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Payne, Richard J, Kilfeather, Aoibheann A, van der Meer, Jaap JM et al. (1 more author) (2005) Experiments on the taphonomy of tephra in peat. Suo. pp. 147-156.

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147SUO 56(4), 2005© Suoseura — Finnish Peatland Society ISSN 0039-5471Helsinki 2005 Suo 56(4): 147–156

Experiments on the taphonomy of tephra in peat

Richard J. Payne, Aoibheann A. Kilfeather, Jaap J.M. van der Meer &Jeffrey J. Blackford

Richard J. Payne, Aoibheann A. Kilfeather, Jaap J.M. van der Meer & Jeffrey J.

Blackford Department of Geography, Queen Mary, University of London, Mile End

Road, London E1 4NS United Kingdom, e-mail: r.j.payne@qmul.ac.uk

Understanding the taphonomy of tephra (volcanic ash) is crucial to the use oftephrochronology in peatlands. This study uses field experiments on a Scottish peatlandto investigate the post-depositional movement of tephra in peat. Experiments weredesigned to investigate the temporal change in tephra profiles over a 24-month studyperiod, the horizontal distribution of tephra and the microscopic distribution of tephraparticles within the peat. Tephra concentration profiles show that the majority of tephrashards are retained within the top cm of peat with small numbers penetrating to amaximum of 6 cm depth. This distribution would be reduced as the peat is compressedwith subsequent accumulation. Examination of thin sections from the plots indicatesthat tephra movement may be dependent on the microscopic structure of the peat, espe-cially porosity. These results provide general support to the use of tephrochronologyalthough further work will be required, particularly in other peat types and environ-ments.

Keywords: tephra, taphonomy, peatland, Achnacree

Introduction

Layers of volcanic ash (tephra) are found pre-served in peatlands throughout the world (Zoltai1988, Botis et al. 1993, Dugmore et al. 1995,Lowe et al. 1999, van den Bogaard & Schmincke2002, Bergman et al. 2004, Payne & Blackford2004). Tephra has been used in many palaeo-ecological studies as it provides a time-specificmarker horizon (isochrone) allowing comparisonbetween sites and, where the age of the eruptionis known, a means of dating sediments (Cham-bers et al. 1997, Hall 1998, Langdon et al. 2003,Lomas-Clarke & Barber 2004). The technique isparticularly valuable for comparing the timing ofpalaeoecological change across large areas (van

den Bogaard et al. 2002, Langdon & Barber 2004)and for investigating the impacts of ancient vol-canic eruptions (Blackford et al. 1992, Hall 2003).

Two fundamental assumptions underlie thiswork: that tephra is deposited instantaneously onthe time-scale under consideration, and that tephradoes not undergo substantial post-depositionalmovement. While the first assumption seems tobe correct in most cases, the second assumptionhas received little consideration. Lake sedimentstudies have observed significant movement oftephra with secondary deposition, biological mix-ing processes and density-related movement withredeposition at lower levels (Anderson et al. 1985,Thompson et al. 1986, Boygle 1999, Beierle &Bond 2002). These problems can impair correla-

148 Payne et al.

tions and dating based on tephras. In peat there isalso evidence that depositional processes are notas straightforward as is often assumed. Bjarnasson(1991) noted the sinking of Icelandic tephrathrough a moss carpet. Tephra may be redepositeddue to wind action, this may be a particular issueif the peat surface is frozen (Bergman et al. 2004).In the sedimentary record, some tephra profileshave been noted to have secondary and subsidi-ary peaks (Caseldine et al. 1998, Richard Payne,unpublished data) and some sites appear to havea background tephra concentration (Charman etal. 1995, Holmes et al. 1999).

Dugmore & Newton (1992) used X-radiog-raphy to examine the horizontal variability of atephra layer in peat. Their study showed an un-even distribution across the palaeo-bog surfacewith greater quantities found in downward pro-jecting pockets, presumably representing tephrawashing into hollows. Caseldine et al. (1999) havealso shown fine-scale horizontal variability intephra concentration by examining the reflectanceproperties of tephra layers in peat. These studiesimply the possibility of post-depositional move-ment of tephra and suggest that an improvedknowledge of tephra taphonomy is essential tothe continued use of tephrochronology. Both ver-

tical and horizontal movement of tephra are po-tentially significant. If tephra moves verticallythrough the peat profile this could result in anerroneous age being assigned to the sediment.Horizontal movement of tephra could potentiallyresult in concentrations being reduced below de-tection limits and a valuable isochrone being lost.

