Chemical Geology 322–323 (2012) 79–90
Contents lists available at SciVerse ScienceDirect
Chemical Geology
journal homepage: www.elsevier.com/locate/chemgeo
Glacier meltwater stream chemistry in Wright and Taylor Valleys, Antarctica:
Significant roles of drift, dust and biological processes in chemical weathering in a
polar climate
A.R. Stumpf a, M.E. Elwood Madden a,⁎, G.S. Soreghan a, B.L. Hall b, L.J. Keiser a, K.R. Marra a
a
b
University of Oklahoma, School of Geology and Geophysics, 100 E. Boyd, Suite 710, Norman, OK 73019, United States
University of Maine, Department of Earth Sciences and Climate Change Institute, 5790 Bryand Global Sciences Center, Orono, ME 04469, United States
a r t i c l e
i n f o
Article history:
Received 6 February 2012
Received in revised form 5 June 2012
Accepted 12 June 2012
Available online 20 June 2012
Editor: J.D. Blum
Keywords:
Proglacial stream
Mineral dissolution
Aqueous geochemistry
a b s t r a c t
This study examines stream chemistry within glacial drainages in Wright and Taylor Valley Antarctica in conjunction with geochemical modeling and sediment leach experiments in order to assess the source of chemical weathering fluxes within polar drainages. The targeted catchments are underlain by granitoid basement,
with streams flowing through glacial drifts of variable composition. Analyses of meltwater from Clark Glacier
in Wright Valley, and Howard Glacier (Delta Stream) in Taylor Valley show increases in solute concentrations
as a function of distance from the glaciers. Surface area normalized weathering rate estimates based on Si and
K fluxes within the streams confirm that significant chemical weathering is occurring in these systems at
rates comparable to field and laboratory rates of silicate weathering under much warmer conditions. Downstream increases in several cations (Ca, Na, K, and Mg) occur within both drainages; however solute concentrations between the two streams differ significantly. Clark Glacier stream contains Na > Ca whereas Howard
Glacier stream contains Ca > Na distally. Geochemical reaction path modeling for Clark Glacier predicts increasing concentrations of Ca, Na, and Mg over the reaction progress, paralleling the results of field data
from Clark Stream. Modeling of Howard Glacier stream also shows an increase in several cations; however
Na concentrations exceed those for Ca, which differs from the observed field data, and suggests that the differences in stream chemistry are not due to differing bedrock mineralogy. Instead, ion-leaching experiments
conducted on drift sediments demonstrate that (1) glacial deposits within Wright Valley and Taylor Valley
may significantly affect stream chemistry through direct water–rock interactions within the stream channel,
and/or (2) the water chemistries are influenced by eolian additions of locally sourced dust to glacier surfaces.
Differences in Fe concentrations between the two stream systems indicate that biological activity may also
significantly influence iron concentrations within Howard Glacier stream.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
The McMurdo Dry Valleys, Antarctica, are the coldest and driest
regions on Earth, making them an extreme end-member for studying
chemical weathering. Streams within the Dry Valleys flow only during
the brief austral summer and are fed primarily by surficial melting of
cold-based glaciers that occur at higher elevations along the valley.
Whereas wet-based glaciers affect the geochemistry of landscapes
through production of fine-grained sediments with high reactive surface
areas (Hallet et al., 1996; Atkins et al., 2002; Anderson, 2007), coldbased (polar) glaciers exist at subfreezing temperatures (−20 °C)
where no basal melt occurs and are therefore frozen to the rock. This
limits production of fine-grained sediments because basal grinding of
⁎ Corresponding author.
E-mail addresses: [email protected] (A.R. Stumpf), [email protected]
(M.E. Elwood Madden), [email protected] (G.S. Soreghan), [email protected] (B.L. Hall),
[email protected] (L.J. Keiser), [email protected] (K.R. Marra).
0009-2541/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.chemgeo.2012.06.009
bedrock does not occur. Since polar glaciers are assumed to be weak
erosive agents relative to temperate glaciers, chemical weathering flux
and denudation rates are expected to be low by comparison.
However, a growing number of studies have suggested that chemical weathering rates within the Dry Valleys are comparable to those of
temperate and humid watersheds (Anderson et al., 1997; Nezat et al.,
2001; Gooseff et al., 2002; Maurice et al., 2002; Lyons et al., 2003;
Fortner et al., 2005). These recent studies challenge the assumption
that minimal chemical weathering occurs within cold (polar) climates
by demonstrating that solute concentrations, including dissolved silica,
increase significantly downstream. This suggests that significant chemical weathering is occurring in these systems. However, the source of
the weathering material within the streams remains unclear. For example, eolian dust trapped on glaciers (cryoconites) and released through
surficial glacier melting may significantly impact supraglacial and
proglacial stream systems in Taylor Valley (Lyons et al., 2003; Fortner
et al., 2011). In addition, mineral–water interactions within the limited
hyporheic zone along the edges of stream channels may also be
80
A.R. Stumpf et al. / Chemical Geology 322–323 (2012) 79–90
responsible for weathering fluxes (Nezat et al., 2001; Gooseff et al.,
2002; Lyons et al., 2002; Green et al., 2005; Harris et al., 2007; Levy et
al., 2011). The objective of this study is to examine glacial meltwater
stream chemistry along a transect from the glacial source downstream
to quantify the chemical weathering occurring within polar drainages
and further investigate the sources for solutes in meltwater streams.
In addition to sampling stream water within glacial drainages in Wright
and Taylor Valleys (Figs. 1, 2A and B), geochemical modeling and ionleaching experiments were conducted in order to assess the weathering
processes occurring in the Dry Valleys. By combining the solute data
from these Dry Valley natural systems with equilibrium and kinetic
reaction-path modeling of bedrock mineralogy, and ion-leaching experiments involving glacial drift sediments, this study aims to clarify the
major sources of chemical weathering within these meltwater streams.
2. Study area and geologic setting
The McMurdo Dry Valleys are located in southern Victoria Land,
Antarctica. The valleys are the largest ice-free area in Antarctica
with a combined area of 4800 km 2 (Wagner et al., 2006). Taylor and
Wright Valleys trend east–west and are bordered by the Ross Sea to
the east and the East Antarctic Ice Sheet (EAIS) to the west (Fig. 1).
The mean annual air temperature for the Dry Valleys is about
−20 °C (Doran et al., 2002), and during the summer, air temperatures hover around freezing (Lyons et al., 2003). Easterly wind directions from the Ross Sea occur during summer while westerly
katabatic winds descend from the EAIS during winter (Clow et al.,
1988; Doran et al., 2002). The Onyx River, Antarctica's largest, drains
from Lake Brownworth in eastern Wright Valley and flows more than
30 km to Lake Vanda in the west (Green et al., 2005).
The geology of the Dry Valleys is varied, consisting of a basement
complex comprising Precambrian to Cambrian metasediments (including schist, argillite, quartzite, and marble) and Paleozoic intrusive
rocks (e.g. granite and granodiorite) (Hershum et al., 2007). Overlying the basement complex is the Beacon Sandstone, which is intruded
by the Jurassic Ferrar Dolerite. Finally, Quaternary lacustrine and glacial drift deposits cover the valley floors (Hall et al., 2000). Clark
Glacier stream (Fig. 2A) flows through unconsolidated drift, underlain
by the Brownworth Pluton, a coarse-grained, homogeneous granite
and quartz monzonite (Peterson and Marsh, 2008). Howard Glacier
rests atop biotite orthogneiss, a quartz and biotite-plagioclase-K-feldspar
gneiss of granitic composition (Peterson and Marsh, 2008), and the
stream draining this glacier (“Delta” stream) flows across the Ross Sea
drift, situated atop the biotite orthogneiss (Fig. 2B). High-salinity brines
exist throughout the valleys, thought to be the remnants of LGM
proglacial lakes (Hall et al., 2001; Wagner et al., 2006).
The hypersaline waters contain major ion ratios that resemble those
of seawater. Glacial Lake Washburn occupied much of Taylor Valley.
Lake level, recorded by perched deltas, was >78 m deep. As lake level
fell, lacustrine waters became concentrated, leading to basin-wide
precipitation of CaCO3 (Hall et al., 2000; Whittaker et al., 2008; Green
and Lyons, 2009). Interpretations of lake chemistry suggest that the
lake level of Glacial Lake Washburn fluctuated, causing solutes from
the saturated brine to diffuse within valley lakes (McKnight et al.,
1991; Lyons et al., 1998).
Unconsolidated sediment deposits from grounded Ross Sea ice
during the LGM and/or previous glacial advances also filled Wright
and Taylor Valleys (Hall et al., 2000; Higgins et al., 2000; Hall and
Denton, 2005). The Clark Glacier drainage system flows over Trilogy,
Brownworth, and Loke drifts (Fig. 2A) (Hall and Denton, 2005). Trilogy
drift, thought to be of early- to mid-Quaternary age, is a sandy
diamicton with stained and ventifacted clasts comprised of mainly
Ferrar Dolerite, Olympus Granite gneiss, and microdiorite. Loke drift,
deposited during the mid-Quaternary, is a sandy diamicton with
patches of stratified sand and gravel composed of Ferrar Dolerite and
Olympus Granite gneiss (Hall and Denton, 2005). Brownworth drift,
the youngest of the three in The Clark Glacier drainage, is a coarse
sand diamicton made up of Ferrar Dolerite, Vida Granite, and microgranite clasts. Within Clark Glacier drainage, Brownworth drift overlies
both the Trilogy and Loke drifts (Fig. 2A). These drifts were all emplaced
before the LGM and are made up of largely coarse‐grained sediments
deposited from cold-based glaciers (Hall and Denton 2005).
