Iron speciation in organic-rich shales from the Upper Triassic Yanchang Formation, Ordos Basin, Northern China: Implications for depositional environment

Abstract

The iron (Fe) cycle is one of the most important marine and lacustrine geochemical cycles and is closely related to environmental redox conditions, which directly affects carbon, sulfur, phosphorus cycling. Analysis of Fe speciation (including Feox, Femag, Fecarb, and Fepy) of organic-rich shale samples provides new insights into the controversial redox conditions during the Chang 7 sedimentary period. Our results show that Fecarb and Fepy of the Chang 7 organic-rich shale are the main forms of reactive Fe pools, indicating that reactive Fe mainly existed in the form of Fe2+ in sediments. With a few exceptions, most samples fall into the ferruginous region in crossplots of Fepy/FeHR versus FeHR/FeT, indicating that pore water environments during early diagenesis were ferruginous accompanied by intermittent euxinic. Previous works examining the size of framboidal pyrite, the molar ratio of organic carbon and total phosphorus (P), and nitrogen isotope values suggest the bottom water environments conditions were oxic-suboxic accompanied by intermittent anoxic. We argue these patterns may be explained by the proximity of the redox boundary of depositional environment to the sediment–water interface during most of the Chang 7 period, with short-term fluctuations. Euxinic pore water conditions correspond to anoxic bottom water conditions and elevated total organic carbon (TOC) content, indicating that these environmental conditions would increase the flux of P diffused from sediments. As a result, lacustrine primary productivity would be elevated, creating conditions conducive to the accumulation of organic matter that is characteristic of the Chang 7 sedimentary period. This work will further our understanding of the accumulation mechanism of organic matter in the Chang 7 shale, the principal source rock for the Mesozoic oil-bearing system in the Ordos Basin.