ÇAĞRILI KONUŞMALAR / KEYNOTES Invited Speeches ... - TPJD

Isotopic and Geochemical Records of the Middle Eocene Climatic Changes in
Central Anatolia, Turkey
Faruk Ocakoğlu 1 , Sanem Açıkalın 1 , İsmail Ömer Yılmaz 2 , Celal Erayık 1
1 Eskişehir Osmangazi University, Department of Geology Engineering, Eskişehir
2 Middle East Technical University, Department of Geology Engineering, Ankara
A 150 m thick lacustrine succession crops out in vicinity of Orhaniye and Güvenç villages in
the 20 km north of Ankara (central Turkey). It is made of generally rhythmically bedded shalelimestone
alternations. High resolution sedimentological logging and isotopic and geochemical
investigation of the samples collected enabled the understanding of climatic and limnological
conditions prevailed during the deposition of succession. Spectral analysis of bed thickness
displayed statistically significant hidden cyclicities at 8.2 m and 16.1 m that would correspond
precession and obliquity Milankovitch bands respectively. This finding, together with the
previously published benthic foraminifera biozone data of overlying marine unit showed that the
target lacustrine unit would have been deposited within 430 ka between 44.53 Ma and 44.1 Ma.
Evaluation of the δ13C and δ18O concentrations indicates relatively high correlation between
them. Carbonate intervals have higher δ18O (>-3 permil) and δ13C compared to shale levels.
An overall negative shift in δ18O up in the section is typical. Proxy elements of Co, V, Ni, Mn
and Zn, characteristic of reducing conditions are enriched in shale levels and more scarce in
carbonates. The upper half of the section is evidently poor in these elements compared to lower
half. Indicators of the weathering intensity in source area, the Na 2 O/Al 2 O3, Na 2 O/TiO 2 and Na 2 O/
K 2 O ratios are generally higher in carbonate intervals. Similarly, the upper part of the section
has lower elemental ratios that may explain more humid and warm climate conditions. δ13C
isotope stratigraphy having a positive shift (5.6 permil) in the lower part and then a negative
shift towards up may support this interpretation. These briefly explained preliminary data proves
that during the deposition of the Güvenç lacustrine sediments in the Middle Eocene climatic
optimum, astronomic forcing may have been prime driver. Probably for this reason, δ18O
stratigraphy of the area has very parallels with the previously published global pattern of this
time. In this scheme, carbonate intervals of the Güvenç succession may have been formed during
cold and dry periods while the shale intervals represent warm and hot period when the lake level
and weathering intensity were higher.
Keywords: Lacustrine, Lutetien, Eocene climatic optimum, stable isotopes, geochemistry
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