ИСТИНА

Skeletal animals appeared abruptly and globally in the terminal Ediacaran, ~548-550 million years (Ma) ago but radiated in the early Cambrian ~530 Ma. The trigger for widespread biomineralisation may have been the rise of oxygenation beyond a critical threshold, but high resolution geochemical and fossil data have not been integrated to demonstrate this cause and effect. Using three sections across a shelf-edge transect from the South-East Siberian platform - a major centre of early animal diversification - we show that prior to ~540 Ma the water column was dominantly anoxic, ferruginous, and Mg-rich as manifest by primary dolomite precipitates (’dolomite seas’) with a very shallow oxic/anoxic chemocline that supported the first soft-bodied metazoans. After ~540 Ma, primary dolomite disappears and is replaced by aragonite suggesting that sea water chemistry changed to an expanded oxic chemocline and lower-Mg state (‘aragonite seas’). This change occurs after the appearance of soft-bodied macrofossils, but is coincident with the rapid diversification of skeletal metazoans and trace fossils. We suggest that this shift is widespread and marks a profound change in global carbon cycling that facilitated the rise of skeletal animals. Oxygenation may have been triggered by a plate tectonic-induced change in weathering rates or products, or a reduction of oceanic salinity which enhanced oxygen-carrying capacity, or by the proliferation of filtrating macrobiota which oxygenated the water column.