The Tonian Period followed a long interval of relative stasis and led into the climatic extremes and biological radiations of multicellular life during the Cryogenian and Ediacaran Periods, respectively. However, despite its pivotal situation, it remains relatively understudied, in large part due to the lack of robust age constraints. A combination of fossil evidence, radiometric ages, and isotopic constraints reveal that carbonate strata on the North China craton were deposited between ca. 980 and ca. 920 Ma, thereby filling a gap in marine archives. Here we present 87Sr/86Sr data from selected calcite microspar cements, which filled early diagenetic “molar tooth” cracks, along with data from demonstrably well-preserved bulk carbonate samples. These new data show that seawater 87Sr/87Sr rose in stages from ∼0.7052 at ca. 980 Ma to ∼0.7063 by ca. 920 Ma, after which a return to low values coincided with the eruption of the Dashigou large igneous province across the North China craton. We also present a new Neoproterozoic seawater 87Sr/86Sr curve, which reveals that the general trend toward higher 87Sr/87Sr during the Tonian Period was checked repeatedly by the input of less-radiogenic strontium from a series of eruptive events, both coincident with and prior to the main breakup of Rodinia. The weathering of Tonian volcanic provinces has been linked to higher carbon burial, glaciation, and oxygenation due to the high phosphorus content of flood basalts. Here we show that the weathering of major volcanic provinces affected material fluxes and ocean chemistry much earlier than previously envisaged.