Geochimica et Cosmochimica Acta, Vol. 63, No. 5, pp. 587–598, 1999 Copyright © 1999 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/99 $20.00 1 .00 Pergamon PII S0016-7037(99)00067-8 Field weathering rates of Mt. St. Helens tephra R. A. DAHLGREN,1,* F. C. UGOLINI,2 and W. H. CASEY1 1 2 Department of Land, Air and Water Resources, University of California, Davis, CA 95616 USA Dipartimento Di Scienza Del Suolo E Nutrizione Della Pianta, Universita Degli Studi, Piazzale Delle Cascine, 15, 50144 Firenze, Italy (Received March 31, 1998; accepted in revised form December 3, 1998) Abstract—The initial stages of chemical weathering in tephra were examined under field conditions in a cool and humid forest ecosystem in the Cascade Mountains of Washington. Unleached tephra from the 1980 eruption of Mt. St. Helens was applied in 5 cm and 15 cm depths to simulate natural tephra deposition. Leachate solutions from the tephra were then collected and analyzed over a 4 year period. Concentrations of dissolved elements were combined with the water fluxes to determine elemental fluxes from tephra and to estimate chemical weathering rates. Solutions leached from the tephra layer indicate incongruent dissolution resulting in formation of a cation-depleted, silica-rich leached layer on glass and mineral surfaces. Measured weathering rates were 1–3 orders of magnitude less than comparable rates reported in the literature for laboratory dissolution studies, but considerably greater than those measured for entire watersheds in field studies. Dissociation of carbonic acid, originating primarily from upward transport of carbon dioxide from the buried soil, was the dominant source of protons for weathering reactions. Weathering rates in the 5 cm treatment were approximately twice those of the 15 cm treatment. A greater flux of CO2 per unit volume of tephra in the 5 cm treatment is believed to be responsible for the differential weathering rates. Copyright © 1999 Elsevier Science Ltd (Shoji et al., 1993a). As a result, soil solutions become oversaturated with respect to several poorly ordered solid phase materials (Dahlgren and Ugolini, 1989a). The presumed rapid precipitation rates of noncrystalline materials relative to crystalline minerals favor formation of these metastable solid phases. These metastable materials are very important because they impart several distinctive physical (e.g., low bulk density, very friable, high water holding capacity) and chemical (e.g., high organic matter accumulation, phosphorus fixation, variable charge characteristics) properties to soils (Shoji et al., 1993a). Laboratory weathering of volcanic glass shows an initial period of rapid hydration and release of cations through surface exchange with aqueous hydrogen ions (White and Claassen, 1980; White, 1983). This incongruent dissolution forms a cation depleted leached layer near the glass surface. The extent of cation depletion and the thickness of the leached layer both increase as the pH decreases (White, 1983). As weathering progresses, dissolution rates are controlled by surface dissolution with concurrent diffusion of cations through the leached layer near the glass surface. A shift from incongruent to congruent dissolution presumably occurs when the increase in the diffusion length equals the rate of retreat of the solution-solid interface (White, 1993). However, many complicated reactions, such as pore formation and hydration of the glass framework, affect the diffusivity of the glassy material (Casey and Bunker, 1990). Short term (,60 d) laboratory glass dissolution studies in the pH range 5–7 exhibit parabolic rate constants between 10214 and 10213 mol/cm2/s0.5 for base cations, such as sodium, and linear rate constants of between 10216 and 10215 mol/cm2/s for silicon and aluminum (White and Claassen, 1980; White, 1983). Similarly, Shoji et al. (1993b) measured rates for aluminum release from colored (basaltic andesite composition) and noncolored (rhyolite composition) glass on the order of 1. INTRODUCTION There is a paucity of information concerning weathering rates of geologic materials under field conditions. Current field weathering rates are often estimated from watershed elemental budgets (e.g., Cronan, 1985; Velbel, 1985; April et al, 1986; Dethier, 1986; White and Blum, 1995) or isotope abundance (e.g., Aberg and Jacks, 1985; Aberg et al., 1989; Blum et al., 1993; Bullen et al., 1997) studies. Time averaged weathering rates are also determined from changes in the elemental composition of soil profiles developed in deposits of known initial elemental composition and age (Brimhall and Dietrich, 1987; Merritts, et al., 1992; White et al., 1996). All of these methodologies suffer from a variety of assumptions and the imprecision of measuring small changes in large elemental pools (Sverdrup, 1990). Dorn and Brady (1995) were able to avoid many of the problems associated with determination of long term (.105 years) weathering rates by measuring microscopic dissolution of plagioclase minerals in exposed rock surfaces. While there is an abundance of laboratory derived weathering rates for various minerals, the efficacy of extrapolating these rates to the field is not well known. Thus, there is a critical need to document weathering rates under a wide range of field conditions. Tephra consists of fragmentary volcanic materials, such as ash, dust, cinders, and volcanic bombs, given off during an eruption. It is composed of remnants of the volcanic vent and materials solidified from magma during the eruption. The weathering of tephra is of particular interest since its weathering in the soil environment commonly leads to formation of metastable noncrystalline materials (Lowe, 1986). Rapid weathering of these glassy particles is believed to release elements faster than secondary crystalline minerals can form *Author to whom correspondence should be addressed. 