Assessing the sources of inorganic carbon in surface-, soil- and non-thermal groundwater in Iceland by δ 13 C and 14 C

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Sveinbjörnsdóttir, Árný E., Stefánsson, Andri, Heinemeier, Jan, Arnórsson, Stefán, Eiríksdóttir, Eydís Salome, Ólafsdóttir, Rósa
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2020
• Subjects: Carbon isotopes; Dissolved inorganic carbon (DIC); Isotope geochemical model; Soil waters; Surface waters; Iceland
• Online Access: https://pure.au.dk/portal/en/publications/83d27768-c37d-4167-ac63-3c331d698e12; https://doi.org/10.1016/j.gca.2020.03.021; http://www.scopus.com/inward/record.url?scp=85083293101&partnerID=8YFLogxK

Chemical weathering and associated atmospheric CO 2 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 (δ 13 C and 14 C) of surface-, soil-, and non-thermal groundwater in Iceland were studied to assess the various sources and reactions of carbon, atmospheric CO 2 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 δ 18 O content. The measured range of δ 13 C DIC and 14 C DIC 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 CO 2 (g) through water-air exchange at the surface, and (3) decomposition of organic matter in soils. The majority of DIC originated from atmospheric CO 2 (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 CO 2 uptake by basalt weathering for selected rivers in East Iceland was assessed to be 0.09–1.56 mol/km 2 /yr or ∼5–40% (av. 17%) lower than total riverine CO 2 fluxes suggest.