Surface water oxygenation and low bioproductivity during deposition of iron formation of the Jacadigo Group (Brazil): Insights from combined cadmium – Chromium isotopes

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The Banda Alta Formation (Urucum district, Mato Grosso do Sul, Brazil) comprises ∼600 Ma Fe and Mn deposits, which are among the world's youngest and largest Neoproterozoic sedimentary Fe and Mn formations (IF; MnF). These have been deposited in a redox-stratified, marine sub-basin (Jacadigo Basin), which was strongly influenced by glacial advance/retraction cycles with temporary influx of continental freshwater and upwelling of metal-enriched deep anoxic seawater. Cr and Cd isotopes measured on meticulously separated hematite mesobands from drill core samples are relatively homogenous throughout the ca. 325 m thick sequence sampled in the Banda Alta Fm., with average authigenic δ53Cr values of +0.93 ± 0.24 ‰ (2σ; n = 23) and δ114Cd values of −0.14 ± 0.14 ‰ (2σ; n = 15). The significant enrichment of Cr, in parallel with the strong enrichments of other redox sensitive elements (U, Mo), attests for effective and efficient reduction removal processes in the surface waters during cycles where upwelling Fe2+-rich waters reached the oxygenated surface layer exposed to the atmosphere during episodic glacier retreat stages. Assuming a similar quantitative and efficient removal pathway of dissolved Cd by iron oxyhydroxides, the so-inferred average δ114Cd signature of −0.14 ± 0.14 ‰ in the Jacadigo Basin surface water is significantly lower than signatures of modern ocean surface waters with a range of δ114Cd of ca. ∼0.4 to ∼1 ‰ and even lower than the signature of modern ocean deep waters with δ114Cd of ∼0.3 ‰. It possibly attests to reduced primary production levels and lower nutrient utilization rates during deposition of the Late Neoproterozoic Jacadigo Group, compared to today. This despite the inferred oxidized surface water layer that must have prevailed during this time, as implied by the strongly positively fractionated Cr isotope signatures and pronouncedly negative Ce-anomalies recorded in the seawater-like, shale-normalized Rare Earth Element and Yttrium (REY) patterns exhibited by the hematite mesobands. Data presented herein speak for: (1) a stable, isotopically heavy Cr input to the Jacadigo Basin at the time of deposition, implying high atmospheric O2 levels in the Late Neoproterozoic (2) likely quantitative, reductive incorporation / adsorption processes of dissolved Cr and Cd, respectively, into/onto precipitating iron oxyhydroxides, and (3) the prevalence of low nutrient concentrations and utilization rates in the Jacadigo Basin during glacier retreat cycles. Banded iron formations are considered suitable archives for reconstructing redox and bioproductivity levels in past marine depositional basin, including those prevalent in Neoproterozoic glacial conditions, via employing the Cr–Cd isotope double tracer to iron-rich mesobands.

Original languageEnglish
Article number122101
JournalChemical Geology
Volume657
ISSN0009-2541
DOIs
Publication statusPublished - 5 Jul 2024

Bibliographical note

Funding Information:
We are thankful to Vectorial Ltd. for providing access to field sites and the drill core sheds and allowing sampling of core STCR-DD-36-32. In particular, we thank Raphael Henson, who provided guidance in the Vectorial mine and shared his knowledge. Toby Leeper, Toni Larsen, Cristina Nora Jensen de Olsen and Martin Heckscher helped with mass spectrometric analyses, ion chromatographic separations and sample preparations, respectively. Ware thankful for the detailed, constructive and insightful comments provided by four anonymous reviewers, and for streamlining suggestions by editor Vasileios Mavromatis. This research is financed by the Independent Research Fund Denmark (grant 1026-00001B to RF), and by CNPq (Proc 2014/01233-0) and FAPESP (Proc 2004/01233-0, Proc 2016/06114-6 and Proc 2020/16140-0) to PCB.

Publisher Copyright:
© 2024 Elsevier B.V.

    Research areas

  • Banded iron formation, Bioproductivity, Brazil, Cadmium isotopes, Chromium isotopes, Jacadigo Group, Ocean redox

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