587 588 R. A. Dahlgren, F. C. Ugolini and W. H. Casey 10212 mol/g/s at pH 4.0. Release rates were 1.5 times greater in the colored (i.e., more mafic) glass and increased approximately 1.5 times for each 10°C increase in temperature between 0 and 30°C. Weathering in the soil environment is strongly affected by soil temperature, leaching intensity, flux of protons, and the pH and complexing ligand concentrations of the percolating soil solution (Lowe, 1986; Shoji et al., 1993a). White et al. (1986) found no detectable changes in the bulk chemistry of tephra in the vicinity of Mt. St. Helens after two years of field weathering; however, they showed extensive chemical change to the surfaces of tephra as detected with X-ray photoelectron spectroscopy. Both calcium and sodium were depleted in the surface layer (.10 nm) with a more rapid depletion of calcium than sodium. A 10 year study examining alterations to previously unweathered tephra in a subalpine forest in the Washington Cascades showed a decrease in pH (;1 unit for a 1:1 tephra:water suspension) and an appreciable accumulation of aluminum and iron alteration products compared to the original unweathered material (Dahlgren et al., 1997). Aluminum accumulated as Al-humus complexes and hydroxy-Al polymers in the interlayer of 2:1 layer silicates while iron accumulated as Fe-humus complexes and ferrihydrite. In contrast to these short term investigations, long term studies of weathering from tephra deposits indicate half lives for volcanic glass fragments of 1650 to 7000 years (Kirkman and McHardy, 1980; Ruxton, 1988; Shoji et al., 1993b). While several studies report short term laboratory weathering and long term field weathering characteristics of tephra and volcanic glass, no previous study has measured actual weathering rates for tephra under field conditions. Thus, the primary objective of this study was to provide a direct measurement of weathering rates from previously unweathered tephra deposits in a cool/humid climatic regime. We sought to answer several questions concerning the initial stages of tephra weathering including (1) what are the initial weathering rates of tephra under field conditions?, (2) does tephra dissolve stoichiometrically or incongruently?, and (3) do weathering rates depend on the depth of the tephra deposit? To answer these questions, 5 cm and 15 cm layers of unweathered tephra from the 1980 eruption of Mt. St. Helens were applied to the surface of a forest soil to simulate natural tephra deposition. Leachates from the tephra were collected and analyzed for a 4 year period and used to calculate elemental fluxes and weathering rates. Companion studies provide a detailed discussion of the effects of tephra deposition on soil processes and changes to the physical, chemical and mineralogical properties of the tephra after 10 years of weathering (Dahlgren and Ugolini, 1989b; Dahlgren et al., 1997). 2. MATERIALS AND METHODS 2.1. Study Area The investigation was conducted at the Findley Lake Reserve and Research Area on the western slopes of the Cascade Range, Washington. This area did not receive airfall tephra during the 1980 eruptive episode of Mt. St. Helens. The study site is located in the upper montane forest zone at an elevation of 1150 m. Mean annual air temperature is 5.5°C and annual precipitation averages 230 cm with approximately 10% occurring during the summer (National Oceanic and Atmospheric Administration, 1982–1986). Winter snowpack com- Table 1. Total elemental analysis and mole fraction of each metal in fresh tephra from the 18 May 1980 eruption of Mt. St. Helens. Element Concentration --- g kg21 --- Mole fraction of metal (%) SiO2 Al2O3 Fe2O3 MgO CaO Na2O K2O TiO2 P2O5 MnO 595 177 67.5 31.5 63.2 44.3 9.7 7.9 1.6 1.1 55.3 19.4 4.7 4.4 6.3 8.0 1.2 0.6 0.1 0.1 Total 998.8 100 monly exceeds 3 m but the soils remain unfrozen due to the insulating effect of the snowpack. Vegetation is dominated by a mature (185 years old at the start of the study) stand of Pacific silver fir (Abies amabilis, Dougl.-Forbes) with mountain hemlock (Tsuga mertensiana, Bong.Carr) as an associated species. Soils are classified as moderately well drained Andic Humicryods (Soil Survey Staff, 1997) and are composed of ;30 cm of tephra deposits overlying andesitic glacial drift. The soils have received 3 identifiable additions of tephra during the Holocene: (1) Mt. Mazama, 7000 – 6700 yr B.P. (Sarna Wojcicki et al., 1983), (2) Mt. St. Helens Yn, 3500 yr B.P., and (3) Mt. St. Helens Wn, 450 yr B.P. (Mullineaux, 1986). 2.2. Tephra Characteristics Tephra was collected from airport runways at Yakima, Washington shortly after the 18 May 1980 eruption. A detailed description of the physical, chemical and mineralogical properties of the unweathered tephra was previously published (Dahlgren et al., 1997); thus, only a summary of important properties is provided here. The mineralogical composition of the very fine sand fraction consisted of volcanic glass (36%), plagioclase (29.9%), glassy aggregates (27.5%), pyroxene-amphibole (4.2%), cristobalite-quartz (1.2%), and opaque oxides (1.2%). The glassy aggregates consist of a variable thickness of glass coating microlites of plagioclase (;70%) and ferromagnesian (;30%) minerals. The volcanic glass is classified as dacitic with a chemical composition very similar to the bulk tephra (Table 1) and the plagioclase was of andesine composition with a Ca/Na atomic ratio of '1.0. Particle size distribution was dominated by sand (2– 0.05 mm 5 86%) with lesser amounts of silt (0.05– 0.002 mm 5 11%) and clay (,0.002 mm 5 3%). The surface area of the unweathered tephra as determined by the BET method (Brunauer et al., 1938) was 0.57 m2/g. 