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Incipient weathering by Stereocaulon vulcani at Réunion volcanic island
Incipient weathering by Stereocaulon vulcani at R´eunion volcanic island J.D. Meunier, S. Kirman, D. Strasberg, O. Grauby, P. Dussouillez PII: DOI: Reference:
S0009-2541(14)00281-2 doi: 10.1016/j.chemgeo.2014.05.033 CHEMGE 17257
To appear in:
Chemical Geology
Received date: Revised date: Accepted date:
17 February 2014 21 May 2014 24 May 2014
Please cite this article as: Meunier, J.D., Kirman, S., Strasberg, D., Grauby, O., Dussouillez, P., Incipient weathering by Stereocaulon vulcani at R´eunion volcanic island, Chemical Geology (2014), doi: 10.1016/j.chemgeo.2014.05.033
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Incipient weathering by Stereocaulon
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vulcani at Réunion volcanic island 1
CEREGE, CNRS, Aix-Marseille Université, Europôle Méditerranéen de l’Arbois, 13545 Aix en Provence, France
UMR PVBMR, Université de La Réunion, CS 92003, 97 744 Saint-Denis Cedex 9 3
CINaM-CNRS-Aix-Marseille Université, Campus de Luminy Case 913, 13288 Marseille Cedex 9, France
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J.D. Meunier1*, S. Kirman1, D. Strasberg2, O. Grauby3, P. Dussouillez1
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* Corresponding author. Phone +33 (0)4 42 97 15 26. E-mail address: [email protected]
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ABSTRACT
The impact of early land plants and fungi in increasing global weathering is still debated, particularly before the advent of vascular plants during the Devonian. Here we present a study of the incipient
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weathering of basalt by Stereocaulon vulcani, a native colonizing lichen on Réunion Island (Indian ocean). We analysed the chemistry and mineralogy of a 24-year-old flow located at low altitude that
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was mostly covered by Stereocaulon vulcani with aboveground biomass of 6249 kg ha-1. The chemical composition of Stereocaulon vulcani showed that besides C and N, Si and Fe were the dominant
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elements. The Si stored in the aboveground pioneer vegetation gives 27 kg ha-1, comparable to the Si stored in the old-growth at the Marelongue Reserve (Meunier et al. 2010). On thin sections, the inner part of Stereocaulon vulcani was mostly composed of Si while Fe coatings observed at the base of the thallus may be the result of wind blow dust interception as suggested by Cochran and Berner (1992). Using BSE images on SEM, we showed evidence of dissolution of the glass matrix at the basaltthallus contact. The quantification of porosity by digital imagery showed a variation between 7% in the unweathered zone to near to 40% at the surface. A maximum denudation rate of 6.7 m y-1 is estimated to fall within the range of the values reported in the literature. Using our data for analog to the past, we suggest that early land plants would have been capable to mobilize Si in a similar proportion as vascular plants and should have significantly affected the weathering of land before the advent of vascular plants. Keywords: Weathering; Lichen; Basalt; La Réunion; Chemical composition; Digital imagery
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ACCEPTED MANUSCRIPT 1. INTRODUCTION Vascular plants are major contributors to the evolution of landscape and global
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geochemical cycles (Berner et al., 2005; Amundson et al., 2007). Plant roots secret organic acids and chelates which enhance the chemical weathering of soil minerals and the release of
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elements in soil solutions (Kelly et al., 1998; Lucas, 2001). Knowing the rate of weathering by plants is important in fundamental issues such as the modeling of CO2 at the geological scale (Berner 1997), the understanding of forest disturbances (Conley et al., 2008; BaloghBrunstad et al., 2008) as well as forest management (Augusto et al., 2001; Jonhson-Maynard
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et al., 2005; Bakker et al., 2004; Meunier et al., 2010) and monument conservation (Almeida et al., 1994; Adamo and Violante, 2000). The acceleration of chemical weathering is well evidenced for vascular plants but many questions remains to be solved for a better
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understanding of the interplay between weathering and biota (Brantley et al., 2011). For instance, Brantley et al. (2011) address the role of biological stoichiometry in weathering and
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makes the hypothesis that biology drives weathering during initial successions. Lichens are pioneer organisms that are frequently observed to colonize rocks exposed to the surface. Lichens result from a symbiotic association between a fungus and a
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photosynthesizing organism; they are known to accelerate the degradation of rock by physical and chemical processes (Banfiel et al., 1999 ; Chen et al., 2000 ; Adamo and Violante, 2000;
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Asta et al., 2001). Penetration of the hyphae of lichens in the voids of the substrate leads to the dismantlement of rocks and porosity increase. Lichen exudates include respiratory CO2,
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organic acids, alkaline metabolic products and complexing agents that are able to increase the dissolution of minerals (Chen et al., 2000; Neaman et al., 2005). Organic acids such as oxalic acid are also able to react with cations dissolved from the substrate and form secondary crystalline products such as oxalates, iron oxides as well as alumino-silicates. The role of lichens in weathering rates and soil formation is therefore complex and its extent in accelerating weathering is still not well understood. Measurements of the enhancement of weathering rates by lichens compared to bare surface show values ranging from x 2 to x 71 using measurement of the weathering rinds and ranging from x 2.5 to x 18 using major element concentration in aqueous solutions (Zambell et al., 2012). Some species are even recognized to protect from the weathering (Silva et al., 1999; Garcia-Valles et al., 2003; Carter and Viles, 2005). The intensity of weathering by organisms such as lichens, fungi and algae is still debated in the issue of the factors that controlled the atmospheric CO2 before the advent of vascular plants (Berner, 1992; Schwartzman and Volk, 1989). According to 2
ACCEPTED MANUSCRIPT Schartzmann and Volk (1989) pre-vascular organisms were able to increase weathering at a considerable rate (10-100 times). This statement was rejected by Berner (1992) who stated that lichens cannot contribute to weathering compared with vascular plants that produce deep
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roots. The disagreement was partly due to different interpretations of the weathering of Hawaiian basalt flows by Stereaucon vulcani, a fruticose lichen. Jackson and Keller (1970)
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found that "the reddish weathering crust is consistently thicker by one or more orders of magnitude on rock surface colonizer by Stereocaulon vulcani than on areas of bare rocks ". Cochran and Berner (1996) re-examined the previous fields using an Electron microprobe and did not find evidence of significant enhanced weathering. They also re-interpreted the
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weathering crust (or rind) as a result of interception of wind-blown dusts. More recently, Brady et al., (1999) used back scattered images of thin section in order to quantify mineral porosity and found that weathering intensity under lichens is 2-18 times greater that the
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abiotic case. Their results are in good agreement with the values ranging from 4 (for Si) to 16 (for Mg) found by Aghamiri and Schwartzman (2002) who used mini watershed experiments
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of weathering of mica-schist by crustose lichens at Hubbard Brook Experimental Forest. However, the studies of Brady et al. (1999) do not provide detailed information about the of the volcanic flow.
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weathering processes and assumed that the duration of the weathering is equivalent to the age At La Réunion island, a "hot spot " volcanic-type tropical island, Stereocaulon vulcani
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is also observed as an early native colonizer on lava flows (Fig. 1a). After decades Stereocaulon vulcani is replaced by ferns and native canopy trees that characterize the
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lowland tropical rainforest of the island (Kirman et al., 2007; Strasberg, 1995; Cadet, 1977). In the first stages of plant succession, pioneer vegetation is confronted with harsh environmental conditions, which is why the nitrogen fixing lichen, Stereocaulon vulcani, dominates the early developmental stages (Cadet, 1977). Stereocaulon vulcani never occur in stage environment like the lowland rain forest remnant at the Marelongue reserve (Kirman et al., 2007) developed above a 500-year-old flow. The same evidence is given at Hawaii by Kurina and Vitousek (1999), who noted a complete disappearance of this species after 140 yrs on lava flows at low altitudes. Kurina and Vitousek (1999) showed that lichen decline on Hawaiian flows is most probably due to light limitation as a result of shading by vascular plants. The study of weathering of La Réunion basalt by Stereocaulon vulcani appears to us a good opportunity to confront the previous results obtained at Hawaii. The aim of this study is to get new insights on the processes and rates of weathering during colonization of lands by early land organisms. 3
ACCEPTED MANUSCRIPT 2. MATERIAL AND METHODS This study was conducted on the 1976 lava flow located on the east slopes of the
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Piton de la Fournaise, the active Hawaiian-type volcano of La Réunion island (21°09'S55°30'E, Indian Ocean). The climate of the study site is characterized by an average annual
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rainfall and temperature of respectively 4000 mm and 23°C (Raunet, 1991). The study site belongs to the lavas that have flowed into a caldeira named “L’Enclos” during the last 3 500 years, opening towards the east and named the Grand Brûlé (Bachelery, 1981). The field analysis and sampling was done at 250 m above sea level during the year 2000, 24 years after
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its eruption, where the flow appeared to be mostly covered by Stereocaulon vulcani (Fig.1b). Herein, the related weathering processes are referred to a 24-year-old lava flow. For the purpose of the study, we analyzed the major element composition of the lichens and the
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associated vegetation of the flow and the composition of the basalt at the interface with lichens. Sampling was set up in a network of four permanent plots of 100 m2 established
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across the 1976 lava flow. In each 100 m2 plot, all the plants were recorded and counted on 5 quadrats of 4 m2. Biomass was estimated using a destructive method: the lichens were harvested on five small plots (100 cm2) and biomass estimations were correlated with cover
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measurements. Since 2007, the lower part of the study site is totally covered by a large new lava flow.
