Microbe-forced Mn‑carbonate direct precipitation in Ediacaran micro-stromatolites of South China

The direct precipitation model of Mn‑carbonate formation, based on investigation of the modern sedimentary record and simulation experiments, is commonly used to explain the genesis of both modern and ancient Mn‑carbonate deposits. This process is considered to be primarily influenced by physicochemical conditions and lacks microbial mediation. Despite the established role of microbes in global Mn cycling and biomineralization, the specific contribution of microbial processes to Mn‑carbonate formation remains understudied. In this study, Ediacaran Mn‑carbonates from South China with well-preserved micro-stromatolites offer a novel insight into understanding the microbes involved in the formation of Mn-deposits. Petrological observations reveal that the laminated Mn-ores mainly consist of alternating layers of dark Mn‑carbonate and light-colored dolomite laminae. Mn‑carbonate minerals, dominantly rod-like rhodochrosite crystals, exhibit a close spatial relationship with micro-stromatolites, suggesting a possible link to microbial activity. Geochemical results display that these Mn‑carbonates document high δ13C values (average = −1.65 ‰) compared to typical diagenetic Mn‑carbonate indicating the seawater likely contributed to the carbon source. Combined with alabandite deposition, small-sized pyrite framboids, and positive Eu anomalies, the Ediacaran Mn‑carbonates may have formed by microbially-mediated direct precipitation in Mn-rich anoxic seawater. The Mn‑carbonate and micro-stromatolite laminae accumulated during a period of enhanced bacterial activity, driven by episodic inputs of hydrothermal Mn2+ and bioessential elements. The present study highlights the microbially-mediated significant role in the primary precipitation pathway of Mn‑carbonate. Direct precipitation of Mn‑carbonate deposits, controlled by ocean conditions as well as enhanced by microbial processes, may account for the formation of other ancient economic manganiferous sedimentary deposits.