In Taylor Valley, Howard Glacier stream flows through Ross Sea
drift, the only widespread drift type within the drainage (Fig. 2B).
ey
all
rV
o
yl
Ta
ss
Ro
a
Se
ey
all
tV
h
rig
W
0
2
4 km
Fig. 1. Satellite image of the Dry Valleys, showing Wright and Taylor Valleys (shown in more detail in Fig. 2). Inset box shows location on the continent.
A.R. Stumpf et al. / Chemical Geology 322–323 (2012) 79–90
A
Biotite Orthogneiss
Ross Sea Drift
Water Sample
Sediment/Drift Sample
Lake Hoare
S
0
1
2km
Lake Fryxell
B
Brownworth Pluton
Trilogy Drift
Hornblende Biotite
Orthogneiss
Brownworth Drift
Denton Pluton
Loke Drift
Water Sample
Biotite Orthogneiss
Sediment/Drift Sample
Clark Glacier
S
0
1
2km
Fig. 2. Geologic maps of the study areas. (A) Taylor Valley sample locations for Howard
Glacier stream, also known as Delta Stream. Bedrock geology is biotite orthogneiss, a
quartz, biotite-plagioclase-K-feldspar gneiss of granitic composition. Ross Sea drift
underlies the drainage where Howard Glacier Stream flows into Lake Fryxell. Circles
represent sampling locations. (B) Wright Valley sample locations for Clark Glacier
and Onyx River. Clark Glacier drainage geology consists of the Brownworth Pluton, a
coarse-grained, homogeneous granite and quartz monzonite. Clark Glacier stream
flows through multiple unconsolidated drifts, all of which exhibit a similar lithology.
The Onyx River flows to the west (inland) from Lake Brownworth.
Ross Sea drift was deposited during the LGM as a lobe of ice from the
Ross Sea carrying glacial rock flour entered the valley mouth (Hall
et al., 2000). It is divided into 5 sedimentary facies that range from
unstratified till to stratified, interbedded silt, sand, and gravel capped
by cobbles and boulders. The drift contains abundant basalt, sandstone, and dolerite erratics, consisting of carbonate coated clasts of
Ferrar Dolerite, Theseus Granodiorite, Olympus Granite Gneiss, and
microdiorite. Owing to its origin from a partially wet-based ice
sheet, the Ross Sea drift contains abundant fines, unlike the drifts
within the Wright Valley study area (Hall et al., 2000).
For 6–12 weeks during the austral summer, glacial meltwater flows
in the stream channels and over unconsolidated drifts. Meltwater from
81
the glaciers is the only source of water to the streams and lakes in the
valley bottom because what little snow accumulates usually sublimates
before contributing to stream flow. The main form of precipitation in
the Dry Valleys is snow, amounting to b100 mm/yr in water equivalence (Chinn, 1993; Lyons et al., 2003). Therefore, initial stream chemistry is controlled by glacial ice and the chemical processes occurring
on the glacier surface and during runoff (Lyons et al., 2003). A permafrost layer that inhibits the flow of subsurface water occurs at a depth
of about 30 cm within the unconsolidated sediment. This confines any
chemical weathering to this near-surface zone where glacial meltwater
has direct contact with the sediment.
As solar radiation warms the surface of the glaciers within Wright
and Taylor Valleys during the austral summer, meltwater flows from
the surface of the glacier directly into the stream channel, limiting the
water–rock interaction to only the hyporheic zone immediately surrounding the stream channel (Hall et al., 2010). Based upon the observation of increasing solute concentrations downstream, previous
studies have suggested that the exchange between glacial meltwater
and the stream is rapid and may provide a significant number of ions
to the stream waters (Nezat et al., 2001; Gooseff et al., 2002).
However, lake deposits (now exposed) formed during the LGM,
brines present in modern lakes, and marine aerosols from the Ross
Sea may provide additional salts to the system. In Wright Valley,
evaporation of a large former lake has resulted in the formation of a
halite and gypsum playa surrounding Don Juan Pond, which is saturated in calcium chloride (Hendy et al., 2000). The bottom waters of
Lake Vanda also have a brine composition similar to that of Don
Juan Pond. In addition, paleo lake sediments within Taylor and
Wright Valleys could influence chemical trends observed in glacial
meltwater streams. For example, the presence of salts within the
drift sediments may contribute to elevated Ca and Na concentrations
observed in some streams (Hendy et al., 2000; Green et al., 2005).
Eolian addition of silicate and salt phases from dust accumulation
on glacier surfaces may also affect chemical weathering fluxes within
streams of the Dry Valleys (Lyons et al., 2003; Fortner et al., 2011).
Supraglacial streams originate from the upper portion of a glacier
and therefore may entrain eolian dust from the glacier surface
(cryoconite). This dust can be a source for additional solutes as
water flows downstream (Tranter et al., 2005). Significant dust deposits within glaciers in Taylor Valley, which vary in abundance
with distance from the Ross Sea, and wind patterns within the valley,
may contribute to the heterogeneities in stream and lake chemistry
observed geographically (Welch et al., 2010)
3. Methods
3.1. Site selection
Sample collection sites within Wright and Taylor Valleys were
pre-selected using aerial photos, drift maps, and geologic maps from
Hall and Denton (2005), Hall et al. (2000), and the National Resources
Research Institute at the University of Michigan. Meltwater channels
considered ideal for sample collection are those with continual flow
from the glacier base to its representative lake or confluence. Sample
sites within Wright Valley include the meltwater stream flowing from
Clark Glacier, and the Onyx River flowing from Lake Brownworth, a
proglacial lake adjacent to Wright Lower Glacier (Fig. 2A). Taylor
Valley collection sites include the drainages flowing from Goldman
Glacier and Howard Glacier (Fig. 2B). In Taylor Valley, Howard Glacier
stream (commonly termed “Delta” stream) was the only drainage
with significant water of the streams sampled during this field campaign. However, the stream was intermittent and throughflow occurred near Howard Glacier. The stream channel remained water
saturated and contained standing water until the stream regained
surface flow where the elevation again decreases.
82
A.R. Stumpf et al. / Chemical Geology 322–323 (2012) 79–90
3.2. Sample collection
Water, sediment, and bedrock samples were collected from 9
January 2010 to 18 January 2010. 50 ml water samples were collected
every 250–500 m (depending on the total length of the stream) using
a 25 ml pipette and placed in a 60 ml Nalgene sample container with
5 ml of 1 N HCl and then frozen to prevent any chemical reactions
from taking place. The pipette was rinsed between each sample site
and a new pipette was used for each stream. Temperature and pH
were recorded at each sample site using a Hanna hand-held pH
meter, and GPS location, elevation, and time were also noted during
each measurement.
3.3. Analytical methods
Once thawed, each sample was filtered through a 0.2 μm Millipore
filter and placed in a new acid-washed Nalgene bottle and refrozen
until testing. Chemical analyses were performed by the Oklahoma
State University Soils Lab for the following major ions: Na, Ca, Mg,
K, SO4, P, Fe, Zn, Mn, and Si using a Spectro CirOs ICP Spectrometer.
Standard deviations of quality control standards were within 2–3%
of the standard concentration. Since the samples were acidified in
the field prior to filtering, the bulk chemistry of the analyzed solution
could potentially include ions originally adsorbed to mineral grains
which were later removed by the filtering process and/or solutes
released due to dissolution of fine-grained material in the acidified
sample bottles. However, since the samples were frozen almost
immediately after acidification and filtered within hours of thawing,
these dissolution effects are likely minimal, an inference confirmed
by our results, as explained further in the discussion.
An ice sample was collected on the surface of Wright Lower Glacier
(WLG) in Wright Valley. Based on the proximity of WLG to Clark Glacier,
the ice chemistry provides a reasonable estimate of soluble salts for
Clark Glacier, which was not accessible for sampling due to calving hazards. Because Clark and Howard Glaciers are polar glaciers, meltwater
originates from the surface of the glacier. Therefore, any accumulation
of eolian sediments on the glacier surface would contribute to solute
concentrations in the stream water chemistry. We subtracted WLG ice
chemistry from Clark Glacier stream concentrations and surface ice
chemistry (Lyons et al., 2003) from Howard Glacier stream concentrations to account for the initial chemistry of the meltwater input into
the streams (see Supplementary materials, Table S1).
Where available, bedrock samples were collected from the sampling
site and thin sections prepared. Point counts of the bedrock were
completed using the Gazzi–Dickinson method (Ingersoll et al., 1984).
