Table
1.
The dry weight of each composition in samples
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| Citation: | Shenggao Cheng, Xumei Mao, Fenglin Wang, Yetang Hong, Yongxuan Zhu, Qi An. Tephra Discovered in High Resolution Peat Sediment and Its Indication to Climatic Event. Journal of Earth Science, 2008, 19(2): 174-183.
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Floating tephra was deposited together with ice core, snow layer, abyssal sediment, lake sediments, and other geological records. It is of great significance to interpret the impact on the climate change of volcanic eruptions from these geological records. It is the first time that volcanic glass was discovered from the peat of Jinchuan (金川) Maar, Jilin (吉林) Province, China. 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 (天池) volcano eruptions, Changbai (长白) Mountain, nor from Jinlongdingzi (金龙顶子) volcano about 1 600 aBP eruption, but maybe from an unknown eruption of Longgang (龙岗) volcano group according to their geochemistry and distribution. Geochemical characters of the tephra are similar to those of Jinglongdingzi, which are poor in silica, deficient in alkali, Na2O content is more than K2O content, and are similar to distribution patterns of REE and incompatible elements, which helps to speculate that they originated from the same mantle magma with rare condemnation, and from basaltic explosive eruption of Longgang volcano group. The tephra, from peat with age proved that the eruption possibly happened in 15 BC–26 AD, is one of Longgang volcano group eruption that was not recorded and is earlier than that of Jinglongdingzi about 1 600 aBP eruption. And the sedimentary time of tephra is during the period of low temperature alteration, which may be the influence of eruption toward the local climate according to the correlativity of eruption to local temperature curve of peat cellulose oxygen isotope.
Volcanic activity is a short and intense geologica phenomenon affecting the environment strongly.The volcanoes erupted a lot of gas, solid scraping, and magma which destroyed villages and farms, even the whole city sometimes.The aerosol from volcanic eruption diffused in atmosphere, and even has a tremendous impact on a certain area or global climate in a certain time(Horn and Schmincke, 2000).Volcano eruption in geological history is regarded as a transient event, and the ejecta form an isochronism surface when scattered on the surface of a certain region deposited with other sediments, then the tephra can be regarded as a known time point for calibration.This is the basic principle for tephra dating as a time calibration point(Barberi et al., 1992).
It is an important investigative material to volcanic glass of tephra for volcanology.The chemical compositions of volcanic glass do not change with the distance to the eruption source, and it is so as to the tephra, therefore, volcanic glass also has the significance of demarcating the stratum isochronism surface(Rose and Chesner, 1987).So the chemical compositions of volcanic glass are used to determine the source of tephra or distinguish the products of different eruptions in the same layer.Presently, the volcanic glass in ice core(Fiacco et al., 1994, 1993), snow layer(Dugmore et al., 1995; Blackford et al., 1992), abyssal sediment(Liu et al., 2006; Mangerud et al., 1984; Ledbetter and Sparks, 1979), loess layer(Berger et al., 1996), and peat bog(Blackford, 2000)has been studied.To date the sediment layer which contains volcanic glass and to discuss the influence of volcano eruption on climate change have obtained a much more satisfactory result.
A section of silt-like material was discovered in the middle of peat layer while cutting the samples from Jinchuan, Jilin Province, Northeast China.It was proved to be tephra whose possible age is 15 BC–26AD(calibration age)with respect to its origin.Hong et al.(2001, 2000)had studied the relationship between oxygen and carbon isotopic compositions and ancient climate change in Jinchuan peat.15 BC–26 AD is the low temperature alteration time shown on the oxygen isotopic temperature variation curve, and it may be the result of the impact of volcanic eruption on the local climate change.
Xidadianzi is a dry Maar, called as Hanlongwan by local people, located on the west of Jinchuan Town, Huinan County, Jilin Province.Its altitude from the floor is 700 m, approximate rotundity, and the acreage is 0.85 km2.The relative altitude of pyroclastic rock ring around Maar is about 10 m, which is made up of basaltic slag and basaltic fulgurite mostly.In Maar peat is 4–5 m thick, and the thickest part reaches about 10 m.