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Biomass harvests were sorted according to species, oven dried several days at 70°C, weighed and ashed (five hours at 600°C). The carbon and nitrogen content were measured
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with an elemental analyzer and standard operating procedures (Fisons Instruments NA 1500) on the oven dried and ground vegetation samples. Major elements were determined by ICPOES (Jobin Yvon Ultima-C) on ash sub-samples after alkaline fusion using standard procedures (Germanique, 1994). A sample from the National Research Center for Certified Reference Materials (NRCCRM) of Bush branches and leaves (GBW07602) was used for calibration (Table 1). The yields ranged from 45.1% for Na to 99.1% for Ti. Fragments of basalts were sampled using a hammer. About 100g of fresh basalt removed from the first centimeter depth in contact with the air was dried and ground before chemical analysis. The major elements were determined using the same procedure used for plants (ICP-OES after alkaline fusion). Polished thin sections of a sample of basalt coated/non coated by lichen were made for SEM and microprobes analyses. The sample coated by lichens was impregnated by epoxy resin (EPON, Merck). Low and high-resolution micrographs of samples were acquired during observation with a scanning electron 4
ACCEPTED MANUSCRIPT microscope (SEM) JEOL JSM-6320F at 15kV equipped with Energy dispersive X-ray spectroscopy (EDS) using a Si-Li detector (Quantax, Brucker AXS Microanalysis GmbH Berlin, German). Using spot mode EDS spectra allowed qualitative elemental analyses at the
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micrometer scale. Major element compositions of minerals were determined in thin sections by EPMA on a CAMECA SX-100 instrument equipped with five wavelength-dispersive X-
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ray spectrometers (WDS) at Service Microsonde Sud (Université Montpellier 2). The analyses were done with 20 kV accelerating voltage, a focused beam of 10 nA and counting times of 20–30 s. Concentrations were obtained from raw intensities using the ‘‘X-PHI’’ quantification procedure (Merlet, 1994). Natural minerals, synthetic oxides and pure metals
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were used as standards.
Using BSE images in SEM, the porosity was quantified at the lichen-basalt interface using digital imagery following a procedure derived from Dorn (1995). The principle is based
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on the property of the pores at the surface of a polished thin section to give a darker image than do minerals. Using MATLAB, we coded in intensity a zone of the lichen-basal interface
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where porosity was higher than in the zone not impacted by lichens. In order to obtain a porosity estimation of the basalt/lichen interface, we combined SEM images and selected a zone where the porosity was the most impacted inside the basalt at the contact with one
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thallus. The quantification of the porosity of the selected zone (Fig. 2) was calibrated using the following steps: first, we selected one porous zone (in black) and two representative
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minerals: plagioclase (in dark grey) and glass (in light grey). For each zone we estimated the probability density function (pdf) as a function of pixel intensity that increases when dark
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pixel decreases (Fig. 3). The porosity was estimated by the area that corresponded to an intensity range distinct from minerals at the threshold of 78. The porosity was therefore calculated using a binary image (0 = minerals; 1 = porosity). 3. RESULTS AND DISCUSSION 3.1. High Si uptake Stereocaulon vulcani contributed to 77.5 % of the total vegetation cover on the 1976 lava flow (Table 2). Other significant endemic species on the flow were the fern Nephrolepis abrupta and the tall sedge Machaerina iridifolila. The first seedlings of the pioneer tree species Agarista salicifolia were seen, represented by 5 750 individuals/ha. For comparison, on the 500 year old flow they represent only 44 individuals ha-1 (Kirman et al., 2007). The composition of the colonizing plants on this 24-year-old flows was similar to other young 5
ACCEPTED MANUSCRIPT undisturbed lava flows studied at low elevations (Cadet, 1977; Strasberg et al., 1995; Strasberg, 1996). The aboveground biomass on the 1976 flow was essentially represented by Stereocaulon vulcani with 6249 kg ha-1. This result was only slightly higher than the values
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around 5000 kg ha-1 obtained by Kurina and Vitousek (1999) at Hawaii for Stereocaulon vulcani grown on a 10-year-flow at 900 m of elevation. The biomass above the 1976 flow was
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however 3 orders of magnitude less than the aboveground biomass which characterized the mature lowland rainforest developed at a similar elevation on a 500-year-old flow (Kirman et al., 2007).
The chemical composition of Stereocaulon vulcani showed that besides C, N and O ,
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Si nd Fe were the dominant elements followed respectively by Al, K, Ca, Mg, Na, Ti, P, Mn (Table 1 and Fig. 4). The values of Si and Al fall in the low range of values given for the same species by Jackson and Keller (1970) at Hawaii while Fe is slightly higher at La relative
abundance
of
the
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Réunion. This relative importance of the elements in the lichen was quite similar to the same
elements
in
the
underlying
parent-rock:
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Si>Fe>Ca>Al>K>Mg>Na>Ti>P>Mn. The fact that the chemical composition of the lichen was similar to the rock substrate may show the presence of parent rock fragment engulfed by the thalli (Lee and Parsons, 1999; Silva et al., 1999). The red coatings observed at the base of
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the thallus were characterized by the presence of Al, Fe and Ti. These coatings may be the result of wind-blown dust accumulation as suggested by Cochran and Berner (1992).
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Observations on thin sections showed that the inner part of Stereocaulon vulcani was mostly composed of Si (Fig. 4). The lack of silicate fragments that would suggest physical
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weathering is a strong evidence for chemical weathering and intake of Si by Stereocaulon vulcani . The relative high proportion of Si in Stereocaulon vulcani is apparently puzzling because, contrary to N, Si is not considered as a nutrient element. Si is known to accumulate in pioneer plants (Ma and Takahashi, 2002) and to increase the plant resistance during environmental stresses such as metal toxicity or pathogenic disease (Cooke and Leishman, 2010; Epstein, 2009; Guntzer et al., 2012). Accordingly, the presence of an elevated proportion of Si in Stereocaulon vulcani is not fortuitous and is probably due to a selective advantage for colonizing new substrates. As a consequence Si constitutes, in the present case study, a good tracer for the quantification of weathering by colonizing plant. Surprisingly, the Si stored in the aboveground pioneer vegetation given by multiplying the biomass (6249 kg ha-1, Table 2) by the Si content (4.35 g kg-1, Table 1) gives 27 kg ha-1 comparable to the Si stored in the mature forests at Marelongue with 18 kg ha-1 (Meunier et al., 2010). This result is explained by the lower concentration of Si in the litter and woods of the mature forest 6
ACCEPTED MANUSCRIPT despite their greater biomass. 3.2. Preferential bio-etching of glass within 24 years
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The chemical analyses (Table 3) showed that the 1976 flow is typical of olivine basalt (Nativel et al., 1979) with phenocrystals or microlithes of olivine, Ca plagioclases, augite,
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iron-titanium oxides and an isotropical glass matrix. In optical microscopy, red coatings were observed (Fig. 4a) on the surface of the basalt similar to the ones observed on Stereocaulon vulcani. Using BSE images (Fig. 5a and b), an increase of intergranular porosity was observed towards the attached thallus while the basalt which was not in contact with the
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thallus did not provide any evidences of porosity increase. At higher magnification in SEM (Fig. 5c), we showed that the porosity had mainly affected the glass matrix. The other silicates had smooth contours showing that they were not affected by weathering. The
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porosity in the glass showed micrometric cavities mainly observed at the interface with the other minerals but also inside the glass mass forming galleries. A mineral phase containing Ca
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was detected as filling some of the cavities (Fig. 5d). Ca-minerals present in the cavities were probably oxalate originating from the excretion of oxalic acids by the mycobiont (Adamo and Violante, 2000). At the contact with Ca minerals, glass presented pitted contours typical of
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chemical weathering. BSE images in the non-weathered zones - i.e. zones not in contact with
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the thallus and zones deeper than 800 m from the surface - showed that the glass had a granophyric-like texture (Fig. 5e). In the weathered zones in contact with the thallus, granophyric-like glass showed a comb texture at the contact with the pores (Fig. 5f).
