Tephra Shards Research Articles

Geochemical analysis of extremely fine-grained cryptotephra: New developments and recommended practices

Tephrochronology is a powerful tool used to synchronise and date stratigraphic records by accurate and precise geochemical analysis of deposited volcanic glass shards. However, in many distal stratigraphic records (e.g., polar ice cores) tephra shards are often extremely fine-grained (<10 μm). Geochemical characterisation of these shards is challenging because conventional preparation and analytical techniques require highly polished glass areas >5 μm for electron probe microanalysis (EPMA) to ensure high analytical totals and minimise alkali element loss. Recent method developments have put forward alternative approaches to accurately measure major oxides of small shards: a smaller 3 μm diameter beam, overlapping large (20 μm) beam areas onto supporting epoxy resin, and using scanning electron microscopy with energy dispersive spectrometry (SEM-EDS). However, there has been no direct intercomparison of these alternative techniques, which to date have only been tested on a limited range of glass compositions and tephras that are much larger than the extremely fine-grained material found in distal archives. These issues complicate decision making about the best analytical approach to take when faced with small shards. Here, we provide a new workflow protocol for the analysis of <10 μm tephra by determining the accuracy and precision of alternative SEM-EPMA methods. By analysing a variety of glass standards including those prepared to replicate fine-grained ice-core cryptotephras, we show that a 3 μm EPMA beam is suitable for use on all glass compositions provided the beam current is reduced to 1 nA. When glass areas are too small for a 3 μm beam we show that overlapping this small beam onto epoxy resin is preferable to SEM-EDS analysis. We also provide evidence confirming that using 3–0.2 μm polishes for <5 min increases analytical precision of the most abundant major oxides by up to three times, whilst, crucially, preserving the smallest shards in a sample. By directly applying these alternative methods to ice-core cryptotephra, we demonstrate the data are of suitable accuracy and precision to make robust geochemical correlations. This workflow can be applied to future tephrochronology studies, significantly increasing the quality and quantity of data that are obtained from cryptotephra horizons in distal records.

Detection of ice core particles via deep neural networks

Abstract. Insoluble particles in ice cores record signatures of past climate parameters like vegetation dynamics, volcanic activity, and aridity. For some of them, the analytical detection relies on intensive bench microscopy investigation and requires dedicated sample preparation steps. Both are laborious, require in-depth knowledge, and often restrict sampling strategies. To help overcome these limitations, we present a framework based on flow imaging microscopy coupled to a deep neural network for autonomous image classification of ice core particles. We train the network to classify seven commonly found classes, namely mineral dust, felsic and mafic (basaltic) volcanic ash grains (tephra), three species of pollen (Corylus avellana, Quercus robur, Quercus suber), and contamination particles that may be introduced onto the ice core surface during core handling operations. The trained network achieves 96.8 % classification accuracy at test time. We present the system's potential and its limitations with respect to the detection of mineral dust, pollen grains, and tephra shards, using both controlled materials and real ice core samples. The methodology requires little sample material, is non-destructive, fully reproducible, and does not require any sample preparation procedures. The presented framework can bolster research in the field by cutting down processing time, supporting human-operated microscopy, and further unlocking the paleoclimate potential of ice core records by providing the opportunity to identify an array of ice core particles. Suggestions for an improved system to be deployed within a continuous flow analysis workflow are also presented.

Volcanoes in the estuaries: Insights into Earth’s oldest (3.22 Ga) terrestrial microbial habitats, Moodies Group, Barberton Greenstone Belt

