Global Change Abstracts The Swiss Contribution - SCNAT

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170 08.1-347 Detailed record of the mid-Oxfordian (Late Jurassic) positive carbonisotope excursion in two hemipelagic sections (France and Switzerland): A plate tectonic trigger? Louis Schmid B, Rais P, Bernasconi S M, Pellenard P, Collin P Y, Weissert H Switzerland, France Paleontology , Oceanography , Geology , Geochemistry & Geophysics The Oxfordian (Late Jurassic) was a time of widespread change in Jurassic marine (carbonate) sedimentation patterns. A marked positive excursion in delta C-13 is dated as Middle Oxfordian in age. In this study we investigate if changes in carbonate sedimentation coincided with altered carbon cycling and climate. We use C-isotope records as a proxy for the evolution of the carbon cycle and compare delta C-13- trends with the evolution of sedimentation in a segment of the opening Tethys seaway. One of the studied sections is located in the Subalpine basin of France (Trescleoux and Oze), the other in the Swiss Jura mountains (Liesberg). Carbon-isotope stratigraphy of carbonate carbon locates the peak values of the Middle Oxfordian positive excursion to the antecedens and parandieri subzones of the plicatilis and transversarium ammonite zones, respectively. Causes of the excursion remain enigmatic. The excursion seems not to coincide with a global oceanic anoxic event, but regionally enhanced organic carbon accumulation during the Early and early Middle Oxfordian may be the cause of the increase in delta C-13. The excursion occurs during a time of progressive climate warming, and its peak values coincide with the first calcareous sediments recurring after a period of reduced carbonate accumulation in the Early and early Middle Oxfordian. The excursion is also time-equivalent to a major reorganisation of global ocean currents: the opening Atlantic and Tethys oceans combined with rising sea level led to the formation of an efficient circumglobal seaway. We conclude that this favoured the widespread recurrence of carbonate producers by providing new habitats. As a result, C-carb burial increased, leading to a lower C- org/ C-carb burial rate and therefore to stabilisation and decrease in delta C-13 in the Middle Oxfordian. Thus, the mid-Oxfordian positive excursion in carbonate C-isotopes may reflect changes in the carbon cycle that were triggered by a copious reorganisation of the ocean current system caused by major plate tectonic movements. Palaeogeography Palaeoclimatology Palaeoecology, 2007, V248, N3-4, MAY 25, pp 459-472. Global Change Abstracts – The Swiss Contribution | Past Global Changes 08.1-348 Impact of methane seeps on the local carbonisotope record: a case study from a Late Jurassic hemipelagic section Louis Schmid B, Rais P, Logvinovich D, Bernasconi S M, Weissert H Switzerland Geology , Paleontology , Geochemistry & Geophysics An Oxfordian (Late Jurassic) hemipelagic succession from Beauvoisin (SE France) contains a pronounced, short-lived negative excursion in the bulk-carbonate carbon-isotope record, with an amplitude of 47 parts per thousand. it was shown previously that the Beauvoisin paleoenvironment was impacted by hydrocarbon seepage. New isotopic data corroborate that methane was a significant constituent of these hydrocarbons. The negative excursion was caused by transient enhanced precipitation of 13 C-depleted carbonate, mediated by anaerobic oxidation of methane. Despite its local diagenetic origin, the Beauvoisin excursion is similar in shape and duration to globally recognized negative C-isotope excursions that have been related to catastrophic, massive dissociation of methane hydrate. Shape and duration of negative excursions therefore cannot be used as an argument when determining their origin if they have not been shown to represent a global perturbation of the carbon cycle. Terra Nova, 2007, V19, N4, AUG, pp 259-265. 08.1-349 Plate tectonic trigger of changes in pCO(2) and climate in the Oxfordian (Late Jurassic): Carbon isotope and modeling evidence Louis Schmid B, Rais P, Schäffer P, Bernasconi S M, Weissert H Switzerland, France Modelling , Paleontology , Meteorology & Atmospheric Sciences , Geochemistry & Geophysics The transition from the Middle to the Late Jurassic was characterized by significant changes in oceanography and climate and by changes in global carbon cycle as shown in the C-isotope record. A prominent mid-Oxfordian positive excursion in bulk carbonate carbon isotope values (delta C-13(carb)) With an amplitude of more than 1% has been documented from many sections in the Northern Tethys realm. In this study we present new bulk organic matter C-isotope data (delta C-13 (org)) from northwestern Tethys that do not record the mid-Oxfordian positive excursion in carbonate carbon. On the contrary, delta C-13 (org) decreases during the interval of the most
