Abstract
The 13C signature is evaluated for various environmental compartments (vegetation, soils, soil gas, rock and groundwater) for three crystalline rock terrains in England and Norway. The data are used to evaluate the extent to which stable carbon isotopic data can be applied to deduce whether the alkalinity in crystalline bedrock groundwaters has its origin in hydrolysis of carbonate or silicate minerals by CO2. The resolution of this issue has profound implications for the role of weathering of crystalline rocks as a global sink for CO2. In the investigated English terrain (Isles of Scilly), groundwaters are hydrochemically immature and DIC is predominantly in the form of carbonic acid with a soil gas signature. In the Norwegian terrains, the evidence is not conclusive but is consistent with a significant fraction of the groundwater DIC being derived from silicate hydrolysis by CO2. A combined consideration of pH, alkalinity and carbon isotope data, plotted alongside theoretical evolutionary pathways on bivariate diagrams, strongly suggests real evolutionary pathways are likely to be hybrid, potentially involving both open and closed CO2 conditions.
Highlights
Hydrogeochemistry, alkalinity and the global CO2 sinkRelatively little recent discussion has taken place in the scientific literature regarding the evolution of carbonate alkalinity in groundwater in crystalline silicate bedrock aquifers, such as granites, gneisses, basalts, dolerites, and schists
Manag. (2019) 5:267–287 and Frengstad 2006; Schulte et al 2011) have suggested that 13C stable isotope determinations might (“Implications for bedrock weathering as a global CO2 sink”, “Carbon isotope fractionation in the groundwater recharge process”, “Hypothetical groundwater 13C signatures in carbonate and silicate weathering systems”) be able to indicate whether this alkalinity is derived from silicate or carbonate weathering and, whether it represents a permanent or temporary CO2 sink
These ions can be derived from water–mineral interaction and it can often be important to separate out marine-derived sodium from lithologically derived sodium
Summary
Little recent discussion has taken place in the scientific literature regarding the evolution of carbonate alkalinity in groundwater in crystalline silicate bedrock aquifers, such as granites, gneisses, basalts, dolerites, and schists. (2019) 5:267–287 and Frengstad 2006; Schulte et al 2011) have suggested that 13C stable isotope determinations might (“Implications for bedrock weathering as a global CO2 sink”, “Carbon isotope fractionation in the groundwater recharge process”, “Hypothetical groundwater 13C signatures in carbonate and silicate weathering systems”) be able to indicate whether this alkalinity is derived from silicate or carbonate weathering and, whether it represents a permanent or temporary CO2 sink. It uses the data to ascertain whether this hypothesis can be readily applied in practice to ascertain if groundwater alkalinity is derived primarily from carbonate weathering (temporary C O2 sink) or silicate weathering (permanent CO2 sink)
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