Bernard Pelletier1 , Gaël ErausoL2, Emmanuelle Gérardo 3, Bénédicte Menezre3 , Christophe Monnin4, Claude Payri 5, Anne Postec2 , Roy Priceet Marianne Quéméneurm 2.

The discovery at the end of 2000, near the mid-Atlantic ridge, of white chimneys of calcium carbonate and magnesium hydroxide from the hydrothermal site of Lost City located on an ultramafic bedrock (Kelley et al., 2001) , came to modify our vision of the conditions in which life develops and could have emerged on our planet. It made it possible to highlight the importance of the role of the serpentinization process in the carbon and hydrogen cycle and in abiotic organic synthesis potentially of interest for prebiotic chemistry, a precursor to the appearance of life. The fluids emitted, linked to the alteration of rocks of mantle origin (peridotites) into serpentine, have a moderate temperature (40-90°C), a high pH (9-10) and are rich in dihydrogen (H2); they host a large community of specific microorganisms. This type of hydrothermal system is supposed to present conditions similar to those prevailing on the primitive Earth several billion years ago. Its study is therefore of great interest for studying abiotic organic synthesis, the emergence of the first forms of life but also the deep carbon cycle, the mechanisms of natural hydrogen production or the alteration mantle rocks related to serpentinization. This type of extreme environment is also of great biotechnological interest because it is potentially rich in microorganisms capable of synthesizing various molecules via original metabolisms.

However, hyperalkaline hydrothermalism had already been reported in Oman and especially observed in coastal oceanic areas, in Prony Bay, at the southern end of the large peridotite massif of New Caledonia (Photo 1 and Fig. 1). Launay and Fontes (1985) had in fact described low-temperature (30-40°C) and hyperalkaline (pH 11) water sources there, as well as precipitates of calcium carbonates and brucite (magnesium hydroxide). , both on land, at the northern end of the bay, on a shoal of Carénage Bay (Japanese Bath) and at the mouth of the Kaoris River (sources already reported by Garnier in 1873), and ‘at sea, in the middle of the northern part of the bay, in the form of the famous Prony needle (Photo 2). This 38 m high needle was previously considered as a coral reef before its discovery as a hydrothermal spring by divers from ORSTOM (now Institut de Recherche pour le Développement, IRD) in February 1975. Launay and Fontes (1985) thus provided a first description of the composition of these particular waters which rise at the level of the foreshore and determined the mineralogical composition of the precipitates on land and at sea (at the level of the foreshore and the submarine needle). Despite these works, this hydrothermal site did not arouse the interest it deserved at the time.

The rediscovery of the Prony Bay hydrothermal field and its extension

It was only in the mid-2000s, during seabed mapping campaigns using multibeam sonar and scuba diving (film “Geological curiosities pique the scientists” on in 2005; Pelletier et al., 2006), that the importance of the Prony Bay hydrothermal site, which has the same mineralogical, chemical and biological characteristics as that of Lost City, has been recognized and that its scope has been revealed.

One evening in May 2004, during an unexpected incursion in Prony Bay to anchor during a mapping and seismic reflection campaign in the southern lagoon of New Caledonia, basins 5 to 6 meters deep and 200 meters in diameter were observed by 45-50 m depth thanks to the multibeam sounder of the oceanographic vessel (N.O.) Alis of the IRD. These basins, located in the southern part of the bay of Prony, included in their center one or several peaks 4 to 5 meters high (Fig. 2). Given the presence of thermal springs on land and the underwater hydrothermal needle located to the north of the bay (Fig. 3), the hypothesis of a hydrothermal origin for these structures is then immediately put forward. Despite the keen interest aroused on board, these observations, outside the objectives of the current campaign, had not been continued.

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