Zr/Hf ratios in Banded Iron Formations as tracers of Early Ocean evolution 

The Zr/Hf ratio of modern seawater (150-3001) is significantly fractionated relative to the chondritic value (32.7-34.22) and magmatic systems. This deviation is driven by the higher particle reactivity of Hf relative to Zr in low-temperature, aqueous systems, resulting in preferential sorption of Hf onto (particle) surfaces. The Zr/Hf ratio of aqueous systems increases from the continents towards the open oceans, varies with water depth and water mass age, making it a powerful tool for tracing water masses. While reasonably well constrained in modern aquatic systems, the Zr/Hf composition of ancient seawater remains poorly understood, but may provide unique insights into the circulation of water masses.To investigate the Zr/Hf evolution of the seawater throughout Earth’s history, banded iron formations (BIFs) represent a viable archive for the Precambrian seawater chemistry because they are chemical sedimentary rocks and reflect the chemistry of the seawater from which they precipitated. Here, we present new high-precision Zr–Hf data from Precambrian BIFs, complemented by available literature data, to evaluate the Zr/Hf ratio as a paleoceanographic tracer of ancient water masses.Archean BIFs predominantly display near-chondritic Zr/Hf ratios, with ratios not exceeding 75. The first super-chondritic Zr/Hf ratios occur in individual BIF-layers at ~2.51 Ga, and the formation showing overall super-chondritic Zr/Hf ratios is the ca. 2.4 Ga Hotazel Formation, indicating widespread Zr/Hf fractionation in marine environments. Formation-scale averages largely remain near-chondritic until ~2.0 Ga, while younger BIFs show predominantly super-chondritic ratios. This secular trend from chondritic towards super-chondritic Zr/Hf ratios in the early to mid Proterozoic likely reflects changing seawater conditions that enabled widespread Zr–Hf fractionation. The increasing availability of Fe–Mn(oxide) particles, based on increasing atmospheric oxygenation but also the progressive development of modern-style estuarine and shelf environments, may have led to global Zr-Hf fractionation in marine systems by that time. Within individual formations, Zr/Hf ratios correlate with Mn/Fe ratios, indicating a link between Zr-Hf fractionation and the redox-evolution of the Earth. Moreover, regional differences among coeval BIFs suggest variable depositional settings and distinct water-mass circulation patterns already in the Neo-archean. Thus, our results highlight the potential of Zr/Hf ratios in BIFs and other chemical sedimentary rocks to trace the redox-evolution of the Earth with the appearance and spatial heterogeneity of oxygenated water masses in Early Earth oceans. 1Godfrey et al., 1996, GCA 60 2Münker et al., 2025, GPL 3