Carbon isotope stratigraphy of Precambrian iron formations and possible significance for the early biological pump

Tsikos, H. et al. (2022) Carbon isotope stratigraphy of Precambrian iron formations and possible significance for the early biological pump. Gondwana Research, 109, pp. 416-428. (doi: 10.1016/j.gr.2022.05.007)

Abstract

The origin of Precambrian iron-formations (IF) remains contentious, particularly with respect to the mineralogy of primary precipitates and the exact processes and conditions leading to their formation. Despite the uncertainties, prevailing hypotheses range from biological precipitation of ferrihydrite to abiotic water-column formation of greenalite. By contrast, iron carbonate minerals (siderite, ankerite) in IF have traditionally been attributed to diagenetic origins based on textural and isotopic relationships. Recent studies on IF from the Neoarchaean-Paleoproterozoic Transvaal Supergroup of South Africa have revealed evidence for apparently primary, low-δ13C, Fe/Mn-bearing Mg calcite as precursor to iron carbonate formation and as a potentially underestimated pathway of isotopically light carbon burial during IF deposition. Here, we present whole-rock δ13C data and carbonate-specific geochemical analyses for samples from five drill cores that capture the entire stratigraphic extent of the Kuruman and Griquatown IF of the Transvaal Supergroup. Our results demonstrate remarkable consistency in stratigraphic profiles among the locations for the trends and magnitudes of bulk δ13C values that are independent of paragenetic association, modal mineralogy, and chemical composition of the bulk carbonate fraction of each sample. We interpret these records as resulting from water-column abiotic carbonate formation that was accompanied by kinetic isotopic effects associated with fluctuating conditions (pH, alkalinity) controlling carbonate supersaturation in ambient seawater. Although our interpretation provides strong support for abiotic, anoxic models for IF genesis prior to the Great Oxidation Event (GOE), it does not entirely preclude additional biological mechanisms of primary ferric oxyhydroxide formation and its possible role in an early biological pump.

• Item Type: Articles
• Additional Information: H. Tsikos acknowledges generous funding by ASSMANG Ltd for the establishment of research unit PRIMOR while he was at Rhodes University in 2014-21. The company SOUTH32 in Hotazel, particularly Mr T. Rambuda and Mr E.P. Ferreira, are thanked for providing unconstrained access to all drill cores used in this study. D.R. Gröcke acknowledges SIBL for covering the organic carbon isotope analyses. T.W. Lyons and P.R.D. Mason acknowledge financial support from Rhodes University through the Hugh Kelly Visiting Fellowship for 2014 and 2016, respectively. The NSF-EAR FESD Program and the NASA Astrobiology Institute under Cooperative Agreement No. NNA15BB03A issued through the Science Mission Directorate Contributions provided support to Lyons.
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Papadopoulos, Vlassis and Boyce, Professor Adrian
• Authors: Tsikos, H., Siahi, M., Rafuza, S., Mhlanga, X.R., Oonk, P.B.H., Papadopoulos, V., Boyce, A.J., Mason, P.R.D., Harris, C., Gröcke, D.R., and Lyons, T.W.
• College/School: College of Science and Engineering; College of Science and Engineering > Scottish Universities Environmental Research Centre
• Journal Name: Gondwana Research
• Publisher: Elsevier
• ISSN: 1342-937X
• ISSN (Online): 1878-0571
• Published Online: 18 May 2022