Relative Weathering Intensity of Calcite Versus Dolomite In
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Article Geochemistry 3 Volume 8, Number 4 Geophysics 10 April 2007 Q04002, doi:10.1029/2006GC001337 GeosystemsG G ISSN: 1525-2027 AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society Relative weathering intensity of calcite versus dolomite in carbonate-bearing temperate zone watersheds: Carbonate geochemistry and fluxes from catchments within the St. Lawrence and Danube river basins Kathryn Szramek Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA Now at Department of Geology, Washington and Lee University, Lexington, Virginia 24450, USA ([email protected]) Jennifer C. McIntosh Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA Now at Department of Hydrology and Water Resources, University of Arizona, Tucson, Arizona 85716, USA Erika L. Williams Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA Now at ARCADIS U.S., Inc., Millersville, Maryland 21108, USA Tjasa Kanduc and Nives Ogrinc Department of Environmental Sciences, ‘‘J. Stefan’’ Institute, Jamova 39, 1000 Ljubljana, Slovenia Lynn M. Walter Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA [1] Calcite and dolomite solubilities in open weathering environments are proportional to pCO2 and inversely proportional to temperature, and dolomite solubility is progressively greater than calcite below 25°C. The continent-scale weathering budget reveals the significance of the Northern Hemisphere (NH) to 2+ 2+ À globally integrated riverine fluxes of Ca ,Mg , and HCO3 . The NH contributes 70% of the global À HCO3 flux while only 54% of the riverine discharge. We present results of a comparative hydrogeochemical study of carbonate mineral equilibria and weathering fluxes in two NH carbonate- rich river basins. Surface water geochemistry and discharge were determined for headwater streams in Michigan and Slovenia within the St. Lawrence and Danube river basins. Michigan watersheds are established atop carbonate-bearing glacial drift deposits derived from erosion of Paleozoic strata with thick soil horizons (100–300 cm). Slovenia watersheds drain Mesozoic bedrock carbonates in alpine and dinaric karst environments with thin soil horizons (0–70 cm). Carbonate weathering intensity is a parameter that À À À2 À1 normalizes river runoff and HCO3 concentration to catchment area (meq HCO3 km s ), summing calcite and dolomite contributions, and is used to gauge the effects of climate, land use, and soil thickness on organic-inorganic carbon processing rates. Importantly, Michigan riverine discharge is one-tenth of Slovenian rivers, providing the opportunity to evaluate the kinetics of carbonate mineral equilibration. The À 2+ 2+ study rivers are HCO3 À Ca À Mg waters, supersaturated for calcite at pCO2 values in excess of the Copyright 2007 by the American Geophysical Union 1 of 26 Geochemistry 3 szramek et al.: calcite and dolomite weathering Geophysics 10.1029/2006GC001337 Geosystems G À 2+ atmosphere. As discharge varies, HCO3 concentrations differ by less than 20% for any location, and Mg / Ca2+ remains relatively fixed for Michigan (0.5) and Slovenia streams (0.4), requiring that dolomite dissolution exceed calcite on a mole basis. The ability of calcite and dolomite dissolution to keep pace with increased discharge indicates carbonate weathering is limited only by water flux and temperature- dependent solubility in these watersheds. Carbonate weathering intensity in Michigan and Slovenia exceeds the world average by factors between 2 and 20, and dolomite weathering intensity, estimated from riverine Mg2+ fluxes, exceeds the world average by factors between 2 and 15. Thus global fluxes of carbonate-related weathering products appear heavily skewed toward carbonate-bearing environments at higher latitudes with relatively low mean annual temperatures and high discharge. Components: 19,887 words, 14 figures, 1 table. Keywords: rivers; carbonate mineral weathering; dolomite; geochemical fluxes; calcite. Index Terms: 1806 Hydrology: Chemistry of fresh water; 1879 Hydrology: Watershed; 1886 Hydrology: Weathering (0790, 1625). Received 14 April 2006; Revised 5 July 2006; Accepted 20 September 2006; Published 10 April 2007. Szramek, K., J. C. McIntosh, E. L. Williams, T. Kanduc, N. Ogrinc, and L. M. Walter (2007), Relative weathering intensity of calcite versus dolomite in carbonate-bearing temperate zone watersheds: Carbonate geochemistry and fluxes from catchments within the St. Lawrence and Danube river basins, Geochem. Geophys. Geosyst., 8, Q04002, doi:10.1029/2006GC001337. 