Bedrock samples with similar mineralogy were grouped together and
one representative sample for each group was selected for electron
microprobe analysis.
3.4. Electron microprobe
Bedrock thin sections for samples W10-CG-7 (Clark Glacier) and
T10-GOLD-1 (near Howard Glacier) were analyzed further to determine
the stoichiometry for plagioclase, potassium feldspar, micas, and amphibole within the bedrock samples. All electron microprobe imaging
and analyses were performed using a Cameca SX50 electron probe
microanalyzer at the University of Oklahoma. Phase identification utilized both qualitative examination of EDXA spectra and standardless
semi-quantitative analysis based upon automated Gaussian peak
deconvolution, background subtraction, and PAP matrix correction procedures (Pouchou and Pichoir, 1985) using 20 kV accelerating voltage
and 10 nA sample current. Images were acquired digitally using
1024 × 1024 pixel arrays. Electron Microprobe Analysis (EMPA) was
performed by Wavelength-Dispersive Spectrometry (WDS) using first
order Kα X-ray emission lines, acquired as simple intensity above
mean background based on a two-point linear background model.
Materials used for standard reference intensities (see Supplementary
materials, Table S2) were well-characterized natural or synthetic compounds. Analytical conditions were 20 kV acceleration, 10 nA sample
current, and a 3 μm spot size. Counting times were 30 s on peak for all
elements, yielding minimum detection levels of 0.01–0.05 wt.% of the
oxide components for all analyzed elements.
3.5. Geochemical modeling
Geochemical modeling was completed using the REACT module of
Geochemists Workbench® (GWB; Bethke, 1996) for the streams
draining Clark and Howard Glaciers, under both simple equilibrium
conditions and with the addition of reaction kinetics. The REACT
model and associated thermodynamic dataset (GWB's standard
LLNL database) were used to test the hypothesis that the solute
chemistry observed within the stream channels could be produced
through chemical weathering of the underlying bedrock. REACT was
used in the batch reactor mode, where a mass of rock (assemblage
of mineral reactants) is titrated into a given volume of water. At
each step in the titration, the model recalculates the minimum
Gibbs free energy (thermodynamically most favored) assemblage of
minerals + fluid in the system. In the simple equilibrium model, the
program adjusts the composition of the fluid and mineral assemblage
to reflect this minimum Gibbs free energy state before adding the
next increment of rock, therefore maintaining thermodynamic equilibrium at each step in the titration; i.e. the system instantaneously
reaches equilibrium at each titration step. In the kinetic model, the
system always moves toward the minimum Gibbs free energy assemblage; however, the release of solutes due to dissolution of metastable
mineral phases in the system is limited by the dissolution rate; likewise, the consumption of solutes due to precipitation of thermodynamically stable phases is limited by nucleation and growth
kinetics. Thus, in the kinetic model, geochemical processes are driven
toward the thermodynamically preferred equilibrium assemblage,
but the transformation is not instantaneous, and is instead limited
by reaction kinetics.
The basis for both the equilibrium and kinetic models included
fixed temperature, pH, atmospheric ƒCO2 and ƒO2, as well as initial
glacial water chemistry (Table 1). The initial water chemistry used
in both models is similar to water from glacier ice in Taylor Valley
(Mager et al., 2009). Model variables included bedrock mineralogy
for the reactants in both the equilibrium and kinetic models, as well
as estimated surface areas and dissolution rates for phases used in
the kinetic models. Clark Glacier bedrock mineralogy was determined
by point-counting bedrock sample W10-CG-7. Sample W10-CG-7 was
selected because it best represents the Brownworth Pluton, a coarsegrained, homogeneous granite and quartz monzonite underlying
Clark Glacier. Howard Glacier bedrock mineralogy was determined
using point-counts from sample T10-GOLD-1, a biotite orthogneiss.
This sample was collected at nearby Goldman Glacier but is representative of the bedrock beneath Howard Glacier. GWB does not contain all
mineral phases; specifically for the micas and amphiboles solid solution
primary phases were selected based upon EMPA (see Supplementary
materials, Table S3). BET surface-area (100,100 cm2/g) from the mud
fraction of the stream sediments collected during this field effort
(Marra et al., 2011) was used in the kinetic model. Appropriate kinetic
dissolution rates for each mineral were selected from the literature
based on temperature and pH relative to our measurements in the
field (see Supplementary materials, Table S4).
3.6. Drift leach experiments
Representative drift samples from both Wright and Taylor Valleys
were split and wet sieved to separate the clay to sand-sized fraction.
A 20 g sample of each drift was placed in a beaker with 200 ml of
ultrapure water and mixed on a stir plate for 90 min at a rate of
A.R. Stumpf et al. / Chemical Geology 322–323 (2012) 79–90
Table 1
Reaction modeling inputs.
Clark Glacier
Howard Glacier
Fixed input
T (°C)
pH
fCO2
fO2
H2O (kg)
5
7.3
3.88 × 10− 4
0.2
1
1
7.3
3.88 × 10− 4
0.2
1
Mineral (cm3)
Quartz
Albite
Anorthite
K-feldspar
Annite
Phlogopite
Clinochlore-14A
Amphibole
32
27
8
22
6
2
2
.5
30
30
4
36
1
0.6
–
–
2.3
0.5
1.1
1.5
2.3
4.3
0.5
1.0
1.0
0.1
Na and Ca show the most pronounced downstream increases, with Na
having the highest concentration. The increase in Na and Ca around
2000 m does not correlate with any change in the underlying drift
(see Fig. 2A) at that distance. Si, Mg, and K initially decrease, and
then increase slightly after approximately 1500 m downstream distance. Fe is the only solute that does not show a net increase downstream. Water pH values decrease from 9.00 to 7.78 with distance
from the glacier and temperature increased during the sampling
throughout the day (Table 3).
A 400
Na
Ca
Mg
K
350
Concentration (µM)
Initial meltwater (mg/l)
Na
Mg
K
Ca
SO4
Cl
NO3
SiO2
Fe
Al
2.3
0.5
1.1
1.5
2.3
4.3
0.5
1.0
1.0
0.1
83
300
SO 4
Fe
Si
Clark Glacier Stream
250
200
150
100
50
0
0
500
1000 1500 2000 2500 3000 3500 4000 4500
Distance from Glacier (m)
Concentration (µM)
B 700
500 rpm. The resulting fluid was filtered through 0.2 μm Millipore filters. Drift sediments from the cation leaching experiment were then
oven dried at 50 °C and 30 ml of 1 N HCl was added to the drift sediment every 20 min for 1 h to determine the acid-leachable solutes
remaining in the drift. The resulting fluid was also filtered through a
0.2 μm filter and both the water and acid leach samples were analyzed with a Spectro CirOs ICP Spectrometer for major cations.
4. Results and discussion
4.1. Clark Glacier stream chemistry
Water samples from Clark Glacier stream show a general increasing trend in cations as a function of distance from the glacier (Fig. 3A).
Howard Glacier Stream
500
400
300
200
100
0
0
500
1000 1500 2000 2500 3000 3500 4000 4500
Distance from Glacier (m)
C
90
Howard Glacier Stream
80
Concentration (µM)
Petrographic analysis, point counting and EMPA analysis of W10CG-7 revealed that the majority of plagioclase consists of albite and
the remainder is anorthite (Table 2). Potassium feldspar grains are
mainly perthite. Phyllosilicates present are slightly chloritized biotite
and chlorite. The sample also contains a trace amount of hornblende.
Sample T10-GOLD-1 contains similar plagioclase mineralogy, with albite more abundant than anorthite. The amount of chloritized biotite
in this sample is far less than that in the Wright Valley sample and the
bedrock sample contains no amphibole (Table 2).
600
70
60
50
40
30
20
10
0
0
500
1000 1500 2000 2500 3000 3500 4000 4500
Distance from Glacier (m)
Table 2
Bedrock mineralogy (vol.%) determined by point counts.
Mineral
Clark
Howard
Quartz
K-Feldspar
Plagioclase
Mica
Amphibole
31.50
24.25
32.63
10.88
0.50
30.25
36.00
31.00
2.75
–
Fig. 3. Water chemistry for Clark and Howard Glacier streams. (A) Clark Glacier water
chemistry corrected for initial meltwater chemistry using solute concentrations measured in surface ice from nearby Lower Wright Glacier. Solutes generally increase as
a function of distance from the glacier, expect for Fe, which decreases slightly. Na
and Ca are highest in concentration, with Na > Ca. (B) Howard Glacier water chemistry
corrected for initial meltwater chemistry using surface ice solute concentrations from
Lyons et al. (2003). All solutes except Fe increase in concentration from the base of
the glacier to Lake Fryxell. Na and Ca significantly increase, with Ca > Na. (C) corrected
Howard Glacier water chemistry excluding Na and Ca such that trends in other solutes
are visible over the range of concentrations presented.
