Carbon and oxygen isotopic composition of Silurian brachiopods (Gotland/Sweden): palaeoceanographic implications

Abstract

One hundred and seventy-two non-luminescent brachiopod shells from the Silurian sequence of Gotland were investigated for their carbon and oxygen isotopic composition. Diagenetic alteration of the shell calcite was excluded by means of petrographic, trace element and cathodoluminescence analysis. Oxygen and carbon isotopic signals vary considerably within the uppermost Llandovery to the Ludlow. Average δ 18O and δ 13C values of well-defined stratigraphic units range from −2.4‰ to −5.9‰ and −1‰ to +8‰, respectively. We observed parallel trends in the carbon and oxygen isotope curves, with the most prominent shifts occurring in the Lower Wenlock, Upper Wenlock and Middle Ludlow. Average δ 18O values (−2.4‰ to −2.9‰) recorded in the Middle Ludlow are among the highest δ 18O values reported for the Early Palaeozoic. The recorded δ 18O variations cannot solely be explained by changes in sea surface temperature and salinity. The variations in the isotopic signals correlate negatively with changes in sea level. Heavy carbon and oxygen signals coincide with sea-level lowstands, whereas light signals are recorded during sea-level highstands. Sea-level highstands were probably characterized by the formation of warm saline waters on subtropical epicontinental shelves, inducing oxygen-deficiency in the basins (alumshale facies). The low δ 18O values are attributed to 18O depletion in surface waters owing to the storage of 18O-enriched saline waters in the basins. Low δ 13C values are explained by a low surface water productivity, since nutrients were not supplied to the photic zone but rather stored in the deeper ocean. During sea-level lowstands, colder waters that formed in higher latitudes may have represented the main intermediate water source. A better ventilation of the basins is documented by the deposition of well-aerated sediments. Whereas the high δ 18O values are explained by the change in oceanic circulation, the high δ 13C values are attributed to an enhanced primary productivity and possibly to an enhanced burial of C org. The model discussed here may provide an attractive scenario for accounting for the recorded short-term variations in the isotopic signals, without the necessity of introducing an isotopic correction factor for Silurian sea water.