Geochemistry of the Upper Jurassic–Lower Cretaceous limestone of the Bilecik Formation in the Dümrek area (Eskişehir, western Central Anatolia): implications for depositional environment and paleotemperature

Upper Jurassic–Lower Cretaceous limestones of the Bilecik Formation are exposed near the village of Dümrek (Eskişehir Province) within the Sakarya Zone. Mineralogical analyses reveal that these limestones are predominantly composed of calcite and include intraclasts, fossil shell fragments, ooids, and peloids, cemented by sparry and micritic calcite. The U/Th, Ni/Co, V/(V+Ni), V/Cr, and Ce/Ce* ratios indicate that these carbonates were deposited in an oxic environment. Furthermore, the Mn/Sr ratios imply no diagenetic alteration, indicating preservation of the original geochemical signature. The PAAS-normalized REE+Y patterns of these limestones display depletion in light rare earth elements, a pronounced positive La anomaly (La/La* = 2.13–6.2), a distinct negative Ce anomaly (Ce/Ce* = 0.18–1.76), a slight positive Eu anomaly (Eu/Eu* = 1–1.41), and an enrichment in Y, all pointing to a welloxygenated depositional environment with negligible detrital input. Stable isotope data show that the average δ13C (V-PDB) value is 2.47‰, while the average δ18O (V-PDB) value is –2.10‰. The δ18O values display slight alteration with negative values, which are interpreted as evidence of early marine diagenesis. Based on these oxygen isotope values, the estimated paleotemperature during carbonate precipitation was approximately 26.15 °C. The calculated paleosalinity index (Z) of 131.40, obtained using the average δ18O and δ13C values, corroborates a marine origin for the carbonates. Furthermore, the paleoclimate proxy ratio Σ(Fe + Mn + Cr + Ni + V + Co)/Σ(Ca + Mg + Sr + Ba + K + Na) ranges between 0.087 and 0.09, indicative of an arid climate during limestone formation. The δ13C (V-PDB) ‰ versus δ18O (V-PDB) ‰ bivariate diagram confirms the marine character of the limestone.