84
A.R. Stumpf et al. / Chemical Geology 322–323 (2012) 79–90
Table 3
Chemistry of water and ice samples (μmol/l) collected from Clark Glacier (CG), Lower Wright Glacier (LWG), Lake Bonney (LB), Denton Glacier (DG), and No Name Glacier (NN)
streams in Wright Valley (W) and Goldman Glacier (GOLD), Howard Glacier (HOW) and Moa Glacier (MOA) in Taylor Valley (T). Standard deviations of quality control standards
were within 2–3% of the standard concentration.
Sample
pH
Temp (°C)
Na
Ca
Mg
K
SO4
Fe
Si
Zn
Mn
ICAP-P
Stream water
W10-CG-1
W10-CG-2R
W10-CG-4R
W10-CG-6R
W10-CG-8R
W10-CG-10R
W10-LB-1
W10-OR-1
W10-OR-4
W10-OR-7
W10-OR-9
W10-OR-16
W10-OR-18
W10-OR-21
W10-DG-3
W10-NN-1
W10-NN-7
T10-MOA-1
T10-GOLDW-1
T10-GOLD-1
T10-GOLD-2
T10-GOLD-4
T10-GOLD-5
T10-HOW-1
T10-HOW-3
T10-HOW-6
T10-HOW-13
9
8.32
7.84
7.77
7.68
7.78
8.55
8.38
8.25
8.01
7.86
7.63
7.66
7.66
7.48
7.85
7.45
8.64
7.9
8.8
8.52
8.34
8.24
9.1
8.37
8.37
8.4
1
3.2
4.9
3.7
7.6
7.8
4.4
3.5
3.2
4.5
6.8
7.4
5.8
4.5
− 0.2
0.9
0
6.1
9
1.8
0.8
0.5
2.2
− 0.5
0
3.7
− 0.2
115.44
91.74
102.70
117.66
303.66
346.41
92.00
90.34
90.52
94.61
97.74
110.09
114.22
116.53
568.91
159.94
507.79
84.43
550.90
50.20
161.51
419.93
145.02
247.89
288.22
208.66
118.23
107.12
96.31
112.95
130.75
220.02
236.69
97.63
52.15
49.58
52.60
55.94
62.58
72.26
75.55
399.92
245.15
172.41
112.23
591.60
391.86
363.57
525.53
524.03
83.34
660.26
382.70
425.47
32.92
91.13
77.97
71.59
115.49
117.26
13.21
12.80
12.63
13.17
14.36
14.81
20.94
17.44
143.06
32.42
127.42
41.35
138.49
17.69
82.58
191.40
69.16
22.46
95.62
66.24
42.91
16.75
47.80
42.12
40.28
72.79
79.54
9.90
9.08
8.08
9.34
11.92
16.39
17.01
15.42
78.24
29.00
75.81
26.88
63.48
21.41
31.79
48.83
38.85
21.31
38.90
25.91
20.67
28.97
17.56
19.98
24.11
41.21
48.72
22.13
18.87
19.20
20.89
21.05
22.13
24.16
24.33
238.75
76.67
82.51
26.85
499.28
23.73
55.57
141.31
50.01
18.62
69.59
51.32
46.77
22.62
114.37
98.39
76.14
88.04
85.54
0.57
13.09
0.34
0.27
0.48
0.98
4.57
0.88
42.60
18.10
132.25
33.72
0.61
1.24
4.05
11.60
0.57
0.59
0.32
1.29
14.74
27.84
112.48
103.54
73.42
125.55
117.61
9.76
11.54
10.68
13.32
15.24
13.85
26.67
21.86
56.01
15.74
108.77
34.07
22.86
22.86
13.71
34.04
31.01
21.86
48.71
41.48
25.46
0.21
0.54
0.31
0.24
0.31
0.37
0.24
0.32
0.28
0.14
0.18
0.24
0.28
0.14
0.24
0.24
0.55
0.26
0.24
0.12
0.21
0.24
0.17
0.15
0.17
0.11
0.21
0.24
1.95
1.29
1.07
1.13
0.96
0.05
0.55
−0.22
− 0.16
− 0.20
− 0.11
−0.13
−0.20
0.47
0.05
1.29
0.24
−0.18
− 0.18
− 0.02
0.00
− 0.22
− 0.22
− 0.22
− 0.20
0.02
1.45
6.81
5.52
5.17
6.39
4.46
0.13
1.84
1.03
1.84
1.13
0.32
0.87
0.81
2.94
1.07
4.39
0.74
0.42
0.55
0.68
1.10
0.39
1.23
0.94
0.61
1.36
Glacier ice
W10-LWG-1
Howard Glaciera
Rhone Glacierb
7.95
–
–
0
–
–
47.98
86
100
38.10
54.4
37.4
13.29
18.2
20.6
11.66
5.4
28.1
9.98
8.2
18.7
16.37
–
–
23.50
–
–
0.15
–
–
− 0.02
–
–
2.62
–
–
a
b
Lyons et al. (2003).
Mager et al. (2009).
4.2. Howard Glacier stream chemistry
Overall, ion concentrations increase from Howard Glacier to Lake
Fryxell (Fig. 3B). Ca has the highest concentration followed by Na
and both increase as a function of distance. Mg, K, SO4, and Si all
have lower concentrations, but also increase downstream (Fig. 3C).
Ca concentrations are higher than Na concentrations in Howard
Glacier stream, showing an opposite trend to that observed in the
Clark Glacier drainage (Wright Valley). The pH ranged from 8.37 to
8.4, and temperature remained relatively constant over the length
of the stream (Table 3).
4.3. Onyx River chemistry
Downstream trends in Onyx River water data are similar to those of
Clark Glacier stream with notable increases in Na and Ca, and Na
concentrations being the highest (Fig. 4A). However, solute concentrations are lower in the Onyx River compared to the Clark and Howard
Glacier streams, perhaps due to higher discharge of the Onyx River.
Onyx River data collected here are comparable to results reported by
Green et al. (2005) (Fig. 4B). Comparison of our Onyx River data to
the previous study shows that, beginning at the outflow west of Lake
Brownworth, our results yield similar trends in solute concentrations.
This confirms that acidifying our samples prior to filtering had little to
no effect on the solute trends.
General comparison of Clark and Howard Glacier streams shows
increasing concentrations of cations and Si downstream which suggests
significant chemical weathering is occurring within this cold-arid environment despite the low temperatures and the glaciers being frozen to
their bedrock. The general increase in solute concentrations in all glacial
streams confirms the influence of silicate weathering over relatively
short distances in polar glacial streams.
4.4. Na and Ca
Na/Ca ratios in the stream waters of Clark and Howard Glaciers
differ significantly (Fig. 3A and B). This contrast in major cation ratios
between the two streams may relate to differences in 1) primary
bedrock mineralogy underlying the streams, 2) water-soluble salts
within the local drift or delivered as aerosols, 3) dissolution of carbonates observed within the drift deposits, or 4) differences in dust
chemistry within the melting glaciers. To test these hypotheses, we
conducted a series of model calculations and experiments.
Chemical weathering models were conducted using Geochemist's
Workbench to test the influence of bedrock mineralogy on stream
solute chemistry. Leach experiments were conducted with ultrapure water and 1 N HCl to compare soluble salt compositions within
the glacial drifts in the two drainages. The influence of dust abundance and chemistry was evaluated using data from the literature
(Lancaster, 2002; Lyons et al., 2003; Fortner et al., 2011).
4.5. Geochemical modeling
Chemical weathering models were conducted using Geochemist's
Workbench to test the influence of bedrock mineralogy on stream
solute chemistry. Thin section petrography and EMPA analysis provided
representative mineralogy for the bedrock components (Tables 1 and
2). Significant differences between the bedrock mineralogy for Clark
and Howard Glacier include differing amounts of albite, anorthite,
K-feldspar, and mica, which we hypothesized could cause differences
A.R. Stumpf et al. / Chemical Geology 322–323 (2012) 79–90
Concentration (µM)
A 140
SO4
Fe
Si
Na
Ca
Mg
K
120
Onyx River, this study
100
80
60
40
20
0
0
2000
4000
6000
8000
10000
Distance from Glacier (m)
B 200
Onyx River, Green et al, 2005
Concentration (µM)
180
2
160
1
140
120
100
80
60
40
20
0
0
5000
10000
15000
20000
25000
30000
85
have been present in the system due to intermittent warming above
0 °C prior to sampling in early-mid January. Howard Glacier kinetic
geochemical modeling results over five days (Table 4) also predict
an increase in Ca of b0.1 μM and a decrease in Fe; all other cations
are not predicted to change. Over a time interval of 50 days, the
model predicts a small increase in Ca (b1 μM) and very small decreases in Fe and SiO2 (aq).
