250 research outputs found
A multidating approach applied to historical slackwater flood deposits of the Gardon River, SE France
A multidating approach was carried out on slackwater flood deposits, preserved in valley side rock cave and terrace, of the Gardon River in Languedoc, southeast France. Lead-210, caesium-137, and geochemical analysis of mining-contaminated slackwater flood sediments have been used to reconstruct the history of these flood deposits. These age controls were combined with the continuous record of Gardon flow since 1890, and the combined records were then used to assign ages to slackwater deposits. The stratigraphic records of terrace GE and cave GG were excellent examples to illustrate the effects of erosion/preservation in a context of a progressively self-censoring, vertically accreting sequence. The sedimentary flood record of the terrace GE located at 10 m above the channel bed is complete for years post-1958 but incomplete before. During the 78-year period 1880-1958, 25 floods of a sufficient magnitude (> 1450 m3/s) have covered the terrace. Since 1958, however, the frequency of inundation of the deposits has been lower:only 5 or 6 floods in 52 years have been large enough to exceed the necessary threshold discharge (> 1700 m3/s). The progressive increase of threshold discharge and the reduced frequency of inundation at the terrace could allow stabilisation of the vegetation cover and improved protection against erosion from subsequent large magnitude flood events. The sedimentary flood record seems complete for cave GG located at 15 m above the channel bed. Here, the low frequency of events would have enabled a high degree of stabilisation of the sedimentary flood record, rendering the deposits less susceptible to erosion. Radiocarbon dating are used in this study and compared to the other dating techniques.Eighty percent of radiocarbon dates on charcoals were considerably older than those obtained by the other techniques in the terrace. On the other hand, radiocarbon dating on seeds provided better results. This discrepancy between radiocarbon dates on charcoal and seeds is explained by the nature of the dated material (permanent wood vs. annual production and resistance to degradation process). Finally, we showed in this study that although the most common dating technique used in paleoflood hydrology is radiocarbon dating, usually on charcoal preserved within slackwater flood sediments, this method did not permitus to define a coherent age model. Only the combined use of lead-210, caesium-137, and geochemical analysis of mining-contaminated sediments with the instrumental flood record can be applied to discriminate and date the recent slackwater deposits of the terrace GE and cave GG.Une approche par datations multiples a Ă©tĂ© menĂ©e sur les dĂ©pĂŽts de sĂ©diments de crue retrouvĂ©s dans les gorges du Gardon (Languedoc). Le plomb-210, le cĂ©sium-137 et lâanalyse gĂ©ochimique des sĂ©diments contaminĂ©s par les mines ont Ă©tĂ© utilisĂ©s pour reconstruire lâhistoire des dĂ©pĂŽts de crue. Cette approche a Ă©tĂ© combinĂ©e avec lâhistorique des crues du Gardon depuis 1890 pour affiner lâapproche gĂ©ochronologique. Au niveau de la cavitĂ© GG et de la terrasse GE les analyses ont permis dâillustrer les mĂ©canismes dâĂ©rosion-redĂ©position pouvant perturber les sĂ©quences de sĂ©diments. Les enregistrements des dĂ©pĂŽts de sĂ©diments de crue sont complets sur la terrasse GE situĂ©e Ă 10 m au-dessus du lit mineur Ă partir de 1958, et incomplets auparavant. Sur la pĂ©riode 1880-1958, 25 crues ont eu un dĂ©bit suffisant (>1450 m3/s) pour recouvrir la terrasse, alors que lâon recense seulement 5 Ă 6 crues depuis 1958 (pour un dĂ©bit de submersion supĂ©rieur Ă 1700 m3/s). Lâaugmentation progressive du dĂ©bit de submersion et la frĂ©quence moindre dâinondation de la terrasse ont pu permettre sur la pĂ©riode rĂ©cente la stabilisation de la couche de surface par la vĂ©gĂ©tation et limiter lâĂ©rosion lors de crues importantes. Les enregistrements semblent complets dans la cavitĂ© GG situĂ©e Ă 15 m au-dessus du lit mineur. La frĂ©quence moindre de submersion Ă ce niveau plus Ă©levĂ© peut expliquer une meilleure stabilisation des dĂ©pĂŽts de crue, rendus moins vulnĂ©rables Ă lâĂ©rosion. La datation au radiocarbone a Ă©tĂ© testĂ©e mĂȘme si elle ne permet pas de dater avec prĂ©cision les dĂ©pĂŽts de crue des derniers siĂšcles. Sur la terrasse GE, 80% des dates au carbone 14 sur charbon de bois sont beaucoup trop vieilles par rapport aux autres techniques de datation, ce qui peut sâexpliquer par une remobilisation/dĂ©stockage de charbons de bois anciens. Les datations radiocarbone sur graines donnent de meilleurs rĂ©sultats, car issues dâune production annuelle, plus facilement dĂ©composable et moins sujette Ă des dĂ©pĂŽts intermĂ©diaires. Au final, il apparaĂźt que la technique de datation par radiocarbone sur des charbons de bois retrouvĂ©s dans les sĂ©quences sĂ©dimentaires nâest pas fiable, alors quâelle est la plus pratiquĂ©e en palĂ©o-hydrologie. Lâutilisation combinĂ©e du plomb-210, du cĂ©sium-137, de lâanalyse gĂ©ochimique des sĂ©diments et des informations sur les crues historiques a permis de discriminer et de dater les diffĂ©rents dĂ©pĂŽts de crue de ce dernier siĂšcle dans la cavitĂ© GG et la terrasse GE
