69. Winter-to-spring evolution of Arctic Ocean acidification state in under-ice water and effect of sea-ice dynamics during N-ICE 2015 ice drift project

Agneta Fransson (1)*, Melissa Chierici (2), Mats Granskog (1), Daiki Nomura (3), Philipp Assmy (1), Paul Dodd (1), Anja Rösel (1), Anna Silyakova (4), Harald Steen (1)

1 Norwegian Polar Institute, Tromsø, Troms, 9296, Norway
2 Institute of Marine Research, Tromsø, Troms, 9294, Norway
3 Hokkaido University, Hakodate, Japan
4 Centre for Arctic Gas Hydrate, Environment and Climate (CAGE), The Arctic University of Tromsø, Norway

Background
Ocean acidification in the Arctic Ocean surface waters is affected by physical and biological processes such as sea-ice processes, freshwater supply, vertical mixing, gas fluxes, primary production and bacterial activity (Chierici et al., 2011; Fransson et al., 2013). However, there are few winter-to-spring investigations of the effect of sea-ice dynamics such as thin ice formation after opening of leads and brine rejection on the carbonate system and ocean acidification (OA) state in the underlying water. During the N-ICE 2015 Arctic Ocean drift study north of Svalbard (latitude 80° to 83°N, longitude 8°E to 28°E) onboard RV Lance, we gained unique data from winter to spring (January to June 2015).

Methods
We collected winter and spring data by sampling different types of sea ice, brine, and seawater. From all samples, we measured total inorganic carbon (DIC), total alkalinity (AT), nutrient concentrations (nitrate, phosphate, silicic acid), salinity and temperature. We calculated carbonate ion concentrations, calcium carbonate saturation state of aragonite and calcite (ΩAr and ΩCa), pH and pCO2 in the water column to investigate the seasonal evolution of sea-ice processes and effects on the carbonate system and OA state.

Findings
From winter (January-April) to spring (May-June), the carbonate system (e.g. DIC, pCO2, ΩAr, ΩCa), nutrients and salinity changed in the upper 100 meters due to changes in physical and biological processes and southward drift from Arctic water to Atlantic water. Vertical mixing, brine rejection, meltwater and primary production influenced the variability of the carbonate system and OA state in the mixed layer during the winter-to-summer season. Spring bloom in May-June caused nutrient uptakes, decreased DIC and increased pH. The calculated partial pressure of CO2 (pCO2) in the upper 100 m showed undersaturation in relation to the atmospheric CO2 level (~400 µatm) throughout the winter and spring.

Conclusions
It was evident that the several processes affected the carbonate system and OA state in the upper 100 m. Although we drifted through both Arctic water and Atlantic water (with different carbonate system composition) and CO2-rich brine was rejected from the ice, the surface-water pCO2 was undersaturated throughout the winter and spring relative to the atmospheric CO2 level. This indicates that during break-up of ice and ice melt there is a potential for uptake of atmospheric CO2. However, we need more investigations to understand the seasonal drivers of the air-sea CO2 fluxes and changes in Ω, in a changing climate.