Mo isotope records from Lower Cambrian black shales, northwestern Tarim Basin (China): Implications for the early Cambrian ocean

The widely developed black shales deposited during the early Cambrian recorded paleoenvironmental information about coeval seawater. Numerous studies have been conducted on these shales to reconstruct the paleomarine environment during this time period. However, most research has been conducted on stratigraphic sections in South China, and equivalent studies of sections from other cratons are relatively rare. Here, we report Mo isotopic compositions as well as redox-sensitive trace-element and iron (Fe) speciation data for black shales of the Lower Cambrian Yuertusi Formation from the Tarim block (i.e., a small craton). The Fe speciation data show high Fe_HR/Fe_T and Fe_py/Fe_HR ratios, indicating roughly sustained euxinic bottom-water conditions during their deposition. Based on Mo isotopic compositions (δ^98/95Mo), we further classified the euxinic black shales into two intervals: a lower interval (0−21.3 m) and an upper interval (21.3−32.3 m). The lower interval is characterized by variable Mo isotopic compositions (−2.12‰ to +0.57‰, mean = −0.52‰ ± 0.72‰), with an obvious negative excursion in its middle portion. The overlying upper interval has relatively heavy δ^98/95Mo values up to +1.42‰ (mean = +0.62‰ ± 0.37‰). We ascribe δ^98/95Mo differences in the lower and upper intervals to inadequate aqueous H₂S concentrations for quantitative thiomolybdate formation under euxinic conditions. The most negative Mo isotope excursion may have been caused by upwelling hydrothermal inputs during a transgression, consistent with significantly elevated total organic carbon (TOC) contents, Mo and U enrichments, and Fe supply. Relatively positive δ^98/95Mo values in the upper interval have roughly similar variations with other coeval sections, indicating such variations were common for early Cambrian euxinic deposits, and they were most likely caused by local differences in [H₂S]_aq. 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.