Detailed oxygen and carbon isotope stratigraphy of the Oxfordian in Central Poland

A collection of 73 belemnites and brachiopods from detailed stratigraphically documented sections in the Polish Jura Chain of Central Poland representing almost the whole Oxfordian Stage was studied for oxygen and carbon isotopes. The delta(18)O and delta(13)C (V-PDB) values range from -3.6 to +0.5parts per thousand and from -1.6 to +2parts per thousand for belemnite rostra, and from -2.5 to +0.4parts per thousand and from +1.6 to +4.3parts per thousand for brachiopod shells, respectively. The delta(13)C temporal trends of belemnite rostra and brachiopod shells show a constant isotopic difference (ca. 2.5-3parts per thousand.), which is interpreted as a vital effect of belemilites or as a result of different life environments. delta(18)O values are similar in both groups of fossils. The delta(18)O values in the Lower and Middle Oxfordian oscillate around 0parts per thousand and decrease in the Upper Oxfordian. This decrease results mainly from diagenetic overprint as proved by the cathodoluminescence studies. The Lower and Middle Oxfordian delta(18)O-derived temperatures were constant in consequence of significant ocean depth. For this reason there are no established links with distribution of ammonite faunas. The delta(13)C temporal trend displays a gentle slope through the Oxfordian and reaches maximum values at the turn of the Plicatilis/Transversarium Zones or the Middle Oxfordian, which is commonly considered as a global event. It can be related to the sea-level high-stand and growth crisis of carbonate platforms. The decrease in delta(13)C values starting in the upper Middle Oxfordian is interpreted to be due to the acceleration of carbonate sedimentation rate during this time period.