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Comparative studies of the lichen–rock interface of four
lichens in Vingen, western Norway
Torbjørg Bjelland
a,
*, Ingunn H. Thorseth
b,1
a
Department of Botany, University of Bergen, Alle
´gaten 41, N-5007 Bergen, Norway
b
Department of Geology, University of Bergen, Alle
´gaten 41, N-5007 Bergen, Norway
Received 12 March 2002; received in revised form 9 July 2002; accepted 18 July 2002
Abstract
Sandstone samples from Vingen, western Norway have been analysed to find whether different lichen taxa may explain
variations in the degree of weathering at this site. The samples were analysed by high performance thin-layer chromatography
(HPTLC)/thin-layer chromatography (TLC), scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray
fluorescence spectroscopy (XRF), followed by descriptive statistics and principal component analysis (PCA).
The study of the weathering rinds beneath four lichens indicates differences in weathering effect between the studied taxa.
Those rinds beneath Ophioparma ventosa, and to a lesser extend Pertusaria corallina, are in general deeper and show a higher
degree of mineral dissolution and crumbling of the rock surface, compared to Fuscidea cyathoides and Ochrolechia tartarea.A
very high concentration of fungal hyphae within the weathering rind beneath O. ventosa and P. corallina, compared to the other
two taxa, may be the cause of the increased biophysical and biochemical weathering. O. ventosa and F. cyathoides contain
similar amounts of oxalate, which also in general are much higher than for P. corallina and O. tartarea. The studied taxa also
contain different lichen compounds. Differences in content of lichen compounds seem thus to be a more likely explanation for
the variations in chemical weathering, than differences in oxalic acid production.
In addition to biological mediated physical and chemical weathering, the water holding capacity of lichens may increase the
chemical dissolution and frost wedging of rock surfaces. The thallus and the endolithic fungal hyphae bind the partly
fragmented rock surface and protect it from abrasion and erosion. However, when the thallus dies, loose mineral grains and
fragments from the upper lichen –mineral interface will be removed and a new surface will be exposed and available for
colonization and biodegradation. It is thus likely that lichens enhance weathering rates, except at locations with extremely high
abrasion, where they may protect the surface.
D2002 Elsevier Science B.V. All rights reserved.
Keywords: Endolithic hyphae; Lichen compounds; Lichen taxa; Oxalate; Sandstone; Weathering
1. Introduction
Biophysical weathering of rocks by lichens results
from penetration of the mycobiont into the weathering
rind and expansion and contraction of these fungal
hyphae due to wetting and drying cycles. This may
0009-2541/02/$ - see front matter D2002 Elsevier Science B.V. All rights reserved.
PII: S0009-2541(02)00193-6
*
Corresponding author. Fax: +47-55-58-96-67.
E-mail addresses: torbjorg.bjelland@bot.uib.no (T. Bjelland),
ingunn.thorseth@geol.uib.no (I.H. Thorseth).
1
Fax: + 47-55-58-94-16.
www.elsevier.com/locate/chemgeo
Chemical Geology 192 (2002) 81 – 98
lead to fragmentation of the rock and its component
minerals (Syers and Iskander, 1973). Biophysical
weathering by lichens was mentioned in the literature
for the first time in 1856 (Lindsay, 1856).Fry (1922,
1927) claimed that the effect of rock-encrusting
lichens on their substrate was restricted to mechanical
action. However, as analytical techniques have
improved, a number of studies have demonstrated
chemical interaction and degradation of the rock
substrate by lichens (Schatz, 1963; Jackson and Kel-
ler, 1970; Ascaso et al., 1976; Hallbauer and Jahns,
1977; Wilson and Jones, 1983, 1984; Adamo and
Violante, 1991; Gehrmann et al., 1992; Adamo et al.,
1993). Biochemical weathering is mediated by acids
and metal-complexing compounds, produced directly
or indirectly by biogenic processes in the lichens.
Respiration by lichens generates CO
2
, which in com-
bination with water, forms carbonic acid. Production
of organic acids may also accelerate weathering by
lowering the pH. The effect on mineral dissolution
rates by metal-complexing biogenic acids is however
most important at near neutral pH. Biochemical
weathering may hence lead to textural and chemical
changes of the rock and its component minerals (Syers
and Iskander, 1973).
The discussion of chemical weathering by lichens
has focused on the role of lichen compounds (includ-
ing lichen acids) and oxalic acid. Lichen compounds
have been shown to increase mineral dissolution rates
in laboratory experiments (Ascaso et al., 1976).Purvis
et al. (1987, 1990) demonstrated that lichen com-
pounds have the ability to complex metal cations
from the mineral substrate. Due to their limited
solubility and the location of some lichen compounds
in the upper part of thallus (cortex and upper
medulla), other researchers conclude that lichen com-
pounds in general do not play an important role in
mineral weathering (Barker and Banfield, 1996).
In recent years, numerous studies have indicated
that the formation of oxalic acid at the lichen – rock
interface is responsible for decomposition of minerals
(e.g. Jones et al., 1980; Galvan et al., 1981; Wilson et
al., 1981; Ascaso et al., 1982; Chisholm et al., 1987;
Purvis et al., 1987; Salvadori et al., 1990; Adamo et
al., 1993; Seaward and Edwards, 1995, 1997). Con-
centrations of the acid may be sufficiently high to
precipitate oxalate salts, and there is a direct relation-
ship between the cation composition of oxalate in the
lichen and the mineralogy and chemical composition
of the substrate (Purvis, 1984). The effect of oxalic
acid has also been investigated under experimental
conditions (Jones et al., 1980; Wilson et al., 1981;
Ascaso et al., 1982; Welch and Ullman, 1993; Stil-
lings et al., 1996; Ullman et al., 1996).Inthe
experimental study of Stillings et al. (1996),the
dissolution rate of plagioclase was shown to increase
by a factor of 2 –15 in 1 mM oxalic acid solutions,
compared to rates measured in inorganic acid solu-
tions of the same pH.
The question of whether a rock surface that is
encrusted by lichens weathers slower or faster than
an identical but lichen-free surface has interested
archaeologists, architects, botanists, chemists, engi-
neers, geographers and geologists. An extensive liter-
ature on lichens and biodeterioration of stonework
exists (see Piervittori et al., 1994, 1996, 1998 for a
literature list; and Adamo and Violante, 2000; Chen et
al., 2000, for review articles). Today, it is commonly
agreed that lichens are able to weather their rock
substrate both biophysically and biochemically, but
the overall effect of lichens in weathering of minerals
and rocks is still a matter of debate. Some researchers
claim that the weathering rates would be faster
beneath lichens (Schwarzmann and Folk, 1989;
Schwarzmann, 1993; Schwarzmann et al., 1997; Rob-
inson and Williams, 2000), which has no effect on the
overall weathering rate (Berner, 1992; Drever, 1994;
Cochran and Berner, 1996). Others state that a li-
chen cover retards weathering by protecting the
surface from external weathering agents such as
wind abrasion, raindrop impact, water flow, temper-
ature variations, salt deposition, and airborne pollu-
tion (Lallemant and Deruelle, 1978; Seaward et al.,
1989; Viles and Pentecost, 1994; Arin
˜o et al., 1995;
Lee and Parsons, 1999; Chen et al., 2000; Motters-
head and Lucas, 2000).
