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

• Published in: Palaeogeography, palaeoclimatology, palaeoecology palaeoclimatology, palaeoecology, 2007-10, Vol.254 (3), p.523-540
• Main Authors: Wierzbowski, H., Joachimski, M.
• Format: Article
• Language: English
• Subjects: Ammonites; Ammonoidea; Bajocian; Bathonian; Belemnites; Bivalves; Marine; Oxygen and carbon isotopes
• ISSN: 0031-0182; 1872-616X
• DOI: 10.1016/j.palaeo.2007.07.010

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 δ 18O values of oysters, belemnites and trigoniids translate into relatively low palaeotemperatures of 10.1, 9.2 and 7.4 °C, respectively. The comparable δ 18O 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 δ 18O (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 δ 18O 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 δ 13C 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 δ 13C DIC values calculated from ammonite δ 13C are lower in comparison to δ 13C DIC values calculated from oyster and belemnite carbon isotope ratios, which in conjunction with ammonite δ 18O values indicates lower salinity of surface or shallow near-coastal waters.