Sequential Leaching of Carbonate Rocks for Reconstructing Ediacaran Barium Systematics, South China

The Ediacaran Period represents a critical transitional interval in Earth history marked by profound environmental change and biological innovation, and the South China Craton preserves one of the most complete records of this transition. Barium (Ba) isotopes and concentrations preserved in marine carbonates potentially provide a promising tracer of biogeochemical processes, for example redox conditions, sulphur cycling and primary productivity. However, the accurate extraction of carbonate-associated barium (CABa) remains challenging due to contamination from non-carbonate fractions. This study aims to reconstruct Ediacaran Ba systematics by developing and applying a sequential leaching protocol to isolate CABa from carbonate rocks of Member Ⅱ and Member Ⅲ of the Doushantuo Formation at the Xiang’erwan section, South China. During the stepwise extraction procedure, an ammonium acetate prewash was first applied to remove the exchangeable fraction, followed by dilute acetic acid to extract the remaining exchangeable component and some CABa. A subsequent dilute acetic acid leach was then used to obtain the most representative, operationally defined CABa fraction. This approach allows for the evaluation of Ba sources and minimizes overprinting from non-carbonate phases. Results demonstrate that the exchangeable Ba fraction is characterized by distinctly high Ba/(Ca+Mg) ratios and low δ¹³⁷/¹³⁴Ba values, highlighting the need to remove it prior to CABa extraction. In the Xiang’erwan section, Ba/(Ca+Mg) ratios of the carbonate fraction remain relatively stable, ranging from 5.0×10-6 to 2.2×10-5, below the onset of the Shuram excursion. During the Shuram event, Ba/(Ca+Mg) ratios increase markedly, reaching values as high as 5.8×10-5. Correspondingly, δ¹³⁷/¹³⁴Ba values show a gradual increase from approximately -0.39 to -0.02‰ in the lower part of the section, followed by a sharp decrease to around -0.51‰ immediately preceding the Shuram excursion. Notably, δ¹³⁷/¹³⁴Ba variations display trends broadly similar to coeval carbonate δ¹³C values. These results suggest that Ba systematics are generally stable from Member Ⅱ to Member Ⅲ of the Doushantuo Formation, supporting the robustness of the CABa signal. The pronounced increase in Ba concentrations during the Shuram excursion may reflect enhanced Ba availability related to changes in ocean redox conditions and barite cycling during this interval.