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32 Years of Sea Ice Physics and Biogeochemistry

32 Years of Sea Ice Physics and Biogeochemistry. S.F. Ackley. Sea Ice Scales. Antarctic Sea Ice Profile. Antarctic Sea Ice Freeboards. Conclusions through Halftime ~1994. Algal incorporation in Frazil Ice exceeded incorporation in Columnar Ice

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32 Years of Sea Ice Physics and Biogeochemistry

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  1. 32 Years of Sea Ice Physics and Biogeochemistry S.F. Ackley

  2. Sea Ice Scales

  3. Antarctic Sea Ice Profile

  4. Antarctic Sea Ice Freeboards

  5. Conclusions through Halftime ~1994 • Algal incorporation in Frazil Ice exceeded incorporation in Columnar Ice • Nearly all Antarctic sea ice contains measurable Chl content but low concentrations • Primary mechanism of incorporation is wave pumping in frazil-pancake ice • Concentrations in ice exceeded that in underlying water column by 10x to 100x

  6. Conclusions through Halftime (~1994) • Algal growth led to nutrient drawdown, so high algal concentrations required renewal of nutrients from surface sea water • Top surface freezing in autumn led to convective overturning and fueled a fall bloom of algae (Fritsen et al 1994 from ISW) • Warming leads to porous sea ice, while cooling can cause brine rejection and upwelling (Golden et al 1997)

  7. Krill under Sea Ice Credit: Klaus Meiners

  8. Krill swarms Krill under sea ice First evidence of extensive krill swarms under ice

  9. 2nd Half (2007 to Present) • SIPEX-E.Antarctic Ice—A Seasonal Progression • SIMBA-Bellingshausen Sea Ice—Temperature Cycling • DMS and DMSP Production • A Sea Ice CO2 Pump

  10. ICESat 1290 ICESat 1305 ICESat 0166 ICESat 0025 ICESat 0055 ICESat 1297 Background: AMSR-E ice conc. 10 Oct. 2007; courtesy of G. Spreen

  11. SIPEX Ice temperature and salinity (iron site) Figure: Delphine Lannuzel Photo: Mats Granskog

  12. SIPEX - Ice algal biomass Chlorophyll a (mg m-2) Relative ice core depth (%) Time → Most of the algal biomass was at the bottom of the cores, except towards the end of the experiment

  13. Time Series Sampling, How does the sea ice at one site change with time? Occupied one floe for 27 days.

  14. Mixture of Ice Types at Ice Station Belgica Icebergs open water lead Liege Site (off photo) Brussels Site - level first year ice- no flooding Deformed thick ice with snow cover- Fabra Site (60 to 80% neg. freeboard)

  15. Brussels site- level ice, thin snow Liege site- mod. roughness, thicker snow Brussels site-smooth, thin snow Fabra site highly deformed, thick snow cover

  16. C. Fritsen C. Fritsen SIMBA Brussels Site Texture G = granular C = columnar FS = froz. snow f = fine m = medium c = coarse s = small l = large M. Lewis

  17. SIMBA Liège Site Texture G = granular C = columnar FS = froz. snow f = fine m = medium c = coarse s = small l = large M. Lewis

  18. Brussels Site IMB Radiometer Buoy (Brussels 1) Thinner snow cover allows cold pulse to penetrate sea ice No flooding at snow/ice interface Ice thickness loss

  19. Thermodynamics at Brussels Site (unflooded)

  20. DMS(P) and Chla evolution at Brussels Site [Chla] measurements by I. Dumont, C. Fritsen, B. Saunders

  21. Liège Site IMB Colder Air Temperatures don’t penetrate thick snow cover Snow/Ice Interface continuously flooded with sea water Sea Ice relatively isothermal Irregular ice bottom affects sonar returns. Bottom pinger reset when CTD removed for repairs.

  22. DMS(P) and Chla evolution at Liège Site [Chla] measurements by I. Dumont, C. Fritsen, B. Saunders

  23. Radiometer indicates changes in irradiance, snow cover thickness, biological growth Opening and closing of Leads in the ice Breakup of the ice floe where IMBs separate and drift independently

  24. SIMBA Accomplishments Antarctic sea ice is a springtime “Biogeochemical Reactor” Driven by Physical Feedback, where thermal changes drive porosity and convection, leading to: • Nutrient Flux =>Enhanced Biological Productivity • Biological Degradation=>DMSP+DMS Flux • Porosity Changes=>CO2 Exchanges • Shelf Sediments and Iceberg Melting as Iron sources

  25. The Sea Ice CO2 Pump

  26. A long lived dogma… • Weiss et al 1979, Gordon et al 1984, Poisson and Chen 1987 • « Weddell Sea pack ice effectively blocks the air-sea exchange of gases » • No evidence of marked ventilation is found in deep waters of the Weddel Sea. Thus sea ice appears to prevent air-sea exchange.

