Microfossils Preserved in Highly Metamorphosed Rocks: a New Window into the Early Biosphere

Circular graphite discs, among abundant irregular graphite particles, have been extracted from ArcheanPaleoproterozoic amphibolite grade metamorphic rocks in the Wutaishan area of North China. Raman spectroscopic analysis indicates that these graphite discs experienced maximum metamorphic temperature of 513 ± 50 °C. The discs bear morphological features related to metamorphism, but they are also characterized by circular morphology, distinct marginal concentric folds, surficial wrinkles, and complex nanostructures that cannot be accounted for by metamorphism alone. Their carbon isotope compositions range from –7.4‰ to – 35.9‰, averaging at –20.3‰, which are consistent with biological affinity. We interpret these graphite discs as deflated and compressed microfossils that were originally spheroidal organic-walled vesicles (acritarchs), representing either large prokaryotic or possibly eukaryotic cells. This interpretation suggests that organic-walled microfossils may survive significant metamorphic heating and a new paleontological window may be open onto the early biosphere. Continuing research, including experimental heating of bona fide organic-walled microfossils as well as focused ion beam ultrastructural analysis, is also highlighted here.