Quaternary tephrochronological and tephrostratigraphical studies: a brief review

2016-03-21ZHAOHongliHALLValerieLIUJiaqi

地球环境学报 2016年3期

ZHAO Hongli, HALL Valerie A, LIU Jiaqi

(1. State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an 710061, China; 2. School of Geography, Archaeology and Palaeoecology, Queen's University Belfast, BT7 1NN UK; 3. Key Laboratory of Cenozoic Geology and Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China)

ZHAO Hongli1, HALL Valerie A2, LIU Jiaqi3

Background, aim, and scopeAs a dating method, Quaternary tephrochronology and tephrostratigraphy have now been employed in many areas of the world. This article presents a brief review of tephra studies in some countries in Europe, New Zealand and the countries around NE China (Japan and Russia). Tephrochronology and tephrostratigraphy are well established in Europe and New Zealand, however only few work has been done in NE China (to date). Since the 1930s, tephrochronological methods have been used extensively to solve problems in Quaternary geology, archaeology, paleopetrology, and paleogeography. A pioneer in the use of tephra layers to establish chronology was Sigurdur Thorarinsson, who began by studying the layers he found in the deposits of his native Iceland. In the early stage of tephra studies, the layers were often mapped with their source volcanoes by observing the colour and petrology of the tephra, as these are characteristics of a particular volcano. Then, the real age of the tephra layers was dated directly with reasonable accuracy using radiometric dating methods such as glass-fission-track, Potassium-Argon (K-Ar), Argon-Argon (39Ar-40Ar), radiocarbon (14C), uranium series, thermo-luminescence and electron spin resonance. Therefore,more and more tephras have been dated indirectly by correlation and now many improved reference sets for indirect dating through tephras from most volcanic centers are accessible in geological databases such as Tephrabase available at http://www.tephrabase.org/. Further information can be added to the Tephrabase in the future, whenever the details are available. Therefore, more and more tephras have been dated indirectly by correlation.Materials and methodsGlass shards which were detected in the sediments need to be concentrated and purified in the lab and then made slides for the geochemical analysis. Laboratory procedures for the determination of loss-on-ignition and extraction of glass shards from the sediments could follow the ashing method, and then for the density separation technique. Sometimes, some of the samples were rich in diatoms, whose presence obscured the glass particles during microscopic examination. The diatoms were dissolved in 5% KOH solution heated in the water bath at 90℃ for an hour and shaken after 30 minutes. The samples were then processed, following procedures in the following chapters. Samples were analyzed under polarized light at ×400 magnification using a microscope. Samples for wave-length dispersive electron probe microanalysis (WDS-EPMA) were prepared and performed on a scanning electron microscope.ResultsTephra and cryptotephra deposits provide records of volcano eruption histories, and their study thus aids volcanic hazard analysis and mitigation. As well, it is evident that tephra sequences may be more comprehensive at medial to distal locations in sediments, including ice, than at proximal locations, and that the interbedding of the layers from multiple sources at such locations uniquely reveals their complex stratigraphic interrelationships.DiscussionAsh can be transported against the prevailing wind fl ow. Some tephras, such as the one found in the Tasman Sea, have travelled several hundreds of kilometers from source to the north and northwest of North Island by southeast trade winds, as well as having an eastern dispersal in the Pacifi c Ocean. This distribution occurred at a higher altitude than the westerlies which drive most of the material to the east. Fluvial systems can deposit nearly pure tephra in units exceeding 20 m in thickness at distances up to 250 km from the vent. Therefore, for the stratigraphic purposes many fl uvial tephra deposits can be considered almost simultaneous with the fall or flow event.ConclusionsTephrochronology, the characterization and use of volcanic-ash layers as a unique stratigraphic linking, synchronizing, and age- equivalent dating tool has become a globally-practised discipline of immense practical value in a wide range of subjects including Quaternary stratigraphy and geochronology, palaeoenvironmental reconstruction, volcanology, geomorphology, archaeology, human evolution, and palaeoanthropology. Although much more work has been done on tephrochrnology and tephrostratigraphy in New Zealand, Europe and other parts of the world (Japan, Russia etc.), in NE China it still has lots of work could be done in the future.Recommendations and perspectivesTephra research in Europe and New Zealand are advanced the benefi t of tephra for effecting high-precision correlations between late Quaternary records has been demonstrated in both the area. These correlations enable the testing of past environmental change where good time control is absent or lacks precision. Northeastern China has a long history of volcanism, so tephra is potentially an important chronological tool in the area for reconstructing past environments.

tephrochoronology; tephrostratigraphy; Quaternary

1 Tephrochronological studies

1.1 Introduction

“Tephrochronology” sensu stricto is the use of tephra layers as isochrons (time-parallel marker beds) to connect and synchronize sequences and to transfer relative or numerical ages to them using stratigraphy and other tools. However, tephrochronology sensu lato has been used more broadly to describe all aspects of tephra studies (Lowe, 2011).

