Origin of carbon dioxide and evolution of CO[2]-rich waters in the West Carpathians, Poland

Paweł M. Leśniak


The research was carried out to determine the origin of carbon dioxide and to track the possible isotopic evolution of carbonated waters of the flysch Carpathians. Carbon isotopic composition (-9.2‰ < δ13CC0[2(g)] <-1.8‰, -7.1 ‰ <δ13CTDC <+5.4‰) of the unusually CO2-rich, cold waters of various salinity, with bicarbonate as the dominant anion, is characteristic for an CO2-water open system. It differs from carbon isotopic composition of waters of the closed-CO2 system with δ13CTDC values reaching up to +28‰, that have chloride as the dominant anion. From the detailed isotopic measurements of CO2(g) (δ13CC0[2(g)]) and of the corresponding total dissolved carbonates (δ13CTDC) it is found which fluids are in isotopic equilibrium with accompanying CO2 gas and which are not. Because of the variability of the δ13CC0[2(g)] values simple assignment of a definite single or multiple CO2 source (s) is rather ambiguous. To evaluate the main factors which influence the isotopic composition, two limiting models of kinetic fractionation occurring during a vertical advective flow of carbonated solution and/or CO2 gas are derived. The models suggest that both the δ13CC0[2(g)] and δ13CHC0[3] values can be modified on the way of the CO2-H20 mixture to the Earth surface. The model of parallel flow of CO2(g) and carbonated solution predicts that the δ13CC0[2(g)] and δ13CHC0[3] values can be only slightly changed towards negative δ13C values at Damkohler number (Da) equal 1. In the one-phase CO2 flow model, due to decreasing CC0[2(g)]/CHC0[3] the δ13CC0[2(g)] and δ13CHC0[3] equilibrium values (Da=l) display a clearly declining trend along the vertical profile. Generally, the isotopic shift with respect to the original input value depends mainly on PCO[2] gradient, Damköhler number and the difference in the velocity of CO2 gas and carbonated solution. Because during both the equilibrium or non-equilibrium fluid flow the δ13CC0[2(g)] values decrease, only the highest δ13CC0[2(g)] values can be considered as non-fractionated and may possibly reveal the origin of CO2. Lower δ13CC0[2(g)] values result from isotopic fractionation and are not useful as indicators of CO2 origin. These findings support the view that low δ13CC0[2(g)] values considered sometimes as an evidence of mantle or organic provenance of carbon dioxide in the West Carpathians or elsewhere are in fact significantly modified during the ascendence flux of CO2 gas to the Earth surface. The common assumption that δ13CC0[2 (g)] remains constant during CO2-H20 flow to the surface has been shown to be invalid. High δ13CC0[2(g)] values around -2‰ suggest that CO2 gas in the CO2-charged waters in the West Carpathians is predominantly derived from the thermal decomposition of the carbonate and silicate rocks. In the closed systems with respect to CO2, (δ13CTDC up to 28‰), carbon dioxide originates from the decarboxylation of acetic acid.

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