Oxygen and carbon isotope composition of Callovian-Lower Oxfordian (Middle-Upper Jurassic) belemnite rostra from central Poland: A record of a Late Callovian global sea-level rise?

摘要

Oxygen and carbon isotope data of well-preserved and stratigraphically well-dated belemnite rostra are presented from a Callovian-Lower Oxfordian succession of the Polish Jura Chain (south-central Poland) that was deposited in a deep shelf environment Palaeotemperatures calculated from belemnite delta O-18 values (9 to 14 degrees C) indicate relatively constant climatic conditions during studied Callovian and Early Oxfordian intervals. Belemnite delta C-13 temporal trends, which are interpreted to record primary variation in the isotope composition of dissolved inorganic carbon reservoir, display a prominent positive carbon isotope shift (up to around 1.7 parts per thousand VPDB) in the Upper Callovian. This excursion is preceded and followed by a drop in average delta C-13 values of belemnite rostra of around 1 parts per thousand. The Late Callovian carbonate carbon isotope excursion is likely linked to the enhanced storage of organic carbon in shelf sediments during a global sea-level rise. The presented data are not compatible with the theory of prolonged global glaciation at the Middle-Late Jurassic transition.