To our knowledge, no systematic and comparative
studies of different taxa on the same substrate at the
same site have been undertaken to date. During
preservation and conservation of rock carvings in
sandstone in western Norway, we observed that the
degree of weathering differed beneath different lichen
taxa. In order to determine whether lichens were
responsible for this weathering, surfaces covered by
four different saxicolous lichen species were system-
atic sampled and analysed.
T. Bjelland, I.H. Thorseth / Chemical Geology 192 (2002) 81–9882
2. Materials and methods
2.1. Study site and sampling
Vingen is situated in a small fjord on the western
coast of Norway in Sogn og Fjordane county at
61j50VNand05j20VE, at the northern edge of out-
crop of Devonian sedimentary rocks of the Hornelen
Basin. The Vingen area has a coastal climate with
mild winters and normally only a few short periods
with temperatures below 0 jC (mean temperature in
January ca. 1.8 jC) (Aune, 1993), and relatively large
amounts of precipitation (annual precipitation ca.
2500 mm) (Førland, 1993). With westerly winds, the
summer temperatures are relatively low (mean tem-
perature in July 12.5 jC) (Aune, 1993), but in calm
weather and with easterly winds, the effect of the
shielding topography may raise maximum temper-
atures to 20– 25 jC(Bjelland, 2001).
The crustose lichens used for this study were
Fuscidea cyathoides (Ach.) V. Wirth and Ve
ˇzda,
Ochrolechia tartarea (L.) A. Massal., Ophioparma
ventosa (L.) Norman, and Pertusaria corallina (L.)
Arnold. Altogether, 117 rock cores (diameter: 2.4 cm;
depth: 3–6 cm) were drilled from 65 individual lichen
thalli in 1997–1999. Thirteen of the cores, each from
a separate thallus, were used for XRD analyses. The
remaining 104 cores, two parallels from 52 individual
thalli, were used for SEM and XRF analyses. Of the
52 thalli, 11 were F. cyathoides, 11 were O. tartarea,
13 were O. ventosa, and 17 were P. corallina The
cores were drilled from rock surfaces that probably
have been subaerially exposed since the deglaciation
10 000 years ago. The selected rock surfaces had
different inclinations, and were situated at 6 – 20 m
above sea level.
2.2. Taxonomic identification
Lichen specimens that could not be identified in
the field were collected for identification in the
laboratory by using the method described by Purvis
et al. (1992). For the identification of lichen com-
pounds, high performance thin-layer chromatography
(HPTLC) was performed using the method described
by Arup et al. (1993), except for O. tartarea samples
which were examined by thin-layer chromatography
(TLC, Culberson, 1972 and later modifications), as
gyrophoric and lecanoric acid were easier to separate
by this method. HPTLC is a modification of the
standard TLC method and utilizes TLC plates com-
prising a thinner layer of smaller grained silica par-
ticles than the standard TLC method. The analyses
showed that O. tartarea contains gyrophoric and
lecanoric acid, F. cyathoides contains fumarprotoce-
traric acid, O. ventosa contains divaricatic, thamnolic,
and usnic acid, while P. corallina contains thamnolic
acid in the thallus (Table 5, factor variable: 0 = no
lichen compound present, factor 1 = lichen compound
present).
2.3. Analytical methods
The 25 variables used in the ordination analysis are
listed in Table 1, with abbreviations, units, data type,
and the analytical method applied. The inclination
between the bedding in the rock and the rock surface
was measured at the site (factor variable: 0 = rock
surface parallel to bedding, 1 = rock surface inclined
to bedding, and 2 = rock surface perpendicular to
bedding).
2.3.1. Scanning electron microscopy (SEM)
SEM was used to study weathered mineral surfa-
ces, depth of dissolution of minerals (X-ray elemental
mapping, both maximum and minimum dissolution
depth of calcite were measured in each sample), grain
size (ranged in categories appropriate for this study;
factor variable: 0 V50 Am, 0.5 = 50 –63 Am, 1 = 63 –
80 Am, 1.5 = 80 – 100 Am, 2 = 100 – 200 Am,
2.5 = 200 –300 Am, and 3 = 300– 500 Am), lamination
(factor variable: 0 = no lamination, 0.5 = weak/diffuse
lamination, and 1 = clear lamination), porosity (image
analysis), and biological material associated with the
weathering rind.
For the SEM observations, a JEOL JSM-6400
scanning electron microscope, equipped with a Tracor
Northern (TN 5600 Series II) energy dispersive spec-
trometer (EDS) system and a backscatter electron
(BSE) detector, was used. The samples were cut in
sections perpendicular to the surface, coated with
Au–Pd, impregnated with epoxy, polished and coated
with carbon. The analyses were performed at an
acceleration voltage of 20 kV. To study the penetra-
tion and amount of biological material within the
weathering rind, some sections were stained with
T. Bjelland, I.H. Thorseth / Chemical Geology 192 (2002) 81–98 83
Pb-citrate, washed, air dried, and re-coated with
carbon.
2.3.2. X-ray diffraction (XRD)
To determine the possible occurrence of secondary
minerals in the weathering rind, 13 rock cores were
analysed by XRD (three cores with F. cyathoides,
three with O. tartarea, four with O. ventosa, and three
with P. corallina). The weathering rind of each drill
core was cut in two to three subsamples parallel to the
surface, ground to a powder in an agate mortar, and
analysed by a Philips PW 1700 diffractometer, using
Cu K-alpha radiation. A standard power diffraction
file (PDF) database connected to a graphical terminal
was used for mineral identification. The samples were
re-analysed after glycol-treatment and after heating to
500 jC. For comparison, one subsample of the fresh
unweathered rock of each drill core was also analysed.
2.3.3. X-ray fluorescence spectroscopy (XRF)
The upper 2–3 mm of the weathering rind and 2 – 3
mm of the fresh underlying rock were cut from each
drill core by sawing, ground to a powder in an agate
mortar and heated at 900 jC for 2 h to determine the
loss on ignition (LOI). The dry powder was mixed
with dilithium-tetraborat, melted to glass beads and
analysed for major elements by a Philips PW 1404
XRF-spectrometer, calibrated with international stand-
ards with recommended or certified values (Govindar-
aju, 1989).
