Unravelling groundwater salinization and flushing in the Canterbury Bight during glacial-interglacial cycles: Insights from paleo-hydrogeochemical modeling

Offshore freshened groundwater (OFG) is well-documented in the shelf sediments of Canterbury Bight (New Zealand), with an estimated maximum volume of 213 km³, extending up to 60 km offshore from the coast. However, the evolution and emplacement dynamics of the OFG system remains poorly constrained. To advance the current state of understanding OFG systems, this study seeks to utilize the previously underutilized IODP geochemical and geological data from the Canterbury Bight to constrain the timing and emplacement mechanisms of the OFG system. Specifically, the main objectives of this paleo-hydrogeochemical transport-reaction modelling study are: (1) to identify key factors/processes influencing groundwater salinization and flushing in the continental shelf; (2) to improve understanding of the influences of OFG on subseafloor biogeochemical processes by transport-reaction modelling; (3) to explore the interactions between paleo-groundwater system and seawater; and (4) to propose a conceptual mode for shelf groundwater system evolution in relation to glacial/interglacial processes.

Preliminary results suggest that present-day recharge does not fully account for the OFG, particularly in the outer shelf, which is the fossil groundwater emplaced during the lowstands since the late Pleistocene. The intensified sulphate depletion observed in freshening sections is attributed to enhanced anaerobic oxidation of dissolved organic matter brought by the OFG. Modern salinity conditions are not in equilibrium with present-day sea level conditions, as the OFG is gradually being salinized through downward solute transport from overlying seawater. Submarine groundwater discharge and OFG volume are interconnected components of the offshore paleo-groundwater system, both closely tied to sea-level fluctuations. The findings from this study are expected to enhance our understanding of the Canterbury Bight’s offshore groundwater system and provide broader insights into OFG formation and evolution under changing climatic and sea-level conditions worldwide.