The use of tree-rings and foliage as an archive of volcanogenic cation deposition
Introduction
Volcanic activity can affect vegetation in numerous direct and indirect ways, from local fumigation (Delmelle, 2003) to perturbations of radiation budgets and climate (Briffa et al., 1998, Gu et al., 2003). Understanding the environmental fate of volcanogenic trace elements may allow the development of reliable proxies for volcanic activity. For example, if a diagnostic chemical signature could be found in tree-rings linked to a causal volcanic event, this would be of great value for reconstructing records of past eruptions.
Multi-elemental dendrochemical analysis (investigation of the composition of tree-rings) has been used extensively to study anthropogenic pollution, commonly using laser-ablation or solution inductively-coupled-plasma mass-spectrometry (ICP-MS) on individual or composited tree-rings (e.g., Hall et al., 1990, Hoffmann et al., 1994). Some authors have argued that dendrochemistry is unsuitable as a pollution archive (e.g., Bindler et al., 2004, Garbe-Schönberg et al., 1997, Zhang et al., 1995), while others conclude it has potential (e.g., Eklund, 1995, McClenahan et al., 1989) though perhaps only qualitatively (Bondietti et al., 1989). Dendrochemical methods have successfully recorded changing pollution levels on decadal to centennial timescales, and across large spatial scales (e.g., Padilla and Anderson, 2002, Tommasini et al., 2000, Watmough, 1999).
If dendrochemistry may be used to investigate anthropogenic pollution sources, then the method may reasonably be applied to active volcanic areas, since volcanoes emit many of the same trace constituents, such as Cu, Zn, Hg and Pb. Dendrochemical methods have been applied to large individual volcanic eruptions (e.g., Laki, 1783 AD and Tambora, 1815 AD; Padilla and Anderson, 2002, Pearson et al., 2005). While these studies demonstrate the potential of tree-rings to record macroenvironmental volcanic impacts, their potential as local archives of persistent but fluctuating volcanic activity remains uncertain. Here, we evaluate the potential of tree-ring and foliar chemistry to record the temporal and spatial influence around a persistently active volcano (Mount Etna, Sicily) in terms of cation signatures preserved in vegetation on sub-annual (foliage) and annual to supra-annual (tree-rings) timescales. This work, especially the leaf cation measurements, complements similar work on volcanogenic fluorine deposition (Bellomo, 2005).
Active volcanoes display a spectrum of behaviour, with persistent non-eruptive degassing common at many systems. This is characterised by the emission of gases (including SO2, HCl and HF), which increase depositional fluxes of anions (e.g., SO42−, Cl− and F−) to local ecosystems. Other trace elements are also released during both eruptive and non-eruptive degassing, and volcanoes may be the major global natural emission source for some key elements (e.g., As, Cd, Cu, Pb; Mather et al., 2003). Trace cations are generally (with some exceptions, e.g., Hg; Pyle and Mather, 2003) carried in the solid phase (Hinkley, 1991), either predominantly in the silicate ash phase (lithophilic elements) or the non-silicate aerosol phase (volatile elements). Volatile cations may be conveniently categorised by their affinity for certain complexing anions as chalcophilic (sulphide-forming) or halide-forming (see Table 1). During eruptions, activity including explosions, fire-fountaining and ashfall (Fig. 1) may introduce large quantities of fresh material rich in lithophilic cations to the environment; during non-eruptive periods, emissions will be dominated more by volatile cations (e.g., Aiuppa et al., 2003b). Such signals might potentially be recorded in dendrochemical archives.
Volcanic cation emission rates are difficult to quantify, but may be estimated using gaseous SO2 fluxes, if cation:SO2 ratios are known (although large variation may occur in these ratios; Mather et al., 2003). Detailed SO2 flux records such as that for Etna since 1987 (Caltabiano et al., 2004) are the best rough proxies for volcanogenic cation release.
Volcanic plumes aid cloud formation, causing acidic rain and fog (Fig. 1). Rainfall is an important controlling factor in the enhanced deposition of cations scavenged from the plume (Aiuppa et al., 2006, Gauthier and Le Cloarec, 1998).
Within a plant, elemental uptake from the environment may occur via the roots, foliage and stem (Lepp, 1975), with subsequent transport through the phloem and into the xylem. Rates of cation absorption and transport, and the degree to which cations move between tree-rings, are determined by element mobility, which varies widely between tree species and over which there is some disagreement (e.g., Bondietti et al., 1989, Prohaska et al., 1998).
