The Oxic Degradation of Sedimentary Organic Matter 1.4 Ga Constrains Atmospheric Oxygen Levels
We studied sediments from the ca. 1400 million-year-old Xiamaling Formation from the northern China Block. The upper unit of this formation (unit 1) deposited mostly below storm wave base and contains alternating black and green-gray shales with very different geochemical characteristics. The black shales are enriched in redox sensitive trace metals, have high concentrations of total organic carbon (TOC), high hydrogen index (HI) and iron speciation indicating deposition under anoxic, mostly
... inic, conditions. In contrast, the green-grey shales show no trace metal enrichments, low TOC, low HI and iron speciation consistent with an oxygenated depositional setting. Altogether, unit 1 displays alternations between oxic and anoxic depositional environments, and differences in carbon preservation consistent with observations from the modern ocean. We combined our TOC and HI index results to calculate the differences in carbon mineralization between the oxygenated and anoxic depositional environments. Through comparisons of these results with modern sedimentary environments, and by use of a simple diagenetic model, we conclude that carbon mineralization under oxygenated conditions at Xiamaling required a minimum of 4&thinsp;% to 8&thinsp;% of present-day oxygen levels (PAL). These oxygen levels are higher than estimates based on chromium isotopes and, furthermore, were likely sufficient for early animal respiration. Therefore, our results reinforce the idea that the environment contained enough oxygen for animals long before their evolution.