Redox‐State and Bioproductivity of the Glaciogenic Early Cryogenian Rapitan Iron Formation (Cranswick River, Canada): Constraints From Combined Cadmium—Chromium Isotopes

Abstract The Rapitan Group (Northwest Territories, Canada) includes banded iron formation (BIF), an unusual sediment type that is associated with late Neoproterozoic glaciations, in this case, the early Cryogenian Sturtian glaciation. New non‐traditional stable isotope data from jasper and hematite iron formation (IF) from the Cranswick River area contribute new insights regarding water chemistry changes in the respective depositional basin during glacier advance and retreat. Cr isotope signatures are characterized by strongly positively fractionated δ 53 Cr values (+0.68–1.47‰), attesting to strongly oxidized surface waters during the entire depositional episode. Relatively weakly fractionated δ 114 Cd isotope signatures (−0.02 to +0.28‰), and the absence of organic matter in the samples, indicate a low bioproductivity during BIF deposition. Detailed fluctuations over an intermittent glacier advance and retreat cycle recorded in the studied section, however, reveal an ephemeral increase in primary production (excursion to slightly elevated δ 114 Cd values) just after deposition of a diamictite layer, which agrees with corresponding elevated δ 53 Cr values. This evidence attests to the sensitivity of the Cd‐Cr double isotope tracer applied to BIFs to reconstruct changes in the surface waters of depositional basins and track climate changes that accompanied the glacial episodes. When compared to data from the younger (Marinoan/Ediacaran) Urucum BIF in Brazil, the Rapitan section differs by lack of negative Ce anomalies in its rare earth element patterns, implying that water column oxidation was not as strong during the early Cryogenian as compared to the Marinoan/Ediacaran glacial episodes.