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

Air-ice CO2 fluxes were measured continuously using automated chambers from the initial freezing of a sea ice cover until its decay. Cooling seawater prior to sea ice formation acted as a sink for atmospheric CO2, but as soon as the first ice crystals started to form, sea ice turned to a source of CO2, 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 CO2. Direct measurements of outward ice-atmosphere CO2 fluxes were consistent with the depletion of dissolved inorganic carbon in the upper half of sea ice. Combining measured air-ice CO2 fluxes with the partial pressure of CO2 in sea ice, we determined strongly different gas transfer coefficients of CO2 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 CO2 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

Determination of air‐sea ice transfer coefficient for CO2: Significant contribution of gas bubble transport during sea ice growth

Air‐ice CO2 fluxes were measured continuously from the freezing of a young sea‐ice cover until its decay. Cooling seawater was as a sink for atmospheric CO2 but asthe ice crystalsformed,sea ice shifted to a source releasing CO2 to the atmosphere throughout the whole ice growth. Atmospheric warming initiated the decay, re‐shifting sea‐ice to a CO2 sink. Combining these CO2 fluxes with the partial pressure of CO2 within sea ice, we determined gas transfer coefficients for CO2 at air‐ice interface for growth and decay. We hypothesize that this difference originates from the transport of gas bubbles during ice growth, while only diffusion occurs during ice melt. In parallel, we used a 1D biogeochemical model to mimic the observed CO2 fluxes. The formation of gas bubbles was crucial to reproduce fluxes during ice growth where gas bubbles may account for up to 92 % of the upward CO2 fluxes

The future of Arctic sea-ice biogeochemistry and ice-associated ecosystems

The Arctic sea-ice-scape is rapidly transforming. Increasing light penetration will initiate earlier seasonal primary production. This earlier growing season may be accompanied by an increase in ice algae and phytoplankton biomass, augmenting the emission of dimethylsulfide and capture of carbon dioxide. Secondary production may also increase on the shelves, although the loss of sea ice exacerbates the demise of sea-ice fauna, endemic fish and megafauna. Sea-ice loss may also deliver more methane to the atmosphere, but warmer ice may release fewer halogens, resulting in fewer ozone depletion events. The net changes in carbon drawdown are still highly uncertain. Despite large uncertainties in these assessments, we expect disruptive changes that warrant intensified long-term observations and modelling efforts

Autumn to spring inorganic carbon processes in pack and landfast sea ice in the Ross Sea, Antarctica

The Ross Sea, the southernmost sea on Earth, presents several iconic features of polar seas: sites of deep water formation, high summer primary production, floating ice shelves, the annual cycle of advance and retreat of sea ice, polynyas and katabatic winds. Furthermore, sea ice in McMurdo sound (western Ross Sea) is one of the most productive marine environments. However, sea ice inorganic carbon dynamics and related air-ice CO2 fluxes have never been documented in the Ross Sea. Two surveys were carried out in the western Ross Sea to bridge over a critical gap in the current understanding of sea ice: autumn and winter processes. The land-based YROSIAE project was a temporal survey from late winter to summer within landfast sea ice. The ship-based PIPERS project was an unique opportunity to study the early stages of sea ice formation (in polynyas) and more common consolidated pack ice in autumn. Based on these two consistent surveys, this work aims to (i) examine the bulk ice pCO2 dynamics in landfast sea ice from late winter to summer (ii) investigate the seasonal pattern (net source vs net sink) and diurnal pattern of air-ice CO2 fluxes (iii) analyse the depth-dependent physical and biogeochemical processes involved in inorganic carbon dynamics (iv) assess the precipitation of calcium carbonate in autumn and during a full bloom season. CO2 fluxes were measured using the chamber technique in autumn, late winter and spring, over open surface water, frazil ice patch, grey unconsolidated ice and consolidated first-year ice. These new autumn and winter data provide a first step to set up the budget of air-ice CO2 fluxes over the year and evaluate the large-scale influence of these fluxes on the annual uptake of CO2 by ice-covered oceans. Our results confirm that sea ice acts as a CO2 source for the atmosphere during ice growth, with enhanced fluxes reported at the early stages of sea ice formation, and shifts to a sink in spring. In late spring, diel pattern superimposed upon this seasonal pattern and was potentially assigned to either ice skin freeze-thaw cycles or diel changes in net community production. The snowpack plays a complex role in CO2 exchanges and can no longer be considered as an inert reservoir lying at the sea ice surface. The main features of the normalized DIC distribution (DIC35) through the ice column were: (i) a marked depletion at the surface from autumn to spring induced by the CO2 releases to the atmosphere (ii) bubble-driven gas enrichment below or within impermeable layers and (iii) an initial DIC35 enrichment in the bottom layer disappearing in spring when the seasonal peak in biomass occurs.At the bottom of landfast ice, in spring, a particular assemblage of microorganisms, the biofilm, led to a massive biomass build-up counterintuitively associated with nutrients accumulation. This biofilm formation may also promote calcium carbonate precipitation. However, in young pack ice or in cold landfast ice in early spring, limited calcium carbonate precipitation was reported. This suggests that calcium carbonate precipitation is not an ubiquitous process, especially in winter and autumn Antarctic sea ice. Comparison of calcium carbonate precipitation and pCO2 measurements advocates that the calcium carbonate precipitation is rather controlled by pCO2 than temperature

