Geochemistry and molybdenum isotopes of the Lower Cambrian Yuertusi Formation in Tarim Basin（China）: Implications for ocean-redox conditions

Abstract

During the Early Cambrian epoch, a sequence of global marine transgressions culminated in the extensive deposition of organic-rich shales across the planet. This phenomenon was especially pronounced over the Tarim Block, where these organic-rich sediments accumulated significantly during this period. However, the paleomarine environment of it is still poorly constrained due to deep burial and the rare samples. We conducted an integrated study of iron speciation, redox-sensitive trace elements(RSTE), and Molybdenum(Mo) isotopes for samples from the Luntan 1 borehole, located in the Lunnan depression. Yuertusi Formation mainly includes mudstone in the lower member (LM) and limestone in the upper member (UM). Iron speciation data (FeHR/FeT, Fepy/FeT) suggest euxinic in the LM and Anoxic-ferruginous conditions in the UM. Whereas the trace-element and Mo isotopic data show more pronounced variation in redox conditions. The LM shows higher trace element concentrations and lower and more variable Mo isotopic composition relative to the UM. Ratios of redox-sensitive trace element(RSTE) concentrations to total organic carbon(TOC) are significantly more variable and higher on average in the LM relative to the UM. The stable and high Mo isotopic composition (about 1.7‰) in the UM may exist Fe-Mn shuttle, varial and lower Mo isotopic composition (about 0.9‰) in the LM reflect to [H2S]aqueous fluctuate with H2S<11μM. The consistency with RSTE/TOC and Mo isotopic composition in the LM implies frequent transient oxygenation during the early Cambrian. Additionally, the Compilation of Mo isotope data from the early Cambrian and earlier times further indicates relatively oxygenated seawater, especially the deep-marine areas during the early Cambrian before reaching a state like modern seawater.