Article pubs.acs.org/est
Climate Change Increasing Calcium and Magnesium Leaching from Granitic Alpine Catchments Jiří Kopácě k,*,† Jiří Kaňa,† Svetlana Bičaŕ ová,‡ Ivan J. Fernandez,§ Josef Hejzlar,† Marie Kahounová,∥ Stephen A. Norton,⊥ and Evžen Stuchlík† Biology Centre CAS, Institute of Hydrobiology, 370 05 Č eské Budějovice, Czech Republic Earth Science Institute, Slovak Academy of Sciences, 059 52 Stará Lesná, Slovak Republic § University of Maine, School of Forest Resources and Climate Change Institute, Orono, Maine 04469, United States ∥ Charles University in Prague, Institute for Environmental Studies, 128 01 Prague, Czech Republic ⊥ University of Maine, School of Earth and Climate Sciences and Climate Change Institute, Orono, Maine 04469, United States † ‡
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ABSTRACT: Climate change can reverse trends of decreasing calcium and magnesium [Ca + Mg] leaching to surface waters in granitic alpine regions recovering from acidification. Despite decreasing concentrations of strong acid anions (−1.4 μeq L−1 yr−1) during 2004−2016 in nonacidic alpine lakes in the Tatra Mountains (Central Europe), the average [Ca + Mg] concentrations increased (2.5 μeq L−1 yr−1), together with elevated terrestrial export of bicarbonate (HCO3−; 3.6 μeq L−1 yr−1). The percent increase in [Ca + Mg] concentrations in nonacidic lakes (0.3−3.2% yr−1) was significantly and positively correlated with scree proportion in the catchment area and negatively correlated with the extent of soil cover. Leaching experiments with freshly crushed granodiorite, the dominant bedrock, showed that accessory calcite and (to a lesser extent) apatite were important sources of Ca. We hypothesize that elevated terrestrial export of [Ca + Mg] and HCO3− resulted from increased weathering caused by accelerated physical erosion of rocks due to elevated climate-related mechanical forces (an increasing frequency of days with high precipitation amounts and air temperatures fluctuating around 0 °C) during the last 2−3 decades. These climatic effects on water chemistry are especially strong in catchments where fragmented rocks are more exposed to weathering, and their position is less stable than in soil.
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temperature can explain elevated dissolution and fluxes of sodium (Na) and silica (Si) from silicate rocks but has a limited effect on dissolution and leaching of calcium (Ca) and magnesium (Mg).10 Some studies, however, showed that Ca and Mg concentrations increased in some lakes with silica-rich bedrock during their recovery from acidification, or at least their rate of decrease slowed compared to the rate of concentration decrease of sulfate (SO42−), and attributed these patterns to climate warming and accelerated weathering.11−13 Hence, a question arises: Are climatic effects responsible for increased weathering and leaching of divalent base cations from catchments in areas with silicate-rich mineralogies? Mast et al.14 suggested an important role of carbonate minerals in controlling the chemistry of surface waters in alpine granitic catchments. Some carbonate minerals may originate
INTRODUCTION There is little doubt that climate change affects the chemical and biological recovery of freshwaters from acidification, although these effects may be ambiguous, i.e., reinforcing the recovery but also resulting in acidic events that delay recovery.1 Numerous studies have shown how precipitation, changes in water flow pathways, summer drought, snowmelt, and sea-salt deposition can affect nitrogen, carbon, and sulfur cycles, as well as cation exchange in soils, resulting in acidic episodes.2−4 Other studies indicate that emerging patterns in long-term weather, involving warming and/or greater precipitation, increase the rate of chemical weathering of soil minerals and accelerate recovery of acidified waters.5−7 Many of the climaterelated chemical changes were attributed to biological processes in soils and lakes. However, elevated concentrations of base cations observed in high elevation lakes, where large parts of their catchments consist of igneous and metamorphic exposed bedrock and scree (broken rock fragments at the base of cliffs and valley shoulders), have provided evidence that these rocky areas may have an important weathering contribution, which can potentially be magnified by climate warming.8,9 Increasing © XXXX American Chemical Society
Received: Revised: Accepted: Published: A
July 16, 2016 December 4, 2016 December 8, 2016 December 8, 2016 DOI: 10.1021/acs.est.6b03575 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology
representative catchment-lake systems (Table SI-1), covering wide ranges in their recent chemistry (mean pH of 5.1−7.4 during 2004−2016) and catchment cover (forest to alpine regions, the latter dominated by exposed bedrock and scree areas). Six lakes (herein denoted as acidic lakes) had ANC < 20 μeq L−1 (one equivalent = one mole of charge) throughout the 13-year period and remained acidic in 2016. They were located in forest, dwarf pine bushes or alpine meadows, and had a large proportion of their catchment covered with soils (Table SI-1). In contrast, 25 lakes (herein denoted as nonacidic lakes) either had already recovered from acidification (their ANC was >20 μeq L−1 in 2004−2016, while it was 30 mm day−1 increased during 1992−2015 (Figure 3f). (3) Frequency of days with minimum air temperature 5 °C continuously increased during 1992−2015 (Figure 3d)
Figure 3. Time series in climate characteristics in the Tatra Mountains (Skalnaté Pleso meteorological observatory, elevation of 1778 m): (a) Annual average air temperature; (b) annual precipitation amount; number of days with (c) no snow cover, (d) minimum air temperature below 0 °C and maximum air temperature above 5 °C, (e) no precipitation, and (f) precipitation amount >30 mm day−1. Solid red lines show significant linear regressions for the 1992−2015 period (full circles); R2 is coefficient of determination, and p indicates the significance of the relationship.
Leaching Experiments. Chemical and mineralogical composition of the granodiorite samples differed: (1) Samples A and B had approximately 50% plagioclase, the remainder being dominated by orthoclase and quartz. Sample C contained nearly equal proportions of quartz, plagioclase, and orthoclase. (2) Samples A and C contained 64 and 46 μmol g−1 carbonate; sample B did not have detectable carbonate (