Since the 1930s, tephrochronological methods have been used extensively to solve problems inQuaternary geology, archaeology, paleopetrology, and paleogeography. A pioneer in the use of tephra layers to establish chronology was Sigurdur Thorarinsson, who began by studying the layers he found in the deposits of his native Iceland (Thorarinsson, 1981). He made significant contributions in many areas of geology, especially volcanology and glaciology, both in Iceland and abroad. The potential for studies employing tephrochronology is much greater than had been previously realized. Many sites have accumulated cryptotephra particles (particles not visible to the naked eye), sometimes in large quantities (e.g. Hall et al, 1994; Dugmore et al, 1995). The problem of using cryptotephra as a chronological tool is that it is often diffi cult to establish in which sites these particles have accumulated. This is especially true when they have been deposited in mineral-rich lacustrine sediment, as is frequently the case in late glacial sequences. The concentration and separation of cryptotephra particles in organic-rich sediments is reasonably straightforward for both the quantification and geochemical identification of shards (Dugmore, 1989; Pilcher and Hall, 1992). However, the approaches used are much less successful for separating cryptotephra from mineral-rich sediments, which can be highly-inorganic and where the inorganic component contained copious phytoliths, diatoms and occasionally tephra particles, since they fail to digest the inorganic matrix.

A technique was successfully applied by Turney et al (1997) and is a further adaptation of the method described by Pilcher and Hall (1992) for application to Holocene peat deposits. It uses a density separation procedure to concentrate any cryptotephra component in lake sediments and was applied to the investigation of a lake sediment succession from a small basin in NE Scotland. Using this approach it was possible to defi ne quantitatively of the presence of the Vedde Ash tephra layer on the British Isles. The Vedde Ash is found widely across Northern Europe and the North Atlantic seaboard, being a widespread marker in these areas. The Vedde Ash has now been traced into continental Europe (e.g. Turney et al, 2006) and is known to have travelled as far east as Russia (Wastegård et al, 2000b), and south into the Netherlands (Davies et al, 2005).

1.2 Dating of tephra

The key to tephrochronology is the ability to recognize tephra from a specific eruption (Sarna-Wojcicki, 2000). In the early stage of tephra studies, the layers were often mapped with their source volcanoes by observing the colour and petrology of the tephra, as these are characteristics of a particular volcano (Naeser et al, 1981). Then, the real age of the tephra layers was dated directly with reasonable accuracy using radiometric dating methods such as glass-fission-track, Potassium-Argon (KAr), Argon-Argon (39Ar-40Ar), radiocarbon (14C), uranium series, thermo-luminescence and electron spin resonance (Naeser et al, 1981; Machida, 2002).

After more than six decades and hard work on regional tephrochronological studies (Thorarinsson, 1981; Walker, 1981; Machida and Arai, 1983; Rose and Chesner, 1987; Sigurdsson and Carey, 1989; Machida et al, 1990; Pilcher and Hall, 1992, 1996; Hall et al, 1994; Donoghue et al, 1995; Donoghue and Neall, 1996; Newton and Metcalfe, 1999; Shane, 2000; Sigurdsson, 2000; Wastegård et al, 2000a, 2000b; Hall and Pilcher, 2002; Machida, 2002; Hall, 2005), many tephras have been dated and now many improved reference sets for indirect dating through tephras from most volcanic centers are accessible in geological databases such as Tephrabase available at http://www.tephrabase.org/ (Newton et al, 2007). Further information can be added to the Tephrabase in the future, whenever the details are available. Suitable materials for direct dating are not always available at all outcrops and more importantly, correlation techniques are typically faster and cheaper through tephra linkage than traditional radiometric dating (Shane, 2000). Therefore, more and more tephras have been dated indirectly by correlation (Shane, 2000).

1.3 Advantages of tephrochronological research

Tephrochronology forms part of the standard suite of Quaternary dating techniques even in areas far removed from the site of volcanic activity (Self and Sparks, 1981). The main advantages oftephrochronology are that the volcanic ash layers can be relatively easily identifi ed in many sediments and that the tephra layers are deposited relatively instantaneously over a wide spatial area. This means they provide accurate temporal marker layers which can be used to verify or corroborate other dating techniques and allow precise correlations between late Quaternary stratigraphic successions widely separated by location into a unifi ed chronology. (eg. Ruddiman and Glover, 1972; Westgate and Gorton, 1981; Björck et al, 1992; Hafl idason et al, 1995; Wohlfarth, 1996).

