THALLIUM ISOTOPIC CONSTRAINTS ON GLOBAL MARINE OXYGENATION DURING THE EARLY NEOPROTEROZOIC FROM THE REEFAL ASSEMBLAGE IN YUKON, CANADA

Estimating the extent of ocean oxygenation during the Neoproterozoic is central to testing hypotheses alleging a critical role for oxygen in the evolution of early animal life. This study provides a constraint on the global extent of marine bottom water oxygenation during the early Neoproterozoic by measuring thallium (Tl) isotope ratios (reported as ε²⁰⁵Tl) for shales from the Tonian Reefal Assemblage in the Coal Creek inlier of Yukon, Canada. Samples analyzed in this study are stratigraphically below a tuff horizon that yielded a zircon U–Pb age of 811.51 ± 0.25 Ma and the ca. 810 Ma Bitter Springs carbon isotope excursion. Thallium isotopes are an emerging proxy for tracking the past global extent of oxygenated oceanic bottom waters. When global bottom waters are oxygenated (cf., today), the heavier-mass Tl isotope (²⁰⁵Tl) is preferentially adsorbed onto seafloor manganese oxides, in-turn driving a complementary enrichment of the lighter-mass Tl isotope (²⁰³Tl) in seawater. Today’s low seawater ε²⁰⁵Tl value is globally homogenous because the residence time of Tl (~19 kyr) is longer than the ocean mixing time. Thallium isotope ratios from modern marine sediments with reducing porewaters mirror the Tl isotopic composition of seawater. Thus, shales deposited under locally anoxic conditions record the Tl isotopic composition of seawater at the time of deposition and can be used to estimate the global extent of marine bottom water oxygenation in the past. Within the Reefal Assemblage, 16 of the 18 samples analyzed have ε²⁰⁵Tl values that are lighter than the continental weathering input value (ε²⁰⁵Tl ~ –2) but heavier than the modern seawater value (ε²⁰⁵Tl ~ –6). Using the current mass balance model and interpretive framework, these values suggest that oxygenated bottom waters were present over regions of the global seafloor at the time of deposition of the Reefal Assemblage. Future work will explore the influence of suboxic water masses and the relative role of atmospheric oxygen, primary productivity, and global climate on the thallium isotopic record.