The nature and rate of weathering by lichens on lava flows on Lanzarote

The nature and rate of weathering by lichens on lava flows on Lanzarote

Geomorphology 47 (2002) 87 – 94 www.elsevier.com/locate/geomorph The nature and rate of weathering by lichens on lava flows on Lanzarote R.C. Stretch...

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Geomorphology 47 (2002) 87 – 94 www.elsevier.com/locate/geomorph

The nature and rate of weathering by lichens on lava flows on Lanzarote R.C. Stretch, H.A. Viles * School of Geography and the Environment, University of Oxford, Mansfield Road, Oxford OX1 3TB, UK Received 21 March 2001; received in revised form 29 September 2001; accepted 4 June 2002

Abstract Samples of lichen-covered and bare lava surfaces from Lanzarote, dating from the 1730 – 1736 eruption, have been analysed using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) to investigate the relative roles of biological and inorganic weathering processes. Lichens here grow preferentially on N and NE facing surfaces and create a range of nanomorphological features as a consequence of their weathering activities. The fruticose lichen Stereocaulon vesuvianum and a mixture of crustose lichen species are found to be particularly effective agents of weathering. Comparison of the thickness of the weathering rind on bare and lichen-covered samples (mean thickness 15.7 and 253.9 Am, respectively) shows a significant difference at the 99% confidence level. Following previous studies in Hawaii by Jackson and Keller [Am. J. Sci. 269 (1970) 466], these results are used to suggest that lichens on Lanzarote lava flows cause a 16 times increase in weathering rates over those found on bare surfaces. Comparison of these results with those from similar lava flows in Hawaii indicates that under the wetter climate of Hawaii both biological and inorganic rates are over double those found in Lanzarote. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Biological weathering; Stereocaulon vesuvianum; Scanning electron microscopy; Nanomorphologies; Lichen weathering

1. Introduction In this paper, we demonstrate the significance of biological weathering, both biophysical and biochemical, in relation to other forms of ‘‘inorganic’’ weathering, using scanning electron microscope (SEM) analysis on samples of lava from dated flows that have been colonised by lichen, in comparison to bare

*

Corresponding author. Tel.: +44-1865-271919; fax: +441865-271-929. E-mail address: [email protected] (H.A. Viles).

lava of the same age. The nature of the weathered surface, the depth and chemical composition of the crust and the microtopography of the rock surface on lichen-covered and bare surfaces are examined to show if there is a statistically significant difference. This research contributes to the current debate regarding the nature and extent of weathering by lichen, and provides an interesting comparison with previous research on lava from Hawaii, with similar substrates and lichen species, but in different climatic conditions (Jackson and Keller, 1970). Lichens are able to weather their substrate physically: rhizines penetrate into the substrate and loosen

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fragments, the thallus can also expand and contract depending on moisture availability, pulling and plucking rock fragments from the surface (Fry, 1927). Biochemical weathering can occur via the production of respiratory carbon dioxide, the secretion of organic acids such as oxalic acid or the production of weak phenolic acids known as lichen acids, which react with the substrate (Syers and Iskandar, 1973; Bland and Rolls, 1998). Many studies have been performed on a variety of substrates with different conclusions regarding the significance and nature of weathering. Some studies show that lichens can have a biophysical and biochemical weathering effect (Fry, 1927; Jones et al., 1980, 1981; Cooks and Otto, 1990; Adamo and Violante, 1991; Adamo et al., 1993; Moses and Smith, 1993; Wasklewicz, 1994; Dorn, 1995; McCarroll and Viles, 1995; Paradise, 1997). However, some studies suggest that lichens may retard inorganic processes and have a protective effect (Galvan et al., 1981; Viles, 1987; Schwartzman and Volk, 1989; Alexander and Calvo, 1990; Benedict, 1993; Viles and Pentecost, 1994; Viles, 1995). Few studies, however, have investigated the relative importance of biological versus other weathering processes. Previous studies of the effects of lichens upon basaltic lava have been performed in Hawaii and Italy; there is a debate as to the nature and extent of the weathering: points of contention include the formation of different secondary products, the abundance of etching and dissolution features and the most susceptible minerals within the lava (Jackson and Keller, 1970; Jones et al., 1980; Adamo and Violante, 1991; Wasklewicz, 1994). In Hawaii, Jackson and Keller (1970) suggest that the weathering crusts under the lichen are several orders of magnitude deeper than the weathering crusts on bare rock, and that their chemical composition has been altered by the action of lichen. Cochran and Berner (1996) however, believe that the significance Jackson and Keller attribute to the action of the lichens is over-exaggerated and they reinterpret the evidence as aeolian material. Viles (1995) developed a conceptual model showing how the effectiveness of biological weathering changes depending upon the stress of the environment. The model suggests that in more arid environments with greater stress levels, the direct effects of lithobiotic communities are more evident and that

biopitting and flaking should be more abundant. Biofilms will dominate in unstressed, i.e. moist environments, and these films can have a protective effect. Our research in Lanzarote, in comparison with the previous studies in Hawaii, provides a test for this climatic-control model.

