Paleoenvironmental changes during the Middle-Late Ordovician transition, Northwestern Tarim Basin, NW China and implications for the great Ordovician Biodiversification Event

Abstract

• Black shales of Saergan Formation had a mainly felsic source region likely in an active continental margin. • The paleoenvironment exhibited a semiarid climate with high salinity and anoxic marine conditions during the Middle–Late Ordovician in the Awati Sag. • The Middle–Late Ordovician was a transition period which the paleoenvironment changed greatly. • The GOBE may be triggered by supergreenhouse climate. The changes in the geological environment during the Middle–Late Ordovician transition are well documented by geochemical evidence. We study the paleoenvironmental changes during the Middle–Late Ordovician and their effects on the Great Ordovician Biodiversification Event (GOBE) through the analysis and testing of the major elements, trace elements and rare earth elements (REEs) present in 21 samples from the Middle–Upper Ordovician Saergan Formation in the Awati Sag, Northwestern Tarim Basin, NW China. Compared with post-Archaean Australian Shale, the Saergan Formation is enriched in Ca, P, U and REEs. The total abundance of REEs ranges from 107.3 to 674.8 ppm, with obvious enrichment of light REEs and negative Eu anomalies, as well as Ce shows a weak negative anomaly. Furthermore, the parent rock of Saergan Formation is mainly felsic. Combined with the provenance attributes and tectonic background, the source area is considered to be mainly in an active continental margin tectonic environment. The black shales underwent moderate weathering and simple cyclical sorting, with an immature composition. Multiple geochemical indicators show that the Middle–Late Ordovician is a transition period and the paleoenvironment had semiarid climate and hypersaline anoxic marine conditions. The supergreenhouse climate may accelerate chemical weathering and increase nutrient inputs. In addition, the warm climate melting of the polar ice sheet which leads to the relative shallowness of the sea surface temperature gradient, sea level rise and expansion of marine habitats. The resultant environmental changes could have accelerated the diversification of benthos, which provides an ancient analogue for anthropogenic climate change.