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

Abstract

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 ‰ <δ¹³C_TDC <+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 δ¹³C_TDC values reaching up to +28‰, that have chloride as the dominant anion. From the detailed isotopic measurements of CO₂(g) (δ¹³C_C0[2(g)]) and of the corresponding total dissolved carbonates (δ¹³C_TDC) it is found which fluids are in isotopic equilibrium with accompanying CO₂ gas and which are not. Because of the variability of the δ¹³C_C0[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 δ¹³C_C0[2(g)] and δ¹³C_HC0[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 δ¹³C_C0[2(g)] and δ¹³C_HC0[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 C_C0[2(g)]/C_HC0[3] the δ¹³C_C0[2(g)] and δ¹³C_HC0[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 P_CO[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 δ¹³C_C0[2(g)] values decrease, only the highest δ¹³C_C0[2(g)] values can be considered as non-fractionated and may possibly reveal the origin of CO₂. Lower δ¹³C_C0[2(g)] values result from isotopic fractionation and are not useful as indicators of CO₂ origin. These findings support the view that low δ¹³C_C0[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 δ¹³C_{C0[2 (g)]} remains constant during CO₂-H₂0 flow to the surface has been shown to be invalid. High δ¹³C_C0[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, (δ¹³C_TDC up to 28‰), carbon dioxide originates from the decarboxylation of acetic acid.