Offshore Groundwater Salinization and Flushing in the Canterbury Bight During Glacial‐Interglacial Cycles

Offshore freshened groundwater (OFG) is a significant yet underexplored component of global water resources, occurring widely along many continental margins. However, its dynamics remain poorly constrained due to limited offshore observations and the absence of robust evaluation models for unambiguous emplacement interpretation. To address this knowledge gap, we integrate existing porewater data and geological constraints from the Canterbury Bight with paleo‐hydrogeochemical modeling to investigate the origin, evolution, and biogeochemical processes related to OFG driven by sea‐level fluctuations. Our simulation results indicate that modern recharge from onshore aquifers contributes only a limited fraction of the present OFG. The majority of OFG has mean groundwater ages of less than ∼300 kyr, suggesting that it mainly originates from local meteoric recharge during sea‐level lowstands since the late Pleistocene. OFG volume and paleo‐submarine groundwater discharge along the continental shelf have varied periodically in response to sea‐level fluctuations throughout glacial‐interglacial cycles. The Canterbury Bight OFG system is currently in a non‐steady state, undergoing gradual salinization by downward intrusion from overlying seawater at an estimated rate of 0.06 cm yr −1 . The long‐term evolution of the OFG system has increased the complexity of biogeochemical processes within shelf sediments through freshwater–seawater mixing. By integrating paleo‐hydrogeological reconstruction, geological archives, and reactive transport modeling, this study provides a comprehensive framework for understanding OFG formation and evolution, advancing beyond previous models focused primarily on salinity.