The dominant plant-cation uptake route is through the soil (Martin and Coughtrey, 1982), although this may be reduced in nutrient poor, shallow or acidic soil (Cutter and Guyette, 1993). Soil processes are highly important in terms of cation transport and uptake to the plant (Fig. 1, Kapusta et al., 2006). Etnean soils are derived from volcanic ash, and have a high cation exchange capacity and are able to receive large amounts of volcanogenic cations. However, intense soil leaching may cause the release of these cations to groundwater (Guicharnaud and Paton, 2005). Acid deposition (e.g., from volcanic sulphate and chloride) increases soil cation leaching and may decrease levels of exchangeable cations such as Ca and Mg (Delmelle et al., 2003, Tendel and Wolf, 1988). Low pH soil solutions may also increase mobility of elements including Zn, Al and Cd (Bondietti et al., 1989, Smolders, 2001). Solute flow rate and contact time between plant roots and soil solution are other important factors determining cation uptake by plant roots.
Trees in poor soils may favour stem or foliar uptake, and show greater sensitivity to environmental changes (Ragsdale and Berish, 1988). Inward lateral transport of cations to wood from bark may be slow unless metals are in soluble form (Martin and Coughtrey, 1982). For Cd and Pb, and perhaps for most heavy metals, bark uptake may offer the clearest signal of changing atmospheric levels (Garbe-Schönberg et al., 1997, Huang et al., 1982). Foliar uptake is enhanced by leaf-surface wetness, which increases cation solubility (Percy and Baker, 1988), but may be inhibited by SO2 (Weinstein, 1977). Volcanic gas fumigation, particularly by SO2 and HF, causes leaf injury through stomatal-pore absorption (Smith, 1990). The response varies with leaf type, species, extent of exposure and atmospheric conditions.
In summary, the main source of temporal and spatial cation depositional variation around a volcano is expected to be due to volcanic activity, supplemented by anthropogenic pollution or sources such as forest fires. This pattern is likely to be modified before uptake into the plant, dominantly by processes within the soil. Modifications within the tree may be introduced due to plant metabolism, element mobility variations, disease, growth rate and wood density differences, and the common systematic changes at the sapwood–heartwood boundary (Chun and Hui-yi, 1992).
Section snippets
Sampling site: Mount Etna, Sicily
Mount Etna covers an area of 11 500 km2, reaching about 3300 m in altitude. Etna's activity is characterised by persistent quiescent degassing punctuated by regular eruptions. Etna is one of the largest volcanic gas point sources globally, with emissions accounting for about 10% of global volcanic CO2 and SO2 (Caltabiano et al., 2004). Records of volcanic activity are available in unique detail (e.g., Bonaccorso et al., 2004), making Etna an ideal study area. During eruptions Etna may contribute
Qualitative dendrochemistry
Laser-ablation data were processed by linear interpolation to the start of each annual growth ring, placing results in the time domain and allowing direct comparison between tree cores. A low band-pass Gaussian filter, applied at 0.1-year sample intervals with a 3-year window, removed high-frequency variation arising from analytical noise and tree-ring density differences. The resulting dendrochemical traces show a distinct temporal variation with elemental peaks occurring over 1 to 10-year
Dendrochemical correlation with volcanic activity
Laser-ablation ICP-MS provided a fast, efficient method of measuring dendrochemical variation and our results reveal the presence of discrete dendrochemical peaks. However, there is no obvious correspondence between volcanic events and element variation (Fig. 4). Synchronous peaks are observed, but often in trees from different parts of the volcano, with other nearby trees showing no correspondence.
Ashfall may provide the greatest time-varying input of volcanogenic cations to the environment,
Conclusions
No correlation between dendrochemical patterns and volcanic activity is observed around the flanks of Mount Etna. Element translocation within and between tree-rings appears to be significant and complex, making results highly dependent on the chosen sample and the orientation of the sampled core. These processes complicate the volcanogenic signature in the system to the extent that results cannot be replicated. Hence, it is impossible to attribute dendrochemical peaks to a causal event with
Acknowledgements
Many thanks to Simon Crowhurst for suggestions during data analysis, and to Tommaso Caltabiano (INGV, Catania, Italy) and Torbjörn Nilsson (Dept. Forest Soils, SLU, Uppsala, Sweden) for the use of data. We acknowledge the NOAA Air Resources Laboratory for the provision of the HYSPLIT transport and dispersion model. Thanks also from SFLW to Fitzwilliam College, Cambridge and the Cambridge European Trust for fieldwork grants, from TAM to the Royal Society for funding, and from DMP to the
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Present address: Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK.