Vivianite verified in early Cambrian strata in northwestern China: Implications for phosphorus recycling in ancient anoxic oceans

In modern low sulfate and anoxic (euxinic) waters, the precipitation of mineral vivianite (an easily oxidized hydrated ferrous-iron phosphate) has played a crucial role in restraining the limiting nutrient element phosphorus (P) recycling back to the water column and consequently decreasing primary productivity. Although such low sulfate and anoxic conditions were widespread in ancient coastal oceans, vivianite has not been directly discovered in the paleo-sediments, which hampers the understanding of P cycling in ancient anoxic environments. Here, we combined techniques of scanning electron microscopy-energy dispersive X-ray spectroscopy, focused ion beam-transmission electron microscopy and P K-edge X-ray absorption near edge structure spectroscopy to analyze samples of P-bearing siliceous rocks and shales from the early Cambrian Yurtus Formation in the Tarim Craton, northwest China. Our results have demonstrated that micron- to nano-scale vivianite crystals are well preserved in the rocks and the vivianite dominates the P phase in some samples. The cherty matrix of the rocks most likely increased the chances of preservation of the oxidation-sensitive vivianite. In light of recent advances, we suggest that vivianite was a crucial P phase in ancient continental margin sediments, spanning most time from the Neoarchean to the early Cambrian. During this interval, the precipitation of vivianite was likely aided by the prevalent dynamic ocean euxinic conditions linked with the seawater sulfate reservoir and the flux of organic matter settling. We propose a negative feedback mechanism in which vivianite precipitation from ancient euxinic waters restricted P availability for biota, reduced marine primary productivity, and possibly abated the rate of Earth's oxygenation and associated evolution of life. This work was financially supported by NSFC projects (grants 42072264, 41730213) and Hong Kong RGC GRF (17307918).