Carbon dioxide transfer at the ice-sea and air-ice interfaces: a step towards the end of a long-lived paradigm ?

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(1)CO2 transfer at the ice-sea and air-ice interfaces: a step towards the end of a long-lived dogma ? 1 Unité Unité d’Océ Océanographie Chimique. Bruno Delille1, Anne J. Trevena2, Delphine Lannuzel3, Marie-Line Sauvée3, Bronte Tilbrook4, Michel Frankignoulle1, Alberto V. Borges1 and Jean-Louis Tison2 For decades, sea ice has been considered as an inert and impermeable cover which prohibits any exchange of gases between underlying water and the atmosphere. In the framework of the Belgian project SIBCLIM (Sea Sea Ice Biogeochemistry in a Climate Change Perspective ) spring dynamics of partial pressure of CO2 (pCO2) within and below fast sea ice, and associated exchanges of CO2 at the ice-sea and air-ice interfaces were investigated in conjunction with the measurement of an extended and comprehensive set of physical, biological, and biogeochemical variables. Our preliminary results show that specific biogeochemical processes occur in sea-ice leading to high CO2 dynamics . Furthermore, CO2 transfer with both underlying water and the atmosphere above have been observed.. CO2 dynamics "Winter" type stations pCO2 (ppmV) 250 300400. pCO 2 (ppmV). 440 600 900. 250 300400 300 Brine Shallow Brine Deep. ice. STATION 3. 440 600 900. ice. #7 STATION 7. 1m. water. STATION 9. 5m. 2000. 2100. 2250. 2265. Amongst the physical properties of the sea ice cover, the temperature profile appears to be the main controlling factor on the CO2 dynamics. Ice below the porosity threshold of about -5°C C (winter type stations) displays the higher pCO2 values, whilst the warmer, more porous, ice ("spring" type station) favours the set up of primary production and hence, shows the lowest pCO2 values.. STATION 12 STATION 13. 30 m. 1900. Preliminary results exhibit strong CO2 dynamics in seasea-ice, mainly driven by internal physical and biogeochemical processes. processes pCO2 in brines ranged from marked undersaturation down to 210 ppmV to oversaturation up to 915 ppmV.. STATION 4 STATION 5. Interface water. DIC (µmol.kg -1) 35. First direct measurements of pCO2 within and below sea-ice have been measured using a new versatile pCO2 measurement unit (SIES – Sea-Ice Equilibrator System) transportable on the field.. "Spring" type stations. 1900. DIC35 (µmol.kg -1). 2000. 2100. 2250. 2265. In "winter" type stations, negative correlation of pCO2 and Dissolved Inorganic Carbon linearized against salinity (DIC35) in the ice upper layer evidence precipitation of calcium carbonate within the coldest ice layer which increase pCO2 towards the air-ice interface.. Brine Shallow Brine Deep Interface 1m. 5m. At the ice-sea interface, spring initial release of dense CO2 rich brines tends to increase pCO2 of the first meters of the water column while the following development of primary production lead to a decrease of pCO2.at the ice-water interface. 30 m. CO2 transfer In order to better assess vertical gradients of pCO2 in the sea ice layer, we have measured PCO2 in the air extracted from crushed ice at several depths. pCO2 from crushed sea-ice evidenced strong vertical gradient of PCO2 with PCO2 values ranging in some cases from oversaturation at the airair-ice interface to undersaturation at the iceice-sea interface. interface. 396.1. 380. 10:50 10:52 Elapsed Time. 10:54. 10:56. 10:58. 0. 70 70 cm cm. 355. 335 13:51. Intermediate. 1.97. Brines: 400 ppmV Station 7. 30 30 cm cm 40 40 cm cm. -0.46. 70 70 cm cm. 08:23. 08:25. 08:27. 376 374 372. 13:56. 14:01. 30 cm 361.2 ppmV 14:06. 14:11. 