Results of the kinetic modeling predict limited addition of cations
to the stream over the relevant time intervals. Indeed, the concentrations predicted by the kinetic model are two orders of magnitude less
than those observed in the field over the course of the stream reaction, even for the highly conservative 50 day models. These results
also suggest that the cations in the streams are not derived from
weathering of the bedrock alone. It is possible that the rates and surface areas used in the model are not appropriate for the reactions
occurring in the stream drainage. However, we consider this unlikely
because we used the measured BET surface area values of the finegrained (b63 μm) fraction collected in the stream sediment (the
most reactive component owing to relatively high surface area;
Anderson, 2007), as well as laboratory determined dissolution rates
which are often much faster than rates observed in the field (White
and Brantley, 2003). An alternative hypothesis is that the primary
source for solutes within the stream is a combination of dust from
the glacier surface (Lyons et al., 2003; Fortner et al., 2011) and
water–sediment interactions in drift sediments within the hyporheic
zone (Nezat et al., 2001; Gooseff et al., 2002; Lyons et al., 2002; Green
et al., 2005; Harris et al., 2007; Levy et al., 2011), including dissolution
of secondary salts and carbonates, rather than bedrock.
Distance from Glacier (m)
4.6. Leaching experiments
Fig. 4. Onyx River water chemistry. (A) Sampling from Lake Brownworth into Wright
Valley shows increasing concentrations in all ions. Na exhibits a higher concentration
than Ca, similar to Clark Glacier. (B) Onyx River water chemistry from Green et al.
(2005). Black line 1 in both graphs corresponds to the confluence of the Onyx River
and Clark Glacier stream. Black line 2 in B represents the extent of our field sampling.
Data are not corrected for glacial surface composition.
in Na and Ca concentrations in the streams. We analyzed the equilibrium
model output only within the early reaction progress from 0 to 0.1 (0–10%
of the reactants added) to evaluate the role of chemical weathering of
primary bedrock within the stream channel. At low temperatures
(5 °C), Clark Glacier fluid composition modeling results predict significant
increases in Na, Mg, and Ca over the entire reaction progress with Na having the highest concentration overall (Fig. 5A). SiO2 (aq), SO4, and Fe are
predicted to decrease in concentration due to precipitation of secondary
phases (Fig. 5B). K increases in concentration significantly by comparison.
Howard Glacier equilibrium model results also predict significant
increases in Na and Ca; however model Na concentrations are higher
than Ca for Howard Glacier, opposite the trend observed in the field
samples for this site (Fig. 5C). This suggests that differences in relative
Na and Ca concentrations between the two streams are not due to
variation in the underlying bedrock composition. The model predicts
decreases in SiO2 and Fe and no change in SO4. Mg and K increase rapidly over the reaction progress (Fig. 5D).
Mineral dissolution rates were included in some simulations in an
attempt to model the chemical reactions occurring within the meltwater streams more accurately. However, kinetic geochemical modeling of primary minerals in both stream systems predicted almost no
change in concentration of aqueous ions over periods of days to
weeks (Table 4). A five‐day reaction time interval (similar to the
period of time water was flowing within the stream prior to sampling) produces a very small (b1 μM) increase in Ca; concentrations
of all other cations remained the same over the duration of the
Clark Glacier model. Over the 50‐day interval, the model predicts
slight increases in Ca (b2 μM), K, and Mg (both b1 μM), while Fe decreases slightly. A 50-day duration is considered reasonable as this
(~6 weeks) is the maximum possible period that liquid water might
Results from the ultrapure water and HCl leaching experiments reflect
the amount of water-soluble salts and acid-leachable carbonates, respectively, present within the drifts which may impact the water chemistry of
the glacial streams. Green et al. (2005) suggested that high concentrations
of Na and Ca within the Onyx River may be related to aerosol inputs from
the Ross Sea and possible evaporites from remnant proglacial lakes. However, water-leachable Na concentrations in the drift from Wright Valley
are less than those from Taylor Valley drift (Fig. 6A). Furthermore,
water-leachable Ca concentrations in leachates from Taylor Valley drift
exceed those of Wright Valley drift. Water leach results for Si, Mg, K and
Fe are shown in Fig. 6B. Overall, Taylor Valley drifts contain more waterleachable ions than Wright Valley drifts. Abundant Si (highest concentration after Na and Ca), was also released during the water leach experiments, likely due to disaggregated colloidal phases small enough to pass
through the 0.2 μm filter. Mg and K were also present at lower concentrations in Wright Valley drifts. All drift samples produced low amounts of
water leachable Fe, with the first Wright Valley drift sample producing
the highest Fe concentration. Potential sources for the water-soluble
ions include dissolution of primary minerals and/or desorption of ions associated with mineral surfaces within the drift, eolian dust, and marine
aerosols (Lyons et al., 2003).
Based on results from the ultrapure water-leach experiment, the
differences in Ca and Na trends in the water from the two valleys
are not attributable to water-soluble salts within the drifts. If marine
aerosols were a major source we would expect to see higher Na
concentrations in the leachable component of drifts from the Clark
Glacier region relative to those from the Howard Glacier region,
owing to the proximity of Clark Glacier drainage to the Ross Sea.
However, this is not the case; rather, concentrations of watersoluble Na and Ca are much higher in the Taylor Valley drift.
HCl was subsequently added to the water-leached drift samples in
an attempt to dissolve potential carbonates in the drift samples.
Wright Valley drift samples became slightly cloudy, but did not effervesce with the addition of HCl. The Taylor Valley drift samples
became dark and cloudy and both samples effervesced after the first
86
A.R. Stumpf et al. / Chemical Geology 322–323 (2012) 79–90
C 35000
Clark Model
Na
Ca
Mg
25000
Na
Ca
30000
Concentration (µM)
Concentration (µM)
A 30000
20000
15000
10000
5000
Howard Model
25000
20000
15000
10000
5000
0
0
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
0
0.01
0.02
0.03
Reaction Progress
B
D
45
Concentration (µM)
Concentration (µM)
30
25
K x 0.005
SO 4
15
Fe
SiO 2 (aq)
10
5
0.06
0.07
0.08
0.09
0.1
35
Clark Model
20
0.05
40
40
35
0.04
Reaction Progress
30
Howard Model
25
20
15
SO 4
Mg x 0.005
K x 0.005
10
Fe
SiO 2 (aq)
5
0
0
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
Reaction Progress
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
Reaction Progress
Fig. 5. Equilibrium modeling results. (A) Clark Glacier modeling results show an overall increase in ions over the reaction progress. Na exhibits the highest concentration and Na, Mg, and
Ca all increase significantly with reaction progress. (B) K, SiO2 (aq), SO4, and Fe isolated from the more dominant cations in the Clark Glacier model; SiO2 (aq), SO4, and Fe decrease slightly,
while K increases significantly by comparison. (C) Equilibrium modeling results for Howard Glacier Na and Ca increase significantly over the reaction progress. (B) K, SiO2 (aq), SO4, and Fe
isolated from the more dominant cations in the Howard Glacier model; SiO2 and Fe decrease slightly, and no change in SO4 is predicted. Mg and K increase significantly over the reaction
progress. K concentrations rise significantly early in the reaction progress, then increase slightly for the remainder of the model reaction.
20 ml of HCl was added. With the second and third additions of HCl,
the effervescence diminished in the Taylor Valley samples. The
filtered HCl solutions contained high concentrations of Ca in Taylor
Valley drifts (Fig. 6C). Wright Valley drift solutions contain an order
of magnitude less HCl-soluble Ca than Taylor Valley drift. Na concentrations in Taylor Valley drifts were also higher than those in Wright
Valley drifts. Fig. 6D shows the HCl leach results for Mg, K, Fe, and Si.
After Ca and Na, Si exhibits the highest concentration in solution,
followed by Fe. Si, Fe, Mg, and K were all more than an order of magnitude higher in the HCl leach solution than in the ultrapure water
leach experiments for the same samples. Wright Valley HCl leaching
results exhibit a similar trend, with Si having the highest concentration, followed by Fe, then Mg, and finally K. Overall concentrations
Table 4
Kinetic model results (μmol/l).
Time (days)
Na
Ca
Mg
K
SO4
Fe
SiO2 (aq)
Clark
0
5
Δ5
50
Δ 50
97.47
97.47
–
97.47
–
35.02
35.20
0.18
36.81
1.79
20.04
20.04
–
20.05
–
27.41
27.41
–
27.43
–
23.33
23.33
–
23.33
–
0.00
0.00
–
–
–
129.40
129.40
–
129.40
–
Howard
0
5
Δ5
50
Δ 50
97.28
97.28
–
97.28
–
34.96
35.04
0.08
35.85
0.89
20.00
20.00
–
20.00
–
27.36
27.36
–
27.36
–
23.28
23.28
–
23.28
–
0.00
0.00
–
–
–
129.20
129.20
–
129.10
−0.10
of these ions were lower in Wright Valley leachates compared to
Taylor Valley drift leach results, but remain an order of magnitude
higher than Wright Valley ultrapure water leach samples.
Based upon these leaching experiments, carbonates observed in
Taylor Valley are likely the source of the high Ca/Na ratios observed in
Howard Glacier stream. This is consistent with field observations;
Ross Sea drift clasts are commonly coated in carbonate (Hall et al.,
2000), and we observed carbonate crusts on clasts throughout Taylor
Valley drifts, specifically within the Ross Sea drift. In addition, rock
flour within the Ross Sea drift also contains abundant carbonates. Core
logs of Lake Fryxell sediments show abundant sand with algae and
CaCO3 rich laminations deposited during lake lowstands (Whittaker et
al., 2008).