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The quantification of the porosity by digital imagery gives a value of around 20 % for the selected zone (Fig. 6). The increase of the porosity from the contact with thallus to a maximum of 800 m inside the basalt was determined as a representative rectangular zone (Fig. 2) and showed a linear variation between 7% in the unweathered zone to near 40% at the surface (Fig. 7). The result that Stereocaulon vulcani has significantly affected the chemical weathering of volcanic glass of the lava flow after 24 years of exposure but not the others silicates minerals appears to differ from what was found in Hawaii with the preferential dissolution of plagioclase (Dorn, 1995; Brady et al., 1999; Gordon, 2005; Cochran and Berner, 1996). Because of the corrosion effect of siderophore, we would also have expected that the most richest iron bearing minerals; i.e. olivine to be preferential altered as this was observed by Callot et al. (1987) who studied the bio etching of amorphous and crystalline silicates by fungi. The preferential dissolution of glass is however in a good agreement with some experimental and field studies showing that basaltic glass dissolves faster than the other 7
ACCEPTED MANUSCRIPT crystalline minerals in basalts (Gislason and Eugster, 1987; Nieuwenhuyse et al., 2000) but opposite to other cases where olivine dissolves first (Nesbitt and Wilson, 1992). The granophyric-like texture with their comb surface when altered (Fig. 5) was probably favorable
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to preferential penetration of corroding solution. The type of solution is not known but because the rate of glass dissolution increases with pH, contrary to the crystalline minerals
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(Fraysse et al., 2009), we suggest that alkaline solutions would have prevailed. 3.3. Global impact of the incipient weathering by lichens
Using the estimated porosity of 20% at a maximum depth of 800 m (Fig. 6), the
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volume of removed material gives 160 10-6 m3 m-2. Because this volume is entirely made of glass assuming a density of 2.5 t m-3, we can calculate the mass of removed material which gives 400 10-6 t m-2. Using Si concentration (230.39 g kg-1, Table 3) for comparison to lichen
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uptake we can calculate the export of Si that gives around t 921 kg Si ha-1. The value is 34 times more than the Si stored by Stereocaulon vulcani on the flow calculated here (27 kg ha). Using the maximum porosity of 20% we can also calculate a denudation rate of 6.7 m y-1
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of basalt within 24 years ((20x800)/(100x24)). This value falls within the higher range of the
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few values available in the literature (Table 4) showing that chemical weathering by lichens can be significant (Zambell et al., 2012). The corresponding mass of glass gives 166.8 kg ha-1
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y-1 and a rate of Si release of 38.4 kg Si ha-1 y-1. For comparison, this Si output is around 2.4 times the average Si output measured in the streams in the mature forest (Marelongue) developed above a 500 years of basalt flow (15 kg Si ha-1 y-1 from the 7-21 range in Meunier
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et al., 2010). Assuming that the depth of porosity created by lichen is half the maximum observed (800 m) we obtain a Si output value of around 19 kg Si ha-1 y-1, falling in the range found in the mature forest. If we use Stereocaulon vulcani as an analogue for pioneer plants that colonized the land during the Paleozoic, a significant source of plant derived-Si might have been available for impacting the global cycle of silica. The earliest land non-vascular plants are dated from the mid Ordovician to early Silurian (approx 476-432 Ma). Fossil lichens are well evidenced 400 my ago but may have appeared back during the late Proterozoic (Taylor et al., 1995; Willis and McElwain, 2002). Between the early Devonian to late Carboniferous (395 to 286 million years ago) early land plants, lichens, fungi and vascular plants built up forest ecosystems with trees as high as 35 m. It is also during the Devonian that the rise of large vascular plants played a major role in weathering and CO2 regulation (Berner, 1997).
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ACCEPTED MANUSCRIPT Meanwhile in the marine environment, it is striking to observe that radiolarians, microorganisms that secrete a Si skeleton, radiated during the early Silurian (Skinner and Jahren, 2003). Besides, Kidder and Erwin (2001) show that the bedded cherts significantly
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increased from the Silurian following the beginning of extensive siliceous deposits of radiolarian skeletons. If we postulate that vascular plants triggered the radiolarian and cherts
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development, we therefore should observe an increase of marine silica deposits from the Devonian and not as earlier as the Silurian. Alternatively, the input of silica during the Silurian was triggered by pioneer lichens, fungi and non vascular plants. Contrary to the observed vegetation succession in La Reunion, early land organisms during the early
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Paleozoic were not replaced by forests of vascular plants probably until several tens of million years. Therefore, input of Si in the ocean during phanerozoic may have been triggered through the acceleration of weathering by early land organisms before the advent of vascular
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plants (Wright, 1985; Schwartzman and Volk, 1989 ; Keller and Wood, 1993; Kendricks and
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Crane 1997). 4. CONCLUSION
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Incipient weathering of basalt by colonizing Stereocaulon vulcani at Réunion is characterized by glass dissolution and precipitation of secondary carbonate and oxides.
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Contrary to Fe, Ti and Al, Si located in Stereocaulon vulcani, is not the result of dust interception but of parent-rock uptake. Although the biomass of Stereocaulon vulcani on the
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24-year-old flow is lower than the biomass measured in mature forests (Meunier et al. 2010) the Si stored is comparable in both ecosystem. This result is explained by a higher Si concentration despite a lower biomass for Stereocaulon vulcani. The dissolution of glass at the plant-rock interface can affect the basalt up to 800 mm depth. Therefore, our results show that chemical weathering by Stereocaulon vulcani is significant at a decadal scale. Assuming that the results are a good analogue to the past, it can be suggested that transfer of Si from continents to the oceans may have been enhanced by early land organisms before the advent of vascular plants.
Acknowledgements This study was supported by the Programme National Sols et Erosion (French programme 2001-2002), the Direction Régionale de l’Environnement-La Réunion, the Office 9
ACCEPTED MANUSCRIPT National des Forêts-La Réunion and the Institut de Recherche pour le Développement; logistic support was provided by the field station of Marelongue, funded by the P.O.E., Reunion National Park and OSU Reunion. Special thanks to Jean-Jacques Motte, Landryne
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Chassagne, Fabrice Colin and Anicet Beauvais for their scientific input.
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Cooke, J., Leishman, M.R., 2010. Is plant ecology more siliceous than we realize? Trends in Plant Science 16, 61-68. Garcia-Valles, M., Topal, T., Vendrell-Saz, M., 2003. Lichenic growth as a factor in the physical deterioration or protection of Cappadocian monuments. Environm. Geol. 43, 776-781. Dorn, R.I., 1995. Digital processing of back-scatter electron imagery: a microscopic approach to quantify chemical weathering. GSA Bulletin 107, 725-741. Epstein, E., 2009. Silicon: its manifold roles in plants. Ann. Appl. Biol 155, 155-160. Fraysse F., Pokrovsky O.S., Schott J., Meunier J.D., 2009. Surface Chemistry and reactivity of plant phytoltihs in aqueous solutions. Chem. Geol. 258, 197-206. Germanique, J.C., 1994. Major, trace and rare-earth elements in fourteen GSL reference samples. Detemination by X-Ray Fluorescence Spectrometry and Inductivelely Couples Plasma Optical Emission Spectrometry. Geostandards Newsletter 18, 91-100. Gislason, S.R.,
Eugster, H.P., 1987. Meteoric water-basalt interactions. I: a laboratory study.
