The effect of seawater freshening on the marine carbonate system variability in the high Arctic fjords

The spatial variability in hydrography (salinity and temperature) and carbonate chemistry (alkalinity - A_T, total inorganic carbon concentration - C_T, pH, CO₂ partial pressure - pCO_2, and the saturation state of aragonite - Ω_Ar) in high meltwater season (summer) was investigated in four Spitsbergen fjords - Krossfjorden, Kongsfjorden, Isfjorden, and Hornsund. It was found that the differences in hydrology entail spatial changes in the CO₂ system structure. A_T decline with decreasing salinity was evident, however, this relationship was highly heterogenous. Significant surface water A_T variability (1889-2261 µmol kg^-1) suggests multiple freshwater sources having different alkalinity end-member values and biological processes occurring in the water column. Most of the A_T values were within the dilution lines of Ocean Water with freshwater having alkalinity from 0 to ~600 μmol kg^-1. However, the distribution of A_T against salinity suggests that locally the freshwater A_T­ maybe even higher. The effect of A_T fluxes from sediments on the bottom water was rather insignificant, despite high A_T values (2288-2666 μmol kg^-1) observed in the pore waters. Low pCO₂ results in surface water (200-295 μatm) points to intensive biological production, which can strongly affect the C_T values, however, is less important for shaping alkalinity. It has also been shown that the freshening of the surface water in the fjords reduces significantly Ω_Ar (an increase in freshwater fraction contribution by 1% causes a decrease in Ω_Ar by 0.022). Although during the polar day, due to low pCO₂, Ω_Ar values are still rather far from 1 (they ranged from 1.4 to 2.5), during polar night, when pCO₂ values are much higher, Ω_Ar may drop markedly.

This study highlights that the use of salinity to estimate the potential alkalinity can carry a high uncertainty, while good recognition of the surface water A_T variability and its freshwater end-members is key to predict marine CO₂ system changes along with the ongoing freshening of fjords waters due to climate warming.