Elemental Mapping Reveals Selective Dolomitization in Tonian Stromatolites: Implications for Early Diagenesis and Paleoenvironmental Proxies

Abstract Stromatolites archive critical information on Precambrian marine environments, but their geochemical signals are often obscured using complex diagenetic processes. Tonian stromatolites from the Weiji Formation, North China, show selective dolomitization in dark stromatolitic laminae, forming zoned dolomites. The zoned dolomite crystals with well‐preserved growth zoning record the evolution of diagenetic fluid, yet their genesis remains controversial. To constrain selective dolomitization processes and zoned dolomite formation in stromatolites, and further assess their reliability as paleoceanographic proxies, we use LA‐ICP‐TOF‐MS elemental mapping to reveal crystallization mechanisms and early diagenetic controls in stromatolitic carbonates. Results show distinct geochemical distribution patterns between dark and light stromatolitic laminae, reflecting complex microbial‐induced diagenetic reactions during early diagenesis. Fe is preferentially concentrated in dark stromatolitic layers, while Mn anomalously accumulates in light layers (with Mn/Fe ratio up to ∼2). Meanwhile, high terrigenous‐indicative elemental contents (Al‐Si‐K) are observed in dark laminae and the adjacent clay‐rich matrix. The anomalous Fe‐Mn distribution is attributed to the redox oscillations and terrigenous pulses (ROTP) model. Selective dolomitization proceeds through an Ion‐Exchange Motor mechanism during penecontemporaneous to early diagenesis, where Mg 2+ derived from seawater undergoes dehydration via clay mineral adsorption and/or microbial mediation. Within ferroan zoned dolomite, elemental zoning exhibits alternating Mn‐enriched bands and Fe‐rich zones, indicating redox‐controlled diagenesis with preferential Mn(IV) reduction prior to Fe(II) mobilization. While Precambrian stromatolites remain valuable proxies for paleo‐ocean chemistry, our results emphasize the critical need for in situ analytical approaches to distinguish primary signals from diagenetic overprints in Precambrian carbonate systems.