Tephrochronology, the characterization and use of volcanic-ash layers as a unique stratigraphic linking, synchronizing, and age-equivalent dating tool has become a globally-practised discipline of immense practical value in a wide range of subjects including Quaternary stratigraphy and geochronology, palaeoenvironmental reconstruction, volcanology, geomorphology, archaeology, human evolution, and palaeoanthropology (Lowe, 2011). Tephrochronological studies began in volcanic countries and the proximal deposits were an early focus. The studies then spread to some other countries including at locations quite remote from volcanoes. The very long distances away from the source of tephra deposited were demonstrated, more than 7000 km in some cases (Lane et al, 2013; Sun et al, 2014). Such distal tephra deposits extend over far greater areas of the globe than previously imagined, covering thousands or even hundreds of thousands of square kilometres, and uniquely provide one of the most robust stratigraphic and dating tools available to geoscience.

1.4 Limitations of tephra studies for site linkage and tephrochronology

Although tephrochronology has many advantages it has limitations. Firstly, analytical targets must be of sufficient size to represent the original components (Shane, 2000), and this is hard to achieve in the case of cryptotephra layers, where the particles are extremely small and often in very low concentrations. Secondly, although organic carbon in underlying or overlying beds may provide a maximum or minimum age, tephras are not often dated directly (Westgate and Evans, 1978; Naeser et al, 1981). Similarity or dissimilarity of radiometric dates may not separate volcanic events and that further information may be needed (Froggatt, 1983). Some researchers doubted if tephras from the same source could be mapped to different eruptive events, even though there was evidence for tephra composition varying between eruptions of the same volcano (Beaudoin and King, 1986).

1.5 Tephra studies in NE China and other countries in the world

Since the paper of Dugmore (1989), the identifi cation of “hidden” distal tephra deposits (cryptotephras) has resulted in a new generation of tephrochronologists undertaking research in northern Europe, Scandinavia, Iceland (Pilcher and Hall, 1992, 1996; Hall et al, 1994; Hall and Pilcher, 2002; Wastegård et al, 2003; Hall, 2005), Japan (Machida et al, 1985, 1987, 1997; Machida, 1999a, 1999b), New Zealand (Froggatt, 1983; Gehrels et al, 2006), Kamchatka (Braitseva et al, 1997) and elsewhere (e.g., Davies et al, 2002; Turney et al, 2004; Wohlfarth et al, 2006). But to date, very little work has been done on tephrochronology for lake sediments in Northeastern China.

1.5.1 Tephra studies in Iceland and Europe

The high-resolution Holocene tephrochronology of Iceland is important for understanding short-term climatic and palaeoecological events in the North Atlantic region (Eiriksson et al, 2000) (Fig.1). Iceland is a volcanic island located close to the Arctic circle in the North Atlantic Ocean, latitude 63° — 67°N and longitude 28° — 13°W. It is situated on the North Atlantic mid-ocean ridge and above a stationary“hotspot”. This setting results in many volcanic eruptions, some of which have produced widespread tephra-markers (e.g., Davies et al, 2001; Hall and Pilcher, 2002).

It was Dugmore's work (1989) that fi rst showed that Holocene Icelandic volcanic ash had reached Caithness, northern Scotland based on tephra layers detectable only using a microscope. This first report was followed by others describing minute welldefined layers of tephra in Holocene peats and lake muds at sites on Shetland (Bennett et al, 1992) and in the north of Ireland (Pilcher and Hall, 1992). Sincethose fi rst publications, Icelandic tephra-markers have been found in the UK and Ireland (Pilcher and Hall, 1992; Hall et al, 1994; Dugmore et al, 1995; Lowe and Turney, 1997; Turney, 1998; Wastegård et al, 2000a; Davies et al, 2001; Plunkett et al, 2004), in Germany (van den Bogaard et al, 2002), in Scandinavia (Mangerud et al, 1984; Zillén et al, 2002; Davies et al, 2003; Wastegård and Davies, 2009; Pilcher et al, 2005), in the Faroe Islands (Mangerud et al, 1986) and even Russia (Wastegård et al, 2000b). Icelandic tephra-markers have also been found in marine sediment cores from the North Atlantic (Ruddiman and Glover, 1972; Lacasse et al, 1998; Austin et al, 2004), in Alps (Blockley et al, 2007) and in Greenland ice cores (Zielinski et al, 1994; Hammer et al, 2003; Mortensen et al, 2005).

