Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon

TLDR

Carbon isotope ratios, especially a high 12C/13C, indicate biogenic carbon, and the earliest measured samples (~3.8 Ga) are isotopically light, suggesting possible early life, but earlier periods can only be probed through mineral grains like zircon. Graphite within a 4.1‑Ga zircon shows a 12C‑rich isotopic signature, is fully encased and undisturbed, indicating it is not a contamination and may represent evidence of life on Earth by that time.

Abstract

Significance Evidence for carbon cycling or biologic activity can be derived from carbon isotopes, because a high 12 C/ 13 C ratio is characteristic of biogenic carbon due to the large isotopic fractionation associated with enzymatic carbon fixation. The earliest materials measured for carbon isotopes at 3.8 Ga are isotopically light, and thus potentially biogenic. Because Earth’s known rock record extends only to ∼4 Ga, earlier periods of history are accessible only through mineral grains deposited in later sediments. We report 12 C/ 13 C of graphite preserved in 4.1-Ga zircon. Its complete encasement in crack-free, undisturbed zircon demonstrates that it is not contamination from more recent geologic processes. Its 12 C-rich isotopic signature may be evidence for the origin of life on Earth by 4.1 Ga.