Magnesium Isotopes Reveal Gradually Intensified Silicate Weathering during the Sturtian Deglaciation

The termination of Cryogenian Snowball Earth glaciations is theoretically predicted to be driven by a massive silicate weathering feedback, which draws down pCO2 and creates the alkalinity required for global cap carbonate precipitation. However, unlike the Marinoan termination which is marked by ubiquitous cap carbonates, the earlier Sturtian termination lacks a comparable carbonate cap, raising fundamental doubts about the efficacy of the weathering feedback during this earlier deglaciation. Here we present high-resolution silicate-component δ26Mg records from a continuous terminal Sturtian siliciclastic succession in South China. We report a systematic, monotonic δ26Mg increase of >0.5‰ across two fining-upward glacial cycles, continuing into the basal post-Sturtian nonglacial strata. This excursion tracks progressive Mg depletion and mineralogical alteration, independent of lithofacies or grain-size variations, signaling a continuous intensification of silicate weathering during the deglaciation. Our results provide direct isotopic evidence that the canonical high pCO2 weathering feedback operated robustly during the Sturtian termination, despite the absence of a global sedimentary carbonate archive. This reveals a fundamental decoupling between continental weathering intensity and marine carbonate precipitation, highlighting the role of oceanographic and sedimentary boundary conditions (e.g., stratification, sediment routing and preservation) in modulating cap carbonate deposition.