Fig.1 Map of the North Atlantic region showing the location of the countries in which Quaternary aged tephra from Iceland has been detected

Microscopic studies of late-Quaternary tephras from Iceland yield a detailed tephrochronology in deposits of Ireland (Tab.1) (Hall and Pilcher, 2002). In Scandinavia, five of the cryptotephra horizons were correlated to the Askja-1875, Hekla-3, Kebister, Hekla-4 and Lairg A tephras respectively. The radiocarbon dates of these tephras were associated with previously published ages from Iceland, Sweden, Germany and the British Isles. The Faroe Islands has recorded several tephra horizons that were contained in Holocene peat sequences (Wastegård, 2002). Geochemical analyses show that at least two dacitic and one rhyolitic tephra layers were erupted from the Katla volcanic system on southern Iceland between 8000 and 5900 cal BP.

1.5.2 Tephra studies in New Zealand

Tephra in the North Island of New Zealand started to be investigated with early soil surveys for agricultural purposes since the 20th century (e.g., Grange, 1931; Taylor, 1933). These studies soon expanded into a stratigraphy for late Quaternary studies (e.g., Vucetich and Pullar, 1969). Taupo Volcanic Zone (TVZ) in central North Island is considered to be the most frequently active, large rhyolitic centre on Earth (Wilson et al, 1995), and has been the focus of numerous large (＞100 km3) caldera-forming ignimbrite and plinian eruptions (Fig.2). The silicic tephra erupted from TVZ were well preserved in New Zealand Quaternary sequences and those of the surrounding oceans over 1000 km distant (Fig.1).

Tab.1 Summary of geochemically determined late-Quaternary tephra layers in the British Isles (Hall et al, 2002)

Tephra studies in New Zealand by Lowe (1990) and Froggatt and Lowe (1990) summarized the stratigraphy of post-65 ka rhyolitic tephra beds. These works have greatly revised New Zealand's late Cenozoicstratigraphy and eruptive history, and extended the tephra record back to the late Miocene. Tephrostratigraphy of the post-65 ka record in New Zealand is well established, however, there are chronologically wellconstrained tephra beds dating back to 10 Ma. In the North Island of New Zealand, some tephra beds covered most areas, some of the tephras were found in the South Island and also travelled as much as 1400 km from vent into the Pacific Ocean (Carter et al, 1995). The ages of the important stratigraphical units such as Kaharoa (665 yr BP), Kawakawa (22 ka), Rangitawa (0.33 Ma), Potaka (1 Ma), Pakihikura (1.6 Ma) and other tephra beds provide a framework for global climatic and sealevel change recorded in New Zealand. They also help to correlate marine and terrestrial sequences (Lowe, 1988; Frogatt and Lowe, 1990).

Fig.2 Map of North Island, New Zealand showing the location of late Cenozoic volcanic centres (Shane, 2000) TVZ is considered to be the most frequently active, large rhyolitic centre on Earth (Wilson et al, 1995), and has been the focus of numerous large (＞100 km3) caldera-forming ignimbrite and plinian eruptions.

1.5.3 Tephra studies in NE China

There are two volcanic fields located in the Northeastern China, one is Changbaishan volcanic field and the other is Longgang volcanic field (Fig.3). Moreover, much is known about volcanism in both area but only little work has been done (todate) on a tephrochronology for the lake sediments within the region. The Longgang volcanic field is volcanically active with a number of eruptions during the Quaternary but the chronology of the eruptions is poorly defi ned. Some tephra layers are well preserved in the annually laminated sediments of maar lakes in the region, and help with the construction of a much improved chronological framework for the volcanic history of the area. The results from Liu et al (2009) showed that three basaltic explosive eruptions occurred at 460 AD, 11460 cal BP and 14000 cal BP, respectively. The eruption happened in 460 AD is the youngest one in this fi eld based on the Liu et al (2009).The tephra distribution and chronological data suggest that this eruption is likely to be from the Jinlongdingzi Volcano which lies in the Longgang Volcanic Field and has been recognized as an active volcano, and as the site of the second largest volcanic eruptions in China during the past 2000 years (Ou and Fu, 1984; Liu, 1999).

Fig.3 Map of volcanic fi elds in the Changbaishan area (modifi ed by Zhao and Valerie, 2015)

Sun et al (2015) presented new stratigraphic, geochemical, varve chronology, and14C geochronological data from the varved sediments in Lake Sihailongwan, Longgang volcanic field, Northeast China, extending the westerly margin of this eruption. The distinctive geochemical characteristic of volcanic glass (ranging from trachyte to rhyolite), similar to those of proximal and distal tephra, confirmed the occurrence of Changbaishan Millennium eruption ash in the lake, illustrating the westward dispersal fan of the ash deposits.

