Abstract
Mapping the history of atmospheric O(2) during the late Precambrian is vital for evaluating potential links to animal evolution. Ancient O(2) levels are often inferred from geochemical analyses of marine sediments, leading to the assumption that the Earth experienced a stepwise increase in atmospheric O(2) during the Neoproterozoic. However, the nature of this hypothesized oxygenation event remains unknown, with suggestions of a more dynamic O(2) history in the oceans and major uncertainty over any direct connection between the marine realm and atmospheric O(2). Here, we present a continuous quantitative reconstruction of atmospheric O(2) over the past 1.5 billion years using an isotope mass balance approach that combines bulk geochemistry and tectonic recycling rate calculations. We predict that atmospheric O(2) levels during the Neoproterozoic oscillated between ~1 and ~50% of the present atmospheric level. We conclude that there was no simple unidirectional rise in atmospheric O(2) during the Neoproterozoic, and the first animals evolved against a backdrop of extreme O(2) variability.