Summertime calcium carbonate undersaturation in shelf waters of the western Arctic Ocean – how biological processes exacerbate the impact of ocean acidification

Abstract. The Arctic Ocean accounts for only 4% of the global ocean area, but it contributes significantly to the global carbon cycle. Recent observations of seawater CO₂-carbonate chemistry in shelf waters of the western Arctic Ocean, primarily in the Chukchi Sea, from 2009 to 2011 indicate that bottom waters are seasonally undersaturated with respect to calcium carbonate (CaCO₃) minerals, particularly aragonite. Nearly 40% of sampled bottom waters on the shelf have saturation states less than one for aragonite (i.e., Ω_aragonite < 1.0), thereby exposing the benthos to potentially corrosive water for CaCO₃-secreting organisms, while 80% of bottom waters present had Ω_aragonite values less than 1.5. Our observations indicate seasonal reduction of saturation states (Ω) for calcite (Ω_calcite) and aragonite (Ω_aragonite) in the subsurface in the western Arctic by as much as 0.8 and 0.5, respectively. Such data indicate that bottom waters of the western Arctic shelves were already potentially corrosive for biogenic and sedimentary CaCO₃ for several months each year. Seasonal changes in Ω are imparted by a variety of factors such as phytoplankton photosynthesis, respiration/remineralization of organic matter and air–sea gas exchange of CO₂. Combined, these processes either increase or enhance in surface and subsurface waters, respectively. These seasonal physical and biological processes also act to mitigate or enhance the impact of Anthropocene ocean acidification (OA) on Ω in surface and subsurface waters, respectively. Future monitoring of the western Arctic shelves is warranted to assess the present and future impact of ocean acidification and seasonal physico-biogeochemical processes on Ω values and Arctic marine ecosystems.