Tracking the Rise of Superchondritic Zr/Hf in Banded Iron Formations: A Novel Redox Tracer?

The modern oceans’ Zr/Hf ratio (150-300¹) is significantly fractionated from the chondritic value (34.4²). This deviation is driven by the higher particle reactivity of Hf relative to Zr, resulting in preferential sorption onto (oxide)-particles in estuaries. The ratio increases from the coastline towards the open ocean, leading to variability between water masses¹. The short residence time of Hf and Zr makes this ratio a powerful tracer for ocean circulation patterns and sources affecting seawater.

The Zr/Hf of ancient seawater is poorly understood, but has great potential to extend this proxy as a paleoceanographic tracer. Banded iron formations (BIFs) may have preserved the Zr/Hf of ancient seawater and could serve as a seawater archive. We present high-precision Zr-Hf data of Precambrian BIFs complemented by available literature data to test this proxy as a paleoceanographic water mass tracer.

Archean BIFs display near-chondritic Zr/Hf, while super-chondritic ratios in individual BIF layers first appear ~2.51 Ga. The locations’ averages predominantly remain near-chondritic until ~2.0 Ga, whereas younger BIFs show mostly super-chondritic ratios. The shift from chondritic towards super-chondritic Zr/Hf ratios of BIFs records changing conditions throughout the Precambrian seawater affecting the fractionation of Zr and Hf. Potential causes include increased occurrence of (Fe)-Mn(oxide)-particles and enhanced riverine discharge. Regional differences among coeval formations suggest varying depositional settings and distinct water mass circulation in the Precambrian ocean. The Zr/Hf ratio measured in BIFs potentially traces the occurrence of Mn-oxides, hence, oxygenated water masses.

¹Godfrey et al., 1996, GCA 60

²Weyer et al., 2002, Chem.Geo. 187