2.3. Collection and Analysis of Tephra Leachates The tephra was collected prior to any rainfall and sealed in watertight containers until it was applied to the study plots. Triplicate plots (2 3 2 m) receiving either a 5 cm or 15 cm thickness of tephra were established in July 1982 as described by Dahlgren and Ugolini (1989b). The bulk density of the tephra layer following application was 1.32 g/cm3. Tension lysimeter plates (7.6 cm diameter) were placed beneath the tephra layer at the time of tephra addition and a 10 kPa tension was applied to collect leachates. The temperature in the middle of the tephra layer was measured during the first 2 years of the study using thermocouples. These data indicated that the temperature of both the 5 cm and 15 cm tephra layer was approximately equal to the mean daily air temperature during the snow free period and near 0°C when the snow pack was present. Canopy throughfall (the solution reaching the soil surface after interaction with the tree canopy) was collected at 6 sites adjacent to the tephra plots using 4 L collection bottles fitted with a funnel containing teflon wool to act as a coarse filter. Tephra leachates and canopy throughfall were collected at an approximately monthly interval or as precipitation and snowmelt events allowed over a 4 year period commencing with addition of tephra in Field weathering rates of Mt. St. Helens tephra July 1982 and ending in July 1986. Tephra leachates were collected continuously between each collection date, thus the chemical composition of the leachate represents an integration (e.g., volume weighted mean) of all leachates over that time period. Solutions were filtered through a prerinsed 0.2 mm polycarbonate membrane filter and refrigerated at 3°C through completion of analyses. The pH of the solutions was measured potentiometrically with a glass electrode. Total concentrations of iron, aluminum, silicon, calcium, magnesium, potassium and sodium were determined by ICP spectroscopy. Major anions (Cl2, 2 22 ortho-POn2 4 , NO3 , and SO4 ) were quantified by ion chromatography and bicarbonate by titration to an endpoint of pH 5 4.5. Dissolved organic carbon was determined using an O.I. Model 700 TOC analyzer (O.I. Corp., College Station, TX). 2.4. Determination of Elemental Fluxes and Weathering Rates Elemental fluxes leaching from the tephra were calculated from measured solute concentrations and estimated water fluxes determined from a water balance. Measured values of canopy throughfall (i.e., the amount of water actually reaching the soil surface) minus losses to evaporation were used to determine the water flux through the tephra layer during the time period when precipitation occurred as rainfall. Evaporation during the snow-free period was estimated using the model of Thornthwaite (1948) and transpiration from the tephra layer was assumed to be negligible compared to the volume of leachate because very few roots were found in the tephra layer. During periods with a snowpack, the water flux through the tephra layer is regulated by the melting of the snowpack. We used daily measurements of the precipitation and snowpack water equivalent from the nearby weather station at Stampede Pass to calculate a water balance for the snowpack and to estimate water flux from the snowpack (National Oceanic and Atmospheric Administration, 1982–1986). Stampede Pass is 30 km from Findley Lake and both sites are at the same elevation. Increases in the water equivalent of the snowpack result from precipitation events. The amount of water entering the snowpack was calculated as the precipitation amount minus canopy interception. Canopy interception (the amount of water evaporated or sublimated directly from the canopy) was measured using snow boards placed beneath the canopy and in the open. Loss of water equivalents from the snowpack occurs by sublimation and melting/drainage. The sublimation rate directly from the snowpack was estimated using published values from the U.S. Army, Corp of Engineers (1956) snow hydrology investigations. A decrease in the snowpack water equivalent that is not attributable to sublimation was taken as the water flux exiting the snowpack and entering the tephra layer. The consistency between snowpack water dynamics at Findley Lake and Stampede Pass was confirmed by several measurements of snowpack water equivalent at the study area throughout the snowpack season. The elemental fluxes released from the tephra layer were calculated as the difference in flux between tephra leachates (flux leaving tephra layer) and the canopy throughfall (flux entering the tephra layer). By the 4th year of the study, approximately 50% of the tephra surface was covered by a thin layer of litter. Leaching of solutes from this incipient litter layer was considered negligible compared to the flux of solutes released from weathering of the tephra. 2.5. Statistical Analyses Statistical analyses were performed using SYSTAT for Windows (SYSTAT Inc., Evanston, IL). Differences in solute concentrations between the 5 cm and 15 cm treatments were tested using a repeated measures ANOVA with tephra depth (5 or 15 cm) as the factor; least significant difference (LSDs) values were computed to test for differences between means as a function of tephra depth (Hoshmand, 1994). Statistical differences between weathering rates for the 5 cm and 15 cm treatments were tested using a t-test. All statistical differences were tested at the p 5 0.05 level. 