Maar Lake is formed by the effect of stream-magma eruption commonly, and it has some outstanding characters compared with other lakes(Liu et al., 2000).(1)Maar Lake is a mini-type close lake and its pooled acreage approaches lake acreage.Water balance is largely controlled by precipitation and evaporation, and there is no runoff to the outside.(2)Lake bed is relatively flat, and the water level is rather deep, which gets along with the lake in a certain percentage, so that it is good to the formation and preservation of laminae, especially the laminae of biological cause.The time resolution of the ancien climate change may be accurate to a year by the existence of laminae.(3)Higher depth of lake basin and its rapid sedimentation at a rate reaching 1 mm/a or even higher are apt to achieve longer and continuous high-resolution records of the ancient environment.(4)The formation of Maar Lake is closely related to volcanic activity, so that it is easy to date its formation.
The peat in Xidadianzi, Jinchuan, is a Holocene herbaceous peat, which has developed continuously with a sedimentation rate around 1 mm/a.The layer depth of Jinchuan peat has a good linear relationship with its14C age, performing as a straight line closed to origin, providing the basis to dating the source of silt-like material.Jinchuan peat is near to Longgang volcanoes, also to east of Changbaishan Tianchi volcano.And the two volcanic regions both had violent eruptions during Holocene(Liu, 2000).Thus it is mostly possible that the ejecta fell into the Maar Lake and deposited in peat.The samples are near to the center of arid Maar in Xidadianzi, and there are no tephra on lava platform(Mao et al., 2002).The peat layer is marsh peat bog deposited after Maar Lake format, which indicates that water in Maar Lake had very weak hydrological condition during its development.Because of the above characters of Maar Lake itself, the possibility that silt-like material in deposited peat layer is brought by adventitious water or Maar rock ring can be excluded.
Two holes separated by a distance of about 20 m near the center site of Xidadianzi with cutting-type handle drill were drilled, which are 8 m and 10 m thick with respect to the holes of JA and JB, respectively.The upside reed-moss-grass peat core contained a section of silt-like material, which is about6 m thick whereas the underside takes filemot to dark gray clay, silt, and coarse gravel layer in turns(Mao et al., 2002).A section of silt-like substances at a depth of 2.5 m recorded with good layer was suspected as volcanic ash during sampling.After carefully rejecting modern plants on the surface, nearly 6 000, a continuous sequence of samples were obtained by the1 cm gap divided peat core.The thickness of two holes in peat is the same, so is the thickness of silt-like material.
Dealt with the four continuous samples from the center of the peat core of hole JB which contain silt-like material, their depths from the surface are 252cm, 253 cm, 254 cm, and 255 cm, respectively(sample numbers are JB252, JB253, JB254 and JB255, respectively)with length of 4 cm totally.The samples of bog peat were transferred into a plastic beaker, adding 0.1 M Na OH 100 m L, approximately, and placed under a constant temperature about 70℃to disperse peat for 12 h, so that the humic acid in peat sample is dissolved.Then most of the plant materials are singled out firstly, and the samples are filtrated by120 mesh separating two groups of solid parts, while light plant residue seen on and underside the sieve are removed.Light plant residue, solid parts are cleaned with some quantity of dilute HCl to neutralize unnecessary Na OH, followed by repeat clean of de-ionized water, yet there are some plant residue left in samples.Consequently, two groups of samples are baked for 2 h at a low temperature(< 500℃), then two groups of solid material(< 120 mesh and > 120 mesh)excluding plant residue are received.More attention should be paid to the use of deionized water and avoiding glass containers in the whole process.
The shapes of samples were observed by binocular microscope, polarizable microscope, and electron microscope; the main chemical compositions of tephra were measured using electron microprobe, the model of electron microprobe is JCXA-733(Japanese electronics), 2.2×10-8 A electrical current, and 20 k V accelerating voltage; trace elements were measured by ICP-MS.All the experiments were carried out in the State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences Guiyang, China.
Weighing pure dry solids of four samples with an electronic scale, the results are shown in Table 1.
