Uncovering framboidal pyrite biogenicity using nano-scale CNorg mapping

Abstract

Framboidal pyrite has been used as a paleo-redox proxy and a biomarker in ancient sediments, but the interpretation of pyrite framboids can be controversial, especially where later overgrowths have obscured primary textures. Here we show how nano-scale chemical mapping of organic carbon and nitrogen (CN org ) can detect relict framboids within Precambrian pyrite grains and determine their formation mechanism. Pyrite grains associated with an Ediacaran fossil Lagerstatte from Newfoundland (ca. 560 Ma) hold significance for our understanding of taphonomy and redox history of the earliest macrofossil assemblages. They show distinct chemical zoning with respect to CN org . Relict framboids are revealed as spheroidal zones within larger pyrite grains, whereby pure pyrite microcrystals are enclosed by a mesh-like matrix of pyrite possessing elevated CN org , replicating observations from framboids growing within modern biofilms. Subsequent pyrite overgrowths also incorporated CN org from biofilms, with concentric CN org zoning showing that the availability of CN org progressively decreased during later pyrite growth. Multiple framboids are commonly cemented together by these overgrowths to form larger grains, with relict framboids only detectable in CN org maps. In situ sulfur isotope data (δ 34 S = ∼−24‰ to −15‰) show that the source of sulfur for the pyrite was also biologically mediated, most likely via a sulfate-reducing microbial metabolism within the biofilms. Relict framboids have significantly smaller diameters than the pyrite grains that enclose them, suggesting that the use of framboid diameters to infer water column paleo-redox conditions should be approached with caution. This work shows that pyrite framboids have formed within organic biofilms for at least 560 m.y., and provides a novel methodology that could readily be extended to search for such biomarkers in older rocks and potentially on other planets.