1,137 research outputs found
Ground- and surface water mass balances to ensure protection of St Lawrence River ecostystems
No abstract available
Global Carbon Cycle
The European Union of Geosciences held its 9th biannual meeting in Strasbourg, March 23â27, 1997. During this meeting, Symposium N8 18, Global carbon Cycle, was held under the sponsorship of the IGCP 1 n8404 on the «Terrestrial Carbon in the past 125 Ka», the INQUA 2 Carbon Commission and the ESCOBA-Biosphere 3 project of the EC Environment and Climate Programme. The «Global Carbon Cycle» Symposium attracted 28 oral and poster presentations and about one hundred participants, reflecting the interest of the Earth Sciences community in the global carbon cycle
Celestial Climate Driver: A Perspective from Four Billion Years of the Carbon Cycle
The standard explanation for vagaries of our climate, championed by the IPCC (Intergovernmental Panel on Climate Change), is that greenhouse gases, particularly carbon dioxide, are its principal driver. Recently, an alternative model that the sun is the principal driver was revived by a host of empirical observations. Neither atmospheric carbon dioxide nor solar variability can alone explain the magnitude of the observed temperature increase over the last century of about 0.6°C. Therefore, an amplifier is required. In the general climate models (GCM), the bulk of the calculated temperature increase is attributed to "positive water vapour feedback". In the sun-driven alternative, it may be the cosmic ray flux (CRF), energetic particles that hit the atmosphere, potentially generating cloud condensation nuclei (CCN). Clouds then cool, act as a mirror and reflect the solar energy back into space. The intensity of CRF reaching the earth depends on the intensity of the solar (and terrestrial) magnetic field that acts as a shield against cosmic rays, and it is this shield that is, in turn, modulated by solar activity.
Cosmic rays, in addition to CCN, also generate the so-called cosmogenic nuclides, such as beryllium-10, carbon-14 and chlorine-36. These can serve as indirect proxies for solar activity and can be measured e.g., in ancient sediments, trees, and shells. Other proxies, such as oxygen and hydrogen isotopes can reflect past temperatures, carbon isotopes levels of carbon dioxide, boron isotopes the acidity of ancient oceans, etc. Comparison of temperature records from geological and instrumental archives with the trends for these proxies may enable us to decide which one of the two alternatives was, and potentially is, primarily responsible for climate variability. This, in turn, should enable us to devise appropriate countermeasures for amelioration of human impact on air quality and climate.
SOMMAIRE
GĂ©nĂ©ralement, les raisons donnĂ©es pour expliquer les caprices de notre climat, les mĂȘmes que celles avancĂ©es par le CICC (ComitĂ© intergouvernemental sur le changement climatique), veulent que ce soient les gaz Ă effet de serre, particuliĂšrement le dioxyde de carbone, qui en soient le moteur principal. RĂ©cemment, une sĂ©rie d'observations empiriques ont ravivĂ© l'intĂ©rĂȘt pour un autre modĂšle voulant que ce soit le soleil qui en soit le moteur principal. Mais seuls, ni le dioxyde ce carbone ni les variations d'activitĂ© solaire ne permet d'expliquer la hausse de tempĂ©rature observĂ©e au cours du siĂšcle dernier, soit environ 0,6 °C. D'oĂč la nĂ©cessitĂ© d'un facteur d'amplification. Dans les modĂšles climatiques gĂ©nĂ©raux (GCM), le gros de l'accroissement calculĂ© de tempĂ©rature est dĂ» à « la rĂ©troaction positive de la vapeur d'eau ». Dans le modĂšle Ă moteur solaire, ce pourrait ĂȘtre le flux de rayonnement cosmique (FRC), ce pourrait ĂȘtre l'effet des particules Ă©nergiques qui en frappant l'atmosphĂšre entraĂźnent une gĂ©nĂ©ration possible de nuclĂ©us de condensation des nuages (NCN). Alors, les nuages se refroidissent et, comme un miroir, rĂ©flĂ©chissent l'Ă©nergie solaire dans l'espace. L'intensitĂ© du FRC atteignant le sol dĂ©pend de l'intensitĂ© des champs magnĂ©tiques du soleil et de la Terre, lesquels agissent comme un bouclier Ă l'endroit des rayons cosmiques, le pouvoir de ce bouclier Ă©tant Ă son tour modulĂ© par l'activitĂ© solaire.
