HYDROTHERMAL ACTIVITIES AFFECTING P/FE RATIO OF IRONSTONE

Phosphorus is a biolimiting nutrient element that ultimately regulates marine primary productivity in the geological scale. Previous studies show that the ratio of phosphorus to iron [expressed as (P/Fe)₁₀₀] in ironstones can be used to track dissolved phosphate concentration in paleoceans, given known dissolved silica concentration in seawater. High P/Fe ratios of ironstones from the Cryogenian Period (720-635 Ma) may imply seawater phosphorus concentration, which might have driven the subsequent oxygenation and evolution in the Ediacaran Period. In this study, we measured the elemental compositions of banded iron formation (BIF) from the basal Fulu Formation in South China. Regional stratigraphic correlation indicates that the lower part of the Fulu Formation represents the Sturtian glacial deposits. Thus, the Fulu BIF might be correlated with the ironstones from other localities (e.g. Tinder Formation). P/Fe of the Fulu BIF ranges from 0.08 to 1.68 (average=0.54), significantly lower than that of ironstones of equivalent age (average=1.98). In addition, Ge/Si ratios of the Fulu BIF range from 13.8 to 189.2 µmol/mol (average =99.19), significantly higher than the modern seawater value of 0.72, suggesting strong influence of hydrothermal fluids with high Ge/Si. There is a negative correlation between P/Fe and Ge/Si, i.e. BIF samples with higher Ge/Si have lower P/Fe. If P was mainly sourced from seawater, hydrothermal activity would elevate Ge/Si and Fe budget in the ocean, and accordingly lowering the P/Fe of ironstones. Therefore, we suggest that P/Fe ratio of ironstones is not only determined by seawater P content, but would also be affected by some local factors, such as hydrothermal activities. Finally, our study may also explain the scattering of the P/Fe data when compiling the global data together.