Geochim. Cosmochim. Acta 51, 2827-2840.
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ACCEPTED MANUSCRIPT Gordon, S.J., 2005. Effect of environmental factors on the chemical weathering of plagioclase in Hawaiian basalt. Physical Geogr. 26, 69-84. Guntzer F., Keller C., Meunier J.D., 2012. Benefits of plan silicon for crops : a review. Agron. Sust. Developm. 32, 201-213.
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Jackson, T.J., Keller, W.D., 1970. A comparative study of the role of lichens and « inorganic » processes in the chemical weathering of recent Hawaiian lava flows. Am. J. Sci. 269, 446-466.
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Jonhson-Maynard, J.L., Graham, R.C., Shouse, P.J., Quideau, S.A., 2005. Base cation and silicon biogeochemistry under pine and scrub oak monocultures: implications for weathering rates. Geoderma 126, 353-365.
Keller C. K., Wood, B.D., 1993. Possibility of chemical weathering before the advent of vascular land
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plants. Nature, 364, 223-225.
Kelly, E.F., Chadwick, O.A., Hilinski, T.E., 1998. The effect of plants on mineral weathering. Biogeochem. 42, 21-53.
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Kendricks, P., Crane, P.R., 1997. The origin and early evolution of plants on land. Nature, 389, 33-39. Kidder, D.L., Erwin, D.H., 2001. Secular distribution of biogenic silica through the Phanerozoic: comparison of silica-replaced fossils and bedded cherts at the series level. The journal of Geol.
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Kirman, S., Strasberg, D., Grondin, V., Colin, F., Gilles, J., Meunier, J.D., 2007. Biomass and litterfall
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in a native lowland rainforest: Marelongue Reserve, La Réunion Island, Indian Ocean. Forest Ecol. Manag. 252, 257-266.
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Kurina, L.M., Vitousek, P.M., 1999. Controls over the accumulation and decline of a nitrogen-fixing lichen, Stereocaulon vulcani, on young Hawaiian flows. J. Ecol. 87, 784-799. Lee, M.R., Parsons, I., 1999. Biomechanical and biochemical weathering of lichen-encrusted granite:
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textural controls on organic-mineral interactions and deposition of silica-rich layers. Chem. Geol. 161, 385-397. Lucas, Y., 2001. The role of plants in controlling rates and products of weathering : importance of biological pumping. Ann. Rev. Earth Planet. Sci. 29, 135-163. McCarroll, D., Viles, H., 1995. Rock-weathering by the lichen Collena auriforma in solution basin development on a carboniferous limestone substrate. Earth Surf. Proc. Landf. 18, 363-368. Merlet, C., 1994. An accurate computer correction program for quantitative electron probe microanalysis. Mikrochim. Acta 114/115, 363–376. Ma, J.F., Takahashi, E., 2002. Soil, fertilizer, and plant silicon research in Japan, Elsevier. Meunier J.D., Kirman S., Strasberg D., Keller C., 2010. The output and biocycling of Si in a tropical rain forest developed on young basalt flows (La Reunion Island). Geoderma 159, 431-439. Nativel, P., Joron, J.L., Treuil, M., 1979. Etude pétrographique et géochimique des volcans de la Réunion. Bull. Soc. Géol. Fr. 7, 427-446. Neaman A., Chorover J., Brantley S.L., 2005. Implications of the evolution of organic acid moieties
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ACCEPTED MANUSCRIPT for basal weathering over geological time, Am. J. Sci. 305, 147-185. Nesbitt, H. W., Wilson, R. E., 1992. Recent chemical weathering of basalts. Am. J. Sci. 292, 740-777. Nieuwenhuyse, A., Verburg, P.S.J., Jongmans, A.G., 2000. Mineralogy of a soil chronosequence on
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andesitic lava in humid tropical Costa Rica. Geoderma 98, 61-82. Nienow, C.A., Friedman, E.I., 1993. Terrestrial lithophytic (rock) communities. In: Friedman, E.I.,
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ed., Antartic Microbiology. Wiley-Liss, New York, 343-412.
Raunet, M., 1991. Le milieu physique et les sols de l'Ile de la Réunion. Région Réunion , Centre de coopération internationale en recherche agronomique pour le développement (Cirad). Schwartzman, D.W and Volk, T., 1989, Biotic enhancement of weathering and the habitability of
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earth. Nature, 340, 457-460.
rocks. Chemosphere 39, 379-388.
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Silva, B., Rivas, T., Prieto, B., 1999. Effects of lichens on the geochemical weathering of granitic Skinner, H.C.W., Jahren, A.H., 2003. Biomineralization. In: Schlesinger, W.H., ed., Biogeochemistry. Treatise on Geochemistry, Vol. 8 (Holland, H.D. and Turekian, K.K., eds), Elsevier-Pergamon,
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117-184.
Strasberg, D., 1995. Processus d’invasion par les plantes introduites à La Réunion et dynamique de la
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vegetation sur les coulees volcaniques. Ecologie 26(3), 169-180. Strasberg, D., Faloya, V., Lepart, J., 1995. Patterns of tree mortality in an island tropical rainforest subjected to recurrent windstorms. Acta Oecologica 16, 237-248.
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Strasberg, D., 1996. Diversity, size composition and spatial aggregation among trees on a 1-ha rain forest plot at La Réunion. Biodiv. Conserv. 5, 825-840.
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Taylor, T.N., Hass, H., Remy,W., Kerp, H., 1995. The oldest fossil lichen. Nature 378, 244. Willis, K.J., McElwain, J.C., 2002. The evolution of plants. Oxford University Press. Wright V.P., 1985, The precursor environment for vascular plant colonization, Phil. Trans. R. Soc. Lond., B 309, 143-145. Zambell, C.B., Adams, J.M., Gorring, M.L., Schwartzman, D.W., 2012. Effect of lichen colonization on chemical weathering of hornblende granite as estimated by aqueous elemental flux. Chem. Geol. 291, 166-174.
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List of Figures
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Fig. 1. Colonization of La Réunion volcanic flows by Stereocaulon Vulcani. a) after 6 years; b) after
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24 years: the study area.
Fig. 2. Assemblage of BSE (SEM) images showing the zone in contact with lichen used for porosity
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measurement. The rectangle represents the zone analysed in Fig. 7. Fig. 3. Probability density function (pdf) as a function of pixel intensity which increases when dark
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pixel decreases: mineral 1= plagioclase (M in Fig. 3) and mineral 2 = glass (G in Fig. 3). Fig. 4. The interface between lichen and the 24-year basalt analysed on thin sections. a) optical
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microphotograph showing Stereocaulon Vulcani fixed on the surface of the basalt (ba) with red coatings (C) b) close-up of a) showing that the lichen is coated by a red fringe (r); c) BSE image on SEM of a close-up of b) with the position of EDS spectra (circles) showing the presence of Si, Al, Ca,
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Fe and Mg in the basalt (EDS-1) in d), Si, Fe and Ti in the red fringe coating the lichen in e) and Si in
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the center of the lichen (EDS-3) in f). Fig. 5. BSE (SEM) images of thin sections from the 24-year basalt. a) fresh basalt (the light zone with microlites) not in contact with lichen (the dark zone to the right); b) zone in contact with the lichen showing porosity (in black, P) between the glass matrix (in white, G) and the microlithes (in light grey, M); c) at higher magnification, the image shows that the porosity affected the glass forming cavities filled with Ca minerals (Ca); d) at the contact with Ca minerals, glass presented pitted (P); e) image in the non weathered zone showing that the glass has a granophyric-like texture; f). in the zone in contact with lichen, granophyric-like textures showed a comb structure at the interface with the porosity. Fig. 6. Digital imagery of the zone presented in Fig.4 showing the calculated porosity (in black). Fig. 7. Digital imagery and evolution of porosity (in black) for a zone of basalt extracted from Fig. 2 at the contact with Stereocaulon Vulcani (to the right) down to 800 m below the surface.
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List of tables
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Table 1. Chemical composition of Stereocaulon Vulcani and reference materials.
Table 2. Composition and biomass of the vegetation growing on the 24-year-old lava flow.
