Reconstruction of late Bajocian–Bathonian marine palaeoenvironments using carbon and oxygen isotope ratios of calcareous fossils from the Polish Jura Chain (central Poland)

Oxygen and carbon isotope analyses were carried out on Upper Bajocian – Bathonian calcitic belemnite rostra and oyster shells as well as aragonitic ammonite, nautiloid and trigoniid shells from the “Ore-bearing Częstochowa Clay Formation” in south-central Poland. All samples were studied by means of cathodoluminscence microscopy, trace element, X-ray, and scanning electron microscope (SEM) analysis in order to select best-preserved material. SEM studies allowed classifying some aragonitic ammonite and nautiloid shells (> 99% aragonite) as diagenetically altered. Average δ18O values of oysters, belemnites and trigoniids translate into relatively low palaeotemperatures of 10.1, 9.2 and 7.4 °C, respectively. The comparable δ18O values of bivalves and belemnites indicate that belemnites had a nectobenthic lifestyle and did not record surface water temperatures. The ammonite fauna is interpreted to have been nektonic, however, palaeotemperatures derived from ammonite δ18O (around 18.6 °C) may be overestimated due to a lower, than normal marine, salinity of surface waters. The reduced salinity is inferred as well from sedimentological observations and faunal composition. The obtained δ18O values indicate cool climatic conditions during the Late Bajocian–Bathonian in south-central Poland, which may have been linked with the occurrence of a cold climatic episode. Despite the fact that all studied shells were not precipitated in carbon isotope equilibrium with ambient waters, the δ13C shell values enabled us to roughly estimate isotope composition of dissolved inorganic carbon (DIC) in the Late Bajocian–Bathonian sea of south-central Poland. Water δ13CDIC values calculated from ammonite δ13C are lower in comparison to δ13CDIC values calculated from oyster and belemnite carbon isotope ratios, which in conjunction with ammonite δ18O values indicates lower salinity of surface or shallow near-coastal waters.