 |
| Titel |
Hydrology of marginal evaporitic basins during the Messinian Salinity Crisis: isotopic investigation of gypsum deposits |
| VerfasserIn |
Aida El Kilany, Antonio Caruso, Francesco Dela Pierre, Marcello Natalicchio, Jean-Marie Rouchy, Catherine Pierre, Vincent Balter, Giovanni Aloisi |
| Konferenz |
EGU General Assembly 2016
|
| Medientyp |
Artikel
|
| Sprache |
en
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250136181
|
| Publikation (Nr.) |
 EGU/EGU2016-17165.pdf |
|
|
|
|
|
| Zusammenfassung |
| The deposition of gypsum in Messinian Mediterranean marginal basins is controlled by basin restriction and the local hydrological cycle (evaporation/precipitation rates and relative importance of continental vs marine water inputs). We are using the stable isotopic composition of gypsum as a proxy of the hydrological cycle that dominated at the moment of gypsum precipitation. We studied the Messinian Caltanissetta (Sicily) and Tertiary Piedmont (north western Italy) basins where we carried out a high-resolution isotopic study of gypsum layers composing gypsum-marl cycles. These cycles are thought to be the sedimentary expression of astronomical precession cycles, lasting approximately 20 kyr, during which the marginal basins experienced a succession of arid and a wet conditions. We determined the isotopic composition of gypsum hydration water (18O and D), of the sulphate ion (34S, 18O) and of Strontium (87/86Sr), all of which are potentially affected by the hydrological cycle. In our samples, the mother water from which gypsum precipitated is considerably lighter (-4.0 < 18OH2O ‰ vs SMOW < 3.1; -34.8 < DH2O ‰ vs SMOW < 25.3) than evaporated marine waters from which gypsum precipitates (18OH2O 6-7 ‰ vs SMOW; DH2O 30-40‰ vs SMOW), suggesting that the marginal basins were receiving an input of continental water during gypsum precipitation. Moreover, the degree of 18O and D-depletion is basin-specific, which is consistent with the geographical distance between the two basins and their independent local climates and watersheds. Continental water inputs are consistent also with most of the Sr data (0.70861 < 87/86Sr < 0.70897), and with previously published low-salinity fluid inclusion data from the same gypsum layers in the Tertiary Piedmont basin (suggesting a salinity lower than 35 ‰ in many cases). However, in the samples from the Caltanissetta basin, the sulphate ion suggests a marine water source (20.7 < δ34S ‰ vs CTD < 22.9). Two interpretations are possible: (1) the continental waters feeding marginal basins were carrying dissolved sulphate ions originating from the dissolution of pre-existing marine gypsum deposits; or (2) gypsum deposits have undergone meteoric water digenesis that has affected the isotopic signature of the gypsum hydration water while keeping that of the SO4 ion unchanged. We are planning a detailed petrographic investigation of gypsum crystals to look for evidence of dissolution/precipitation processes at the micro-scale. This is an essential step in interpreting the isotopic signals of gypsum because we can expect the 18O and D composition of Messinian continental input to be not too dissimilar from that of modern meteoric waters involved in diagenetic processes. |
|
|
|
|
|
|