The Moodies Group of the Paleoarchean Barberton Greenstone Belt (BGB; ca. 3.22 Ga) provides an exceptional window into Archaean sedimentary and magmatic processes interacting with microbial ecology. Its magmatic component, the Moodies Igneous Complex (MIC), has to date been largely overlooked. The MIC consists of several mafic-to-intermediate sills, peperitic dike stockworks, extensive basaltic lava flows, and various volcaniclastic deposits and tuffs, all of Moodies depositional age. Excellent outcrop exposures illustrate diverse and dynamic interactions between volcanic activities and shallow-water sedimentary processes; they constrain the conditions in which microbial communities thrived in estuarine and tidal environments: (1) When the Moodies Basaltic Lava flooded the basin, some lava fragments and pillows were reworked as boulder-sized conglomerate clasts and in gravelly sandstones while still plastic and presumably hot. (2) Aerodynamically shaped ejecta of mafic lava and common tephra shards in cross-bedded sandstones demonstrate explosive lava-water interactions. (3) Dacitic (lapilli) tuffs up to several m thick were eroded by estuarine channels and filled back-beach lagoons, modifying and smothering concurrent aqueous transport of epiclastic sediment. (4) The paleo-groundwater table and the MIC interacted vigorously by in situ fluidization of poorly to unconsolidated sediment. This led to entrainment of grains and sandstone xenoliths in mafic (sub-)volcanics, widespread peperite formation, and formation of lava-filled fractures and abundant quartz veins in surficially consolidated sandstones.The proximity of numerous shallow intrusives, lavas, and volcanic tephra to the tidal-zone habitat of abundant benthic microbial mats provided microorganisms with readily accessible thermal and chemical energy. Heavy mineral lags in the estuaries, combined with potentially aggressive weathering of felsic to ultramafic volcanic rock, released ample nutrients as colloidally-bound transition metals and phosphorus. Drying-and-wetting cycles on tidal flats and coastal plains, the high porosity and permeability of the well-sorted quartzose sand in shallow water, the variably energetic current regime, and the anoxic, reducing-to-oxidizing chemistry of these habitats also benefitted microbial communities greatly.

Effects of Holocene climate change, volcanism and mass migration on the ecosystem of a small, dry island (Brava, Cabo Verde)

AbstractAimPalaeoecological data provide an essential long‐term perspective of ecological change and its drivers in oceanic islands. However, analysing the effects of multi‐scalar and potentially co‐occurring disturbances is particularly challenging in dry islands. Here, we aim to identify the ecological consequences of the integrated impacts of a regional drying trend, volcanic eruptions and human mass migrations in a spatially constrained environment—a small, dry oceanic island in Macaronesia.LocationBrava Island, Republic of Cabo Verde.TaxaTerrestrial vegetation and fungi.MethodsWe use palaeoecological analyses applied to a caldera soil profile that dates back to 9700 cal yr BP (calibrated years before the present). Analyses include pollen (vegetation history), non‐pollen palynomorphs (changes in fern and fungal communities), grain‐size distribution, loss‐on‐ignition and geochemistry (sedimentology and erosion regimes), microscopic tephra shards (volcanic ash deposition) and charcoal (fire regime).ResultsA regional drying trend after c. 4000 cal yr BP caused increased erosion but had limited immediate impacts on highland grassland vegetation. The expansion of fern‐rich woody scrubland was contemporaneous with significant deposition of volcanic ash and erosion between 1800 and 650 cal yr BP. About 300 cal yr BP, exogenous plants expanded, grazing and fires increased, and there was a decrease of native vegetation cover.Main conclusionsThroughout the Holocene, highland vegetation in Brava was characterized by the presence of open landscapes dominated by herbaceous species (e.g. Poaceae, Forsskaolea), with some presence of woody native taxa (e.g. Ficus, Dodonaea). A regional drying trend was a driver of erosion since the Mid Holocene but did not have an immediate influence on highland vegetation. Tephra deposition is a possible driver of vegetation change. Inter‐island mass migration after volcanic events in Fogo Island c. 1680 CE potentially triggered land use change and intensification, causing a reduction of native vegetation in Brava.

