The oceanic primary productivity variations during the Ordovician and Silurian transtion

The surface productivity variations are still unclear through the Ordovician-Silurian crisis, which is belong to one of the “Big Five” extinction. Here, we present barium (Ba) concentration and isotope data from organic matter-enriched anoxic siliceous sediments of various facies (X sites from proximal to distal regions) during the OST from South China. Our data show that both raw Ba and ratios of Ba to aluminum (Ba/Al) are higher than that in numerous ancient black shales and modern high productivity area, document elevated Ba accumulations during this interval. Besides, a larger gradient (~ 1 ‰) of Ba isotope (¹³⁸Ba) gradient between the shallow-water to deep-water sites, additional support higher marine productivity was the reason of the higher Ba burial in these sediments. These data provide evidence that elevated organic carbon fluxes from the surface ocean (other than redox conditions) was likely the main control on accumulation of these organic matter-enriched sediments, and thus provide the sources of “shale gas” during this interval.