Stable Carbon and Oxygen Isotopes of Early Silurian Pentameride Brachiopods from Québec, Ontario, and Nunavut, and Paleoenvironmental Implications

Jordan N. Siewnarine, The University of Western Ontario

Abstract

Analysis of low-Mg calcite brachiopod shells for δ¹⁸O and δ¹³C signatures provide quantitative independent geochemical data for paleoenvironmental interpretations. We analyzed 59 brachiopod shells from lower Silurian carbonate rocks of Anticosti Island, the Hudson Bay Lowlands and Niagara regions for spaciotemporal isotopic trends for the early Silurian. Screening tests using scanning electron microscopy, cathodoluminescence and trace element analysis demonstrate exceptional preservation, suggesting retention of primary isotopic values.

δ¹³C and δ¹⁸O values of shells range from 0.2‰ to 2.9‰ and -6.9‰ to -4.3‰ (VPDB), respectively. Temporally, increases in δ¹³C and δ¹⁸O values occurred in response to the Late Aeronian glaciation. Paleogeographically, δ¹⁸O values were not significantly different between regions while δ¹³C was tied to open ocean vs epeiric sea settings. We argue for ocean δ¹⁸O value of ~ -1.5‰ (VSMOW), producing paleotemperatures of 30 °C to 36 ⁰C, consistent with the proposition that ocean δ¹⁸O has remained constant over geologic time.