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Table 3. Chemical composition of the studied rock samples.
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Table 4. Denudation rates on various rock types covered by lichens.
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Figure 1
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Figure 2
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Figure 3
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Figure 5
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Figure 6
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Figure 7
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Sample
Si
Al
Ti
Na
Mg
Fe
K
Ca
Mn
P
C
N
g kg -1 5.8
2.14 0.095 11.0 2.87 1.02 8.5 22.2 0.06 0.83
Yield % (2)
89.1 98.4 99.3 78.8 86.9 84.2 45.1 97.0 92.9 90.7
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Standard (1)
S Vulcani: avg. 12.9 7.08 and range
2.6
2.1- 1.8-
0.4-
at Hawaii (4) 36.8 15.6
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S Vulcani (3) 4.35 2.63 0.45 0.69 1.06 4.34 1.9 1.69 0.03 0.26 385 6.30
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(1) = Certified values given by NRCCRM for sample GBW07602 (Bush branches and leaves) (2) = 100 x measured value/certified value; (3) Values corrected using (2); (4) From Jackson and Keller (1970).
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Table 1
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Blechnum tabulare 125(*) <1
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Stoebe passerinoides 250(*) <1
Boehmeria penduliflora 6125(*) nd
Tristemma mauritiana 125(*) nd
Rubus alceifolius 125(*) nd
Stereocaulon vulcani 77,5(**) 6249
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Agarista Nephrolepis Machaerina Psilotum salicifolia abrupta iridifolia nudum Density 5750(*) 59875(*) 2750(*) 2875(*) -1 Biomass (kg ha ) 837 197 1 <1 (* ) = number of individuals/ha; (**) = % of the covered lava area (s.d. = 16.18)
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Table 2
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Sample
Analysis
Si
Al
Ti
Na
Mg
Fe
K
Ca
Mn
ICP/AES
76.26
15.77 20.03 37.03 100.79 6.14 78.83 1.24
Pyroxene Microprobe 240.15
18.03
7.80
1.80
93.16
53.85 0.02 149.48 1.10
5.80
6.61
1.73
0.33
5.00
6.68
0.03
Feldspar Microprobe 236.71
158.99
0.67
24.63
1.15
5.25
1.44
97.98
0.04
0.78
0.01
0.23
0.01
0.10
0.03
1.09
0.00
n=10 (sd)
n=19 (sd)
Microprobe 184.36
0.39
0.30
1.55
0.46
0.08
Microprobe 230.39
67.32
n=7 (sd) Glass
4.64
7.08
0.12
7.91
23.57 21.37 29.52 1.86
6.31
9.91
5.54
0.23
2.43
2.05
0.17
0.16
115.00 9.59 69.36
1.75
15.33
0.25
10.67 0.02
2.19 13.26
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n=25 (sd)
0.12 244.92 153.18 0.02
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Olivine
14.82
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1976 flow
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221.05
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g kg -1
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Table 3
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This study
McCarroll and Viles (1995) Moraine 1.2
Aghamiri and Schwartzman (2002) Mica-shist 1-10
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Basalt 6.7
Nienow and Friedman (1993) Sandstone 10
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Rock type Denudation -1 rate (m y )
Lee and Parsons (1999) Granite 2-3
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Table 4
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Major elements from Stereaucolon Vulcani and rocks were analysed on a 24-years-old lava flow at Réunion island Si stored in Stereaucolon Vulcani is comparable to Si stored in the mature forest above a 500 years-old lava flow In basalt at the contact with Stereaucolon Vulcani, the glass matrix has been preferentially weathered We estimate a significant denudation rate of about nearly 7 m y-1 Early land plants may have contributed significantly to weathering before the advent of vascular plants
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S0009-2541(14)00281-2 doi: 10.1016/j.chemgeo.2014.05.033 CHEMGE 17257
To appear in:
Chemical Geology
Received date: Revised date: Accepted date:
17 February 2014 21 May 2014 24 May 2014
Please cite this article as: Meunier, J.D., Kirman, S., Strasberg, D., Grauby, O., Dussouillez, P., Incipient weathering by Stereocaulon vulcani at R´eunion volcanic island, Chemical Geology (2014), doi: 10.1016/j.chemgeo.2014.05.033
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Incipient weathering by Stereocaulon
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vulcani at Réunion volcanic island 1
CEREGE, CNRS, Aix-Marseille Université, Europôle Méditerranéen de l’Arbois, 13545 Aix en Provence, France
UMR PVBMR, Université de La Réunion, CS 92003, 97 744 Saint-Denis Cedex 9 3
CINaM-CNRS-Aix-Marseille Université, Campus de Luminy Case 913, 13288 Marseille Cedex 9, France
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2
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J.D. Meunier1*, S. Kirman1, D. Strasberg2, O. Grauby3, P. Dussouillez1
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* Corresponding author. Phone +33 (0)4 42 97 15 26. E-mail address: [email protected]
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ABSTRACT
The impact of early land plants and fungi in increasing global weathering is still debated, particularly before the advent of vascular plants during the Devonian. Here we present a study of the incipient
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weathering of basalt by Stereocaulon vulcani, a native colonizing lichen on Réunion Island (Indian ocean). We analysed the chemistry and mineralogy of a 24-year-old flow located at low altitude that
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was mostly covered by Stereocaulon vulcani with aboveground biomass of 6249 kg ha-1. The chemical composition of Stereocaulon vulcani showed that besides C and N, Si and Fe were the dominant
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elements. The Si stored in the aboveground pioneer vegetation gives 27 kg ha-1, comparable to the Si stored in the old-growth at the Marelongue Reserve (Meunier et al. 2010). On thin sections, the inner part of Stereocaulon vulcani was mostly composed of Si while Fe coatings observed at the base of the thallus may be the result of wind blow dust interception as suggested by Cochran and Berner (1992). Using BSE images on SEM, we showed evidence of dissolution of the glass matrix at the basaltthallus contact. The quantification of porosity by digital imagery showed a variation between 7% in the unweathered zone to near to 40% at the surface. A maximum denudation rate of 6.7 m y-1 is estimated to fall within the range of the values reported in the literature. Using our data for analog to the past, we suggest that early land plants would have been capable to mobilize Si in a similar proportion as vascular plants and should have significantly affected the weathering of land before the advent of vascular plants. Keywords: Weathering; Lichen; Basalt; La Réunion; Chemical composition; Digital imagery
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ACCEPTED MANUSCRIPT 1. INTRODUCTION Vascular plants are major contributors to the evolution of landscape and global
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geochemical cycles (Berner et al., 2005; Amundson et al., 2007). Plant roots secret organic acids and chelates which enhance the chemical weathering of soil minerals and the release of
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elements in soil solutions (Kelly et al., 1998; Lucas, 2001). Knowing the rate of weathering by plants is important in fundamental issues such as the modeling of CO2 at the geological scale (Berner 1997), the understanding of forest disturbances (Conley et al., 2008; BaloghBrunstad et al., 2008) as well as forest management (Augusto et al., 2001; Jonhson-Maynard
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et al., 2005; Bakker et al., 2004; Meunier et al., 2010) and monument conservation (Almeida et al., 1994; Adamo and Violante, 2000). The acceleration of chemical weathering is well evidenced for vascular plants but many questions remains to be solved for a better
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understanding of the interplay between weathering and biota (Brantley et al., 2011). For instance, Brantley et al. (2011) address the role of biological stoichiometry in weathering and
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makes the hypothesis that biology drives weathering during initial successions. Lichens are pioneer organisms that are frequently observed to colonize rocks exposed to the surface. Lichens result from a symbiotic association between a fungus and a
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photosynthesizing organism; they are known to accelerate the degradation of rock by physical and chemical processes (Banfiel et al., 1999 ; Chen et al., 2000 ; Adamo and Violante, 2000;
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Asta et al., 2001). Penetration of the hyphae of lichens in the voids of the substrate leads to the dismantlement of rocks and porosity increase. Lichen exudates include respiratory CO2,
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organic acids, alkaline metabolic products and complexing agents that are able to increase the dissolution of minerals (Chen et al., 2000; Neaman et al., 2005). Organic acids such as oxalic acid are also able to react with cations dissolved from the substrate and form secondary crystalline products such as oxalates, iron oxides as well as alumino-silicates. The role of lichens in weathering rates and soil formation is therefore complex and its extent in accelerating weathering is still not well understood. Measurements of the enhancement of weathering rates by lichens compared to bare surface show values ranging from x 2 to x 71 using measurement of the weathering rinds and ranging from x 2.5 to x 18 using major element concentration in aqueous solutions (Zambell et al., 2012). Some species are even recognized to protect from the weathering (Silva et al., 1999; Garcia-Valles et al., 2003; Carter and Viles, 2005). The intensity of weathering by organisms such as lichens, fungi and algae is still debated in the issue of the factors that controlled the atmospheric CO2 before the advent of vascular plants (Berner, 1992; Schwartzman and Volk, 1989). According to 2
ACCEPTED MANUSCRIPT Schartzmann and Volk (1989) pre-vascular organisms were able to increase weathering at a considerable rate (10-100 times). This statement was rejected by Berner (1992) who stated that lichens cannot contribute to weathering compared with vascular plants that produce deep
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roots. The disagreement was partly due to different interpretations of the weathering of Hawaiian basalt flows by Stereaucon vulcani, a fruticose lichen. Jackson and Keller (1970)
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found that "the reddish weathering crust is consistently thicker by one or more orders of magnitude on rock surface colonizer by Stereocaulon vulcani than on areas of bare rocks ". Cochran and Berner (1996) re-examined the previous fields using an Electron microprobe and did not find evidence of significant enhanced weathering. They also re-interpreted the
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weathering crust (or rind) as a result of interception of wind-blown dusts. More recently, Brady et al., (1999) used back scattered images of thin section in order to quantify mineral porosity and found that weathering intensity under lichens is 2-18 times greater that the
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abiotic case. Their results are in good agreement with the values ranging from 4 (for Si) to 16 (for Mg) found by Aghamiri and Schwartzman (2002) who used mini watershed experiments
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of weathering of mica-schist by crustose lichens at Hubbard Brook Experimental Forest. However, the studies of Brady et al. (1999) do not provide detailed information about the of the volcanic flow.
