Preliminary Mo isotope data of Phanerozoic clastic sediments from the northern margin of the Yangtze block and its implication for paleoenvironmental conditions

Mo isotopes along with Nd isotopes and other geochemical characteristics of the Phanerozoic clastic sediments from the northern margin of the Yangtze block were analyzed by MC-ICP-MS and some other techniques, spanning the period from Cambrian to Jurassic. The δ⁹⁸Mo values and ε_Nd(t) in these sedimets were observed to exhibit a large range of variation (−0.65‰2+1.87‰, −1.46–−10.90, respectively). Specifically, the sample from Late Permian Maokou Formation has relatively positive values in both δ⁹⁸Mo and ε_Nd(t) (+0.97‰, −1.46, respectively). These elevated values are proposed to relate to the input of a large quantity of H₂S and HS⁻ into the contemporaneous seawater by the volcanic eruption of the Emeishan mantle plume. Introduction of these sulfide gases would cause MoO₄ ²⁻ to be transferred into MoS₄ ²⁻, favoring the preferential deposition of the heavy Mo isotope as MoS₄ ²⁻ and thus leaving a positive Mo isotope value. The sample from Late Permian Changxing Formation has a significantly elevated δ⁹⁸Mo value of +1.87‰, in association with the relatively negative ε_Nd(t) (−10.90) and remain the relatively higher content of both TOC and total sulfur (TS) (1.52%, 2.02%, respectively). This infers the presence of the enhanced primary productivity and high organic burial during the Changxing period. The production of a huge quantity of HS⁻ and H₂S by the sulfate-reducing reaction led to the existence of the elevated seawater δ⁹⁸Mo values during the Changxing period. The preliminary Phanerozoic data reported here allow us to propose that the Mo isotope signature preserved in reductive sediments is related to the regional tectonic settings and the associated environmental conditions, and thus the measurement of δ⁹⁸Mo promotes our understanding on the evolution of the paleoenvironmental event in the Earth history. Supported by the program for the Ministry of Education of China and National Natural Science Foundation of China (Grant Nos. B07039, 40673020 and 40521001) and the SinoPec project (Grant No. G0800-06-ZS-319)