Cheng et al (2008) investigated some volcanic glass from a peat of Jinchuan Maar and it is in situ sediments from a near-source explosive eruption according to particle size analysis and identification results. The tephra were neither from Tianchi volcano eruption nor from Jinlongdingzi volcano from about 1600 a BP eruption (Fan and Sui, 2000), but maybe from an unknown eruption in the Longgang volcano group according to their geochemistry and distribution. The tephras are poor in silica, deficient in alkali, Na2O content is more than K2O content. The tephra, from peat of known age proved that the eruption happened in 15 BC — 26 AD (1934 — 1965 cal BP) and is one of Longgang volcano group eruption that was not recorded and that is earlier than that of the Jinlongdingzi eruption.

In Zhao et al (2015), it showed that (1) The vegetation succession history of Gushantun since 10000 cal a BP and Hanlongwan since 4000 cal a BP both have the obvious regularity. It showed the alternating characteristics between the coniferous tree dominated and mixed coniferous and broadleaved tree dominated. (2) The fi re history in Gunshantun during the Holocene showed a highly fi re prone environment. These charcoal peaks could be caused by volcanic eruptions that were remote from the study site. In this study, the fi re event identifi ed at a depth 40 — 43 cm seems associated with the Tianchi volcano eruption. There is one fi re event appeared in Hanlongwan record between 1566 — 1806 cal a BP (299 — 300 cm), which corresponded with a high charcoal peak, which could have been caused by the fire event resulted by the Jinlongdingzi volcano eruption.

In the new study (Zhao and Valerie, 2015b), few rhyolitic glass shards found in the Gushantun peat bog were considered coming from the Tianchi volcano eruption in 1702 AD, 1668 AD and 1597 AD. Three tephra layers and sublayers were detected in Sihailongwan are considered to have come from Jinlongdingzi volcano eruption. A powerful method of establishing precise correlations between local and distal records can be provided by the identifi cation and geochemical characteristics of tephra layers within Quaternary sediments.

1.5.4 Tephra studies in Japan

Violent volcanic activities often cause volcanic hazards, which have claimed more than 260000 lives in the world since 1600 AD (Tilling, 1989). In Japan, there are over 250 Quaternary volcanoes, which have frequently erupted. Large quantities of lava and tephra, mainly of rhyolitic and andesitic composition, have erupted throughout the Quaternary in those areas. Within the 1500 years of historic records, severe damages and casualties by volcanic hazards have been reported have been reported in Japan Meteorological Agency, 2005.

From the 1950s tephra studies in Japan has developed steadily. Since Machida and Arai (1976) published the first catalog of tephras in Japanese Islands, new data and findings urged frequently revision and augmentation of the catalogs (Machida and Arai, 1992; Machida, 1999a, 1999b). This enabled identification and distinction of many individual tephra layers. Due to the large scale eruptions that happened in Kyushu and Hokkaida Islands in Japan from Middle Pleistocene to Holocene, the estimated average frequency is once in several ten thousand years (Machida and Arai, 1976). Their impacts on human society and ecosystems have been very severe (Machida, 1984, 2002; Kuwahata, 2002).

Japanese tephrochronology has evolved under the influence of several geoscientific disciplines. Since 1970s, the long-distance correlation of tephra became possible (Machida and Arai, 1976). The widespread marker tephras have been identified andgreatly facilitated the Quaternary tephra studies over extensive areas which included Japanese islands and the adjacent seas. In their papers, Machida and Arai (1976, 1983) described two widespread tephra layers from gigantic explosive volcanism of the southern Kyushu caldera volcanoes Aira and Kikai. These two layers have been identified throughout Japan and the Sea of Japan as well as over the fl oor of the Pacifi c Ocean to the South of Honshu. The two tephra layers named Aira-Tn (AT) and Kikai-Akahoya (KAh) with age 26000 — 29000 cal BP and 7300 cal BP respectively (Machida and Arai, 1976) have helped many applications of Quaternary sciences in the region of Japan. These results have encouraged the identifi cation of many other widespread tephra layers. For example, there is a tephra layer (B -Tm) 4 — 6 cm thick in the islands of north Japan, which has been recognized as coming from the millennium eruption of Tianchi Volcano, Changbaishan Volcanic Field in China (Fig.4) (Machida and Arai, 1992; Machida, 1999a, 1999b).