Heat flow from the Southeast Indian Ridge flanks between 80°E and 140°E: Data review and analysis
International audienceWe analyze available heat flow data from the flanks of the Southeast Indian Ridge adjacent to or within the Australian-Antarctic Discordance (AAD), an area with patchy sediment cover and highly fractured seafloor as dissected by ridge- and fracture-parallel faults. The data set includes 23 new data points collected along a 14-Ma old isochron and 19 existing measurements from the 20- to 24-Ma old crust. Most sites of measurements exhibit low heat flux (from 2 to 50 mW mâ2) with near-linear temperature-depth profiles except at a few sites, where recent bottom water temperature change may have caused nonlinearity toward the sediment surface. Because the igneous basement is expected to outcrop a short distance away from any measurement site, we hypothesize that horizontally channelized water circulation within the uppermost crust is the primary process for the widespread low heat flow values. The process may be further influenced by vertical fluid flow along numerous fault zones that crisscross the AAD seafloor. Systematic measurements along and across the fault zones of interest as well as seismic profiling for sediment distribution are required to confirm this possible, suspected effect
Groundfish overfishing, diatom decline, and the marine silica cycle : lessons from Saanich Inlet, Canada, and the Baltic Sea cod crash
Author Posting. © American Geophysical Union, 2009. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 23 (2009): GB4032, doi:10.1029/2008GB003416.In this study, we link groundfish activity to the marine silica cycle and suggest that the drastic mid-1980s crash of the Baltic Sea cod (Gadus morhua) population triggered a cascade of events leading to decrease in dissolved silica (DSi) and diatom abundance in the water. We suggest that this seemingly unrelated sequence of events was caused by a marked decline in sediment resuspension associated with reduced groundfish activity resulting from the cod crash. In a study in Saanich Inlet, British Columbia, Canada, we discovered that, by resuspending bottom sediments, groundfish triple DSi fluxes from the sediments and reduce silica accumulation therein. Using these findings and the available oceanographic and environmental data from the Baltic Sea, we estimate that overfishing and recruitment failure of Baltic cod reduced by 20% the DSi supply from bottom sediments to the surface water leading to a decline in the diatom population in the Baltic Sea. The major importance of the marginal ocean in the marine silica cycle and the associated high population density of groundfish suggest that groundfish play a major role in the silica cycle. We postulate that dwindling groundfish populations caused by anthropogenic perturbations, e.g., overfishing and bottom water anoxia, may cause shifts in marine phytoplankton communities.We acknowledge the VENUS Project, University of Victoria, for supporting the ship and submersible time for field experiments and USGS, CMGP, for support to J.C. Additional funding from NSERC Canada and from the Canada Research Chairs Foundation to V.T.; a Rothschild fellowship to G.Y.; and a Yohay Ben-Nun fellowship and Moshe Shilo Center for Marine Biogeochemistry fund to T.K. are also acknowledged
Comparing glacial and Holocene opal fluxes in the Pacific sector of the Southern Ocean
Author Posting. © American Geophysical Union, 2009. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 24 (2009): PA2214, doi:10.1029/2008PA001693.The silicic acid leakage hypothesis (SALH) predicts that during glacial periods excess silicic acid was transported from the Southern Ocean to lower latitudes, which favored diatom production over coccolithophorid production and caused a drawdown of atmospheric CO2. Downcore records of 230Th-normalized opal (biogenic silica) fluxes from 31 cores in the Pacific sector of the Southern Ocean were used to compare diatom productivity during the last glacial period to that of the Holocene and to examine the evidence for increased glacial Si export to the tropics. Average glacial opal fluxes south of the modern Antarctic Polar Front (APF) were less than during the Holocene, while average glacial opal fluxes north of the APF were greater than during the Holocene. However, the magnitude of the increase north of the APF was not enough to offset decreased fluxes to the south, resulting in a decrease in opal burial in the Pacific sector of the Southern Ocean during the last glacial period, equivalent to approximately 15 Gt opal kaâ1. This is consistent with the work of Chase et al. (2003a), and satisfies the primary requirement of the SALH, assuming that the upwelled supply of Si was approximately equivalent during the Holocene and the glacial period. However, previous results from the equatorial oceans are inconsistent with the other predictions of the SALH, namely that either the Corg:CaCO3 ratio or the rate of opal burial should have increased during glacial periods. We compare the magnitudes of changes in the Southern Ocean and the tropics and suggest that Si escaping the glacial Southern Ocean must have had an alternate destination, possibly the continental margins. There is currently insufficient data to test this hypothesis, but the existence of this sink and its potential impact on glacial pCO2 remain interesting topics for future study.Funding for this research was provided in