2. Study area Lanzarote is the most northerly of the Canary Islands, which are located in the Atlantic Ocean to the west of Morocco. This chain of volcanic islands experiences a sub-Saharan climate as a result of its location. In the summer months, there are, on average, 11 h of sunshine/day, average maximum temperatures ranging from 18 to 28 jC; in the winter months, there is an average of 6 h of sunlight and average maximum temperatures are 14 –21 jC. Average annual rainfall is less than 250 mm and exhibits a seasonal pattern; whilst average annual evaporation is 1600 mm, and mean relative humidity is 60%. Prevailing winds are from the Northeast and can reach speeds of 70 km h 1 (Goulding and Goulding, 1989; Puebla et al., 1997). This provides an interesting comparison with Hawaii, where annual precipitation can range from 510 to 1910 mm a 1. The five-phase eruption of 1730– 1736 in Lanzarote occurred along a narrow 14-km-long fissure, orientated in a NNE – SSW direction, in the area now designated as the Timanfaya National Park (lat. 28j58V30W–29j04V00W, long. 13j49V30W– 13j52V10W) (Arana and Carracedo, 1979; Scarth, 1994). These eruptions covered about one third of the island (167 km2) with basaltic lava, composed of melaphenites (SiO2 undersaturated), rich in ultrabasic (peridotic) inclusions to the Northwest; with alkaline basalts evolving towards olivine theoleiites in the Southwest and East (Carracedo et al., 1992). The substrate studied includes pahoehoe (pillow), aa and blocky forms of lava; the different forms are a result of the different viscosities of the lava prior to cooling; pahoehoe lava has a lower viscosity and a smoother form, aa lava is more rugged and spinose. The lava flows in Lanzarote have since been colonised by over 71 species of lichen (Puebla et al., 1997). A pilot study on the island showed that the main species were Stereocaulon vesuvianum (a white

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Fig. 1. Mixed lichen community dominated by S. vesuvianum on aa lava flow.

fruticose species), Ramalina bourgeana (a light-green fructicose lichen) and a mosaic of four white crustose species (from the genera Lecanora and Rhizocarpon). Characteristic lichen covers on aa and pahoehoe lava flows are shown in Figs. 1 and 2. SEM analysis suggests there may also be a cryptoendolithic lichen

community. All three lichen species studied preferentially colonise sites which face North/Northeast, as a result of microclimatic factors—lower insolation levels permit retention of higher moisture levels in sites with a northerly aspect and the prevailing winds act as a source of moisture (Puebla et al., 1997).

Fig. 2. Detail of lichen community and weathering crust on pahoehoe lava flow.

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3. Methods Fifty samples of lava, 25 with a lichen cover, and 25 without, were collected from a variety of sites in Lanzarote, for SEM analysis. The samples were obtained from 12 sites chosen to cover a variety of aspects, lava types and altitudes, well away from any disturbance from roads. Samples of bare lava and lava with a lichen cover were collected at random within each site by chiselling from the main lava flow; 11 sites were on the main 1730 –1736 lava flow and one site was on the 1824 eruption. A hammer and chisel were used to break off smaller transverse sections (max. 10  10  10 mm) from the samples taken. At least two lichen-covered and two bare samples were prepared from each of the sites for SEM analysis. Transverse sections were fixed onto aluminium stubs, gold-coated, and then observed using a Cambridge Stereoscan 90 to examine nanomorphologies and evidence of weathering such as flaking, pitting, cracking and etching (Viles and Moses, 1998). The depth of the weathering crust beneath the lichen thallus was measured at three random points along each SEM sample. The boundary between weathered zone and unaltered rock was

usually fairly clear, with a sharp break visible. The depth of penetration of rhizines into the sample (as illustrated in Fig. 3) was also noted. The depth of the weathering crust was taken to be a reliable indicator of the amount of weathering (Jackson and Keller, 1970). A Mann – Whitney U-test (Ebdon, 1985) was performed on the data set in order to see if there was a statistically significant difference between the depth of the weathering crust on the lichen-encrusted samples and the depth of the weathering crust on bare rock. Rates of weathering were calculated by dividing the mean depth of the weathering crust by the age of the lava flow. This assumes that the lava was colonised immediately after the cessation of the 1736 eruption, so the rates calculated are therefore a minimum figure. Additional lichen-covered samples from half the sites were also prepared with 6% hydrogen peroxide to remove the organic material and permit observation of the underlying surface, to investigate the nature of weathering further. In conjunction with SEM, a Link Systems 860 EDS system was used to study the chemical composition of samples, in order to see if there was a difference between the mineralogy of the weathering crusts beneath lichens and on bare rock, and between

Fig. 3. SEM image of thick layer of S. vesuvianum penetrating into porous lava. Scale bar = 200 Am.

