Extending the utility of the Tl isotope paleoredox proxy to carbonates

Thallium isotope ratios ( 205 Tl/ 203 Tl) 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 (ε 205 Tl) that are either indistinguishable from today’s globally homogenous seawater value (ε 205 Tl = –6.0 ± 0.3‱) or within 0.4‱. Older carbonates from the deeper and diagenetically altered cores also oftentimes capture seawater ε 205 Tl values. Carbonates with high dolomite contents are the most reliable archives, revealing with only minor exception ε 205 Tl values within analytical error of modern seawater. Meteoric diagenesis predictably drives carbonate ε 205 Tl values higher and toward the average composition of global freshwaters (ε 205 Tl ≈ –2‱). Our findings suggest that properly screened ancient carbonates can be viable archives of past seawater ε 205 Tl values. These seawater ε 205 Tl 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.