Responses of marine carbonate system to warming and sea-ice retreat in the pacific sector of the Arctic Ocean
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The Arctic Ocean, particularly the Pacific sector of the Arctic Ocean, plays an important role in the global carbon cycle. To evaluate temporal and spatial variations and controlling mechanisms of this ocean’s marine carbonate system in the responses to rapid sea ice retreat, two Chinese Arctic Research Expedition (CHINARE) cruises were conducted to measure dissolved inorganic carbon (DIC), total alkalinity (TAlk), and partial pressure of carbon dioxide (pCO2) in summer 2008 and 2010. Through a direct high accuracy and precision comparison of field measurements of pCO2 to pCO2 values calculated from bottle measurements of DIC and TAlk and seven sets of dissociation constants, we concluded that calculated values agreed best with field measurements using the carbonic acid dissociation constants of Mehrbach (1.5 ± 5.7 µatm) or the constants of Lueker (2.3 ± 5.4 µatm) in a temperature range of –1.5 to 10.5 °C and a salinity range of 25.8 to 33.1. Quasi-conservative surface TAlk distribution indicated that Rivers contributed the majority of freshwater on shelves while sea-ice meltwater dominated in the Canada Basin. Low TAlk river water and ice meltwater determined the majority of the distribution pattern of the carbonate system. Biologically related pCO2 and DIC were decreased by biological production on shelves and in the partially ice-covered basin areas, particularly around the margins of sea ice cover. Both mixing model approach and field observations revealed that atmospheric CO2 invasion is the controlling mechanism which sustains high surface pCO2 in an ice-free Canada Basin. Continuously increased surface pCO2 due to air-sea gas exchange rapidly decreased the uptake capacity of CO2 from the atmosphere. Additionally, carbon cycling in the water column has latitudinal features and responded to various stages of sea ice melt, showing CO2 was fixed at the beginning of sea ice melt, then the carbon cycling was modified due to accumulation of freshwater in the surface with limited air-sea gas exchange, and finally, carbon uptake capacity was decreased due to limited nutrient supply and enhanced stratification and atmospheric CO2 invasion.