252 Climate Transitions in the Paleozoic

This session will examine the major climate transitions in the Paleozoic Era, the paleoclimate evidence for their identification, their causes, and their consequence to the Earth System (e.g., ocean and atmospheric chemistry, sedimentology, biodiversity).

Wednesday, 22 October 2025
8:00 AM - 12:00 PM
HBGCC, 303C

Calcium isotopes across the Devonian–Mississippian Climate Transition

Paper number: 252-7
Program: Technical Sessions
Time: 9:40 AM – 9:55 AM

Authors

Abstract

The Devonian-Mississippian Climate Transition (DMCT) marks the shift from a warm, greenhouse climate in the Early Paleozoic to cooler, icehouse conditions of the Late Paleozoic. This major climatic shift is widely attributed to a decline in atmospheric pCO2 driven by the diversification of land plants, increased orogenic activity, rifting, and enhanced continental weathering. The DMCT is evidenced by a pronounced positive δ18O excursion in marine carbonates. However, the underlying cause of this excursion remains contested, with the interpretations invoking either a primary temperature signal, changes in seawater δ18O and oceanic crustal cycling, or artifacts related to restricted seawater circulation. Calcium isotopes (δ44/40Ca) in marine carbonates offer an additional proxy for reconstructing paleoenvironmental conditions, as they are tightly coupled to the global calcium cycle, crustal processes, carbon cycling, and the dynamics of CaCO₃ formation. In this study, we aim to refine the δ44/40Ca record across the DMCT, test hypotheses regarding the influence of crustal cycling on δ18OSW and develop a more robust understanding of Paleozoic calcium isotope trends. We present new δ44/40Ca data from well-preserved brachiopod shells spanning the DMCT, collected from various sites across the U.S. Midcontinent, Canada, and the Russian Platform. Each specimen was thin-sectioned and microsampled in areas without luminescence when viewed with cathodoluminescence microscopy. The new measurements are integrated with previously published δ44/40Ca datasets across this interval to construct a more comprehensive record of seawater calcium isotope variability. Preliminary results reveal a δ44/40CaSW baseline of ~-0.7‰ with a maximum up to -0.2‰ higher around 378 Ma, which cannot be explained by changes in calcium fluxes due to the long residence time of Ca in seawater. We interpret this variation as reflecting a calcium isotope fractionation change, potentially influenced by restricted basin circulation. Additionally, our findings offer new insights into the long-term evolution of the Phanerozoic calcium cycle and place tighter constraints on the δ44/40 CaSW record across the DMCT.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-414370

© Copyright 2025 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.