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weathering processes and assumed that the duration of the weathering is equivalent to the age At La Réunion island, a "hot spot " volcanic-type tropical island, Stereocaulon vulcani
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is also observed as an early native colonizer on lava flows (Fig. 1a). After decades Stereocaulon vulcani is replaced by ferns and native canopy trees that characterize the
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lowland tropical rainforest of the island (Kirman et al., 2007; Strasberg, 1995; Cadet, 1977). In the first stages of plant succession, pioneer vegetation is confronted with harsh environmental conditions, which is why the nitrogen fixing lichen, Stereocaulon vulcani, dominates the early developmental stages (Cadet, 1977). Stereocaulon vulcani never occur in stage environment like the lowland rain forest remnant at the Marelongue reserve (Kirman et al., 2007) developed above a 500-year-old flow. The same evidence is given at Hawaii by Kurina and Vitousek (1999), who noted a complete disappearance of this species after 140 yrs on lava flows at low altitudes. Kurina and Vitousek (1999) showed that lichen decline on Hawaiian flows is most probably due to light limitation as a result of shading by vascular plants. The study of weathering of La Réunion basalt by Stereocaulon vulcani appears to us a good opportunity to confront the previous results obtained at Hawaii. The aim of this study is to get new insights on the processes and rates of weathering during colonization of lands by early land organisms. 3
ACCEPTED MANUSCRIPT 2. MATERIAL AND METHODS This study was conducted on the 1976 lava flow located on the east slopes of the
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Piton de la Fournaise, the active Hawaiian-type volcano of La Réunion island (21°09'S55°30'E, Indian Ocean). The climate of the study site is characterized by an average annual
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rainfall and temperature of respectively 4000 mm and 23°C (Raunet, 1991). The study site belongs to the lavas that have flowed into a caldeira named “L’Enclos” during the last 3 500 years, opening towards the east and named the Grand Brûlé (Bachelery, 1981). The field analysis and sampling was done at 250 m above sea level during the year 2000, 24 years after
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its eruption, where the flow appeared to be mostly covered by Stereocaulon vulcani (Fig.1b). Herein, the related weathering processes are referred to a 24-year-old lava flow. For the purpose of the study, we analyzed the major element composition of the lichens and the
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associated vegetation of the flow and the composition of the basalt at the interface with lichens. Sampling was set up in a network of four permanent plots of 100 m2 established
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across the 1976 lava flow. In each 100 m2 plot, all the plants were recorded and counted on 5 quadrats of 4 m2. Biomass was estimated using a destructive method: the lichens were harvested on five small plots (100 cm2) and biomass estimations were correlated with cover
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measurements. Since 2007, the lower part of the study site is totally covered by a large new lava flow.
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Biomass harvests were sorted according to species, oven dried several days at 70°C, weighed and ashed (five hours at 600°C). The carbon and nitrogen content were measured
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with an elemental analyzer and standard operating procedures (Fisons Instruments NA 1500) on the oven dried and ground vegetation samples. Major elements were determined by ICPOES (Jobin Yvon Ultima-C) on ash sub-samples after alkaline fusion using standard procedures (Germanique, 1994). A sample from the National Research Center for Certified Reference Materials (NRCCRM) of Bush branches and leaves (GBW07602) was used for calibration (Table 1). The yields ranged from 45.1% for Na to 99.1% for Ti. Fragments of basalts were sampled using a hammer. About 100g of fresh basalt removed from the first centimeter depth in contact with the air was dried and ground before chemical analysis. The major elements were determined using the same procedure used for plants (ICP-OES after alkaline fusion). Polished thin sections of a sample of basalt coated/non coated by lichen were made for SEM and microprobes analyses. The sample coated by lichens was impregnated by epoxy resin (EPON, Merck). Low and high-resolution micrographs of samples were acquired during observation with a scanning electron 4
ACCEPTED MANUSCRIPT microscope (SEM) JEOL JSM-6320F at 15kV equipped with Energy dispersive X-ray spectroscopy (EDS) using a Si-Li detector (Quantax, Brucker AXS Microanalysis GmbH Berlin, German). Using spot mode EDS spectra allowed qualitative elemental analyses at the
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micrometer scale. Major element compositions of minerals were determined in thin sections by EPMA on a CAMECA SX-100 instrument equipped with five wavelength-dispersive X-
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ray spectrometers (WDS) at Service Microsonde Sud (Université Montpellier 2). The analyses were done with 20 kV accelerating voltage, a focused beam of 10 nA and counting times of 20–30 s. Concentrations were obtained from raw intensities using the ‘‘X-PHI’’ quantification procedure (Merlet, 1994). Natural minerals, synthetic oxides and pure metals
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were used as standards.
Using BSE images in SEM, the porosity was quantified at the lichen-basalt interface using digital imagery following a procedure derived from Dorn (1995). The principle is based
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on the property of the pores at the surface of a polished thin section to give a darker image than do minerals. Using MATLAB, we coded in intensity a zone of the lichen-basal interface
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where porosity was higher than in the zone not impacted by lichens. In order to obtain a porosity estimation of the basalt/lichen interface, we combined SEM images and selected a zone where the porosity was the most impacted inside the basalt at the contact with one
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thallus. The quantification of the porosity of the selected zone (Fig. 2) was calibrated using the following steps: first, we selected one porous zone (in black) and two representative
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minerals: plagioclase (in dark grey) and glass (in light grey). For each zone we estimated the probability density function (pdf) as a function of pixel intensity that increases when dark
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pixel decreases (Fig. 3). The porosity was estimated by the area that corresponded to an intensity range distinct from minerals at the threshold of 78. The porosity was therefore calculated using a binary image (0 = minerals; 1 = porosity). 3. RESULTS AND DISCUSSION 3.1. High Si uptake Stereocaulon vulcani contributed to 77.5 % of the total vegetation cover on the 1976 lava flow (Table 2). Other significant endemic species on the flow were the fern Nephrolepis abrupta and the tall sedge Machaerina iridifolila. The first seedlings of the pioneer tree species Agarista salicifolia were seen, represented by 5 750 individuals/ha. For comparison, on the 500 year old flow they represent only 44 individuals ha-1 (Kirman et al., 2007). The composition of the colonizing plants on this 24-year-old flows was similar to other young 5
ACCEPTED MANUSCRIPT undisturbed lava flows studied at low elevations (Cadet, 1977; Strasberg et al., 1995; Strasberg, 1996). The aboveground biomass on the 1976 flow was essentially represented by Stereocaulon vulcani with 6249 kg ha-1. This result was only slightly higher than the values
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around 5000 kg ha-1 obtained by Kurina and Vitousek (1999) at Hawaii for Stereocaulon vulcani grown on a 10-year-flow at 900 m of elevation. The biomass above the 1976 flow was
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however 3 orders of magnitude less than the aboveground biomass which characterized the mature lowland rainforest developed at a similar elevation on a 500-year-old flow (Kirman et al., 2007).
