The role of submarine groundwater discharge in the ocean carbon budget

Ocean chemistry is dictated by weathering and transporting elements from the land to the ocean and their removal through precipitation and adsorption. While the role of rivers was established many decades ago, other sources of elements such as mid-ocean ridge hydrothermal systems, discharge from forearcs of subduction zones, and submarine groundwater discharge (SGD) have been recognized more recently. SGD may release large amounts of trace metals, nutrients, carbon, and other dissolved species to the coastal ocean. The element fluxes may be comparable to surface water flow due to groundwater interaction with the aquifer sediments and the high ratio between rock and water. To better understand the coastal water budgets, it is crucial to assess all sources and sinks, including SGD. Recent attempts to calculate the dissolved inorganic carbon (DIC) and alkalinity ocean budgets have shown that riverine DIC input and marine carbonate burial cannot be balanced by alkalinity delivered via submarine groundwater. This deficit in the ocean's DIC and alkalinity mass balance may be attributed to insufficient knowledge of the carbonate and alkalinity contributions through SGD, particularly seawater circulation in the aquifer.

The DIC and alkalinity fluxes are influenced by the mechanisms driving groundwater flow in the subsurface (fresh and saline water), affecting the flow paths, residence times, and redox states. Therefore, we expect DIC enrichment or depletion to vary among different environmental settings. Thus, it is unclear if coastal aquifers serve as a source or a sink for DIC, depending on the settings. Our study shows a significant contribution of alkalinity by fresh groundwater discharge and also during long-term seawater circulation in the East Mediterranean coastal aquifer. Even on a relatively short distance like the Israeli Mediterranean coastline (~150 km), we observed differences in alkalinity and DIC derived from the shift in the aquifer's rocks as carbonate amounts drop and sand levels increase from north to south. To comprehensively and globally understand alkalinity fluxes through SGD, we generated an extensive data archive on coastal aquifers worldwide. This data is used to characterize the interactions between groundwater and country rocks depending on the type of rock and how they may impact groundwater alkalinity and DIC. Most of the groundwater samples lie below the 1:1 Alkalinity-DIC ratio line, which may suggest that the major processes affecting the carbonate system contribute more DIC than Alkalinity. Many sandy and carbonate aquifers have a DIC greater than alkalinity, which indicates a significant amount of CO₂ and low pH levels. In contrast, alluvial aquifers have a minor trend, and basaltic aquifers usually have DICs equal to alkalinities (the most common species is bicarbonate). To better understand climate feedback mechanisms, it is crucial to know the ocean's alkalinity buffer system budget and its carbon sources and sinks.