Isotope constraints on nutrient supply, bio-productivity and redox dynamics in the Mesoproterozoic McArthur Basin, Australia: Insights from δ114Cd, δ13C and δ15N proxies

• Cadmium and nitrogen isotopes trace Mesoproterozoic nutrient cycling • Mafic weathering enhanced P and Cd delivery to the McArthur Basin • Redox stratification promoted nutrient regeneration and productivity • Nitrate scarcity drove biological N₂ fixation in anoxic waters • Nutrient fluxes fostered conditions for early eukaryotic diversification Low nutrient availability and oxygen levels in the Mesoproterozoic Ocean are often viewed as barriers to early eukaryotic diversification. However, the links between nutrient inputs and cycling, primary productivity, and redox evolution during this time remain poorly understood. Here we present new cadmium isotope data (δ 114 Cd) from the organic-rich shales of ∼1430–1360 Ma Velkerri Formation shales (McArthur Basin, Australia), integrated with nitrogen (δ 15 N), organic carbon (δ 13 C org ) isotope proxies, total organic carbon (TOC) and nitrogen (TN), and selected redox-sensitive trace element (Mo, U) concentration data. We observe a pronounced positive δ 114 Cd excursion in the B-organofacies (up to +0.2‰) that correlates with peak TOC values (up to 9 wt%) and elevated δ 13 C org , supporting enhanced primary productivity likely sustained by mafic-derived nutrient inputs, the latter suggested by previously published Nd and Os isotope data. These coupled isotope trends are also accompanied by persistently low δ 15 N values (1–2‰), consistent with nitrate limitation and dominant biological N 2 -fixation under anoxic/euxinic marine conditions. In contrast, a decline of δ 114 Cd values (to -0.15‰) in the overlying C-organofacies, accompanied by lower δ 13 C org , points to reduced bio-productivity and photosynthetic activity and thus a purported shift toward greater chemosynthetic productivity. Together, these results demonstrate that tectonically enhanced weathering of mafic sources, and by inference key nutrients, drove increased P and Cd delivery to the McArthur Basin, stimulating local bio-productivity and expanding basin’s redox stratification. By linking the Cd, C and N isotope proxy records and δ 114 Cd excursions directly to nutrient cycling and bio-productivity in the McArthur Basin, this study establishes Cd isotopes as a robust proxy for Mesoproterozoic bio-productivity changes and reconstruction. Overall, this study highlights how nutrient influx sustained organic carbon production, preservation and burial in the greater McArthur Basin, with implications also for the coeval marine paleo-redox changes and the early eukaryotic diversification during this critical interval in Earth’s geological and biological evolution.