28 Paleozoic Events and Processes: Sedimentary Geology, Paleontology, and Geochemistry

The focus of this session is short- to long-term geologic, paleobiologic, and geochemical events and processes, recorded in Paleozoic sedimentary rocks.

Monday, 23 March 2026
3:25 PM - 5:30 PM
CCC, Room 27

Triple oxygen isotope analyses of late Ordovician to early Silurian brachiopods from Anticosti Island, Canada, reveal complex diagenesis and paleohydrology

Paper number: 28-1
Program: Technical Sessions
Time: 3:30 PM – 3:50 PM

Authors

Abstract

The late Ordovician glaciation is the first of two icehouse intervals within the Paleozoic. While temperature estimates obtained from geochemical proxy data (e.g. δ18O and Δ47 values of biogenic carbonate materials) vary for low-latitude regions (~30°C C to ~60°C), they are generally substantially higher than those predicted by climate model simulations. In this study, we use triple oxygen isotope measurements of carbonate brachiopod shells from Anticosti Island, Canada, to estimate their formation conditions over the late Ordovician-early Silurian (448-436 Ma). Triple oxygen isotope values are paired δ17O-δ18O measurements (hereafter referred to as Δ′17O) that allow for identification of whether a mineral precipitated in equilibrium with seawater or was impacted by either kinetic effects or alteration after burial. We can use Δ′17O values to understand when a fossil is preserving environmental conditions and, potentially, more accurately constrain ocean temperatures of the early Paleozoic.

Based on the Δ′17O and δ18O values, all brachiopod fossils in our dataset experienced diagenesis, despite appearing well-preserved under cathodoluminescence. We use a fluid-rock mixing model to estimate the alteration and primary formation conditions of our samples. Assuming the δ18O seawater value associated with open ocean environments (-1‰; for ice-free conditions and +1‰; for glaciated conditions) results in unrealistically low formation temperatures between 0 and 16°C. Instead, we consider the evaporative waters of a restricted basin to yield temperatures above 15°C.

Such results amplify the complex diagenetic and paleohydrological conditions represented in Anticosti Island strata and demonstrate the caution needed when interpreting paleoclimate reconstructions derived from Anticosti Island material, especially those that extrapolate results to diagnose global climate trends. To evaluate seawater temperatures and δ18O trends in the global ocean across the Ordovician-Silurian, sample Δ′17O-δ18O values from additional locations are needed. More broadly, our data show the utility of triple oxygen isotope measurements to identify diagenetic overprinting and regional palaeohydrological differences in Paleozoic fossil carbonate.

Geological Society of America Abstracts with Programs. Vol. 58, No. 2

doi: 10.1130/abs/2026NE-411380

© Copyright 2026 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.