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We studied sediments from the ca. 1400 million-year-old Xiamaling Formationfrom the North China block. The upper unit of this formation (unit 1)deposited mostly below storm wave base and contains alternating black andgreen-gray shales with very distinct geochemical characteristics. The blackshales are enriched in redox-sensitive trace metals, have highconcentrations of total organic carbon (TOC), high hydrogen index (HI) andiron speciation indicating deposition under anoxic conditions. In contrast,the green-gray shales show no trace metal enrichments, have low TOC, low HIand iron speciation consistent with an oxygenated depositional setting.Altogether, unit 1 displays alternations between oxic and anoxicdepositional environments, driving differences in carbon preservationconsistent with observations from the modern ocean. We combined our TOC andHI results to calculate the differences in carbon mineralization andcarbon preservation by comparing the oxygenated and anoxicdepositional environments. Through comparisons of these results with modernsedimentary environments, and by use of a simple diagenetic model, weconclude that the enhanced carbon mineralization under oxygenated conditionsin unit 1 of the Xiamaling Formation required a minimum of 4 to 8 % ofpresent-day atmospheric levels (PAL) of oxygen. These oxygen levels are higher than estimates based on chromium isotopes and reinforce the idea that theenvironment contained enough oxygen for animals long before their evolution

The oxic degradation of sedimentary organic matter 1400 Ma constrains atmospheric oxygen levels