Slow Ascent of Unusually Hot Intermediate Magmas Triggering Strombolian to Sub-Plinian Eruptions

AbstractTo assess whether magma ascent rates control the style of volcanic eruption, we have studied the petrography, geochemistry and size distribution of microlites of plagioclase and pyroxene from historical eruptions from Tongariro, Ruapehu and Ngauruhoe volcanoes located in the southern Taupo Volcanic Zone, New Zealand. The studied deposits represent glassy andesitic and dacitic tephra shards from the Mangamate, Mangatawai, Tufa Trig and Ngauruhoe tephra formations, ranging in age from 11 000 years bp to ad 1996. Covering a range in eruption styles and sizes from Strombolian to Plinian, these samples provide an excellent opportunity to explore fundamental volcanic processes such as pre-eruptive magma ascent processes. Our quantitative petrographic analysis shows that larger microlites (&amp;gt;30 µm) display complex growth zoning, and only the smallest crystals (&amp;lt;30 µm) have formed during magma ascent in the conduit. Using a combination of orthopyroxene geothermometry, plagioclase hygrometry, and MELTS modelling, we show that these microlites nucleated at maximum pressures of 550 MPa (c. 16·5 km) from hot andesitic magmas (1010–1130 °C) with low H2O content (0–1·5 wt%). Size distributions of a total of &amp;gt;60 000 microlites, involving 22 tephras and 99 glass shards, yield concave-up curves, and the slopes of the pyroxene microlite size distributions, in combination with well-constrained orthopyroxene crystal growth rates from one studied tephra, indicate microlite population growth times of ∼3 ± 1 days, irrespective of eruption style. These data imply that microlites form in response to cooling of melts ascending at velocities of &amp;lt;5 cm s–1 prior to H2O exsolution, which occurs only at &amp;lt;33 MPa. Maximum magma ascent rates in the upper conduit, calculated using the exsolution of water during final decompression, range between 3 and 12 m s–1; that is, at least an order of magnitude lower than the hypersonic vent velocities typical of Vulcanian or sub-Plinian eruptions (up to 400 m s–1). This implies that magma ascent from depths of an average of 4 km occurs in dykes, and that vent velocities at the surface are controlled by a reduction of conduit cross-section towards the surface (e.g. dyke changing to cylindrical conduit).

Standard chemical‐based tephra extraction methods significantly alter the geochemistry of volcanic glass shards

ABSTRACTThe chemical compositions of tephra shards are widely utilised in a myriad of disciplines, including volcanology, petrology, tephrochronology, palaeoecology and climate studies. Previous research has raised concerns over the possible chemical alteration of microscopic (&lt;100 µm) volcanic glass shards through standard extraction procedures, such as the widely used acid digestion method. This study subjects 10 samples of well‐characterised volcanic glasses ranging from basalt to rhyolite to three common methods used in the extraction of volcanic material from lake sediments and peats. The major element geochemistry of each sample was analysed and compared with a control group. The results of this test indicate that basaltic and andesitic glasses are highly susceptible to chemical alteration, particularly to the concentrated corrosive materials used in acid and base digestion techniques. PERMANOVA analysis of the variation within groups suggests that the oxides most susceptible to variation are alkalis from groups I and II (K2O, Na2O, CaO, MgO) and SiO2, and the most stable oxides are Al2O3 and FeO. Felsic glasses are considerably less susceptible to alteration by both acidic (HCl, HNO3, H2SO4) and alkaline (KOH) digestions. Our findings have important implications for interpreting the geochemistry of volcanic glasses. Copyright © 2019 John Wiley &amp; Sons, Ltd.

40Ar/39Ar dating of the Thorsmork ignimbrite and Icelandic sub-glacial rhyolites

Robust correlation of paleoclimatic archives (e.g. marine sediments, ice, speleothems) necessitates an increase of the number of independent and accurately dated stratigraphic markers using numerical methods. Icelandic rhyolitic volcanism has a strong chronological interest because these products are found as tephra in the North Atlantic marine archives as well as in Greenland ice cores. In this study, we have dated, using the 40Ar/39Ar method, subglacial rhyolites of the Tindfjallajökull, Kerlingarfjöll and Torfajökull volcanoes which are the main sources of rhyolitic tephra identified in marine and glaciological records.The Thorsmork ignimbrite (Tindfjallajökull) which correlates with the rhyolitic component (II-RHY-1) of the North Atlantic Ash Zone II (NAAZ II) layer is dated at 55.6 ± 2.4 ka, an age that agrees with the ones given by the ice core chronologies (GICC05: 55.4 ± 1.2 ka). The eruptions of Rauðfossafjöll and Fannborg are respectively dated at 77.0 ± 3.0 ka and 149.3 ± 3.4 ka.These new ages along with the published ones confirm, as already concluded by Flude et al. (2010) that there is no clear evidence, at least at the latitude of Iceland, that climate warming is the main mechanism that triggered rhyolitic eruptions.Combining major and trace element compositions with our new age determinations, it appears that the Rauðfossafjöll and the Snaefell rhyolitic eruptions have not yet been discovered in marine sediment cores. Nevertheless, given the accuracy and precision of our dating, these rhyolitic eruptions have the potential to become chronostratigraphic markers, once tephra shards corresponding to these eruptions are discovered in ice and or marine sediment records. The Fannborg eruption might be recorded in the marine core LINK 16 during MIS 6 prior to the transition to MIS5e.

Rhyolitic and dacitic component of the Askja 1875 tephra in southern and central Finland: first step towards a Finnish tephrochronology

ABSTRACTTephra from Icelandic volcanic eruptions is frequently dispersed to northern Europe, but so far tephra has not been detected in Finnish sedimentary records. We report the first finding of a cryptotephra layer in southern and central Finland. Sediment samples from five lakes and 10 peatlands, most of them located in the tephra fallout zone of the Hekla 1947 eruption, were investigated for the presence of cryptotephra to assess the potential for tephrochronology in Finland. Tephra shards were extracted from their host matrix and electron probe microanalysis was conducted on single shards for geochemical characterization. Our results confirm the presence of Askja 1875 tephra in Finland, thus extending the known dispersal area of the Askja 1875 tephra eastwards. Most of the shards are rhyolitic, but a minor concentration of tephra with dacitic composition forms a narrow dispersal zone extending from Sweden to southern Finland. This zone possibly represents the main dispersal axis of the tephra in the distal area. Our results suggest that Finnish environmental research could benefit greatly from adding tephrochronology to the array of dating methods commonly used. Additionally, the absence of Hekla 1947 tephra in the previously inferred fall‐out zone is an important result indicating the complexity of tephra deposition and preservation.

Extending the known distribution of the Vedde Ash into Siberia: occurrence in lake sediments from the Timan Ridge and the Ural Mountains, northern Russia

Tephra shards from the Vedde Ash eruption have been identified in two lakes from northwestern Russia and the Polar Ural Mountains. This is the most distal and easternmost occurrence of this regional tephra marker horizon found so far and it extends the area of the Vedde Ash tephra more than 1700 km further east than previously documented. This means that particles the size of fine sand have travelled more than 4000 km from the Katla volcano source, south Iceland. These findings offer a new possibility to correlate archives over a very long distance in the time period around the Younger Dryas.

The 3.6 ka Aniakchak tephra in the Arctic Ocean: a constraint on the Holocene radiocarbon reservoir age in the Chukchi Sea

Abstract. The caldera-forming eruption of the Aniakchak volcano in the Aleutian Range on the Alaskan Peninsula at 3.6 cal kyr BP was one of the largest Holocene eruptions worldwide. The resulting ash is found as a visible sediment layer in several Alaskan sites and as a cryptotephra on Newfoundland and Greenland. This large geographic distribution, combined with the fact that the eruption is relatively well constrained in time using radiocarbon dating of lake sediments and annual layer counts in ice cores, makes it an excellent stratigraphic marker for dating and correlating mid–late Holocene sediment and paleoclimate records. This study presents the outcome of a targeted search for the Aniakchak tephra in a marine sediment core from the Arctic Ocean, namely Core SWERUS-L2-2-PC1 (2PC), raised from 57 m water depth in Herald Canyon, western Chukchi Sea. High concentrations of tephra shards, with a geochemical signature matching that of Aniakchak ash, were observed across a more than 1.5 m long sediment sequence. Since the primary input of volcanic ash is through atmospheric transport, and assuming that bioturbation can account for mixing up to ca. 10 cm of the marine sediment deposited at the coring site, the broad signal is interpreted as sustained reworking at the sediment source input. The isochron is therefore placed at the base of the sudden increase in tephra concentrations rather than at the maximum concentration. This interpretation of major reworking is strengthened by analysis of grain size distribution which points to ice rafting as an important secondary transport mechanism of volcanic ash. Combined with radiocarbon dates on mollusks in the same sediment core, the volcanic marker is used to calculate a marine radiocarbon reservoir age offset ΔR = 477 ± 60 years. This relatively high value may be explained by the major influence of typically carbon-old Pacific waters, and it agrees well with recent estimates of ΔR along the northwest Alaskan coast, possibly indicating stable oceanographic conditions during the second half of the Holocene. Our use of a volcanic absolute age marker to obtain the marine reservoir age offset is the first of its kind in the Arctic Ocean and provides an important framework for improving chronologies and correlating marine sediment archives in this region. Core 2PC has a high sediment accumulation rate averaging 200 cm kyr−1 throughout the last 4000 years, and the chronology presented here provides a solid base for high-resolution reconstructions of late Holocene climate and ocean variability in the Chukchi Sea.

Raman spectroscopy for the discrimination of tephras from the Hekla eruptions of AD 1510 and 1947

Tephrochronology (the dating of sedimentary sequences using volcanic ash layers) is an important tool for the dating and correlation of sedimentary sequences containing archives and proxies of past environmental change. In addition, tephra layers provide valuable information on the frequency and nature of ash fallout from volcanic activity. Successful tephrochronology is usually reliant on the correct geochemical identification of the tephra which has, until now, been based primarily on the analysis of major element oxide composition of glass shards using electron probe microanalysis (EPMA). However, it is often impossible to differentiate key tephra layers using EPMA alone. For example, the Hekla AD 1947 and 1510 tephras (which are found as visible layers in Iceland and also as ‘crypto-tephra’ microscopic layers in NW Europe) are currently indistinguishable using EPMA. Therefore, other stratigraphic or chronological information is needed for their reliable identification. Raman spectroscopy is commonly used in chemistry, since vibrational information is specific to the chemical bonds and symmetry of molecules, and can provide a fingerprint by which these can be identified. Here, we demonstrate how Raman spectroscopy can be used for the successful discrimination of mineral species in tephra through the analysis of individual glass shards. In this study, we obtained spectra from minerals within the glass shards – we analysed the microlites and intratelluric mineral phases that can definitely be attributed to the tephra shards and the glass itself. Phenocrysts were not analysed as they could be sourced locally from near-site erosion. Raman spectroscopy can therefore be considered a valuable tool for both proximal and distal tephrochronology because of its non-destructive nature and can be used to discriminate Hekla 1510 from Hekla 1947.

Holocene vegetation change in relation to fire and volcanic events in Jilin, Northeastern China

Volatiles erupted from large-scale explosive volcanic activities have a significant impact on climate and environmental changes. As an important ecological factor, the occurrence of fire is affected by vegetation cover, and fire can feed back into both vegetation and climatic change. The causes of fire events are diverse; and can include volcanic eruptions. The amount of charcoal in sediment sequences is related to the frequency and intensity of fire, and hence under good preservation conditions fire history can be reconstructed from fossil charcoal abundance. Until now, little research on the role of fire has been carried out in northeastern China. In this study, through research on charcoal and tephra shards from Gushantun and Hanlongwan, Holocene vegetation change in relation to fire and volcanic events in Jilin, Northeastern China, was investigated. Where tephra shards are present in Gushantun it is associated with low level of both conifers and broadleaved trees, and is also associated with a pronounced charcoal peak. This suggests forest cover was greatly reduced from a fire caused by an eruption of the Tianchi volcano. We also detected one tephra layer in Hanlongwan, which also has the almost same depth with low level forest pollen values and one charcoal peak. This was caused probably by an eruption of the Jinlongdingzi volcano.