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As shown in Table 1, the weight of four samples bigger than 120 mesh(coarse)decreased from bottom to up, among which crude components of bottom JB255 is the most, and the top JB252 is the least whereas the weight of the group smaller than 120 mesh(refined)are different in each sample, among which those of middle two are more than others especially JB252 is the least.These rules seem to indicate that the material is of volcanic origin with a near-source eruption, or be the product of severa mixed eruptions or a certain continuous eruption persisting for years.Therefore, further identification was carried out by the microscope.
The origin of the volcanic substance focuses on identifying the volcanic glass.The origin of the volcanic substances will be confirmed based on the presence or absence of volcanic glass.
The observations of samples, through binocular microscope, greater than 120 mesh show that the particle size is not equal(0.1–0.5 mm), and the granule has irregular corner, obvious concavity and well developed pore with very loose surface and slag-like structure(Fig. 1a), which seems not to be handled by ground.Glass litter and crystal debris are the main elements, among which glass litter accounts to 60%and others are crystal or ceramic-matrix coexistence, aggregation of feldspar, pyroxene, and quartz decrystalized in varying degrees.The glass material can be found through observations on four samples in groups more than 120 mesh by polarizing microscope(Fig. 1b).
Observations by electron microscope on the groups less than 120 mesh show a great deal of volcanic glass(Fig. 2).
Each sample has the same composition of volcanic glass by the spectroscopy analysis compared to the four samples under the electron microscope.The composition of volcanic glass can be broadly divided into two types according to their elements and relative content of elements on the basis of spectra graph(Fig. 3): one is basaltic type(Fig. 3a), thecontents of O, Al, K, Ca, Fe are as high as that of Si, also includes Na, Ti etc., the other is quartz type(Fig. 3b), and its main contents are Si and O with few other elements.
Therefore, it is concluded that these solid materials are of volcanic origin, namely, tephra on the basis of the granule surface and the confirmation of volcanic glass by polarized light microscopy and scanning electron microscopy.
The main chemical compositions of tephra are shown in Table 2, the content of Si O2 in tephra studied is close to slag of Jinlongdingzi both under 55%(Fan et al., 2000, 1999), especially the approximative gross contents of Al2O3 and Fe, and even the equal gross contents of Na2O and K2O with higher content of Na2O than that of K2O.It is obvious that the characters of tephra composition are very similar to those of Jinlongdingzi eruption, Longgang.They are both poor in Si O2, rich in alkali, more Na2O content than K2O content, and the composition of feldspar crystal is calcoligoclase.The above composition helps to speculate that it is of alkaline basalt category originated from the basaltic magma.The total quantity in quartz crystal is found to be lower through electron microprobe with possibility that the content of volatile part is high.The reason is that amounts of eject produced by explosive eruption of magma mixed with near surface water formed massive volcanic glass, feldspar, and minicrystal in the course of rapid chilling, whereas relatively higher crystallization point of quartz crystal and rapid crystallization made the volatile matter be preserved in it without enough escape time.
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Sample JB252 is not sufficient for the analysis of REE and trace elements, so the other three samples of tephra are analyzed by ICP-MS in the Institute of Geochemistry, CAS(Table 3).
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Quite similar REE and incompatible element distribution patterns shown in Fig. 4 prove that tephra of the three samples are derived from the same single eruption undoubtedly.
Comparing the distribution of trace element from the tephra studied with Jinlongdingzi volcanoes(erupting about 1 600 a BP)in Longgang area(Fan et al., 1999), they have extremely similar distribution of rare earth element, which shows faint positive Eu anomalies and absence of Sr and Ba positive anomalies found in shield basalt of Tianchi volcano and Wangtiane volcano.And relative similar distribution of incompatibility element reveals they are from the same mantle derived magma with rare contamination, namely, the tephra having their origin in the Longgang volcano area.It is possible that the magma suffered a fractional crystallization in a mantle magma chamber to a certain extent, whereas the composition of the magma hardly changed during the course of rising and eruption indicating that there is no stage of crystallization in a crystal chamber and a possibility of single mantle magma chamber existing in the area of Longgang.But distribution of trace element from tephra studied is slightly different from that of Jinlongdingzi volcanics in Longgang area(erupting about 1 600 a BP), whose distribution of REE is lower than that of Jinlongdingzi, Longgang and partial incompatibility elements changed slightly in distribution pattern.It is conceivable that the magma formed the tephra that interacted with near-surface water during eruption and led an explosive and steam-magma eruption, which made some of the incompatibility elements anomalistic in the course of rapid chilling, and contents of rare earth and major elements relatively low attributing to high content of volatile matter.
