Abstract
Abstract. 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).
Highlights
In 2000, the discovery of the submerged Lost City hydrothermal field (LCHF) along the mid-Atlantic ridge has focused attention on the role of ultramafic rock alteration in the global geochemical cycles of carbon and hydrogen as well as in producing favorable environmental conditions for the synthesis of prebiotic molecules (FruhGreen et al, 2003; Kelley et al, 2001; Kelley et al, 2005)
The Lost City site focused the attention of the scientific community, low-temperature, high-pH hydrothermal systems have been previously reported on land in ophiolites at several locations over the world, such as in Oman (Barnes and O’Neil, 1969; Chavagnac et al, 2013b; Neal and Stanger, 1984b; Stanger, 1985), in Bosnia (Barnes et al, 1978), in northern Italy, Ligurian Alps, (Chavagnac et al, 2013b; Cipolli et al, 2004), in New Caledonia (Launay and Fontes, 1985), and more recently in Ontario (Sader et al, 2007), in Newfoundland (Szponar et al, 2012) and in California (Morrill et al, 2013)
The present results show that the hyperalkaline waters emitted at all sites in Prony Bay are of meteoric origin and that these high-pH fluids may mix with seawater at the discharge, but not in the subsurface
Summary
In 2000, the discovery of the submerged Lost City hydrothermal field (LCHF) along the mid-Atlantic ridge has focused attention on the role of ultramafic rock alteration (i.e. serpentinization) in the global geochemical cycles of carbon and hydrogen as well as in producing favorable environmental conditions for the synthesis of prebiotic molecules (FruhGreen et al, 2003; Kelley et al, 2001; Kelley et al, 2005). The highly reducing conditions of the serpentinizing environment allows the development of abiotic organic compounds (Lang et al, 2010) and hosts a large and specific community of microorganisms (Brazelton et al, 2010, 2006) Such hyperalkaline hydrothermal systems may have been numerous on the early Earth, as well as on the Martian surface (Szponar et al, 2012) and may have been the locus of the emergence of earliest forms of life (Muntener, 2010; Russell, 2007; Russell et al, 2010). Lost City and Mariana forearc hyperalkaline fluids have salinities close to that of seawater and mix with deep seawater at the vents Such hyperalkaline systems can be characterized by the type of environment in which these peculiar high-pH fluids discharge and the type of local waters with which they can mix (Table 1). Chavagnac et al (2013); Neal and Stanger (1984) Marques et al (2008) Fujii et al (2010) Cipolli et al (2004); Boschetti and Toscani (2008), Chavagnac et al (2013) Neal and Shand (2002) Morrill et al (2013)
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