Assessment
of Chemical Denudation in Cold Environments
Achim A. Beylich*,
Else Kolstrup*, Tage Thyrsted**, Niklas Linde***, Laust B. Pedersen***
*Department of
Earth Sciences, Environment and Landscape Dynamics,
**Harbacken, Stavby, Alunda, Sweden
***Department of Earth Sciences, Geophysics, Uppsala University, Uppsala, Sweden
Process
Rapp’s (1960)
paper on Kärkevagge in northern Swedish Lapland stands out as the study, which
opened awareness of chemical denudation in cold regions and is today a classical
paper on chemical denudation in periglacial regions. Following Rapp’s studies
investigations have been undertaken in other cold climate areas such as for
example in Colorado (Caine, 1979; Caine and Thurman, 1990), the northern Cascade
Mountains (Reynolds and Johnson, 1972), Alaska (Dixon et al., 1984) and Iceland
(Beylich 1999, 2000) and besides, in later years investigations have been
undertaken again in Swedish Lapland (e.g., Darmody et al., 2001; Thorn et al.,
2001; Campbell et al., 2001, 2002; Beylich et al., 2003; 2004a; 2004b;
submitted). Yet, even if investigations show that chemical denudation does take
place in subpolar, polar and alpine areas to various extent, there is still only
little information to reliably assess the general importance of chemical
weathering in relation to specific environmental factors such as climate,
topography, lithology, regolith thickness and ground frost. Therefore a
quantitative investigation of denudation rates requires an integrated approach,
combining different methods.
To improve upon the
knowledge and understanding of how dilution processes, chemical denudation and
water chemistry are related to individual environmental parameters,
investigations are needed in a number of confined drainage basins with different
but individually homogeneous lithology. In turn, these basin should differ
internally with regard to aspect and radiation, slope angles, regolith
thickness, ground frost conditions and other factors of importance.
This paper gives an outline of different methods that have been applied and integrated with assessment of chemical denudation in the Latnjajaure confined drainage basin, an arctic-oceanic periglacial test area in northernmost Swedish Lapland. The methods have been presented previously in different contexts together with the related results (Beylich et al., 2003; 2004a; 2004b; submitted) and further details as well as some of the figures and tables referred to can be found in those papers. The present outline focuses on a description of the methods and their combination.
Figure
1. Location of the Latnjavagge drainage basin in northernmost Swedish Lapland.
The Latnjavagge
drainage basin
The Latnjavagge drainage basin (Fig. 1) is ca 9 km2, its altitude is between 950 m and 1440 m a.s.l. and the location is 68o20’N, 18o30’E. The mean annual temperature in this arctic-oceanic periglacial environment is -2,3 oC (1993-2001), the mean annual precipitation is 818 mm (1990-2001) (Beylich, 2003). The basin has relatively homogeneous bedrock of mica schist and the regolith is predominantly of local origin. In addition to the small size of the area this makes it possible to regard lithological composition and climate as principally constant parameters over the area. Sub-catchments and sampling sites within the drainage basin (Fig. 2) were selected by means of aerial photographs and detailed field work so that different slope angles, aspects to radiation, regolith thickness, snow cover duration and ground frost conditions are represented.
Figure 2.
Location of investigation areas, sampling sites and geophysical profiles within
the Latnjavagge drainage basin.
Techniques
Quantification
of solute inputs
Calculation of net
chemical denudation rates requires quantification of atmospheric solute inputs
into the defined catchment area. The basis for a reliable quantification of
solute inputs is measurement of total annual precipitation. Precipitation was
measured with a Hellmann totalisator (surface area 200 cm2) installed 1 m above
ground and with wind shelter (Molau, 2001; 2004). Snow cores from the annual
snow pack were taken along defined profiles (Fig. 2) before the beginning of
snow melt in spring (Beylich et al., submitted). The precipitation samples and
snow core samples were filtered in the field laboratory with a portable pressure
filter and Munktell quantitative filter papers (OOH).
