- Email: [email protected]
14C dating of sediment samples
Nuclear Instruments
and Methods in Physics Research B 123 (1997)455-459
Beam interactions with Materials 6 Atoms
ELSEVIER
14Cdating of sediment samples W. Kretschmer a** , G. Anton a, M. Bergmann a, E. Finckh a, B. Kowalzik ‘, M. Klein ‘, M. Leigart a, S. Merz a, G. Morgenroth a, I. Piringer a, H. fister b, R.D. Low b,l, T. Nakamura ’ a Physikulisches Institut. Universitd Erlangen-Niimberg. D-91058 Erlangen. Germany b InstitutJiir Vor- und Frchgeschichte. Universitiit Miinchen, D-80992 Miinchen. Germany ’ Dating and Materials Research Center, Nagoya University, Chikusu. Nugoya 464, Jupan
Abstract We report on the 14C dating of sediment samples from Bavaria using the Erlangen AMS facility. The absolute time calibration of different sediment profiles together with pollen analyses should establish a better chronology of climate and vegetation during the Holocene in Bavaria. The methods to prepare AMS samples from both bulk sediments and pollen grains are discussed. Finally we report on the dating of volcanic sediment samples from Southern Kyushu (Japan) which is interesting for the eruptive history of the Sakurajima volcano and its predecessor.
1. Introduction Since the last glacial period the temperature in Southem Germany has increased by about 10°C and vegetation has developed from only few species to a huge variety. In that time period of increased warming both animals and men have influenced the vegetation considerably. To investigate the evolution of biosphere in the Holocene the research program “change of geo- and biosphere during the last 15000 years” has been started in Germany. As part of this program we perform 14C dating of sediment profiles from Bavaria using the Erlangen AMS facility. In this paper we discuss methods for sample preparation and report on first results for two continuous sediment cores covering the time interval mentioned above. Both cores have been investigated by pollen analysis which gives some information on the vegetation history of the region they have been taken.
2. Methods The sediment cores have been taken by H. Kiister from bog sites, which developed from ancient lakes and fens. The cores were cut into pieces of half a meter and stored frozen to temperatures below 0°C to avoid the interaction with microorganisms containing “modem” carbon. For
* Corresponding author. Tel. + 49 913 1 857075, fax + 49 9 I3 1 15249, e-mail: [email protected]. ’ Present address: Center for Ancient Studies, Minneapolis, USA. 0168-583X/97/$17.00
the 14C measurements the samples which are extracted from the inner part of the core, have to be treated chemically to convert them to graphitized carbon (see Fig. I). In a chemical pretreatment two major components which may obscure the 14C results are removed: 1) carbonates arising from the erosion of limestone are removed by heating the material with HCI and 2) humic acid which may be younger due to its high mobility is removed by heating it together with NaOH. The second procedure is repeated until the brown color of the solution vanishes. This AAA pretreatment is finished by another heating in HCI, washing in distilled water and drying the remaining material in an oven. The residue is put into a quartz tube together with copper oxide and silver wool. After evacuation the tube is sealed by melting. The conversion to carbon dioxide is performed by heating the sealed tube to 900°C for about an hour. After a slow cooling the tube is cracked in the reduction facility, where graphitization takes place by heating an appropriate mixture of hydrogen and carbon dioxide at 625°C with iron powder of IO pm as a catalyst. Details of the reduction procedure are described in Ref. [I]. One problem with the radiocarbon dating of peat samples is the “hard water effect”, since the plants and possibly algae on which the peat is based can assimilate CO, both from the air and from the water. Depending on the site and on the pH value of the moor old carbonates from the lower stratum may be dissolved in the water with the consequence that the dating of the bulk sediments can be obscured by this effect. To obtain more reliable results the dating of macrofossils and pollen grains is being performed in addition to that of the bulk sediments. For the
0 1997 Elsevier Science B.V. A11rights reserved
PII SOl68-583X(96)00603-9
XII. ENVlRONMENTAL/PALEOCLIMATIC
STUDIES
456
W. Kretschmer er al./Nucl.
AAA-treatment
Insrr. and Meth. in Phys. Res. B 123 (1997) 4.55-459
Oxidation and reduction
conversionto CO, with
lnHCIat80T (dissolving of carbonale)
CuO at 903 “C
V
I--
reductionto carbon
repeated treatment with 1 n NaOH at 80 “C
with H, and iron at 625 “C
___..--
I
solid carbonfor sputtertargets
1 n HCI at 80 “C I-
extraction of the pollen grains from the sediment we follow a procedure described in Refs. [2,3] shown schematically in Fig. 2. The material is heated in 1N NaOH at 80°C for the deflocculation and for the removal of humic acids, then it is sieved with a 100 pm nylon mesh. The remaining material is treated like the bulk sediments described above, the filtrate is repeatedly heated with NaOH followed by HCl to remove carbonates. For the removal of silicates we use cold 10% HF and hot 1N HCl, the cellulose is removed with H,SO, and lastly for the deflocculation of amorphous organic material a treatment with NaOCl is performed. After each step the dissolved material is separated from the pollen by sieving with a 20 pm nylon mesh. The efficiency of this separation method is finally checked under the microscope. The conversion to sputter targets is done in the usual manner described above. But due to the small sample size the reduction is performed in a newly built reduction apparatus with a volume of only 3.8 cmj described in Ref. [l]. The resulting material contains 50-500 pg carbon. Since samples of this size usually have more modem carbon background [3] both calibration- and graphite- samples with comparable carbon content are used for the AMS measurements.
