Seawater barium and sulfide removal improved marine habitability for the Cambrian Explosion of early animals

Abstract

An increase in atmospheric <i>p</i>O₂ has been proposed as a trigger for the Cambrian Explosion at ∼539-514 Ma but the mechanistic linkage remains unclear. To gain insights into marine habitability for the Cambrian Explosion, we analysed excess Ba contents (Ba_excess) and isotope compositions (δ¹³⁸Ba_excess) of ∼521-Myr-old metalliferous black shales in South China. The δ¹³⁸Ba_excess values vary within a large range and show a negative logarithmic correlation with Ba_excess, suggesting a major (>99%) drawdown of oceanic Ba inventory via barite precipitation. Spatial variations in Ba_excess and δ¹³⁸Ba_excess indicate that Ba removal was driven by sulfate availability that was ultimately derived from the upwelling of deep seawaters. Global oceanic oxygenation across the Ediacaran-Cambrian transition may have increased the sulfate reservoir via oxidation of sulfide and concurrently decreased the Ba reservoir by barite precipitation. The removal of both H₂S and Ba that are deleterious to animals could have improved marine habitability for early animals.