A quick method to
calculate total dissolved solids (TDS) in precipitation and melted snow samples
is to measure the electric conductivity of the water. In Latnjavagge this was
done by means of a portable conductivity meter (Cond 315i/SET, WTW Weilheim)
corrected to 25 oC. The conductivity values (µS/cm) multiplied by a factor (in
our area 0.7, see also Strömquist and Rehn, 1981; Darmody et al., 2000) can
provide information on total dissolved solids (TDS) [mg/l] values. Atmospheric
solute input rate (mean weighted TDS concentration in wet deposition [kg km-2
y-1]) can then be calculated from the annual precipitation for an area of known
size (Barsch et al., 1994).
Atmospheric input of solutes (wet deposition) over four years in Latnjavagge is given in Table 1 (see below).
Figure 3. . Illustration of the geophysical equipment: Schematic of the field layout for RMT measurements.
Quantification
of solute yields and annual net denudation rates
Calculation of net
chemical denudation rates requires information on atmospheric solute inputs and
quantification of solute concentrations in surface water and of runoff from the
defined catchment area. This requires information on channel discharge and
surface water chemistry.
Hydrological
measurements and sampling for chemical analysis were conducted over the entire
arctic summer from the beginning of snowmelt in late May until the beginning of
September. The analyses for discharge and water chemistry presented below are
simple and therefore applicable in the extreme climatic and topographical
conditions of the remote test area. The methods allow a high spatial density of
daily measurements but are work intensive. The techniques were used according to
the general regulations given in, for example, Barsch et al. (1994).
Discharge from
several channels and at sub-basin outlets was measured three times daily with an
Ott-propeller C2 (Ott GmbH & Co. KG, Kempten) immediately prior to sampling
of water. The stream velocity was measured at defined channel cross-sections at
horizontal distances of 10 cm along each cross-section and at 60% depth of the
total water depth at each measuring point. Velocity isolines over the entire
channel cross-sections were calculated by interpolation. Discharge [m3 sec-1]
was calculated by multiplying the velocity [m sec-1] by the corresponding
cross-section area. Daily discharge [m3 d-1] for each channel was estimated by
interpolating the three daily measurements. Daily specific runoff [mm d-1] was
then calculated from the daily discharge data in relation to the contributing
(sub-)catchment areas (see Beylich, 1999). Installation of fixed gauge stations
was not possible because of the characteristics of the channels (bedrock and/or
larger blocks, shifting channels during snowmelt, slush flows) and the remote
locations of the measuring sites. At each of the sampling sites (Figure 2)
surface water electric conductivity was measured by the portable conductivity
meter mentioned above. Total dissolved solids of surface water was calculated in
the same way as for the input water, and mean daily TDS values [mg/l] could then
be found. Annual gross yields [kg km-2 yr-1] for confined sub-areas as
well as for the entire catchment could then be calculated.
The resulting annual
net yields for the basin (= chemical denudation rates [kg km-2 yr-1]) were found
from the difference between total annual output and input values and a yearly
mean was calculated for a measurement period of three years.
Water and rock chemistry
Following the
conductivity measurements water chemistry analysis was done to detect the
composition of ions for comparisons between precipitation and surface water (Beylich
et al., 2004b).
A total of 205
samples from the different surface water sampling sites (sampling with 1000 ml,
wide-necked polyethylene bottles), the precipitation (collected by the Hellmann-Totalisator)
and from the snow packs were stored in 200 ml polyethylene bottles in a freezing
box after the samples had been filtered (see above). The samples were kept
frozen until they were analyzed in the laboratory for different ions. Na+ and K+
were determined with a “Flammenphotometer Eppendorf Elex 6361”; Ca2+, Mg2+,
Fe2+, Mn2+ were determined with an AAS Perkin-Elmer 5000. SO42-, Cl- and NO3-
contents were measured with an ion chromatograph (DX 100 Dionex) and PO43- with
an autoanalyzer II Technicon.
Two fresh and two
weathered representative rock samples (mica schist) were collected from,
respectively, exposed bedrock and debris at the W- and E-facing valley slopes
and were chemically analysed for various major and rare earths elements by SGAB
Analytica, Luleå Technical University according to standard G-5. Densities (kg
m-3) of the same samples were calculated according to the pycnometer method
according to Swedish Standard SS 13 21 24 at the Swedish Geological Survey (SGU),
Uppsala. The densities of the fresh samples were clearly higher than the
weathered ones and also the chemical components of the two sets differed
somewhat (Beylich et al., 2004b). These results were compared to the contents of
the water samples for detection of rock mineral components that could have been
lost to the water (e.g. CaO). The lanthanides and some other elements in the
rocks show a clear loss from fresh to weathered samples, but these components
can not be compared with the ion content results from the water analysis and are
therefore not included here.