One possible means of investigating tephrataphonomy is to use an experimental approach.Rowley & Rowley (1956) and Clymo & Mackay(1987) investigated the taphonomy of pollen insurface peat using experimental approaches andillustrated substantial movement. These studiescannot be directly applied to tephra due to thedifferences in tephra morphology and size range;however similar methodologies may be appliedto a study of tephra taphonomy. This study usesthree field experiments to investigate thetaphonomy of tephra in peat. Experiments weredesigned to investigate the movement of tephrathrough peat over time, the horizontal distribu-tion of tephra and the microscopic distributionof tephra particles using thin sections.

Material and methods

The site chosen for this study is the Moss ofAchnacree, a large raised bog in Argyll and Bute,western Scotland (Grid Reference NM9134:Fig.1). Peat deposits cover an area of almost 7Km2

averaging around 1.9m depth; the site receivesan annual rainfall of approximately 1500mm(Scottish Environmental Protection Agency, un-published data). The peatland has been damagedand its extent reduced through a combination ofgrazing, drainage and peat cutting and is borderedby local roads on four sides (Fig. 1: Scottish Natu-ral Heritage and Scottish Wildlife Trust, unpub-lished data). A series of 1m2 plots was establishedon the site as part of a related ecological study(Payne & Blackford 2005). Tephra was appliedto the plots at concentrations equivalent to Ice-landic tephra found in northern British peatlands(Dugmore & Newton 1992, Dugmore et al. 1995).Results from three plots are reported here: Plot15 (50g tephra per m2), Plot 9 (50g tephra perm2) and Plot 21 (300g tephra per m2). Full detailsof applications and sampling are shown in Table

Fig. 1. Location of Moss of Achnacree and sampling area.

Kuva 1. Kokeen sijainti Achnacree -suolla Skotlannissa,

jossa tutkittiin tulivuoren tuhkan käyttöä turpeen ajoitta-

misessa.

Loch

Etive

N

Moss of Achnacree

Experimental

Plots

2 kilometres

Connel

149SUO 56(4), 2005

1. Plots 15 and 9 are hummock sites and havesimilar vegetation, dominated by Calluna vul-

garis and Eriophorum vaginatum with anunderstorey of Cladonia portentosa, Hypnum

cupressiforme, Aulacomnium palustre andOdontoschisma sphagni. The surface peat in theseplots is compact and well humified with a de-fined upper surface. Plot 21 is a hollow site andhas a Sphagnum -dominated vegetation commu-nity including Sphagnum magellanicum, Sphag-

num recurvum, Eriophorum vaginatum andOdontoschisma sphagni. The surface layer in thisplot consists of a Sphagnum carpet and is moreporous than the highly humified surface peat inPlots 15 and 9.

Tephra was obtained from proximal air-falldeposits of the Öraefajökull 1362 eruption insouthern Iceland (Ellershaw 2004). Tephra ap-plied to Plots 9 and 21 passed through a 150µmsieve while that applied to Plot 15 was in the sizerange 150–300µm. The tephra was applied insuspension using a domestic watering can; ply-wood sheets were placed around the plots to keepall the tephra within the plot. Tephra was appliedto Plots 9 and 15 in May 2002. To investigatetephra movement down through the peat column,Plot 15 was sampled again at five intervals overthe next two years in June 2002 (1 month), Sep-tember 2002 (4 months), November 2002 (6months), June 2003 (13 months) and May 2004(24 months). Twenty centimetre long cores wereextracted using a 5cm-bore Russian corer (Aaby& Digerfeldt 1986). Coring locations were se-lected using a random number and grid system.In June 2003 tephra was applied to Plot 21, afterthree days a monolith block (10x10x20cm) was

extracted from the centre of this plot and anotherfrom Plot 9. These blocks were used for the prepa-ration of thin-sections to examine the microscopicdistribution of tephra particles within the peat.At the same time, four cores were taken along atransect of Plot 9 to investigate the horizontalvariability of tephra; cores 9-A, 9-B and 9-D arefrom lower elevation regions of the plot and core9-C from a higher region of the plot.