The chemical composition of Stereocaulon vulcani showed that besides C, N and O ,
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Si nd Fe were the dominant elements followed respectively by Al, K, Ca, Mg, Na, Ti, P, Mn (Table 1 and Fig. 4). The values of Si and Al fall in the low range of values given for the same species by Jackson and Keller (1970) at Hawaii while Fe is slightly higher at La relative
abundance
of
the
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Réunion. This relative importance of the elements in the lichen was quite similar to the same
elements
in
the
underlying
parent-rock:
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Si>Fe>Ca>Al>K>Mg>Na>Ti>P>Mn. The fact that the chemical composition of the lichen was similar to the rock substrate may show the presence of parent rock fragment engulfed by the thalli (Lee and Parsons, 1999; Silva et al., 1999). The red coatings observed at the base of
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the thallus were characterized by the presence of Al, Fe and Ti. These coatings may be the result of wind-blown dust accumulation as suggested by Cochran and Berner (1992).
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Observations on thin sections showed that the inner part of Stereocaulon vulcani was mostly composed of Si (Fig. 4). The lack of silicate fragments that would suggest physical
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weathering is a strong evidence for chemical weathering and intake of Si by Stereocaulon vulcani . The relative high proportion of Si in Stereocaulon vulcani is apparently puzzling because, contrary to N, Si is not considered as a nutrient element. Si is known to accumulate in pioneer plants (Ma and Takahashi, 2002) and to increase the plant resistance during environmental stresses such as metal toxicity or pathogenic disease (Cooke and Leishman, 2010; Epstein, 2009; Guntzer et al., 2012). Accordingly, the presence of an elevated proportion of Si in Stereocaulon vulcani is not fortuitous and is probably due to a selective advantage for colonizing new substrates. As a consequence Si constitutes, in the present case study, a good tracer for the quantification of weathering by colonizing plant. Surprisingly, the Si stored in the aboveground pioneer vegetation given by multiplying the biomass (6249 kg ha-1, Table 2) by the Si content (4.35 g kg-1, Table 1) gives 27 kg ha-1 comparable to the Si stored in the mature forests at Marelongue with 18 kg ha-1 (Meunier et al., 2010). This result is explained by the lower concentration of Si in the litter and woods of the mature forest 6
ACCEPTED MANUSCRIPT despite their greater biomass. 3.2. Preferential bio-etching of glass within 24 years
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The chemical analyses (Table 3) showed that the 1976 flow is typical of olivine basalt (Nativel et al., 1979) with phenocrystals or microlithes of olivine, Ca plagioclases, augite,
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iron-titanium oxides and an isotropical glass matrix. In optical microscopy, red coatings were observed (Fig. 4a) on the surface of the basalt similar to the ones observed on Stereocaulon vulcani. Using BSE images (Fig. 5a and b), an increase of intergranular porosity was observed towards the attached thallus while the basalt which was not in contact with the
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thallus did not provide any evidences of porosity increase. At higher magnification in SEM (Fig. 5c), we showed that the porosity had mainly affected the glass matrix. The other silicates had smooth contours showing that they were not affected by weathering. The
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porosity in the glass showed micrometric cavities mainly observed at the interface with the other minerals but also inside the glass mass forming galleries. A mineral phase containing Ca
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was detected as filling some of the cavities (Fig. 5d). Ca-minerals present in the cavities were probably oxalate originating from the excretion of oxalic acids by the mycobiont (Adamo and Violante, 2000). At the contact with Ca minerals, glass presented pitted contours typical of
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chemical weathering. BSE images in the non-weathered zones - i.e. zones not in contact with
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the thallus and zones deeper than 800 m from the surface - showed that the glass had a granophyric-like texture (Fig. 5e). In the weathered zones in contact with the thallus, granophyric-like glass showed a comb texture at the contact with the pores (Fig. 5f).
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The quantification of the porosity by digital imagery gives a value of around 20 % for the selected zone (Fig. 6). The increase of the porosity from the contact with thallus to a maximum of 800 m inside the basalt was determined as a representative rectangular zone (Fig. 2) and showed a linear variation between 7% in the unweathered zone to near 40% at the surface (Fig. 7). The result that Stereocaulon vulcani has significantly affected the chemical weathering of volcanic glass of the lava flow after 24 years of exposure but not the others silicates minerals appears to differ from what was found in Hawaii with the preferential dissolution of plagioclase (Dorn, 1995; Brady et al., 1999; Gordon, 2005; Cochran and Berner, 1996). Because of the corrosion effect of siderophore, we would also have expected that the most richest iron bearing minerals; i.e. olivine to be preferential altered as this was observed by Callot et al. (1987) who studied the bio etching of amorphous and crystalline silicates by fungi. The preferential dissolution of glass is however in a good agreement with some experimental and field studies showing that basaltic glass dissolves faster than the other 7
ACCEPTED MANUSCRIPT crystalline minerals in basalts (Gislason and Eugster, 1987; Nieuwenhuyse et al., 2000) but opposite to other cases where olivine dissolves first (Nesbitt and Wilson, 1992). The granophyric-like texture with their comb surface when altered (Fig. 5) was probably favorable
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to preferential penetration of corroding solution. The type of solution is not known but because the rate of glass dissolution increases with pH, contrary to the crystalline minerals
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(Fraysse et al., 2009), we suggest that alkaline solutions would have prevailed. 3.3. Global impact of the incipient weathering by lichens
Using the estimated porosity of 20% at a maximum depth of 800 m (Fig. 6), the
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volume of removed material gives 160 10-6 m3 m-2. Because this volume is entirely made of glass assuming a density of 2.5 t m-3, we can calculate the mass of removed material which gives 400 10-6 t m-2. Using Si concentration (230.39 g kg-1, Table 3) for comparison to lichen
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uptake we can calculate the export of Si that gives around t 921 kg Si ha-1. The value is 34 times more than the Si stored by Stereocaulon vulcani on the flow calculated here (27 kg ha). Using the maximum porosity of 20% we can also calculate a denudation rate of 6.7 m y-1
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1
of basalt within 24 years ((20x800)/(100x24)). This value falls within the higher range of the
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few values available in the literature (Table 4) showing that chemical weathering by lichens can be significant (Zambell et al., 2012). The corresponding mass of glass gives 166.8 kg ha-1
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y-1 and a rate of Si release of 38.4 kg Si ha-1 y-1. For comparison, this Si output is around 2.4 times the average Si output measured in the streams in the mature forest (Marelongue) developed above a 500 years of basalt flow (15 kg Si ha-1 y-1 from the 7-21 range in Meunier
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et al., 2010). Assuming that the depth of porosity created by lichen is half the maximum observed (800 m) we obtain a Si output value of around 19 kg Si ha-1 y-1, falling in the range found in the mature forest. If we use Stereocaulon vulcani as an analogue for pioneer plants that colonized the land during the Paleozoic, a significant source of plant derived-Si might have been available for impacting the global cycle of silica. The earliest land non-vascular plants are dated from the mid Ordovician to early Silurian (approx 476-432 Ma). Fossil lichens are well evidenced 400 my ago but may have appeared back during the late Proterozoic (Taylor et al., 1995; Willis and McElwain, 2002). Between the early Devonian to late Carboniferous (395 to 286 million years ago) early land plants, lichens, fungi and vascular plants built up forest ecosystems with trees as high as 35 m. It is also during the Devonian that the rise of large vascular plants played a major role in weathering and CO2 regulation (Berner, 1997).
