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 Lagerstätte 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 (d 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.

Sedimentary pyrite δ34S differs from porewater sulfide in Santa Barbara Basin: Proposed role of organic sulfur

“…Recent work by MacLean et al (2008) provides evidence for the formation of framboidal pyrite on organic templates within microbial biofilms (Wacey et al, 2014). The pyrites observed in Santa Barbara Basin sediments are therefore consistent with biologically induced pyrite mineralization in association with sulfate-reducing microbial communities.…”

Composition and pore characteristics of black shales from the Ediacaran Lantian Formation in the Yangtze Block, South China

“…Each pyrite crystal was enclosed by a nano-porous organic rim, all of which were similar to the framboids that occur in a matrix of bacteria and biopolymers in modern sediments (MacLean et al, 2008). This suggests that a structural and chemical environment is provided by biofilms for the growth of pyrite framboids (MacLean et al, 2008;Wacey et al, 2015). Most pyrite framboids or irregular clusters in the Lantian shales showed neither structures enclosed by OM nor an organic rim around individual crystals; rather, the 2D images showed them apparently floating on the nano-porous OM matrix.…”

“…4a, c, e). A recent study of Ediacaran siltstone using both FIB-SEM imaging and nanoscale secondary ion mass spectrometry (NanoSIMS) mapping (Wacey et al, 2015) found clear evidence of pyrite framboids enclosed by almost spherical nano-porous OM zones. Each pyrite crystal was enclosed by a nano-porous organic rim, all of which were similar to the framboids that occur in a matrix of bacteria and biopolymers in modern sediments (MacLean et al, 2008).…”

Advances and Opportunities in Ore Mineralogy

“…Opportunity exists to use nanoSIMS to address other outstanding issues in the mineralogy of compositionally-zoned minerals, trace element partitioning, and in mineral processing. In one innovative application of nanoSIMS, Wacey et al (2015) [98] have used nanoSIMS mapping of organic carbon and nitrogen in framboidal pyrite, elucidating mechanisms of framboid formation in terms of organic biofilms.…”