Distinguishing cellular from abiotic spheroidal microstructures in the ca. 3.4 Ga Strelley Pool Formation

Area of Science:

• Geology
• Paleontology
• Geochemistry

Background:

• The origin of carbonaceous microstructures in Archean rocks is debated, with potential for abiotic formation mimicking microfossils.
• Mineral growths can obscure or mimic cellular structures, complicating the identification of early life.
• Previous criteria for microfossil identification, such as continuous carbonaceous walls, are challenged by new findings.

Purpose of the Study:

• To investigate the morphogenesis of spheroid microstructures in the 3.4 Ga Strelley Pool Formation chert.
• To analyze carbonaceous matter distribution and its relationship with quartz crystallographic textures.
• To differentiate between abiotic and potential biotic origins of these ancient microstructures.

Main Methods:

• High-resolution in situ imaging and analysis of spheroid microstructures.
• Examination of carbonaceous matter distribution and quartz crystallographic textures.
• Comparison of siliceous microstructures with pyritic spheroids from the same sample.

Main Results:

• Five new types of spheroids were identified, with evidence for carbonaceous matter migration onto abiotic siliceous spherulites.
• Nanoparticulate walls cutting across quartz crystals suggest migration before silica recrystallization.
• Some clustered spheroids exhibit features consistent with post-mortem cell alteration, though abiotic origins are not ruled out.
• Pyritic spheroids did not appear to be precursors to the siliceous structures.

Conclusions:

• New textural evidence supports abiotic morphogenesis for some Archean spheroids.
• The absence of these features in other spheroids leaves the microfossil hypothesis open for distinct structures.
• Distinguishing these characteristics is crucial for identifying genuine microfossils and understanding early life.
• Further research on metamorphosed rocks and in vitro experiments is recommended.