Measuring low concentrations of Th in water and sediment     

James T. Waples  Kent A. Orlandini  Kim M. Weckerly  David N. Edgington  J. Val Klump 《Marine Chemistry》2003,80(4):661

²³⁴Th/²³⁸U disequilibria have been used extensively in studies of particle dynamics and the fate and transport of particle-reactive matter in marine environments. Similar work in low salinity, estuarine, and freshwater systems has not occurred primarily because the lower concentrations of both parent and daughter nuclides that are typical of these systems often render established methods for the analysis of ²³⁴Th inadequate. The application of this radionuclide tracer technique to these systems, however, has great potential. To this end, we present a method for measuring low activities of ²³⁴Th in relatively small samples (<200 l) using low background gas-flow proportional counters, a ²²⁹Th yield monitor, and empirical corrections for the interferences from real and apparent betas that are emitted by other thorium isotopes and their progeny. For samples with low ²³⁴Th/²²⁸Th activity ratios, we improve upon current beta counting methodologies that rely on immediate sample counting, weak beta absorption, or multiple beta counts so that, using the analytical approach outlined here, it should be possible to measure ²³⁴Th activities (i) as low as 1.5 dpm/total sample, (ii) up to 2 weeks after radiochemical purification of thorium, and (iii) with only one sample count for alpha and beta activity.  相似文献   

A phytoplankton class-specific primary production model applied to the Kerguelen Islands region (Southern Ocean)     

Julia Uitz  Hervé Claustre  F. Brian Griffiths  Joséphine Ras  Nicole Garcia  Valérie Sandroni 《Deep Sea Research Part I: Oceanographic Research Papers》2009,56(4):541-560

As part of the KErguelen: compared study of the Ocean and the Plateau in Surface water (KEOPS) project in late summer 2005, we examine the phytoplankton community composition and associated primary production in the waters surrounding the Kerguelen Archipelago, with the emphasis on two contrasted environments: (i) the Kerguelen Plateau, where a large bloom occurs annually, and (ii) the high-nutrient low-chlorophyll (HNLC) offshore waters. A biomarker pigment approach was used to assess the community composition in terms of chlorophyll biomass of three phytoplankton size classes, namely micro-, nano-, and picophytoplankton. The second objective was to evaluate a global class-specific approach for estimating the contribution of the three pigment-based size classes to the primary production in the study area. To do so, primary production rates associated with each phytoplankton class were computed from the class-specific chlorophyll biomass coupled to a class-specific primary production model, and compared with in situ measurements of size-fractionated ¹³C-based primary production. The iron-enriched bloom region was dominated by microphytoplankton (diatoms), which contributed 80–90% to the total primary production (of ≈1 g C m^?2 d^?1). In the HNLC area, the primary production was about 0.30 g C m^?2 d^?1, mainly (65%) achieved by small diatoms and nanoflagellates. The model results show a good overall agreement between predicted and measured total primary production rates. In terms of size classes, agreements were higher for the bloom region than for the HNLC waters. Discrepancies in this complex iron-limited area may be explained essentially by the smaller size of diatoms, or a different set of photophysiological properties.  相似文献   

Implementing ICZM: The experience of Northwest Europe     

Rhoda Ballinger  Valerie Cummins  Hance Smith 《Ocean & Coastal Management》2010,53(12):725-726

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Nutrient dynamics at the sediment-water interface in a Mediterranean lagoon (Thau, France): influence of biodeposition by shellfish farming activities   总被引：2，自引：0，他引：2  

Mesnage V  Ogier S  Bally G  Disnar JR  Lottier N  Dedieu K  Rabouille C  Copard Y 《Marine environmental research》2007,63(3):257-277

The Thau Lagoon, a French Mediterranean shallow lagoon, is a site where extensive shellfish farming occurs. The aim of the present work is to evaluate the role of this activity on nutrient exchange at the sediment-water interface in relation to organic matter (OM) sedimentation and degradation. Two stations inside (C5) and outside (C4) of the shellfish farming areas were sampled at three seasons. Porewater chemistry surveys and calculated diffusive fluxes were used to evaluate the trophic status of the Thau lagoon. Quantitative (Particulate Organic Carbon) as well as qualitative OM (Hydrogen Index, Carbohydrates) analyses were performed on sediments to assess OM characteristics. Results emphasized that surficial sediments at C5 are always more enriched in OM. Porewater nutrient concentrations are 10-20 times higher at C5 than at C4. In June 2003, the porewater profiles exhibit a sharp gradient at the bottom waters, indicating a hypereutrophic status, leading to an anoxic crisis.  相似文献   