3. RESULTS AND DISCUSSION Concentrations of solutes in canopy throughfall and tephra leachates are shown in Figs. 1–3. Significant differences 589 (p , 0.05) between solute concentrations of 5 cm and 15 cm tephra leachates were indicated by ANOVA for all solutes except iron and chloride. Differences were most prominent in the first 2 years (1982–1984) after which time the differences were generally not significant. Temporal variation was prominent for most solutes due primarily to the seasonal pattern in water flux. Little leaching (i.e., water drainage) from the tephra layer occurs during the summer period (July–October) while 50%– 80% of the annual water flow through the tephra layer occurs during the melting of the snowpack (April–May) (Table 2). Maximum solute concentrations in solutions draining from the tephra layer occur during the July–October period as weathering products accumulate during the dry summer months and are mobilized by the first rains of the fall. In contrast, minimum solute concentrations generally occur during the January to May period when cold temperatures attenuate weathering reactions and melting of the snowpack further dilutes weathering products. Over the 4 year study period, there is a distinct decrease in the annual peak concentrations of several solutes, such as calcium, sodium, silicon, sulfate, and bicarbonate. The pH of the tephra leachates decreases by about one unit during the first year followed by values within a relatively consistent range for the remainder of the study (Fig. 1). The higher initial pH values are believed to result in large part from the rapid release of cations (e.g., Ca21 & Na1) associated with H1 exchange for cations during the formation of a cation depleted leached layer at the particle surfaces, especially from the amorphous glass phase (White, 1983). Rapid dissolution of ultrafine particles and/or high energy sites may also contribute to rapid consumption of H1 through silicate hydrolysis. As the leached layer develops and ultrafine particles are consumed, fewer H1 ions will be consumed resulting in a decrease in the leachate pH. Leachate pH values average 0.2 units lower for the 5 cm treatment than for the 15 cm treatment; however, few differences exceeded the LSD value. The lower pH of the 5 cm tephra leachates is consistent with a statistically significant (p , 0.05) lower pH of equilibrated tephra:water suspensions (1:1) for the 5 cm treatment (Dahlgren et al., 1997). Aqueous iron and aluminum concentrations are low because of their low solubility in the pH range 6 –7 (Fig. 1). As a result, they display low mobilities and accumulate in the solid phase (Dahlgren et al., 1997). Aqueous concentrations of iron and aluminum appear to increase during the last two years which may result from their enhanced mobility due to complexation by soluble organic acids. Dissolved organic carbon concentrations show a similar increase during this time period as an incipient forest floor begins to develop on the surface of the tephra (Fig. 3). Given the pH of the leachate, it is believed that most of the iron and aluminum leached from the tephra is in the form of mobile organo-metal complexes (Dahlgren and Ugolini, 1989c). Silicon concentrations show a strong seasonal variation with the highest concentrations during the July–October period and lowest concentrations during the January–April period (Fig. 1). Maximum silicon concentrations appear to decrease with each successive year (from 450 to 150 mmol/L); however, the minimum values are relatively stable in the vicinity of 100 mmol/L over the entire period. This decrease in silicon concentrations shows the same general trend as bicarbonate concentrations (formed from carbonic acid dissociation) suggesting that 590 R. A. Dahlgren, F. C. Ugolini and W. H. Casey Fig. 1. Mean pH and iron, aluminum, and silicon concentrations for canopy throughfall (TF) and leachates from the 5 and 15 cm tephra treatments during the initial 4 years of weathering. The LSD bar provides the least significant difference (p , 0.05) between mean concentrations of the 5 and 15 cm treatments. weathering rates are decreasing over time. There was no evidence for neoformation of aluminosilicates minerals or opaline silica that would result in retention of silicon within the solid phase (Dahlgren et al., 1997). Silicon concentrations are often greater in the 15 cm treatment but not by a factor of 3 as might be expected for a threefold greater volume of tephra. The concentrations of sulfate and base cations (Cb 5 Ca, Mg, K and Na) showed a large initial pulse followed by a Field weathering rates of Mt. St. Helens tephra 591 Fig. 2. Mean concentrations of calcium, magnesium, potassium, and sodium for canopy throughfall (TF) and leachates from the 5 and 15 cm tephra treatments during the initial 4 years of weathering. The LSD bar provides the least significant difference (p , 0.05) between mean concentrations of the 5 and 15 cm treatments. second spike in concentrations during the July–October period in 1983 (Fig. 2 & 3). These high initial concentrations are attributed to the presence of soluble salts on the fresh tephra (e.g., Fruchter et al., 1980; Smith et al., 1982; White et al., 1986). These salts formed from interaction of acidic gases (e.g., H2SO4 and lesser HCl) with the tephra during the eruption and subsequent tephra fallout. The acidic volatiles were effectively neutralized by H1 exchange for base cations at the mineral surfaces and silicate hydrolysis leading to the formation of soluble salts. Therefore, the initial leaching of the tephra resulted in mobilization of high concentrations of the calcium, sodium, magnesium, and potassium cations with the sulfate and chloride anions (Fig. 2 & 3). Chloride salts were removed during the first 3 months while the less soluble sulfate salts 592 R. A. Dahlgren, F. C. Ugolini and W. H. Casey Fig. 3. Mean concentrations of chloride, sulfate, bicarbonate, and dissolved organic carbon (DOC) for canopy throughfall (TF) and leachates from the 5 and 15 cm tephra treatments during the initial 4 years of weathering. The LSD bar provides the least significant