Thallium isotope fingerprints (ε²⁰⁵Tl) of redox dynamics in the Early Cretaceous Yamama Formation

Thallium isotope compositions (ε²⁰⁵Tl, ‱) from the Early Cretaceous Yamama Formation provide a calibrated paleoredox proxy that links Mn-oxide burial efficiency to bottom-water oxygenation. We analyzed four cores from the Ratawi field, southern Iraq (RT-3, RT-4, RT-6, RT-14), pairing bulk carbonates with staged leaches to track the transition from primary carbonate precipitation to diagenetic modification in meteoric, marine-phreatic, and burial settings. Pelagic clays from the Gulf of Oman yield ε²⁰⁵Tl of − 5.8 ± 0.3‱ (SL167) and − 6.1 ± 0.4‱ (SL163/MC681), matching modern seawater (− 6.0 ± 0.3‱) within uncertainty and providing external analytical controls. Within Yamama, deeper dolomite-rich intervals retain seawater-like ε²⁰⁵Tl within the reconstructed Early Cretaceous range ( ≈ − 6.0 ± 0.3‱; 2SD), whereas upper intervals exhibit meteoric overprint with ε²⁰⁵Tl shifted toward less-negative values ( ~ − 2‱), consistent with limited freshwater mixing. After petrographic and geochemical screening (Mn/Sr, Fe/Sr; targeted weak/reductive leaches to minimize Mn-oxide influence), screened carbonates preserve ε²⁰⁵Tl indistinguishable from coeval seawater. Expressed as Δε²⁰⁵Tl (sample − modern seawater), the Yamama trajectory across ~ 120–112 Ma shows coherent variability that parallels published Tethyan–Atlantic ε²⁰⁵Tl trends within age uncertainty, indicating basin-scale modulation of Mn-oxide burial and near-bottom oxygenation rather than local diagenetic control. Collectively, these results validate screened carbonate platforms, particularly dolomite-rich horizons, as robust ε²⁰⁵Tl archives and extend the thallium-isotope paleoredox framework beyond shale and Fe–Mn repositories. The Yamama Formation thus provides a reference for reconstructing Early Cretaceous redox evolution and demonstrates the utility of ε²⁰⁵Tl (‱) for integrating carbonate-dominated and siliciclastic records in basin-scale reconstructions.