Hydroclimatic variations over the last two glacial/interglacial cycles at Lake Urmia,Iran     

Lora R. Stevens  Morteza Djamali  Valérie Andrieu-Ponel  Jacques-Louis de Beaulieu 《Journal of Paleolimnology》2012,47(4):645-660

A coarse-resolution, absolutely dated isotope record spanning the last 180 ka was constructed from aragonite-rich fecal pellets in a sediment core from Lake Urmia, Iran. The isotope record is not continuous as a consequence of detrital calcite that contaminates the pellets at certain depths. An isotopic correction was applied to samples > 50% aragonite, using the δ¹⁸O values of calcite-only pellets. Absolute dates were derived from U-Th analyses on pellets > 80% aragonite. The resultant δ¹⁸O record and sediment lithofacies confirm that the Penultimate Glacial was cold and dry, with pronounced interstadials at ca. 150 and >175 ka. Deglaciation began at ca. 130 ka, but climate was unstable and supported a probable no-analog vegetation assemblage. The onset of true interglacial conditions occurred at ca. 126 ka. Peak moisture conditions lasted for 5–6 ka before the climate became drier, with the loss of some tree taxa towards the end of the Last Interglacial. A period of greater moisture occurred between 116 and 108 ka, overlapping in timing with interstadial conditions recorded in the Soreq Cave speleothem record. In general, stadial to interstadial transitions (OIS 5d-5c, 5b-5a) are marked by decreasing δ¹⁸O values and rising lake levels. The nature of the OIS 5 to OIS 4 transition cannot be resolved in this study because of the loss of 5 m of core and imprecise dates. According to the present chronology, δ¹⁸O values, and inferred lake levels, arid conditions began at 60 ka and persisted until Termination I. Episodic increases in moisture, characterized by layers of aragonite-rich pellets, ostracodes, and the pollen of trees and/or aquatic plants, occurred during the Last Glacial. Deglaciation associated with the Late-glacial Interstadial began at 14 ka but was interrupted by a marked Younger Dryas event that resulted in renewed aridity. The Holocene began at approximately 10 ka. Despite large errors in the chronology, the overall timing and pattern of effective moisture matches the record from Soreq Cave, Israel, suggesting that the two regions were climatically in-phase.  相似文献   

Characteristics of cloud-to-ground lightning in warm-season thunderstorms in the Central Great Plains     

Stacy A. Fleenor  Christopher J. Biagi  Kenneth L. Cummins  E. Philip Krider  Xuan-Min Shao 《Atmospheric Research》2009,91(2-4):333-352

In July 2005, a field campaign was conducted in the Central Great Plains to obtain 60-field/s video imagery of lightning in correlation with reports from the U.S. National Lightning Detection Network (NLDN) and broadband electric field waveforms from the Los Alamos Sferic Array (LASA). A total of 342 GPS time-stamped cloud-to-ground (CG) flashes were recorded in 17 different sessions, and 311 (91%) of these were correlated with reports from the NLDN. Only 6 of the 17 recording sessions were dominated by flashes that lowered negative charge to ground, and 11 were dominated by positive CG flashes. A total of 103 flashes recorded on video were correlated with at least one NLDN report of negative CG strokes, 204 video flashes were correlated with one or two positive stroke reports, and 4 had bipolar reports. In this paper, we will give distributions of the estimated peak current, I_p, as reported by the NLDN, of negative and positive first strokes that were recorded on video, the multiplicity of strokes that were recorded on video, and the number of ground contacts per flash that were resolved on video. 41 (40%) of the negative flashes produced just a single-stroke on video, and 62 (60%) showed two or more strokes. The observed multiplicity of negative flashes averaged 2.83, which becomes about 3.14 after correcting for the finite time-resolution of the video camera. 195 (96%) of the positive flashes produced just a single-stroke on video, and 9 (4%) showed two strokes; therefore, the observed multiplicity of positive flashes averaged 1.04. Five out of 9 (56%) of the positive subsequent strokes re-illuminated a previous channel, and 4 out of 9 (44%) created a new ground contact. Simultaneous video, LASA, and NLDN measurements also allowed us to examine the classification of NLDN reports during 3 single-cell storms (one negative and two positive). Based on the LASA waveforms, a total of 204 out of 376 (54%) NLDN reports of CG strokes were determined to be for cloud pulses. The misclassified negative reports had |I_p| values ranging from 3.8 kA to 29.7 kA, but only 58 (24%) of these had |I_p| > 10 kA, and only one misclassified positive report had I_p > 20 kA. Radar analyses showed that most of the negative and positive CG strokes that were recorded on video were produced within or near the convective cores of storms. The radar imagery also showed that single-cell storms tended to produce one polarity of CG flashes at a time, and that such storms could switch rapidly from negative to positive CG flashes when the reflectivity was near maximum. Multiple-cell storms produced both negative and positive flashes over a broad region, but each polarity tended to cluster near regions of high-reflectivity.  相似文献   