2.4. Statistical methods
Descriptive statistics were performed using the
SPLUS statistical package (version 4.0, Statsci,
1997). The computer program CANOCO for Win-
dows (version 4.0) was used for all ordinations (ter
Braak and S
ˇmilauer, 1998), and the ordination results
were graphically analysed using CANODRAW (S
ˇmi-
lauer, 1993). Detrended Correspondence Analysis
(DCA), with detrending by segments and log-trans-
formation, was performed to estimate the amount of
compositional turnover in standard deviation (Hill and
Gauch, 1980). The first DCA axis was 0.86 SD,
suggesting that linear-based methods like principal
component analysis (PCA) should be used (ter Braak
and Prentice, 1988). PCA with default settings, except
for centering by variables, was used.
Table 1
List of recorded variables, with abbreviations, units, data type, and
the analytical method used to measure each variable
Variables measured
(abbreviations)
Units Data type Analytical
method
Divaricatic
acid in thallus
(divaricatic)
factor (1/0) HPTLC
Fumarprotocetraric acid in
thallus (fumarprotocetraric)
factor (1/0) HPTLC
Gyrophoric acid
in thallus (gyrophoric)
factor (1/0) TLC
Lecanoric acid in
thallus (lecanoric)
factor (1/0) TLC
Thamnolic acid
in thallus (thamnolic)
factor (1/0) HPTLC
Usnic acid
in thallus (usnic)
factor (1/0) HPTLC
Bedding (bedding) factor (0 – 2)
Lamination (lamination) factor (0 – 1) SEM
Grain size (grain size) Am factor (0 – 3) SEM
Apatite, depth
of dissolution
(apatite)
mm continuous SEM
Calcite, maximim
depth of dissolution
(MAX-calcite)
mm continuous binocular/
SEM
Calcite, minimum
depth of dissolution
(MIN-calcite)
mm continuous binocular/
SEM
Chlorite, depth
of dissolution
(chlorite)
mm continuous SEM
Porosity (porosity) vol.% continuous SEM
Concentration
of Al
2
O
3
(Al2O3)
wt.% continuous XRF
Concentration
of CaO (CaO)
wt.% continuous XRF
Concentration
of FeO
t
(FeOt)
wt.% continuous XRF
Concentration
of K
2
O (K2O)
wt.% continuous XRF
Concentration
of MgO (MgO)
wt.% continuous XRF
Concentration
of MnO (MnO)
wt.% continuous XRF
Concentration
of Na
2
O (Na2O)
wt.% continuous XRF
Concentration
of P
2
O
5
(P2O5)
wt.% continuous XRF
Concentration
of SiO
2
(SiO2)
wt.% continuous XRF
Concentration
of TiO
2
(TiO2)
wt.% continuous XRF
Loss on ignition
(LOI)
wt.% continuous XRF
T. Bjelland, I.H. Thorseth / Chemical Geology 192 (2002) 81–9884
Table 2
Average chemical composition (wt.%) and loss on ignition (L.O.I.) of unweathered and weathered sandstone
SiO
2
TiO
2
Al
2
O
3
FeO
t
MnO MgO CaO Na
2
OK
2
OP
2
O
5
LOI MAX-calcite MIN-calcite Apatite Chlorite Porosity
Unweathered sandstone
Min 55.07 0.23 8.94 1.61 0.04 0.88 1.00 1.03 0.06 0.05 2.24
Max 74.91 1.95 15.27 15.53 0.10 3.09 10.10 2.18 4.28 3.07 9.04
Mean 67.99 0.62 10.88 3.64 0.06 1.80 6.00 1.79 2.41 0.19 5.45
Median 68.23 0.59 10.42 4.41 0.06 1.66 6.19 1.80 2.18 0.10 5.45
SD 3.53 0.33 1.39 1.99 0.01 0.59 2.66 0.26 0.72 0.41 1.80
Skewness 0.87 * 2.21 * 1.01 * 4.32* 0.58 * 0.37 0.23 0.82 * 0.03 1.49 * 0.18
Kurtosis 2.18 * 6.51 * 0.90 25.01 * 0.54 0.86 0.80 0.64 1.34 3.11 * 0.69
n52 52 52 52 52 52 52 52 52 52 52
Weathered sandstone
Min 64.66 0.25 7.65 0.91 0.01 0.23 0.43 0.85 1.45 0.01 1.77 3.00 2.00 0.00 0.00 6.00
Max 81.98 1.72 12.71 10.59 0.06 2.42 2.19 2.42 3.76 0.23 16.74 24.00 21.00 7.27 6.00 31.50
Mean 73.06 0.61 10.59 2.94 0.03 1.10 0.70 1.63 2.57 0.06 7.51 10.80 8.20 2.76 1.90 14.92
Median 72.89 0.52 10.85 2.48 0.03 1.06 1.08 1.60 2.44 0.04 7.13 10.00 7.50 2.58 1.77 15.00
SD 3.97 0.29 1.10 1.47 0.01 0.56 0.53 0.32 0.61 0.05 3.06 4.93 3.79 1.89 1.59 5.26
Skewness 0.09 1.52 * 0.49 2.90 * 0.62 * 0.51 0.66 * 0.02 0.30 1.35 * 0.80 * 0.80 * 0.81 * 0.47 0.64 * 0.62 *
Kurtosis 0.43 3.00 * 0.19 13.50 * 0.59 0.30 0.78 0.12 1.06 1.48 1.58 0.06 1.11 0.40 0.25 1.05
n52 52 52 52 52 52 52 52 52 52 52 52 52 52 52 52
See Table 1 for abbreviations.
*p-values < 0.05.
T. Bjelland, I.H. Thorseth / Chemical Geology 192 (2002) 81–98 85
PCA is a restructuring of the information in the
observations with the purpose of reducing the
dimensions inherent in the data. The axes span
the variance, and subsequent axes are orthogonal
allowing independent interpretation. The first axis is
the direction of the observations that includes the
highest variance, i.e. the distribution of sites along
this axis is most likely caused by some external
factor. The following axes incorporate less subse-
quently variance, until all variances in the original
data are extracted. To simplify the interpretation, we
are interested in the major trends in our observa-
tions. Therefore, we constrain our interpretation to
those axes that incorporates most of the total
variance, i.e. the first couple of axes.
3. Results
3.1. Unweathered bedrock
The bedrock in Vingen is an arkosic metasandstone
where quartz (45 – 55%), plagioclase (15 – 40%)
and potassium feldspar (15–5%) are the domi-
nant minerals. Other minerals present are mus-
covite (5–10%), Fe-rich chlorite (7–12%), epidote
(3–5%), and accessories including apatite, zircon,
sphene, iron- and titanium-oxide. The detrital grains
are mainly cemented by calcite (5 – 12%), but occa-
sionally a quartz cement also occurs.
