An interglacial period existed between the Sturtian (ca. 720 ∼ 660 Ma) and the Marinoan glacial period (ca. 650 ∼ 635 Ma) in the Cryogenian. It is manifested as the Mn-bearing black rock series of the Datangpo Formation in the Nanhua rift basin, South China. In the Datangpo Formation in the Xiangtan area, a continuous mercury-rich stratum nearly 60 m thick was found, while in a previous study of the Wuhe section of the shallow sea shelf area and the Daotuo section of the semi-deep water slope area, no such thick mercury-rich stratum was reported. To explain this problem, comprehensive analyses were performed mainly focused on mercury and its isotopes, combined with total organic carbon (TOC), total sulfur (TS), and major and trace elements. The results show that the Hg content of all samples of the Datangpo Formation in the Xiangtan section is highly correlated with TOC and TS. Among them, the sandstone and Mn-ore units (Interval I) at the bottom are low in mercury content (19.78 ∼ 297.38 ppb), TOC and TS content, with the mercury isotope of Δ¹⁹⁹Hg close to 0 ‰ (0.02 ∼ 0.19‰, with 0.07‰ on average), which is relatively lower in the whole section. The black shale in the middle (Interval II) and upper parts (Interval III) of the section has high mercury contents (121.00 ∼ 721.58 ppb) and high TOC and TS contents, with positive Δ¹⁹⁹Hg values (0.20‰ on average). This reveals that a large dissolved organic carbon (DOC) reservoir existed in the deep-water area during the Datangpo period. The release of organic carbon and Hg is mainly controlled by the redox environment and is not affected by paleoproductivity. The persistent high Hg value and positive Δ¹⁹⁹Hg values can trace the existence of this DOC reservoir. Additionally, the sandstone-black shale-siltstone sequence of the Datangpo Formation from bottom to top shows a complete interglacial climate pattern characterized by initial warming followed by cooling, as well as a rise and subsequent fall in sea level. This study will help us to establish a more complete Hg cycle and redox process for the Datangpo period in the Nanhua basin.