Radio
magnetotelluric (RMT)-geophysical method
To assess chemical
denudation rates for a defined area the regolith volume needs to be known
because it largely determines the contact surface between water and mineral
particles during drainage. At the start of the investigation there was no
information on regolith thickness in Latnjavagge, but it was known that the
regolith consists of poorly sorted material of all grain sizes. It was also
uncertain in how far there was permafrost within the basin. Since coring in this
stony soil would not have given reliable information, it was decided to make
geophysical profiles along selected lines during the late part of the summer
when the ground had thawed as much as possible before freezing set in again. The
photos in the appendix
(photos 1-7) illustrate the geophysical equipment and the variation in
landscape and regolith composition along the geophysical profiles.
Radio magnetotelluric (RMT)-geophysical method
Geoelectrics are
generally suitable in studies of permafrost as frozen ground has a high
resistivity as compared to unfrozen sediments. Furthermore, the thickness of the
regolith can be determined in case of resistivity contrast between regolith and
basement. A comprehensive description of EM geophysics is found in Nabighian
(1987, 1991).
A newly developed
tensor radio magnetotelluric instrument, EnviroMT (Fig 3) (Bastani, 2001), was
applied (see also appendix, photos 1-7) to obtain information on regolith
thickness and frozen ground conditions across selected parts of the Latnjavagge
drainage basin. The tensor radio magnetotelluric (RMT) method makes use of the
electromagnetic fields from distant radio transmitters working in the frequency
range 10-250 kHz. At all frequencies unique transfer functions exist (i.e. the
impedance tensor) dependent only on the earth`s electrical properties. The
impedance tensor enters into the linear relation between the horizontal electric
field and the horizontal magnetic field. The transfer functions are immediately
displayed in the field as apparent resistivities and phases for the two
perpendicular measuring directions used. This allows a first impression of the
resistivity structure and the data quality. The system allows 1-D inversion in
the field, giving a more direct impression of the resistivity structure than the
raw data. The data is later typically inverted using a 2-D inversion code such
as REBOCC (Siripunvaraporn and Egbert, 2000) to get a final image of the
resistivity structure in the ground as a function of depth and profile
direction. Inversion is the process of finding a model that fits the data to a
certain level under certain constraints (e.g. smoothness). Even though the earth
is generally 3-D, 2-D or even 1-D interpretations are often used because of lack
of data or negligible 3-D/2-D effects. A useful introductory text in inversion
theory is Menke`s (1984) book.
In the most populated areas of Europe there are usually between 25 and 35 radio transmitters from which sufficiently strong signals in the band 14-2509 kHz can be received. In less populated areas such as parts of the Northern Hemisphere the number is considerably lower. For example, the signal power distributions from Latnjavagge and Central Greenland (68o44’N, 51o12’E) respectively, show that 10-15 and about 6 different transmitters, could be recieved more or less continuously (Figure 4). In spite of this, the number of VLF (Very Low Frequency Band: 3-30 kHz) and LF (30-300 kHz) transmitters is sufficient to make the results reliable for Northern Europe and Greenland.
Figure 4:
Signal to noise ratio of the horizontal magnetic field as a function of
frequency at Latnjavagge, Northern Sweden, and Central Greenland during August
2001 and summer 2002 respectively. The signal to noise ratio is given in decibel
units, i.e.
The station spacing
in Latnjavagge was 10 m. A TSVD (Truncated Singular Value Decomposition)
processing scheme (Bastani and Pedersen, 2001) was used to estimate reliable
transfer functions based on the original data. Inversion was carried out using
the determinant of the impedance tensor, because it is believed to be less
affected by 3-D effects than other choices for inversion, using the REBOCC code
(Siripunvaraporn and Egbert, 2000). The TSVD data, the predicted data from the
model, and the residuals for one of the profiles are given in Beylich et al.
(2003, plate 1) as an illustration. The resulting model is given in the same
paper.