part by the U.S. NSF (grant OPP02-30268). We thank the core repository at
LDEO and the Antarctic Research Facility at FSU for providing samples
Tracing dust input to the global ocean using thorium isotopes in marine sediments: ThoroMap
Increased nutrient supply to the Southern Ocean during the Holocene and its implications for the pre-industrial atmospheric CO<sub>2</sub> rise
A rise in the atmospheric CO2 concentration of ~20 parts per million over the course of the Holocene has long been recognized as exceptional among interglacials and is in need of explanation. Previous hypotheses involved natural or anthropogenic changes in terrestrial biomass, carbonate compensation in response to deglacial outgassing of oceanic CO2, and enhanced shallow water carbonate deposition. Here, we compile new and previously published fossil-bound nitrogen isotope records from the Southern Ocean that indicate a rise in surface nitrate concentration through the Holocene. When coupled with increasing or constant export production, these data suggest an acceleration of nitrate supply to the Southern Ocean surface from underlying deep water. This change would have weakened the oceanâs biological pump that stores CO2 in the ocean interior, possibly explaining the Holocene atmospheric CO2 rise. Over the Holocene, the circum-North Atlantic region cooled, and the formation of North Atlantic Deep Water appears to have slowed. Thus, the âseesawâ in deep ocean ventilation between the North Atlantic and the Southern Ocean that has been invoked for millennial-scale events, deglaciations and the last interglacial period may have also operated, albeit in a more gradual form, over the Holocene
Archives sĂ©dimentaires dans les lagunes du Golfe dâAigues-Mortes
Avec lâaugmentation rĂ©cente de la population mondiale et des infrastructures en domaine littoral, les tempĂȘtes et les cyclones sont devenus un des risques majeurs Ă lâĂ©chelle globale. La MĂ©diterranĂ©e, et plus particuliĂšrement le Golfe du Lion, ne sont pas Ă©pargnĂ©s par ces phĂ©nomĂšnes. La tempĂȘte de 1982 par exemple, de catĂ©gorie 2 sur lâĂ©chelle de Saffir-Simpson est responsable du dĂ©cĂšs de quinze personnes et de plus de 400 millions dâeuros de pertes Ă©conomiques. La prĂ©vision de ces Ă©vĂ©nements Ă partir de modĂšles climatiques est devenue primordiale. Le problĂšme majeur dans cette approche est le manque de recul dans le temps pour calibrer ces modĂšles. Notre approche a donc Ă©tĂ© de reconstituer ces tempĂȘtes du passĂ© Ă partir de lâĂ©tude dâarchives sĂ©dimentaires (palĂ©otempestologie). Les lagunes du littoral languedocien sont les rĂ©ceptacles de matĂ©riels terrigĂšnes provenant des bassins versants en pĂ©riode de crues, mais aussi de sables transportĂ©s par la mer en pĂ©riode de tempĂȘtes. LâĂ©tude des archives sĂ©dimentaires dans lâĂ©tang de Pierre Blanche nous a permis dâidentifier et de reconstituer lâalternance des Ă©vĂ©nements les plus extrĂȘmes au cours des 2000 derniĂšres annĂ©es. La frĂ©quence de ces tempĂȘtes semble varier dans le temps et une augmentation significative est constatĂ©e durant la fin du Petit Ăge Glaciaire.Hurricane and cyclone, are one of the most alarming natural hazard due to the recent concentration of resources and population in coastal areas. Let us consider the last storm events having affected the south of France (Gulf of Lion, Mediterranean Sea) like in 1982, category 2 in SaffirâSimpson scale, this storm caused the death of fifteen persons and economic losses estimated at 400 million euros, it is necessary to examine the past decadal- to millennial-scale variability of storm activity. Nevertheless the lack of instrumental long time series does not allow to determine the frequency of the most extreme events, therefore we have to resort to other methods. Paleotempestology was used, in this study, to record the past overwash in sediment cores. To identify and characterize the record of environmental changes due to these hurricanes, this work focused on the wetland complex of the Aigues-Mortes gulf (central part of the Gulf of Lion). The filling of these coastal lagoons was due to riverine particulate input during floods and marine sediment input during storm events. Thus cores sediments study of this lagoon allows to characterize the record of these paleostorm events for the last 2000 years. Little Ice Age seems to affect the storm frequency with an increase of hurricanes landfall probabilities from 0.06 % to 0.86 % during this cold period
Les archives sĂ©dimentaires : mĂ©moire des Ă©vĂ©nements extrĂȘmes en MĂ©diterranĂ©e
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