Palaeoecological implications of stable isotope analyses of late Valanginian ammonites from central Poland

Abstract

Analyses of δ18O and δ13C of original shells of late Valanginian (verrucosum to triptychoides ammonite zones) ammonites preserved in clay at localities in central Poland, and of some Berriasian samples from well cores have been carried out for the first time. Analyses of carbon stable isotope composition have revealed diverse values, representing vital effects rather than recording the palaeoenvironment. In turn, analyses of oxygen stable isotope composition in samples from the verrucosum and triptychoides ammonite zones clearly indicates the connection between particular species and stable isotope ratios, and thus palaeotemperature. The interpretation of ammonite palaeoecology in the present paper is based on estimated palaeotemperatures and supports other interpretations of ammonite behaviour, mainly based on functional morphology of shells. There are two main groups of ammonites associated with lower and higher temperatures. The first group of δ18 O values indicates lower palaeotemperatures. Genera analysed include Bochianites, Dichotomites and Prodichotomites; a nekto-benthic (demersal) habitat is suggested for these. The second group, characterised by higher palaeotemperatures, includes Saynoceras, Valanginites and Olcostephanus, with a suggested nektonic (or planktic) habitat (free-floating), Neohoploceras and Neocomites with a suggested nektonic habitat (free-swimming) and a seasonal migration from warmer (Tethys) to cooler waters of the Polish Basin or lived in the Polish Basin for several summer and winter seasons. Co-occurring bivalves, echinoid spines and scaphopods were analysed for comparison with results obtained from ammonites for a better overview of the palaeoenvironment. The estimated palaeotemperatures for the Polish Basin are between 12 and 13 °C (at the sea floor) to 21–24 °C (in the upper water column), while for the Tethys or summer surface waters of the Polish basin, these were in the range of 26–30 °C.