A layer of silt-like substance contained in middle of peat of Xidadianzi, Jinchuan is tephra.If the water or wind had moved them from the surface, there would have been a few layers of these tephra, accordingly, simple component and angular-granule surface of tephra deposited in environment of Maar Lake indicate they are in-situ sediment rather than transportation.Developed pore on the surface, high content of volcanic glass and large numbers of components whose diameters exceed 0.1 mm help to speculate that they are the outcome of an explosive eruption of near-source volcano.Moreover, regulation that granule shows different diameters and similar component of volcanic glass and crystal reveals the tephra have the identical origin of eruption.
It is a common way to use the age of sediment for dating the sediment source.It is a good linear relationship between depth of Jinchuan peat layer to its 14C age, which can make use of the 14C age of peat layer including volcanic ash to speculate the age of the volcanic eruption.After drilling in the peat layer from top to bottom and collecting total of 20 samples for 14C dating, the peat was dealt with in the State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences to extract cellulose, and then sent to the Japan Accelerator Mass Spectrometry Laboratory, University of Tokyo for analysis.The 14C ages in the upper and lower layers of peat cellulose samples for ash location are 1 783 a BP(layer depth of 230 cm) and 2 169 a BP(layer depth of 274 cm), respectively, and according to which the linear interpolation can be drawn from the volcanic ash layer of median age, namely 2 002 a BP–1 976 a BP(layer depth of 255–252cm)(Fig. 5).
It is significant to confirm the eruption in sediment of peat of Xidadianzi, Jinchuan.On the one hand, more reliable exact calibration curve of 14C age for peat is able to gain after calibrating the 14C age of peat containing the tephra, provided the age of them is known.On the other hand, it proposes a new method to date volcano eruption and the age of tephra in it according to the 14C age of peat.
Tianchi volcano of Changbai Mountain has erupted fiercely about 5 000 a and 800 a ago in Holocene, and fallout pumice and pyroclastic brought by the eruption gather around the Tianchi volcano and its eastem area.However, the ejecta did not reach Xidadianzi located to the west of Tianchi volcano in170 km owing to the west wind.Therefore, there is little probability that the ejecta of Tianchi volcano fell into Xidadianzi and deposited in the peat sediment, and the ejecta are not found in the corresponding time horizons.Furthermore, there is even big difference between component of ejecta studied and that of Tianchi, Changbai Mountain, especially high content of Si O2 in that of Tianchi, Changbai Mountain, which came from acid magma, and this reveals that they have different erupting sources(Liu et al., 1998).Besides, comparing with the time Tianchi, Changbai mountain erupted, there is tremendous distance between it and there are two eruptions in 5 000 a BP and 800 a BP of Tianchi volcano.Above all, the ejecta are impossibly derived from Tianchi, Changbai Mountain.
Longgang volcanic cluster has erupted several times since it formed, one of which known at present Jinlongdingzi erupted about 1 600 a ago.Because of the scale of eruption and strong west wind, its tephra sheet did not reach the Xidadianzi(Mao et al., 2002), and ejecta are not found in corresponding peat sediment.So the ejecta are impossibly derived from the eruption 1 600 a ago.
The volcanic ejecta found in Xidadianzi peat deposition are almost coarse particles and should come from near source of volcanic eruption according to particle size analysis and identification results.But there are only Tianchi volcano, Changbai Mountain, and Longgang volcano group nearby which had eruptions during last ten thousands years.Geochemical and chronological results excluded the possibility from Tianchi volcano, Changbai Mountain, and impossible from Jinlongdingzi volcano about1 600 a BP eruption either.Those ejecta found in peat sediment of Xidadianzi have similar components which are rich in alkali, deficient in Si O2, and more Na2O than K2O with those of Jinlongdingzi in Longgang area, indicating their origin is identical to basaltic magma source.The ejecta characterized with volcanics in Longgang area may be from another eruption of volcanic cluster in Longgang area earlier than that of Jinlongdingzi about 1 600 a BP ago, and eruption time of the volcanic cluster is likely to be 15BC–26 AD(calibration age)according to 14C age of ejecta(2 002 a BP–1 976 a BP).
