HR: 0800h
AN: B21D-0911    [Abstracts]
TI: The carbon isotope behaviour during sea water evaporation in salinas as related to the biogeochemical cycle of carbon.
AU: * pierre, c
EM: cat@lodyc.jussieu.fr
AF: LODYC-UPMC, 4 Place Jussieu, Paris, 75252 France
AU: PUEYO MUR, J
EM: juanj@natura.geo.ub.es
AF: Universitat de Barcelona, Zona Universitaria de Pedralbes, Barcelona, 08028 Spain
AU: Fritz, B
EM: bfritz@illite.u-strasbg.fr
AF: Universit‚ Louis Pasteur, 1 rue Blessig, Strasbourg, 67084 France
AB: Field measurements and sampling of hypersaline solutions have been realized in the salinas of Santa Pola (Spain) during June 2000 and May 2001. We have followed the geochemical and isotopic evolutions of the solutions along the sea water evaporitic pathway and during a nycthemeral cycle in four evaporitic basins (two carbonate basins, one gypsum basin and one halite basin). There are important differences from one year to another : during May 2001, the \delta$^{13}$C values of DIC were enriched by 1\permil to 5\permil compared to June 2000, indicating organic productivity levels higher in May 2001 than in June 2000. The alkalinity, dissolved oxygen, dissolved organic carbon (DOC) and \delta$^{13}$C values of DIC show large changes from night to day with variable amplitude in the different basins, being more important in the first evaporitic basins where carbonate organic-rich sediments are deposited. The large interannual and nycthemeral variabilies of the \delta$^{13}$C values of DIC in marine evaporitic settings show that these environments are very sensitive to the external and internal constraints which drive the evaporation rate, the mineral precipitation, as well as the organic productivity-regeneration levels. Because inorganic and biological processes in evaporating marine solutions control the carbon cycle and the carbon isotope fractionations, they are both recorded in the geochemistry of the solutions and in the \delta$^{13}$C values of DIC. During sea water evaporation up to halite saturation, the \delta$^{13}$C values of DIC vary in a wide range between -5\permil and +10\permil, following four major steps . 1) carbonate precipitation causes the initial drop of the \delta$^{13}$C values ; 2) algal production (photosynthesis) is responsible for $^{13}$C enrichments in the solutions while organic matter remineralization in the sediment release $^{13}$C -poor CO$_{2}$ in the overlying solutions causing major decreases of \delta$^{13}$C values of DIC in the solutions ; 3) In the basins where gypsum and halite are deposited, the formation of hard evaporitic crusts at the basin floor inhibits gas and ion diffusion from the sediment toward the overlying solutions ; 4) large $^{13}$C enrichments in the heaviest brines results mostly from CO$_{2}$ evasion during evaporation and probably also from the $^{13}$C -poor CO$_{2}$ uptake by the bacterial biomass.
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting