The impact of high tephra loading on late-Holocene carbon accumulation and vegetation succession in peatland communities

Quaternary Science Reviews 67 (2013) 160e175

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The impact of high tephra loading on late-Holocene carbon accumulation and vegetation succession in peatland communities P.D.M. Hughes a, *, G. Mallon a, A. Brown a, H.J. Essex a, J.D. Stanford a, b, S. Hotes c a

Palaeoecology Laboratory (PLUS), Geography and Environment, University of Southampton, Highfield, Southampton, Hampshire SO17 1BJ, UK Department of Geography, Wallace Building, Swansea University, Singleton Park, Swansea SA2 8PP, UK c Philipps-University, Faculty of Biology, Department of Ecology, Karl-v.-Frisch-Str. 8, 35043 Marburg, Germany b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 August 2012 Received in revised form 5 January 2013 Accepted 16 January 2013 Available online

Peatlands are major terrestrial stores of carbon (C) of importance to the global climate system. Recent studies have made progress in understanding the climatic controls on the C cycle; however, important interactions between volcanic deposition and peatland C stores remain to be addressed. This study uses a 3000-year peatland record from northern Japan to examine the interactions between carbon accumulation, vegetation community succession and volcanic ash deposition. Plant macrofossil and testate amoebae records are presented alongside records of total organic carbon, nitrogen and phosphorous. Ageedepth models are developed using a Bayesian approach, with seven AMS radiocarbon dates and two identified historical tephras from Baitoushan (AD 969 (981 cal. BP)), and HokkaidoKomagatake (AD 1640 (310 cal. BP)) volcanoes. Results show that moderate to high tephra loading can shift peatland plant communities from Sphagnum to monocotyledon domination. This vegetation change is associated with increased peat humification and reduced carbon accumulation. Where tephra deposition and reworking has occurred, the apparent rate of carbon accumulation can be halved while high tephra loading of the mire surface is sustained. Sphagnum species vary in their tolerance to tephra deposition. After each ash fall Sphagnum magellanicum disappeared from the plant macrofossil record, whereas Sphagnum papillosum showed apparent continuity of development through the 1856 (94 cal. BP) Ko-c1 tephra. High rates of carbon accumulation (peaking at >100 g m
2 yr
1), 2e3 times faster than the average for northern peatlands, were recorded in the Sphagnum communities that established after the cessation of tephra deposition and reworking from the AD 969 Baitoushan ash fall (B-Tm tephra). This peak in C accumulation was coincident with a radical shift in mire nutrient cycling most probably caused by the interaction of S. magellanicum with leachates from the underlying tephras. The phase of high C accumulation continued for over 300 years, offsetting the initial negative impact of the B-Tm tephra on peatland C accumulation. These results suggest that management for ash-tolerant Sphagnum species could be a highly effective strategy for minimising volcanic disruption to peatland carbon accumulation. The study also shows that consideration of volcanic impacts on peatlands is essential for development of more realistic terrestrial carbon balance models in volcanically active regions. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Plant macrofossils Tephra Testate amoebae Carbon accumulation Holocene Bog Hokkaido Japan

1. Introduction Peatlands are an important component of the global carbon (C) cycle since they presently accumulate 100 Tg of carbon annually (Dean and Gorham, 1998) and they are estimated to store 547,000 Tg C (547 Gt) (Yu et al., 2010). The rate of carbon accumulation in peatlands is dependent upon the balance between primary production and peat decomposition (Charman, 2002).

* Corresponding author. Tel.: þ44 (0)23 80 592489; fax: þ44 (0)23 80 593295. E-mail address: [email protected] (P.D.M. Hughes). 0277-3791/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.quascirev.2013.01.015

Changes in this balance are of importance for modelling of future climates and to understand the likely sensitivity of peatland systems to regional environmental and climatic change. Whilst we are beginning to understand the broad-scale climatic controls that govern feedbacks between carbon accumulation and climate change (e.g. Yu et al., 2010; Charman et al., 2012), there is still considerable uncertainty over key regional controls such as the impact of volcanism on peatland communities and C stores. Extensive tracts of boreal peatland occur throughout the Pacific Rim region from northern Japan and eastern Russia (e.g. Hokkaido, Kamchatka and eastern Siberia) to the Pacific seaboard of North America. These C-rich peatlands lie in close proximity to chains of

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active volcanoes that intermittently deposit widespread, heavy ash falls (Machida, 2002). For example, the AD 969  20 eruption of Baitoushan, on the border of North Korea and China, deposited tephras throughout northern Japan with visible ash fall 1000 km from the eruption centre (Horn and Schmincke, 2000). Some larger eruptions from the Pacific Rim spread tephras well beyond the region. The largest Holocene volcanic eruptions of western continental North America have distributed ash across much of the continent (Pyne-Donnell et al., 2012), with the greatest of these eruptions from Mt. Mazama (7627  150 GISP2 years BP), depositing >4 cm of ash (compacted depth) 1100 km from the eruption centre (Hoblitt et al., 1987). In the late-Holocene Alaskan tephras such as Aniakchak II and the White River Ash eastern lobe distributed tephras widely across North America (Pyne-Donnell et al., 2012). Such impacts on peatlands are also found widely, for example, in Northern Europe e originating from Iceland (e.g. Charman et al., 1995), throughout much of South America (e.g. Kilian et al., 2006) and in the forested peatlands of Southeast Asia (e.g. Jago and Boyd, 2005). Modern observations suggest that depositions of tephra in peatlands may be sufficient to alter mire functioning, nutrient status and vegetation communities (e.g. Wolejko and Ito, 1986) through enhanced nutrient delivery, smothering of vegetation and the deposition of plant toxins. These impacts have the potential to affect the carbon balance of peatlands because they alter both primary production and litter decomposition rates. Commonly the addition of major nutrients (Nitrogen (N), Phosphorous (P) and Potassium (K)) to ombrotrophic peatlands is observed to shift plant communities from species producing more recalcitrant litter, such as Sphagnum, to more labile litter such as that produced by vascular plants, reducing the capacity for peatland carbon accumulation (e.g. Malmer et al., 2003). Very similar short-term impacts following tephra deposition have been noted by Hotes et al. (2006, 2010) in Japan and Payne and Blackford (2008) in Alaska. These tephra-induced shifts in plant communities (see Section 1.2 for more detail) are also accompanied by changes in testate amoebae communities and wider microbial activity that may be important for peat decomposition (Payne et al., 2010; Payne, 2012) and C storage. However, long-term (multicentennial-scale) changes in peatland plant communities and C stores impacted by high volcanic deposition have received little attention. 1.1. Aims of this study The aim of this paper is to examine the impact of high tephra loading on multi-centennial-scale mire succession and C accumulation, using an example from Hokkaido, Japan. The study will test the hypothesis that heavy tephra deposition (>5 cm thickness) reduces peatland carbon accumulation over multi-centennial timescales. 1.2. Effects of volcanism on peatlands The likely impacts of distal volcanic activity on peatlands have been reviewed by Payne and Blackford (2008). Here we briefly update that review to provide the necessary context for the present study. 1.2.1. Ecological studies Few studies have made direct observations of the impact of natural tephra deposition on mire plant communities and to overcome this lack of data several field experiments have been performed. Hotes et al. (2004, 2010) used a combination of natural tephra from Tarumai volcano, Hokkaido (Ta-a) and ground glass to

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simulate deposition of ash with three distinct ranges of grain sizes from medium silt-sized particles (glass e median size 9 mm), through coarse sand (Ta-a tephra e median size 855 mm) to coarse sand/fine gravel (ground glass e median size 2400 mm) with three application depths of 1, 3 and 6 cm. Hotes et al. (2004, 2010) found that thicker, finer-grained tephra layers had greater effects on vegetation and that leaching of fine-grained glass shards caused sustained changes in peat pore waters and increases in pH, electrical conductivity and concentrations of sodium, potassium and silicon dioxide. In contrast, oxygen saturation decreased. They found that some plant species disappeared from treatment plots and that mosses were more severely affected than vascular plants but most species survived and where Sphagnum carpets were smothered they were able to re-establish by growing through the natural tephra and ground glass treatments. Hotes et al. (2010) also concluded that whilst mire communities could display a degree of resilience to tephra deposition, some plots subject to tephra deposition developed to an alternative state with a high cover of the dwarf shrub Myrica gale. The mechanism behind the emergence of these alternative equilibria in community composition remains to be clarified. Payne and Blackford (2005) also treated mire vegetation plots using tephra, with the addition of acid applications, to simulate the Hekla-4 eruption. Their application rate was much smaller than that applied by Hotes et al. (2010) and at this dose they found that impacts on vegetation were only associated with the acid applications. While experimental studies may be very useful in studying tephra impacts on peatlands, they are limited by the short duration of observations relative to the rate of vegetation succession and the life span of the plant species involved (Walker, 1970; Pickett et al., 2009) and, as noted by Payne and Blackford (2008), there is some uncertainty over the degree of realism that tephra/ground glassloading experiments represent. Similarly, the realism of the acid application in the experiments by Payne and Blackford (2005) has also been discussed in relation to the quantity of acid applied (Payne et al., 2013). 1.2.2. Palaeoecological studies of plant communities Palaeoecological studies provide a means of tracking long-term vegetation change because peatlands preserve a detailed record of their past biodiversity (Barber, 1993). Previous plant and macrofossil studies have reported variable mire vegetation responses to tephra loading in ombrotrophic and blanket bogs from Alaska, Europe and Hokkaido (Edwards et al., 2004; Hotes et al., 2001, 2006; Payne and Blackford, 2008). In these sites macrofossil records showed both increases and decreases of individual macrofossil types following tephra deposition; however, Hotes et al. (2006) noted that there was no clear response pattern to tephra deposition and even in cases where tephras ranging in thickness between 0.5 cm and >25 cm fell on Sphagnum-dominated vegetation ‘no fundamental shifts to new plant communities were found’ (p. 561). In cores from Alaska Payne and Blackford (2008) found that some macrofossil records showed little response to tephra loading, while in others the vegetation appeared to shift from Sphagnum dominance towards monocotyledon-rich communities immediately following tephra deposition, especially where tephra loading was relatively high. Palynological studies of tephra impacts on vegetation have also recorded a variety of vegetation changes during volcanic events. For example, Charman et al. (1995) found that, of three tephras present at Kildonan, only one was associated with a major vegetation change, as indicated by a marked decline in Cyperacaeae pollen representation. Similarly, in Iceland Edwards et al. (2004) found that, whilst shifts in pollen assemblages from two small peatland basins occurred across phases of tephra deposition, they may not

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have been caused by direct impacts from the eruptions and could be explained by changes in land management practices designed to mitigate the impact of the eruptions on pastures. In the British Isles the Hekla 4 (H4) tephra is a widespread isochrone in peat deposits from Scotland Ireland and northern England. High resolution pollen studies across the H4 horizon in Scotland showed co-incidence between the rapid decline of Pinus sylvestris pollen and the Hekla 4 eruption (Blackford et al., 1992). However, a subsequent study of the Hekla 4 tephra horizon using pollen and dendrochronological analyses suggested no temporal link between H4 ash deposition and the decline of pine pollen in Ireland (Hall et al., 1994). This latter research was complicated by uncertainty surround the presence of P. sylvestris locally (Edwards et al., 1996; Hall et al., 1996); however, Hall (2003) also found no temporal link to vegetation change for other tephras. Most recently, Payne et al. (2013) have concluded that palynological evidence for volcanic impacts on vegetation in the British Isles is weak because of a lack of suitable evidence. Many of the palaeoecological studies from European sites relate to crypto-tephras and the possible roll of volcanic impacts on wetland and non-wetland vegetation communities, mediated through the climate system. These should be distinguished from the studies that have considered direct impacts of high tephra loads on mire vegetation such as those conducted by Hotes et al. (2006) in Japan. The equivocal evidence for long-term vegetation change following direct tephra loading may be partly a function of whether tephra applications were sufficient to cross community-specific ecological thresholds or the result of differences in ash quality and partly a result of the low taxonomic precision used in previous macrofossil and pollen studies. Classification of plants into broad functional groups or sections (e.g. Monocotyledons, Bryopsida, Sphagnum sect Acutifolia) may not be sufficient to fully distinguish the changes in floristic composition caused by volcanic impacts. Another issue is the potentially high spatial variability of vegetation and macrofossil composition at the centimetre scale that influences the outcome of stratigraphic analyses using peat cores. A number of questions remain to be answered including the following: (1) Can tephra loading cause shifts from nutrient/mineral-poor bog towards richer ‘fen-like’ communities?, (2) How resilient are oligotrophic Sphagnum communities to high tephra loads and what are the mechanisms causing changes in species composition or dominance patterns?, (3) What happens to peatland C accumulation immediately after a heavy tephra fall and how does C accumulation change in the subsequent decades to centuries? 1.3. Probable modes of volcanic impact on peatlands Payne and Blackford (2008) classified volcanic impacts on peatlands into five modes: (1) indirect climatic impacts from atmospheric cooling and changes in air mass circulation/precipitation patterns caused by sulphuric acid aerosols and tephra particles; (2) the physical impact of tephras on vegetation, including abrasion of plant tissues, blocking of stomata and smothering; (3) impacts on peatland hydrology; (4) chemical impacts of tephra and tephra leachates, including both plant nutrients and toxins and (5) impacts of volcanic gas and acids through wet and dry deposition and adherence to the surfaces of tephra shards. Recent research has focused on the geochemical impacts of tephra deposition on peatlands. De Vleeschouwer et al. (2008) have shown that fine-grained tephras can act as a barrier to the downward movement of leachates and weathering products from tephras. This blocking can lead to strata, lying above tephras, enriched in metals such as iron (Fe) at concentrations that may reduce the performance of ombrotrophic Sphagna by reducing the

solubility and availability of phosphorous (Silfverberg and Hartman, 1999). 1.4. Mire classification and terminology in this paper The distribution of mire types in Japan is controlled by a distinctive set of environmental conditions particular to the archipelago, requiring a specific classification system for the region (Wolejko and Ito, 1986). The tectonically active geological setting has generated steep relief and frequent volcanism, as well as land movements and tsunamis that all affect mire formation and development. Differentiation of mire types is also controlled by the peculiarly low organic salt solute loads of rivers in the region and the strong climatic zonation that exists throughout the archipelago. The deposition of volcanic products is also recognised as an important control on mire vegetation communities, leading Wolejko and Ito (1986) to coin the term ‘tephratrophic mire’ to describe peatlands that have developed under the influence of volcanism. Several studies have begun the process of classifying Japanese mires (e.g. Suzuki, 1977; Gimingham, 1984; Wolejko and Ito, 1986; Damman, 1988; Hotes, 2002; Fujita et al., 2009), although this effort is not yet complete (Fujita et al., 2009) and terminology is inconsistent in the literature. The terms fen, transitional bog and raised bog have been applied previously in Japan solely on the basis of their floristic composition or physiognomy and without reference to the original hydrological and geomorphological meanings (Fujita et al., 2009). This has led to some confusion in the classification of raised bogs, which has been addressed by Hotes (2002). In this study we use the terms in their original sense, following Moore (1984) and we use the term ‘mire’ to mean a peat-forming system (peatland) following Moore and Bellamy (1974). 2. Study location: Utasai Bog, Hokkaido, Japan Hokkaido is situated on the North American Plate at two major subduction boundaries with the Pacific and Eurasian Plates (Yamagishi, 1996). These subduction zones have created an arc of Quaternary volcanoes, many of which have erupted in the Holocene and in historical times. In the south west of the island seven active volcanoes lie within 150 km of the study site at Utasai Bog (42 380 00.3200 N, 140180 26.7900 E; Fig. 1). Hokkaido-Komagatake volcano, located 70 km south of Utasai, produced one of the largest historical eruptions on Hokkaido in AD 1640. This event was closely followed by a major eruption of Usu (35 km SSE of Utasai) in AD 1663 (287 cal. BP) and by large eruptions of Tarumai in AD 1667 (283 cal. BP) and AD 1739 (211 cal. BP). Three further Plinian eruptions of Hokkaido-Komagatake occurred in 1694, 1856 and 1929 (256, 94 and 21 cal. BP, respectively). Numerous active volcanic cones exist further afield, to the north and south, throughout the Japanese archipelago and many of these could have contributed distal ash to the study site during the Holocene. Utasai Bog is an oligotrophic lowland Sphagnum bog near the town of Kuromatsunai. The mire is small, covering 4.5 ha, and has been bisected by a road; however, it still preserves extensive areas of uncut peat surface and bog vegetation typical of the region (Wolejko and Ito, 1986). Utasai is included in the listing, ‘500 Important Wetlands in Japan’ because it is representative of the original bog vegetation of south-western Hokkaido. The modern plant communities include Moliniopsis japonica e Sphagnum papillosum and M. japonica e Carex lasiocarpa var. occultans. Low hummock species include Ledum palustre var. diversipilosum, Ilex crenata var. paludosa and Empetrum nigrum var. japonicum. In the hollows Rhynchospora faurei e R. alba communities are also present. The dry land areas surrounding the bog have been disturbed by road construction in recent times and they now support Betula

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two adjacent boreholes, and each core drive was overlapped by 5 cm to ensure recovery of the complete peat sequence. The samples were transferred to plastic gutter sections, wrapped in carbonstable, airtight plastic bags and stored at 4  C. 3.2. Chronological methods The UTS-06 peat profile was dated using three lines of evidence; the uncut peat surface, Accelerator Mass Spectrometer radiocarbon dating (hereafter AMS 14C dating) and geochemical fingerprinting of tephra shards to historical eruption events. 3.2.1. Radiocarbon dating Seven AMS 14C dates were placed in the core to establish an outline ageedepth model and to date major stratigraphic events. The aerial parts of identified plant macrofossils were used for AMS 14 C dating. Sphagnum stems were selected in preference to other macrofossils but where these were not available the identifiable leaf remains of sedges and shrubs were used. For each assay 300 mg of material was picked from 1 cm3 sub-samples of peat and cleaned in distilled water to remove contaminants. Sphagnum stems were split to check for fungal remains. Before submission all samples were stored at 4  C in distilled water mixed with 5 ml of 10% hydrochloric acid. Radiocarbon dates were calibrated using Oxcal 4.1.7 (Bronk Ramsey, 2008, 2009) and the dendrochronological calibration curve INTCAL09 (Reimer et al., 2009) and they are quoted in the text as the 2-sigma (2s) calibrated range before present (present ¼ 1950). The results of the AMS 14C assays are presented in Table 1 and in the ageedepth model (Fig. 2).

Fig. 1. Location of Utasai Bog in Hokkaido, Japan (inset middle) and in relation to the settlement of Toyohoro (main map). ‘’ marks the location of core UTS-06 on the site plan (inset bottom).

platyphylla e Sasa spp. communities (The Ministry of the Environment, Japan, 2002; Fujita et al., 2009). Wolejko and Ito (1986) recognise five broad mire/marsh zones across Japan including: (1) Mountain mire zone of Hokkaido, (2) Lowland bog zone of Hokkaido, (3) Mountain mire and upland bog zone of northern Honshu, (4) Mires of a transitional zone, (5) Peatless mire (marsh) zone of southern Japan. Utasai Bog, together with Shizukari Mire, defines the southern limit of the Lowland bog zone in Hokkaido, although the potential bioclimatic region for this zone extends further south (Fujita et al., 2009). 3. Methods

3.2.2. Tephra stratigraphy Major phases of tephra deposition were recorded in the field since the most concentrated deposits of ash were visible as lighter bands in the stratigraphy. Stratigraphic boundaries were refined further with the aid of the sieved sediment fractions prepared for plant macrofossil analysis. The abundance of shards greater than 125 mm was noted on a 5-point scale of abundance. This record was found to correspond closely with the loss-on-ignition curve throughout the peat profile. LOI is therefore plotted alongside the tephra stratigraphy profile in Fig. S1 to provide an indication of tephra abundance since the high numbers of shards throughout the record made individual shard counts impractical. 3.2.3. Glass chemistry Tephra chemistries were established using a Cameca Wavelength Dispersive Spectrometer (WDS) electron microprobe (Edinburgh University) set to an accelerating voltage of 10 kV with a beam current of 10 nA and a minimum spot size of 4 mm. The contamination mark method was used to confirm the spot size with reference to a pure metal standard.

Table 1 Radiocarbon dates from profile UTS-06 (0e254 cm depth). Lab no.

Material

14 Depth C date d13C(&) below peat BP (uncal.) surface (cm)

Beta-294520 Beta-294521 Beta-306243 Beta-294522 Beta-306244 Beta-294523 Poz-21098

Sphagnum Sphagnum Sphagnum Sphagnum Sphagnum Sphagnum Sassa node

41 61 80 110 116 136 254

3.1. Field sampling at Utasai Bog The bog was depth-probed using crossed transects to locate the deepest peat deposits. A coring location was chosen that contained the greatest number of visible tephras. The stratigraphy of the core location was described using the Tröels-Smith (1955) sediment classification system before taking a master core using a 9  30 cm Russian pattern corer. The master core, UTS-06, was extracted using

160 220 280 460 650 1010 2820

      

30 30 30 30 40 30 35


27.9
28.1
27.1
28.2
27.4
26.9
27.0

      

2s calibrated range (cal. BP) 0.1 290e0 0.1 310e0 0.1 453e155 0.1 530e490 0.1 672e552 0.1 960e910 0.1 3060e2845

Calibrated using Oxcal version 4.1.7 (Bronk Ramsey, 2008, 2009).

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0

Surface

Ko-c1 tephra*

Ko-c2 tephra* Beta-294520 50

Ko-d tephra Beta-294521

Beta-306243

100 Beta-294522

Depth (cm)

Beta-306244

B-Tm tephra Beta-294523

150

200

250 Poz-21098

0

500

1000

1500

2000

2500

3000

3500

Age (calibrated years BP) Fig. 2. Bayesian ageedepth model for core UTS-06, using two known tephras (Ko-d and B-Tm), seven radiocarbon dates and the uncut peat surface. Radiocarbon dates are shown with the 2-sigma calibrated range before present, where present ¼ AD1950. Tephras are shown as vertical bars. *Ko-c1 and Ko-c2 were not used in the ageedepth model.

The microprobe was configured to use a TAPeLPETeLLiFeTAPe LTAP crystal setup on five spectrometers. Three measurement cycles were employed. In the first cycle the TAP crystal was used to determine silicon (Si) and aluminium (Al) with LTAP measuring sodium (Na). The LPET and LLiF crystals measured potassium (K) and manganese (Mn), respectively. Peak counting times were 20 s for Si, Al, K and Mn and 30 s for Na. To account for Na mobilisation caused by beamesample interaction during analysis the peak intensities of Na were sub-counted at 6 s intervals and extrapolated to time zero by calculating a decay curve (Neilsen and Sigurdsson, 1981). Phosphorous (P), chlorine (Cl), iron (Fe), and magnesium (Mg), were measured in the second cycle for peak counting times of 30 s,

20 s, 20 s, and 20 s respectively, whereas calcium (Ca) (LPET) and titanium (Ti) (LLiF) were measured in the last cycle for peak times of 10 s and 20 s respectively. The total background counting times for all elements were matched with their respective peak counting times. Primary line calibrations were made using a set of well characterised natural minerals, pure metals and synthetic oxides: Na e jadeite, Mg and Al e spinel, Si and Ca e wollastonite, P e apatite, Cl e NaCl, K e orthoclase, Ti rutile, Mn e pure metal and Fe e fayalite. Secondary standards of basaltic and rhyolitic glass (Hunt and Hill, 1996; Potts et al., 2002) were measured alongside the Holocene tephra chemistries to assess machine performance. PEAKSIGHT software, which uses the PAP matrix correction algorithm, was used

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to process the data (Pouchou and Pichoir, 1991). Tephras were identified with reference to glass chemistries in Tokui (1989), Nakagawa and Ohba (2002) and Nanayama et al. (2003). 3.3. Plant macrofossil analysis Plant macrofossil species counts were made on processed 4 cm3 samples of peat, which were taken at 4 cm intervals throughout the core. Standard processing procedures were applied following the protocol of Mauquoy et al. (2010/11) with the exception that plant remains were not boiled or stained. Vegetative macrofossils forming the bulk of the sample were quantified using the Quadrat-andLeaf-Count (QLC) method of Barber et al. (1994), whereas fruits, seeds and other small remains were assessed using a 5-point scale where 1 ¼ rare, 2 ¼ occasional, 3 ¼ frequent, 4 ¼ very frequent and 5 ¼ abundant. Plant identifications were made with the aid of the University of Southampton’s herbarium collection and with reference to Daniels and Eddy (1990) for Sphagna. 3.4. Testate amoebae analysis Counts of testate amoebae species abundances were made at 4 cm intervals throughout the core. Extraction procedures followed the protocol of Charman et al. (2000). Bulk 1 cm3 peat samples were boiled in distilled water for 10 min and sieved using 15 mm and 300 mm mesh sizes. Following sieving the size fraction between 15 mm and 300 mm was used to identify testate amoebae. A total count of 150 tests per sample was used and identifications were made with reference to Charman et al. (2000). 3.5. Peat bulk density and loss-on-ignition The bulk density of the peat profile was measured on 2 cm3 peat slices taken in contiguous 1 cm depth increments throughout the peat core. Sample volumes were then measured by water displacement. The samples were transferred to clean, dry, preweighed crucibles, weighed and dried overnight at 105  C. They were reweighed and then sub-sampled for the measurement of nitrogen (N), phosphorous (P), total organic carbon (TOC) and losson-ignition analyses (LOI). The LOI sub-samples were weighed into fresh pre-weighed crucibles and burnt at 550  C for 2 h. Finally, the ashed samples were weighed. 3.6. Determination of total organic carbon, nitrogen and phosphorous Dried peat samples were milled to a fine powder (<130 mm) using a pestle and mortar. Approximately 30 mg of powdered sediment was added to 30 ml of 0.22 M HCl and left to react for two hours in order to purge the samples of inorganic carbon. Samples were then ultrasonicated and sieved at 130 mm to remove coarse material that couldn’t be analysed. Material >130 mm was oven dried overnight and weighed and the dry weight was subtracted from the initial weight to ensure an accurate calculation of initial sediment concentration in the analysed solution. The TOC and N content of the acid suspension was analysed by catalytic high temperature oxidation in an Analytik Jena 3100 C/N analyser. A set of standards was prepared from a series of dilutions of a potassium hydrogen phthalate master solution and these were used for calibration of results. Samples were run alongside a set of blanks. All measurements were completed in triplicate and the average values were used. The P (PO4) content of 10 mg sub-samples of powdered peat was determined using the Magnesium persulphate high-temperature oxidation (MOPHTO) method of Ormaza-González and Statham

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(1996). Following this, finely crushed Palintest tablets (LR water test) were added to the extract and the colour reaction was allowed to develop for 10 min. The resultant solutions containing the samples were measured for their colour using a spectrophotometer set to 640 nm. The average of three readings was used to calculate the concentration of PO4 in mg/l. 3.7. Calculation of carbon accumulation The long-term apparent rate of carbon accumulation (LORCA) was calculated using the dry bulk density and the total organic carbon measurements taken on the peat samples, following Mäkilä (1997). First, the long-term apparent rate of peat accumulation was calculated using the equation:

A ¼ r  r  1000 Where A ¼ accumulation of dry mass (g m
2 yr
1), r ¼ rate of peat accumulation (mm yr
1), r ¼ dry peat bulk density (g cm
3). The rate of peat accumulation (r) was calculated by dividing the distance (in mm) between dated levels by the difference in years. This calculation contains two inherent uncertainties derived from the radiocarbon dating errors and the likely variability of peat accumulation between dated points. The high dating density in the upper part of the peat profile aims to increase the precision of the estimated rates of peat accumulation; however these remain best estimates. LORCA (g C m
2 yr
1) was then calculated from the long-term apparent rate of peat accumulation using the measured percentage of TOC for each sample. 4. Tephrostratigraphy, tephra identification and ageedepth modelling 4.1. Tephrostratigraphy Six primary deposition events at 188.5e187.5 cm (UTS-6), 174e 173 cm (UTS-5), 134e130 cm (UTS-4), 57e46 cm (UTS-3), 40e38 cm (UTS-2) and 24e20 cm (UTS-1) depth (Fig. S1) are discernible as visible horizons or as peaks in shard abundance in the sieve residues acquired during the preparation of macrofossil samples. Two phases of post-depositional reworking of tephras are apparent between UTS-6 and -5 and after the UTS-4 tephra falls (Fig. S1). The protracted phase of tephra input to the mire between UTS-6 and -5 probably results from the redistribution of shards from the two primary horizons and possibly from offsite sources through water table fluctuations and/or inflows to the fen. Geochemical analysis of the UTS-5 tephra shards from 173 to 174 cm (Table 2) depth gave consistent glass chemistries, indicative of an air-fall event; however, preliminary analyses from 180 cm depth produced a mixture of contrasting shard chemistries, suggesting reworking, although more work is required to clarify this. After the main deposition of UTS-4 there are two clear peaks apparent in the mineral content record at 126 and 120 cm depth (Fig. S1). The sieve residues showed that these were tephra layers and typing of shards from these levels showed that they mostly had the same chemical composition as the shards from the main deposition event at 134e130 cm depth; although unlike the main deposition, c. 5% of shards had contrasting glass chemistries, suggesting some reworking. Some downward movement of shards is also apparent in the peat horizons below UTS-4 and UTS-3 as shown by increasing mineral content before the main tephra horizons; however, these levels have not been analysed.

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Table 2 Tephra composition recalculated to 100% with oxides/elements expressed as wt%. UTS code

Proposed tephra ID

Depth

SiO2

TiO2

Al2O3

FeO

MnO

MgO

CaO

Na2O

K2O

Cl

Total

n

UTS-1 UTS-2 UTS-3 n/a UTS-4a UTS-4b UTS-4a UTS-4b UTS-4a UTS-4b UTS-5

Ko-c1 Ko-c2 Ko-d Unknown B-Tm* B-Tm* B-Tm* B-Tm* B-Tm* B-Tm* Unknown

20e24 38e40 46e57 120 120 120 132 132 134 134 173e174

75.75 75.84 75.73 72.82 67.79 74.57 66.55 74.59 67.93 74.45 69.04

0.45 0.43 0.48 0.29 0.38 0.26 0.46 0.24 0.36 0.25 0.39

12.35 12.30 12.42 11.66 14.48 10.57 14.84 10.50 14.21 10.55 15.42

2.49 2.53 2.46 4.31 4.75 4.16 5.03 4.18 4.83 4.18 2.82

0.10 0.10 0.10 0.12 0.14 0.09 0.14 0.10 0.16 0.09 0.12

0.55 0.52 0.53 0.11 0.12 0.02 0.24 0.02 0.11 0.02 0.78

2.65 2.61 2.60 0.76 1.14 0.31 1.38 0.28 1.09 0.33 2.59

3.52 3.37 3.62 5.00 5.57 5.04 5.69 5.13 5.72 5.20 4.02

1.96 2.06 1.80 4.54 5.47 4.50 5.44 4.45 5.40 4.43 4.82

0.16 0.16 0.17 0.38 0.19 0.49 0.16 0.50 0.19 0.50 0.52

100 100 100 100 100 100 100 100 100 100 100

11 13 19 8 24 19 17 20 15 22 15

n ¼ number of shards. Individual shard chemistries for the identified tephras can be found in supplementary Table S1. * Note that UTS-4, the Baitoushan tephra (B-Tm) is bimodal. The two distinct shard chemistries have been averaged separately and they are displayed as UTS-4a and UTS-4b. UTS-6 was not analysed.

Analysis of the macrofossil sieve residues showed that the basal mineral input between 250 and 235 cm contained very few tephra shards. 4.2. Tephra identification Tephras were identified on the basis of their stratigraphic position and glass composition (Table 2; Fig. S1, Table S1) with the exception of Tephras UTS-5 and UTS-6 which have not been identified at present. Tephra horizon UTS-4 is very distinctive because it contains two distinct shard populations. Analysis of the glass composition shows that the ash contains both rhyolitic and trachydacitic shards. The two populations of shards contain approximately 10.5 and 14.5 wt% Al2O3, respectively. Levels of K2O and FeO are relatively high at 4.43e5.47 and 4.16e5.03 wt%, respectively. MgO (0.02e0.24 wt%) and CaO (0.28e1.38 wt%) are relatively low compared with other tephras in the UTS-06 core. Comparisons with published WDS microprobe data (Machida, 1999; Nanayama et al., 2003 e see Fig. S2) show that the tephras match well with the bimodal B-Tm tephra which was erupted in ca AD 969 from the Korean volcano Baitoushan (Baekdusan), located 1000 km to the west of Utasai. Note that the Na2O levels in the shards from the UTS-06 core are consistently slightly higher (0.27e0.45 wt%) than those recorded by Machida (1999); however, this may be a result of differences in machine performance and correction for Na migration. Tephras UTS-3, UTS-2 and UTS-1 are all rhyolitic tephras and radiocarbon dating of the UTS-06 peat core shows that they were all deposited within the last 400 years. A comparison with reference glass chemistry data from recent widespread tephra layers in Hokkaido (Fig. S3; Table S1) shows that all three of these UTS tephras are most likely to have originated from the HokkaidoKomagatake volcano. Distinctions between tephras from Hokkaido can be made using K2O and TiO2 contents (Aoki and Machida, 2006). UTS-1 e 3 tephras have K2O contents of between 1.71 and 2.2 wt% (Table S1), consistent with the Ko-d, Ko-c2 and Ko-c1 eruptions of Hokkaido-Komagatake (Fig. S3), whereas tephras from Tarumai (Ta-a and Ta-b) have higher K2O contents of around 2.6 wt% and the Usu-b tephra has significantly lower K20 and TiO2 contents averaging 1.33 wt% and 0.14 wt%, respectively (Nanayama et al., 2003). The stratigraphic positions of the three uppermost tephras in the UTS-06 core (Fig. S1) suggest that UTS-3 represents the Kod tephra which deposited during the eruption of HokkaidoKomagatake in AD 1640 (Katsui et al., 1975), while UTS-2 represents tephra Ko-c2 which was erupted 54 years later in AD 1694 (Furukawa et al., 1997) and UTS-1 represents Ko-c1 from 1856 (Nanayama et al., 2003).

The attribution of UTS-3 to the Ko-d tephra is supported by the radiocarbon chronology (Fig. 2) and by the glass geochemistry data (Fig. S3). These latter data show that all of the UTS-3 shards have K20 contents in the range 1.71e1.90 wt% most closely matching the Ko-d reference shards, whereas the overlying UTS-2 shards cluster in the range 1.9e2.22 wt%, more closely matching the reference shard chemistries for Ko-c2 and Ko-c1 (which cannot be readily separated on shard composition alone). The geochemical differences between UTS-3 and UTS-2 support the interpretation that these layers represent two separate eruptions as suggested above, nevertheless, the identifications cannot be definitive because all three Ko tephras have partially overlapping geochemical compositions. Although UTS-2 and UTS-1 have very similar glass chemistries, these two tephra layers most probably represent the Ko-c2 and Ko-c1 eruptions because they are separated in the stratigraphic record by 10 cm of peat that has a relatively low mineral content (Fig. S1). 4.3. The ageedepth model The ageedepth model (Fig. 2) uses a Bayesian approach to model ten datable points, including all seven AMS radiocarbon dates (Table 1), two known tephras (Ko-d and B-Tm) and the uncut peat surface. The ageedepth model was prepared using Oxcal 4.1.7 (Bronk Ramsey, 2008, 2009) which can combine multiple types of chronological data and provide error estimates. Tephras UTS-2 and UTS-1 have not been used in the model because of the difficulty of separating these layers. 5. Biostratigraphic and geochemical results 5.1. Plant macrofossils and testate amoebae The results of the plant macrofossil analyses are presented in Fig. 3 and the associated zones are described in Table 3. For testate amoebae results are displayed in Fig. 4 and zones are described in Table 4. The macrofossil assemblages show that peat accumulation began with a Phragmites australis swamp. Few other plant species are associated with this swamp community; however, this could be a function of high rates of decay and differential losses from the macrofossil record which are suggested by the levels of unidentifiable organic matter (U.O.M.) in the assemblage. After the pioneer community, Utasai Bog followed a pattern of succession towards increasing acidification and falling water table levels, first developing through a shrubby fen with Carex species and then to a mesotrophic poor fen with Sphagnum palustre and later Sphagnum magellanicum.

P.D.M. Hughes et al. / Quaternary Science Reviews 67 (2013) 160e175 Fig. 3. Plant macrofossil profile for core UTS-06. Species are arranged approximately in order of appearance in the record. Macrofossils quantified with the QuadrateLeafeCount technique (sensu Barber et al., 1994) are shown as linked histograms, whereas remains scored using a 5-point scale of abundance are shown as unlinked histograms. Shaded grey bars indicate air-fall tephras.

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Table 3 Plant macrofossil sub-zones for core UTS-06, 0e254 cm. Macrofossil Depth (cm) Zone description zone UTSm-I

UTSm-H

0e18

18e46

UTSm-G

46e114

UTSm-F

114e134

UTSm-E

134e166

UTSm-D

166e190

UTSm-C

190e204

UTSm-B

204e235

UTSm-A

235e254

Fresh Sphagnum papillosum remains (80e90%) occur with Moliniopsis japonica, Vaccinium oxycoccos and Ilex crenata. U.O.M. and smooth monocotyledon rootlets peak early in the zone. Very humified monocotyledon remains with leaves of Moliniopsis japonica and Eriophorum vaginatum occur in the upper half of the zone. Fresh Sphagnum magellanicum remains dominate and occur with traces of Vaccinium oxycoccos and Ericaceae wood fragments. U.O.M. drops to less than 5% of the assemblage and Eriophorum vaginatum occurs sporadically. The mineral content of the peat increases sharply in the lowermost part of the zone. Few macrofossil types are present and smooth monocotyledon rootlets dominate. Unidentified organic matter peaks at 48% in the middle of the zone. Smooth monocotyledon rootlets dominate and Moliniopsis japonica leaves account for 15e30% of the assemblage. Sphagnum palustre reappears in the lower half of the zone before disappearing from the record at 152 cm depth. Sphagna disappear to be replaced by Carex spp. rootlets and leaves. Small quantities of Moliniopsis japonica occur in the upper part of the zone together with smooth monocotyledon rootlets. Wood/woody roots decline to <10% at the lower zone boundary. Sphagnum palustre increases and peaks at 55%. Sphagnum magellanicum appears in the record for the first time at the upper zone boundary. Smooth monocotyledon rootlets increase through the zone to a peak of 65% at the upper boundary. Highly humified wood fragments occur throughout the zone, peaking at 50%. Woody roots form 10e25% of the assemblage. Carex spp. roots and smooth monocotyledon roots are consistently present with a trace of Sphagnum palustre. Phragmites australis leaves and abundant smooth monocotyledon rootlets with some humified wood fragments.

The first significant tephra deposition event (188.5 cm depth) immediately overlies the poor fen levels. The occurrence of the tephra (Tephra UTS-6; Fig. S1) is accompanied by major shifts in both the plant and testate amoebae assemblages. Sphagnum is replaced by remains of Carex spp. leaves/roots which are found together with Juncus spp. seeds. Simultaneously, the previously dominant testate amoeba, Hyalosphenia subflava declines sharply, whilst the abundance of Cryptodifflugia oviformis increases. This pattern of dominance by C. oviformis is also apparent during the deposition and reworking of the B-Tm tephra at 118e134 cm depth and during the deposition of the Ko-d tephra and Ko-c2 tephras at 57e46 cm and 40e38 cm depth, respectively. Macrofossil Zone UTSm-G (Testate amoebae Zone UTSt-F) represents a phase of reduced tephra deposition. In this zone S. magellanicum is the dominant peat building species and it is found together with a range of other oligotrophic species typical of raised bog communities, including several members of the Ericaceae family and Eriophorum vaginatum. The zone is also notable for the increased diversity of testate amoebae species present when compared with preceding zones containing tephra deposition (UTSt-C, UTSt-E). Arcella catinus, Difflugia pulex, Heleopera sylvatica, Hyalosphenia elegans and Hyalosphenia papilio all contribute to the testate amoebae assemblage, indicating a dry mid-hummock microtope.

Table 4 Testate amoebae sub-zones for core UTS-06, 0e240 cm. Testate amoebae zone

Depth (cm)

UTSt-H

0e26

UTSt-G

26e58

UTSt-F

118e138

UTSt-E

118e138

UTSt-D

138e170

UTSt-C

170e186

UTSt-B

186e234

UTSt-A

234e240

Zone description

Testate amoebae diversity increases substantially in the uppermost zone where a range of Nebela and Euglypha species are accompanied by Heleopera sylvatica, Assulina muscorum and Corythion e Trinema type. Cryptodifflugia oviformis increases to 48% of the assemblage at the lower zone boundary. Overlying this level is a tephra layer with few tests. Cryptodifflugia oviformis again forms much of the testate amoebae assemblage above the tephra layer together with Heleopera sylvatica. A diversity of testate amoebae is present throughout the zone. In the lower levels Hyalosphenia subflava dominates, whereas Difflugia pulex is most abundant in the mid zone, followed by peaks in Hyalosphenia papilio, Hyalosphenia elegans and Heleopera sylvatica in the upper levels of the zone. Testate amoebae diversity decreases and the assemblage is almost completely dominated by Cryptodifflugia oviformis. Difflugia pulex replaces Cryptodifflugia oviformis early in the zone. Hyalosphenia subflava increases through the zone and it is joined by small amounts of Arcella catinus-type, Heleopera sylvatica and Trigonopyxis arcula-type. Hyalosphenia subflava declines through the zone to <10%, whereas Cryptodifflugia oviformis increases, peaking at >90% at the upper zone boundary. Diversity is very low. A marked change to dominance by Hyalosphenia subflava (60e90%). Arcella catinus-type peaks in the middle of the zone followed by Assulina muscorum. Low testate amoebae diversity. Difflugia pulex dominates and Hyalosphenia subflava occurs at 30e45%.

In zone UTSt-G an 11 cm deep visible tephra layer (UTS-3, Kod Tephra; Fig. S1) is present in the peat stratigraphy from 57 cm to 46 cm depth. Within the tephra layer very few testate amoebae are found and the plant macrofossils are mainly of S. magellanicum (top of UTSm-G), suggesting that the eruption deposited onto an actively accumulating bog surface. In the plant macrofossil zone immediately above Ko-d (UTSmH) there is a major change in the assemblage. S. magellanicum is completely replaced by Cyperaceae species including Moliniopsis japonica and Eriophorum vaginatum with traces of S. papillosum. The latter species increases through the zone and becomes dominant in the uppermost macrofossil zone despite the occurrence of another tephra layer (Ko-c1) at 20e24 cm depth. 5.2. Total organic carbon and major plant nutrients (N and P) The analytical results for TOC, N and P are shown in Fig. 5aec, respectively. The TOC content of the peat varies between 40 and 50 wt% and shows little overall trend through the first 2000 years of the record. A low TOC value is recorded in the basal peat sample because of the deposition of silts and clays during early peat formation. However, the lowest TOC values occur during the deposition of the major tephras in AD 969 and again in AD 1640 and during the tephra reworking phases that followed the AD 1640 eruption. By contrast, peak TOC values >50 wt% occur in the S. magellanicum-dominated strata that accumulated in the centuries leading up to the AD 1640 eruption. In the early part of the UTS-06 record nitrogen values peak at 1.4 wt% and subsequently decline to a low value of 0.6 wt% at

%) ent (

t ion

ral c Min e

T es

t co

nce

ont

ntra

. spp Depth (cm)

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240

UTSt-H

UTSt-G

UTSt-F

UTSt-E

UTSt-D

UTSt-C

UTSt-B

P.D.M. Hughes et al. / Quaternary Science Reviews 67 (2013) 160e175

Zone

UTSt-A 20

20

20

50

100

150

20

20

40

60

20

40

20

20

20

40

20

20

40

20

40

60

80 100

20

40

20

20

20

500 1000 1500

20

40

60

Fig. 4. Testate amoebae profile for core UTS-06. Taxa are presented as percentages of the total assemblage, together with test concentration and percentage mineral content of the sample. Shaded grey bars indicate air-fall tephras.

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a.

b.

c.

UTS-2 UTS-1 UTS-3

UTS-5 UTS-4

UTS-6

Fig. 5. The UTS-06 record of change in peat Total Organic Carbon (a), Nitrogen (b) and Phosphorous (c) over the last 3000 years cal. BP. Vertical bars indicate air-fall tephras.

2500 cal. BP, before recovering to vary within the range 0.75e1.2 wt % through the middle of record until 700 cal. BP. After this time generally lower values are recorded (0.2e0.8 wt%) with two short-lived peaks of 1.1e1.2 wt% just before and after the Kod tephra. Values of P register a modest rising trend through the record until 500 cal. BP, with some minor reversals. After this time the trend in peat P content increases sharply and variability between samples increases. P peaks at 0.19 wt% immediately after the deposition of the Ko-d tephra. 6. Discussion 6.1. Mire vegetation and peat decomposition changes following tephra deposition Previous palaeoecological studies of the impacts of volcanic ash deposition on mires note few discernible patterns in vegetation response (e.g. Charman et al., 1995; Hotes et al., 2001, 2006). Hotes et al. (2001) tabulated mire vegetation changes across tephra horizons from 33 cores sampled in Kiritappu mire, northern Hokkaido, and found that sometimes there were examples of vegetation change from apparently more eutrophic to oligotrophic conditions and vice versa. Likewise, there were both wetting and drying trends, as well examples of no apparent change in

vegetation. Similarly, Payne and Blackford (2008) found variation in vegetation responses following tephra loading in Alaskan peatlands, with some tephras causing little apparent impact whilst others were associated with vegetation change; these differences were tentatively attributed to season-dependent variations in the response of vegetation to volcanic impacts or to differences in prevailing meteorological conditions. At Utasai Bog there is also a variable pattern of response to the six tephra deposition events and several phases of reworking; however, detailed analyses of the peat stratigraphy using multiple lines of evidence give important insights into the impacts of each tephra on the mire system. Analyses of sub-fossil plant assemblages and testate amoebae to genus and species level, together with the record of carbon accumulation, show that there are distinctive changes in mire functioning across all of the tephra deposition events that have occurred over the last 3000 years (see Figs. 3e6), although not all are accompanied by clear changes of peat type (Fig. 3). Deposition of the UTS-6 tephra at 188.5 cm depth is associated with the displacement of the first S. magellanicum/S. papillosum community to be recorded at the core site, by Carex species. This event is also associated with increased peat humification as indicated by a major increase in U.O.M. The UTS-6 tephra was deposited when the bog was on the cusp of the fenebog transition. The change from S. palustre to S. magellanicum in Zone UTSm-C of the macrofossil record (Fig. 3) suggests that the bog was following a typical seral pathway from a mesotrophic fen with a pH of c. 5.5 (Daniels and Eddy, 1990) to a more oligotrophic community before the ash fall. The alteration of the succession to a more eutrophic Carex community following tephra deposition (early in Zone UTSmD) accords with the modern observations of Wolejko and Ito (1986), Shoji et al. (1966), Tachibana and Sato (1983) and Damman (1988). These authors suggest that the eutrophication response may be a result of tephras leaching nutrients and minerals into surrounding mire waters. Indeed Wolejko and Ito (1986) coin the term ‘tephratrophic mire’ to describe a type of mire resembling ‘Hochmoor’ (raised bog), with vegetation communities more typical of fens, that have developed under the influence of nutrient and mineral loading from tephras. Hotes et al. (2010) have shown in experimental treatments that nutrient levels, pH and conductivity all increase in bog waters after tephra applications and De Vleeschouwer et al. (2008) also report evidence of leaching from naturally-deposited tephras in Icelandic peats. However, the clear response of the mire vegetation to the relatively modest input of the UTS-6 tephra contrasts with other palaeoecological records of long-term vegetation change following a similar scale of deposition (e.g. Charman et al., 1995; Hotes et al., 2001). The response recorded in the present study may have occurred because the mire was in the process of crossing a critical threshold in the hydrosere, namely the transition from fen to bog. At this point the mire vegetation may have been particularly vulnerable to disturbance from nutrient enrichment. The re-establishment of ‘fen-like’ communities does not necessarily imply a reversal to groundwater-fed conditions, however, since the tephra deposition was an air-fall event and the bog most probably remained ombrogenous. In the years between the UTS-6/UTS-5 tephras and the Baitoushan eruption (B-Tm tephra (UTS-4)) in AD 969 the mire vegetation at the UTS-06 core site had passed back through a brief S. palustre phase and had developed into a phanerogam community with Moliniopsis japonica. The eruption, although 1000 km distant from the study site, was a large one (Volcanic Explosivity Index (VEI) ¼ 7; Global Volcanism Program, Smithsonian Institution, 2012) and deposited a c. 4 cm thick air-fall tephra (with later transport and reworking increasing that depth to 16 cm at the core site, Fig. S1). In the macrofossil record this disturbance is followed

P.D.M. Hughes et al. / Quaternary Science Reviews 67 (2013) 160e175

Age (calibrated years BP) 0

500

1000

1500

2000

2500

3000

100

% mineral content

80

a.

60 40 20 1.2 1.0

b.

0.8 0.6 0.4 0.2

100

0.0

C accumulation (mg cm -2 yr -1 )

0

90

C : N ratio

80

c.

70 60 50 40

50

30 20

40 30 20 N : P > 16 = P limited N : P < 14 = N limited

Testate amoebae DCA Axis 1 sample scores

4 3

N : P ratio

d.

10 0

e.

2 1 0 UTS-2 UTS-1

0

UTS-3

500

UTS-4

1000

UTS-5 -

1500

UTS-6

2000

2500

3000

Age (calibrated years BP) Fig. 6. Data synthesis for core UTS-06 showing change in %mineral content (a), Carbon accumulation (b), C:N ratio (c), N:P ratio [N and P-limiting thresholds follow Koerselman and Meuleman, 1996] (d) and testate amoebae Detrended Correspondence Analysis (DCA) Axis 1 samples scores (e). Low DCA scores show mineral-enriched conditions. Vertical bars indicate air-fall tephras.

by the disappearance of identifiable Moliniopsis remains, and greatly increased peat humification, as shown by a peak in U.O.M. to 50% of remains. Following the eruption, the ageedepth model (Fig. 2) shows that the rate of peat accumulation slowed sharply, possibly representing a hiatus in peat formation and this is also visible as a deceleration in the apparent carbon accumulation record (Fig. 6b). The combination of species level identifications in the macrofossil record with peat carbon accumulation data clearly reveal the major impact of the eruption on the bog; however, this impact would be much less obvious if pollen or low-taxonomic-

171

resolution macrofossil records had been used since monocotyledon species dominated before and after the eruption. Nevertheless, the precise nature of the changes to the vegetation caused by the Baitoushan eruption are hard to gauge because there may be up to 500 years of low peat accumulation, with the possibility of a hiatus in the record after the tephra deposition, and the preservation of the sub-fossil plant material is poor. Similar hiatuses in peat accumulation may account for some of the apparent variability in vegetation responses noted in previous palaeoecological studies. At the time of the Ko-d tephra the bog had developed to a wellestablished oligotrophic community dominated by S. magellanicum and was most likely ombrotrophic. Here the sensitivity of S. magellanicum to tephra impacts seems to be demonstrated again. Following tephra deposition S. magellanicum was replaced by a brief phase of monocotyledon domination with Moliniopsis japonica, Eriophorum vaginatum and some Carex species.; however, it was S. papillosum that established at the core site after the ash deposition rather than re-establishment by S. magellanicum. One explanation for the replacement of S. magellanicum by S. papillosum may be that the latter species has a greater tolerance of very high nutrient loading. Daniels and Eddy (1990) and Anderson et al. (1995) show that S. papillosum has a wider tolerance to eutrophic conditions than S. magellanicum. Equally, S. papillosum may be the more resistant of the two species to damage from the blocking of pores and smothering by ash and to toxic metals. Hotes et al. (2004) recorded the ratios of S. magellanicum to S. papillosum on experimental plots two years after treatments with natural tephras and ground glass. The ratios of 9:4, 8:9, 10:9 show that both species were still present in the experimental plots after two years and there was no consistent pattern of dominance by one species. However, the experiments may need to replicate natural conditions more closely over a longer timescale to fully characterise the response of these two species to tephra loading. Nevertheless, the resilience of S. papillosum to tephra deposition is clearly demonstrated in the UTS-06 core at 20e24 cm depth with the deposition of the Ko-c1 tephra (UTS-1). This ash layer seems to have presented little or no barrier to the expansion of S. papillosum at the UTSm-H/I zone boundary (see Fig. 3). At the time of the Ko-c1 tephra S. papillosum had already colonised the core site and it seems to have been capable of growing through the 3e4 cm of ash deposition. Hotes et al. (2004, 2010) noted in their experimental plots that S. papillosum rapidly grew through tephra/glass applications by producing spindly shoots. In this instance, whilst there appears to be species change across the Ko-c1 tephra horizon, S. papillosum may well have risen to dominance anyway since it started increasing before the tephra deposition. The direct long-term observations of vegetation community change afforded by species-level palaeoecological analyses help to explain some of the variable responses to tephra deposition noted in previous studies. However, the detailed focus makes extensive studies difficult to undertake. Further analyses will be needed to establish how representative the changes noted in UTS-06 are of raised mires more generally, since Hotes et al., 2004 show very clearly that there is considerable variety in vegetation succession following tephra deposition, partly arising from tephra impacts but also from stochastic processes (Hotes et al., 2010). 6.2. The response of Cryptodifflugia to tephra layers Fig. 4 shows an apparent link between the abundance of Cryptodifflugia oviformis and the amount of tephra found in the peat matrix. During periods of substantial tephra deposition through UTS-1, UTS-2, UTS-3, UTS-4, UTS-5 and UTS-6, percentage abundances of C. oviformis increase and return to negligible levels after

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the tephras are gone. These changes in abundance may relate to food availability. Some testate amoebae species feed on particulate organic matter, fungi, cyanobacteria, plant cells, and other microorganisms (Gilbert et al., 2000), although many largely feed on bacteria while some species such as Difflugia oviformis, are obligate bacterivores (Gilbert et al., 2000). De Vleeschouwer et al. (2008) demonstrated that pores in tephra shards deposited in peat are confined spaces for bacterial growth that may be fed by the leaching of nutrients from the tephra. These bacterial communities can form films around the individual shards and provide food for bacterivore testate amoebae. Furthermore, volcanic sulphates, which have been shown to be positively linked to the abundance of C. oviformis (Payne et al., 2010), may increase the quantity of sulphate reducing bacteria. Therefore, the positive response of C. oviformis during these phases of increased availability of bacterial prey may suggest that this species is a bacterivore, although this hypothesis remains to be tested. 6.3. Initial impacts of tephra deposition on peatland carbon accumulation The datasets presented in this study suggest that tephra loading of mire vegetation can trigger major changes in peatland C dynamics (Fig. 6), most likely mediated through the physical effects of lying ash and through nutrient-induced changes in plant and microbial communities. The larger episodes of tephra deposition are associated with subsequent decelerations in the apparent rate of C accumulation (Fig. 6b). For example, after the Baitoushan tephra was deposited in AD 969 carbon accumulation fell from 25 g C m
2 yr
1 to <10 g C m
2 yr
1 (Fig. 6b). This change represents a shift in the balance between net primary production (NPP) in the mire community and annual litter decay. Both sides of this equation may be impacted by tephra deposition. The ash may supply plant nutrients (especially P and K; De Vleeschouwer et al., 2008) promoting NPP; however, the plant macrofossil assemblages (Fig. 3) suggest this effect is more than offset in the Utasai record by the change to a sedge-dominated community, accompanied by enhanced decay processes e U.O.M. is elevated in all of the macrofossil zones that supersede ash-fall events and phases of ash reworking. This enhanced peat decay could be a response to a number of interlinked processes including: (1) thermal effects on the peat surface/subsurface caused by changed vegetation structure and mire surface specific heat capacity; (2) enhanced microbial activity following fertilisation; (3) altered evapo-transpiration after plant community change and (4) changes in litter quality. The previous discussion of the plant communities has shown that tephra deposition can shift the competitive advantage in favour of monocotyledons (in this case, Moliniopsis and Carex species); an effect that is also noted following the fertilisation of oligotrophic bogs by aerial deposition of pollutants, including eroded soil dust (e.g. Chambers et al., 2007; Ireland and Booth, 2012). This plant community shift would be likely to have a drying effect on the bog rooting zone (Wendel et al., 2011) through the development of more extensive vascular root systems and enhanced evapotranspiration. Wendel et al. (2011) also found that increased monocotyledon growth following fertiliser applications resulted in increased peat temperatures at depth because of more efficient thermal conductivity in drier peat. Both of these fertilisation feedbacks would promote peat decay. Added to these effects, a blanketing layer of tephra over the peat surface may have promoted thermal conductance by reducing the specific heat capacity of the mire surface. Litter quality could also play an important part in the observed differences in C accumulation before and after tephra deposition. Cellulose decomposition rates tend to be significantly higher in

phanerogam-dominated, ‘fen-like’ communities than in Sphagnum-dominated peatlands (Verhoeven et al., 1990). Many studies (e.g. Aerts et al., 1999; Aerts et al., 2001; Scheffer et al., 2001) have shown that nutrient availability is an important factor in promoting litter decay. Indeed Scheffer et al. (2001) have suggested that nutrient availability and microbial community adaptation to nutritional and other environmental conditions may be the main regulators of carbon and nutrient cycles in the peatlands that they studied. Sub-fossil testate amoebae assemblages can be used as a proxy for the study of microbial community change (Payne, 2012). The major shifts in testate amoebae assemblages, noted in Section 6.2, suggest that the broader microbial community re-organised in response to each ash fall. Microbial activity tends to be enhanced with fertilisation (Ireland and Booth, 2012) and might be expected to increase decay. Counterbalancing this contention, some modern fertilisation studies show that a more efficient microbial community might accelerate C uptake; however, this effect is thought to be short-lived and may be offset by the production of more readily decomposable plant litter (Basiliko et al., 2006). 6.4. Recovery of C accumulation following heavy tephra deposition The major acceleration in apparent C accumulation from 10 g m
2 yr
1 (see Fig. S1) following the Baitoushan eruption to a peak of >100 g m
2 yr
1 at c. 500 cal. BP (Fig. 6b) is a striking feature of the UTS-06 record and quite unlike the C accumulation records of undisturbed northern peatlands (e.g. Gorham, 1991; Tolonen et al., 1992; Mäkilä, 1997). Gorham (1991) reports average C accumulation rates of 34.3 g C m
2 yr
1 for Sphagnum peat and 29 g C m
2 yr
1 for Carex peat in northern peatlands, whereas the equivalent values for the UTS-06 record are 27 g C m
2 yr
1 for sedge peat under minimal tephra loading, between 9 g and 25 g C m
2 yr
1 for the same peat type after heavy tephra loading and 67 g C m
2 yr
1 for Sphagnum-dominated peat. The apparent increase in carbon accumulation is driven by both more rapid peat accumulation (Fig. 2) and increasing carbon content of the peat (Fig. 5a); however, further radiocarbon dating would be required to characterise the exact timing of the shift. Even then uncertainties in the age models propagate to the C accumulation calculations; therefore, changes in C accumulation are interpreted at multidecadal to centennial timescales. The C accumulation peak in the UTS-06 record is co-incident with the main period of S. magellanicum dominance on the mire (Fig. 3, Zone UTSm-G) and follows a phase of low C accumulation that occurred immediately after the deposition of the Baitoushan tephra and through the subsequent phase of tephra reworking. Since nutrient cycling is most likely to be a key regulator of C accumulation (Aerts et al., 2001), explanations for the observed acceleration in C accumulation may be developed by exploring the role of nutrient limitation in peat bog functioning. Ombrotrophic peatlands are often considered to be N-limited; however, the literature contains examples of both N-limitation (e.g. Bragazza et al., 2004) and P-limitation (e.g. Malmer and Wallen, 2005) in these peatlands. Complicating this picture further, both inter-specific differences in N and P limitation as well as intra-specific differences between peatland sites have been identified from modern studies (Koerselman and Meuleman, 1996). However, both continental and global-scale reviews of macro nutrient limitation have shown that N and P limitation may be co-limiting in a wide range of freshwater, terrestrial and marine ecosystems (Bedford et al., 1999; Elser et al., 2007) and this pattern accords with the findings of Hotes (2004) for Sarobetsu mire in Hokkaido. The apparent complexity of the modern evidence highlights the difficulty of interpreting past changes in peatland macro-nutrients, nevertheless an

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assessment of the interactions between tephras, nutrient cycling and carbon accumulation is required. Ratios of macro-nutrients provide diagnostic tools for the study of ecosystem functioning (Koerselman and Meuleman, 1996). C:N and N:P ratios are reported in Fig. 6c and d to explore nutrient dynamics. At Utasai Bog the C:N ratio is principally driven by shifts in plant community composition from monocotyledon to Sphagnum domination with the exception of basal assemblages where the ratio also reflects changes in shrub species (see Figs. 3 and 6c). The N:P ratio shows a declining trend from over 30:1 at the core base to 3:1 near the bog surface. Koerselman and Meuleman (1996) suggest that N:P ratios > 16 represent P limitation, whereas values < 14 represent N-limitation (N:P values between 14 and 16 represent either N or P limitation or co-limitation) although these thresholds are uncertain (Bragazza et al., 2004 estimate that bogs in Europe change from N-limitation to K and P co-limitation at an N:P ratio <30). The thresholds set by Koerselman and Meuleman (1996) imply that Utasai began as a P - limited system (see Fig. 6d) and that after c. 600 cal. BP Utasai became N-limited (or more strongly Nlimited, following Bragazza et al., 2004). This change is significant because it co-insides with the rapid acceleration in C accumulation (Fig. 6b). The evidence presented above suggests that once heavy tephra deposition and reworking subsided after the AD 969 Baitoushan eruption, succession towards oligotrophic S. magellanicum bog was associated with a major change in nutrient limitation. With the development of oligotrophic Sphagnum carpets low redox potentials occur, inhibiting both mineralisation and microbial incorporation of P (Wilson and Fitter, 1984). Normally these conditions would result in low P concentrations; however, with leaching of K and P from the underlying Baitoushan tephra and a lack of microbial immobilisation of P to organic forms, both K and inorganic P would have accumulated while N declined through efficient uptake by Sphagna - see Fig. 5b and c. Where N is low relative to P and K, Sphagnum litter decay may be very slow and this has the potential to have a major affect on carbon accumulation (Hogg et al., 1994; Aerts et al., 2001). The high rate of peat accumulation in the S. magellanicum community at Utasai may be also partly caused by rapid Sphagnum growth since the genus can respond strongly to K and P fertilisation (e.g. Limpens et al., 2004) and it can out compete vascular plants in low N conditions (Malmer et al., 2003). However, the response of Sphagnum to experimental fertiliser application is strongest in regions with a high atmospheric nitrogen load and may be rather less pronounced or absent where N loads are low (e.g. Aerts et al., 1992). In summary, the interactions between K and P-bearing tephras and Sphagnum most likely had a radical impact on the Utasai peatland carbon balance in favour of greatly accelerated C accumulation. This effect lasted for at least 300 years, and with an average C accumulation rate of 67 g C m
2 yr
1, it offset the c. 400year phase of low C accumulation (9 g C m
2 yr
1 - down from a pre-eruption average of 27 g C m
2 yr
1) that followed the B-Tm tephra. These carbon accumulation rates suggest that over multidecadal to multi-centennial timescales the indirect effects of the B-Tm tephra had a substantial net positive impact on C accumulation that ended only when S. magellanicum communities were smothered by the heavy Ko-d tephra fall of AD 1640. The uppermost macrofossil assemblage (UTSm-I) records the recent recovery of the Sphagnum community after a phase of monocotyledon domination caused by the deposition of the Ko-d, Ko-c2 and Ko-c1 tephras; however, since Sphagnum recovery is very recent and the uppermost strata represent the unconsolidated acrotelm, it is not yet possible to discern whether a second sustained burst of carbon accumulation will follow these recent tephra depositions at Utasai.

173

7. Conclusions Long-distance transport of tephras from high magnitude volcanic eruptions such as Baitoushan (AD 969) can cause major changes in peatland vegetation communities and C accumulation. Moderate to high tephra loading can shift the competitive balance of mire communities in favour of monocotyledons such as Moliniopsis and Carex species. This may be a fertilisation effect caused by enhanced mineralisation of peat and/or liberation of plant nutrients such as P and K from the surface coatings of tephras and from leaching of the volcanic glass. Mire sensitivity to tephra loading may vary dependent upon successional stage. At Utasai modest tephra loading at the fenebog transition is associated with a change of succession towards more eutrophic ‘fen-like’ conditions, although the mire may have remained ombrogenous. Later heavier deposition onto S. papillosum shows continuity of peat accumulation and no reversal of succession. S. papillosum appears to be tolerant of tephra loading and associated nutrient enrichment, whereas S. magellanicum declines even after modest loading. Species-level identifications of plant macrofossils are therefore critical to understand the impact of tephras on past vegetation communities. The immediate impact of moderate to heavy tephra deposition on peatland carbon accumulation is negative. Where heavy tephra deposition occurs for a sustained period because of reworking and re-deposition (e.g. after deposition of the B-Tm tephra at Utasai) the rate of apparent carbon accumulation can be halved for as a long as there is high tephra delivery to the mire surface. The re-establishment of ombrotrophic Sphagna after tephra deposition can lead to greatly accelerated carbon accumulation. At Utasai S. magellanicum domination is associated with a change in peatland nutrient cycling towards N-limitation. Where N becomes less available relative to P and K the accumulation of decay resistant Sphagnum litter is strongly promoted and this most probably affected the carbon balance of Utasai Bog. Also, fertilisation effects of tephra leachates on Sphagnum growth rates may be partly responsible for promoting high rates of C accumulation. The Utasai record shows that C accumulation in tephra-impacted Sphagnum communities can be 2e3 times faster than the average for northern peatlands, more than offsetting the initial impact of volcanic products on C storage over multi-centennial timescales. The findings reported here suggest that management of tephraimpacted peatlands for Sphagnum could be a highly successful strategy for mitigating the effects of tephra deposition on peatland carbon stores. Sphagnum cover is most likely to be re-established where pre-existing water table management favoured nearsurface water levels and where tephra re-working is minimised. This study also indicates that a consideration of the impact of tephras on peatland nutrient cycling and vegetation community dynamics will be essential for the development of more realistic regional carbon balance models for volcanically active regions. Acknowledgements The authors thank the Natural Environment Research Council (NERC) for funding support under Grant NE/D006899/1 and via the NERC TAU microprobe facility. David Steele and Chris Hayward are thanked for their assistance and advice in the use of the NERC microprobe at Edinburgh University. We thank Kaori Aoki for assistance in sourcing Yumi Tokui’s MSc. thesis and Dmitri Mauquoy is thanked for his help with the ageedepth modelling. We are grateful to Ryuta Furukawa for supplying reference glass chemistry data and to Peter Morgan and Anne Stringfellow for assistance with laboratory sample preparation for TOC analysis. We also thank the University of Southampton Cartographic Unit for the preparation of

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