Fluid chemistry of the low temperature hyperalkaline hydrothermal system of Prony Bay (New Caledonia)

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, SO 4 , dissolved inorganic carbon, SiO 2 (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 highpH fluid, the lagoon seawater and the river water from the nearby Rivière du Carénage. The chemical compositions of Published by Copernicus Publications on behalf of the European Geosciences Union. C. Monnin et al.: Low temperature hyperalkaline hydrothermal system of Prony Baythe 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 H 2 -rich gas. The calculation of the mineral saturation states carried out for the Na-K-Ca-Mg-Cl-SO 4 -DIC-SiO 2 -H2O system shows...

Metagenomic and PCR-Based Diversity Surveys of [FeFe]-Hydrogenases Combined with Isolation of Alkaliphilic Hydrogen-Producing Bacteria from the Serpentinite-Hosted Prony Hydrothermal Field, New Caledonia

“…The range of the H 2 , CH 4 , and N 2 gas contents collected at the intertidal PHF sites (RK and BdJ) were 19–24%V, 6–13%V, and 67–69%V, respectively (Monnin et al, 2014). The fluids discharged by PHF sites had high pH-values ranging from 10.1 (ST07) to 11.1 (BdJ).…”

Geochemical bioenergetics during low‐temperature serpentinization: An example from the Samail ophiolite, Sultanate of Oman

“…In general, temperatures of all spring sources were elevated relative to air temperatures in January, possibly reflecting values closer to the average ambient temperature or possibly some heating from the geothermal gradient during circulation in the subsurface. The temperature and pH data obtained in this study fall within the ranges of previous studies [ Neal and Stanger , , ; Stanger , ; Boulart et al ., , ; Chavagnac et al ., ] and are in line with investigations of other hyperalkaline spring systems resulting from serpentinization [ Barnes et al ., ; Neal and Shand , ; Cipolli et al ., ; Boschetti and Toscani , ; Marques et al ., ; Boulart et al ., , ; Morrill et al ., ; Monnin et al ., , ; Cardace et al ., ; Woycheese et al ., ]. In general, as pH increases so do the concentrations of reduced species while those of oxidized species decrease.…”

“…• Data Set S18 • Data Set S19 Boschetti and Toscani, 2008;Marques et al, 2008;Wheat et al, 2008Wheat et al, , 2010Lang et al, 2010;Paukert et al, 2012;Boulart et al, 2012Boulart et al, , 2013Morrill et al, 2013;Monnin et al, 2014aMonnin et al, , 2014bCardace et al, 2015;Woycheese et al, 2015;Miller et al, 2016], and may be a mechanism for life to infiltrate deep into subduction zones. Arguments for the ultramafic nature of the crust and upper mantle of the early Earth lead to the suggestion that the emergence of life was hosted in serpentinizing systems [Sleep et al, 2004[Sleep et al, , 2011Russell, 2007;Martin et al, 2008;Russell et al, 2010;Lane et al, 2010;Lane, 2010Lane, , 2014Lane and Martin, 2012;Sousa et al, 2013;Herschy et al, 2014;Martin et al, 2014;Weiss et al, 2016].…”

Climate engineering by mimicking natural dust climate control: the iron salt aerosol method

“…Moreover, any THC decrease would further trigger the acidification of the ocean surface by lowering or preventing the neutralization of dissolved CO 2 and HCO − 3 due to the alkalinity decrease from hydrothermal sources (Monnin et al, 2014;Orcutt et al, 2011).…”