Chemostratigraphy of the Mesoproterozoic Shennongjia Group, Yangtze Craton (South China): Implications for oxidized shallow seawaters

The relatively low Mesoproterozoic oxygen concentrations in atmospheric and marine environments remained controversial because the marine chemical evolution has not been sufficiently researched. To provide a better understanding of the secular redox conditions of shallow-water sedimentary environments and to perform global isotopic comparisons, we present new data of rare earth elements and yttrium (REE + Y), redox-sensitive elements (RSEs), and carbonate carbon-oxygen isotopes from Mesoproterozoic marine succession in the Shennongjia Group of the Yangtze Shennongjia area. Seawater-like REE + Y distribution patterns with significantly negative Ce anomalies, which had previously been screened to avoid detrital effects in the leached carbonates, illustrate the oxidation of shallow seawaters during the deposition of this group. Although the Ce/Ce* and (Pr/Yb)(SN) values remain relatively low in carbonate samples throughout the Mesoproterozoic, these values are higher than those of the modern oxygenated ocean, indicating less intense oxygenation. This conclusion is supported by the high enrichment factors of the RSEs in screened samples resulting from a considerable marine reservoir for dissolved RSEs in a constantly oxidized environment. Under these circumstances, the delta C-13(carb) chemostratigraphy is moderately varied, with values near +3.5 parts per thousand and lower than +5 parts per thousand, and more dynamic variability is observed over time, which is related to progressively smaller marine dissolved inorganic carbon (DIC) reservoirs. In addition, the significant negative excursion of delta C-13(carb) near to or less than 0 parts per thousand in this lower subgroup is globally matched in different blocks and may correspond to the oxidation of isotopically light organic matter in oceans. A worldwide comparison indicates that the delta C-13(carb) chemostratigraphies combined with limited geochronological data may constrain this extended negative excursion to ca. 1.3-1.2 Ga and the Shennongjia Group to ca. 1.3-1.1 Ga.