Prior to 2.5 billion years ago, the Earth system was dramatically different from today, and the Archean oceans were largely anoxic except for local oxygen oases in shallow seas. This study reports a complete molybdenum (Mo) stable isotope chemostratigraphy based on two correlative cored drill holes through Earth’s earliest thick carbonate platform, preserved in the 2.87-billion-year-old (Ga) Ball Assemblage of the Red Lake Greenstone Belt (Ontario, Canada). This site is particularly notable as negative Ce anomalies and Mo isotope data obtained from outcrop samples suggests the presence of a Mesoarchean oxygen oasis (McIntyre and Fralick, 2017; Thoby et al., 2019) which is further supported by La-Ce geochronological data indicating that the Ce anomalies are syn-genetic (Patry et al., in review).

δ^98/95Mo values across diverse lithologies, including stromatolitic dolomites, microbialitic calcites, oxide-facies banded iron formations, black shales, and sulfidic black shales, range from -2.22 ‰ to 0.53 ‰, clearly demonstrating important redox-driven Mo isotope fractionation as far back as 2.87 Ga. Despite non-negligible authigenic Mo enrichments in carbonate and iron formation (IF) samples relative to the crust, Mo concentrations determined by isotope dilution remain significantly below crustal values (mean values of ca. 300 ppb and 60 ppb for IF and carbonates, respectively). Mo stable isotopes in carbonate samples are generally unfractionated from crustal values, indicating a small Mesoarchean seawater Mo reservoir of near-zero δ^98/95Mo composition. The few outcrop carbonate samples previously analyzed from Red Lake showing heavy Mo isotope compositions (Thoby et al., 2019) may therefore represent a false positive for the presence of O₂ due to recent (<20 Ka) surface contamination. In contrast, shales and IF are generally characterized by isotopically lighter values, consistent with fractionation during Mo adsorption onto Mn-/Fe-oxides or partial uptake by reducing sediments, respectively, from a dissolved seawater reservoir that was near 0 ‰. The systematic presence of mild positive Ce anomalies in all samples showing the lightest Mo isotope compositions point to a role for Mn-oxide shuttling during deposition, consistent with previous reports of the establishment of an oxygen oases at Red Lake. Combined, this evidence suggests that any oxidative processing of Mo occurring during the deposition of the Red Lake platform did not result in the establishment of an isotopically heavy seawater Mo reservoir, as occurs today. Instead, the results point to a largely unfractionated seawater Mo reservoir derived from weathering under low O₂ conditions and/or from hydrothermal sources. To investigate the conditions enabling such a reservoir, we employ a steady-state isotope mass balance model to examine the Mesoarchean Mo cycle. Our results suggest that even in the face of incipient oxidative processing in the marine realm, a well-mixed Mo reservoir with a δ^98/95Mo near zero is the likely result for a Mesoarchean ocean subject to reduced riverine Mo fluxes, modern to elevated hydrothermal Mo input fluxes, and for a variety of plausible reservoir sizes, providing important perspective on the application of Mo isotope redox proxies at the onset of Earth system oxygenation.