difference (p , 0.05) between mean concentrations of the 5 and 15 cm treatments. were evident throughout the study. Following the initial spike in chloride concentrations due to leaching of soluble salts, chloride concentrations in the tephra leachates are similar in magnitude to that of the canopy throughfall. The leaching pattern between base cations was similar throughout the study (Fig. 2). The high initial concentrations were associated with leaching of sulfate and chloride salts along with bicarbonate formed from hydrolysis reactions in- volving carbonic acid. The spike in cation concentrations during July–October 1983 and the smaller spike during July– October 1984 is due in part to the continued leaching of sulfate salts. Following removal of the sulfate salts, cation concentrations were subsequently regulated by weathering reactions with carbonic acid. The pH of the downward percolating solutions decreased over time as less carbonic acid becomes neutralized. The rate of Field weathering rates of Mt. St. Helens tephra 593 Table 2. Estimated temperature of tephra layer, monthly precipitation, and percentage of each month with snow pack. Data are from the nearby Stampede Pass weather station (National Oceanic and Atmospheric Administration, 1982–1986). Water year 1982–83 Month Temp C PPT cm July Aug Sept Oct Nov Dec Jan Feb Mar April May June Mean/Total 12.6 13.2 10.4 2.1 0.5 0.5 0.5 0.5 0.5 0.5 3.5 8.6 4.4 8.1 4.1 16.4 18.4 16.4 21.5 34.7 13.0 15.2 5.7 7.6 8.9 170 1983–84 Snow % 50 100 100 100 100 100 100 55 Temp C PPT cm 11.0 13.7 8.1 5.3 0.5 0.5 0.5 0.5 0.5 0.5 2.5 8.0 4.3 15.1 4.7 11.9 7.4 37.9 18.4 39.9 17.6 20.9 13.8 22.0 15.6 225 consumption of acidity decreases with time as weathering rates slow down. Weathering reactions slow because the highly reactive surfaces are covered with leached layers and ultrafine particles have been consumed. At this point, the pH of the reacting solution decreases because there are fewer sinks for protons. Correspondingly, both bicarbonate and base cation concentrations decrease as shown by the data. 3.1. Elemental Fluxes Elemental fluxes from the tephra layer were calculated as the difference in flux between tephra leachates (outputs) and canopy throughfall (inputs) (Table 3). Low fluxes of iron and 1984–85 Snow % 100 100 100 100 100 100 25 Temp C PPT cm 13.7 12.9 8.4 1.0 0.5 0.5 0.5 0.5 0.5 0.5 1.6 9.6 4.2 0.6 2.7 9.6 25.9 27.8 31.6 2.5 18.1 15.7 19.3 9.0 11.2 174 1985–86 Snow % Temp C PPT cm 16.9 12.7 8.1 2.2 0.5 0.5 0.5 0.5 0.5 0.5 1.4 12.1 4.7 0.5 2.3 18.3 41.6 35.6 4.4 22.3 32.2 15.7 13.6 13.4 4.1 204 67 100 100 100 100 100 100 67 Snow % 32 100 100 100 100 100 100 80 aluminum indicate that these elements are relatively immobile and accumulate in the tephra. In contrast, silicon released by weathering is mobile and is leached from the tephra. Base cations show a large release during the first year associated with leaching of salts and the presumed formation of a cation depleted leached layer at the surface of glass and minerals. The flux of Cb is decreased by a factor of 2 during the second year and by a factor up to 8.4 during the fourth year of the study. Sulfate fluxes were very high during the first 3 years as sulfate salts were leached, but showed a substantial decrease during the fourth year indicating depletion of this pool. Over the entire study period, sulfate fluxes from the 15 cm treatment were 2.84 Table 3. Elemental fluxes (mean 6 SEM) for 5- and 15-cm tephra treatments and total leachate for the first four years following tephra addition to soils. Leachate (cm) Element Fe Al Si Ca Mg K Na Cl SO4 HCO3 1982–83 120 Tephra depth (cm) 5 15 5 15 5 15 5 15 5 15 5 15 5 15 5 15 5 15 5 15 1983–84 169 1984–85 146 1985–86 126 1.5E-2 (5.0E-3) 1.2E-2 (4.3E-3) 0.09 (0.02) 0.06 (0.02) 1.17 (0.26) 1.95 (0.30) 0.76 (0.09) 1.67 (0.10) 0.25 (0.03) 0.40 (0.04) 0.23 (0.02) 0.34 (0.03) 0.41 (0.07) 1.03 (0.12) 20.01 (0.08) 0.03 (0.09) 0.39 (0.09) 1.28 (0.02) 1.38 (0.04) 2.26 (0.13) 1.6E-2 (5.6E-3) 1.4E-2 (4.1E-3) 0.09 (0.03) 0.05 (0.01) 0.73 (0.16) 1.19 (0.02) 0.38 (0.08) 0.60 (0.07) 0.21 (0.05) 0.38 (0.01) 0.20 (0.06) 0.22 (0.05) 0.38 (0.03) 0.80 (0.03) 0.04 (0.04) 0.06 (0.03) 0.16 (0.01) 0.26 (0.02) 1.39 (0.13) 2.10 (0.34) Elemental flux kmol/ha 3.6E-3 (1.6E-3) 4.3E-3 (3.6E-4) 2.5E-2 (6.7E-3) 2.0E-2 (7.4E-4) 1.95 (0.28) 2.73 (0.18) 2.87 (0.61) 5.01 (0.51) 0.51 (0.09} 0.91 (0.11) 0.57 (0.09) 0.80 (0.04) 1.18 (0.17) 2.37 (0.32) 0.10 (0.07) 0.28 (0.03) 0.92 (0.19) 2.78 (0.37) 6.01 (0.84) 8.97 (0.62) 3.4E-3 (5.4E-4) 4.7E-3 (1.8E-4) 3.3E-2 (3.7E-3) 2.7E-2 (1.9E-3) 1.59 (0.16) 2.38 (0.32) 1.39 (0.13) 2.69 (0.46) 0.32 (0.01) 0.53 (0.05) 0.28 (0.05) 0.56 (0.02) 0.60 (0.07) 1.19 (0.17) 0.06 (0.02) 0.07 (0.02) 0.68 (0.08) 1.80 (0.24) 2.84 (0.18) 4.49 (0.30) 594 R. A. Dahlgren, F. C. Ugolini and W. H. Casey times greater than the 5 cm treatment (6.12 versus 2.15 kmol/ ha) indicating a nearly equivalent release of sulfate from both tephra treatments. Similarly, chloride fluxes were about 2.9 times greater (0.28 versus 0.09 kmol/ha) during the first year of the study. Following the first year of the study, net chloride fluxes from the tephra layer were near zero, indicating a balance between chloride inputs from canopy throughfall and outputs in the tephra leachate. This near balance in chloride flux provides a degree of validation for the estimated water fluxes used to calculate the elemental fluxes. The release of large fluxes of nutrients from the tephra help replenish plant-available nutrient pools (e.g., exchangeable cations, adsorbed anions) in these strongly leached soils. Nutrients released from the tephra are largely retained within the rooting zone of the soil resulting in enhanced nutrient availability (Dahlgren and Ugolini, 1989b). Thus, periodic addition of nutrients associated with tephra addition helps to maintain the nutrient status of soils formed in volcanic ash and explains in part why these soils are among the most productive in the world (Leamy, 1984). 