Coronene spikes in the Ediacaran Doushantuo Formation, South China: A combustion origin?

Abstract

Paleo-wildfires evidenced by high contents of the polycyclic aromatic hydrocarbon coronene (C₂₄H₁₂), black carbon, charcoal, and inertinite are mainly known from strata younger than ∼430 Ma, following colonization of continents by vascular plants. One exception is a record of elevated coronene concentrations from Ediacaran strata in Australia, which have been interpreted as a product of wildfires ignited by a bolide impact (i.e., Acraman in South Australia). However, this inference is at odds with both the timing of terrestrial vegetation expansion and inferred atmospheric oxygen levels prior to the Phanerozoic, which are generally regarded as too low to have sustained wildfires. Here, we report two middle Ediacaran coronene spikes from the Doushantuo Formation in the Sixi section, South China. These coronene spikes may have an origin related to (1) aerosols generated by the Acraman impact and subsequently carried to South China by winds, or (2) ignition of a terrestrial microbial organic layer by regional volcanism or lightning strike. Our results are significant in demonstrating the presence of large amounts of coronene in Ediacaran strata of South China, making this the oldest known deposit having elevated coronene concentrations, with possible implications for the timing of onset of wildfires in Earth history. Similar to the transient “whiffs” of oxygen that characterized the otherwise anoxic conditions of the Late Archean world, we hypothesize that transient “blasts” of oxygen production occurred in the generally low-oxygen Late Neoproterozoic world.