1. Introduction [3] Chemical weathering in the terrestrial environ- ment is often quantified by using dissolved ele- [2] Mineral weathering is an important regulator of mental riverine fluxes. Early studies attempted to global atmospheric CO2 concentrations. During quantify solute fluxes to the world’s oceans and silicate weathering, riverine HCOÀ is derived en- therefore focused on the largest drainage basins, 3 6 2 tirely from the atmosphere and is a net long-term typically greater than 1 Â 10 km , such as the sink for CO2. Carbonate weathering draws down Amazon, Congo, and Mississippi Rivers [e.g., even greater amounts of atmospheric CO2, but the Kempe, 1982; Meybeck, 1979, 1987]. Such large withdrawal is relatively short term because CO2 is river basins integrate varying climates, lithology, then returned to the atmosphere by carbonate soil composition, soil depth and land use types precipitation in the oceans. The sequestration of (e.g., agricultural, urban, and forested). In order to atmospheric CO2 stored in freshwater and shallow examine these individual factors, which can have a oceans thus can be significant on timescales of large influence on river chemistry, previous studies hundreds of years making the carbonate system in smaller scale catchments attempt to control for dynamics of surface waters germane to the increas- lithology [Bluth and Kump, 1994; Gaillardet et al., ing anthropogenic perturbations to the global car- 1999; Jacobson et al., 2002], others for lithology in bon cycle. As CO2 levels increase, riverine specific climates [White and Blum, 1995; Huh et chemical fluxes (e.g., dissolved organic and inor- al., 1998; Huh and Edmond, 1999; He´lie et al., ganic carbon, nutrients) may be influenced by local 2002; Millot et al., 2003], while a few studies changes in organic-inorganic carbon transforma- investigate a link between soil and land use type tion rates, vegetation, and rainfall. Watershed [Telmer and Veizer, 1999; Finlay, 2002]. adjustments, in response to climate change, will [4] On a global scale, the importance of mineral likely be most evident in the smaller headwater weathering on different landmasses is determined streams. Herein, we present a hydrogeochemical by the runoff and the proportions of silicate versus database of the headwater reaches of two major carbonate rock within the drainage basins [e.g., northern hemisphere drainage basins, the St. Law- Gibbs and Kump, 1994; Amiotte Suchet and rence and the Danube, which we use to better Probst, 1995; Ludwig et al., 1998; Amiotte Suchet quantify the processes controlling rates and path- et al., 2003]. The average continental, riverine ways of carbonate mineral weathering in carbonate- À À1 HCO3 concentrations (in meq L ) in ascending dominated watersheds of very different geologic order are: S. America, 0.40; Africa 0.45; Oceania, and climatic settings. 1.10; Asia, 1.10; N. America, 1.20; and Europe 2of26 Geochemistry 3 szramek et al.: calcite and dolomite weathering Geophysics 10.1029/2006GC001337 Geosystems G À 1.40 [e.g., Berner and Berner, 1996]. Bicarbonate highest HCO3 flux. Aridity and silicate cratonic À1 À values of <0.5 meq L are generally observed in bedrock contribute to the low African HCO3 flux. pure silicate catchments while higher values reflect À the increasing contribution of carbonate weather- [5] The normalization of HCO3 fluxes to drainage À area allows for a comparison of weathering ‘‘in- ing. The continental HCO3 fluxes are influenced by both lithology and runoff. The Asian continent tensities’’ between drainage basins [see Amiotte À Suchet and Probst, 1993]. The weathering ‘‘inten- contributes the highest total annual flux of HCO3 15 À À1 sity’’ is a proxy for lithology (e.g., silicate versus (15 Â 10 meq HCO3 year ), while N. America, S. America, Europe and Africa contribute, in carbonate), age and development of bedrock/soils 15 15 (e.g., how deeply weathered), land type/use, and descending order, 6.5 Â 10 ,5.6Â 10 ,4.3Â 15 15 climate. For a given lithology, high runoff values 10 and 2.3 Â 10 , respectively. Abundant car- bonate available for weathering coupled with high are associated with increasing weathering fluxes annual precipitation contributes to Asia having the [White and Blum, 1995; Ludwig et al., 1998; Millot et al., 2003]. Amiotte Suchet and Probst [1993] note that rivers draining carbonate terrain have a À HCO3 flux 17 times higher than that for silicate lithologies at the same runoff value. These relations are illustrated in Figure 1a, which shows carbonate À À2 À1 weathering intensity (meq HCO3 km s ) versus specific runoff (L kmÀ2 sÀ1) for the different con- tinents. The areas dominated by carbonate mineral weathering (N. America, Asia, Europe) have the highest carbonate weathering ‘‘intensities’’, while those draining silicate-dominated terrains (Africa and S. America) have lower carbonate weathering