Early Devonian Marine Isotopic Signatures: Brachiopods from the Upper Gaspe Limestones, Gaspe Peninsula, Quebec, Canada

Abstract

ABSTRACT Brachiopods from the Upper Gaspe Limestones (Gaspe Peninsula, eastern Canada) document the isotopic geochemistry (18O, 13C, and 87Sr/86Sr) of marine calcites for the Siegenian and Emsian stages (Early Devonian). Nonluminescent brachiopods (NLB) from five species of a deep-water benthic community show a narrow range of 13C values (+1.3 to + 1.7, average + 1.5) and a much wider range of 18O values (-2.7 to -4.6, average -3.7) that partially overlaps the field of luminescent brachiopods. This is probably related to retention of fabric during diagenesis despite oxygen-isotope exchange. The heaviest 18O values for nonluminescent brachiopods (-3.3 Siegenian; -2.7, Emsian) are interpreted as the signature of marine low-Mg calcite. The 18O values for the Gas e brachiopods suggest a negative 18O shift at the end of the Early Devonian. The heaviest 18O values documented by previous workers for late Early Devonian (Emsian) brachiopods cluster around -4.5 a figure that differs by 1.8 from the heaviest value for Gaspe brachiopods. This difference is most likely related to the growth of the Gaspe brachiopods in deeper and cooler seawater, as suggested by independent sedimentologic and biologic evidence, rather than a 7°C warming of ocean surface temperatures or a rapid (< 7 ma) shift in the 18O of seawater in a period of nonglaciation. The 87Sr/86 data for Early Devonian brachiopods fill the Early Devonian gap in the Burke et al. (1982) curve for Devonian seawater. The Siegenian value is slightly more radiogenic than the younger Emsian ones, and support a continuous decrease in 87Sr/86Sr values from latest Silurian to Middle Devonian time.