Nitrogen in strongly peraluminous granites reveals a significant increase in biomass burial and O₂ production prior to the Neoproterozoic oxygenation event

Strongly Peraluminous Granites (SPGs) are mostly formed by the partial melting of Al-rich sedimentary rocks and can reflect the chemical properties of their sedimentary protoliths. Recent work suggests that the nitrogen content ([N]) of SPGs preserve changes in biomass burial over time (Mikhail et al., 2024). This mechanism occurs because in unaltered samples, SPGs with elevated [N] are thought to reflect the elevated [N] of their sedimentary protolith, and this additional nitrogen would come from sediment-hosted biomass. This archive reveals an increase in biomass burial by a factor of 5- to 8-fold between 1.4–0.5 Ga. In this study, we analysed the nitrogen abundances and isotopic values of SPG samples from 1.0 to 0.7 Ga to better resolve the period when biomass burial increased. We find that SPG [N] increases 2.4-fold across the Mesoproterozoic-Neoproterozoic boundary at 1.0 Ga and 2.8-fold across the Neoproterozoic-Phanerozoic boundary at 0.5 Ga. Therefore, with consideration of the time lag from biomass burial to SPG formation, we suggest that biomass burial first began to increase in the late Mesoproterozoic. Biomass burial removes organic carbon, a reductant, from Earth’s surface, causing a net production of O₂. Therefore, these data permit us to calculate an increase in O₂ production from biomass burial starting in the Mesoproterozoic, yielding an additional 6.3 × 10²⁰ to 30 × 10²⁰ moles of O₂ throughout the Neoproterozoic, possibly contributing to the Neoproterozoic Oxygenation Event and to the chain of events which resulted in an environment capable of supporting animal life in the Cambrian.