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 Czestochowa 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 delta O-18 values of oysters, belemnites and trigoniids translate into relatively low palaeotemperatures of 10.1, 9.2 and 7.4 degrees C, respectively. The comparable delta O-18 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 delta O-18 (around 18.6 degrees 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 delta O-18 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 6 delta C-13 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 delta C-13(DIC) values calculated from ammonite delta C-13 are lower in comparison to delta C-13(DIC) values calculated from oyster and belemnite carbon isotope ratios, which in conjunction with ammonite delta O-18 values indicates lower salinity of surface or shallow near-coastal waters. (C) 2007 Elsevier B.V. All rights reserved.