The cores were sampled from the peat sur-face down. For the hummock sites (Plots 15 and9) the surface was taken to be the top of the com-pact peat layer with the living portion of themosses and vascular plants removed. For the hol-low site there is no such defined interface betweenthe peat and the living vegetation and the surfacewas taken to be the upper surface of the Sphag-

num carpet. Sub-samples were taken at 1cm reso-lution from the surface to 10cm depth. Tephrasamples were prepared from the nine cores fol-lowing the method of Pilcher & Hall (1992). Sub-samples were oven dried and then combusted at550°C for at least 12 hours. Samples wereweighed pre- and post-burning and these dataused to calculate loss on ignition (LOI), whichcan provide an aid to locating the largest micro-tephra layers. The remaining inorganic residuewas cleaned in 10% HCl and stored in distilledwater. To allow tephra shard concentration cal-culations a Lycopodium inoculum (1 tablet, ap-proximately 13400 spores) was added to the sam-ple (Stockmarr 1971). Slides were made up us-ing a small amount of sediment in glycerol andexamined microscopically at 400x magnification.Tephra shards could be identified by their dis-tinctive morphology. A minimum of 200 Lyco-

Table 1. Experimental treatment and sampling of plots

Taulukko 1. Tuhkakäsittelyt ja näytteenottoajankohdat ja -käytännöt koealoittain

–––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––Plot Microform Treatment Time of application Sampling–––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––9 Low Hummock 50g tephra May 2002 Monolith and cores

(< 150µm) after 13 months15 Low Hummock 50g tephra May 2002 Cores at 5 intervals

(150–300µm) over 24 months21 Hollow 300g tephra June 2003 Monolith after 3 days

(<150µm)–––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––

150 Payne et al.

podium grains were counted in the majority ofsamples and the number of tephra shards re-corded.

To examine the microscopic distribution oftephra in two dimensions, thin sections were pre-pared from Plots 9 and 21. A 10 × 7cm thin-sec-tion tin was cut into a cleaned face of the ex-tracted monolith. The samples were dried in ac-etone and then impregnated with crystic resin ina vacuum chamber. After impregnation, the sam-ples were left to harden for six weeks. Sampleswere mounted on glass and ground to a thicknessof approximately 25µm (see Murphy (1986) forfull details of the laboratory procedures). The pre-pared thin-sections were examined at 400× mag-

nification. Tephra shard identification is moredifficult in these samples than in the ashed sam-ples as the optical quality is somewhat poorer;shards are often partially obscured by organicmaterial, may be shown in cross-section and can-not be moved or rotated. Due to these difficultiesinterpretation is based on clusters of shards. Theclusters highlighted here are where two or moreprobable shards occur in a single field of vision.These clusters therefore show regions wheretephra shards are present but provide only a semi-quantitative indication of overall tephra distribu-tion.

Fig. 2. Tephra concentration (solid line) and LOI profiles (dotted line) across Plot 9 on a hummock site. Sampling is 1cmcontiguous blocks; points are marked at the mid-point of their depth range. Figures are labelled as distance in centimetresfrom the western margin of the plot. Note differences in scale between plots.

Kuva 2. Turpeen tuhkapitoisuus (partikkelia /gramma, kiinteä viiva) ja LOI profiili (hehkutushäviö, rasteriviiva) koealal-

la 9 mätäspinnalla. Kuvien numerointi perustuu näytteiden etäisyyteen (cm) koealan länsireunasta. Näytteet on otettu

turveprofiilista 1 cm välein. Huomaa kuvien erilainen mittakaava.

