Assessing the sources of inorganic carbon in surface-, soil- and non-thermal groundwater in Iceland by δ13C and 14C

Abstract

Chemical weathering and associated atmospheric CO2 uptake by surface water and transport of dissolved inorganic carbon (DIC) by rivers to the ocean and subsequent carbonate deposition has been suggested to play a major role in long-time climate changes. However, DIC in surface-, soil- and groundwater may also be sourced from decomposition of organic matter, direct leaching of carbon from the rocks themselves and deep degassing in volcanic areas and modified by chemical reactions. Carbon isotopes (δ13C and 14C) of surface-, soil-, and non-thermal groundwater in Iceland were studied to assess the various sources and reactions of carbon, atmospheric CO2 uptake upon basalt weathering, and water apparent radiocarbon age. The waters had a pH of ∼6.5–10.9, dissolved inorganic carbon (DIC) of 3.2–194 ppm, and were of meteoric origin based on the δD and δ18O content. The measured range of δ13CDIC and 14CDIC in the waters was large, −25.9 to +2.2‰ (average −8.2‰, n = 334) and 7.9–126 pMC (average 95 pMC, n = 334), respectively. Insignificant annual and seasonal variations in DIC and isotope composition were generally observed for a given location. Applying isotope modeling and comparing the results with the dataset revealed three major sources of DIC: (1) leaching of carbon from basaltic rocks, (2) atmospheric CO2(g) through water-air exchange at the surface, and (3) decomposition of organic matter in soils. The majority of DIC originated from atmospheric CO2(g) (15–100%, av. 64%) and decomposition of organic matter (0–85%, av. 34%) with the highest values observed for soil waters. Carbon from rock leaching was generally insignificant (av. 2%) except for some groundwater (3–100%). The atmospheric CO2 uptake by basalt weathering for selected rivers in East Iceland was assessed to be 0.09–1.56 mol/km2/yr or ∼5–40% (av. 17%) lower than total riverine CO2 fluxes suggest.