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| Titel |
Eemian seasonal temperature variations recorded by very high-resolution analyses of a MIS 5e stromatolite from Caours (Somme Basin, Northern France): combining petrography and stable isotopes. |
| VerfasserIn |
Julie Dabkowski, J. Andrews, P. Antoine, N. Limondin-Lozouet, A. Marca-Bell |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250058608
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| Zusammenfassung |
| In many tufa formations, very well crystallised deposits called stromatolites are preserved.
They generally present successive laminations thought to be linked to seasonal climatic
and environmental variations in modern to sub-fossil deposits. They thus represent
a huge potential for very high resolution records of Pleistocene climate. One of
the very first investigations in this way has been performed on a 2.5Â cm-radius
stromatolite from the Eemian sequence of Caours (Somme Basin, Northern France),
where precise petrographical observations have been combined with stable isotope
analyse.
Lamina succession observed in macroscopic scale has been shown in thin section to be
linked to two major facies. The first is built by well developed, elongate calcite spar crystals
including imprints of cyanobacteral bushes. This facies alternates with laminae composed by
fractured crystals or micritic layers. The first facies could be linked to summer development
of cyanobacteria under optimal temperature and light conditions and the ‘degraded’ or
micritic facies to less favourable winter conditions. Stable oxygen and carbon isotope
analyses performed on 69 intra-lamina samples show variation strongly concordant with
lamina succession. Facies with developed sparry crystals is associated to minimal isotopic
values whereas the ‘degraded’ facies clearly record the highest. At seasonal resolution,
carbon isotopic composition has been previously shown in modern tufa to covary positively
with oxygen isotopic composition probably driven by temperature dependent phenomena in
the aquifer: thus the positive correlation observed in the Caours stromatolites between the
isotopic ratios is not surprising. Oxygen stable isotope composition in tufa calcite
is known at this resolution to be directly linked to water temperature: increasing
temperature related to decreasing values. This interpretation of geochemical results is thus
strongly consistent with facies attribution to seasonal variations. Indeed, maximal
temperatures are recorded in well developed calcite laminas assigned to summer
conditions.
These preliminary results indicate that seasonal palaeoclimatic information should be
available in fossil Pleistocene stromatolites. High-resolution studies combining
petrographical and intra-laminae stable isotope analyses allow data comparison and
strengthen their interpretations. At this resolution, the oxygen stable isotope composition
is thermo-dependent and could be use to quantify water temperature. Analyses
of a larger stromatolite from Caours covering several decades would provide a
strong record of summer-winter water temperature variations during the Eemian. |
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