3.2. Weathering Rates Three independent measures of weathering rates were determined based on fluxes of silicon, base cations (Cb), and bicarbonate from the tephra layer. The underlying assumption is that these components are not consumed by reactions subsequent to weathering; that is, these components are freely leached from the tephra layer and do not recombine with the solid phase or interact with biota. This assumption is believed to be met because no appreciable roots were observed in the tephra layer for plant uptake of these elements and these components are not strongly retained by sorption in the tephra. Similarly, there was no evidence (e.g., by TEM and XRD) for neoformation of aluminosilicates minerals or opaline silica that indicates retention of silicon within the solid phase (Dahlgren et al., 1997). The amount of Cb released by “current” weathering processes was determined by using the quantity Cb 2 Ca which corrects for the equivalents of Cb associated with anions of strong acids (Ca 5 Cl2 1 SO22 4 ) comprising the soluble salts. Carbonic acid is the dominant proton donor in the chemical weathering of the tephra and thus the flux of bicarbonate is a direct measure of the amount of protons consumed by weathering reactions. Similarly, silicon is a product of weathering and is commonly assumed to reflect weathering rates in watershed elemental budget studies (Cronan, 1985). We assume that the silicon released to solution by dissolution is transported away because we are not close to saturation with the likely products of homogeneous precipitation, such as amorphous silica. This assumption is not equivalent to the statement that silicon does not accumulate at the weathering particle surfaces. Field weathering rates for each tephra treatment were calculated on a land area basis (kmol or kmolc/ha), a mass basis (mmol or mmolc/g), and a surface area basis (mol/cm2/s) for comparison to various studies from the literature (Table 4). Weathering rates on a surface area basis were calculated for individual elements (Si, Na and Ca). Calcium release rates were corrected for the dissolution of CaSO4 salts by subtracting an equivalent concentration of sulfate (i.e., Ca 2 SO4). Over the 4 year study period, weathering rates decreased by a factor of 3 to 5 based on Cb2 Ca and bicarbonate fluxes and by a factor of 2 to 3 based on silicon fluxes. The largest portion of the decrease occurred between the first and second years and weathering rates were generally similar in the third and fourth years. The large initial decrease is probably attributable to formation of the cation-depleted leached layer and to the rapid dissolution of easily weatherable (e.g., glass) and high surface area microparticles. The release of weathering products on a land area basis was generally greater for the 15 cm treatment compared to the 5 cm treatment; however, the ratios were much less than the factor of 3 difference in the amount of tephra applied. Conversion of weathering rates to a mass or surface area basis indicates that weathering rates were significantly (p , 0.05) greater in the 5 cm treatment. Over the study period, weathering rates were 1.8 to 2.1 times greater in the 5 cm treatment. All three parameters provided remarkably similar ratios between weathering rates in the 2 tephra depths. Field weathering rates in this study, determined as a yearly average, ranged between 10218 and 10217 mol/cm2/s for sodium, calcium and silicon (Table 4). These rates are 1 to 3 orders of magnitude less than for glass (Si 5 10216 – 10215 mol/cm2/s) (White and Claassen, 1980; White, 1983) and plagioclase minerals at steady state (10216 – 10214 mol/cm2/s) (e.g., Wollast and Chou, 1985; Sverdrup, 1990; White, 1995; Blum and Stillings, 1995) in laboratory dissolution experiments at pH 5–7 and 25°C. The 1 to 3 orders of magnitude discrepancy is consistent with other estimates of natural weathering rates from soils, watersheds, and groundwater aquifers (e.g., Claassen and White, 1979; Sverdrup, 1990; Swodoba-Colberg and Drever, 1993; Velbel, 1993; White, 1995; White et al., 1996). The principal factors contributing to the discrepancy between field and laboratory weathering rates are availability of water, temperature, and the extent of solution-solid mixing which serves to reduce the influence of solute diffusion on the rates. Laboratory dissolution studies typically involve vigorous mixing and high solution/solid ratios to minimize the influence of diffusion. In contrast, field weathering rates may be limited by the lack of mixing and low solution/solid ratios (#1), so that solutions approach equilibrium with secondary phases. Previous studies examining the effects of precipitation and temperature on field chemical weathering rates show a linear increase in rates with precipitation and an exponential increase with temperature (Jenny, 1941; White and Blum, 1995). Of the 230 cm of average annual precipitation, 50%– 80% of the water flux passes through the soil within the 1 to 2 month period associated with snow melt when soil temperatures are near 0°C. In contrast, little leaching occurs during the summer months when soil temperatures are highest and water availability is limited. While laboratory studies are usually performed at 25°C, the mean annual temperature of the tephra layer was 4°C–5°C, with temperatures near 0°C for more than half of the year (Table 3). Shoji et al. (1993b) showed an increase in weathering rates for volcanic glass of 1.5 times for each 10°C increase in temperature between 0°C–30°C. A review of weathering rate data for plagioclase minerals by Sverdrup (1990) similarly showed