Submarine groundwater discharge governs dissolved carbon cycling: Multi-scale evidence from a large river-dominated groundwater-saltmarsh-estuary system.

The groundwater-saltmarsh-estuary continuum plays a critical role in regulating coastal carbon cycling, but the influence of submarine groundwater discharge (SGD) on carbon dynamics, particularly in large river estuaries, remains poorly understood. To address this knowledge gap, this study presented a comprehensive reassessment of carbon budgets along the Yangtze River Estuary continuum. Based on the radium-derived SGD measurements, dissolved carbon through SGD was significantly lower than carbon burial within small-scale tidal creek systems, but its contribution escalated to 1.5 times carbon burial when expanding to the entire saltmarsh wetland scale, underscoring the disproportionate role of SGD in modulating saltmarsh carbon sequestration dynamics. Remarkably, SGD contributed to ∼65 % of the total carbon sources in the estuary, indicating its profound control over carbon budgets throughout the continuum. These findings emphasize the presence of SGD-derived carbon outwelling as a novel marine carbon cycling mechanism in the continuum. This study not only enhances our understanding of the carbon dynamics in the Yangtze River Estuary but also provides valuable insights into the significance of the groundwater-saltmarsh-estuary continuum in major river estuaries globally.