BEYOND ANOXIA: EXPLORING SEDIMENTARY THALLIUM ISOTOPES IN PALEO-REDOX RECONSTRUCTIONS FROM A NEW CORE TOP COLLECTION

The thallium isotopic composition (ε²⁰⁵Tl) of seawater has been identified as a promising proxy for global redox conditions due to its close association with Mn oxide burial under oceanic oxygenation changes. Currently the preferred archives for past seawater Tl isotope reconstructions are from anoxic (no oxygen detected) and/or sulfidic (no oxygen detected and free sulfide present) environments that may suffer from basin restriction. Here we conduct a comprehensive modern calibration of authigenic sedimentary ε²⁰⁵Tl with core tops covering a wide range of bottom water oxygenation and sediment compositions. We show that quantitative Tl removal from the water column is associated with the presence of reducing porewaters (e.g., Mn reduction) at/near the sediment-water interface. Even if the bottom water is not anoxic, a shallow oxygen penetration depth under high export productivity and/or high sedimentation rates may still allow rapid oxygen consumption and buildup of reducing porewaters conducive for complete Tl removal. Thus, applications of the Tl isotope redox proxy can be greatly expanded beyond the anoxic conditions into places with open-ocean connections and high mass accumulation rates for high-resolution temporal reconstructions. We present a decision tree using the sediment enrichment factors of Mn, Ba, and U to determine whether sediment archives are likely to record the seawater ε²⁰⁵Tl. We additionally provide an estimate of authigenic Tl burial fluxes in environments with quantitative Tl removal, which is roughly double the size of the ‘euxinic/anoxic’ sink in previously published global marine Tl mass balance estimates.