Autumn to spring inorganic carbon processes in pack and landfast sea ice in the Ross Sea, Antarctica

The Ross Sea, the southernmost sea on Earth, presents several iconic features of polar seas: sites of deep water formation, high summer primary production, floating ice shelves, the annual cycle of advance and retreat of sea ice, polynyas and katabatic winds. Furthermore, sea ice in McMurdo sound (western Ross Sea) is one of the most productive marine environments. However, sea ice inorganic carbon dynamics and related air-ice CO2 fluxes have never been documented in the Ross Sea.Two surveys were carried out in the western Ross Sea to bridge over a critical gap in the current understanding of sea ice: autumn and winter processes. The land-based YROSIAE project was a temporal survey from late winter to summer within landfast sea ice. The ship-based PIPERS project was an unique opportunity to study the early stages of sea ice formation (in polynyas) and more common consolidated pack ice in autumn. Based on these two consistent surveys, this work aims to (i) examine the bulk ice pCO2 dynamics in landfast sea ice from late winter to summer (ii) investigate the seasonal pattern (net source vs net sink) and diurnal pattern of air-ice CO2 fluxes (iii) analyse the depth-dependent physical and biogeochemical processes involved in inorganic carbon dynamics (iv) assess the precipitation of calcium carbonate in autumn and during a full bloom season.CO2 fluxes were measured using the chamber technique in autumn, late winter and spring, over open surface water, frazil ice patch, grey unconsolidated ice and consolidated first-year ice. These new autumn and winter data provide a first step to set up the budget of air-ice CO2 fluxes over the year and evaluate the large-scale influence of these fluxes on the annual uptake of CO2 by ice-covered oceans. Our results confirm that sea ice acts as a CO2 source for the atmosphere during ice growth, with enhanced fluxes reported at the early stages of sea ice formation, and shifts to a sink in spring. In late spring, diel pattern superimposed upon this seasonal pattern and was potentially assigned to either ice skin freeze-thaw cycles or diel changes in net community production. The snowpack plays a complex role in CO2 exchanges and can no longer be considered as an inert reservoir lying at the sea ice surface.The main features of the normalized DIC distribution (DIC35) through the ice column were: (i) a marked depletion at the surface from autumn to spring induced by the CO2 releases to the atmosphere (ii) bubble-driven gas enrichment below or within impermeable layers and (iii) an initial DIC35 enrichment in the bottom layer disappearing in spring when the seasonal peak in biomass occurs. At the bottom of landfast ice, in spring, a particular assemblage of microorganisms, the biofilm, led to a massive biomass build-up counterintuitively associated with nutrients accumulation. This biofilm formation may also promote calcium carbonate precipitation. However, in young pack ice or in cold landfast ice in early spring, limited calcium carbonate precipitation was reported. This suggests that calcium carbonate precipitation is not an ubiquitous process, especially in winter and autumn Antarctic sea ice. Comparison of calcium carbonate precipitation and pCO2 measurements advocates that the calcium carbonate precipitation is rather controlled by pCO2 than temperature.Doctorat en Sciencesinfo:eu-repo/semantics/nonPublishe

ULB-ULg : Sea ice biogeochemistry work plan

1. Manuscript submitted to Elementa Last year we submitted a manuscript about air-ice CO2 fluxes measured in continuous with a chamber over the ice during INTERICE V experiment. The results show that sea ice shifts from: (i) a sink during ice crystals formation, (ii) a source during ice growth, (iii) return to a sink during ice melt. We attempt to reproduce these fluxes with the 1Dimension model developed by Martin and Sebastien in Moreau et al. (2015). The inversion between outward CO2 fluxes during ice growth and inward CO2 fluxes during ice melt depicts well the observations. However, the model strongly underestimates the fluxes during the cold phase if the formation rate of gas bubbles is low. Since ice is permeable throughout the cold phase, higher gas bubble formation rates lead to higher CO2 fluxes. The contribution of gas bubble buoyancy to upward flux was the main hypothesis of this manuscript. 2. TA-DIC compilation With the code developed by Martin (and others), we computed profile of DIC normalized to the mean ice salinity. We observe a reverse C shape with a depletion at the surface and more scattered data at the bottom. It’s striking to observe that at mid-depth (0.5 m), all data sounds to converge at the same value (around 480 µmol/kg). It makes us confident with the fact that we can gather data and compare them. The mean DIC value in the middle of the cores is similar to the sea surface water DIC in Antarctica. Our idea is that these value are due to simple brine rejection and that there is a depletion at the top and at the bottom. The bottom depletion is subject to biogeochemistry processes. While the top depletion may be due to the CO2 release during ice formation which lead to a potential CO2 flux out of the ice. For the time beeing, we aim to derive a budget of CO2 flux from this compilation. This will be presented at the next BEPSII meeting. 3. Further studies and perspectives (PhD thesis of Fanny and Marie) Sea ice production of N2O and halocarbons and their contribution to atmospheric concentrations. Development of a flux chamber in process