The RMT technique
provides a good resolution of electrically conducting sediments and it is
favourable for studying the geometry of the regolith and variations therein
because it is fast and has low power consumption owing to the fact that existing
radio-transmitters are used as a source, the latter being of particular
importance in isolated and remote areas. The well conducting mica shists in
Latnjavagge form ideal conditions for mapping the bottom of the regolith with EM
methods. A more resistive basement (e.g. granite or gneiss) would make the
transition between the regolith and the basement less well determined.
Refraction seismics
with explosives at the groundwater level could have been an alternative
methodology especially in case of a resistive basement. However, refraction
seismics is invasive and trained staff must be employed to carry out the field
measurements. DC Geoelectrics is often better in determining high resistivity
features, such as permafrost or resistive basements. An inductive method is in
this case superior as it would be difficult to get ground contact among the bare
bedrock. GPR measurements would be much affected by scattering due to dominance
of boulders in the upper part of the regolith. Furthermore, the depth to the
basement would probably not be penetrated. In the absence of transmitters
controlled source tensor magnetotellurics (CSTMT) is an alternative. CSTMT is
comparatively slow, the equipment is heavy and more power is needed. The method
would yield little extra information in this application. Other inductive
methods are possible as reported by Hauck et al. (2001). Methods such as the VLF
method or EM31 are faster than the EnviroMT method, but yield much less
information as they only measure at one frequency, whereby the depth to the
basement is only weakly constrained. However, such methods are extremely useful
for recognisance studies and to assess qualitative changes in the resistivity
structure.
EnviroMT works well
in remote areas, data and preliminary models can be overviewed in the field, it
is non-invasive and could be used to study occurrence of permafrost. However,
the equipment is at the present stage relatively heavy and requires two to three
persons for operation. It is also difficult to measure in very rough terrain, or
where larger creeks and streams must be crossed. Finally, the system does not
yet have a correction for changes in altitudes along profiles (Beylich et al.,
2004a).
Integration
of hydrological, chemical and geophysical data: Examples of results
As illustration of
results Figure 5 presents daily precipitation, daily specific runoff, daily
water temperatures, daily discharge weighted mean concentrations and daily gross
yields of dissolved solids over the 2001 field season at the outlet of
Latnjavagge. Table 2 in Beylich et al. (2003) provides water chemistry data from
precipitation and snow pack samples together with chemistry data from surface
water samples for selected sites within the basin.
The geophysical
profiles (see Beylich et al., 2003, Plate 2; Beylich et al., 2004a, Fig. 3) are
related to the hydrological measurement and sampling sites. For example, profile
2 which crosses the delta is related to site 18, and profile 3+4 along the
west-facing slope of the basin relates to sites 13, 11, 9 and 7.
The solute
concentrations and chemical denudation rates in Latnjavagge were generally found
to be low but there were significant differences between sub-areas. The
geophysical data show a clear distinction between the upper moderately resistive
part of the profiles and the lower highly conductive part that represents the
bedrock and it thus provides an estimate of regolith thickness along the
profiles. The thickness seems to be only a few meters in many places, an
impression confirming field observations there. Further, the resistivity of the
regolith is too low to indicate presence of large bodies of frozen ground
(compare e.g., Isaksen et al., 2000; see also Kling, 1996, 2004; Beylich et al.,
2004a).
The combined
information provided by the chemical and geophysical data can be used with data
along the profiles but also, by extrapolation, to areas further upstream or
upslope. For example, pure water with constantly low ion concentrations over the
summer and a mean annual denudation rate of only 2333 kg km-2 yr-1 (Beylich et
al., 2003; submitted) came from the subcatchment north of Latnjajauare (site 18,
Table 2). Geophysical profile 2 (Plate 2 in Beylich et al., 2003) shows that
across the delta there was no permafrost in August. Yet, the ground in most of
the basin was frozen during the early part of the summer. As the water yield and
TDS values of August do not differ much from those of previous months after snow
melt it seems that (most of) the thin regolith of this upper region would have
to be frozen during the whole field season in order to maintain such low values.
This is in agreement with the shaded position and the longer duration of snow
cover in part of this sub-catchment, which makes up the coldest part of the
Latnjavagge catchment (Beylich et al., 2003; 2004b; submitted). The water
chemistry data indicate that there must have been some chemical weathering in
this upper catchment as the TDS values and the chemical composition of the
drainage water deviate from those of the rain water (see also Beylich et al.,
2004b).
