206 / 2025-04-14 21:39:10

An expansive Ediacaran lagoon: clay mineralogy and geochemistry evidences for the spatiotemporal extent of evaporitic conditions in the Doushantuo Formation

authigenic clay,Ediacaran Doushantuo Formation,restricted lagoon,evaporitic condition

Theme 4: Microbial and Biogeochemical Evolution through Time > Session 4c: Reconstructing Biogeochemical Cycles in Deep Time: Assumptions, Bias, Constraints, and Directions

The Ediacaran Doushantuo Formation (ca. 635–551 Ma) in South China has yielded a broad array of microfossil and isotope proxy records which underpin much of our understanding of the Earth System and biospheric evolution during a critical time period, while the depositional environment is still debated. Abundant authigenic Mg-rich saponite clay has been reported in the Yangtze Gorges Area as a proxy for local restricted, evaporitic lagoon setting ¹, while the spatiotemporal extent of restricted, evaporitic conditions across the Yangtze Block remains largely unexplored. Here we use mineralogical and petrographic approaches to systematically document spatiotemporal variations of clay mineralogy in the Yangtze Block, providing new constraints for depositional environments and further implication on early earth system.



Our results find that the open marine slope and basin settings do not contain Mg-rich clays and are dominated by illite clay, consistent with previous work which suggests that formation of Mg-rich clays require evaporitic, restricted conditions. In contrast, Mg-rich clays persistently occur in all shelf settings studied, including saponite, talc, sepiolite and palygorksite, implying that evaporitic restricted conditions were much more expansive than previous assumed. In addition to this spatial pattern, we identify a clear stratigraphic progression where the basal and uppermost Doushantuo Formation contains illite but Mg-rich clays appear in between, shortly above the cap carbonate. The results suggest an extensive carbonate rimmed, evaporitic lagoon developed on the Yangtze shelf shortly after the deposition of the cap carbonate, consistent with existing paleographic reconstructions ². Open marine conditions on the shelf only became re-established following erosion of the carbonate rim, most likely during the Gaskiers glaciation, and subsequent marine transgression.



Interestingly, we found the intervals containing authigenic Mg-rich clay, indicating evaporitic conditions, show more positive carbonate carbon isotope values and elevated carbonate Sr concentration. The geochemical variations are correlated to our clay mineralogical results, indicating different water chemistry between lagoon and the open ocean. Stable Sr isotope work is currently underway to further test the scenario, following the reconnaissance studies in a modern lagoonal system ³.



In summary, our findings illuminate hitherto unsuspected temporal changes in the depositional environment of the Doushantuo Formation. Restricted, evaporitic conditions were much more expansive than previously assumed, and likely influenced biogeochemical cycles and biological radiations of the Doushantuo Biota.