En plus d'entraĂźner la formation de NCN, les rayons cosmiques, gĂ©nĂšrent aussi ce qu'on appelle des nuclĂ©ides cosmogĂ©niques, comme le bĂ©ryllium-10, le carbone-14 et le chlore-36. Ces nuclĂ©ides peuvent servir d'indicateurs indirects de l'activitĂ© solaire puisqu'on peut en mesurer la teneur dans des sĂ©diments anciens, des arbres, et des coquilles, par exemple. D'autres indicateurs indirects comme les isotopes d'oxygĂšne et d'hydrogĂšne peuvent reflĂ©ter les tempĂ©ratures de jadis, les isotopes de carbone peuvent reflĂ©ter les niveaux de dioxyde de carbone, les isotopes de bore peuvent reflĂ©ter l'aciditĂ© des anciens ocĂ©ans, etc. La comparaison entre des registres de mesures de tempĂ©rature directes et d'archives gĂ©ologiques, avec les courbes de tendance de tels indicateurs indirects peut nous permettre de dĂ©cider laquelle de deux options Ă©tait et continue possiblement d'ĂȘtre la cause principale des variations climatiques. On pourrait alors dĂ©cider de contre-mesures appropriĂ©es permettant d'attĂ©nuer l'impact des activitĂ©s humaines sur la qualitĂ© de l'aire et sur le climat
Ruthenium/Iridium Ratios in the Cretaceous-tertiary Boundary Clay: Implications for Global Dispersal and Fractionation Within the Ejecta Cloud
Ruthenium (Ru) and iridium (Ir) are the least mobile platinum group elements (PGE's) within the Cretaceous-Tertiary (K-T) boundary clay (BC). The Ru/Ir ratio is, therefore, the most useful PGE interelement ratio for distinguishing terrestrial and extraterrestrial contributions to the BC. The Ru/Ir ratio of marine K-T sections (1.77 +/- 0.53) is statistically different from that of the continental sections (0.93 +/- 0.28). The marine Ru/Ir ratios are chondritic (C1 = 1.48 +/- 0.09), but the continental ratios are not. We discovered an inverse correlation of shocked quartz size (or distance from the impact site) and Ru/Ir ratio. This correlation may arise from the difference in Ru and Ir vaporization temperature and/or fractionation during condensation from the ejecta cloud. Postsedimentary alteration, remobilization, or terrestrial PGE input may be responsible for the Ru/Ir ratio variations within the groups of marine and continental sites studied. The marine ratios could also be attained if approximately 15 percent of the boundary metals were contributed by Deccan Trap emissions. However, volcanic emissions could not have been the principal source of the PGE's in the BC because mantle PGE ratios and abundances are inconsistent with those measured in the clay. The Ru/Ir values for pristine Tertiary mantle xenoliths (2.6 +/- 0.48), picrites (4.1 +/- 1.8), and Deccan Trap basalt (3.42 +/- 1.96) are all statistically distinct from those measured in the K-T BC
Terrestrial and fluvial carbon fluxes in a tropical watershed: Nyong basin, Cameroon
The Nyong watershed, with an area of 27 800 km2 and a mean annual discharge of 390 m3 sâ1, is the second largest river in Cameroon. The Nyong watershed serves as an outstanding study area for the examination of carbon fluxes in humid tropical environments because of its limited anthropogenic impact and homogeneous silicate bedrock. Between April 2005 and April 2007, we sampled water at seven stations, from the small watershed of the Mengong (0.6 km2) to the Nyong at Edea (24 500 km2), and monitored temperature, pH, dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) contents, as well as the isotopic composition of DIC (ÎŽ13CDIC)andDOC(ÎŽ13CDOC).We estimated terrestrial net ecosystemproductivity in theNyong River watershed and measured fluvial fluxes of carbon to the ocean and the atmosphere. The Nyong River basin sequesters significant amounts of carbon on an annual basis: ~7 920 000t C yearâ1 (300 g C mâ2 yearâ1). The combined dissolved organic, dissolved inorganic and atmospheric fluxes of carbon from the Nyong River only export 3% of this flux fromthe basin on an annual basis. This includes a minimumCO2 outgassing of 1487 g Cmâ2 yearâ1, comparable to 115% of the annual flux of DOC and four times greater than the flux of DIC
