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Estimates of Ikaite Export from Sea Ice to the Underlying Seawater in a Sea Ice–Seawater Mesocosm

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Estimates of Ikaite Export from Sea Ice to the Underlying Seawater in a Sea Ice–Seawater Mesocosm The Cryosphere, 10, 2173–2189, 2016 www.the-cryosphere.net/10/2173/2016/ doi:10.5194/tc-10-2173-2016 © Author(s) 2016. CC Attribution 3.0 License. Estimates of ikaite export from sea ice to the underlying seawater in a sea ice–seawater mesocosm Nicolas-Xavier Geilfus1,2, Ryan J. Galley1, Brent G. T. Else3, Karley Campbell1, Tim Papakyriakou1, Odile Crabeck1, Marcos Lemes1, Bruno Delille4, and Søren Rysgaard1,2,5 1Centre for Earth Observation Science, University of Manitoba, Winnipeg, Canada 2Arctic Research Centre, Aarhus University, Aarhus, Denmark 3Department of Geography, University of Calgary, Calgary, Canada 4Unité d’Océanographie Chimique, Université de Liège, Liège, Belgium 5Greenland Climate Research Centre, Greenland Institute of Natural Resources, Nuuk, Greenland Correspondence to: Nicolas-Xavier Geilfus ([email protected]) Received: 19 January 2016 – Published in The Cryosphere Discuss.: 11 February 2016 Revised: 12 August 2016 – Accepted: 15 August 2016 – Published: 21 September 2016 Abstract. The precipitation of ikaite and its fate within sea sea ice began to melt. Ikaite crystal dissolution in the water ice is still poorly understood. We quantify temporal inorganic column kept the seawater pCO2 undersaturated with respect carbon dynamics in sea ice from initial formation to its melt to the atmosphere in spite of increased salinity, TA and TCO2 in a sea ice–seawater mesocosm pool from 11 to 29 Jan- associated with sea ice growth. Results indicate that ikaite uary 2013. Based on measurements of total alkalinity (TA) export from sea ice and its dissolution in the underlying sea- and total dissolved inorganic carbon (TCO2), the main pro- water can potentially hamper the effect of oceanic acidifica- cesses affecting inorganic carbon dynamics within sea ice tion on the aragonite saturation state (aragonite) in fall and were ikaite precipitation and CO2 exchange with the atmo- in winter in ice-covered areas, at the time when aragonite is sphere. In the underlying seawater, the dissolution of ikaite smallest. was the main process affecting inorganic carbon dynamics. Sea ice acted as an active layer, releasing CO2 to the atmo- sphere during the growth phase, taking up CO2 as it melted and exporting both ikaite and TCO2 into the underlying sea- 1 Introduction water during the whole experiment. Ikaite precipitation of up to 167 µmolkg−1 within sea ice was estimated, while its ex- Currently, each year, 7 Pg anthropogenic carbon is released port and dissolution into the underlying seawater was respon- to the atmosphere, 29 % of which is estimated to be taken sible for a TA increase of 64–66 µmolkg−1 in the water col- up by the oceans through physical, chemical and biologi- umn. The export of TCO2 from sea ice to the water column cal processes (Sabine et al., 2004). The Arctic Ocean takes −1 −1 increased the underlying seawater TCO2 by 43.5 µmolkg , up −66 to −199 TgCyear , (where a negative value indi- suggesting that almost all of the TCO2 that left the sea ice cates an uptake of atmospheric CO2) contributing 5–14 % was exported to the underlying seawater. The export of ikaite to the global ocean CO2 uptake (Bates and Mathis, 2009), from the ice to the underlying seawater was associated with primarily through primary production and surface cooling brine rejection during sea ice growth, increased vertical con- (MacGilchrist et al., 2014). However, polar ocean CO2 up- nectivity in sea ice due to the upward percolation of seawa- take estimates consider sea ice as an impermeable barrier, ter and meltwater flushing during sea ice melt. Based on the ignoring the potential role of ice-covered areas on gas ex- change in TA in the water column around the onset of sea ice change between the ocean and atmosphere. Recent studies melt, more than half of the total ikaite precipitated in the ice have shown that sea-ice-covered areas participate in the vari- during sea ice growth was still contained in the ice when the able sequestration of atmospheric CO2 into the mixed layer below the ice (e.g., Papakyriakou and Miller, 2011; Geil- Published by Copernicus Publications on behalf of the European Geosciences Union. 2174 N.