Abstract
Abstract. Distribution of calcium carbonate saturation state (Ω) was observed in the Chukchi Sea in autumn 2012 and early summer 2013. Ω in bottom water ranged from 0.3 to 2.0 for aragonite and from 0.5 to 3.2 for calcite in 2012. In 2013, Ω in bottom water was 1.1–2.8 for aragonite and 1.7–4.4 for calcite. Aragonite and calcite undersaturation was found in high productivity regions in autumn 2012 but not in early summer 2013. Comparison with other parameters has indicated that biological processes – respiration and photosynthesis – are major factors controlling the regional and temporal variability of Ω. From these ship-based observations, we have obtained empirical equations to reconstruct Ω from temperature, salinity and apparent oxygen utilization. Using 2-year-round mooring data and these equations, we have reconstructed seasonal variation of Ω in bottom water in Hope Valley, a biological hotspot in the southern Chukchi Sea. Estimated Ω was high in spring and early summer, decreased in later summer, and remained relatively low in winter. Calculations indicated a possibility that bottom water could have been undersaturated for aragonite on an intermittent basis even in the pre-industrial period, and that anthropogenic CO2 has extended the period of aragonite undersaturation to more than 2-fold longer by now.
Highlights
During the last decade, ocean acidification due to uptake of anthropogenic carbon dioxide (CO2) has emerged as an urgent issue in ocean research (e.g., Raven et al, 2005; Orr et al, 2005)
Considering that the mooring site is located in a biological hotspot where the lowest was observed in autumn (Figs. 1 and 3), the total time of undersaturation estimated here is likely at a maximum within the Chukchi Sea
In the future case with an atmospheric pCO2 of 650 ppm, the period of undersaturation is estimated to increase further to > 11 months for aragonite and half a year for calcite. This analysis indicates that anthropogenic CO2 has a significant impact on the duration of CaCO3 undersaturation in the bottom water even though seasonal and interannual variations of are mainly controlled by biological processes
Summary
Ocean acidification due to uptake of anthropogenic carbon dioxide (CO2) has emerged as an urgent issue in ocean research (e.g., Raven et al, 2005; Orr et al, 2005). Cold water dissolves more CO2, large freshwater inputs from rivers and sea ice melt reduce calcium ion concentrations and alkalinity, the buffering capacity of seawater to added CO2 (Salisbury, 2008; Yamamoto-Kawai et al, 2011), and respiration at the bottom of a salt stratified water column accumulates CO2 in bottom water (Bates et al, 2009) Because of these characteristics, both surface and bottom waters of Arctic shelf seas exhibit naturally low compared to other ocean waters (e.g., Fabry et al, 2009; Mathis et al, 2015; Yamamoto-Kawai et al, 2013). By comparing with distributions of physical and biogeochemical parameters, factors controlling will be discussed Based on these results, we attempt to reconstruct seasonal variations of in the bottom water in the southern Chukchi Sea by using data from a 2-year-round mooring observation between July 2012 and July 2014
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