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ACCEPTED MANUSCRIPT Meanwhile in the marine environment, it is striking to observe that radiolarians, microorganisms that secrete a Si skeleton, radiated during the early Silurian (Skinner and Jahren, 2003). Besides, Kidder and Erwin (2001) show that the bedded cherts significantly
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increased from the Silurian following the beginning of extensive siliceous deposits of radiolarian skeletons. If we postulate that vascular plants triggered the radiolarian and cherts
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development, we therefore should observe an increase of marine silica deposits from the Devonian and not as earlier as the Silurian. Alternatively, the input of silica during the Silurian was triggered by pioneer lichens, fungi and non vascular plants. Contrary to the observed vegetation succession in La Reunion, early land organisms during the early
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Paleozoic were not replaced by forests of vascular plants probably until several tens of million years. Therefore, input of Si in the ocean during phanerozoic may have been triggered through the acceleration of weathering by early land organisms before the advent of vascular
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plants (Wright, 1985; Schwartzman and Volk, 1989 ; Keller and Wood, 1993; Kendricks and
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Crane 1997). 4. CONCLUSION
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Incipient weathering of basalt by colonizing Stereocaulon vulcani at Réunion is characterized by glass dissolution and precipitation of secondary carbonate and oxides.
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Contrary to Fe, Ti and Al, Si located in Stereocaulon vulcani, is not the result of dust interception but of parent-rock uptake. Although the biomass of Stereocaulon vulcani on the
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24-year-old flow is lower than the biomass measured in mature forests (Meunier et al. 2010) the Si stored is comparable in both ecosystem. This result is explained by a higher Si concentration despite a lower biomass for Stereocaulon vulcani. The dissolution of glass at the plant-rock interface can affect the basalt up to 800 mm depth. Therefore, our results show that chemical weathering by Stereocaulon vulcani is significant at a decadal scale. Assuming that the results are a good analogue to the past, it can be suggested that transfer of Si from continents to the oceans may have been enhanced by early land organisms before the advent of vascular plants.
Acknowledgements This study was supported by the Programme National Sols et Erosion (French programme 2001-2002), the Direction Régionale de l’Environnement-La Réunion, the Office 9
ACCEPTED MANUSCRIPT National des Forêts-La Réunion and the Institut de Recherche pour le Développement; logistic support was provided by the field station of Marelongue, funded by the P.O.E., Reunion National Park and OSU Reunion. Special thanks to Jean-Jacques Motte, Landryne
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Chassagne, Fabrice Colin and Anicet Beauvais for their scientific input.
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List of Figures
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Fig. 1. Colonization of La Réunion volcanic flows by Stereocaulon Vulcani. a) after 6 years; b) after
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24 years: the study area.
Fig. 2. Assemblage of BSE (SEM) images showing the zone in contact with lichen used for porosity
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measurement. The rectangle represents the zone analysed in Fig. 7. Fig. 3. Probability density function (pdf) as a function of pixel intensity which increases when dark
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pixel decreases: mineral 1= plagioclase (M in Fig. 3) and mineral 2 = glass (G in Fig. 3). Fig. 4. The interface between lichen and the 24-year basalt analysed on thin sections. a) optical
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microphotograph showing Stereocaulon Vulcani fixed on the surface of the basalt (ba) with red coatings (C) b) close-up of a) showing that the lichen is coated by a red fringe (r); c) BSE image on SEM of a close-up of b) with the position of EDS spectra (circles) showing the presence of Si, Al, Ca,
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Fe and Mg in the basalt (EDS-1) in d), Si, Fe and Ti in the red fringe coating the lichen in e) and Si in
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the center of the lichen (EDS-3) in f). Fig. 5. BSE (SEM) images of thin sections from the 24-year basalt. a) fresh basalt (the light zone with microlites) not in contact with lichen (the dark zone to the right); b) zone in contact with the lichen showing porosity (in black, P) between the glass matrix (in white, G) and the microlithes (in light grey, M); c) at higher magnification, the image shows that the porosity affected the glass forming cavities filled with Ca minerals (Ca); d) at the contact with Ca minerals, glass presented pitted (P); e) image in the non weathered zone showing that the glass has a granophyric-like texture; f). in the zone in contact with lichen, granophyric-like textures showed a comb structure at the interface with the porosity. Fig. 6. Digital imagery of the zone presented in Fig.4 showing the calculated porosity (in black). Fig. 7. Digital imagery and evolution of porosity (in black) for a zone of basalt extracted from Fig. 2 at the contact with Stereocaulon Vulcani (to the right) down to 800 m below the surface.
14
List of tables
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Table 1. Chemical composition of Stereocaulon Vulcani and reference materials.
Table 2. Composition and biomass of the vegetation growing on the 24-year-old lava flow.
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Table 3. Chemical composition of the studied rock samples.
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Table 4. Denudation rates on various rock types covered by lichens.
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Figure 1
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Figure 2
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Figure 3
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Figure 5
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Figure 6
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Figure 7
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Sample
Si
Al
Ti
Na
Mg
Fe
K
Ca
Mn
P
C
N
g kg -1 5.8
2.14 0.095 11.0 2.87 1.02 8.5 22.2 0.06 0.83
Yield % (2)
89.1 98.4 99.3 78.8 86.9 84.2 45.1 97.0 92.9 90.7
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Standard (1)
S Vulcani: avg. 12.9 7.08 and range
2.6
2.1- 1.8-
0.4-
at Hawaii (4) 36.8 15.6
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3
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S Vulcani (3) 4.35 2.63 0.45 0.69 1.06 4.34 1.9 1.69 0.03 0.26 385 6.30
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(1) = Certified values given by NRCCRM for sample GBW07602 (Bush branches and leaves) (2) = 100 x measured value/certified value; (3) Values corrected using (2); (4) From Jackson and Keller (1970).
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Table 1
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Blechnum tabulare 125(*) <1
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Stoebe passerinoides 250(*) <1
Boehmeria penduliflora 6125(*) nd
Tristemma mauritiana 125(*) nd
Rubus alceifolius 125(*) nd
Stereocaulon vulcani 77,5(**) 6249
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Agarista Nephrolepis Machaerina Psilotum salicifolia abrupta iridifolia nudum Density 5750(*) 59875(*) 2750(*) 2875(*) -1 Biomass (kg ha ) 837 197 1 <1 (* ) = number of individuals/ha; (**) = % of the covered lava area (s.d. = 16.18)
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Table 2
24
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Sample
Analysis
Si
Al
Ti
Na
Mg
Fe
K
Ca
Mn
ICP/AES
76.26
15.77 20.03 37.03 100.79 6.14 78.83 1.24
Pyroxene Microprobe 240.15
18.03
7.80
1.80
93.16
53.85 0.02 149.48 1.10
5.80
6.61
1.73
0.33
5.00
6.68
0.03
Feldspar Microprobe 236.71
158.99
0.67
24.63
1.15
5.25
1.44
97.98
0.04
0.78
0.01
0.23
0.01
0.10
0.03
1.09
0.00
n=10 (sd)
n=19 (sd)
Microprobe 184.36
0.39
0.30
1.55
0.46
0.08
Microprobe 230.39
67.32
n=7 (sd) Glass
4.64
7.08
0.12
7.91
23.57 21.37 29.52 1.86
6.31
9.91
5.54
0.23
2.43
2.05
0.17
0.16
115.00 9.59 69.36
1.75
15.33
0.25
10.67 0.02
2.19 13.26
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n=25 (sd)
0.12 244.92 153.18 0.02
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Olivine
14.82
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1976 flow
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221.05
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g kg -1
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Table 3
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This study
McCarroll and Viles (1995) Moraine 1.2
Aghamiri and Schwartzman (2002) Mica-shist 1-10
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Basalt 6.7
Nienow and Friedman (1993) Sandstone 10
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Rock type Denudation -1 rate (m y )
Lee and Parsons (1999) Granite 2-3
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ACCEPTED MANUSCRIPT Highlights
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Major elements from Stereaucolon Vulcani and rocks were analysed on a 24-years-old lava flow at Réunion island Si stored in Stereaucolon Vulcani is comparable to Si stored in the mature forest above a 500 years-old lava flow In basalt at the contact with Stereaucolon Vulcani, the glass matrix has been preferentially weathered We estimate a significant denudation rate of about nearly 7 m y-1 Early land plants may have contributed significantly to weathering before the advent of vascular plants
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