A copper isotope investigation of methane cycling in Late Archaean sediments

The rise of oxygenic photosynthesis arguably represents the most important evolutionary step in Earth history. Recent studies, however, suggest that Earth's pre-oxidative atmosphere was also heavily influenced by biological feedbacks. Most notably, recent geochemical records propose the existence of a hydrocarbon haze which periodically formed in response to enhanced biospheric methane fluxes. Copper isotopes provide a potential proxy for biological methane cycling; Cu is a bioessential trace metal and a key element in the aerobic oxidation of methane to carbon dioxide (methanotrophy). In addition, Cu isotopes are fractionated during biological uptake. Here, we present a high-resolution Cu isotope record measured in a suite of shales and carbonates from core GKF01, through the similar to 2.6-2.5 Ga Campbellrand-Malmani carbonate platform. Our data show a 0.85 parts per thousand range in Cu isotope composition and a negative excursion that predates the onset of a haze event. We interpret this excursion as representing a period of enhanced aerobic methane oxidation before the onset of the Great Oxidation Event. This places valuable time constraints on the evolution of this metabolism and firmly establishing Cu isotopes as a biomarker in Late Archaean rocks.

Automated summary

The rise of oxygenic photosynthesis is considered the most important evolutionary step in Earth's history, but recent studies suggest that biological feedbacks also heavily influenced Earth's pre-oxidative atmosphere. Copper isotopes in Late Archaean rocks can potentially serve as biomarkers, providing valuable insights into the evolution of aerobic methane oxidation.