The evolution of Earth's biota is intimately linked to the oxygenation of the oceans and atmosphere. We use the isotopic composition and concentration of molybdenum (Mo) in sedimentary rocks to explore this relationship. Our results indicate two episodes of global ocean oxygenation. The first coincides with the emergence of the Ediacaran fauna, including large, motile bilaterian animals, ca. 550-560 million year ago (Ma), reinforcing previous geochemical indications that Earth surface oxygenation facilitated this radiation. The second, perhaps larger, oxygenation took place around 400 Ma, well after the initial rise of animals and, therefore, suggesting that early metazoans evolved in a relatively low oxygen environment. This later oxygenation correlates with the diversification of vascular plants, which likely contributed to increased oxygenation through the enhanced burial of organic carbon in sediments. It also correlates with a pronounced radiation of large predatory fish, animals with high oxygen demand. We thereby couple the redox history of the atmosphere and oceans to major events in animal evolution.T he concentration of O 2 in the Earth's atmosphere and oceans has increased over time from negligible levels early in Earth history to the 21% we have in the atmosphere today (1-3). Our understanding of this history is indirect, based mainly on a series of geochemical proxies reflecting chemical interactions between O 2 and other oxidation-reduction (redox) sensitive elements. These proxies include the isotopic compositions and concentrations of elements such as S, Fe, and Mo preserved in sedimentary rocks (3-7). Several of these proxies (4,5,8,9) indicate an increase in oceanic O 2 during the Ediacaran Period (635 to 542 million years ago, Ma), roughly synchronous with the emergence of large, motile bilaterian animals and, therefore, suggestive of a physiological link between Ediacaran evolution and environmental change. Despite this, the distribution of organic-rich shales (10, 11), the ratio of pyrite sulfur to organic carbon in shales (12), and modeling of the sulfur isotope record (13,14) all indicate that large tracts of subsurface ocean remained anoxic well into the early Phanerozoic Eon (the "age of visible animals," since 542 million years ago). Levels of ocean and atmospheric oxygenation, however, are unquantified from these proxies. Indeed, the most comprehensive history of Phanerozoic oxygenation has been inferred from biogeochemical models. These models diverge, however, on their predictions for the Paleozoic, suggesting either low (15) or high levels of atmospheric oxygen (16,17). Here, we present an independent record of ocean oxygenation history derived from the isotopic composition and concentration of Mo in black shales.The geochemical behavior of Mo is controlled by the relative availability of dissolved H 2 S and O 2 in the oceans. In oxic waters, Mo is soluble and exists as the molybdate anion, MoO 4 2− . In sulfidic waters, molybdate reacts with H 2 S to form particlereactive oxythiomo...

The Mesoproterozoic Eon [1,600-1,000 million years ago (Ma)] is emerging as a key interval in Earth history, with a unique geochemical history that might have influenced the course of biological evolution on Earth. Indeed, although this time interval is rather poorly understood, recent chromium isotope results suggest that atmospheric oxygen levels were <0.1% of present levels, sufficiently low to have inhibited the evolution of animal life. In contrast, using a different approach, we explore the distribution and enrichments of redox-sensitive trace metals in the 1,400 Ma sediments of Unit 3 of the Xiamaling Formation, North China Block. Patterns of trace metal enrichments reveal oxygenated bottom waters during deposition of the sediments, and biomarker results demonstrate the presence of green sulfur bacteria in the water column. Thus, we document an ancient oxygen minimum zone. We develop a simple, yet comprehensive, model of marine carbon−oxygen cycle dynamics to show that our geochemical results are consistent with atmospheric oxygen levels >4% of present-day levels. Therefore, in contrast to previous suggestions, we show that there was sufficient oxygen to fuel animal respiration long before the evolution of animals themselves.atmosphere | Mesoproterozoic | oxygen minimum zone | trace metals | biomarkers

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