151SUO 56(4), 2005

Results

To investigate the horizontal distribution of tephrawithin the plots, a series of cores was extractedfrom Plot 9 and examined for tephra; tephra con-centration and LOI plots are shown in Fig.2. Alltephra profiles show a rapidly declining tephraconcentration with depth, the majority of tephrashards remaining within the uppermost cm ofpeat. Core 9-C also shows a pronounced second-ary tephra peak at 5–6cm. The maximum depthof tephra penetration varies from 3cm in cores 9-A and 9-D to 4cm in core 9-B and 6cm in core 9-C. Both the maximum and the overall concentra-tions vary greatly between cores. Much the great-est concentrations are found in core 9-B and par-ticularly core 9-C with substantially lower con-centrations in cores 9-A and 9-D. LOI plots forcores 9-A and 9-D do not show any distinct rela-tionship to tephra concentration. However, theLOI in the uppermost cm of cores 9-B and 9-C ismarkedly reduced.

Fig.3 shows the results from Plot 15 investi-gating the vertical movement of tephra throughthe study period. In common with Plot 9, all tephraprofiles show a rapid decline with the highestconcentrations in the uppermost centimetre andtephra shards penetrating to 3cm (cores 15-B, 15-D and 15-E) or 4cm (cores 15-A, 15-C). Tephraconcentrations vary greatly between the cores.There is an apparent trend of increasing tephraconcentration through the study period withgradually increasing values through the first foursamples and a particularly dramatic increase tothe final sample. There is no apparent relation-ship between tephra concentration and LOI forcores 15-A to 15-D, however LOI in the upper-most cm of plot 15-E (79.6%) is much the lowestof any core examined here.

In nine investigated cores the overwhelmingmajority of tephra shards were contained withinthe uppermost centimetre of peat. The maximumdepth of tephra shard penetration varied from 3to 6cm and averaged 3.6cm. The smaller tephrashards in Plot 9 generally penetrated further thanthe larger shards in Plot 15. In all cores tephraconcentration showed a rapid decay with depth.Secondary peaks were noted in core 15-A and 9-C. The secondary peak in core 15-A only repre-

sents a single shard and is therefore unimportant,however the secondary peak in core 9-C repre-sents more shards and appears noteworthy. Themajority of LOI profiles do not appear to showany relationship to tephra concentration presum-ably as the increased concentration of inorganicmaterial was too small to be detected above thebackground variability. However, the reducedvalues in the uppermost cm of cores 15-E, 9-Band 9-C may well represent the high tephra con-centrations in these samples. The LOI values ofaround 95–97% in most of these samples arebroadly typical of ombrotrophic peatlands.

Fig. 4 shows thin-section images from Plots21 and 9. The Plot 9 thin-section shows a rela-tively compact peat matrix. The peat contains finechannels (generally <5mm diameter) many ofwhich have a roughly vertical orientation; theseare most obvious in the lower half of the thin sec-tion. The centre of the slide contains a prominentroot structure, which is most likely Calluna vul-

garis. Tephra shards in this core are concentratedin the upper portions of the peat. Ten clusters wereidentified; these are all located on the right-handside of the slide and reach a maximum depth of5mm below the surface.

The thin-section from Plot 21, a Sphagnum-dominated hollow, shows a notably more porousstructure than Plot 9. A sub-vertical band of largeinterconnected pores occupies the centre of thethin section. The slide bisects some roots but doesnot contain the large root structures of Plot 9.Tephra shards are widely distributed in this thin-section. The greatest concentration of clustersoccurs on the left-hand side of the slide, approxi-mately 5–13mm below the surface of the thin-section. A further six clusters occur towards theright-hand side of the slide between 14 and 32mmbelow the surface. Three clusters occur in thelower half of the slide at around 52, 68 and 74mm.It is interesting to note that all these lower clus-ters occur close to the channel feature in the cen-tre of the slide.

Discussion

The results for horizontal distribution of tephrado not provide any significant evidence of tephra

152 Payne et al.

Fig. 3. Tephra concentration (solid line) and LOI profiles (dotted line) for Plot 15 at five intervals over the 24-month studyperiod on a hummock site. Sampling is 1cm contiguous blocks; points are marked at the mid-point of their depth range.Note differences in scale between plots.

Kuva 3. Turpeen tuhkapitoisuus (partikkelia /gramma, kiinteä viiva) ja LOI profiili (hehkutushäviö, rasteriviiva) koealal-

la 15 mätäspinnalla. Näytteet on otettu viitenä eri ajankohtana 24 kk ajanjakson aikana turveprofiilista 1 cm välein.

Huomaa kuvien erilainen mittakaava.

153SUO 56(4), 2005

movement. Tephra shard concentrations are attheir highest in the cores towards the centre ofthe plot (cores 9-B & 9-C) and lower in the pe-ripheral cores (9-A & 9-D). The most likely ex-planation for this is unevenness in tephra appli-cation making any movement impossible to as-sess. During application, efforts were made toensure even tephra application across the plot,however this is difficult to judge by eye and it islikely that areas at the centre of the plots receivedmore. A further contributory factor may also bethat tephra pathways through the overlying veg-etation will serve to concentrate tephra in certainregions. These results provide no evidence for ahorizontal movement of tephra towards the lowerregions of the plot; concentrations are very simi-lar in core B from a lower elevation point andcore C from a higher elevation point.

The results from Plot 15 show an apparenttrend of increasing tephra concentration throughthe two-year study period. Results from Plot 9suggest an uneven distribution of tephra acrossthis plot, so it is possible that this trend is purelycoincidental. However the consistency of thisincrease suggests that this is unlikely; a moreprobable reason for the trend is that tephra movedfrom the overlying vegetation (which was notincluded in tephra preparations) into the upperpeat during the course of the experiment. Al-though tephra concentration in the peat apparentlyincreases through the study period, the tephra doesnot penetrate significantly further into the peat.The smaller tephra shards in Plot 9 penetrate fur-ther into the peat than the larger shards appliedto Plot 15, presumably as the smaller shards canpass through smaller pores in the peat matrix and

Fig. 4. Thin-sections from A) Plot 9 and B) Plot 21 showing location of tephra clusters. See text for notes on thin-sectionpreparation and examination.

Kuva 4. Turveprofiilin ohutnäytteet koealalta 9 (A) ja koealalta 21(B), joihin merkitty tuhkarakeiden sijainti. Näytteiden

valmistaminen esitetty tarkemmin tekstissä.

De

pth

(c

m)

154 Payne et al.

are less likely to be retained near the surface. Thismay suggest that tephra particle size can providea further indication of the isochrone location,which may be particularly valuable where con-centration profiles are complex. Further experi-ments would be required to investigate this.

The vertical distribution of tephra shown inthese experiments would be reduced as peat ac-cumulates and the tephra-containing peat is com-pressed on entering the catotelm. The large voidswhich occupy much of the thin-sections in theseplots are absent from thin-sections taken fromdeeper peats, illustrating this compression (Payneet al. unpublished data).

The thin-section from Plot 9 is in generalagreement with the pattern shown in the concen-tration plots. All tephra clusters located in thisslide occur in the uppermost cm of peat. Giventhe smaller volume of material in the thin-sec-tion compared to the ashing preparations it isunsurprising that the ‘tail’ of declining tephraconcentrations with depth is not represented inthese results. The thin section sample from plot21 shows a rather more complex pattern. In thisthin section tephra penetrates significantly fur-ther into the peat, this has occurred over a veryshort period of time. The lowest shards recov-ered lie close to what appears to be a vertical chan-nel; this is interesting as it suggests that such fea-tures may provide pathways through the peat. Onthe left side of the thin section, the greatest con-centrations are in the uppermost cm, in keepingwith results from other plots. However, on theright-hand side of the thin section, the greatestconcentrations are somewhat below the surface,the reason for this difference is unclear. Thegreater tephra penetration in this slide is mostlikely due to the Sphagnum-dominated vegeta-tion and more porous peat structure. Although itis possible that some distortion of tephra distri-bution has occurred during sample preparation,there is no indication that this has taken place.

