Glacial meltwater impacts the marine carbonate system and acidification on the continental shelf off Iceland

Increased meltwater from glaciers may drive localized ocean acidification and CO₂ uptake, but the carbonate system dynamics in glacially impacted marine environments remains poorly understood. Here we investigate dissolved inorganic carbon (DIC) and total alkalinity (TA) at the land-ocean interface along Iceland’s glacially impacted shelf waters in June 2023. We examine the state and drivers of ocean acidification and air-sea CO₂ fluxes. TA in surface shelf waters varied between 1290 and 2340 μmol kg^-1 whereas DIC varied between 1460 and 2120 μmol kg^-1. The lowest values of both TA and DIC occurred by the outlet of a marine-terminating glacial lagoon. The shelf waters were a net CO₂ sink, taking up 2.4±1.4 μmol CO₂ m^-2 day^-1 on average. Aragonite saturation states (Ω_Ar) ranged between 0.1 - 5.8 (median = 2.4) in surface shelf waters with the majority of stations above 1, the threshold for solid CaCO₃ stability. Ω_Ar<1 were found by the outlets of the marine-terminating glacial lagoon and the largest glacial river. Freshwater inputs such as glacial rivers and groundwater discharging onto the shelf displayed [TA-DIC] values lower than those of the shelf, implying their potential to reduce the buffering capacity of the coastal ocean. The wide range of TA and DIC content of the freshwater endmembers (90-1230 μmol kg^-1 TA, 130-1220 μmol kg^-1 DIC) makes it challenging to disentangle the contributions of these endmembers to the shelf. Overall, the study improves our understanding of the marine carbonate system across a glacier-ocean continuum, indicating glacial meltwater as a major driver of acidification in these continental shelf waters.