Origin of gases and waters from a hypersaline, carbonate spring on Anticosti Island, Québec, Canada

Abstract

Hypersaline spring water (TDS ∼ 85 g/L) and gas are escaping from the top of a calcite travertine mound on Anticosti Island. This mound is located in the Chaloupe River area above the Jupiter Fault which cuts through the Ordovician formations in the subsurface, including the oil and gas-rich Macasty Shale Formation. The Cl content is approximately 3 times that of seawater, but the geochemical profile indicates it to be a Paleozoic basin brine (geogenic He = 2E–7 cc/cc) evaporatively enriched beyond halite saturation (Br:Cl molar ratio = 0.0022 ± 0.00013) that has been diluted by some 73 ± 6% with meteoric waters. Measurable tritium (3.4 TU) indicates this to be a mixture of modern and Holocene recharge. The radioiodine (129I < 10 million atoms/L) is a three-component mixture of geogenic and post-nuclear contributions with low levels in pre-nuclear Holocene recharge. Degassing at the vent (1% to 2% CO2, PCO2 = 0.02 atm, >90% CH4) provokes calcite precipitation and travertine mound formation. Methane comprises a thermo-catalytic component and a biogenic component produced at depth in the organic-rich Macasty Formation by reduction of surface-derived dissolved inorganic carbon (DIC; a14C = 3.5 pMC), producing enriched δ13CDIC values recorded in the travertine. The occurrence of basin-derived brine and gas discharging in association with the surface expression of the Jupiter fault implies that this fault provides a pathway for deep circulation of meteoric waters, likely driven by the relief on the island.