<p>The Shuram/DOUNCE event, the largest negative carbonate carbon isotope excursion in Earth’s history, has been linked to global ocean oxygenation and early animal evolution. While this anomaly is often attributed to dissolved organic carbon oxidation, the feasibility of this hypothesis remains debated due to uncertain oxidant supply. Here, we construct a refined global seawater δ¹³C curve for the event using an extensive carbonate δ¹³C database and published geochronological constraints. Applying an inverse approach within an Earth Evolution Model, we reassess the dissolved organic carbon oxidation timeline and oxidant demand. Our results show that oxidizing ~9×10¹⁹mol of dissolved organic carbon could account for the negative δ¹³C excursion, but cessation of oxidation alone cannot explain the rapid recovery, which requires enhanced productivity. These findings support continental sulfate as the major, sufficient oxidant source, while suggesting that a later period of enhanced productivity drove the rapid δ¹³C rebound.</p>