Dolomite Versus Calcite Weathering in Hydrogeochemically Diverse Watersheds Established on Bedded Carbonates (Sava and Soča Rivers, Slovenia)

Abstract

The relative contributions of dolomite to calcite weathering related to riverine fluxes are investigated on a highly resolved spatial scale in the diverse watersheds of Slovenia, which previous work has shown have some of the highest carbonate-weathering intensities in the world and suggests that dolomite weathering is favored over limestone weathering in mixed carbonate watersheds. The forested Sava and Soča River watersheds of Slovenia with their headwaters in the Julian Alps drain alpine regions with thin soils (<30 cm) and dinaric karst regions with thicker soils (0 to greater than 70 cm) all developed over bedded Mesozoic carbonates (limestone and dolomite), and siliclastic sediments is the ideal location for examining temperate zone carbonate weathering. This study extends previous work, presenting geochemical data on source springs and documenting downstream geochemical fluctuations within tributaries of the Sava and Soča Rivers. More refined sampling strategies of springs and discrete drainages permit directly linking the stream Mg2+/Ca2+ ratios to the local bedrock lithology and the HCO3 concentrations to the relative soil depths of the tributary drainages. Due to differences in carbonate source lithologies of springs and tributary streams, calcite and dolomite weathering end members can be identified. The Mg2+/Ca2+ ratio of the main channel of the Sava River indicates that the HCO3 concentration can be attributed to nearly equal proportions by mass of dolomite relative to calcite mineral weathering (e.g., Mg2+/Ca2+ mole ratio of 0.33). The HCO3 concentration and pCO2 values increase as soil thickness and alluvium increase for discrete spring samples, which are near equilibrium with respect to calcite. Typically, this results in approximately 1.5 meq/l increase in HCO3 from the alpine to the dinaric karst regions. Streams in general do not change in HCO3 , Mg2+/Ca2+, or Mg2+/HCO3 concentrations down course, but warming and degassing of CO2 produce high degrees of supersaturation with respect to calcite. Carbonate-weathering intensity (mmol/km2-s) is highest within the alpine regions where stream discharge values range widely to extreme values during spring snowmelt. Overall, the elemental fluxes of HCO3 , Ca2+, and Mg2+ from the tributary watersheds are proportional to the total water flux because carbonates dissolve rapidly to near equilibrium. Importantly, dolomite weathers preferentially over calcite except for pure limestone catchments.

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Acknowledgments

We acknowledge the support of the National Science Foundation (EAR-0208182 and EAR-05-18965) and the Slovenian Research Agency. We thank the following people for their invaluable assistance in the field and laboratory: Corey Lambert, Nina Carranco, Lixin Jin, and Jennifer Macintosh. We extend our thanks to the anonymous reviewers who helped improve this manuscript.

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Correspondence to Kathryn Szramek.

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Szramek, K., Walter, L.M., Kanduč, T. et al. Dolomite Versus Calcite Weathering in Hydrogeochemically Diverse Watersheds Established on Bedded Carbonates (Sava and Soča Rivers, Slovenia). Aquat Geochem 17, 357–396 (2011). https://doi.org/10.1007/s10498-011-9125-4

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Keywords

  • Dolomite
  • Carbonate
  • Weathering
  • Rivers
  • Slovenia
  • Watersheds
  • Fluxes