-X. Geilfus et al.: Estimates of ikaite export from sea ice to the underlying seawater fus et al., 2012, 2014, 2015; Nomura et al., 2013; Delille and drive a CO2 uptake from the atmosphere to the ice. Such et al., 2014). Studies are required to elucidate the processes a mechanism could be an effective sea ice pump of atmo- responsible for this as well as their magnitudes, both tempo- spheric CO2 (Delille et al., 2014). In addition, ikaite stored rally and spatially. in the ice matrix could become a source of TA to the near- The carbonate chemistry in sea ice and brine is spatially surface ocean upon its subsequent dissolution during sea ice and temporally variable, which leads to complex CO2 dy- melt (Rysgaard et al., 2007, 2009). namics with the potential to affect the air–sea CO2 flux (Par- The main air–sea fluxes of CO2 and TCO2 are driven mentier et al., 2013). Release of CO2 from sea ice to the at- by brine rejection to the underlying seawater and its con- mosphere has been reported during sea ice formation from tribution to intermediate and deep-water formation (Semile- open water (Geilfus et al., 2013a) and in winter (Miller et tov et al., 2004; Rysgaard et al., 2007, 2009; Fransson et al., 2011; Fransson et al., 2013), while uptake of CO2 by al., 2013) or below sea ice in ice tank studies (e.g., Killawee sea ice from the atmosphere has been reported after sea ice et al., 1998; Papadimitriou et al., 2004). As sea ice thick- melt onset (e.g., Semiletov et al., 2004; Nomura et al., 2010, ens, reduced near-surface ice temperatures result in reduced 2013; Geilfus et al., 2012, 2014, 2015; Fransson et al., 2013). brine volume content, increased brine salinity and increased In combination, these works suggest that the temporal cy- solute concentration in the brine. In the spring–summer, as cle of sea ice formation and melt affects atmospheric CO2 the ice temperature increases, sea ice brine volume increases uptake by the ocean in variable ways. Sea ice may also and sea ice becomes vertically permeable to liquid (Golden act as an important control on the partial pressure of CO2 et al., 2007), enhancing the potential CO2 exchange between (pCO2) in the sea surface through a sea ice pump (Rys- the atmosphere, sea ice and ocean. Eventually internal ice gaard et al., 2007). During the earliest stages of sea ice for- melt promotes brine dilution, which decreases brine salin- mation, a small fraction of CO2-supersaturated brine is ex- ity, TA and TCO2, and leads to lower pCO2 in the brine. pelled upward onto the ice surface, promoting a release of In addition, the dissolution of ikaite decreases brine pCO2 CO2 to the atmosphere (Geilfus et al., 2013a). As sea ice (Reaction R1) (Geilfus et al., 2012, 2015). These conditions forms and grows thicker, salts are partly rejected from the all favor sea ice as a sink for atmospheric CO2 (Nomura et sea ice to the underlying seawater and partly trapped within al., 2010, 2013; Geilfus et al., 2012, 2015). Melting sea ice the sea ice structure, concentrated in brine pockets, tubes and stratifies surface seawater, leading to decreased TA, TCO2 channels. As a result, the concentration of dissolved salts, in- and pCO2, in the sea surface, enhancing air–sea CO2 fluxes cluding inorganic carbon, increases within the brine and pro- (Rysgaard et al., 2007, 2009). motes the precipitation of calcium carbonate crystals such as Although we now have a basic understanding of the key ikaite (CaCO3 6H2O) (Marion, 2001). These crystals have mechanisms of carbon cycling in sea ice, significant un- been reported inq both natural (Dieckmann et al., 2008; No- knowns remain. One of the major unknowns is the fate of mura et al., 2013; Søgaard et al., 2013) and experimental sea ikaite, TCO2 and CO2 released from sea ice during winter. ice (Geilfus et al., 2013b; Rysgaard et al., 2014) and have It is unclear what proportion of precipitated ikaite crystals in been suggested to be a key component of the carbonate sys- sea ice remains in the matrix to be released upon melt or what tem (Rysgaard et al., 2007; Fransson et al., 2013; Delille et proportion is expelled with brine drainage during ice forma- al., 2014). tion and growth. Examining the chemical signatures of the During ikaite precipitation within sea ice, total alkalinity water column beneath sea ice may provide an indication of (TA) in brine is reduced by 2 moles due to the reduction of the importance of the different processes. However, the sig- − bicarbonate (HCO3 ) while total dissolved inorganic carbon nal of carbon components released from 1 to 2 m of sea ice (TCO2) in brine is only reduced by 1 mol (Reactions R1– growth is difficult to detect in a water column several hun- R3): dred meters deep. In this study, we followed the evolution of the inor- 2C C − C C Ca 2HCO3 5H2O CaCO3 6H2O CO2; (R1) ganic carbon dynamics within experimental sea ice from − h 2−i q sea ice formation to melt in a sea ice–seawater mesocosm TCO2 D HCO C CO C [CO2]; (R2) 3 3 pool (∼ 435 m3) at the University of Manitoba, Winnipeg, h i D − C 2− C − C − − C TA HCO3 2 CO3 B.OH/4 OH H : (R3) Canada. The benefits of this type of environment are multi- ple. An artificial pool equipped with a movable bridge makes The specific conditions leading to ikaite precipitation, as well it possible to collect undisturbed samples from thin growing as the fate of these precipitates in sea ice, are still not fully sea ice.
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