Using ocean chemical budgets and groundwater flow models to quantify SGD individual component fluxes

Coastal aquifers encompass a variety of flow patterns, with circulating seawater being a prominent phenomenon driven by diverse mechanisms of varying spatial and temporal scales. Understanding the volume of seawater circulation in aquifers is of great interest due to water-rock interactions and consequent modification of seawater composition. While the influence of circulating seawater on ocean biogeochemistry and ecology is well recognized, quantifying its actual effect is challenging due to the involvement of multiple mechanisms.

In this study, we employed element and isotope ocean budgets and groundwater flow modeling to quantify fluxes through specific mechanisms, enabling us to determine solute fluxes from coastal aquifers into the sea. Our budgets included all known ocean sources and sinks, such as river fluxes, mid-ocean ridge hydrothermal circulation, basalt weathering, and diffusive fluxes. We used major element budgets and δ²⁶Mg and ⁸⁷Sr/⁸⁶Sr budgets to constrain the long-term SGD flux. Our sensitivity tests included steady-state conditions and scenarios where the hydrothermal fluxes and rivers were over or underestimated, and precipitation of carbonates was over-estimated. We found the steady-state, underestimation of both hydrothermal and river fluxes, and overestimation of carbonate fluxes reasonable scenarios.

Our findings, using groundwater flow models sensitivity tests and geospatial databases, reveal that benthic wave-driven circulation contributes the largest volume of circulating seawater, while other mechanisms such as nearshore circulation, tidal pumping, and density-driven circulation are 2-3 orders of magnitude smaller. However, the short duration (minutes) of water-rock interaction under the wave-driven circulation limits its potential to modify seawater. Long-term density-dependent circulation emerges as the most significant mechanism influencing ocean chemistry, primarily due to its extended time scale of water-rock interaction. The prevailing water-rock interaction process in coastal aquifers is identified as ion exchange, wherein circulating seawater returning to the sea becomes enriched in calcium and depleted in potassium and sodium. The annual volume of seawater circulating through the long-term process is calculated to be approximately 1000-2000 km³/y, resulting in calcium and potassium fluxes of 17 ± 6 and -3.36 ± 1.6 Tmol/y, respectively. Notably, these fluxes are of the same magnitude as solute fluxes from rivers.