A new contribution to the Late Quaternary tephrostratigraphy of the Mediterranean: Aegean Sea core LC21

Tephra layers preserved in marine sediments can contribute to the reconstruction of volcanic histories and potentially act as stratigraphic isochrons to link together environmental records. Recent developments in the detection of volcanic ash (tephra) at levels where none is macroscopically visible (so-called ‘crypto-tephra’) have greatly enhanced the potential of tephrostratigraphy for synchronising environmental and archaeological records by expanding the areas over which tephras are found. In this paper, crypto-tephra extraction techniques allow the recovery of 8 non-visible tephra layers to add to the 9 visible layers in a marine sediment core (LC21) from the SE Aegean Sea to form the longest, single core record of volcanic activity in the Aegean Sea. Using a novel, shard-specific methodology, sources of the tephra shards are identified on the basis of their major and trace element single-shard geochemistry, by comparison with geochemical data from proximal Mediterranean volcanic stratigraphies. The results indicate that the tephra layers are derived from 14 or 15 separate eruptions in the last ca 161 ka BP: 9 from Santorini; 2 or 3 from Kos, Yali, or Nisyros; 2 from the Campanian province; and one from Pantelleria. The attributions of these tephra layers indicate that 1) inter-Plinian eruptions from Santorini may have produced regionally significant tephra deposits, 2) marine tephrostratigraphies can provide unique and invaluable data to eruptive histories for island volcanoes, and 3) tephra from both Pantelleria and Campania may be used to correlate marine records from the Aegean Sea to those from the Tyrrhenian, Adriatic and Ionian Seas.

Tephra analysis in ombrotrophic peatlands: A geochemical comparison of acid digestion and density separation techniques

ABSTRACTOmbrotrophic peatlands often lack any substantial local mineral component and therefore present exceptional opportunities for the extraction and identification of tephra horizons. Following concerns of shard geochemistry alteration during isolation using acid digestion, an alternative methodology using heavy liquids has been proposed. However, harsh basic and acidic digestions are rarely required in organic‐rich, ombrotrophic peatlands. Consequently, this study tested for geochemical differences between tephra shards from a known rhyolitic eruption, isolated from the same stratigraphic depth of a peat sequence using density separation and an acid digestion methodology typically applied to ombrotrophic peat samples and analysed using electron probe microanalysis. Findings suggest that the two samples could not be geochemically distinguished using visual and statistical methods and that, in the context of rhyolitic tephras in ombrotrophic peats, acid digestion can be used without significant effects on the interpretation of results. Density separation may still be required in peatlands with high mineral input and further work is required to examine the effect of the acid digestion technique on tephra of non‐rhyolitic compositions.

Locating cryptotephra in lake sediments using fluid imaging technology

We report a new approach to locate and quantify cryptotephra in sedimentary archives using a continuously-imaging Flow Cytometer and Micro- scope (FlowCAM). The FlowCAM rapidly photo- graphs particles flowing in suspension past a microscope lens and performs semi-automated ana- lysis of particle images. It has had primarily biological applications, although the potential sedimentological applications are numerous. Here we test the ability of this instrument to image irregularly shaped, vesicular glass shards and to screen sediment samples for the presence of cryptotephra. First, reference samples of basalt and rhyolite tephra (sieved \63 lm) were analyzed with the FlowCAM, demonstrating the ability of the instrument to photograph individual tephra shards. The highest-quality images were used to create a reference library of tephra particles, against which other particle morphologies could be automat- ically compared. Lake sediment samples with known concentrations of tephra were then analyzed. The tephra image library was used to perform pattern recognition calculations, automatically distinguishing tephra-like images from other particles in the sediment samples. The number of tephra shards identified by the FlowCAM technique was compared to manual count- ing using a polarizing light microscope, demonstrating that this rapid approach is capable of determining the relative concentrations of tephra in a given sediment sample. However, the FlowCAM systematically underestimates tephra concentrations relative to man- ual counts. We conclude that with a well-developed image library, the FlowCAM can be an effective tool for cryptotephra and sedimentological applications, but it may be inappropriate for large volume samples or if particle morphologies are outside the range of the image library.

