Trends in Nutrient Supply, Productivity and Atmosphere Oxygenation through the Boring Billion

English Abstract:

The redox sensitive and nutrient trace elements in marine sedimentary pyrites in Proterozoic black shales have been used to provide clues to nutrient supply, productivity and atmosphere oxygenation during the boring billion years. Using the nutrient trace elements adsorbed in marine pyrite (Zn, Cd, Cu, Ni, Mo and Se) as a proxy, reveals that following the GOE1, nutrient supply to the oceans dropped to a minimum around 1650 Ma, peaked at 1300 to 1000 Ma dropping again to a second minimum around 850 Ma, before rising gradually and then steeply toward the end of the Proterozoic. Using uranium in the shale matrix as a biological productivity proxy reveals a similar pattern with productivity at a minimum between 1400 and 1600 Ma, with a second minimum at 800 to 1000 Ma. The opposing patterns of Se (oxidation proxy) and Co (reduction proxy), indicates the Se/Co ratio in marine pyrite maybe a useful atmosphere oxygenation proxy. The pattern of this proxy compared with Mo and Se in pyrite suggests atmosphere O₂ was at a relative maximum at the start of the boring billion and declined in several steps to a relative minimum around 1000 Ma, corresponding with the second minimum in nutrient supply and productivity. These patterns may possibly relate to the supercontinent cycles, tectonic activities (plate collision) leading to uplift, erosion and increased nutrient supply to the ocean.