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

摘要

The Arctic Ocean accounts for only 4% of the global ocean area, but itcontributes significantly to the global carbon cycle. Recent observations ofseawater CO₂-carbonate chemistry in shelf waters of the western ArcticOcean, primarily in the Chukchi Sea, from 2009 to 2011 indicate that bottomwaters are seasonally undersaturated with respect to calcium carbonate(CaCO₃) minerals, particularly aragonite. Nearly 40% of sampledbottom waters on the shelf have saturation states less than one for aragonite(i.e., Ω_aragonite < 1.0), thereby exposing the benthos topotentially corrosive water for CaCO₃-secreting organisms, while 80%of bottom waters present had Ω_aragonite values less than1.5. 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 asmuch as 0.8 and 0.5, respectively. Such data indicate that bottom waters ofthe western Arctic shelves were already potentially corrosive for biogenicand sedimentary CaCO₃ for several months each year. Seasonal changes inΩ are imparted by a variety of factors such as phytoplanktonphotosynthesis, respiration/remineralization of organic matter and air–seagas exchange of CO₂. Combined, these processes either increase or enhancein surface and subsurface waters, respectively. These seasonal physical andbiological processes also act to mitigate or enhance the impact ofAnthropocene ocean acidification (OA) on Ω in surface and subsurfacewaters, respectively. Future monitoring of the western Arctic shelves iswarranted to assess the present and future impact of ocean acidification andseasonal physico-biogeochemical processes on Ω values and Arcticmarine ecosystems.