Volcanoes emit a lot of dust and gas, which make great influence on surrounding environment(Prather, 1992; Devine et al., 1984).Extrusive eruption closes over the sky and weakens sun shine, which makes the local temperature decrease obviously, for so much volcanic ash floats in air during a certain period in a certain area, and extrusive gases poison the surroundings and lead to the death of lots of plants and animals.The isotopic composition of peat cellulose oxygen has good response to local temperature change for example, the greater the value of peat oxygen isotope composition of Jinchuan, Jilin, the higher the temperature at that time(Hong et al., 2000).Comparing the local oxygen isotope values of Xidadianzi peat cellulose to the volcano eruptions of this study, Jinlongdingzi volcano about 1 600 a BP eruption, Tianchi volcano about 5 000 a BP and about800 a BP eruptions, these four times of volcano eruptions correspond to relatively cold period in oxygen isotope curves, and it seems to indicate that the local volcanic eruptions lower the temperature, namely, Tianchi volcano about 800 a BP eruption impacts the global climate greatly, and it leads to a decrease of the global mean temperature range of 3 to5℃(Horn and Schmincke, 2000), which is consistent with the result of oxygen isotope curves.Especially, Jinlongdingzi volcano, about 1 600 a BP eruption in the oxygen isotope curve, is consistent with the corresponding period of low temperature, which seems to indicate that the eruption impacts the local temperature significantly.Sediment time of the tephra is a period of low temperature alteration, which may be the influence of eruption toward the local climate, through correlativity of the eruption on temperature curve from peat cellulose oxygen isotope(Fig. 6).
After treatment, analysis, component determination, and comparison of silt-like material deposited in peat sediment of Xidadianzi, Jinchuan in Jilin Province, the conclusions are as follows.
(1) Silt-like material deposited in the middle of peat sediment from 255 cm to 252 cm of Xidadianzi, Jinchuan in Jilin Province is tephra, which is preserved in sealed environment of Maar Lake.Characters of the tephra such as homogeneous component, content of volcanic glass(about 60%), high content of component whose diameter exceed 0.1mm, identical content of major elements, resemblance of rare earth, and the distribution pattern of incompatibility elements help to confirm that they are tephra fallout sediment originating from a blast-eruption of near-source volcano.
(2) The tephra from volcano cluster in Longgang are an outcome of its eruption beyond the record and research rather than that of Jinlongdingzi about 1 600a BP.And the eruption is characterized with the basalt-water explosive eruption that is typical in volcanic area of Longgang.
(3) The tephra are similar with volcanics of Jinlongdingzi, which are rich in alkali, deficient in SiO2, more Na2O content than K2O content, faint positive Eu anomalies, and absence of Sr and Ba positive anomalies possessing in shield basalt of Tianchi volcano and Wangtiane volcano.All the characters reflect that they come directly from identical and relatively original mantle magma chamber without contamination.
(4) The 14C age of peat tephra situated reveals that the time of newly found eruption is 2 002 a BP–1 976 a BP, so there are at least two eruptions of Longgang volcano cluster since Holocene—small eruption in the period of 2 002 a BP to 1 976 a BP(14C age) and eruption of Jinlongdingzi about 1 600 a BP(14C age).It is worth paying attention to that depositional period of tephra is a period of low temperature alteration, which may be the influence of the eruption toward local climate.
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Shenggao Cheng, Xumei Mao, Fenglin Wang, Yetang Hong, Yongxuan Zhu, Qi An. Tephra Discovered in High Resolution Peat Sediment and Its Indication to Climatic Event. Journal of Earth Science, 2008, 19(2): 174-183.
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