weathering rates to increase 3–5 times between 0°C and 25°C. The difference in temperature (;20°C) between our field study and laboratory studies could contribute to approximately one order of magnitude difference in rates. Field weathering rates of Mt. St. Helens tephra 595 Table 4. Weathering rates (mean 6/SEM) for 5- and 15-cm tephra treatments reported on a land area, mass and surface area basis for the first four years following tephra addition. The ratio of weathering rates between the 5- and 15-cm treatments is also shown. Year 1 Land area baiss Year 2 Depth (cm) Si HCO3 Cb-Ca 5 15 5 15 5 15 HCO3 Cb-Ca 1.95 (0.28)** 2.73 (0.18)** 6.01 (0.49)* 8.97 (0.62* 6.56 (0.71)** 9.18 (0.83)** 1.59 (0.15)** 2.38 (0.32)** 2.84 (0.18)* 4.49 (0.31)* 2.88 (0.25)** 4.52 (0.70)** Na Ca-SO4 1.17 (0.25) 1.95 (0.30)* 1.38 (0.04)* 2.26 (0.13)* 1.89 (0.11)* 2.91 (0.32)* 0.72 (0.16)** 1.20 (0.02)* 1.39 (0.13) 2.10 (0.34) 1.40 (0.29)* 2.39 (0.18)* mmol or mmolc/g 5 15 5 15 5 15 2.95 (0.42)* 1.38 (0.09)* 9.11 (0.74)* 4.53 (0.31)* 9.94 (1.07)* 4.63 (0.42)* 2.42 (0.24)* 1.20 (0.16)* 4.31 (0.27)* 2.27 (0.15)* 4.37 (0.38)* 2.28 (0.35)* 1.78 (0.39) 0.98 (0.15) 2.09 (0.06)* 1.14 (0.07)* 2.86 (0.17)* 1.47 (0.16)* 1.10 (0.24) 0.60 (0.01) 2.11 (0.20)* 1.06 (0.17)* 2.13 (0.44) 1.20 (0.09) mol/cm2/s Surface area basis Si Year 4 kmol or kmolc/ha Mass basis Si Year 3 5 15 5 15 5 15 1.64 6 0.23E-17* 7.66 6 0.51E-18* 9.97 6 1.46E-18 6.66 6 0.89E-18 1.64 6 0.14E-17* 6.27 6 0.44E-18* 1.34 6 0.13E-17* 6.67 6 0.90E-18* 5.08 6 0.60E-18 3.33 6 0.46E-18 5.94 6 0.85E-18* 2.50 6 0.65E-18* 9.87 6 2.15E-18 5.48 6 0.83E-18 3.47 6 0.62E-18 2.88 6 0.33E-18 3.07 6 0.19E-18* 1.10 6 0.25E-18* 6.10 6 1.35E-18 3.35 6 0.06E-18 3.24 6 0.23E-18* 2.25 6 0.09E-18* 1.90 6 0.54E-18 9.37 6 1.92E-19 2.1 2.0 2.1 2.0 1.9 1.9 1.8 1.8 1.9 1.8 2.0 1.8 Ratio of 5/15 cm weathering rates Si HCO3 Cb-Ca * p , 0.05; ** p , 0.10 Thus, our field weathering rates are greatly attenuated because of the inverse relationship between optimal temperature and water conditions: (1) when moisture is most abundant, the soil temperatures are very low (,1°C); and (2) when soil temperatures are highest, soil water availability and leaching are at their lowest. This point is important because of recent conjecture about the role that silicate weathering can play in mitigating climate change (e.g., Brady and Carroll, 1994). We find that, although increased temperatures can induce more rapid CO2 uptake by weathering, the net effect maybe complicated by soil drying at sites similar to ours in the western Cascades. An alternative approach is to compare tephra weathering rates to cation denudation rates from watershed mass balance studies. Cation denudation rates for the tephra range from 1.20 – 6.38 kmolc/ha/yr for the 5 cm treatment to 1.73–9.38 kmolc/ha/yr for the 15 cm treatment. In reviews of weathering rates based on watershed mass balance studies, Sverdrup and Warfvinge (1988) and Sverdrup (1990) showed that most cation denudation rates were within the range 0.1–1.5 kmolc/ha/yr. Thus, weathering rates from the 5 cm and 15 cm tephra treatments often exceed the amount of weathering for entire watersheds, especially during the first 2 years. This striking difference implies that the initial rates of tephra weathering are much more rapid compared to those of other parent materials. Weathering rates in the 5 cm tephra layer were approxi- mately a factor of 2 greater than the 15 cm layer based on all 3 measures of weathering rates (Table 4). This difference may result from variations in (1) leaching intensity (i.e., efficiency of solute removal), or (2) carbonic acid flux per unit volume of tephra. The leachate volume to tephra volume ratio is 3 times greater for the 5 cm treatment resulting in a greater efficiency of weathering product removal. Mean solute concentrations were about 1.5 times greater in the 15 cm treatment compared to the 5 cm treatment over the study period. The effect of ionic strength changes on the dissolution rate of albite is small and generally less than 10% (Chou and Wollast, 1985; Wollast and Chou, 1985). Based on these data, the higher Na and Si concentrations associated with the 15 cm treatments would result in approximately a 10% reduction in weathering rates rather than the 50% reduction measured. Thus, differences in transport efficiency do not appear to account for the differential weathering rates. An alternative explanation involves the flux of gaseous CO2 from the buried soil, which hydrates to produce carbonic acid and release protons. These protons exchange for base cations near solid surfaces resulting in the transport of base cations with bicarbonate from the tephra layer into the underlying soil. The majority of the CO2 in the tephra layer results from upward transport of CO2 from the organic rich surface horizons of the buried soil. There is little CO2 in solutions entering the soil from precipitation because these solutions are near equilibrium 596 R. A. Dahlgren, F. C. Ugolini and W. H. Casey Fig. 4. Schematic representation of the CO2/H2CO3/HCO2 3 weathering/transport cycle occurring between the tephra and Oa horizon of the buried soil. with atmospheric CO2 (pCO2 ;1023.5 atm). In contrast, concentrations of CO2 in the Oa horizon (10 cm depth) of nontephra buried soils ranged between 4 and 15 times atmospheric levels over the course of a year. Elevated concentrations of CO2 beneath the tephra layer originate from biological respiration (e.g., roots and microorganisms) and from protonation of HCO2 3 leaching from the overlying tephra layer. Cations released by weathering in the tephra layer (pH 6 –7) migrate downward with bicarbonate to the acidic organic horizon (pH'4.0) where H2CO3 reequilibrates with the high pCO2 (Fig. 4). This carbon transports upward as gaseous CO2 and is then available to take part in another cycle of weathering and transport. Thus, the buried organic-rich soil beneath the tephra acts as an acidity pump and the overall process of tephra weathering appears to be controlled by solute/gas transport and not by the conventional sense of reaction at a leached layer. If the flux of CO2 transported upwards from the buried soil is similar beneath the 5 cm and 15 cm treatments, the amount of H2CO3 available for weathering on a per unit volume or mass basis of tephra would be 3 times greater for the 5 cm treatment. The fact that the measured weathering rates in the 5 cm treatment are only a factor of 2 greater may be related to kinetic limitations of weathering reactions (i.e., solution residence time) or to depletion of easily weatherable minerals in the 5 cm treatment. 