Re-evaluation and extension of the Marine Isotope Stage 5 tephrostratigraphy of the Faroe Islands region: The cryptotephra record

Previous studies of marine sequences from the Faroe Islands region have identified a series of coarse-grained tephra horizons deposited during Marine Isotope Stage (MIS) 5. Here we reassess the MIS 5 tephrostratigraphy of the Faroe Islands region and focus on the cryptotephra deposits preserved within the fine-grained fraction of marine core LINK 16. We also extend the record to encompass the late MIS 6 and early MIS 4 periods. A density separation technique, commonly used for tephra investigations in lacustrine settings but rarely applied to marine sediments, is utilised to explore the fine-grained material and EPMA and LA-ICP-MS are employed to determine the major and trace element composition of individual tephra shards. In total, 3 basaltic and 3 rhyolitic Icelandic cryptotephra deposits with homogeneous geochemical compositions are identified — all of which have the potential to act as isochronous tie-lines. Geochemical results highlight that the Grímsvötn volcanic system of Iceland is the predominant source of the basaltic horizons and the Öraefajökull or Torfajökull systems are the likely sources of the rhyolitic deposits. Three of the horizons have been previously recognised in Faroe Islands region marine sequences, with two of these deposits traceable into a Norwegian Sea sequence. An early MIS 4 rhyolitic horizon is the most widespread deposit as it can be traced into the Norwegian Sea and to the south into a record from the Rockall Trough. Basaltic and rhyolitic horizons deposited during late MIS 6 have not been recognised in other sequences and represent new additions to the regional tephrostratigraphy.

Re-deposited cryptotephra layers in Holocene peats linked to anthropogenic activity

Tephra layers can form useful age-equivalent stratigraphic markers for correlating palaeoenvironmental sequences and they provide information about the spatio-temporal nature of past volcanic ash fall events. The use of microscopic ‘cryptotephra’ layers has both increased the stratigraphic resolution of tephra sequences in proximal areas and extended the distal application of tephrochronology to regions of the world situated far from volcanoes. Effective tephrochronology requires the discrimination between in situ tephra deposited directly from volcanic plumes and tephras that have been remobilised since their initial deposition. We present tephrostratigraphic and glass chemistry data from two proximal peat profiles (one lowland, one upland) from the Shetland Islands, UK. Both profiles contain the Hekla-Selsund tephra (deposited c. 1800–1750 cal. bc), whilst the Hekla 4 ash (c. 2395–2279 cal. bc) is present in the upland record. Overlying the Hekla-Selsund tephra are a number of distinct peaks in tephra shard abundance. The geochemistry of these layers shows that they represent re-working of the Hekla 4 and Hekla-Selsund layers rather than primary air-fall deposits. Pollen analysis of the peat sequences illustrates that these re-deposited tephra layers are coincident with a rise in heather-dominated vegetation communities (heath and/or moorland) and a subsequent intensification of burning in the landscape. We suggest that burning caused increased erosion of peats resulting in the remobilisation of tephra shards. The study demonstrates both the need for caution and the opportunities created when applying tephrochronologies in regions heavily affected by past human activity that contain both reworked tephra layers and in situ fallout.

