Multimillennial climate-freshwater legacy beneath a large-river delta-front estuary

Offshore relatively fresh groundwater (ORFG; <10 grams per liter) represents an underexplored frontier in addressing global freshwater scarcity, yet its origin, persistence, and vulnerability remain poorly constrained. Here, we reveal a vast climate-driven freshwater reservoir beneath the Pearl River estuary and adjacent shelf. Integrating hydrochemical profiling, isotopic tracing, and groundwater dating, we demonstrate that this ORFG is a relic of ancient meteoric recharge, emplaced during Late Pleistocene sea-level lowstands and preserved beneath the seafloor for tens of millennia. Secular equilibrium in offshore ²²⁶Ra/²³⁰Th activity ratios, consistent ¹⁴C ages of 6 to 10 thousand years in onshore groundwater, and pronounced onshore-offshore contrasts in salinity and δ¹⁸O collectively confirm its fossil nature rather than slowly circulating modern groundwater. Numerical modeling further constrains the timescale of natural degradation, underscoring the system's resilience. These findings establish ORFG as not only as a long-lived archive of paleohydrological cycles but also as a potential strategic freshwater reserve, with substantial implications for coastal water security in a changing climate.