system model for U-series age determinations of fossil corals. Earth and Planetary Science Letters210, 365-381; Villemant B., and Feuillet N. (2003) Dating open systems by the ²³⁸U-²³⁴U-²³⁰Th method: application to Quaternary reef terraces. Earth and Planetary Science Letters210, 105-118; Scholz D., Mangini A., and Felis T. (2004) U-series dating of diagenetically altered fossil reef corals. Earth and Planetary Science Letters218, 163-178].Here, we assess the age variability of both conventional ²³⁰Th/U-dating and the three models by application to different sub-samples of individual coral specimens. The age variability, estimated as the 2σ-standard deviation on the individual ages, is compared with the errors quoted by the different methods. Our results show that the errors of conventional ²³⁰Th/U-dating as well as those of the method of Thompson et al. (2003) do not account for the true age variability. The age variability of both methods is in the range of the errors given by the models of Villemant and Feuillet (2003) and Scholz et al. (2004).Furthermore, we show that the widely used reliability criteria are not sufficient to identify all diagenetically altered corals. In contrast, analysis of different sub-samples of one coral specimen allows (i) to estimate the real age variability, (ii) to test if the assumptions of the models are fulfilled, and (iii) to investigate the diagenetic processes in more detail. Thus, this method should generally be applied to obtain more reliable U-series coral ages and errors.  相似文献   

Relative current effect on short wave growth     

Guimarães  Pedro Veras  Ardhuin  Fabrice  Perignon  Yves  Benetazzo  Alvise  Bouin  Marie-Noëlle  Garnier  Valerie  Redelsperger  Jean-Luc  Accensi  Mickael  Thomson  Jim 《Ocean Dynamics》2022,72(8):621-639

Ocean Dynamics - Short waves growth is characterized by nonlinear and dynamic processes that couple ocean and atmosphere. Ocean surface currents can have a strong impact on short wave steepness and...  相似文献   

Place-making at the frontier of Brazilian agribusiness     

Antonio Augusto Rossotto Ioris 《GeoJournal》2018,83(1):61-72

The intricacies of one of the most relevant agribusiness frontiers in the world today—the north of the State of Mato Grosso, in the southern section of the Amazon, Brazil—are considered through a critical examination of place-making. Vast areas of Amazon rainforest and savannah vegetation were converted there, since the 1970s, into places of intensive soybean farming, basically to fulfil exogenous demands for land and agriculture production. The present assessment goes beyond the configuration of new places at the agricultural frontier, and starts with a qualitative intellectual jump: from place-making on the frontier to place-making as an ontological frontier in itself. It means that, instead of merely studying the frontier as a constellation of interconnected places, we examine the politicised genesis of the emerging places and their trajectory under fierce socio-ecological disputes. The consideration of almost five decades of intense historic-geographical change reveals an intriguing dialectics of displacement (of previous socio-ecological systems, particularly affecting squatters and indigenous groups, in order to create opportunities for migrants and companies), replacement (of the majority of disadvantaged farmers and poor migrants, leading to land concentration, widespread financialisation and the decisive influence of transnational corporations) and misplacement (which is the synthesis of displacement and replacement, demonstrated by mounting risks and a pervasive sentiment of maelstrom). Overall, there was nothing inevitable in the process of rural and regional development, but the problems, conflicts and injustices that characterise its turbulent geographical trajectory were all more or less visible from the outset.  相似文献   

Hydrological response of a small mediterranean agricultural catchment     

Joan Estrany  Celso Garcia  Ramon J. Batalla 《Journal of Hydrology》2010,380(1-2):180-190

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Evaluating explosive eruption risk at European volcanoes     

Peter J. Baxter  Augusto Neri  Russell Blong 《Journal of Volcanology and Geothermal Research》2008

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Comparative biomonitors of coastal trace metal contamination in tropical South America (N. Brazil)     

e Silva CA  Smith BD  Rainbow PS 《Marine environmental research》2006,61(4):439-455

Samples of 5 bivalve molluscs (Crassostrea rhizophorae, Mytella charruana, Anomalocardia brasiliana, Anadara ovalis, Phacoides pectinata), 2 barnacles (Fistulobalanus citerosum, Balanus amphitrite) and leaves of the mangrove tree Rhizophora mangle were collected from up to 11 sites in two estuaries in Natal, Brazil--the comparatively contaminated Potengi estuary and the comparatively uncontaminated Curimataú estuary. Specimens were analysed for the trace metals Zn, Cu, Cd, Fe, Mn and Ni, and a comparative assessment made of the power of the different species as trace metal biomonitors. Four of the 5 bivalves (not P. pectinata) take up metals from solution and suspended material (food source), while P. pectinata as a lucinid with symbiotic chemosynthetic bacteria takes up metals from dissolved sources only. The organisms with the strongest net accumulation of particular metals showed the greatest discrimination between trace metal bioavailabilities between sites. Barnacles (F. citerosum) showed the best discrimination, but oysters (C. rhizophorae) are particularly recommended as biomonitors given their strong accumulation patterns for many trace metals, their large size and their local abundance.  相似文献   

Recording inter-annual changes on a boulder beach in Galicia,NW Spain using an unmanned aerial vehicle     

Alejandro Gómez-Pazo  Augusto Pérez-Alberti  Alan Trenhaile 《地球表面变化过程与地形》2019,44(5):1004-1014

An unmanned aerial vehicle (UAV) was flown over a boulder beach (area 20 000 m²) on the southern coast of Galicia (northwestern Spain) in May 2016, continuing earlier work based on flights over the same beach in July 2012, May 2013, and late March 2014. Digital surface models (DSMs) with 1.8 cm resolution were constructed from the 2014 and 2016 data to identify changes in beach morphology over the intervening period. Analyses were conducted using a Limit of Detection (LoD) of 0 cm and 3.71 cm. In both cases, the analyses showed that erosion dominated over 19% of the beach area. Accretion occurred over the rest of the beach, which acquired an additional 1500 m³ of material over the study period. Re-analysis of the data from earlier flights suggested that erosion dominated on the beach in 2012–2013 and deposition in 2013–2014. Without any clear relationship between beach behaviour and storm severity during each winter period, it is proposed that gravitationally induced erosion and storm-wave induced deposition are the result of perturbations about an equilibrium beach gradient. The UAV data also suggested that an essentially random component modulates regional patterns of movement. © 2018 John Wiley & Sons, Ltd.  相似文献   

An initial evaluation of drone‐based monitoring of boulder beaches in Galicia,north‐western Spain          下载免费PDF全文

Augusto Pérez‐Alberti  Alan S. Trenhaile 《地球表面变化过程与地形》2015,40(1):105-111

Low altitude flights by a micro‐drone were made in 2012 and 2013 over two boulder beaches in north‐western Spain. Geographical information system software was used to map the data. Boulder outlines from the first flight were recorded on 4796 clasts at Laxe Brava and 2508 clasts at Oia. Changes in location were identified by overlaying these outlines on the 2013 images. About 17.5% of the boulders (mean surface area 0.32 m²) moved at Laxe Brava and about 53% (mean surface area 0.23 m²) at Oia. Most movement on both beaches was between the mid‐tide to about 2 m above the high tidal level. The location and elevation of the highest points were also recorded on the 2012 images on 4093 boulders at Laxe Brava and 3324 boulders at Oia. These elevations were compared with the elevations at the same locations in 2013. The occurrence and scale of the elevational changes were generally consistent with changes in the boulder outlines. The study confirmed that boulder beaches can be cheaply and effectively monitored using high resolution, micro‐drone technology. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献