Global Change Abstracts – The Swiss Contribution | Past Global Changes 171 rapid increase in delta C-13(carb). We demonstrate that this decrease is not due to a changing marine-terrestrial organic carbon partitioning but that the contrasting isotope trends record peculiar environmental and climate changes which occurred near the beginning of the Late Jurassic. Using a simple carbon cycle model we show that an increase in atmospheric p CO(2) starting at modem levels could be the cause of contrasting trends in delta C-13(carb) and delta C-13(org). We suggest that a reorganisation of ocean currents related to the opening and/or widening of the Tethys-Atlantic-Pacific seaway, and a massive spread of shallow-sea carbonate production led to higher pCO(2). Model simulations indicate that this increase in pCO(2) may have triggered changes in the biological carbon pump and in organic carbon burial that can explain the Middle Oxfordian C-isotope record. Earth and Planetary Science Letters, 2007, V258, N1-2, JUN 15, pp 44-60. 08.1-350 New constraints on the gas age-ice age difference along the EPICA ice cores, 0-50 kyr Loulergue L, Parrenin F, Blunier T, Barnola J M, Spahni R, Schilt A, Raisbeck G, Chappellaz J France, Switzerland Paleontology , Meteorology & Atmospheric Sciences , Cryology / Glaciology Gas is trapped in polar ice sheets at similar to 50-120 m below the surface and is therefore younger than the surrounding ice. Firn densification models are used to evaluate this ice age-gas age difference (Delta age) in the past. However, such models need to be validated by data, in particular for periods colder than present day on the East Antarctic plateau. Here we bring new constraints to test a firn densification model applied to the EPICA Dome C (EDC) site for the last 50 kyr, by linking the EDC ice core to the EPICA Dronning Maud Land (EDML) ice core, both in the ice phase (using volcanic horizons) and in the gas phase (using rapid methane variations). We also use the structured Be-10 peak, occurring 41 kyr before present (BP) and due to the low geomagnetic field associated with the Laschamp event, to experimentally estimate the Delta age during this event. Our results seem to reveal an overestimate of the Delta age by the firn densification model during the last glacial period at EDC. Tests with different accumulation rates and temperature scenarios do not entirely resolve this discrepancy. Although the exact reasons for the Delta age overestimate at the two EPICA sites remain unknown at this stage, we conclude that current densification model simulations have def- icits under glacial climatic conditions. Whatever the cause of the Delta age overestimate, our finding suggests that the phase relationship between CO 2 and EDC temperature previously inferred for the start of the last deglaciation (lag of CO 2 by 800 +/- 600 yr) seems to be overestimated. Climate of the Past, 2007, V3, N3, pp 527-540. 08.1-351 Comparison of techniques for dating of subsurface ice from Monlesi ice cave, Switzerland Luetscher M, Bolius D, Schwikowski M, Schotterer U, Smart P L Switzerland, England Paleontology , Cryology / Glaciology The presence of cave ice is documented in many karst regions but very little is known about the age range of this potential paleoclimate archive. This case study from the Monlesi ice cave, Swiss Jura Mountains, demonstrates that dating of cave ice is possible using a multi-parameter approach. Ice petrography, debris content and oxygen isotope composition have the potential for identification of annual growth layers, but require a continuous core from the ice deposits, limiting application of this approach. Furthermore, complete melting of ice accumulations from individual years may occur, causing amalgamation of several annual bands. Use of H-3 content of the ice and C-14 dating of organic debris present in the ice proved to be of limited utility, providing rather broad bounds for the actual age. Initial estimates based on Pb-210 analyses from clear ice samples gave results comparable to those from other methods. The most reliable techniques applied were the determination of ice turnover rates, and the dating of anthropogenic inclusions (a roof tile) in the ice. These suggest, respectively, that the base of the cave ice was a minimum of 120 and a maximum of 158 years old. Therefore, our data support the idea that mid-latitude and low-altitude subsurface ice accumulations result from modern deposition processes rather than from presence of Pleistocene relict ice. Journal of Glaciology, 2007, V53, N182, pp 374-384. 08.1-352 Detailed sedimentary N isotope records from Cariaco Basin for terminations I and V: Local and global implications Meckler N A , Haug G H, Sigman D M, Plessen B, Peterson L C, Thierstein H R Switzerland, Germany, USA Geology , Geochemistry & Geophysics , Oceanography , Paleontology
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