 |
| Titel |
Coupled measurement of δ18O/δD in gypsum hydration water and salinity of fluid inclusions in gypsum: A novel tool for reconstructing parent water chemistry and depositional environment |
| VerfasserIn |
Nick Evans, Fernando Gázquez, Alexandra Turchyn, Hazel Chapman, David Hodell |
| Konferenz |
EGU General Assembly 2015
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250104503
|
| Publikation (Nr.) |
 EGU/EGU2015-3924.pdf |
|
|
|
|
|
| Zusammenfassung |
| The measurement of oxygen and hydrogen isotopes in gypsum hydration water
(CaSO4•2H2O) is a powerful tool to determine the isotopic composition of the parent
fluid from which gypsum precipitated. To be useful, however, the hydration water
must retain its original isotope signal and not have undergone postdepositional
exchange. We developed a novel method to ascertain whether hydration waters
have secondarily exchanged by coupling oxygen and hydrogen isotopes of gypsum
hydration water with the salinities of fluid inclusions. Salinity is obtained through
microthermometric analysis of the same gypsum crystals measured for hydration
water by freezing the sample and then measuring the melting point of the fluid
inclusions.
We apply the method to Messinian gypsum deposits of Cycle 6 within the Yesares
Member, Río de Aguas section, Sorbas Basin (SE Spain). After correction of oxygen and
hydrogen isotopes of gypsum hydration water for fractionation factors, the estimated range of
the mother water is –1.8‰ to 2.8‰ for δ18O and –12.5‰ to 16.3‰ for δD. In the same
samples, estimated salinity of primary fluid inclusions range from 18 to 51ppt.
Salinity is highly correlated with δ18O and δD, yielding an r2 of 0.88 and 0.87,
respectively. The intercepts of the regression equations (i.e., at zero salinity) define the
isotope composition of the freshwater endmember, and average –4.4±1.3oÂfor
δ18O and –28.9±8.7oÂfor δD. These values are within error of the average isotope
composition of precipitation and groundwater data from the local region of Almería today
(–4.3oÂand –22.2oÂfor δ18O and δD, respectively). This agreement provides strong
evidence that the gypsum hydration water has retained its isotope composition
and has not undergone postdepositional exchange. Furthermore, the isotope and
salinity values indicate a significant contribution of meteoric water during gypsum
deposition. This observation contrasts with sulfur and oxygen isotopes in sulfate (21.9
>Âδ34S >Â23.3‰ ; 11.3 >Âδ18OSO4 >Â14.5o) and strontium isotopes (0.708942
>Â87Sr/86SrÂ>Â0.708971) that are similar to those measured in other Messinian evaporites of
the Mediterranean. We suggest sulfate and strontium isotopes are relatively insensitive to
freshwater influence because of the high concentrations of sulfate and strontium in
seawater.
The cyclic alternation of gypsum and marl in the Yesares Member has been interpreted as
reflecting changing climate related to Earth’s precession cycle, but to date direct
evidence linking depositional environment and orbital forcing has been lacking. We
demonstrate that the δ18O, δD and salinity of the parent brine increased from low
values at the base of the cycle to a maximum in the massive gypsum palisade, and
decreased again to lower values in the supercones at the top of the cycle. This pattern is
consistent with precession-driven changes in climate with wetter conditions during
precession minima (insolation maxima) associated with the interbedded marls and
drier climate during gypsum precipitation with the driest conditions during the
precession maxima (insolation minima) associated with gypsum palisade formation. |
|
|
|
|
|
|