The bedding generally strikes NE – SW and dips
45jSE. The different beds and laminae range from
Table 3
Summary of each taxon on each continuous variable measured in the unweathered samples, including minimum, maximum, mean, median,
standard deviation, and number of cores
Species SiO
2
TiO
2
Al
2
O
3
FeO
t
MnO MgO CaO Na
2
OK
2
OP
2
O
5
LOI
Fuscidea cyathoides
Min 65.01 0.28 9.31 1.93 0.04 1.05 3.63 1.39 1.9 0.05 3.87
Max 73.12 0.76 12.27 4.32 0.07 2.45 9.84 2.18 3.47 0.21 8.48
Mean 69.46 0.42 10.28 2.73 0.06 1.47 6.31 1.91 2.36 0.10 5.54
Median 70.31 0.39 10.11 2.48 0.05 1.2 6.16 1.97 2.09 0.09 5.37
SD 3.30 0.17 0.91 0.78 0.01 0.50 1.83 0.25 0.56 0.05 1.47
n11 11 11 11 11 11 11 11 11 11 11
Ochrolechia tartarea
Min 61.32 0.35 8.94 2.05 0.06 1.09 2.28 1.15 1.75 0.07 3.06
Max 70.98 1.19 13.1 5.63 0.09 3.04 10.07 2.06 3.36 0.25 9.04
Mean 65.79 0.615 10.64 3.48 0.08 1.96 7.49 1.69 2.46 0.14 6.45
Median 65.06 0.5 10.45 2.98 0.07 1.67 8.46 1.65 2.19 0.1 7.19
SD 2.87 0.29 1.21 1.23 0.01 0.68 2.34 0.25 0.62 0.06 1.86
n11 11 11 11 11 11 11 11 11 11 11
Ophioparma ventosa
Min 55.07 0.35 8.94 2.43 0.05 0.88 1.00 1.03 1.34 0.07 2.32
Max 71.36 1.95 14.23 15.53 0.10 2.59 9.72 2.05 3.77 0.36 7.95
Mean 68.07 0.76 11.59 4.84 0.06 1.96 4.52 1.71 2.47 0.14 4.25
Median 69.76 0.61 11.89 3.81 0.06 2.17 3.56 1.71 2.26 0.12 3.81
SD 4.39 0.47 1.63 3.34 0.02 0.55 2.72 0.29 0.77 0.08 1.75
n13 13 13 13 13 13 13 13 13 13 13
Pertusaria corallina
Min 64.84 0.23 9.34 1.61 0.05 0.88 1.17 1.18 0.06 0.06 2.24
Max 74.91 1.13 15.27 5.31 0.08 3.09 10.1 2.17 4.28 0.2 8.65
Mean 68.39 0.56 10.87 3.41 0.06 1.78 5.97 1.83 2.36 0.13 5.65
Median 68.13 0.59 10.69 3.44 0.06 1.66 6.35 1.81 2.18 0.14 5.61
SD 2.88 0.22 1.43 1.08 0.01 0.56 2.86 0.23 0.88 0.05 1.63
n17 17 17 17 17 17 17 17 17 17 17
See Table 1 for abbreviations.
T. Bjelland, I.H. Thorseth / Chemical Geology 192 (2002) 81–9886
medium to very fine sand grade (500 – 63 Am), but silty
laminae ( < 63 Am) are also present. Detrital muscovite
grains are generally oriented parallel to the lamination.
The plagioclase is albitic in composition and usually
contains numerous inclusions of minute sericite crys-
tals ( < 10 Am). Unweathered sandstone has a dark
Fig. 1. (a) SEM – BSE image showing a transverse section through the weathering rind and into fresh sandstone. (b) SEM image showing
crystallographic controlled etch marks in plagioclase. (c) SEM image showing etch marks in quartz.
Fig. 2. Mean depth of dissolution of apatite, calcite, and chlorite beneath F. cyathoides,O. tartarea,O. ventosa, and P. corallina.
T. Bjelland, I.H. Thorseth / Chemical Geology 192 (2002) 81–98 87
greenish grey colour due to the abundance of chlorite
and epidote and virtually no porosity.
The average chemical composition (wt.%) of
unweathered sandstone is listed in Table 2. The CaO
mainly comes from calcite, but sphene and epidote also
contribute. Apart from some adsorbed and crystallo-
graphic H
2
O, CO
2
release from calcite accounts for
most of the LOI.
3.1.1. Variation in unweathered rock composition
within and between taxa
Descriptive statistical parameters for the chemical
composition of unweathered sandstone beneath the
different lichen taxa are summarized in Table 3. These
variations reflect the heterogeneous mineralogical
composition of the sandstone. It is noteworthy that
the mean CaO concentration beneath O. ventosa is low
(4.52 wt.%), indicating lower carbonate content com-
pared to the other taxa. The concentration of CaO in
unweathered sandstone is greatest beneath O. tartarea
(7.49 wt.%). However, the PCA analysis does not show
any differences in the chemical composition of
unweathered sandstone beneath different lichen taxa.
3.2. Weathered bedrock
The effects of weathering beneath the lichens are
represented by a porous grey to beige rind. The thick-
ness of the rind varies from 3 to 24 mm and its porosity
decreases inwards (Fig. 1a). As no secondary minerals
Table 4
Summary of each taxon on each continuous variable measured in the weathered samples, including minimum, maximum, mean, median,
standard deviation, and number of cores
Species SiO
2
TiO
2
Al
2
O
3
FeO
t
MnO MgO CaO Na
2
OK
2
OP
2
O
5
LOI MAX-
calcite
MIN-
calcite
Apatite Chlorite Porosity
Fuscidea cyathoides
Min 67.76 0.30 9.45 1.54 0.01 0.45 0.46 0.85 1.83 0.03 2.73 3.00 2.00 0.00 0.00 6.00
Max 80.26 0.81 12.64 4.63 0.05 2.42 1.57 2.42 3.42 0.18 8.37 12.00 11.00 5.60 4.36 18.00
Mean 74.22 0.46 10.67 2.52 0.03 1.13 1.00 1.84 2.56 0.08 5.87 8.45 6.91 2.47 1.58 11.82
Median 74.05 0.40 10.26 2.38 0.03 0.98 0.76 1.87 2.42 0.06 6.35 10.00 7.00 2.50 1.46 12.00
SD 3.61 0.18 0.95 0.92 0.01 0.60 0.43 0.41 0.61 0.05 1.64 3.05 3.14 1.76 1.32 3.94
n11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11
Ochrolechia tartarea
Min 65.52 0.35 7.65 1.35 0.01 0.23 0.47 1.19 1.83 0.01 3.47 6.00 2.00 0.00 0.00 14.00
Max 79.24 1.18 12.30 4.29 0.05 2.24 2.06 2.08 3.63 0.10 11.10 19.00 12.00 6.25 6.00 24.50
Mean 73.26 0.67 10.66 3.03 0.03 1.25 1.06 1.58 2.51 0.06 7.39 9.98 6.41 2.10 1.39 16.82
Median 73.13 0.62 10.93 2.94 0.03 1.34 0.86 1.58 2.24 0.05 7.46 8.00 6.00 1.50 0.44 16.00
SD 4.41 0.29 1.35 0.96 0.01 0.58 0.54 0.23 0.68 0.03 2.41 4.66 2.50 2.10 1.99 3.20
n11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11
Ophioparma ventosa
Min 68.50 0.43 8.89 1.86 0.02 0.39 0.43 1.08 1.45 0.01 2.27 7.00 6.00 1.90 1.50 11.50
Max 77.39 1.72 11.51 10.59 0.04 1.40 2.19 1.85 3.46 0.07 12.30 24.00 21.00 7.27 5.58 31.50
Mean 72.89 0.72 10.43 3.29 0.02 0.85 0.96 1.43 2.71 0.03 7.68 15.69 12.15 3.98 3.08 19.65
Median 72.99 0.57 10.54 2.34 0.02 0.87 0.85 1.33 2.69 0.02 7.41 15.00 12.00 4.00 3.00 18.50
SD 3.48 0.38 0.94 2.34 0.01 0.35 0.55 0.25 0.63 0.02 2.88 4.64 3.67 1.43 1.02 5.47
n13 13 13 13 13 13 13 13 13 13 13 13 13 13 13 13
Pertusaria corallina
Min 64.66 0.25 8.00 0.91 0.01 0.24 0.44 1.07 1.70 0.01 1.77 5.00 3.00 0.00 0.00 6.00
Max 81.98 1.07 12.71 5.40 0.06 2.31 2.14 2.06 3.76 0.23 16.74 20.00 13.00 6.79 5.09 19.00
Mean 72.29 0.59 10.62 2.88 0.30 1.17 1.24 1.67 2.52 0.08 8.51 9.12 7.18 2.43 1.52 12.09
Median 72.18 0.60 10.88 2.95 0.03 1.34 1.16 1.62 2.28 0.07 7.74 8.00 7.00 2.00 1.00 13.00
SD 4.39 0.26 1.21 1.21 0.01 0.63 0.57 0.26 0.58 0.07 3.91 3.90 2.90 1.85 1.47 3.71
n17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17
See Table 1 for abbreviations.
