Early Devonian marine isotopic signatures; brachiopods from the upper Gaspe limestones, Gaspe Peninsula, Quebec, Canada

Brachiopods from the Upper Gaspe Limestones (Gaspe Peninsula, eastern Canada) document the isotopic geochemistry (delta ¹⁸ O, delta ¹³ C, and ⁸⁷ Sr/ ⁸⁶ Sr) 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 delta ¹³ C values (+1.3 per thousand to + 1.7 per thousand , average + 1.5 per thousand ) and a much wider range of delta ¹⁸ O values (-2.7 per thousand to -4.6 per thousand , average -3.7 per thousand ) that partially overlaps the field of luminescent brachiopods. This is probably related to retention of fabric during diagenesis despite oxygen-isotope exchange. The heaviest delta ¹⁸ O values for nonluminescent brachiopods (-3.3 per thousand Siegenian; -2.7 per thousand , Emsian) are interpreted as the signature of marine low-Mg calcite. The delta ¹⁸ O values for the Gaspe brachiopods suggest a negative delta ¹⁸ O shift at the end of the Early Devonian. The heaviest delta ¹⁸ O values documented by previous workers for late Early Devonian (Emsian) brachiopods cluster around -4.5 per thousand a figure that differs by 1.8 per thousand 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 degrees C warming of ocean surface temperatures or a rapid (< 7 ma) shift in the delta ¹⁸ O of seawater in a period of nonglaciation. The ⁸⁷ Sr/ 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 ⁸⁷ Sr/ ⁸⁶ Sr values from latest Silurian to Middle Devonian time.