From soil
temperature measurements at Latnjajaure Field Station (Molau 2001; 2004) it was
known that the lower slope segment along the eastern shore of Latnjajaure had
ground temperatures above 0oC shortly after the melting of the snow in early
June (Beylich et al., submitted). In the measurement sites (e.g., 9 and 13) on
this slope the TDS values were comparatively high already from the early part of
the season and further increased until late July after which they remained at
about the same level, although fluctuating. The geophysical profile indicates
that there was probably no frozen ground in August along the lower slope
segment, and it is therefore likely that this radiation exposed area thawed
comparatively early and remained unfrozen over the summer, and that even the
upper parts of the slope had thawed by late July. This impression is confirmed
by the chemical composition of the water from this slope: There are relatively
higher values of ions that are not related to rain water, especially of Ca2+,
and as a consequence, the chemical weathering in this part was comparatively
intense.
In the southeastern
part of the Latnjavagge drainage basin, in sub-catchment A (Fig. 2), the mean
annual chemical denudation rate is 7894 kg km-2 yr-1. In the surface water at
sampling site 7, which integrates values from the sub-catchment, the ion
concentrations became gradually higher during the summer to reach relatively
high concentrations during the late part of the season. The geophysical
investigation suggests a relatively thick unfrozen regolith for August and from
the combined data, including the increasing TDS values during the field season,
it therefore seems that the ground had had time to gradually thaw during the
summer and that water could percolate freely through the whole regolith during
the late part of the season.
It is, however, also
possible to distinguish differences within sub-catchments: Sampling site 9
compares well with site 13 in having high TDS and ion values over the summer. In
contrast, a comparison between data at the inlet to Latnjajaure (site 18,
geophysical profile 2) and the corresponding data at site 11 (within
sub-catchment A) shows that even if there is thick unfrozen regolith in A as
indicated by the geophysical profile, the chemical values at site 11 are more
similar to site 18 than to site 9; i.e., site 11 is regarded as representative
of the relatively inactive conditions further upstream where there was still
snowmelt during August. The combined data sets can thus help to point out areas
with comparably high chemical activity such as radiation-exposed and/or
relatively gently sloping west-facing slopes with early snow melt and thick,
unfrozen and possibly also rather warm vegetated regolith on the one hand and
areas of thinner and colder regolith in areas of more shade and/or greater
elevation on the other, even if such areas are not directly crossed by the
geophysical profile.
A further potential of the combined methods could be to relate the regolith volume and the ground temperatures in defined sub-areas to the TDS values measured in outlets along the slope and/or from sub-catchments in order to arrive at more quantitative results on factors controlling chemical denudation rates. Yet, the variable regolith thickness along terraced slope systems and along differentially eroded slopes require a high number of geophysical and ground temperature measurements.
Figure
5. Daily precipitation, daily specific runoff, daily water temperatures, mean
daily discharge weighted mean concentrations (TDS) and daily gross yields of
dissolved solids over the 2001 field season at the outlet of the
Latnjavagge drainage basin (sampling site 8 in Fig. 2).
Conclusions
The combination of
hydrological, chemical and geophysical methods applied in Latnjavagge stresses
the advantages of combining different investigation methods when studying
temporal and spatial variability within even small catchments of homogeneous
lithology. The combination can be used with assessment of regolith thickness in
well-defined areas as well as of presence or absence of frozen ground
conditions. Further, integration of the methods can provide a basis to assess
surface water and subsurface discharge conditions within confined areas; and
changes in the TDS values over the summer in combination with geophysical data
representative of the late part of the season can also be used to detect how
ground frost conditions have changed over the season not only along the location
of the sampling sites and geophysical profiles, but also in upslope and upstream
source areas.
Finally, comparisons
between sub-areas of homogeneous lithology by means of the applied methods can
help to assess the importance of individual environmental parameters such as for
example the influence of slope aspect to radiation, ground frost or regolith
thickness for the intensity of chemical weathering and the rate of chemical
denudation in selected periglacial areas.
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Table
1. Atmospheric solute inputs in Latnjavagge (1999/2000 – 2002/2003).
Latest update: 6. April 2004.