14:16. 14:21. 40 cm 355.1 ppmV 14:26. 14:31. 14:36. 370 13:02 14:41. 14:46. 20 cm/-5.9°C 383.5 ppmV. 30 cm/-4.7°C 377.4 ppmV. 40 cm/-4.0°C 375.9 ppmV. 50 cm/-3.5°C 364.5 ppmV. 375. 13:06. 13:08. 13:10. 13:12. 13:14. 13:16. Elapsed Time. 60 cm/-2.7 365.8 ppmV. 70 cm/-2.3 361.7 ppmV. 385. 13:04. 382 380. 395. 378 376 374 372. 355 03:37 03:39 03:41 03:43 03:45 03:47 03:49 03:51 03:53 03:55 03:57 03:38 03:40 03:42 03:44 03:46 03:48 03:50 03:52 03:54 03:56 03:58. 60 60 cm cm. Summer. 08:21. 365. 50 50 cm cm. 9. 08:19. 378. elapsed rime. STATION 7. 08:17. 380. atm. 365. 20 cm 366.2 ppmV. Biology. 08:15. Elapsed Time. 345. 60 60 cm cm. 08:13. 382. 50 50 cm cm. 20 20 cm cm. 7. 374. 11:00. 385. 375. Station 5. 10:48. pCO2 (ppmV). STATION 9. Brines: 208 ppmV. 10:46. pCO2 (ppmV). Winter. 376. 370 08:11 10:44. 405. 13. 378. 372. 40 40 cm cm. CO2 fluxes (mmol.m-².d-1). 28cm. 393.1. 389. 369 10:42. 30 30 cm cm. Type. 25 cm. 409. 3300ccm m. 20 20 cm cm. Station. 382. 16 cm 392.9. pCO2 (ppmV). Station13. PCO 2 evolution in the chamber placed above snow. 429. pCO2(ppmV). Strong gradients of pCO2 have been observed at the air-ice interface either positive or negative, depending primarily on the temperature profile. These gradients can drive exchanges of CO2 (measured with the chamber technique) up to 2.0 mmol.m mmol.m-2.d-1 , at the airair-ice interface depending of the snow cover and the ice temperature.. 20 20 cm cm. PCO 2 gradients in sea ice ("spike" mesurement of PCO2 form crushed ice). Brines above – 5°C isotherm : 916 ppmV pCO2(ppmV). STATION 13. pCO2 (ppmV). www.ulg.ac.be/oceanbio/co2/. Université Université de Liè Liège Belgium (Bruno.Delille@ulg.ac.be) 2 Glaciology Unit (DSTE) Université Université Libre de Bruxelles Belgium 3 Laboratoire d'Océ d'Océanographie Chimique Université Université Libre de Bruxelles Belgium 4 CSIRO – Marine Research Australia. Biology. Elapsed Time. 370 03:18 03:20 03:22 03:24 03:26 03:28 03:30 03:32 03:34 Elapsed Time. Conclusions and perspectives In autumn and winter, temperature decrease and salinity increase in sea ice brines lead to calcium carbonate precipitation. precipitation This latter processes, and possible bacterial activity, are responsible of a strong increase in pCO2 within sea ice. When theses processes occur, sea ice temperature is too low (below –5°) to allow brines exchange with the underlying water and gases exchange with the atmosphere (station 9). In spring as the sea ice temperature increases, increases brines and gases can be exchanged with respectively underlying water and the atmosphere. Brine release in the underlying water lead to a small increase of the pCO2 of the water column, while oversaturation of pCO 2 at the air-ice interface (station 7) drives CO2 release to the atmosphere (under favourable temperature and snow conditions). However, at the same time, brine circulation initiates the onset of high primary production that is responsible of a dramatic decrease of pCO2 and a large undersaturation. undersaturation Consequently, sea ice begins to act as a CO2 sink, sink firstly in the underlying water since generally phytoplankton starts to bloom at the ice-water interface, then for the atmosphere as the bloom reaches the sea ice upper layer. Thereafter, it appears that spring Antarctic pack ice can either act as a source or as a sink of CO2 for both the atmosphere and the underlying water, in close connection with its thermal and biogeochemical seasonal history. For decades, sea ice was seen as a simple inert stopper for air-sea exchange of CO2; this long-lived paradigm should be revisited in some part..

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