5th Swiss Geoscience Meeting, Geneva 2007
Multi-isotope study of Miocene foraminifers from the
Mediterranean
László Kocsis*, Torsten W. Vennemann*, Denis Fontignie**, Claudia
Baumgartner*** and Alessandro Montanari****
* Institut de Minéralogie et Géochimie, University of Lausanne, Switzerland,
laszlo.kocsis@unil.ch, torsten.vennemann@unil.ch
** Département de Minéralogie, University of Geneva,
Switzerlanddenis.fontigniel@unige.ch
*** Institut de Géologie et Paléontologie, University of Lausanne, Switzerland,
claudia.baumgartner@unil.ch
**** Osservatorio Geologico di Coldigioco Frontale di Apiro, Italy, sandro.ogc@fastnet.it
The composite section of the Umbria-Marche region in Italy represents a
continuous marine pelagic sequence of the Mediterranean, deposited during the
Oligocene up to the Middle Miocene (Montanari et al., 1997). At this time the
Mediterranean was an open seaway between the Atlantic and the Indo-Pacific
provinces, which closed towards the end of the Middle Miocene (e.g. Rögl,
1998). The main aim of this study is to constrain changes in the oceanic
circulation and climate in this part of the Mediterranean in order to examine the
possibility of a regional influence on climate related to the tectonism of the
circum-Alpine region.
To achieve this aim, well-preserved carbonaceous fossils and the carbonate
free, detrital fraction of the sediments were analyzed for their Nd and Sr isotopic
compositions. While isotopic compositions of the marine fossils can provide
information on the composition of the seawater, those of the detrital sediments
help to constrain the local silicate inputs for Nd and Sr from the hinterland. In
addition, planktic and benthic foraminifera were analyzed separately for their O
and C isotope compositions in order to trace habitat conditions, climatic
changes and help to evaluate possible diagenetic alteration of the fossils
Oxygen isotope compositions of benthic and planktic foraminifera preserved
their typical offset related to their habitat and also vary in concert with those
used for evaluating global changes in climate. Carbon isotope compositions
also reflect differences that are typical for the carbon isotope compositions of
dissolved inorganic carbon in bottom and surface waters. Furthermore, at
between 17-13 Ma a positive 13C excursion is noted and this correlates well
with the globally recognized Monterey carbon isotope event. These results
support preservation of the stable isotope compositions of the fossils.
According to the results of the radiogenic isotope measurements, the UmbraMarche composite section can be divided into three periods:
1) During the period from 25 to 19 Ma the fossils are characterized by high Nd
values, with values similar to those of the silicate fraction of the sediments. The
87Sr/86Sr ratios are lower than those of the global open ocean at the time and
are also low for the silicate fraction. These compositions are interpreted to result
from intense volcanism due to the opening of the western part of the
Mediterranean Sea (e.g., Gueguen et al., 1998).
5th Swiss Geoscience Meeting, Geneva 2007
2) Between 19-13 Ma the sediments have all have low Nd values of about -10,
while the fossils have an average value of -8.5, slightly increasing throughout
this section though. There is a short period at around 15.2 Ma where the Nd of
the fossils deviate from the above described trend with a rise to a Nd value of 7, followed by a decrease to the background sediment Nd value of - 10 again.
The Sr isotope ratios of the fossils are still lower than those for the Miocene
global ocean.
The short change at 15.2 Ma is related to a sporadic volcanic event, also
supported by elevated Nd and low 87Sr/86Sr values of the detrital sediment. The
following decrease in the Nd value can be ascribed to an increase in the
terrestrial input from the hinterland which also coincides with the TB 2.3-2.4
sea-level decrease cycle (Haq et al., 1987). The low 87Sr/86Sr of the fossils may
be explained by weathering and erosion of Mesozoic carbonates, known to be
abundant in the hinterland, and an influence thereof on the composition of the
local marine basin. The warm, humid climate during this time was also favorable
for the weathering of carbonates, as indicated by the oxygen isotope data.
3) During the third time interval (13-7 Ma) the Sr isotope ratios of the fossils
reflect those of the Middle Miocene open ocean, with one exception given by
the youngest sample. The average Nd value of the sediment remains the same,
but those of the fossils vary considerably. Around 12 to 11 Ma a change in the
Nd values to between -10 and -9 can be observed for the fossils. At 10 Ma this
is directly followed by an increase to Nd values of between -8 and -7. Towards
the end of the section, at about 8 Ma, the Nd value decreases to -9 again. This
fluctuation parallels the changes in global sea-level.
The strong negative shift in Nd value concurs with transgression and indicates a
strengthening Atlantic water inflow into the Mediterranean. During regressions,
however, water with Nd values similar to the Miocene Indian Ocean was
unlikely to have entered the marine basins in this region, on the basis of
paleontological data that indicates that the Indo-Pacific influence in the region
has ceased (Rögl, 1998). Therefore, seawater via the remnants of the
Paratethys, which is known to have had such relatively high Nd values during
the Middle Miocene (Kocsis et al., submitted) may have entered the
Mediterranean. Alternatively, increased erosion of rocks elsewhere in the
Mediterranean region having higher Nd values could have influenced this water
mass.
REFERENCES
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(submitted). Paleoceanography and paleoclimate of the Miocene Paratethys:
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5th Swiss Geoscience Meeting, Geneva 2007
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