Although the origin of these carbonate crusts remains under investigation, one source could be biologically produced carbonate deposited
during the LGM when large proglacial lakes filled the valley (Lawrence
and Hendy, 1989; Green and Lyons, 2009). Modern valley lakes host a
diverse community of organisms (Green and Lyons, 2009; Wadham et
al., 2010) and carbonate precipitation and sulfate reduction are important
processes in Lake Fryxell (Lawrence and Hendy, 1989; Whittaker et al.,
2008; Green and Lyons, 2009). Algal communities cause CaCO3 to precipitate as their demand for CO2 during photosynthesis raises the pH of the
surrounding lake waters resulting in calcium carbonate precipitates that
accumulate on the lake floor (Hendy et al., 2000). Carbonate production
occurred within Lake Washburn during the LGM (Lawrence and Hendy,
1989). Once the Ross Sea ice sheet retreated from the valley, carbonate
deposits remained among the unconsolidated drift in Taylor Valley.
These deposits would cause high Ca concentrations in the glacial stream
chemistry as they are weathered. Field observations indicate that Howard
Glacier stream also contains abundant algae, whereas Clark Glacier
stream does not exhibit evidence of macroscale biological activity.
A.R. Stumpf et al. / Chemical Geology 322–323 (2012) 79–90
B
Concentration (µM)
12000
10000
160
Ultrapure Water
140
Mg
K
Fe
Si
120
100
Ultrapure Water
80
60
40
20
0
Wright Valley Drift
C 14000
180
Na
Ca
Concentration (µM)
Concentration (µM)
500
450
400
350
300
250
200
150
100
50
0
Wright Valley Drift
Taylor Valley Drift
D 8000
Na
Ca
7000
Concentration (µM)
A
87
HCl
8000
6000
4000
2000
6000
5000
Taylor Valley Drift
Mg
K
Fee
Si
HCl
4000
3000
2000
1000
0
0
Wright Valley Drift
Taylor Valley Drift
Wright Valley Drift
Taylor Valley Drift
Fig. 6. Drift leach experiments. (A) Ultrapure water leach results indicate that Na and Ca concentrations in Taylor Valley drift are higher than those for Wright Valley drift samples. If
aerosol and salt crusts drive the differences in concentrations of Na and Ca between the two valleys, then both would be higher in Wright Valley than in Taylor Valley, however the
opposite occurs. (B) Ultrapure water leach results for Mg, K, Fe, and Si; Taylor Valley drift samples contain more leachable ions than Wright Valley drifts. (C) HCl leach results; Ca
concentrations in Taylor Valley drift exceed those of Wright Valley drift by an order of magnitude. The presence of carbonate crusts in Taylor Valley drifts is likely responsible for the
high Ca in Howard Glacier stream field data. (D) HCl leach results for Mg, K, Fe, and Si also indicate that Taylor Valley drift samples leach more ions. Fe concentrations in the HCl
leach results are an order of magnitude higher than those in the ultrapure water leach results.
Therefore, the difference in carbonate formation could also be related to
the differences in modern biological activity between the two valleys.
Eolian dust released from the glacier surface (cryoconites) and
transported by meltwater downstream could also provide a significant source of weatherable material in the Dry Valley streams
(Lyons et al., 2003; Fortner et al., 2011). Additionally, material from
drifts within the valleys may be redistributed into surrounding
glaciers via eolian processes (Atkins and Dunbar, 2009). Therefore,
it is difficult to discern the source of the weathering material within
the stream, i.e. whether the cause is water–rock interactions with
the drift deposits within the stream channel, or dissolution of finegrained dust released from the glacier surface during downstream
transport. Howard Glacier received 0.28 g/m 2/yr eolian silt and clay
between 1997 and 2000, while Lake Brownworth, located near Clark
Glacier, received 0.19 g/m 2/yr eolian silt and clay over the same
time period (Lancaster, 2002). In addition, eolian materials deposited
on Howard Glacier are strongly enriched in Ca compared to other
glaciers in the region, based on samples collected from the glacier
surface (Lyons et al., 2003; Witherow et al., 2010; Fortner et al.,
2011). While previous studies of eolian dust composition focused on
Taylor Valley, our sample of Wright Glacier surface ice suggests that
similar Ca-rich dust does not occur in the area near Clark Glacier.
This suggests that local drift may significantly influence the composition of cryoconite locally, making these two potential sources difficult
to distinguish with the available data.
is occurring within the stream system. In both streams, Mg>K, with
higher Mg and K concentrations observed in Clark Glacier stream relative
to Howard Glacier stream (Fig. 3). The ultrapure water leach results show
K>Mg in three of the four drift samples (Fig. 6), suggesting that watersoluble salts within the drifts are not the major source of K and Mg in
the streams. However, Mg concentrations exceed K in all of the HCl leachates, and both components occur at high concentrations compared to
both the stream chemistry and the ultrapure water leachates, suggesting
derivation of the Mg from carbonates in the drift.
Equilibrium modeling of water–bedrock interactions within Clark
Glacier stream predicts Mg > K throughout the reaction, with Mg
predicted as the second highest solute concentration (after Na). This
suggests that the relatively high Mg concentration in Clark Glacier
stream could be the result of chloritized biotite, chlorite, and amphibole
weathering from primary bedrock or clasts within the drift. The trends
for K in equilibrium models for both streams are very similar,
suggesting that K-feldspar and mica dissolution may account for the increasing K concentrations downstream. However, equilibrium modeling for Howard Glacier predicts K > Mg (Fig. 5), while Mg was ~2
times greater than K in field water samples. The Ross Sea drift in Taylor
Valley contains basaltic erratics delivered to Taylor Valley by grounded
ice from Ross Island (Hall et al., 2000); these erratics were not considered in the equilibrium water–bedrock model. Weathering of these
mafic erratics may be supplying additional Mg, as well as Ca to the
stream. Higher concentrations of Mg relative to K were also observed
in surface ice samples representative of both glaciers (Table 3), therefore, transport and dissolution of eolian dust may also contribute to increasing Mg and K concentrations downstream.
4.8. Mg and K
4.9. Si
Mg and K concentrations generally increase as a function of distance
from the glacier, providing additional evidence that chemical weathering
Glacier stream water analyses show aqueous Si increases with distance downstream. In Clark Glacier stream, Si concentration initially
4.7. Eolian dust
88
A.R. Stumpf et al. / Chemical Geology 322–323 (2012) 79–90
decreases then increases in samples taken beyond ~ 2000 m from the
glacier, with concentrations ranging from ~75 to 150 μM (Fig. 3A). In
the Onyx River (both field and published data) and Howard Glacier
stream, Si increases linearly over the course of the stream, though
concentrations remain relatively low (Figs. 3C and 4A). However,
equilibrium models for both Clark and Howard Glacier streams
predict decreasing SiO2 (aq) as secondary minerals precipitate. The
ultrapure water and HCl leach experiments both released more Si
from Taylor Valley drifts compared to Wright Valley drifts. However,
Si concentrations in the Howard stream samples were overall lower
than observed in Clark stream, suggesting that addition of soluble
silica and colloids from the drift does not supply the primary source
for silica in the stream waters. Instead, increasing Si along the downstream transects likely reflects silicate mineral dissolution with little
to no precipitation of secondary clays in the system. Potential
sources for the weathering silicates include bedrock, clasts within
the drifts, and eolian dust released into the meltwater from the
glacier surface.
4.10. Fe
Clark Glacier stream contains significant Fe concentrations (~50–
100 μM) that decrease distally, whereas Howard Glacier stream contains relatively low Fe concentrations (b20 μM) throughout (Fig. 3).
Fe concentrations in both equilibrium models are buffered at very
low concentrations (b1 μM) by precipitation of secondary iron oxides
(Fig. 5). Ultrapure water leach results contained low amounts of Fe
(b20 μM) for all drifts. Within Clark stream, both pH and Fe concentration decrease downstream perhaps due to oxidation of ferrous
iron released from primary minerals and subsequent precipitation
of iron (hydr)oxides. Higher concentrations of Fe in the Clark stream
data compared to the equilibrium model could be the result of Fe
remaining in solution in the natural system, either due to redox disequilibrium or slow nucleation kinetics for iron oxides in the system.
Alternatively, nanoparticulate iron oxide phases may be more prevalent proximally in the Clark Glacier stream, resulting in high iron
concentrations due to acidification during sampling. If so, then differences in composition and grain size of eolian dust from the glacier
surface may have a significant effect on the iron load within the
streams.
While differences in drift composition may strongly influence Na,
Ca, and perhaps also K and Mg concentrations observed in the
streams, differences in drift chemistry cannot account for the observed differences in Fe concentrations between the two streams.
Water leachable Fe concentrations in Clark and Howard streams
are roughly equivalent; however, the Ross Sea drift underlying Howard Glacier stream contains significantly more acid-leachable Fe,
likely the result of dissolution of the iron oxides and iron-rich carbonates (Fig. 6). Therefore, if the drift were the major control on Fe
concentrations in the streams, we should see higher Fe levels within
Howard Glacier stream, rather than Clark.
