Mt. St. Helens Tephra Weathering Rates: A Field Study

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. Biological activity in the buried soil and cycling of CO2/H2CO3/
HCO2
3 between the tephra and organic soil horizon in the
buried soil are the primary sources of CO2 for weathering in the
tephra layer. The greater flux of CO2 per unit volume of tephra
597
in the 5 cm treatment is believed to be responsible for the
differential weathering rates between the 2 treatments. Our
study has an importance that extends well beyond the specific
reactions and rates proposed because such a large uncertainty
exists about estimates of heterogeneous reaction rates in the
field.
Acknowledgments—Partial support for this research was from the U.S.
NSF grants BSR80-22189, BSR84-07710 and EAR 96-25663.
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