The SPICE carbon isotope excursion in Siberia: a combined study of the upper Middle Cambrian-lowermost Ordovician Kulyumbe River section, northwestern Siberian Platform
An integrated, high-resolution chemostratigraphic (C, O and Sr isotopes) and
magnetostratigraphic study through the upper Middle Cambrianâlowermost Ordovician shallowmarine
carbonates of the northwestern margin of the Siberian Platform is reported. The interval was
analysed at the Kulyumbe section, which is exposed along the Kulyumbe River, an eastern tributary
of the Enisej River. It comprises the upper Ustâ-Brus, Labaz, Orakta, Kulyumbe, Ujgur and lower
Iltyk formations and includes the Steptoean positive carbon isotopic excursion (SPICE) studied here
in detail from upper Cambrian carbonates of the Siberian Platform for the first time. The peak of the
excursion, showing ÎŽ13C positive values as high as+4.6â°and least-altered 87Sr/86Sr ratios of 0.70909,
is reported herein from the Yurakhian Horizon of the Kulyumbe Formation. The stratigraphic position
of the SPICE excursion does not support traditional correlation of the boundary between theOrakta and
Labaz formations at the Kulyumbe River with its supposedly equivalent level in Australia, Laurentia,
South China and Kazakhstan, where the Glyptagnostus stolidotus and G. reticulatus biozones are
known to immediately precede the SPICE excursion and span the MiddleâUpper Cambrian boundary.
The CambrianâOrdovician boundary is probably situated in the middle Nyajan Horizon of the Iltyk
Formation, in which carbon isotope values show a local maximum below a decrease in the upper
part of the Nyajan Horizon, attributed herein to the Tremadocian Stage. A refined magnetic polarity
sequence confirms that the geomagnetic reversal frequency was very high during Middle Cambrian
times at 7â10 reversals per Ma, assuming a total duration of about 10 Ma and up to 100 magnetic
intervals in the Middle Cambrian. By contrast, the sequence attributed herein to the Upper Cambrian
on chemostratigraphic grounds contains only 10â11 magnetic intervals
A Cenozoic-style scenario for the end-Ordovician glaciation
The end-Ordovician was an enigmatic interval in the Phanerozoic, known for massive glaciation potentially at elevated CO2 levels, biogeochemical cycle disruptions recorded as large isotope anomalies and a devastating extinction event. Ice-sheet volumes claimed to be twice those of the Last Glacial Maximum paradoxically coincided with oceans as warm as today. Here we argue that some of these remarkable claims arise from undersampling of incomplete geological sections that led to apparent temporal correlations within the relatively coarse resolution capability of Palaeozoic biochronostratigraphy. We examine exceptionally complete sedimentary records from two, low and high, palaeolatitude settings. Their correlation framework reveals a Cenozoic-style scenario including three main glacial cycles and higher-order phenomena. This necessitates revision of mechanisms for the end-Ordovician events, as the first extinction is tied to an early phase of melting, not to initial cooling, and the largest ÎŽ13C excursion occurs during final deglaciation, not at the glacial apex
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