Paleobathymetry of a Silurian shelf based on brachiopod assemblages: an oxygen isotope test

Abstract

Primary δ18O signals of 97 brachiopod shells from the Lower Silurian (Llandovery) carbonate succession of Anticosti Island were used to test the hypothesis of water-depth and water-temperature gradient for the Silurian onshore–offshore benthic assemblages (BA1–BA5). The analyzed shells were from the Pentamerus palaformis, Pentamerus oblongus, Stricklandia planirostrata, Ehlersella davidsonii, and Triplesia anticostiensis communities, which have been interpreted to occupy different water depths. Screening of the shells, using petrographic and chemical criteria, confirmed good preservation of original material. Shells of P. palaformis, P. oblongus, E. davidsonii, and T. anticostiensis have very similar mean δ18O values (–5‰ Vienna Peedee Belemnite (VPDB)), while shells of S. planirostrata have a somewhat lighter value (–5.6‰). The δ18O values, in corroboration with taphonomic and paleoecologic data, suggest the following: (1) that the water mass of the Anticosti carbonate shelf was warm and well mixed vertically during the Early Silurian; (2) that the pentamerid brachiopod paleocommunities that lived at ~20–90 m of water-depth show little δ18O differentiation in their shell composition; and (3) that the Pentamerus,Stricklandia, and Triplesia communities need to be investigated at the species level, as both taphonomic and oxygen isotopic data indicate that the Stricklandia planirostrata Community most likely lived in a notably warmer, shallower water than the Pentamerus palaformis Community and that Triplesia anticostiensis (BA5) lived at water temperature comparable to that of the Pentamerus habitat (BA3). This may suggest either that, at the generic level, the Pentamerus, Stricklandia, and Triplesia (Clorinda equivalent) communities may have had substantial overlap in their spatial distribution in the BA3–BA4 paleobathymetric settings or that the isotope signals are too faint to record water depth differences on a tropical shelf.