Instead, the low Fe concentrations within Howard Glacier stream
may reflect differences in biological activity between the two stream
systems. Microbial mats are abundant in the Howard Glacier system,
but are relatively rare in Clark Glacier stream. Therefore, biological
consumption of iron may significantly reduce the concentration of
iron in the stream system. If this observed difference in Fe concentrations between the two streams is primarily the result of biological activity, aqueous iron concentrations may be a useful biosignature in
polar environments.
Differences in Fe concentrations in eolian dust between the two
systems may also contribute to differences in Fe chemistry within
the streams. However, Fe was not included in previous published
analyses of dust chemistry within Taylor valley, so this hypothesis
cannot be tested with data currently available.
4.11. Weathering rates
Si and K weathering rates were estimated using the constant
weathering rate contribution (CRC) model described by Gooseff et al.
(2002) for Clark Glacier stream, Howard glacier stream, and Onyx
River data collected in this study (Table 5). Two endmember crosssectional areas (minimum area 1 m 2 = 2 m wide × 0.5 m deep; maximum area 30 m2 = 30 m wide and 1 m deep) were chosen based on
the width of the stream channels (≤2 m), the width of the wet sediment zone adjacent to the stream channel (2–30 m), and the depth to
frozen ground adjacent to the stream channel (0.2–1 m) to represent
a minimum and maximum possible extent of the hyporheic zone associated with the stream channels. The geometric surface area of mud to
gravel particles was calculated based on mass percentage of each component in the sediment (determined by wet-sieving sediments collected in the stream channel) assuming spherical grains. This geometric
surface was compared with the measured nitrogen-absorption BET surface area of the b62.5 μm (mud) fraction to determine the effective
range in reactive surface area of the sediment within the hyporheic
zone, assuming solid density= 2700 kg/m 3 and 33% porosity within
the unconsolidated sediment (Table 5). Ratios of BET/geometric surface
area varied from 67 for Clark glacier stream to 400 for Howard glacier
stream, similar to values reported elsewhere for unconsolidated sediments. Flow rates (Q, l/s) were estimated using historic data reported
in the McMurdo Dry Valleys LTER database — www.mcmlter.org and
flow rates reported for the Onyx River in Green et al. (2005).
Si and K weathering rates determined for Howard Glacier stream
data are similar to the geometric and BET rates presented by Gooseff
et al. (2002) for Guerard Stream, which is also located in Taylor Valley
and flows over the same Ross Sea drift. However, weathering rates
found in this study for Clark Glacier stream and the Onyx River in
Wright Valley are roughly two orders of magnitude faster than apparent
rates in Howard Glacier and Guerard Streams. This difference in
weathering rates between the streams in the two valleys is due to
both higher concentrations of solutes, especially in Clark Glacier stream,
as well as relatively lower surface areas (both measured BET and calculated geometric surface areas) of sediments in Wright Valley. The lower
surface area of Wright Valley sediments may be due to the cold-based
nature of the glaciers that formed the drift there, in contrast to the
LGM Ross Sea drift in Taylor Valley that resulted from a thicker and partially wet-based ice sheet, which yielded rock flour with higher surface
area (Hall et al., 2000; Hall and Denton, 2005).
However, generally higher concentrations of solutes in Clark Glacier
stream relative to the other streams in the region, including streams
Table 5
Weathering rates.
dK/dx
(mol l− 1 m− 1)
dSi/dx
(mol l− 1 m− 1)
BET SA (m2 g− 1)
GEOM
SA
(m2 g− 1)
Q (l/s)
A (m2)
BET r K
(mol/m2/s)
BET r Si
(mol/m2/s)
GEOM r K
(mol/m2/s)
GEOM r Si
(mol/m2/s)
a
Clark
Onyx
Howard
Guerard
Streama
1.4E-08
1.4E-09
2.5E-09
3.7E-09
2.0E-08
2.0E-09
3.5E-09
6.1E-09
0.1
0.0015
0.1
0.0015
4
0.01
1.23
0.0038
Min
1
1
2.6E15
3.7E15
1.7E13
2.5E13
Max
100
30
7.9E12
1.1E11
5.2E10
7.5E10
Min
40
1
1.0E14
1.5E14
7.0E13
1.0E12
Max
150
30
1.2E12
1.7E12
7.9E11
1.1E10
Average values are from Gooseff et al. (2002).
Min
1
1
1.2E17
1.6E17
4.7E15
6.5E15
Max
100
30
3.5E14
4.9E14
1.4E11
2.0E11
Min
1.4
1.34
4.5E16
7.4E16
1.4E13
2.4E13
Max
11.8
5.3
1.5E14
2.5E14
4.8E12
7.9E12
A.R. Stumpf et al. / Chemical Geology 322–323 (2012) 79–90
which flow through sediment with higher BET and geometric surface
areas suggest that other processes besides chemical weathering of primary minerals may be affecting solute concentrations within the
stream channel. In addition, weathering rates estimated for Clark
Glacier stream are equal to or greater than mineral dissolution rates
measured in the laboratory, as well as the field (Table S3 in the supplemental materials for this study and Table 7 in Gooseff et al., 2002). The
higher solute concentrations, and thus the higher apparent weathering
rates in Clark Glacier stream may be partially attributed to the timing of
the stream sampling. Clark Glacier stream was sampled on January 12,
2010, the second day that the stream was flowing during our sampling
period. However, liquid water may have been previously active within
the hyporheic zone, leading to localized chemical weathering of sediment prior to the seasonal onset of stream flow. Therefore, pore waters
within the hyporheic zone may have had significantly higher concentrations of solutes immediately prior to stream flow. If this was the
case, relatively dilute water from the glacier surface would have
mixed with more saline pore waters within the hyporheic zone leading
to a short initial period of higher solute fluxes due to hyporheic exchange, as suggested previously by Gooseff et al. (2002). However, further field data are required to test this hypothesis since Clark Glacier
stream was sampled only once during the field season and pore water
samples were not collected from the sediments surrounding the stream
channel.
5. Conclusions
This research provides the first published data on the Clark and
Howard Glacier drainages in terms of stream chemistry as a function
of distance from the glaciers. The data confirm results from other
weathering studies in the Dry Valleys that demonstrate the occurrence
of significant chemical weathering despite frigid conditions. Geochemical modeling results, combined with drift leach experiments indicate
that differences in the relative concentrations of Ca and Na in the
streams of the two valleys cannot be attributed to differences in bedrock
mineralogy. Instead, differences in compositions of local glacial drifts
likely exert a significant influence on stream chemistry via weathering
that occurs in the hyporheic zone. Additionally, glacier surfaces within
the valleys trap eolian dust deflated from local drifts, and these trapped
dusts (cryoconites) are subsequently released into proglacial streams
via supraglacial melting during the summer. The high surface area of
this very fine-grained material facilitates weathering that progresses
downstream within the proglacial systems. Estimates of surface area
normalized chemical weathering rates are roughly equivalent to slightly faster than weathering rates observed in Guerard Stream, Taylor
Valley (Gooseff et al., 2002). Higher apparent weathering rates in Clark
Glacier stream relative to the three other Dry Valley streams are likely
due to a combination of 1) initial stream flow flushing out higher salinity
pore waters within the hyporheic zone, 2) lower surface area sediments
within the stream, resulting in a higher apparent surface area normalized rate, and 3) possible solubilization of fine grained cryconite material
during stream transport and/or sample acidification.
Differences in Fe concentrations between the two streams may be
a result of biological activity in Howard Glacial stream, providing a
potential biosignature within polar weathering environments. However, differences in cryconite chemistry may also supply varying
amounts of iron to the streams via melting glacier surfaces.
Acknowledgments
Thanks to A. Madden for the helpful discussions. The authors also
appreciate comments from co-editor J. Blum and two anonymous
reviewers whose feedback strengthened the manuscript significantly.
Funding for this project was provided by NSF Antarctic Earth Sciences
Program (ANT-0842639).
89
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.chemgeo.2012.06.009.
References
Anderson, S.P., 2007. Biogeochemistry of glacial landscape systems. Annual Review of
Earth and Planetary Sciences 35, 375–399.
Anderson, S.P., Drever, J.I., Humphrey, N.F., 1997. Chemical weathering in glacial environments. Geology 25 (5), 399–402.
Atkins, C.B., Dunbar, G.B., 2009. Aeolian sediment flux from sea ice into Southern
McMurdo Sound, Antarctica. Global and Planetary Change 69, 133–141.
Atkins, C.B., Barrett, P.J., Hicock, S.R., 2002. Cold glaciers erode and deposit: evidence
from Allan Hills, Antarctica. Geology 30 (7), 659–662.
Bethke, C.M., 1996. Geochemical Reaction Modeling. Oxford University Press, New
York, Oxford.
Chinn, T.J., 1993. Physical hydrology of the Dry Valley Lakes. ANTSA4, 59, pp. 1–51.
Clow, G.D., McKay, C.P., Simmons, G.M., Wharton Jr., R.A., 1988. Climatological observations and predicted sublimation rates at Lake Hoare, Antarctica. Journal of Climate
1, 715–728.
