Isotopic composition (O, C, Sr, and Nd) and trace element ratios (Sr/Ca, Mg/Ca) of Miocene marine and brackish ostracods from North Alpine Foreland deposits (Germany and Austria) as indicators for palaeoclimate


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Isotopic composition (O, C, Sr, and Nd) and trace element ratios (Sr/Ca, Mg/Ca) of Miocene marine and brackish ostracods from North Alpine Foreland deposits (Germany and Austria) as indicators for palaeoclimate
Palaeogeography, Palaeoclimatology, Palaeoecology
Janz H., Vennemann T.W.
Statut éditorial
Date de publication
The Molasse basin, constituting the western part of the Paratethys,
formed as a consequence or the Alpine orogeny during the Tertiary. To
address the influence of the Alpine upliftment and associated geographic
changes on the regional climate, the faunal assemblages and geochemistry
of Miocene marine and brackish ostracods of the Molasse sediments were
investigated. Eighteen sites from the Bavarian Molasse to the Vienna
basin were studied. ranging in age from the Eggenburgian to the
Pannonian: (20 to 8.5 Ma). A total of 176 species of 71 genera were
found, with the highest species diversity observed for Loxoconcha (12),
Aurila (11), Cyprideis (9), and Cytheridea (7). On the basis of the
different genera, the bathymetry of sedimentation can be characterized
as infra- to circalittoral. From the Eggenburgian to the Early Sarmatian
(20 to 13 Ma) open marine conditions are indicated by the faunal
assemblages. For the Late Sarmatian and Pannonian (12 to 8.5 Ma), the
occurrence of Cyprideis spp. and candonid species indicate brackish
conditions. Chemical and isotopic analyses concentrated an valves of
Cytheridea spp, and Cyprideis spp., but valves from other ostracod
species were also analyzed to evaluate the effects of the environment of
formation and vital effects. For the period from 20 to 13 Ma, the Sr
isotope ratios (Sr-87/Sr-86 between 0.70850 and 0.70883) and delta(18)O
values (range from 28.3 parts per thousand to 32.8 parts per thousand,
VSMOW) are compatible with open marine conditions. Nd isotope ratios for
this period are quite different for different localities (between
0.51216-0.51225), indicating localized control on seawater Nd-143/Nd-144
ratios. Mean delta(13)C values for this period range from -5.2 parts per
thousand to +1.4 parts per thousand (VPDB), Sr/Ca ratios from 0.0039 to
0.0067, and Mg/Ca ratios from 0.0083 to 0.0287, For the period from 12
to 8.5 Ma the variations in Sr-87/Sr-86 (0.70872-0.70883), delta(18)O
(24.1 parts per thousand to 30.2 parts per thousand) and delta(13)C
values (-7.8 parts per thousand to -2.6 parts per thousand), as well as
Sr/Ca (0.0021 to 0.0059), and Mg/Ca (0.0116 to 0.0284) all increase. For
this period the Nd isotope compositions are quite homogeneous
(0.51218-0.51220), indicating a similar source lithological control on
hydrochemistry which may allow the Sr/Ca and Mg/Ca ratios to be
interpreted in terms of changes in water temperature and mineralogy of
the precipitating phases. Geochemical changes for this period are
compatible with alternating influences of seawater and brackish
conditions as well as changes resulting from evaporation. For all
periods delta(18)O and delta(13)C values as well as Sr/Ca and Mg/Ca of
different species from the same samples generally have similar trends
with small offsets. By comparison to records of climatic change from the
global oceans, the delta(18)O values for marine fossils of the Molasse
between 20 and 13 Ma are interpreted to reflect global changes in
climate. This is also true for the increasingly brackish conditions up
to 11 Ma, where combined geochemical records are still compatible with
the global changes in temperatures. However, increasing complexity of
the brackish system makes detailed comparisons with global changes
difficult, particularly after 9.5 Ma (Late Pannonian stages D/E). (c)
2005 Elsevier B.V. All rights reserved.
Création de la notice
29/09/2012 16:23
Dernière modification de la notice
20/08/2019 16:05
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