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Table 1 Results of the SEM analysis: the difference between the depths of weathering crust on bare lava and under different species of lichen

Mean depth (Am) Max. depth (Am) Min. depth (Am) Weathering rate (Am a

1

)

Bare

Lichen (all species)

S. vesuvianum

R. bourgeana

Crustose species

15.68 303 0.73 0.06

253.91 1040 3.51 1.05

325.13 1040 34.7 1.23

239.30 325 120 0.91

33.51 121 3.51 0.16

the weathering crusts and the underlying rock. X-ray diffraction (XRD) was also used to identify minerals in and beneath the lichen thalli.

4. Results—the nature and extent of weathering The mean depth of the weathering crust of bare samples (15.7 Am) is considerably less than that of lichen-covered samples (253.9 Am) (Table 1). A onetailed Mann – Whitney U-test showed a statistically significant difference between these values at the 99% confidence level. Calculations of the rate of weathering showed that organic processes weather the substrate at a rate of 1.046 Am a 1; 16 times faster than the rate of inorganic processes; weathering by inorganic processes occurs at a rate of 0.064 Am a 1.

The weathering crust on bare samples is chemically very similar to the substrate, with silica and aluminium dominant and some calcium. Iron is present in the weathering crust of the majority of lichen-covered samples. Increase in amounts of iron under lichens has been found to occur as a result of organic complexation and reduction reactions (Adamo et al., 1993). XRD analysis on the reddish-yellow (7.5YR 8/6 on the Munsell soil colour chart) material trapped under the thallus of S. vesuvianum showed the presence of maghemite (Fe2O3) and magnetite (Fe3O4), although the main peak remained unidentified. Comparing the weathering crust under different species shows it is deepest under S. vesuvianum and thinnest under the crustose species. The rhizines of S. vesuvianum and R. bourgeana (with a mean width of 213.1 Am) can penetrate up to 3.5 mm into the rock.

Fig. 4. SEM image of chemical etching under lichen thalli. Scale bar = 200 Am.

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This suggests a possible biophysical weathering effect of the lichen. Analysis of weathering features using SEM analysis reveals that flaking is more common underneath lichen covered samples, and removal of organic material with hydrogen peroxide shows that other features such as indentations, striations and pits are present under the lichen thallus and may therefore be a result of organic weathering processes (see Fig. 4). Etching of mineral grains in the rock was not as common as had been found in previous studies in moister environments. Cracking is more common on bare samples, perhaps indicating the prevalence of physical weathering through thermoclasty. The crustose species and S. vesuvianum seem to cause equal amounts of cracking, etching and flaking on the surface of samples; these features are not so common on the surface of samples beneath R. bourgeana, suggesting that Ramalina is a less potent weathering agent. Other features such as indentations and pits are visible on the surface of samples once the crustose species have been removed by hydrogen peroxide. The medulla of the four crustose species are able to absorb more water than the other lichens and are in direct contact with the substratum; this facilitates chemical weathering, making dissolution features more common under these species. There seems to be more iron in the weathering crust under S. vesuvianum suggesting greater chemical change, but this may just be a result of the nature of the substrate, the availability of moisture and the surface area of the lichen. Thus, S. vesuvianum appears to be the most effective agent of weathering on Lanzarote lava flows, producing a thick, iron-rich weathering crust with extensive evidence of biochemical and biophysical effects.

5. Discussion—the significance of organic weathering The mean depth of the weathering crust under lichen is significantly greater than the weathering crust on bare lava at the 99% confidence level. This suggests that the lichen accelerates the weathering of the substrate. In a similar study in Hawaii, Jackson and Keller (1970) believe the deeper weathering crust to be a result of organic weathering processes, but they list alternative explanations, which they believe to be theoretically possible, but in practice improb-

