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| Titel |
Fluid chemistry of the low temperature hyperalkaline hydrothermal system of Prony Bay (New Caledonia) |
| VerfasserIn |
C. Monnin, V. Chavagnac, C. Boulart, B. Ménez, M. Gérard, E. Gerard, C. Pisapia, M. Quéméneur, G. Erauso, A. Postec, L. Guentas-Dombrowski, C. Payri, B. Pelletier |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 11, no. 20 ; Nr. 11, no. 20 (2014-10-16), S.5687-5706 |
| Datensatznummer |
250117643
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| Publikation (Nr.) |
 copernicus.org/bg-11-5687-2014.pdf |
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| Zusammenfassung |
The terrestrial hyperalkaline springs of Prony Bay (southern lagoon, New
Caledonia) have been known since the nineteenth century, but a recent high-resolution bathymetric survey of the seafloor has revealed the existence of
numerous submarine structures similar to the well-known Aiguille de Prony,
which are also the location of high-pH fluid discharge into the lagoon.
During the HYDROPRONY cruise (28 October to 13 November 2011), samples of
waters, gases and concretions were collected by scuba divers at underwater
vents. Four of these sampling sites are located in Prony Bay at depths up
to 50 m. One (Bain des Japonais spring) is also in Prony Bay but
uncovered at low tide and another (Rivière des Kaoris spring) is on land
slightly above the seawater level at high tide. We report the chemical
composition (Na, K, Ca, Mg, Cl, SO4, dissolved inorganic carbon,
SiO2(aq)) of 45 water samples collected at six sites of high-pH water
discharge, as well as the composition of gases. Temperatures reach
37 °C at the Bain des Japonais and 32 °C at the spring of
the Kaoris. Gas bubbling was observed only at these two springs. The emitted
gases contain between 12 and 30% of hydrogen in volume of dry gas, 6 to
14% of methane, and 56 to 72% of nitrogen, with trace amounts of
carbon dioxide, ethane and propane.
pH values and salinities of all the 45 collected water samples range from the
seawater values (8.2 and 35 g L−1) to hyperalkaline freshwaters of the
Ca-OH type (pH 11 and salinities as low as 0.3 g L−1) showing that the
collected samples are always a mixture of a hyperalkaline fluid of meteoric
origin and ambient seawater. Cl-normalized concentrations of dissolved major
elements first show that the Bain des Japonais is distinct from the other
sites. Water collected at this site are three component mixtures involving
the high-pH fluid, the lagoon seawater and the river water from the nearby
Rivière du Carénage. The chemical compositions of the hyperalkaline
endmembers (at pH 11) are not significantly different from one site to the
other although the sites are several kilometres away from each other and are located
on different ultramafic substrata. The very low salinity of the hyperalkaline
endmembers shows that seawater does not percolate through the ultramafic
formation.
Mixing of the hyperalkaline hydrothermal endmember with local seawater
produces large ranges and very sharp gradients of pH, salinity and dissolved
element concentrations. There is a major change in the composition of the
water samples at a pH around 10, which delimitates the marine environment
from the hyperalkaline environment. The redox potential evolves toward
negative values at high pH indicative of the reducing conditions due to
bubbling of the H2-rich gas. The calculation of the mineral saturation
states carried out for the Na-K-Ca-Mg-Cl-SO4-DIC-SiO2-H2O system
shows that this change is due to the onset of brucite formation. While the
saturation state of the Ca carbonates over the whole pH range is typical of
that found in a normal marine environment, Mg- and Mg-Ca carbonates
(magnesite, hydromagnesite, huntite, dolomite) exhibit very large
supersaturations with maximum values at a pH of around 10, very well marked for
the Bain des Japonais, emphasizing the role of water mixing in mineral
formation.
The discharge of high-pH waters of meteoric origin into the lagoon marine
environment makes the hydrothermal system of Prony Bay unique compared
to other low temperature serpentinizing environments such as Oman (fully
continental) or Lost City (fully marine). |
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