Reconstructions of paleo-seawater chemistry are largely inferred from biogenic records of epicontinental seas. Recent studies provide considerable evidence for large-scale spatial and temporal variability in the environmental dynamics of these semi-restricted seas that leads to the decoupling of epicontinental isotopic records from those of the open ocean. We present conodont apatite δ¹⁸O_PO4 and ⁸⁷Sr/⁸⁶Sr records spanning 24 Myr of the late Mississippian through Pennsylvanian derived from the U–Pb calibrated cyclothemic succession of the Donets Basin, eastern Ukraine. On a 2 to 6 Myr-scale, systematic fluctuations in bioapatite δ¹⁸O_PO4 and ⁸⁷Sr/⁸⁶Sr broadly follow major shifts in the Donets onlap–offlap history and inferred regional climate, but are distinct from contemporaneous more open-water δ¹⁸O_PO4 and global seawater Sr isotope trends.

A −1 to −6‰ offset in Donets δ¹⁸O_PO4 values from those of more open-water conodonts and greater temporal variability in δ¹⁸O_PO4 and ⁸⁷Sr/⁸⁶Sr records are interpreted to primarily record climatically driven changes in local environmental processes in the Donets sea. Systematic isotopic shifts associated with Myr-scale sea-level fluctuations, however, indicate an extrabasinal driver. We propose a mechanistic link to glacioeustasy through a teleconnection between high-latitude ice changes and atmospheric pCO₂ and regional monsoonal circulation in the Donets region. Inferred large-magnitude changes in Donets seawater salinity and temperature, not archived in the more open-water or global contemporaneous records, indicate a modification of the global climate signal in the epicontinental sea through amplification or dampening of the climate signal by local and regional environmental processes. This finding of global climate change filtered through local processes has implications for the use of conodont δ¹⁸O_PO4 and ⁸⁷Sr/⁸⁶Sr values as proxies of paleo-seawater composition, mean temperature, and glacioeustasy.