able. It is possible that lichens shield material from erosion; however chemical change within the weathering crust under lichen suggests the lichen has a weathering effect. Cochran and Berner (1996) suggest that the material under the lichen thallus may be trapped aeolian material; however if this was true then aeolian material would be expected to collect in the hollows of bare samples, and this is not the case with the samples studied from Lanzarote. Jackson and Keller (1970) also provide the alternative explanation that a thicker crust is formed by the rapid inorganic weathering of unstable iron-rich material and the lichen prefers to colonize these areas. There is no obvious difference in the underlying mineralogy of the lichen-covered and bare rock in our study, suggesting that this is not true in Lanzarote. The statistically significant difference in the depth of the weathering crusts and the chemical change in the crust under lichen, suggests that it is therefore reasonable to attribute the deeper weathering crust to the action of the lichen. The fruticose species produce a deeper weathering crust and have deeper, thicker rhizines, suggesting the importance of biophysical weathering by the action of these species. Dissolution features are more common underneath the crustose species. The medulla of the four crustose species is able to retain more water and is in direct contact with the substratum, suggesting biochemical weathering prevails under these species. Further research needs to be done to elucidate the weathering role of the cryptoendolithic lichens. Experimental studies using lichens and lichen acids on fresh lava would confirm whether weathering features identified using the SEM are indeed produced by lichen activity. Organic processes have been shown to have a significant effect on the substrate in Lanzarote; the rate of weathering of the lava underneath lichen is 16 times greater than the rate of weathering due to inorganic processes. However, inorganic processes are still important in weathering the lava here as flaking is evident on bare samples and cracking is more common on bare samples. This is likely to be a result of insolation weathering, as rock surfaces in Lanzarote can experience daily temperature fluctuations of 27 jC. The lichen may play a protective role against insolation weathering as hyphae bind the substrate together and prevent cracking. Organic pro-

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cesses are greater on rock faces with a North/Northeasterly aspect where microenvironmental conditions result in a higher percentage cover. Organic processes are likely to have a greater role in the winter when precipitation levels are high; insolation weathering is likely to be more dominant in the summer as the lichens will be subject to desiccation and there are greater temperature ranges to induce weathering on the lava. Our research shows the danger of extrapolating results from other climates and environments. The mean depths of the weathering crusts found in Lanzarote are larger than those found in Hawaii, which are 132 Am under lichen and 2 Am on bare lava (Jackson and Keller, 1970); the lava in Lanzarote has been exposed to weathering for longer periods of time than the younger lava flows of Hawaii. The rate of weathering by S. vesuvianum in Hawaii is 2.88 and 0.134 Am a 1 on bare basaltic lava (Jackson and Keller, 1970). The corresponding figures calculated for Lanzarote are 1.24 Am a 1 (1.05 Am a 1 if all species of lichen are taken into account) and 0.06 Am a 1—weathering in Hawaii is therefore just over twice as fast as in Lanzarote. The difference in rates is likely to be a result of the different climates. Greater levels of precipitation in Hawaii will contribute to greater level of chemical weathering, which produces more extensive dissolution features and weakens the substrate, facilitating biophysical weathering (Cooks and Otto, 1990). Studies in Hawaii (Wasklewicz, 1994; Cochran and Berner, 1996) show that a silica glaze covers bare lava surfaces. There was no evidence of this being present on the samples from Lanzarote; perhaps because a higher level of precipitation is required for its formation. Jackson and Keller (1970) demonstrated that the weathering crusts of bare samples in Hawaii are composed of hematite and that the weathering crusts under S. vesuvianum are composed of a-Fe2O3 and hferric oxide. In comparison weathering crusts on bare surfaces in Lanzarote were dominated by silica and aluminium with some calcium, and iron enrichment seemed to occur only under S. vesuvianum. These differences may reflect differences in substrate mineralogy or different levels of precipitation between Lanzarote and Hawaii. Etching is not as common on the samples from Lanzarote as on samples from Hawaii. This may be a result of the lower amount of

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precipitation in Lanzarote, which results in only a limited dissolution of minerals by biochemical activity. Within Hawaii itself, etching was less common on sites with precipitation levels of less than 100 mm a 1 (Jackson and Keller, 1970). Our results seem to contradict Viles’ (1995) conceptual model of the weathering effects of lithobiotic communities, which states that biological contributions to weathering should be greatest in arid environments where climatic stress encourages penetration of the surface. Indeed, comparisons of data from Lanzarote and Hawaii highlight the importance of precipitation in aiding biological weathering processes. However, our results may not provide a good test of Viles’ (1995) model as Lanzarote probably does not experience arid enough conditions to produce the high rates of endolithic activity found in many desert environments. Further research on lichen weathering of volcanic rocks in more arid conditions would be of great interest.

Acknowledgements We would like to thank Juan Carlos Carracedo of the Volcanology Department at the University of La Laguna, and Aurelio Centellas Bodas and the Park Rangers at the National Park Authority of Timanfaya, for providing valuable information and granting permission to work within the National Park. We would also like to thank Chris Jackson at the School of Geography, Oxford University and Dr. Chris Grosvenor in the Materials Department, Oxford University for invaluable laboratory assistance.

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