Petrographic carbon in ancient sediments constrains Proterozoic Era atmospheric oxygen levels

Significance Earth progressed from an oxygen-free planet at birth to the well-oxygenated planet we have today. Life enabled Earth’s oxygenation through the evolution of cyanobacteria, and the subsequent history of oxygen enabled further developments in the evolution of eukaryotic life; mammals, for example, need relatively high oxygen levels to breathe. The relationship between oxygen and life, however, is hampered by uncertainties in the history of oxygen concentrations, particularly during the middle of the Proterozoic Eon when eukaryote ecosystems were first emerging. We explore ancient organic matter cycling through Raman spectrographic analysis of nine geologic formations spanning 1 billion years of Proterozoic time. Our results, when combined with modeling, show that emerging eukaryote ecosystems were not limited by oxygen concentrations. Oxygen concentration defines the chemical structure of Earth's ecosystems while it also fuels the metabolism of aerobic organisms. As different aerobes have different oxygen requirements, the evolution of oxygen levels through time has likely impacted both environmental chemistry and the history of life. Understanding the relationship between atmospheric oxygen levels, the chemical environment, and life, however, is hampered by uncertainties in the history of oxygen levels. We report over 5,700 Raman analyses of organic matter from nine geological formations spanning in time from 742 to 1,729 Ma. We find that organic matter was effectively oxidized during weathering and little was recycled into marine sediments. Indeed, during this time interval, organic matter was as efficiently oxidized during weathering as it is now. From these observations, we constrain minimum atmospheric oxygen levels to between 2 to 24% of present levels from the late Paleoproterozoic Era into the Neoproterozoic Era. Indeed, our results reveal that eukaryote evolution, including early animal evolution, was not likely hindered by oxygen through this time interval. Our results also show that due to efficient organic recycling during weathering, carbon cycle dynamics can be assessed directly from the sediment carbon record.