The Hekla 1947 tephra in the north of Ireland: regional distribution, concentration and geochemistry

AbstractA cryptotephra layer from the eruption of Hekla in 1947 has recently been discovered in Irish peatlands. This tephra layer represents the most recent deposition of volcanic ash in the UK prior to the eruption of Eyjafjallajökull in 2010. Here we examine the concentration and geochemistry of the Hekla 1947 tephra in 14 peat profiles from across Northern Ireland. Electron probe microanalysis of individual tephra shards (n = 91) reveals that the tephra is of dacitic–andesitic geochemistry and is highly similar to the Hekla 1510 tephra, although spheroidal carbonaceous particle profiles can be used for successful discrimination of the two layers. The highest concentrations of Hekla 1947 are found in western sites, probably reflecting the pathway of the ash fall event due to the prevailing wind direction. Comparable tephra concentrations from two cores (1 km apart) from a single bog and from nearby sites may suggest that tephra shard concentrations in peat profiles reflect ash fallout densities across a specific region, rather than site‐specific factors associated with peatlands. This paper firmly establishes Hekla 1947 as a useful chronostratigraphic marker for the twentieth century, although within a restricted zone. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Cryptotephra sedimentation processes within two lacustrine sequences from west central Sweden

Distal tephra horizons, particularly within lacustrine sequences, are increasingly being used as time-synchronous marker horizons within palaeoclimatic and palaeoenvironmental investigations. As sedimentary features marking the presence of these so-called cryptotephras are absent to the naked eye, it is of some importance that the stratigraphic position representing primary airfall, and likewise the timing of the volcanic event, is accurately and consistently recorded amongst these deposits. Often tephra shards from a cryptotephra deposit can be found spanning several centimetres of sediments within lacustrine and peat sequences. Very few studies, however, have looked in detail at the sedimentation and vertical distribution of cryptotephra deposits within such sequences and, more importantly, the criteria for defining the correct stratigraphic position of the volcanic event. Two sediment cores from Lake Spåime and Lake Getvaltjärnen, west central Sweden are employed to investigate in detail the vertical distribution of the tephra shards derived from the AD 1875 eruption of the Askja volcano in Iceland. Detailed geochemical analysis of shards from both records indicate that products of the Askja eruption are present for at least 120 years and thus emphasize the importance of carefully identifying the correct horizon that marks the timing of the volcanic event rather than shards resulting from a period of reworking or downward migration. Both sites yield contrasting shard concentration profiles and thus raise a number of questions regarding the influence of site-specific processes on cryptotephra sedimentation, particularly the role of snow-beds acting as tephra traps, the possibility of reworking, and downward migration of shards in soft sediment. A second tephra is also identified at Lake Getvaltjärnen and is believed to originate from the AD 1477 Veidivötn eruption and represents the first occurrence of this tephra outside of Iceland.

First discovery of cryptotephra in Holocene peat deposits of Estonia, eastern Baltic

This article reports the first discovery of middle Holocene cryptotephra from a peat sequence in Estonia, eastern Baltic. Two sequences, Mustjärve and Parika (located 110 km apart), were chosen for a pilot study aimed at finding traces of tephra fallout during the middle Holocene. Peat accumulation at both sites started in the early Holocene (c. 9500–9000 14C yr BP; c. 11 000–10000 cal. yr BP) and continued throughout the whole Holocene. The radiocarbon-dated intervals between c. 2000 and 5000 14C yr BP (c. 2000–5500 cal. yr BP) were chosen from both sites for the study. Colourless tephra shards were identified at 312–316 cm below the peat surface in the Mustjärve peat sequence, while no tephra was found in peat of the same age at Parika. Electron microprobe analyses suggest a correlation with the initial phase of the Hekla-4 eruption (c. 4260 cal. yr BP), although the age-depth model indicated an age around 4900 cal. yr BP. Small concentrations of colourless to light brown tephra shards at 266–270 cm in the Mustjärve sequence indicate that the Kebister tephra (c. 3750 cal. yr BP) might also be present, but geochemical analyses were not possible. The low concentration and small size of the tephra particles indicate that Estonian bogs are probably on the verge of where tephrochronology is possible in northwestern Europe. Further studies of full Holocene sequences are required in order to discover traces of other ash plumes reaching as far east as the eastern Baltic area.