Improving resolution and accuracy of mean sea surface from kinematic GPS,Vanuatu subduction zone     

Marie-Noëlle Bouin  Valérie Ballu  Stéphane Calmant  Bernard Pelletier 《Journal of Geodesy》2009,83(11):1017-1030

Satellite altimetry provides an ocean mean sea surface (MSS) map with global coverage and overall excellent precision. However, in some areas, like the Vanuatu archipelago, the coverage and resolution of this tool are not sufficient to correctly map the short scale undulations of the sea surface, due to numerous islands and to strong lithospheric and mantle heterogeneities. New applications such as seafloor geodesy may require local mean surface representation with improved resolution in specific areas. We used sea surface height collected with kinematic GPS during three different cruises in 2004, 2006 and 2007 to reconstruct a homogeneous sea surface map around Santo Island, Vanuatu. We assess the accuracy of this GPS-derived local sea surface to 6–19 cm and evaluate the quality of existing altimetry and gravity-derived MSSs on the Vanuatu archipelago. Observed short scale undulations are interpreted as due to local geodynamics.  相似文献   

Offshore petroleum and commercial fishery interactions: The case of Long Point,Port Dover,Lake Erie     

Val  Erik  Nelson  J. G. 《GeoJournal》1983,7(3):247-260

GeoJournal - From a human ecological perspective, research was undertaken on interactions between Lake Erie fisheries and offshore petroleum (gas) development from 1913 to summer, 1981. In an...  相似文献   

Chemical characterization of porewaters in an intertidal mudflat of the Seine estuary: relationship to erosion-deposition cycles     

Bally G  Mesnage V  Deloffre J  Clarisse O  Lafite R  Dupont JP 《Marine pollution bulletin》2004,49(3):163-173

A seasonal field study was carried out in the Seine estuary to determine the chemistry of sediment porewaters using the 'peeper' technique and changes in the elevation of the mudflats using the 'Altus' technique. This approach allowed us to evaluate the release of nutrients and to link these releases to the sediment hydrodynamics. Our results show that nutrient and organic matter cycling in a Seine estuary mudflat exhibits a seasonal behaviour, which is mainly influenced by variations in hydrodynamics. Sediments, rich organic matter, were input during floods and they were mineralized during summer and autumn, releasing nutrients and dissolved organic carbon into the sediment porewaters. The nutrient release, including ammonium, is mainly linked to the mineralization of organic matter, while the release of phosphate is delayed. The delay could be the result of phosphate association with organic matter and/or its co-precipitation with calcium and iron.  相似文献   

Analyse isotopique de l'air piégé dans la glace pour quantifier les variations de température     

Amaelle Landais  Nicolas Caillon  Jeff Severinghaus  Jean-Marc Barnola  Céline Goujon  Jean Jouzel  Valérie Masson-Delmotte 《Comptes Rendus Geoscience》2004,336(11):963-970

Isotopic measurements in polar ice core have shown a succession of rapid warming periods during the last glacial period over Greenland. However, this method underestimates the surface temperature variations. A new method based on gas thermal diffusion in the firn manages to quantify surface temperature variations through associated isotopic fractionations. We developed a method to extract air from the ice and to perform isotopic measurements to reduce analytical uncertainties to 0.006 and 0.020$\text{‰}$ for δ¹⁵N and δ⁴⁰Ar. It led to a 16±1.5 °C surface temperature variation during a rapid warming ($\text{?70}\text{000}$ yr). To cite this article: A. Landais et al., C. R. Geoscience 336 (2004).  相似文献