3.3. Elemental Ratios in Leachates Ratios of Ca/Na, Si/Ca and Si/Na in the solid phase tephra and in the solutions draining the tephra were examined to determine whether the tephra was dissolving in a congruent or incongruent manner. The ratios were based on the summation of fluxes leached over the 4 year study, including the amount dissolved by acidic volatiles during the eruption and retained in the tephra as soluble salts. The Si/Ca and Si/Na ratios in the solutions draining the tephra were in the range 0.8 to 1.0 and 2.1 to 1.5, respectively (Table 5). These ratios compare to values of 8.8 and 6.9 for Si/Ca and Si/Na, respectively, in the solid phase tephra (Table 5). Comparison of these values suggests that calcium and sodium are selectively removed from the solid phase relative to silicon. This discrepancy provides evidence supporting the formation of a silica-rich, cation-depleted layer at the surface of the minerals during the initial weathering period (White, 1983; White et al., 1986). The nonstoichiometric removal of silicon may be affected by cold soil temperatures. There was no evidence (e.g., by TEM and XRD) of silicon incorporation into secondary minerals (Dahlgren et al., 1997). The mean Ca/Na ratio of the tephra leachates over the 4 year period ranged between 1.9 and 2.1. The ratio of Ca/Na in the Table 5. Elemental ratios in tephra leachates compared to the elemental ratios in the solid-phase tephra. Solid-phase Si/Na Si/Ca Tephra leachates Si/Na Cumulative (4 years) Year 1 Year 2 Year 3 Year 4 Si/Ca Cumulative (4 years) Year 1 Year 2 Year 3 Year 4 6.9 8.8 5 cm 15 cm 2.1 1.7 2.7 2.9 1.9 1.5 1.2 2.0 1.9 1.5 1.0 0.7 1.1 1.5 1.9 0.8 0.5 0.9 1.2 2.0 Field weathering rates of Mt. St. Helens tephra glass was about 0.8 and the andesine plagioclase has a ratio of about 1.0 (Fruchter et al., 1980). If the primary source of dissolved Ca and Na is from amorphous glass and plagioclase, these data would suggest that calcium is preferentially removed relative to sodium. Once a steady-state leached layer is formed, dissolution should proceed by congruent dissolution and leachate elemental ratios should be equal to that of the solid phase. Preferential release of calcium may result because the divalent charge of calcium destabilizes the glass or mineral structure by limiting the number of Si-O-Si linkages. This is consistent with the Goldich (1938) mineral weathering sequence for soils in which anorthite is less stable than albite. Greater removal of calcium relative to sodium was shown by White et al. (1986) during a 2 year field weathering study of Mt. St. Helens tephra. Shoji et al. (1993b) also showed that colored (Ca rich) glass weathered 1.5 times faster than noncolored (Ca poor) glass due to destabilization of the Si-O-Si framework in the glass by cations. Another factor regulating the Ca/Na ratio may be the rapid dissolution of pyroxene and amphibole which would contribute to higher Ca concentrations in solution. Pyroxene and to a lessor extent amphibole have been observed to weather rapidly in deposits of volcanic ash (Yamada et al., 1978; Shoji et al., 1993a). 4. CONCLUSIONS Tephra leachate chemistry indicates incongruent dissolution of the solid phase during the initial 4 year weathering period. Base cations and silicon released by weathering were leached from the tephra while aluminum and iron were immobile and accumulated in the tephra. This geochemical behavior is expected because the major proton donor is H2CO3. Base cation/ silicon ratios indicate that base cations were preferentially released relative to silicon leading to formation of a cationdepleted, silica-rich layer at mineral and glass surfaces. Weathering rates ranged between 10218–10217 mol/cm2/s for sodium, calcium and silicon. These rates are 1–3 orders of magnitude less than laboratory dissolution studies of glass and albite at 25°C. Cold soil temperatures (mean annual temperature of 4°C–5°C) when the soils are moist and a moisture deficit when soil temperatures are highest contribute to low weathering rates as compared to laboratory determined rates. Therefore, using laboratory determined weathering rates to estimate field weathering rates greatly over estimates actual rates. In contrast, weathering rates for the 5 cm and 15 cm tephra layers (1.4 –9.2 kmolc/ha/yr; based on bicarbonate and Cb–Ca fluxes) exceed those of entire watersheds (0.1–1.5 kmolc/ha/yr) in field studies reported in the literature. This difference suggests that weathering rates of tephra, especially “fresh” tephra, are rapid compared to other types of parent materials. Weathering rates of the 5 cm tephra treatment were approximately twice those of the 15 cm tephra treatment when calculated on a mass basis or surface area basis. Since dissociation of H2CO3 is the dominant source of protons during the initial 4 years of weathering, processes regulating CO2 fluxes are believed to be responsible for determining weathering rates. 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