Can seafloor carbonate weathering buffer ocean acidification in northwestern Greenland?

Arctic fjord ecosystems are sensitive to environmental changes such as accelerating warming and ocean acidification. In northwestern Greenland, fjords show strong contrasts in temperature, salinity, and pH due to differences in bathymetric sill depth and ocean circulation. Measurements conducted during the 2019 Ryder expedition onboard I/B Oden revealed a warm (<4 °C) and acidic (pH down to ~7.5) surface ocean from the outer part of Sherard Osborn Fjord. The surface ocean of Petermann Fjord, on the other hand, was colder (~0 °C) and less acidic (pH>8). These hydrographic differences could potentially affect baseline carbonate chemistry and sediment–water interactions. Here, we examine whether natural ocean acidification as a result of meltwater input can be buffered by carbonate weathering on the seafloor of northwestern Greenland, using controlled high-pressure flow-through incubations to simulate in-situ seafloor conditions.

Experiments were conducted at ca. 102 (+/- 1) bar and 6 (+/- 0.5) °C, hypothetical conditions simulating the intrusion of warm water. A custom-built high-pressure system consisting of three incubators, each containing paired upper and lower sand-packs (6 sand-packs in total), was used. The system was equipped with sapphire-window visual cells and fiber-optic pH sensors, enabling continuous monitoring of pH, temperature, pressure, and flow rate. Four sediment sand-packs (incubators 1 and 2) were filled with mixed sediments collected from the Petermann Fjord collected during the 2024 GEOEO North of Greenland Expedition onboard I/B Oden, spanning depths from the surface to 53 cm below seafloor, and incubated with IAPSO standard seawater. Sediment-free IAPSO seawater was incubated under the same conditions in incubator 3 as a control. During the three-month long experiments, inlet seawater was gradually acidified from pH 8.3 to 7.5 using 37% high purity HCl. Fluids were automatically collected six consecutive days per week using an auto-sampler throughout the experiments. Fluids were analyzed for major cations and anions by dual-channel ion chromatography (IC-dual). System performance and integrity were verified prior to the actual experiments by using silica gel and IAPSO seawater.

Preliminary results indicate a systematic increase in the Ca/Cl ratio responding to lower pH values in sediment-containing incubators compared to the controlled incubation, suggesting enhanced carbonate dissolution. Temporal variability in solute concentrations suggests dynamic fluid–sediment interactions and potentially coupled dissolution–precipitation processes during acidification. These findings provide experimental evidence that ongoing ocean acidification may promote carbonate dissolution in northwestern Greenland fjord sediments, with implications for coastal carbon cycling under ongoing ocean acidification.