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| Titel |
The effect of strong salinity and temperature gradients on transport processes and the formation of bathyal authigenic gypsum at a marine mud volcano |
| VerfasserIn |
L. Haffert, M. Haeckel |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250060636
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| Zusammenfassung |
| A thermodynamic activity model (Pitzer approach) applicable to extreme environmental
pTS-conditions (up to 1000 bar, 200 Ë C and 6 M NaCl) coupled to an extensive mineral
database has been developed. The advantage of this code over existing ones, such as Phreeqc,
Wateq, Minteq, is the additional integration of a comprehensive pressure correction, as well
as the flexibility on the choice of input datasets, allowing fine-tuning of the model according
the relevant pTS range.
An example of the successful application of the model is the interpretation of near-surface
pore water profiles from the Mercator mud volcano in the Gulf of Cadiz. These profiles
are intriguing for two reasons. First, they are characterised by a strong salinity
gradient in the upper 1-2 mbsf created by the mixing of upward advecting hypersaline
(halite and gypsum saturated, S=360) mud volcano fluids and seawater (S=35) and,
second, the pore water profiles encompass various types of authigenic gypsum
(CaSO4-
2H2O) and anhydrite (CaSO4) crystals, which usually form only in evaporitic
environments.
It was found that while high Ca and SO4 concentrations from dissolution of an underlying
diapir provide gypsum saturated fluids, the occurrence of supersaturation and thus
authigenic gypsum (or anhydrite) precipitation is only possible through the reduction
of temperature. In addition to the strong temperature control, the salinity has an
important impact on the resultant composition of the precipitating CaSO4 minerals.
Increasing salinity significantly lowers the activity of water, thereby raising the
gypsum-anhydrite transition zone from >1 km to about 500 m sediment depth at the
MMV and during heat pulses (> 30 Ë C) even to within a few metres below the
seafloor.
Another effect of the strong salinity gradient is its influence on the diffusive
transport of solutes. When comparing the activity and concentration profiles of
dissolved species at the Mercator mud volcano, it becomes obvious that here the
true transport property, the chemical potential, respectively activity, of a species
has to be considered, i.e. the Maxwell-Stefan equation. An interpretation solely
based on Fick’s law using the concentration profiles would lead to erroneous results. |
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