TY - JOUR T1 - The chemistry of hyperalkaline springs in serpentinizing environments: 1. the composition of free gases in New Caledonia compared to other springs worldwide. A1 - Monnin,Christophe A1 - Quéméneur,Marianne A1 - Price,Roy A1 - Jeanpert,Julie A1 - Maurizot,Pierre A1 - Boulart,Cedric A1 - Donval,Jean-Pierre A1 - Pelletier,Bernard AD - Géosciences Environnement Toulouse (CNRS/UPS/IRD/CNES) 14 Avenue E Belin31400 Toulouse ,France AD - Aix Marseille Univ. Université de Toulon CNRS IRD MIO Marseille ,France AD - School of Marine and Atmospheric Sciences Stony Brook NY, USA AD - DIMENC Direction de l’Industrie, des Mines et de l’Energie Nouméa,Nouvelle‐Calédonie AD - AD2M UMR 7144 (CNRS Sorbonne Université) Station Biologique de Roscoff Place Georges Tessier, 29680 Roscoff, France AD - IFREMER Unité Géosciences Marines/Laboratoire Cycles Géochimiques ZI Pointe du Diable. CS 1007029280 Plouzané ,France AD - Géoazur (UCA, IRD, CNRS, OCA) Centre IRD de Nouméa BP A5 Nouméa, Nouvelle‐Calédonie UR - https://archimer.ifremer.fr/doc/00718/83047/ DO - 10.1029/2021JG006243 KW - serpentinization KW - methane KW - hydrogen KW - New Caledonia KW - Sabatier reaction KW - Prony Bay N2 - Serpentinization is a natural process that transforms ferromagnesian minerals such as olivine into serpentine and that produces waters at very high pH and gases enriched in methane (CH4) and hydrogen (H2). We report the composition of gases venting at two springs (Bain des Japonais and Rivière des Kaoris) of the serpentinizing environment of the Prony Bay (New Caledonia) collected eight times between 2011 and 2014, along with in situ measurements (temperature, pH, oxydo-reduction potential, dissolved oxygen content) of on-land alkaline springs of the Southern New Caledonia ophiolite. Venting gases are mainly composed of H2, CH4 and N2 and their composition has slightly varied during the 4-year field survey. An elevated oxygen (O2) content in a high-pH water sample is due to air uptake during surface flow. O2-corrected gas compositions along with those published for gas data obtained at similar serpentinizing environments (Italy, Turkey, Philippines, Oman) show that the H2 and CH4 concentrations display a linear correlation with a slope close to the value corresponding to the CH4 production from carbon dioxide rather from a less oxidized carbon such as carbon monoxide. Although these data are consistent with the stoichiometry of the Sabatier reaction, as such in the gas phase, it is also possible that microbial hydrogenotrophic methanogenesis takes place in the aqueous phase followed by degassing. A diagram is proposed that outlines the partitioning of H2 and CH4 between the gaseous and aqueous phases and the need to consider a two-phase flow in the hydrology of these hyperalkaline environments. Y1 - 2021/09 PB - American Geophysical Union (AGU) JF - Journal Of Geophysical Research-biogeosciences SN - 2169-8953 VL - 126 IS - 9 ID - 83047 ER -