252-7 Calcium isotopes across the Devonian–Mississippian Climate Transition

Session: Climate Transitions in the Paleozoic

Presenting Author:

Ayush Sharma

Authors:

Sharma, Ayush¹, Nana Yobo, Lucien², Barney, Bryce B³, Li, Shihan⁴, Zhang, Shuang⁵, Grossman, Ethan L.⁶, Day, James E.⁷, Joachimski, Michael M.⁸, Zaton, Michal⁹

(1) Department of Geology and Geophysics, Texas A&M University, College Station, TX, USA, (2) Department of Geology & Geophysics, Texas A&M University, College Station, TX, USA, (3) Department of Geology & Geophysics, Texas A&M University, College Station, TX, USA, (4) Department of Oceanography, Texas A&M University, College Station, TX, USA, (5) Department of Oceanography, Texas A&M University, College Station, TX, USA, (6) Department of Geology & Geophysics, Texas A&M University, College Station, TX, USA, (7) Department of Geography, Geology & the Environment, Illinois State University, Normal, IL, USA, (8) GeoZentrum Nordbayern, Friedrich-Alexander Universität of Erlangen-Nürnberg, Erlangen, Germany, (9) Institute of Earth Sciences, University of Silesia in Katowice, Katowice, Poland,

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 (δ^44/40Ca) 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 δ^44/40Ca record across the DMCT, test hypotheses regarding the influence of crustal cycling on δ¹⁸O_SW and develop a more robust understanding of Paleozoic calcium isotope trends. We present new δ^44/40Ca 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 δ^44/40Ca datasets across this interval to construct a more comprehensive record of seawater calcium isotope variability. Preliminary results reveal a δ^44/40Ca_SW 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 δ^44/40 Ca_SW record across the DMCT.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10395

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.