Global Change Abstracts The Swiss Contribution - SCNAT
Global Change Abstracts The Swiss Contribution - SCNAT
Global Change Abstracts The Swiss Contribution - SCNAT
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170<br />
08.1-347<br />
Detailed record of the mid-Oxfordian (Late<br />
Jurassic) positive carbon- isotope excursion in<br />
two hemipelagic sections (France and Switzerland):<br />
A plate tectonic trigger?<br />
Louis Schmid B, Rais P, Bernasconi S M, Pellenard P,<br />
Collin P Y, Weissert H<br />
Switzerland, France<br />
Paleontology , Oceanography , Geology , Geochemistry<br />
& Geophysics<br />
<strong>The</strong> Oxfordian (Late Jurassic) was a time of widespread<br />
change in Jurassic marine (carbonate) sedimentation<br />
patterns. A marked positive excursion<br />
in delta C-13 is dated as Middle Oxfordian in age.<br />
In this study we investigate if changes in carbonate<br />
sedimentation coincided with altered carbon<br />
cycling and climate. We use C-isotope records as<br />
a proxy for the evolution of the carbon cycle and<br />
compare delta C-13- trends with the evolution of<br />
sedimentation in a segment of the opening Tethys<br />
seaway. One of the studied sections is located in<br />
the Subalpine basin of France (Trescleoux and<br />
Oze), the other in the <strong>Swiss</strong> Jura mountains (Liesberg).<br />
Carbon-isotope stratigraphy of carbonate<br />
carbon locates the peak values of the Middle Oxfordian<br />
positive excursion to the antecedens and<br />
parandieri subzones of the plicatilis and transversarium<br />
ammonite zones, respectively. Causes of<br />
the excursion remain enigmatic. <strong>The</strong> excursion<br />
seems not to coincide with a global oceanic anoxic<br />
event, but regionally enhanced organic carbon<br />
accumulation during the Early and early Middle<br />
Oxfordian may be the cause of the increase in<br />
delta C-13. <strong>The</strong> excursion occurs during a time of<br />
progressive climate warming, and its peak values<br />
coincide with the first calcareous sediments recurring<br />
after a period of reduced carbonate accumulation<br />
in the Early and early Middle Oxfordian.<br />
<strong>The</strong> excursion is also time-equivalent to a major<br />
reorganisation of global ocean currents: the opening<br />
Atlantic and Tethys oceans combined with rising<br />
sea level led to the formation of an efficient<br />
circumglobal seaway. We conclude that this favoured<br />
the widespread recurrence of carbonate<br />
producers by providing new habitats. As a result,<br />
C-carb burial increased, leading to a lower C- org/<br />
C-carb burial rate and therefore to stabilisation<br />
and decrease in delta C-13 in the Middle Oxfordian.<br />
Thus, the mid-Oxfordian positive excursion<br />
in carbonate C-isotopes may reflect changes in the<br />
carbon cycle that were triggered by a copious reorganisation<br />
of the ocean current system caused by<br />
major plate tectonic movements.<br />
Palaeogeography Palaeoclimatology Palaeoecology,<br />
2007, V248, N3-4, MAY 25, pp 459-472.<br />
<strong>Global</strong> <strong>Change</strong> <strong>Abstracts</strong> – <strong>The</strong> <strong>Swiss</strong> <strong>Contribution</strong> | Past <strong>Global</strong> <strong>Change</strong>s<br />
08.1-348<br />
Impact of methane seeps on the local carbonisotope<br />
record: a case study from a Late Jurassic<br />
hemipelagic section<br />
Louis Schmid B, Rais P, Logvinovich D, Bernasconi S<br />
M, Weissert H<br />
Switzerland<br />
Geology , Paleontology , Geochemistry &<br />
Geophysics<br />
An Oxfordian (Late Jurassic) hemipelagic succession<br />
from Beauvoisin (SE France) contains a<br />
pronounced, short-lived negative excursion in<br />
the bulk-carbonate carbon-isotope record, with<br />
an amplitude of 47 parts per thousand. it was<br />
shown previously that the Beauvoisin paleoenvironment<br />
was impacted by hydrocarbon seepage.<br />
New isotopic data corroborate that methane was<br />
a significant constituent of these hydrocarbons.<br />
<strong>The</strong> negative excursion was caused by transient<br />
enhanced precipitation of 13 C-depleted carbonate,<br />
mediated by anaerobic oxidation of methane.<br />
Despite its local diagenetic origin, the Beauvoisin<br />
excursion is similar in shape and duration to globally<br />
recognized negative C-isotope excursions that<br />
have been related to catastrophic, massive dissociation<br />
of methane hydrate. Shape and duration<br />
of negative excursions therefore cannot be used<br />
as an argument when determining their origin if<br />
they have not been shown to represent a global<br />
perturbation of the carbon cycle.<br />
Terra Nova, 2007, V19, N4, AUG, pp 259-265.<br />
08.1-349<br />
Plate tectonic trigger of changes in pCO(2)<br />
and climate in the Oxfordian (Late Jurassic):<br />
Carbon isotope and modeling evidence<br />
Louis Schmid B, Rais P, Schäffer P, Bernasconi S M,<br />
Weissert H<br />
Switzerland, France<br />
Modelling , Paleontology , Meteorology & Atmospheric<br />
Sciences , Geochemistry & Geophysics<br />
<strong>The</strong> transition from the Middle to the Late Jurassic<br />
was characterized by significant changes<br />
in oceanography and climate and by changes in<br />
global carbon cycle as shown in the C-isotope<br />
record. A prominent mid-Oxfordian positive excursion<br />
in bulk carbonate carbon isotope values<br />
(delta C-13(carb)) With an amplitude of more than<br />
1% has been documented from many sections in<br />
the Northern Tethys realm. In this study we present<br />
new bulk organic matter C-isotope data (delta<br />
C-13 (org)) from northwestern Tethys that do not<br />
record the mid-Oxfordian positive excursion in<br />
carbonate carbon. On the contrary, delta C-13<br />
(org) decreases during the interval of the most