EXPLORING THE POTENTIAL OF THE RE ISOTOPE PALEOREDOX PROXY

Re isotopes may be a unique proxy of the extent of suboxic conditions on the seafloor. Suboxia is defined as a water column in which O₂ is extremely low or absent and H₂S is not present. Re isotopes may be uniquely sensitive to the extent of suboxia compared to other isotope paleoredox proxies such as Mo, Tl, or U because suboxic settings are an especially important sink for Re from the oceans. If Re isotopes fractionate during suboxic removal, and if seawater Re isotope compositions can be tracked in black shales or other sediments, then this unique geochemical cycling - together with its long ocean residence time - could make Re a powerful global ocean proxy.

To explore this paleoredox potential, we present the first Re isotope data (δ¹⁸⁷Re) from the ~2.5 Ga Mt. McRae Shale drill core. Prior work on black shales from this core provided evidence for a transient increase (or ‘whiff’) of environmental O₂ prior to the Great Oxidation Event. This evidence includes Mo and Tl isotope variations recently interpreted to indicate shallow, oxygenated water columns in margin settings during the ‘whiff’ interval (Ostrander et al., 2019). This scenario of increased O₂ in the shallow oceans implies that the extent of suboxic sedimentation, in slightly deeper waters, could also have increased coincident with Mo and Tl isotope variations. If so, correlated variations in δ¹⁸⁷Re are expected and could be used to quantify changes in the extent of suboxic seafloor during this time.

We measured δ¹⁸⁷Re in black shales before, during, and after the ‘whiff’ interval. The data exhibit a shift of δ¹⁸⁷Re to lighter values coincident with increasing Re, Mo, and total organic carbon concentrations, and correlated with Mo and Tl isotope variations. This δ¹⁸⁷Re variation can be interpreted as evidence of an increased importance of suboxic sedimentation, as expected. Alternatively, or additionally, it is possible that the δ¹⁸⁷Re variation reflects changes in δ¹⁸⁷Re of river inputs to the oceans due to the onset of oxidative weathering on the continents. To differentiate between these and other hypotheses, and quantify the implications for either the extent of suboxic sedimentation or continental weathering, we are systematically examining the modern Re isotope budget.