Fig.4 Map of the countries around North China and volcanic ash from the 1199 AD Tianchi volcano, Changbaishan area eruption was found in a layer of 4 — 6 cm in thickness on the islands of north Japan (by Machida et al, 1990; Machida and Arai, 1992)

1.5.5 Tephra studies in Russia

Tephra layers are also widely spread in Russia, such as in Kamchatka Peninsula which hosts more than 20 active volcanoes. Kliuchevskoi (56°03'N, 160°38'E; 4750 m) is Kamchatka's most active volcano. Its cone and surroundings are dotted with numerous flank vents. Flank eruptions were dated with the help of previously studied regional marker ash layers (Braitseva et al, 1997). The most important marker ash layers at the base of Kliuchevskoi are from Shiveluch (SH1, 250 a BP; SH2, 950 a BP; SH3, 1400 a BP; SH5, 2550 a BP; SH2800, 2800 a BP); Ksudach (KS1, 1800 a BP), and Bezymianny volcanoes (BI, 1600 a BP, and B, 3900 a BP). The studied marker tephra layers provide a record of the most voluminous explosive eruptions in Kamchatka during the Holocene and register volcanic input into the atmosphere (Braitseva et al, 1997).

Two sites lake LM and lake LP located in northwestern Russia were chosen by Wastegård et al (2000b) for testing if the Vedde Ash could be found east of the Baltic Sea. Electron microprobe analysis of tephra shards from the two lakes confirmed their origin. This fi nding of the Vedde Ash in northwestern Russia once again shows the potential use of tephrochronology as a tool for correlation and dating of Quaternary sedimentary sequences.

2 Tephrostratigraphical studies

2.1 Introduction

Tephrostratigraphy is the use of tephra layers, in particular volcanic ash, as a correlation tool in the study of stratigraphic sequences. It is one of the best tools for correlating long sedimentary records of environmental and climatic changes (e.g., Hodgson et al, 1998; Newnham and Lowe, 1999; Newton and Metcalfe, 1999; Litt et al, 2003). The identification and characterization of ash layers in continuous sedimentary environments provides a detailed history of the past volcanic activity of the study region and allows time correlation of sites across extensive areas (e.g., Juvigné, 1993; Boygle, 1999; Schmidt et al, 2002; Shane and Hoverd, 2002; Shane et al, 2002; Wulf et al, 2004).

Early studies demonstrated that airfall tephra deposits consist of volcanic glass shards (includingpumice), minerals and lithic fragments (e.g. Froggatt, 1992; Ortega-Guerrero and Newton, 1998; Boygle, 1999). The composition of the minerals and glass shards contained in tephra layers remains the best means of correlating tephra layers, despite different changes in the relative abundance of minerals with distance to the source (Juvigné and Shipley, 1983; Juvigné, 1993).

2.2 Andesite tephrostratigraphy in New Zealand

Two andesitic centres, Tongariro Volcanic Centre and Egmont (Taranaki) volcano which both comprise several vents and volcanoes, have been active throughout much of the late Pleistocene and Holocene in North Island. Tephrostratigraphies based on field characteristics have been established for Egmont Volcano back to 28 ka (Alloway et al, 1995), for the Tongariro centre back to 75 ka (e.g. Topping and Kohn, 1973; Donoghue et al, 1995; Cronin and Neall, 1997; Nairn et al, 1998). Geochemical analysis on 43 macroscopic tephra layers at Suckaland from Egmont volcano which span 10 — 70 ka has been obtained (Shane, 2005). Fourteen well-studied and mostly welldated rhyolitic tephra layers are interbedded with them that provide timing constraints (Shane, 2000).

2.3 Tephrostratigraphical studies in other part of the world

Many Pleistocene and Holocene distal tephras in Europe were described from Icelandic volcanoes (e.g. Thorarinsson, 1981), the Eifel volcanic field in south-western Germany (e.g. van den Bogaard and Schmincke, 1985), and the Massif Central in France (e.g. Juvigné et al, 1992). Another famous region for tephrostratigraphic studies is the Central-Eastern Mediterranean (Mellis, 1954). It is made up of the Italian, Aegean and Anatolian three main volcanic areas. During the Swedish Deep-Sea Expedition of 1947 — 1948, the first distal tephras in the Central-Eastern Mediterranean were detected and described in deep-sea sediments (Mellis, 1954; Norin, 1958). Based on a new study from the Monticchio, the distal tephrostratigraphy of both high- and low-intensity explosive eruptions of south Italian volcanoes for the last 20000 years were recorded. This work also allowed other terrestrial and marine sequences in the Central Mediterranean (Wulf et al, 2008) to be compared.