  27. A long lived dogma challenged?… Science 282: 2238 • Kelley & Gosink 1970s-80s • « unlike ices from pure freshwater, sea ice is a highly permeable medium for gases » • They found rate of CO2 penetration about 60 cm h-1 at -7°C • « gas migration through sea ice is an important factor in ocean-atmosphere winter communication particularly when the surface temperature is > -10° » (Gosink et al., 1976. Nature 263: 41) Golden et al., 1998 • Theoretical and experimental evidence that sea ice permeability increases considerably above -5°C, the so-called “law of fives”(Golden et al., 1998. Science 282: 2238)

  28. Sea Ice Phase Diagram

  29. CaCO3 dissolution/precipitation 2HCO3- + Ca2+ CaCO3 + CO2 • Thompson and Nelson 1956 showed that at a temperature just below the freezing point calcium carbonate begins to precipate from the entrained brines in sea ice and remains in the ice. • Weiss et al 1979 observed alkalinity anomalies in the surface water of the Weddell Sea • Jones et al. 1983 observed a transfer of CaCO3 from the ice to surface waters in the Arctic Ocean • Papadimitriou et al. 2004 and Dieckmann et al. observed CaCO3 precipitation in artificial and natural sea ice and identified it as Ikaite • Rysgaard et al. 2007 suggested that CaCO3 precipitation in sea ice might act as a sink for CO2.

  30. 45 SEA ICE PUMP Is CO2 released to the atmosphere anytime new ice forms? (Answer on Next Slide) D. Nomura, 2006 A potential abiotic CaCO3 Carbon source fall/winter In winter, precipitation of CaCO3 occurs within sea ice. Produced CO2 is expelled with brine, while CaCO3 is trapped within brine channels Brine sink rapidly carrying CO2 (see Brine Drainage Slide) Part of CO2 which passes below the pycnocline is « removed » from the system. 2HCO3- + Ca2+ CaCO3 + CO2 CO2 Rysgaard et al., 2007, Delille et al., in prep.

  31. Upward CO2 Flux over New Ice at the SIMBA site Figure 17:Temperature at the ice interface and CO2 fluxes over ice and snow (positive fluxes correspond to efflux to the atmosphere) at 5 sites of the second "frost flowers" station.

  32. Brine Drainage at SIMBA

  33. GAS COMPOSITION IN SEA ICE 46 A potential abiotic CaCO3 Carbon pump fall/winter spring In spring, CaCO3 trapped within sea ice dissolves. This process consumes CO2. Budget of winter and spring processes is a net sink of CO2. It depends on: ratio of CaCO3 trapped vs CO2 expelled (?) quantity of CO2 which pass below the pycnocline during the autumn-winter (?) CaCO3 + CO2 2HCO3- + Ca2+ 2HCO3- + Ca2+ CaCO3 + CO2 CO2 CO2 Rysgaard et al., 2007, Delille et al., in prep.

  34. Conclusions Sea ice exhibits marked CO2 dynamics controlled by (i) salinity (ii) precipitation/dissolution of CaCO3 and (iii) primary production CaCO3 precipitation occurs within sea ice and can be a very efficient pathway for atmsopheric CO2 uptake Sea ice in spring exchanges CO2 with the atmosphere. Sea ice acts first as a source of CO2 to the atmosphere then as a sink. This spring sink of the antarctic sea ice is about -0.025 PgC yr. It would represent an additional sink of 50% to the CO2 sink of the Southern Ocean Taking into account CaCO3 precipitation and particular sea-ice processes especially discrepency in salt:gas rejection (loose et al. 2009) and possible artefacts in the transcient hallogen tracers, it might deserve to revisite C anthropogenic computation in sea ice covered waters

  35. Where are we? • Strong Coupling between the biogeochemistry and physics of the ice, related to growth, thermal driving, snow • Climatically active gases, DMS and CO2, are important new developments • Time series from IMBs with additional sensors like radiometers, oxygen, pCO2 look important • Modeling of fluid flow looks critical • Subtle differences in conditions can lead to big differences in the biogeochemistry

  36. It’s been fun Thanks to Elizabeth for the invite.

  37. 48 GAS COMPOSITION IN SEA ICE CO2 : the ”trouble maker” Summary • pCO2 is a difficult variable to measure in the sea ice environment • a suite of measurement techniques are still in development and in the need of proper validation • pCO2 is generally supersaturated in most of the sea ice cover during the winter • pCO2 is highly undersaturated in the whole sea ice cover during spring and summer • Potential and concurrent mechanisms for the pCO2 drawdown during the summer are: • Dilution of brines • Dissolution of calcium carbonate • Primary Production • There is a potential inorganic CaCO3 CO2 pump associated to sea ice growth and decay

  38. From land fast sea ice to multiyear pack ice Ispol drift experiment R.V. Polarstern Nov-Dec 2004 First and multi-year pack ice AA03-V1 cruise R.V. Aurora Australis Sep-Oct 2003 First year pack ice Simba drift experiment R.V. N.B. Palmer Oct 2007 First year pack ice

  39. 47 GAS COMPOSITION IN SEA ICE 66°38’ S 66°38‘30 S 66°39' S (from Solubility equations) (from Oxygen production) (from Total alcalinity anomaly) CO2 : the ”trouble maker” Balancing the effects… Dumont d’Urville, 1999 It works!...and all three processes contribute! Delille al. , 2007

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