Life on a Mesoarchean marine shelf \u2013 insights from the world\u2019s oldest known granular iron formation

Albertus J B Smith,Nicolas J Beukes,Jens Gutzmer,Clark M Johnson,Frikkie De Beer,Stanley M Awramik,Andrew D Czaja,Noah Nhleko,Jakobus W Hoffman

doi:10.1038/s41598-020-66805-0

Abstract

The Nconga Formation of the Mesoarchean (~2.96–2.84 Ga) Mozaan Group of the Pongola Supergroup of southern Africa contains the world’s oldest known granular iron formation. Three dimensional reconstructions of the granules using micro-focus X-ray computed tomography reveal that these granules are microstromatolites coated by magnetite and calcite, and can therefore be classified as oncoids. The reconstructions also show damage to the granule coatings caused by sedimentary transport during formation of the granules and eventual deposition as density currents. The detailed, three dimensional morphology of the granules in conjunction with previously published geochemical and isotope data indicate a biogenic origin for iron precipitation around chert granules on the shallow shelf of one of the oldest supracratonic environments on Earth almost three billion years ago. It broadens our understanding of biologically-mediated iron precipitation during the Archean by illustrating that it took place on the shallow marine shelf coevally with deeper water, below-wave base iron precipitation in micritic iron formations.

Highlights

Iron formation (IF, a chemical sedimentary rock containing at least 15 wt% iron1,2) has been at the centre of debate with regards to early life on Earth[3,4] with numerous authors having suggested bacterially mediated deposition of iron-rich mineral precursors[5,6,7,8,9,10,11]
The Mozaan Group contains ten iron-rich sedimentary beds[3], of which the basal IF in the Vlakhoek Member has been the subject of several recent studies about pre-Great Oxidation Event (GOE) oxygen oases[20,21,22,23,24]
Less studied is the Nconga Formation, which occurs towards the top of the Mozaan Group

Summary

Introduction

Iron formation (IF, a chemical sedimentary rock containing at least 15 wt% iron1,2) has been at the centre of debate with regards to early life on Earth[3,4] with numerous authors having suggested bacterially mediated deposition of iron-rich mineral precursors[5,6,7,8,9,10,11]. Our evidence comes from the exceptionally well preserved - and oldest known GIF of the Nconga Formation in the Mesoarchean Mozaan Group of the Pongola Supergroup of southern Africa. The complex structure of the granule rims in the Nconga Formation GIF makes thin section petrography by optical and scanning electron microscope difficult due to the unavoidable bias introduced by two dimensional assessments of three dimensional features (Supplementary Video 3 and 4). The large difference in density between the magnetite in the granule rims and the quartz in the granule cores and matrix, presented a unique opportunity to conduct μXCT studies on the Nconga Formation GIF in order to reveal external morphology and textural details

Methods

Results

Discussion

Conclusion