Through the Clermont crater-lake sediments and surface studies, Vernet et al (1998) presented a tephrostratigrapy of the Limagne for the past 160 ka. 118 ash-falls, recorded in the crater-lake sediments of the Clermont maar lake, showed that basaltic phreatic activity developed along the western fault-scarp of the Limagne between 160 ka and 70 ka (Vernet et al, 1998). Moreover, several acid eruptions occurred between 160 ka and 40 ka. From Older Dryas to Atlantic, between 15 ka and 7 ka, fourteen ash-falls occurred and in some cases the source volcanoes have been identifi ed (Vernet et al, 1998).

3 Tephra preparation and geochemical analysis

In order to get glass shards from the sediment, some methods were taken for the different types. Here are a few steps for observing the glass shards from some sediments of Longgang volcanic fi eld (Zhao et al, 2015; Zhao and Valerie, 2005).

3.1 Concentrate tephra

Dry weights were measured (0.2 g) and samples transferred into a pre-heated furnace at 550℃ for 2 h. After burning, the crucibles were placed directly into a desiccator, which prevented the uptake of moisture during cooling. 5 mL of 10% HCl solution and Lycopodium (batch no. 483216 spores 18583 ± 1708) were added to each 15 mL tube containing the ashing residue, and left overnight. (Immersion in 10% HCl for approximately 16 h facilitated breakdown of the solid sediment matrix and dissolved any carbonates.) And then, each sample was centrifuged at 2500 rpm for 5 min and then washed with distilled water twice. The remaining material was sieved on 10 μm polyester mesh using distilled water and then the residues were returned to the centrifuge tube. Samples were then mounted directly onto microscope slides without any further processing. Samples which required further concentration were subject to density separation.

3.2 Using KOH as an alternative procedure

Some of the samples were rich in diatoms,whose presence obscured the tephra particles during microscopic examination. The diatoms were dissolved in 5% KOH solution (Rose et al, 1996) heated in the water bath at 90℃ for an hour and shaken after 30 minutes. The samples were then processed, following procedures outlined in section 3.1.

3.3 Flotation for sample purifi cation

The removal of organic material is most often achieved by ashing (Hall and Pilcher, 2002) or by chemical digestion (Dugmore et al, 1995). Flotation is an alternative method for separating components of sediment samples, in particular minerogenic components from organic-rich sediments (Turney, 1998; Blockley et al, 2005).

The procedure is as follows:

The 0.2 g sample was treated as described in section 3.1 and then put into individual marked tubes (12 mL). According to the difference between every sample, sodium polytungstate was added with different density and mixed thoroughly. Samples were then centrifuged at 3000 rpm for 10 minutes. Afterwards, the float was decanted into a separate tube, so that the sample was separated into two parts, one being the float and the other the residue. Both the float and residue were carefully examined. The fl oat and residue were diluted with distilled water and centrifuged at 3000 rpm for 5 minutes. This process was applied three times. The light and heavy shards should have been separated and concentrated in the fl oat and residue, in turn.

The microprobe slides were ground (which also makes it easy to number the slides using a pencil) and one drop of residue was placed in the centre of a ground glass slide. The slides were placed on the heating platform until all water has been evaporated, then follow the same procedure for preparation of the surface tephra samples.

3.4 Geochemical analysis

Accurate geochemical characterization is crucial for enabling robust tephra-based correlations. Distal tephrochronological studies are reliant upon precise and accurate geochemical quantification. Precise geochemical analysis is critical for correlating tephras and linking them to volcanic sources. Each eruption may emit material of a distinct composition, but in the deposits from the same volcano the differences may be subtle (Hunt and Hill, 1993). The individual shards are analyzed by electron microprobe; it is the smallest particles of a size not more than 2 mm that can travel hundreds to thousands of kilometers from a volcano. For example, the geochemical data which from Sihailongwan, Hanlongwan and Guanshtun of Longgang volcanic field in NE China (Zhao et al, 2015a, b) were obtained through using The JEOL FEGSEM 6500F analytical system at Queen's University of Belfast (Fig.5). Operating conditions were 15 kV, 20 nA with a raster beam current of 5μm. Lipari natural glass (Hunt and Hill, 1996, 2001) was used as an intermediate standard. Totals of 95% and above for the analysis of the reference samples were retained.

Fig.5 The facility of JEOL FEGSEM 6500F at Queen's University Belfast

4 Dispersal and distribution of tephra deposits

The value of tephra beds as stratigraphic markers and recorders of volcanism is largely due to their wide dispersal. The distribution of ash is influenced by the strength of wind and its direction at the time of eruption, and deposition largely depends on the energy of the event particle size and meteorological conditions (including precipitation) (Carey and Sparks, 1986; Lacasse, 2001). The fi ne ash component of tephra is the most widely dispersed. Plinian eruptions are explosive, with the energy to drive material to high altitude,creating conditions for wide dispersal of tephra, mostly by tropospheric winds (e.g. Scasso et al, 1994). Rhyolitic tephra are valuable in such studies because they result from high elevation plinian columns, and ash columns can be dispersed away from the vent (e.g. Rose and Chesner, 1987; Shane et al, 1995).

