13-1 Revisiting Paleoproterozoic Environmental Conditions Using Sr and Ca Isotope Compositions of Huronian Carbonates

Session: Earth Life Sciences across the Cordillera

Presenting Author:

Authors:

Abstract:

The early Paleoproterozoic (Huronian) glaciations represent the oldest series of extensive glacial events that affected the Earth and comprises three discrete episodes. Within the Huronian Supergroup (Ontario, Canada), carbonate deposition is restricted to two stratigraphic intervals: the ca. 2.4 Ga Espanola Formation, which overlies diamictites of the second glacial event, and the ca. 2.31 Ga Gordon Lake Formation, deposited after the termination of all three glacial episodes. These glaciations have been interpreted as continental-scale ice advances extending to low latitudes and reaching sea level, followed by intervals of greenhouse conditions. We present stable and radiogenic strontium isotope compositions (δ⁸⁸/⁸⁶Sr, ⁸⁷Sr/⁸⁶Sr), together with stable calcium isotope data (δ⁴⁴/⁴⁰Ca), for the Espanola and Gordon Lake formation carbonates. The highly radiogenic ⁸⁷Sr/⁸⁶Sr values for the Espanola Formation have previously been interpreted as reflecting lacustrine depositional setting. Here, we suggest that these ratios were affected by secondary addition of radiogenic ⁸⁷Sr derived from the decay of ⁸⁷Rb hosted in clay minerals and siliciclastic phases within the unit, with strontium mobility enhanced during thermal events recorded by the Huronian Supergroup. This secondary process is not expected to modify stable strontium isotope composition (δ⁸⁸/⁸⁶Sr), as it does not affect the relative abundances of ⁸⁸Sr and ⁸⁶Sr. Carbonates from the Espanola Formation display heavy δ⁸⁸/⁸⁶Sr values (average ~0.38‰), indicating slow precipitation rates and possible episodes of dissolution. These conditions contrast with the rapid precipitation inferred for Neoproterozoic cap carbonates. In contrast, δ⁴⁴/⁴⁰Ca values exhibit a secular increase upsection (average ~0.99‰), suggesting early alteration dominated by seawater-derived fluids, potentially driven by progressive shoaling towards the top of the formation. The decoupling between stable Sr and Ca isotope systematics indicates that these proxies record different stages in the depositional history: stable Sr isotope composition preserves the primary signal, whereas Ca isotope composition records basin evolution and early seawater-derived fluid-buffered alteration. In contrast, the Gordon Lake Formation shows lighter stable Ca (average ~0.84‰) and Sr (average ~0.29‰) isotope compositions and near-primary ⁸⁷Sr/⁸⁶Sr values, consistent with contemporaneous seawater, indicating high precipitation rates in a open-marine setting and providing a reference for comparison with the underlying Espanola Formation. Together, these records indicate a different from the Cryogenian response to extreme greenhouse conditions in the aftermath of the Huronian glaciations.

Geological Society of America Abstracts with Programs. Vol. 58, No. 3, 2026

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.