T. Bjelland, I.H. Thorseth / Chemical Geology 192 (2002) 81–9888
have been detected by XRD, apart for traces of ver-
miculite and goethite, the porosity of the weathering
rind reflects the degree of dissolution of the minerals in
the rock.
Calcite is very susceptible to chemical weathering,
and X-ray elemental mapping confirms that calcite is
largely absent throughout the weathering rind. In the
transition zone between unweathered and weathered
sandstone, the calcite has been only partly dissolved.
In the lower part of the weathering rind, the porosity
(5–12 vol.%) is caused totally by the dissolution of
calcite. Due to differences in grain size and minera-
logical composition of individual beds and lamina, the
dissolution depth of calcite may vary with 2 – 3 mm
(MAX and MIN calcite) in one sample. This is
evident where the bedding is inclined relative to the
rock surface, and there is an alternation of medium
grained lamina rich in calcite, and silty lamina with a
lower calcite content.
After calcite, apatite and chlorite are the least stable
minerals. Apatite is only present in low concentrations
in these rocks, and its dissolution has little impact on
the porosity. Chlorite is more abundant and the dis-
solution of this mineral may contribute to the total
porosity of the weathering rind. In addition to calcite
and apatite, chlorite is frequently completely dis-
solved in the upper part of the weathering rind and
the porosity is therefore higher in this part (12 – 20
vol.%). The zone with total dissolution of chlorite
varies in thickness from zero to 6–7 mm. Due to
dissolution of chlorite, the upper part of the weath-
ering rind is frequently pale grey to beige in colour.
Plagioclase shows partial dissolution at the same
depths as chlorite. The plagioclase grains have been
dissolved along grain boundaries, crystallographically
determined planes, and around sericite inclusions.
Etch pits and channels on grain surfaces are common.
As a result, plagioclase grains have been detached
from other grains and have partly crumbled (Fig. 1b).
Grains of epidote show similar features. Due to
dissolution of plagioclase, the porosity in plagio-
clase-rich surface layers may reach be 32 vol.%.
Table 5
Summary of each taxon on each factor variable measured for each sample
Factor Fuscidea
cyathoides
Ochrolechia
tartarea
Ophioparma
ventosa
Pertusaria
corallina
Bedding 0 4 2 7 11
13 1 4 1
24 8 2 5
Grain size 0.0 0 0 0 2
0.5 1 1 0 0
1.0 0 2 5 5
1.5 0 0 2 2
2.0 7 7 4 7
2.5 2 0 2 0
3.0 1 1 0 1
Lamination 0 9 7 6 13
0.5 1 2 4 1
11 2 3 3
Divaricatic acid 0 11 11 0 17
10 0 13 0
Fumarprotocetraric acid 0 0 11 13 17
111 0 0 0
Gyrophoric acid 0 11 0 13 17
10 11 0 0
Lecanoric acid 0 11 0 13 17
10 11 0 0
Thamnolic acid 0 11 11 0 0
10 0 13 17
Usnic acid 0 11 11 0 17
10 0 13 0
See Materials and methods for the explanation of each factor.
T. Bjelland, I.H. Thorseth / Chemical Geology 192 (2002) 81–98 89
Quartz is the most resistant mineral in the sand-
stones to dissolution and displays a few etch pits (Fig.
1c). Potassium feldspar and muscovite also show
minor dissolution. At the rock surface, muscovite
has commonly been fractured along the basal cleav-
age.
The average chemical composition (wt.%) of
weathered sandstone is listed in Table 2. Lower FeO
t
and MgO concentrations in comparison to the
unweathered rock is the result of dissolution of
chlorite, and lower P
2
O
5
and Na
2
O concentrations
reflect dissolution of apatite and plagioclase, respec-
tively. Greater concentrations of SiO
2
and K
2
Oin
weathered as opposed to unweathered rock due to the
relative resistance of quartz, potassium feldspar and
muscovite to chemical weathering. A part for some
adsorbed and crystallographic H
2
O, biological mate-
rial in the weathering rind mainly accounts for the
LOI.
3.2.1. Variation in weathered rock composition within
taxa
The average depth of total dissolution of calcite,
apatite, and chlorite in weathering rinds beneath the
different taxa is presented in Fig. 2. The variation in
chemical composition, dissolution depth and porosity
beneath different taxa is summarized in Table 4. A list
of the factor variables is given in Table 5. It is worth
noting that the grain size beneath F. cyathoides is
slightly greater than beneath the other taxa. There are
few parallels for each taxon; it is hence not possible to
tell if the total data set or the sub-sets have a normal
distribution.
3.2.2. Variation in weathered rock composition
between taxa
The first four PCA axes reflect most of the varia-
tion in the weathering rind between the four taxa,
describing 27.6%, 18.8%, 11.7%, and 10.9% of the
variability, respectively. Thus, 69.0% of the total
variation is represented along these axes (Table 6).
The most important variable in the PCA-diagram,
dissolution depth of apatite, is negatively correlated
with PCA axis 1 and strongly negatively correlated
with the P
2
O
5
and MgO concentration and positively
correlated with the maximum and minimum depth of
Table 6
The results of the PCA analysis
Axes 1 2 3 4
Eigenvalues 0.276 0.188 0.117 0.108
Cumulative
percentage
variance of
species data
27.6 46.4 58.1 69.0
Fig. 3. Graphic presentation of the variables measured for each core.