Doran, P.T., McKay, C.P., Clow, G.D., Dana, G.L., Fountain, A.G., Nylen, T., Lyons, W.B.,
2002. Valley floor climate observations from the McMurdo Dry Valleys, Antarctica,
1986–2000. Journal of Geophysical Research-Atmospheres 107.
Fortner, S.K., Tranter, M., Fountain, A., Lyons, W.B., Welch, K.A., 2005. The geochemistry
of supraglacial streams of Canada Glacier, Taylor Valley (Antarctica), and their
evolution into proglacial waters. Aquatic Geochemistry 11, 391–412.
Fortner, S.K., Lyons, W.B., Olesik, J., 2011. Eolian deposition of trace elements onto
Taylor Valley Antarctic glaciers. Applied Geochemistry 26, 1897–1904.
Gooseff, M.N., McKnight, D.M., Lyons, W.B., Blum, A.E., 2002. Weathering reactions
and hyporheic exchange controls on stream water chemistry in a glacial meltwater stream in the McMurdo Dry Valleys. Water Resources Research 38 (12),
1279.
Green, W.J., Lyons, W.B., 2009. The saline lakes of the McMurdo Dry Valleys, Antarctica.
Aquatic Geochemistry 15, 321–348.
Green, W.J., Stage, B.R., Preston, A., Wagers, S., Shacat, J., Newell, S., 2005. Geochemical
processes in the Onyx River, Wright Valley, Antarctica: major ions, nutrients, trace
metals. Geochimica et Cosmochimica Acta 69, 839–850.
Hall, B.L., Denton, G.H., 2005. Surficial geology and geomorphology of eastern and
central Wright Valley, Antarctica. Geomorphology 64, 25–65.
Hall, B.L., Denton, G.H., Hendy, C.H., 2000. Evidence from Taylor Valley for a Grounded
Ice Sheet in the Ross Sea, Antarctica. Geografiska Annaler 82, 275–303.
Hall, B.L., Denton, G.H., Overturf, B., 2001. Glacial Lake Wright, a high-level Antarctic
lake during the LGM and early Holocene. Antarctic Science 13, 53–60.
Hall, B.L., Denton, G.H., Fountain, A.G., Hendy, C.H., Henderson, G.M., 2010. Antarctic
lakes suggest millennial reorganizations of Southern Hemispheric atmospheric
and ocean circulation. PNAS 107 (50), 21355–21359.
Hallet, B., Hunter, L., Bogen, J., 1996. Rates of erosion and sediment evacuation by
glaciers: a review of field data and their implications. Global and Planetary Change
12, 213–235.
Harris, K.J., Carey, A.E., Lyons, W.B., Welch, K.A., Fountain, A.G., 2007. Solute and isotope
geochemistry of subsurface ice melt seeps in Taylor Valley, Antarctica. Geological
Society of America Bulletin 119 (5–6), 548–555.
Hendy, C.H., Sadler, A.J., Denton, G.H., Hall, B.L., 2000. Proglacial lake-ice conveyors: a
new mechanism for deposition of drift in polar environments. Geografiska Annaler
82, 249–270.
Hershum, T.G., Marsh, B.D., Simon, A.C., 2007. Contact partial melting of granitic country rock, melt segregation, and re-injection as dikes into Ferrar Dolerite Sills,
McMurdo Dry Valleys, Antarctica. Journal of Petrology 48, 2125–2148.
Higgins, S.M., Hendy, C.H., Denton, G.H., 2000. Geochronology of Bonney Drift, Taylor
Valley, Antarctica: evidence for interglacial expansions of Taylor Glacier.
Geografiska Annaler 82, 391–409.
Ingersoll, R.V., Bullard, T.F., Ford, R.L., Grimm, J.P., Pickle, J.D., Sares, S.W., 1984. The
effect of grain-size on detrital modes; a test of the Gazzi–Dickinson pointcounting method. Journal of Sedimentary Petrology 54 (1), 103–116.
Lancaster, N., 2002. Flux of eolian sediment in the McMurdo Dry Valleys, Antarctica: a
preliminary assessment. Arctic, Antarctic, and Alpine Research 34 (3), 318–323.
Lawrence, M.J.F., Hendy, C.H., 1989. Carbonate deposition and Ross Sea ice advance,
Fryxell Basin, Taylor Valley, Antarctica. New Zealand Journal of Geology and Geophysics 32 (2), 267–277.
Levy, J.S., Fountain, A.G., Gooseff, M.N., Welch, K.A., Lyons, W.B., 2011. Water tracks and
permafrost in Taylor Valley, Antarctica: extensive and shallow groundwater connectivity in a cold desert ecosystem. Geological Society of America Bulletin 123
(11–12), 2295–2311.
Lyons, W.B., Welch, K.A., Sharma, P., 1998. Chlorine-96 in the waters of the McMurdo
Dry Valleys lakes, southern Victoria Land, Antarctica: revisited. Geochimica et
Cosmochimica Acta 62, 185–191.
Lyons, W.B., et al., 2002. Strontium isotopic signatures of the streams and lakes of
Taylor Valley, southern Victoria Land, Antarctica: chemical weathering in a polar
climate. Aquatic Geochemistry 8 (2), 75–95.
Lyons, W.B., Welch, K.A., Fountain, A.G., Dana, G.L., Vaughn, B.H., McKnight, D.M., 2003.
Surface glaciochemistry of Taylor Valley, southern Victoria Land, Antarctica and its
relationship to stream chemistry. Hydrological Processes 17, 115–130.
90
A.R. Stumpf et al. / Chemical Geology 322–323 (2012) 79–90
Mager, S., Fitzsimons, S., Frew, R., Samyn, D., Lorrain, R., 2009. Composition and origin
of amber ice and its influence on the behavior of cold glaciers in the McMurdo Dry
Valleys, Antarctica. Journal of Glaciology 55 (190), 363–372.
Marra, K., Keiser, L., Soreghan, G., Elwood Madden, M., 2011. Trends in grain size and
surface area of glacial and non-glacial alluvial sediments: Building a paleoclimate
proxy for deep-time, abstract EP51A-0823 presented at 2011 Fall Meeting, AGU,
San Francisco, Calif., 5–9 Dec.
Maurice, P.A., McKnight, D.M., Leff, L., Fulghum, J.E., Gooseff, M., 2002. Direct observations
of aluminosilicate weathering in the hyporheic zone of an Antarctic Dry Valley
stream. Geochimica et Cosmochimica Acta 66 (8), 1335–1347.
McKnight, D.M., Aiken, G.R., Smith, R.L., 1991. Aquatic fulvic acids in microbially based
ecosystems: results from two desert lakes in Antarctica. Limnology and Oceanography
36, 998–1006.
Nezat, C.A., Lyons, W.B., Welch, K.A., 2001. Chemical weathering in streams of a polar
desert (Taylor Valley, Antarctica). GSA Bulletin 113 (11), 1401–1408.
Peterson, D.M., Marsh, B.D., 2008. Bedrock Geologic Map of the McMurdo Dry Valleys,
Southern Victoria Land, Antarctica. University of Minnesota Duluth Natural Resources Research Institute.
Pouchou, J.L., Pichoir, F., 1985. “PAP” ϕ(ρz) correction procedure for improved microanalysis. In: Armstrong, J.T. (Ed.), Microbeam Analysis. San Francisco Press, California.
Tranter, M., Skidmore, M., Wadham, J., 2005. Hydrological controls on microbial communities in subglacial environments. Hydrological Processes 19 (4), 995–998.
Wadham, J.L., Tranter, M., Skidmore, M., Hodson, A.J., Priscu, J., Lyons, W.B., Sharp, M.,
Wynn, P., Jackson, M., 2010. Biogeochemical weathering under ice: size matters.
Global Biogeochemical Cycles 24.
Wagner, B., Melles, M., Doran, P.T., Kenig, F., Forman, S.L., Pierau, R., Allen, P., 2006.
Glacial and postglacial sedimentation in the Fryxell basin, Taylor Valley, southern
Victoria Land, Antarctica. Palaeogeography, Palaeoclimatology, Palaeoecology
241, 320–337.
Welch, K.A., Lyons, W.B., Whisner, C., Gardner, C.B., Gooseff, M.N., McKnight, D.M.,
Priscu, J.C., 2010. Spatial variations in the geochemistry of glacial meltwater
streams in the Taylor Valley, Antarctica. Antarctic Science 22 (6), 662–672.
White, A.F., Brantley, S.L., 2003. The effect of time on the weathering of silicate minerals: why do weathering rates differ in the laboratory and field? Chemical Geology 202, 479–506.
Whittaker, T.E., Hall, B.L., Hendy, C.H., Spaulding, S.A., 2008. Holocene depositional environments and surface-level changes at Lake Fryxell, Antarctica. Holocene 18 (5),
775–786.
Witherow, R.A., Lyons, W.B., Henderson, G.M., 2010. Lithium isotopic composition of
the McMurdo Dry Valleys aquatic systems. Chemical Geology 275 (3–4), 139–147.