Calcium carbonate saturation in the surface water of the Arctic Ocean: undersaturation in freshwater influenced shelves

Abstract. In the summer of 2005, we sampled surface water and measured pH and total alkalinity (A_T) underway aboard IB Oden along the Northwest Passage from Cape Farewell (South Greenland) to the Chukchi Sea. We investigated the variability of carbonate system parameters, focusing particularly on carbonate concentration [CO₃²⁻] and calcium carbonate saturation states, as related to freshwater addition, biological processes and physical upwelling. Measurements on A_T, pH at 15°C, salinity (S) and sea surface temperature (SST), were used to calculate total dissolved inorganic carbon (C_T), [CO₃²⁻] and the saturation of aragonite (ΩAr) and calcite (ΩCa) in the surface water. The same parameters were measured in the water column of the Bering Strait. Some surface waters in the Canadian Arctic Archipelago (CAA) and on the Mackenzie shelf (MS) were found to be undersaturated with respect to aragonite (ΩAr<1). In these areas, surface water was low in A_T and C_T (<1500 μmol kg⁻¹) relative to seawater and showed low [CO₃²⁻]. The low saturation states were probably due to the likely the effect of dilution due to freshwater addition by sea ice melt (CAA) and river runoff (MS). High A_T and C_T and low pH, corresponded with the lowest [CO₃²⁻], ΩAr and ΩCa, observed near Cape Bathurst and along the South Chukchi Peninsula. This was linked to the physical upwelling of subsurface water with elevated CO₂. The highest surface ΩAr and ΩCa of 3.0 and 4.5, respectively, were found on the Chukchi Sea shelf and in the cold water north of Wrangel Island, which is heavily influenced by high CO₂ drawdown and lower C_T from intense biological production. In the western Bering Strait, the cold and saline Anadyr Current carries water that is enriched in A_T and C_T from enhanced organic matter remineralization, resulting in the lowest ΩAr (~1.2) of the area.