Abstract: Subterranean estuaries (STEs) are critical biogeochemical reaction zones at the land-ocean interface, but their role in the lithium (Li) cycle and the associated isotope fractionation remains largely unexplored. Here we present a comprehensive investigation of Li cycling in a sandy STE at Qiaogang Beach (Beihai, China), based on integrated measurements of dissolved and solid-phase Li concentrations, stable isotope ratios (δ⁷Li), and key groundwater physicochemical parameters. Our results reveal pronounced non-conservative behavior of Li accompanied by resolvable δ⁷Li variations from +29.1‰ to +34.4‰ across the salinity gradient. Sequential extraction data demonstrate that Li is predominantly hosted in non-silicate phases, with carbonates (22–33 μg g⁻¹; δ⁷Li ≈ +15‰ to +18‰) and Fe (oxyhydr)oxides (1–9 μg g⁻¹; δ⁷Li ≈ +1‰ to +7‰) constituting the two major reservoirs. Redox-driven dissolution and reprecipitation of Fe (oxyhydr)oxides exert a first-order control on Li isotope fractionation, whereas elevated Li concentrations in the freshwater-dominated zone are primarily supplied by terrestrial freshwater recharge rather than in situ carbonate dissolution. After correcting for net non-conservative shifts, fresh submarine groundwater discharge (FSGD) exports Li at a rate of 1.94 ± 0.08 × 10⁶ g yr⁻¹ with a δ⁷Li value of +29.1‰, equivalent to ~19% of the local riverine Li flux. More significantly, total SGD delivers a substantially larger Li flux (4.32 ± 0.58 × 10⁸ g yr⁻¹) than riverine inputs, with a bulk δ⁷Li of approximately +30.2‰. By identifying this previously overlooked heavy-isotope source, we propose that STEs function as active "Li reactors" that provide a mechanistic explanation for the persistently heavy isotopic composition of seawater relative to known continental inputs. Although our estimates derive from a single site, they highlight a process that warrants explicit consideration in future global-scale oceanic Li budget models.
Keywords: Subterranean estuary; Lithium isotopes; Iron (oxyhydr)oxide cycling; Carbonate weathering; Marine Li budget

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