In this study, stable C and O isotope compositions of bulk rock samples from Barremian-Aptian platform carbonates of the Taurides, the Pontides, and of their pelagic counterparts of the Sakarya continent in Turkey were measured for the first time. Platform carbonates and pelagic successions including a prominent black shale interval are composed of distinct sedimentary cycles in the form of meter-scale shallowing- upward sequences and limestone-marl/shale couplets, respectively. The biostratigraphy and the δ¹³C values allow this black shale interval to be correlated with the "Selli level". Variations in lithofacies and δ¹³C and δ¹⁸O values with the magnitudes of about 2 ‰ within the cycles indicate high frequency fluctuations in sea level, probably in response to alternate periods of cooling and warming occurring on Milankovitch-cycle scales even during the greenhouse-type Cretaceous. A good overall correlation between sedimentary cyclicity and variations in δ¹⁸O values suggests preservation of the primary isotopic signal.