Ash can be transported against the prevailing wind flow, for example, Mortensen et al (2005) reported the Icelandic tephra has been deposited to the west despite the dominant wind direction from west to east in Greenland. Some tephras, such as the one found in the Tasman Sea, have travelled several hundreds of kilometers from source to the north and northwest of North Island by southeast trade winds, as well as having an eastern dispersal in the Pacifi c Ocean (Nelson et al, 1986). This distribution occurred at a higher altitude than the westerlies which drive most of the material to the east (Shane, 2000). Fluvial systems can deposit nearly pure tephra in units exceeding 20 m in thickness at distances up to 250 km from the vent (Shane and Froggatt, 1991; Shane et al, 1996; Shane, 2000). Therefore, for the stratigraphic purposes many fluvial tephra deposits can be considered almost simultaneous with the fall or fl ow event.

The ash may distribute and deposit horizontally as well as vertically. Bog surfaces are rarely uniform and shards may accumulate in hollows (Holmes, 1998), so sometimes some tephras seem to disappear during the re-sampling and analysis of the material in core sections close to previously rich tephra units (e.g. Davies et al, 2005). Pyne-O'Donnell (2004) has reported patchy tephra deposition in some environments.

5 The potential of tephra for high-precision correlation

Tephra and cryptotephra deposits provide records of volcano eruption histories, and their study thus aids volcanic hazard analysis and mitigation. For example, Newnham et al (2010) identifi ed an increase in respiratory-related mortality in Auckland and Hamilton in New Zealand as a possible consequence of the effects of diffuse, fine-grained ash fallout (as cryptotephra) from the 1996 Mt Ruapheu eruption. As well, it is evident that tephra sequences may be more comprehensive at medial to distal locations in sediments, including ice, than at proximal locations, and that the interbedding of the layers from multiple sources at such locations uniquely reveals their complex stratigraphic interrelationships (Lowe, 2011).

Tephra research in Europe and New Zealand are advanced the benefi t of tephra for effecting high-precision correlations between late Quaternary records has been demonstrated in both the area. These correlations enable the testing of past environmental change where good time control is absent or lacks precision. Northeastern China has a long history of volcanism, so tephra is potentially an important chronological tool in the area for reconstructing past environments.

6 Summary and conclusion

Acknowledgement:Many thanks to Prof. John Dodson, from Australian Nuclear Science and Technology Organisation, for his thoughtful reviews and comments. Thanks to Prof. Pilcher Johnathan, Prof. Paula Reimer and Dr Maarten Blaauw, Queen's University Belfast UK, for their help in calibrating radiocarbon dates and thoughtful comments.

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火山灰年代学与地层学研究简述

赵宏丽1，HALL Valerie A2，刘嘉麒3

（1. 中国科学院地球环境研究所黄土与第四纪地质国家重点实验室，西安 710061；2. School of Geography, Archaeology and Palaeoecology, Queen's University Belfast，BT7 1NN UK；3. 中国科学院地质与地球物理研究所新生代地质与环境重点实验室，北京 100029）

火山灰的研究作为探讨古气候和古环境的一种手段，近十年来引起了国内外学者的广泛关注。气候学和古环境学的研究表明，火山喷发可以导致某一区域内短期的气候变化或者对冰期产生强烈影响。同时，火山灰层的成因机制决定了其空间分布的等时性和广泛性，因此，火山灰的地层学特征和精确定年，具有重要的地层对比意义。目前，全球各地已经开展了许多火山灰年代学与地层学的研究。在中国，尤其是东北地区，虽然有着丰富的火山资源，但是火山灰年代学与地层学的研究工作依然比较匮乏，本文简要回顾了一下中国东北、欧洲、新西兰、日本、俄罗斯等地区的火山灰研究工作，旨在为以后的研究提供一定的参考资料。

火山灰年代学；火山灰地层学；第四纪

2015-12-28；录用日期：2016-03-28

国家自然科学基金项目（41202260）；Overseas Research Scholarship（UK, 2007 — 2010）

赵宏丽，E-mail: zhaohl@ieecas.cn

10.7515/JEE201603001

Received Date:2015-12-28;Accepted Date:2016-03-28

Foundation Item:National Natural Science Foundation of China (41202260); Overseas Research Scholarship (UK, 2007 — 2010)

ZHAO Hongli, E-mail: zhaohl@ieecas.cn