02 Nov 2022
02 Nov 2022
Status: this preprint is currently under review for the journal BG.

Alkalinity generation from carbonate weathering in a silicate-dominated headwater catchment at Iskorasfjellet, northern Norway

Nele Lehmann1,2,3, Hugues Lantuit2,4, Michael Ernst Böttcher5,6,7, Jens Hartmann8, Antje Eulenburg2, and Helmuth Thomas1,3 Nele Lehmann et al.
  • 1Institute of Carbon Cycles, Helmholtz-Zentrum Hereon, Geesthacht, D-21502, Germany
  • 2Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Potsdam, D-14473, Germany
  • 3Institute for Chemistry and Biology of the Marine Environment (ICBM), University of Oldenburg, Oldenburg, D-26129, Germany
  • 4Institute for Geosciences, University of Potsdam, Potsdam, D-14476, Germany
  • 5Geochemistry and Isotope Biogeochemistry, Leibniz Institute for Baltic Sea Research (IOW), Warnemünde, D-18119, Germany
  • 6Marine Geochemistry, University of Greifswald, Greifswald, D-17489, Germany
  • 7Interdisciplinary Faculty, University of Rostock, Rostock, D-18059, Germany
  • 8Institute for Geology, Center for Earth System Research and Sustainability, University Hamburg, Hamburg, D-20146, Germany

Abstract. The weathering rate of carbonate minerals is several orders of magnitude higher than for silicate minerals. Therefore, small amounts of carbonate minerals have the potential to control the dissolved weathering loads in silicate-dominated catchments. Both weathering processes produce alkalinity under the consumption of CO2. Given that only alkalinity generation from silicate weathering is thought to be a long-term sink for CO2, a misattributed weathering source could lead to incorrect conclusions about long- and short-term CO2 fixation. In this study, we aimed to identify the weathering sources responsible for alkalinity generation and CO2 fixation across watershed scales in a degrading permafrost landscape in northern Norway, 68.7–70.5° N, and on a temporal scale, in a subarctic headwater catchment on the mountainside of Iskorasfjellet, characterized by sporadic permafrost and underlain mainly by silicates as the alkalinity-bearing lithology. By analysing total alkalinity (AT) and dissolved inorganic carbon (DIC) concentrations, as well as the stable isotope signature of the latter (δ13C-DIC) in conjunction with dissolved cation and anion loads, we found that AT was almost entirely derived from weathering of the sparse carbonate minerals. We propose that in the headwater catchment, the riparian zone is a hotspot area of AT generation and release due to its enhanced hydrological connectivity, and that the weathering load contribution from the uphill catchment is limited by insufficient contact time of weathering agent and weatherable material. By using stable water isotopes, it was possible to explain temporal variations in AT concentrations following a precipitation event due to surface runoff. In addition to carbonic acid, sulphuric acid, probably originating from pyrite oxidation, is shown to be a potential corrosive reactant. An increased proportion of sulphuric acid as a potential weathering agent may have resulted in a decrease in AT. Therefore, carbonate weathering in the studied area should be considered not only as a short-term CO2 sink, but also as a potential CO2 source. Finally, we found that AT increased with decreasing permafrost probability, and attributed this relation to an increased water storage capacity associated with increasing contact of weathering agent and rock surfaces, and enhanced microbial activity. As both soil respiration and permafrost thaw are expected to increase with climate change, increasing the availability of weathering agent in the form of CO2 and water storage capacity, respectively, we suggest that future weathering rates and alkalinity generation will increase concomitantly in the study area.

Nele Lehmann et al.

Status: open (until 16 Dec 2022)

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  • RC1: 'Comment on bg-2022-205', Anonymous Referee #1, 07 Dec 2022 reply

Nele Lehmann et al.

Nele Lehmann et al.


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Short summary
Riverine alkalinity in the silicate-dominated headwater catchment at subarctic Iskorasfjellet, northern Norway, was almost entirely derived from weathering of minor carbonate occurrences in the riparian zone. The uphill catchment appeared limited by insufficient contact time of weathering agent and weatherable material. Further, alkalinity increased with decreasing permafrost extent. Thus, with climate change, alkalinity generation is expected to increase in this permafrost-degrading landscape.