(a) The variables (arrows) in a PCA ordination diagram, axes 1 and
2. (b) The samples represented by symbols on the same PCA axes as
in (a). The arrows show in which direction the greatest variance for
the represented variable. The length of the arrow indicates the
variance of the variable, i.e. long arrow more influential on the
position of the samples.
T. Bjelland, I.H. Thorseth / Chemical Geology 192 (2002) 81–9890
calcite dissolution (MAX-calcite and MIN-calcite)
and the depth of chlorite dissolution (Fig. 3a,Table
7). Many of the measured variables correlate (Fig. 3a).
The dissolution depth of apatite, calcite, and chlorite
shows a strong negative correlation with the concen-
tration of P
2
O
5
, CaO, and MgO, respectively, because
these oxides mainly reflect the concentration of the
aforementioned minerals in the rock. In addition to the
dissolution depth of apatite, the concentrations of
SiO
2
, porosity, grain size, divaricatic, and usnic acid
show a negative correlation with the first axis. The
concentrations of TiO
2
,Al
2
O
3
, FeO
t
, MnO and CaO
are positively correlated. Only the 16 mentioned
variables correlate significantly with axis one (Table
7).
The negative end of the diagram reflects samples
with high dissolution depth of apatite, chlorite and
calcite, while the positive end represents samples with
high content of P
2
O
5
, MgO, and CaO, thus indicating
a shallow weathering rind. The samples of O. ventosa
tend to be represented at the left end of axis one, while
the other taxa are more continuously distributed along
this axis (Fig. 3b).
Na
2
O concentration is the most influential variable
along PCA axis 2 (Fig. 3a). In addition to the con-
centration of Na
2
O, the second axis shows a ne-
gative correlation with the amount of SiO
2
, bedding,
grain size, and fumarprotocetraric acid, whereas
the concentration of TiO
2
,K
2
O, LOI, dissolu-
tion depth of calcite (MIN and MAX), porosity, lami-
nation, divaricatic acid, thamnolic acid, and usnic acid
are positively correlated. Fifteen variables correlate
significantly with axis two (Table 7).
There is also a separation of O. ventosa, as well as
of P. corallina, from the other taxa along the second
PCA axis. All the samples of O. ventosa, and most of
the P. corallina samples, are represented at the pos-
itive end of axis 2. These samples are characterised by
a low Na
2
O content, indicating high dissolution of
Fig. 4. SEM – BSE images showing a section through a thallus of O.
tartarea and the upper part of the weathering rind (a) before and (b)
after staining with Pb-citrate.
Table 7
The correlation of the PCA axes with the measured variables
Axis 1 Axis 2 Axis 3 Axis 4
SiO
2
0.581 * 0.554 * 0.020 0.016
TiO
2
0.487 * 0.657 * 0.383 * 0.153
Al
2
O
3
0.430 * 0.072 0.812 * 0.088
FeO
t
0.533 * 0.522 0.091 0.109
MnO 0.838 * 0.227 0.001 0.044
MgO 0.830 * 0.182 0.408 * 0.006
CaO 0.638 * 0.264 0.547 * 0.187
Na
2
O0.171 0.707 * 0.231 0.272
K
2
O0.179 0.289 * 0.918 * 0.145
P
2
O
5
0.844 * 0.080 0.147 0.270
LOI 0.001 0.347 * 0.445 * 0.037
Chlorite 0.873 * 0.104 0.272 0.009
Apatite 0.885 * 0.059 0.210 0.005
MAX-calcite 0.591 * 0.425 * 0.387 * 0.286 *
MIN-calcite 0.649 * 0.483 * 0.131 0.048
Bedding 0.002 0.398 * 0.135 0.411 *
Porosity 0.348 * 0.536 * 0.174 0.442 *
Grain size 0.445 * 0.500 * 0.256 0.092
Lamination 0.010 0.309 * 0.196 0.266
Divaricatic 0.491 * 0.699 * 0.097 0.007
Thamnolic 0.265 0.634 * 0.00 0.448 *
Usnic 0.475 * 0.631 * 0.158 0.010
Gyrophoric 0.229 0.262 0.142 0.897*
Lecanoric 0.229 0.262 0.142 0.897 *
Fumarprotocetraric 0.092 0.504 * 0.142 0.354 *
See Table 1 for abbreviations.
*
p-Values < 0.05.
T. Bjelland, I.H. Thorseth / Chemical Geology 192 (2002) 81–98 91
plagioclase, and high porosity. The most weathered
samples are hence situated between the negative end
of axis one and the positive end of axis two (Fig. 3b).
Several of the variables correlate significantly with
axes three (6) and four (7) (Table 7), but a plot of
these axes does not show any new information about
the data set.
3.3. The lichen – mineral interface
Even if a diverse community of bacteria, algae, and
fungi inhabits the lichen – mineral interface, fungal
hyphae constitute more than 90% of the biomass. Fig.
4shows SEM–BSE micrographs of a transverse sec-
tion through a representative lichen – mineral interface
before (Fig. 4a) and after (Fig. 4b) staining with Pb-
citrate. The contact between lichen and substrate is very
intimate. Mineral grains or fragments are embedded in
the interior of the thallus and fungal hyphae penetrate
into the sandstone. At the uppermost part of the weath-
ering rind, the pores are filled with hyphae, giving the
false impression of a very low porosity in the upper
weathering zone (Fig. 5a). The abundance of hyphae
decreases inwards in the weathering profile. Deeper
within the weathering rinds, hyphae are still abundant
in the pores, but are not as compact as in the upper part
(Fig. 5b). Only a few or a single hyphae can be seen in
the pores at the bottom of the weathering rind (Fig. 5c).
Fungal hyphae seem to occur throughout the porous
rind, independent of weathering depth.
O. ventosa and P. corallina usually have a thicker
thallus ( f2.0 mm) than F. cyathoides and O. tartarea
(f1.0 mm). Fungal hyphae are also more abundant in
the uppermost part of the weathering rind beneath
Fig. 5. (a) SEM image showing one pore filled with fungal hyphae in the upper part of the weathering rind beneath O. ventosa. (b) SEM image
showing one pore with fungal hyphae in the middle of the weathering rind beneath O. ventosa. (c) SEM image showing one pore with a fungal
hyphae at the bottom of the weathering rind beneath O. ventosa. (d) SEM– BSE image showing fungal hyphae infiltrating a muscovite grain
along the basal cleavage.
T. Bjelland, I.H. Thorseth / Chemical Geology 192 (2002) 81–9892
O. ventosa and P. corallina, in comparison to the other
two taxa.
