The impact of volcanism on eutrophication and water column in a freshwater lacustrine basin: A case study of Triassic Chang 7 Member in Ordos Basin

Large-scale organic-rich shales are usually formed in saline basins rather than freshwater basins. However, the Ordos Basin, as a typical freshwater lacustrine basin, has a maximum total organic carbon content (TOC) of 30 % in its Triassic Chang 7 Member shale, way above the average TOC content of shales in saline basins, leaving the main controlling factors a hot topic for discussion. The multiple tuff layers occurred frequently in high TOC sections of the member indicate intense volcanic events and a subtle connection between the events and the high TOC value. Analysis of main and trace elements of the shale confirms the impact of volcanic events as indicated by the relatively higher content of elements enriched in clay minerals like Al and K, of elements as proxy of paleo-productivity and reducing environment including Ni, Cr and V, as well as of high field strength elements (Zr, Th, and Hf). The upper parts of these tuff are even richer in organic matter with increasing hydrocarbon generation intensity that indicates the elevated paleoproductivity. There are trends of Fe_HR/Fe_T ≥ 0.38 and Fe_py/Fe_HR ≤ 0.8 in organic-rich shale but with Fe_py/Fe_HR up to 0.8 with the increase of TOC. The (EF_Mo/EF_U) (auth) ratios is 1-3 when the TOC is greater than 6 %. Both the iron speciation and (EF_Mo/EF_U) (auth) ratios indicate that there was an euxinic environment for Mo and Fepy enrichment, but the sulfate reduction strength was low (SRI ≤ 1.375). In summary, the input of volcanic materials and inorganic elements into the freshwater increased paleoproductivity and promoted the formation of a reducing environment. This is favorable for the organic-rich matter accumulation and preservation. The upper shales of the tuff-bearing section are suggested to be one of the key targets for future exploration and development in the basin.