Prolonged aragonite undersaturation in bottom water of a biological hotspot in the Chukchi Sea, Arctic Ocean

Abstract

<p><strong>Abstract.</strong> Distribution of calcium carbonate saturation state (Ω) was observed in bottom waters of the Chukchi Sea in autumn 2012 and early summer 2013. Aragonite and calcite undersaturation were 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 factor controlling 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 two-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. Results showed prolonged undersaturation for both aragonite and calcite even during winter period, not only in 2012 but also in 2013. Calculations also suggest that bottom water in the hotspot could have been undersaturated for aragonite on an intermittent basis even in the pre-industrial period. Anthropogenic CO<sub>2</sub> has extended the period of aragonite undersaturation to two- or three-fold longer by now. When atmospheric pCO<sub>2</sub> increases to 650 ppm, occupation of aragonite and calcite undersaturation can be as long as two-third and one-third of a year, respectively. Anthropogenic CO<sub>2</sub> has significant impact on duration of CaCO<sub>3</sub> undersaturation in the bottom water of the Chukchi Sea, even though horizontal and seasonal variability in Ω is controlled by biological processes.</p>