Sources of dissolved inorganic carbon in rivers from the Changbaishan area, an active volcanic zone in North Eastern China

Abstract

Major elements and carbon isotopes of dissolved inorganic carbon (DIC) have been measured in the waters of Changbaishan mountain, a volcanic area in northeastern China, between June and September 2016 to decipher the origin of the CO₂ involved in chemical weathering reactions. Spatial variations of major elements ratios measured in water samples can be explained by a change of the chemical composition of the volcanic rocks between the volcanic cone (trachytes) and the basaltic shield as evidenced by the variations in the composition of these rocks. Hence, DIC results from the neutralization of CO₂ by silicate rocks. DIC concentrations vary from 0.3 to 2.5 mmol/L and carbon isotopic compositions of DIC measured in rivers vary from −14.2‰ to 3.5‰. At a first order, the DIC transported by rivers is derived from the chemical weathering’s consumption of CO₂ with a magmatic origin, enriched in ¹³C (−5%) and biogenic soil CO₂ with lower isotopic compositions. The highest δ¹³C values likely result from C isotopes fractionation during CO₂ degassing in rivers. A mass balance based on carbon isotopes suggest that the contribution of magmatic CO₂ varied from less than 20% to more than 70%. Uncertainties in this calculation associated with CO₂ degassing in rivers are difficult to quantify, and the consequence of CO₂ degassing would be an overestimation of the contribution of DIC derived from the neutralization of magmatic CO₂ by silicate rocks.