Carbon-isotope stratigraphy of the Liesberg Beds Member (Oxfordian, Swiss Jura) using echinoids and crinoids


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Carbon-isotope stratigraphy of the Liesberg Beds Member (Oxfordian, Swiss Jura) using echinoids and crinoids
Eclogae Geologicae Helvetiae
Bill M., Baumgartner P.O., Hunziker J.C., Sharp Z.D.
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The Liesberg Beds form the transition between the lower Oxfordian dark
coloured marls (Renggeri Member and the Terrain a Chailles Member) and
the middle Oxfordian reefal limestones (St-Ursanne Formation). Both
lithofacies and biofacies are diverse and evolve rapidly up-section.
Stable isotope studies of whole-rock samples are therefore excluded. In
search for a convenient isotopic marker, we measured carbon isotope
compositions of several fossil groups and chose crinoid stems of
Millericrinus spp and echinoid spines of Paracidaris spp because of
their abundance throughout the section and the small variations of
delta(13)C within one fossil and between fossils from the same
stratigraphic level.
The delta(13)C values of echinoderms largely reflect earliest diagenetic
conditions at the seawatersediment interface. The porous stereome
structure secreted of high Mg-calcite by echinoderms has a high reactive
surface/volume ratio, which triggers the precipitation of very early
syntaxial cements.
In the four studied sections reproducible carbon isotope shifts were
observed both for Millericrinus spp stems and Paracidaris spp spines. A
negative delta(13)C shift of 1-1.5 parts per thousand was observed near
the base of the section, just above the transition from Terrain a
Chailles Member, where the first corals occur. In the middle and upper
part of the four sections, characterised by a stepwise increase of
corals and the macrofossils, a positive delta(13)C Shift of about 2
parts per thousand was observed.
Despite the highly variable lithologic composition of the Liesberg
Beds;Member, carbon isotope shifts seem to be consistent and warrant an
interpretation as an original signal, controlled by the isotopic
composition of dissolved carbonic acid in seawater.
We explain the heavy delta(13)C values (approximate to 2-2.3 parts per
thousand) in the lower Liesberg Beds as a transition from an
oxygen-limited environment (Terrain a Chailles Member) to the Liesberg
Beds Member. The lowest delta(13)C values (approximate to 1-1.5 parts
per thousand) correspond to a large input of dissolved nutrients to the
platform under oxidizing conditions. The ensuing positive shift (between
2.5 and 3.5 parts per thousand), however, seems to correspond to a
general trend of opening up of the platform and connection to open
marine waters. Positive delta(13)C values in the upper Liesberg Beds is
interpreted as a result of important accelareted extraction of organic
carbon from the ocean reservoir, that occurred possibly during periods
of warm and humid climate.
Create date
06/10/2012 17:23
Last modification date
20/08/2019 17:23
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