Dynamique de l’oxyde nitreux dans la glace de mer

Fluctuations in greenhouse gases (GHGs) concentration alter the energetic budget of the climate system. There is high confidence that natural systems related to snow, ice and frozen ground (including permafrost) are affected. Nitrous oxide (N2O) is one of the potent GHG naturally present in the atmosphere, but witch has seen his concentration growing since industrial era. N2O has a lifetime in the atmosphere of 114 years and a global warming potential (GWP) of 298 to be compared to carbon dioxide that has a GWP of 1. N2O is also describe as the dominant ozone-depleting substance emitted in the 21st Century. Yet, there are still large uncertainties and gaps in the understanding of the cycle of this compound through the ocean and particularly in sea ice. Sources and sinks of N2O are therefore still poorly quantified. The main processes (with the exception of transport processes) involved in the N2O cycle within the aquatic environment are nitrification and denitrification. To date, only one study by Randall et al. present N2O measurements in sea ice. Randall et al. pointed out that sea ice formation and melt has the potential to generate sea-air or air-sea fluxes of N2O, respectively. Study on ammonium oxidation and anaerobic bacterial cultures shows that N2O production can potentially occur in sea ice. Denitrification can act as a sink or a source of N2O. In strictly anaerobic conditions, N2O is removed by denitrification. However, denitrification can also occur in presence of O2 at trace level concentrations (<0.2 mg L-1), and in these conditions there is a large N2O production. Recent observations of significant nitrification in Antarctic sea ice shed a new light on nitrogen cycle within sea ice. It has been suggested that nitrification supplies up to 70% of nitrate assimilated within Antarctic spring sea ice. Corollary, production of N2O, a by-product of nitrification, can potentially be significant. This was recently confirmed in Antarctic land fast ice in McMurdo Sound, where N2O release to the atmosphere was estimated to 4 µmol.m-2.yr-1. This assessment is probably an underestimate since it only accounts for dissolved N2O while a significant amount of N2O is likely to occur in the gaseous form like N2, O2 and Ar. Finally, nitrification produces little N2O in oxygenated waters but the N2O production yield from nitrification strongly increases as O2 levels decrease. Hence, it is not possible to distinguish the sources of N2O solely based on bulk N2O concentrations or environmental conditions, while deepened knowledge of processes is needed to well understand N2O emissions

La dynamique du N2O dans la glace de mer

Fluctuations in greenhouse gases (GHGs) concentration alter the energetic budget of the climate system. There is high confidence that natural systems related to snow, ice and frozen ground are affected. Nitrous oxide (N2O) is one of the potent GHG naturally present in the atmosphere, but witch has seen his concentration growing since industrial era. N2O has a lifetime in the atmosphere of 114 years and a global warming potential 300 time higher than that of CO2. Yet, there are still large uncertainties and gaps in the understanding of the cycle of this compound through the ocean and particularly in sea ice. Sources and sinks of N2O are therefore still poorly quantified. The main processes (except the transport processes) involved in the N2O cycle within the aquatic environment are nitrification and denitrification. To date, only one study by Randall et al. 2012 present N2O measurements in sea ice. Randall et al. pointed out that sea ice formation and melt has the potential to generate sea-air or air-sea fluxes of N2O, respectively. Study on ammonium oxidation and anaerobic bacterial cultures shows that N2O production can potentially occur in sea ice. Denitrification can act as a sink or a source of N2O. In strictly anaerobic conditions, N2O is removed by denitrification. However, denitrification can also occur in presence of O2 at trace level concentrations (<0.2 mg L-1), and in these conditions there is a large N2O production. Recent observations of significant nitrification in Antarctic sea ice shed a new light on nitrogen cycle within sea ice. It has been suggested that nitrification supplies up to 70% of nitrate assimilated within Antarctic spring sea ice. Corollary, production of N2O, a by-product of nitrification, can potentially be significant. This was recently confirmed in Antarctic land fast ice in McMurdo Sound, where N2O release to the atmosphere was estimated to 4 µmol.m-2.yr-1. This assessment is probably an underestimate since it only accounts for dissolved N2O while a significant amount of N2O is likely to occur in the gaseous form like N2, O2 and Ar. This poster address the issue related to the production of N2O within sympagic microorganisms. What process is dominant and how much N2O is produced? The determination of the isotopic composition of N2O using cavity enhanced laser absorption spectroscopy technique (Off-axis ICOS) will allow us to determine the origin of these processes. It will be based on the relative isotope abundance values and site preference data in previous studies