Air-ice carbon pathways inferred from a sea ice tank experiment

Abstract

Given rapid sea ice changes in the Arctic Ocean in the context of climate warming, better constraints on the role of sea ice in CO 2 cycling are needed to assess the capacity of polar oceans to buffer the rise of atmospheric CO 2 concentration. Air-ice CO 2 fluxes were measured continuously using automated chambers from the initial freezing of a sea ice cover until its decay during the INTERICE V experiment at the Hamburg Ship Model Basin. Cooling seawater prior to sea ice formation acted as a sink for atmospheric CO 2 , but as soon as the first ice crystals started to form, sea ice turned to a source of CO 2 , which lasted throughout the whole ice growth phase. Once ice decay was initiated by warming the atmosphere, the sea ice shifted back again to a sink of CO 2. Direct measurements of outward ice-atmosphere CO 2 fluxes were consistent with the depletion of dissolved inorganic carbon in the upper half of sea ice. Combining measured air-ice CO 2 fluxes with the partial pressure of CO 2 in sea ice, we determined strongly different gas transfer coefficients of CO 2 at the air-ice interface between the growth and the decay phases (from 2.5 to 0.4 mol m −2 d −1 atm −1). A 1D sea ice carbon cycle model including gas physics and carbon biogeochemistry was used in various configurations in order to interpret the observations. All model simulations correctly predicted the sign of the air-ice flux. By contrast, the amplitude of the flux was much more variable between the different simulations. In none of the simulations was the dissolved gas pathway strong enough to explain the large fluxes during ice growth. This pathway weakness is due to an intrinsic limitation of ice-air fluxes of dissolved CO 2 by the slow transport of dissolved inorganic carbon in the ice. The best means we found to explain the high air-ice carbon fluxes during ice growth is an intense yet uncertain gas bubble efflux, requiring sufficient bubble nucleation and upwards rise. We therefore call for further investigation of gas bubble nucleation and transport in sea ice.

Commentary on the outputs and future of Biogeochemical Exchange Processes at Sea-Ice Interfaces (BEPSII)

“…In situ and temperature-controlled short-term radiotracer incubations identified a significant correlation between sea-ice temperature and intracellular iron uptake by Antarctic sea-ice algae as well as indications of luxury iron uptake (Lannuzel et al, 2017). Sea-ice tank experiments showed that sea ice is a source of CO 2 during ice formation and a sink during ice-melt periods, with a suggested role for gas-bubble formation to explain the measured outflux of CO 2 (Kotovitch et al, 2016).…”

Tracer Measurements in Growing Sea Ice Support Convective Gravity Drainage Parameterizations

“…We also include three diffusive parameterizations ( Table 2). The first is VC10, retained in spite of its drawbacks because it was used in several biogeochemical studies (Kotovitch et al, 2016;Moreau et al, 2015;Vancoppenolle et al, 2010). We also include J11_EMD and J11_MLD (Jeffery et al, 2011).…”

Carbon Dynamics During the Formation of Sea Ice at Different Growth Rates

“…A small part of the brine supersaturated in CO 2 is then predicted to be expelled to the sea-ice surface and to release CO 2 to the atmosphere (Rysgaard et al, 2011). Small, positive CO 2 fluxes out of young, newly formed sea ice have indeed been detected in the field (Else et al, 2011;Geilfus et al, 2013;Barber et al, 2014;Nomura et al, 2018) and in artificial sea ice (Nomura et al, 2006;Geilfus et al, 2016;Kotovitch et al, 2016). However, while the CO 2 flux out of the young ice itself is likely positive (upward), the overall CO 2 flux during ice formation in the marginal sea ice zone may be negative, depending on the presence of open water, including leads and polynyas, which may enhance CO 2 uptake from the air (e.g., Anderson et al, 2004;Loose et al, 2011).…”

“…Therefore, TIC and salinity decoupling could be more pronounced in colder, and thus more saline, brine. Nevertheless, experimental and model studies on newly forming sea ice have found that ikaite precipitation and CO 2 outgassing to the atmosphere are small compared to the efflux of dissolved inorganic carbon with brine to the underlying water (Rysgaard et al, 2007;Sejr et al, 2011;Moreau et al, 2015;Kotovitch et al, 2016). This indicates that the transport in dissolved form is predominant, and the TIC signal should largely follow salinity.…”