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

Author(s)

Frei, Robert ; Gaucher, Claudio ; Boggiani, Paulo Cesar ; Frederiksen, Jesper Allan ; Walker, Samantha Renee ; Fernandes, Henrique Albuquerque ; Caxito, Fabricio

• Document type: Journal article
• Source: CHEMICAL GEOLOGY; v. 657, p. 19-pg., 2024-04-29.

Abstract

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 delta 53Cr values of +0.93 +/- 0.24 %o (2 sigma; n = 23) and delta 114Cd values of -0.14 +/- 0.14 %o (2 sigma; 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 delta 114Cd signature of -0.14 +/- 0.14 %o in the Jacadigo Basin surface water is significantly lower than signatures of modern ocean surface waters with a range of delta 114Cd of ca. -0.4 to -1 %o and even lower than the signature of modern ocean deep waters with delta 114Cd of -0.3 %o. 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. (AU)

• FAPESP's process: 16/06114-6 - The Neoproterozoic Earth System and the rise of biological complexity
• Grantee: Ricardo Ivan Ferreira da Trindade
• Support Opportunities: Research Projects - Thematic Grants
• FAPESP's process: 20/16140-0 - Paleoproterozoic and Neoproterozoic phosphorite depositional systems in Brazil, genetic processes and application in agriculture
• Grantee: Paulo César Boggiani
• Support Opportunities: Regular Research Grants