By comparison to the results of Clymo andMackay (1987), it appears that vertical movementof tephra through peat is probably no greater thanthat of pollen and possibly rather less. However,the differences in methodology and peat compo-sition make an accurate comparison difficult. Dueto the angular morphology and sharp edges of

tephra shards it would seem reasonable to sup-pose that tephra is less liable to movement thangenerally sub-rounded pollen grains. Hall &Pilcher (2002) have suggested that tephra pro-files may aid interpretation of other signals suchas pollen. This may be true, however pollen andtephra particles cannot be assumed to react in thesame way to environmental conditions. It seemslikely that where a tephra profile is complex thismay urge caution in interpretation of pollen sig-nals, however a simple tephra profile cannot betaken as indication that pollen taphonomy is alsosimple.

Overall, the results provide generally goodsupport for the use of tephrochronology inpeatlands. In the majority of cores and plots,tephra remains in the uppermost centimetre ofpeat producing an isochrone which accuratelyrepresents the location of the peat surface at thetime of tephra-deposition. However, some resultsare more complicated; core 9-C shows a second-ary tephra peak and the thin-section of Plot 21shows a complicated tephra distribution. The rea-sons for these complications are uncertain andwill require further work.

This study highlights the importance of con-sidering taphonomy in tephrochronological stud-ies; tephra concentration profiles should alwaysbe constructed and where the profile deviates froman ideal unimodal distribution the isochrone shouldbe treated with caution. This study has only used alimited number of experiments on a single site overa comparatively short period of time. Further ex-periments are necessary to improve our under-standing of the dynamics of tephra taphonomy.Future studies should adopt an alternative approachto tephra application, should include vegetation intephra preparations, investigate tephra movementin a greater variety of peat environments and couldintegrate experiments in more controlled labora-tory conditions. Further studies using imagingtechniques to investigate the micro-structure ofpalaeo-tephra layers may also prove valuable.

This study provides the first experimentalevidence for the extent of post-depositional move-ment of tephra in peatlands. Results demonstratethat tephra movement is generally limited but thatthis may be dependent on the vegetation and peatstructure at a locality.

155SUO 56(4), 2005

Acknowledgements

Our thanks to Mr & Mrs McIntyre for permission to usethe site. The tephra used in this study was kindly suppliedby Dr Megan Ellershaw. The map was prepared by EdOliver. RJP’s work was carried out during the course of aQMUL Westfield studentship.

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Tulivuoren purkaukset levittävät vulkaanista tuhkaa, joka turpeeseen kerrostuessaan muodostaa po-tentiaalisen mahdollisuuden arvioida turpeen ikää, kun purkauksen ajankohta tunnetaan. Tuhka voikuitenkin liikkua turpeessa syvyyssuunnassa laskeutumisensa jälkeen ja siten aiheuttaa epävarmuut-ta turpeen ajoittamiseen. Tuhkan liikkuminen turpeessa on ollut kuitenkin heikosti tunnettua. Tässätutkimuksessa suoritettiin kenttäkokeita eräällä Skotlantilaisella suolla tuhkan liikkuvuuden tutkimi-seksi turpeessa. Kokeisiin sisältyi kolme koealaa, joilta tutkittiin tuhkan vertikaalista ja horisontaa-lista liikkuvuutta pintaturpeessa 24 kuukauden ajan sekä tuhkapartikkelien mikroskooppista vaihte-lua. Turpeen tuhkapitoisuusprofiilit osoittivat, että valtaosa tuhkapartikkeleista jäi ylimpään sentti-metrin paksuiseen turvekerrokseen ja syvimmilläänkin tuhkapartikkeleita vajosi 6 cm:n syvyyteen.Ohutleikenäytteistä tehdyt analyysit osoittivat, että tuhkan liikkuminen riippuu suuresti turpeen ra-kenteesta; varsinkin sen huokoisuudesta. Tutkimuksen perusteella vulkaanista tuhkaa voidaan melkoluotettavasti käyttää turvekerroksen ajoittamiseen, mutta lisätutkimuksia tarvittaisiin, jotta voitaisiinsanoa miten hyvin menetelmä soveltuu käytettäväksi laajemmin mm. erityyppisissä turpeissa ja ym-päristöolosuhteissa, joissa kerrostuminen on tapahtunut.

Received 15.4.2005, Accepted 30.9.2005

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