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Tephra layers and early warning signals for critical transitions

Tephra layers and early warning signals for critical transitions

Project aims

This project aims to establish how vegetation cover affects a) the stabilisation of fine, pyroclastic fragments (tephra) deposited during volcanic eruptions and b) the incorporation of this material into the soil as tephra layers. The ultimate goal of the research is to infer past ecological conditions from tephra layers preserved in the soil.

Background

Recent research suggests that vegetation plays a key role in the formation of tephra layers. If this is the case, then these strata could, in theory, be used as a proxy for ancient vegetation cover. For example, variability in the thickness of a given tephra layer could indicate patchiness in vegetation cover at the time of the deposition (Fig. 1). Patchiness in vegetation may, in certain circumstances, be an early indicator of abrupt ecosystem transitions (e.g. the onset of rapid soil erosion) (Kéfi et al. 2014). It is therefore possible that high-resolution measurements of dated tephra layers could provide a record of changing ecological resilience over timescales of decades to centuries.

Tephra layers

Figure 1: Tephra layers (dark grey/black) in an Icelandic soil section (the soil is orange-brown). This section is from the excavation of an old Norse farm; the trench is about 1.5 m deep. The bottom of the trench is formed by a tephra layer emplaced in the Eldgjá eruption of 934 AD. Note the variability in the thickness of the layers.

Study area

The research was conducted in southern Iceland in June 2014 and 2015. Iceland is an excellent location to study the relationship between vegetation cover, tephra layer variability and ecological resilience because tephra-producing eruptions are frequent (see, for example, the 2009 eruption of Eyjafjallajökull) and the history of human colonisation and land use is well known.

Tephra layers

Figure 2: The 2014 study sites a) Fossdalur (grass-moss heath); b) Kalfafell (moss heath); c) Kalfafell (the grassy top of an erosion feature known as a rofabard); d) Blomsturvellir (grass-moss heath with patches of woolly willow, Salix lanata). In June 2015, the team returned to Iceland to survey a site dominated by birch cover, a vegetation type that covered much of Iceland prior to the Norse colonization in the Ninth Century.

Methods

The project is multidisciplinary and measurements were made across spatial scales (ranging from centimetres to 100s of metres). The project team conducted high resolution surveys of vegetation cover and tephra layer thickness, focussing on the ash produced by the 2011 eruption of Grímsvötn (known as G2011 layer). Sites with distinct vegetation types (moss-dominated, grass-dominated, heathland with shrubs and dwarf birch woodland) were selected for study. Detailed photogrammetric surveys of vegetation structure and birch canopy density were conducted and remote sensing data were collected with a UAV.

Early findings and implications of research

The key findings are that a) structural variation in vegetation cover played a major role in determining the configuration of tephra deposits in the sedimentary section and b) he important aspects of vegetation structure could be captured with relatively simple metrics (e.g. vegetation height) derived from photogrammetric analysis.

The thickness of the G2011 tephra layer was correlated with the vegetation structure present at the time of deposition: broadly, tephra layers were thicker under taller vegetation (Fig. 3). This relationship was found to apply at a landscape scale when different vegetation types were compared (Cutler et al. 2016a). Small- (metre- and sub-metre) scale relationships were also found, although the effect was obscured by the presence of a continuous birch canopy that 'smeared-out' the influence of ground layer vegetation (Cutler et al. 2016b). The results of the study have led to the formulation of conceptual model of tephra-vegetation interactions (Fig. 4).

Our findings have implications for the interpretation of tephra layers, whether this work involves the analysis of ancient volcanic eruptions or archaeological/palaeoenvironmental reconstructions. Furthermore, it is possible that small-scale variability in tephra layers, rather than being interpreted and unhelpful 'noise', could be used as a proxy for palaeo-vegetation structure.

Figure 4

Figure 3: The relationship between vegetation height (expressed here as U0.7, or the height below which 70% of vegetation structure occurs) and the mean thickness of the G2011 tephra layer. The tephra layer is thickest under the tallest vegetation. Each point represents a quadrat survey. The points are coloured-coded according to site/vegetation type: e.g. the green points are from the Kalfafell (moss) sites; the orange points are from the Kalfafell (grass) sampling location, for example.



Figure 5

Figure 4: A conceptual model illustrating the impact of varying vegetation cover on a tephra layer resulting from the same initial deposit (a) in open, short-statured vegetation; (b) a ground layer with scattered shrubs; (c) a continuous shrub canopy.

Funding and collaborators

This project is funded by the National Science Foundation, via a grant to Prof Andrew Dugmore of the University of Edinburgh, and conducted in collaboration with Dr Richard Bailey (University of Oxford) and Dr Richard Streeter (University of St Andrews).

References

  • Cutler, N.A., Bailey, R.M., Hickson, K.T., Streeter, R.T. & Dugmore, A.J., 2016a. Vegetation structure influences the retention of airfall tephra in a sub-Arctic landscape. Progress in Physical Geography, 40, pp 661-675.
  • Cutler, N.A., Shears, O.M., Streeter, R.T. & Dugmore, A.J., 2016b. Impact of small-scale vegetation structure on tephra layer preservation. Scientific Reports, 6, 37260.
  • Kefi, S. et al. (2014) Early warning signals of ecological transitions: methods for spatial patterns. PLoS ONE, 9, e92097.