Besides occupying the pores within the weathering
rind, fungal hyphae have also penetrated into individ-
ual mineral grains through fractures and etch pits and
channels. The BSE image in Fig. 5d shows how
fungal hyphae have penetrated into a muscovite grain
below an O. ventosa thallus. Where the rock substrate
contains abundant mica grains, oriented parallel to the
rock surface, a lifting of the ‘‘surface’’ is frequently
observed beneath all the studied taxa. SEM – BSE
images however indicate that a relatively greater
portion of the upper rock surface beneath O. ventosa
and P. corallina shows evidence of fragmentation and
crumbling than beneath F. cyathoides and O. tartarea.
4. Discussion
Results show that the weathering rind is thickest
beneath O. ventosa. The mean depth below the
lichen–rock interface at which weathering effect can
no longer be detected is f16.0 mm beneath O.
ventosa (MAX-calcite), and f8.5, 10.0, and 9.1
mm beneath F. cyathoides,O. tartarea, and P. cor-
allina, respectively (Table 4). The mean dissolution
depths of apatite (3.98 mm) and chlorite (3.08 mm)
are also greatest beneath O. ventosa (Table 4,Fig. 2).
Furthermore, a higher porosity and a lower Na
2
O
content in the weathering rinds beneath O. ventosa
than other taxa indicate greater dissolution of plagio-
clase, than beneath the other taxa (Table 4,Fig. 3). F.
cyathoides,O. tartarea, and P. corallina are more
difficult to separate with respect to compositional
differences in the weathering rind. However, the
PCA analysis indicates that P. corallina could weather
the substrate at a greater rate than F. cyathoides and O.
tartarea (Fig. 3b).
Even though the samples have been collected at the
same locality within a range of f500 500 m,
several other factors than today’s individual lichen
cover may have helped in determining the degree of
weathering that is recorded today. These factors
include mineralogical heterogeneity of the sandstone
and environmental and vegetational changes follow-
ing deglaciation. However, our results show that the
variation in weathering beneath the taxa exceeds the
heterogeneity of the sandstone. In addition, the PCA
analysis of the unweathered samples indicates no
special preferences in sandstone composition between
taxa. It is therefore not possible that O. ventosa and P.
corallina prefer to grow on very Ca-rich substrates
and therefore weather the bedrock more readily.
As different lichen taxa have colonized rock sur-
faces during the postglacial period (10 000 years), the
weathering seen beneath the taxa today is probably
the result of weathering from several generations of
lichens. Differences between the taxa in age could
also influence on the degree of weathering, as old
lichens would be in contact with the substrate longer
than lichens with a short lifespan. It may be possible
that O. ventosa colonized the area before the other
taxa. If this were the case, then any sampling of
rock colonized by O. ventosa would falsely indicate
enhanced weathering. Nevertheless, since the ob-
served difference in degree of weathering beneath
the studied samples is based on a considerable num-
ber, we suggest that this is caused by differences in the
biochemical and biophysical weathering effect of the
lichens colonising the rock surface today. As there are
no lichen-free reference surfaces in Vingen, it is not
possible to determine if a rock surface that is
encrusted by lichens weather slower or faster than
an identical but lichen-free surface. It is only possible
to compare the relative weathering effect between
taxa.
4.1. Chemical weathering by lichens
One possible explanation of the differences in
weathering beneath different taxa is that they produce
different major lichen compounds. O. ventosa and P.
corallina both contain thamnolic acid (Table 5).In
addition, O. ventosa contains divaricatic and usnic
acid. However, very little is actually known about the
effect of different lichen compounds on rates of rock
and mineral weathering. Ascaso et al. (1976) found
experimentally that a mixture of four lichen com-
pounds, atranorin, usnic acid, stictic acid and norstic-
tic acid had the same weathering effect as stictic acid
alone. The number and nature of the polar groups in
the molecules influence their solubility in water and
hence the role of lichen compounds in chemical
weathering (Iskandar and Syers, 1971). Further exper-
imentals are needed to determine whether the major
lichen compounds in O. ventosa and P. corallina can
T. Bjelland, I.H. Thorseth / Chemical Geology 192 (2002) 81–98 93
weather rocks and minerals more rapidly than lichen
compounds in the other taxa studied here.
The lichen-forming fungi release their lichen com-
pounds in an as yet unknown soluble form into the
apoplast (on the surface of the fungal hyphae) where
they are passively carried along by the main fluxes of
solutes until they reach their site of crystallization on
the hyphae within the thallus (Honegger, 1997). The
lichen compounds are not evenly distributed through-
out the thallus. Some lichen compounds are restricted
to, or have their main occurrence in, special parts of
the thallus (e.g. in the cortical-, medullary layer,
apothecia, soralia) (Culberson, 1969; Tønsberg,
1992). By passive translocation, the unknown soluble
forms also reach the photobiont cells and crystallize
on their surfaces (Honegger, 1986). It is thus also
possible that the soluble forms diffuse into the weath-
ering rind and contribute to the weathering process.
Some of the precipitated crystals may also dissolve
and locally influence the pH. It is known that certain
lichen compounds are slightly soluble in water and
might form soluble metal complexes (Iskandar and
Syers, 1972).
Usnic acid is mainly a cortical lichen compound,
whereas the other compounds in the studied taxa
occur in the medulla. A study of biomineralisation
products in the same four lichen taxa from Vingen
proved that lichen compounds can also occur within
the weathering rind (Bjelland et al., 2002b) and are
thus in contact with mineral surfaces. However, only
divaricatic acid was observed within the weathering
rind beneath O. ventosa, whereas thamnolic and usnic
acid only were present in thallus. No lichen substan-
ces were found in the weathering rind beneath P.
corallina and F. cyathoides, whereas gyrophoric and
lecanoric acid was found in the weathering rind
beneath O. tartarea.
Another explanation of the variations in the degree
of weathering beneath the different species could be
the production of oxalic acid. Oxalate salts can
precipitate extracellularly on the surface of thallus,
within the lichen, and/or at the lichen rock interface.
Little is still known about the factors affecting oxa-
late production in lichens, but several studies have
indicated that the amount and occurrence of oxalate
vary within and between taxa, and that environ-
mental factors have an influence (Edwards et al.,
1995; Holder et al., 2000; Prieto et al., 2000). Com-
munity photosynthetic metabolism is suggested to be
involved in oxalate production since rates of produc-
tion are substantially greater in light (Johnston and
Vestal, 1993; Modenesi et al., 1998). In addition, a
comparative study of the oxalate content in Acaro-
spora from the Antarctic and the Mediterranean,
showed that more calcium oxalate dihydrate were
produced in an arid Antarctic habitat than in a moister
Mediterranean location (Holder et al., 2000).Bjelland
et al. (2002b) compared the amount of oxalate within
and between thallus of F. cyathoides, O. tartarea, O.
ventosa and P. corallina in Vingen, and found that
calcium oxalate crystals (whewellite) occurred in all
taxa, but were especially common in O. ventosa and F.
cyathoides. This indicates a higher oxalic acid pro-
duction in O. ventosa and F. cyathoides than in the
other two species. However, huge differences within
and between thalli of the same taxon were observed.
The formation of metal–lichen acid complexes
and/or metal–oxalates within lichens does not neces-
sarily imply the direct action of the acids on the
underlying substrate. Lichens can also obtain metals
from run-off to precipitate metal – lichen acid com-
plexes or metal–oxalate salts (Czehura, 1977; Purvis,
1984; Ascaso and Wierzchos, 1994).Ourresults
support these suggestions. If the lichens extract cal-
cium directly from the sandstone in Vingen, they have
to extract it from the transition zone between weath-
ered and unweathered sandstone, as calcium is more-
or-less absent from the weathering rind. The weath-
ering rind is much deeper beneath O. ventosa than
beneath F. cyathoides, and they both contain high
amounts of whewellite. It is hence more likely that the
lichens absorb calcium or other metals from run-off or
water within the weathering rind.
Compared to the other studied species, O. ventosa
produces the greatest number of biomineralisation
products. As both F. cyathoides and O. ventosa
contain a large amount of whewellite in Vingen, but
not P. corallina, the results indicate that it is most
likely the content of different major lichen compounds
and not differences in oxalic acid production, which
explain variations in chemical weathering effects of
the studied lichens.
Another way in which lichens can accelerate rates
of chemical weathering is by increasing the residence
time of water at the lichen–rock interface. The chem-
ical weathering reactions can hence proceed for longer
T. Bjelland, I.H. Thorseth / Chemical Geology 192 (2002) 81–9894
time periods in a lichen rock interface, compared to
bare rock not colonized by lichens. However, the
water holding capacity varies strongly between lichen
taxa (Bjelland, 2001), and the effect will probably be
most efficient with a large number of fungal hyphae
within the weathering rind.
4.2. Physical weathering by lichens
The rock substrate is more fragmented beneath O.
ventosa and P. corallina than beneath F. cyathoides
and O. tartarea. This may indicate more effective
biophysical weathering caused by the large concen-
tration of fungal hyphae within the weathering rind
beneath the two first mentioned taxa. However, effec-
tive physical weathering requires chemical weathering
since penetration of fungal hyphae is facilitated by
dissolution of minerals along grain boundaries, cleav-
ages, and cracks, which increase porosity and perme-
ability. Since these weathering processes interact and
enhance each other effects, it is impossible to sepa-
rately quantify the role of each process.
Quartz is resistant to physical weathering, owing to
its the lack of cleavage. Plagioclase weathers more
readily than potassium feldspar owing to a greater
abundance of mica inclusions in the former mineral.
Muscovite and chlorite are especially susceptible to
physical weathering owing to their closely spaced and
perfect cleavages. This is clearly demonstrated in
samples rich in detrital muscovite oriented parallel
to the surface, where a lifting of the surface due to the
expansion and contraction of the fungal hyphae within
and between mineral grains can be seen.
Continued fungal growth separates grains and lifts
them from the substratum. This results in accumula-
tion of mineral grains within the lichen thallus, which
gradually causes a lowering of the lichen – rock inter-
face. The origin of most of the minerals within the
thalli is hence from the surface below the thallus, but a
few particles may also be windblown. When thallus
dies, mineral grains and fragments from the upper
lichen–mineral interface will be removed and a new
surface will be exposed, available for re-colonisation
and biodegradation.
The general growth rate for crustose species is ca.
0.5–2.0 mm year
1
(Hale, 1973), but the exact
growth rate of the taxa studied here is not known.
Differences between taxa in growth rate could have an
influence on the effects of physical weathering. Spe-
cies with fast vertical growth would have more impact
than those with very slow growth rate. The rate of
growth of a lichen thallus is however subject to
considerable variation and apparently identical thalli
in an apparently uniform habitat may grow at different
rates. This variability may be due to small-scale
concentrations of nutrient availability, such as bird
droppings, as well as variations in microclimate
(Hawksworth and Hill, 1984).
In addition, with a deep fungal hyphal penetration
into the rock, surface water can reach a considerable
depth and cause ice-wedging by freezing expansion of
the thallus and the surrounding microenvironment
(Creveld, 1981; Fiedmann, 1985; Friedmann and
Weed, 1987; Schroeter and Scheidegger, 1995; Arin
˜o
et al., 1997).
5. Conclusions
A comparative study of the weathering rinds
beneath four lichens demonstrates differences in
weathering effects between the studied taxa. Weath-
ering rinds beneath O. ventosa and, to a lesser extent, P.
corallina, are deeper and show a higher degree of
mineral dissolution and physical degradation of the
rock surface compared to F. cyathoides and O. tartarea.
The variation in weathering effects between taxa is
greater than the variation in mineralogy of the sand-
stone as well as effects of other weathering factors
during the postglacial period. A greater degree of
biophysical and biochemical weathering due to the
high concentration of fungal hyphae within the weath-
ering rind beneath O. ventosa and P. c o r a l l i n a ,in
comparison to the other two taxa can be one explan-
ation. O. ventosa and F. cyathoides contain a similar
amount of oxalate, which is greater than for P. corallina
and O. tartarea (Bjelland et al., 2002a,b). Thus, differ-
ences in the type and abundance of lichen compounds
is a more likely explanation for the variations in
chemical weathering than differences in oxalic acid
production. While O. ventosa contains usnic, divari-
catic, and thamnolic acid, P. corallina contains only
thamnolic acid in the thallus. However, divaricatic acid
has also been observed within the weathering rind, and
may thus directly interact with the rock minerals (Bjel-
land et al., 2002a,b). Further experimental studies are
T. Bjelland, I.H. Thorseth / Chemical Geology 192 (2002) 81–98 95
however needed to determine the weathering effect of
the different lichen compounds in the studied taxa.
As there are no lichen-free reference surfaces in
Vingen, it is not possible to determine if a rock surface
that is encrusted by lichens weather slower or faster
than an identical but lichen-free surface. It is only
possible to compare the relative weathering effect
between taxa. However, this study clearly indicates
that lichens weather their rock substrate by physical
mechanisms, and chemical mechanisms. In addition,
the water holding capacity of lichens may eluate rates
of chemical weathering and frost wedging of lichen
covered rock surfaces. Conversely, the thallus and
endolithic fungal hyphae also bind the partly frag-
mented rock surface and protect it from abrasion and
erosion. When the thallus dies, loose mineral grains
and fragments from the upper lichen – mineral inter-
face will be removed and a new surface will be
exposed and available for colonization and biodegra-
dation. It is thus likely that lichens in general increase
the weathering processes, except at locations with
extremely high abrasion, where they may protect the
surface.
Acknowledgements
We thank the Norwegian Research Council (NFR)
and The Norwegian Directorate for Cultural Heritage
for financial support, and Rolf Birger Pedersen, Per
Magnus Jørgensen, and Tor Tønsberg for the valuable
comments on an early draft of this paper. We further
thank the referees of this paper (M. Lee and J. Fein)
for the useful comments and suggestions. [EO]
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