Extending the utility of the Tl isotope paleoredox proxy to carbonates

Thallium isotope ratios (²⁰⁵Tl/²⁰³Tl) recovered from ancient marine shales are able to provide novel information about Earth’s ocean oxygenation history. Here we demonstrate the potential to extend this utility to the ancient carbonate record. We present Tl concentration and isotope ratio data for modern and geologically recent carbonates from the Great Bahama Bank that capture initial carbonate deposition to diagenesis on multiple scales. Young carbonates from bank-top cores reveal bulk Tl isotope compositions (ε²⁰⁵Tl) that are either indistinguishable from today’s globally homogenous seawater value (ε²⁰⁵Tl = –6.0 ± 0.3‱) or within 0.4‱. Older carbonates from the deeper and diagenetically altered cores also oftentimes capture seawater ε²⁰⁵Tl values. Carbonates with high dolomite contents are the most reliable archives, revealing with only minor exception ε²⁰⁵Tl values within analytical error of modern seawater. Meteoric diagenesis predictably drives carbonate ε²⁰⁵Tl values higher and toward the average composition of global freshwaters (ε²⁰⁵Tl ≈ –2‱). Our findings suggest that properly screened ancient carbonates can be viable archives of past seawater ε²⁰⁵Tl values. These seawater ε²⁰⁵Tl values are set by past marine Tl isotope mass-balance, which is sensitive to manganese oxide burial on the seafloor and hence past levels of global bottom water ocean oxygenation.