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

Abstract

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., Ωaragonite < 1.0), thereby exposing the benthos to potentially corrosive water for CaCO3-secreting organisms, while 80% of bottom waters present had Ωaragonite values less than 1.5. Our observations indicate seasonal reduction of saturation states (Ω) for calcite (Ωcalcite) and aragonite (Ωaragonite) 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.

Improved routines to model the ocean carbonate system: mocsy 2.0

“…Although f CO 2 and pCO 2 are typically measured or calculated at the surface, e.g., to estimate air-sea CO 2 fluxes, a limited number of studies also focus on those quantities below the surface (e.g., Brewer and Peltzer, 2009;Cocco et al, 2013;Bates et al, 2013). Here we recommend that future studies concerned with subsurface values should choose one of two options: (1) compute in situ f CO 2 and pCO 2 after making pressure corrections to K 0 and the fugacity coefficient following Weiss (1974) using total in situ pressure and in situ temperature or (2) calculate potential f CO 2 and pCO 2 at 1 atm pressure while using potential instead of in situ temperature.…”

A Time-Series View of Changing Ocean Chemistry Due to Ocean Uptake of Anthropogenic CO2 and Ocean Acidification

“…The slow acidification of the surface ocean is evident at the seven ocean CO 2 time-series sites (Figures 4c and 7; Bates et al, 2009Bates et al, , 2013.…”

State of the Climate in 2016

“…In some areas like the Arctic, the pH conditions observed today are now corrosive to biologically important carbonate minerals. Some studies indicate that these corrosive conditions can cover up to 40% of the Chukchi Sea benthos seasonally (Bates et al 2013), and persist for 80% of the year in some hotspots (Cross et al 2017).…”

“…Recently, several comprehensive data synthesis products (Bates 2015;Cross et al 2017;Semiletov et al 2016;Qi et al 2017) were published using much of the available OA data collected in the Arctic Ocean. Several trends have emerged that clearly elucidate the rapid progression of OA across the Arctic Basin, including rapid CO2 uptake from the atmosphere and increasing carbonate mineral corrosivity (e.g., Evans et al 2015).…”