3. Sediment samples from Bavaria wash with distilled water and dry in an oven
Fig. 1. Sample preparation of bulk sediments for AMS measurements.
The two sediment cores from Bavaria discussed in this paper are taken from bog sites close to lake Stamberg and to lake Chiemsee, respectively, both located in a short distance north of the Alps. Both cores have been roughly
Procedure of Dollen extraction
-1 I
l--E;
sieving at 1OOpm I
c1OOpm
Fig. 2. Extraction of pollen grains from sediment samples.
W. Kretschmer et al./Nucl.
457
Instr. and Meth. in Phys. Res. B 123 (1997) 455-459
I
-,--,1,-~-,-r.-,-.-,~~-~,-,-~,-~~,
75
100
125
150
175
200
225
250
275
300
325
350
375
400
425
450
475
‘0
120% 80 ,s LO; 75
100
150
200
250 depth
Fig. ‘3. Absolutetime
scale of the Leutstetten
.2mo~:;;.:.:.::;:..:~~.:::~:::::;:~ co
%?a
cm
300
350
(cm)
sediment core (top) compared
to the resuks of a
ea
700
LOO pOkn
9ca
06 5 a adySiS
(bottom).
,003
depth [cm]
80
6
60
2
40
% 0 g = E_
20 400
560
660
760 depth
Fig. 4. Absolute time scale of the Eggs&t
800
900
0 Id00
(cm)
sediment core (top) compared
to the results of a pollen analysis (bottom).
XII. ENVIRONMENTAL/PALEOCLIMATIC
STUDIES
458
W. Kretschmer et al./Nucl.
Instr. and Meth. in Phys. Res. B 123 (1997) 455-459
predated by pollen analysis [4,5], but due to local variations of vegetation history this method has an estimated error of up to 2000 years. The first site, Leutstetten, is a bog close to the river Wiirm, the northern outflow of lake Stamberg. In the last glacial period this region was covered by glaciers extending northward from the Alps and terminated by a final moraine up to this position. In the late glacial period of increased warming the glaciers retreated, leaving lake Stamberg extending some kilometers more to the north and to the south than today. The formation of the bog originates from a decreasing lake-water level but due to drastic temperature changes the level of lake Stamberg could have changed considerably [5]. The corresponding periods of flooding and drying would in turn result in an irregular sedimentation rate of the core consisting of peat in the upper layer of 1.60 m and mainly of lake marl in the lower part. The result of our AMS measurement, calibrated age versus depth, for the Leutstetten sediment core [6] is shown in the upper part of Fig. 3. This is compared with a pollen distribution for beech, hazel and pine for the same profile, shown in the lower part of Fig. 3. The sediment core shows periods of slow and fast growth rate reflecting the above mentioned changes of the lake water level. Especially between 8000 BC and 12000 BC the increase in sedimentation is extremely slow indicating a rather dry period for this site. The pollen distribution shows a sharp increase for pine pollen at a depth of 3.90 m corresponding to an age of about 12000 BC. With increased warming hazel (after 7000 BC) and later beech (after 3000 BC) grew more frequently close to this place. The second site, the Eggst;itt moor, is located northwest of lake Chiemsee, in a kettle hole with no in- or outflow of water. Therefore a rather regular growth mte. of sedimentation is expected. The bog is now covered by a 3.50 m thick layer of water followed by a 0.50 m thick layer of humus, the sediment core thus beginning at 4 m below ground level. The results of both our AMS measurement [7] and the pollen analysis are shown in Fig. 4. For this core three fractions have been dated separately: pollen grains with a diameter between 20 pm and 100 pm, residues with a size larger than 100 pm and bulk sediments corresponding to the remaining material after the AAA treatment. Except for the sample taken at the deepest level the results agree within the experimental uncertainties supporting the relia-
Table 1 14Cage for charred wood samples from the Aim tephm Sample
‘k age yr BP
Statistical error
A: Osumi pumice fall B: Ito ignimbrite C: Tarumizu pumice fall
24423 24900 25 197
+ 128 f 133 f133
bility of the age determination. For the sample at a depth of 9.70 m, slightly above the moraine material, the bulk sediment is 1400 years older than the corresponding pollen grains, which might be an indication for the hard water effect. The pollen distribution shows a sharp rise for pine at 9.60 m corresponding to an age of about 12000 BC. The increase of hazel pollen and the simultaneous decrease of pine pollen at 8.80 m indicates that the pine population has been replaced by the hazel at about 8000 BC in the region around lake Chiemsee. The rise of beech pollen at 6.40 m corresponds to the appearance of beech tree at about 3500 BC. ‘“C dating with a simultaneous pollen analysis of sediment cores from bog sites represent a great potential for the deduction of vegetation history since last glacial period. For this discussion we have chosen three species of plants typical for the area. The pollen distributions of the other plants, e.g. grass, herbs and all sorts of trees, also analyzed in this sediment core, will be discussed in subsequent publications. Although the pollen record reflects largely the vegetation according to local conditions we have deduced as a common factor that at both sites the time sequence is very similar for these three species.
4. Volcanic sediment samples The Sakurajima volcano, a post caldera cone of the Aira caldera located in southern Kyushu, is one of the most active volcanoes in Japan. Tephra layers from large scale eruptions of this volcano and its predecessor provide excellent time markers in this region and have been investigated extensively [8,9]. The Aira tephra (AT), the product of the latest large scale ignimbrite eruption from the Aira caldera, is subject to a controversy concerning its age ranging from 21 to 25 ka BP. Therefore three charred wood samples from this AT layer, collected at different locations, have been dated using the Erlangen AMS facility. The wood samples and the graphite used for background determination were both treated with the AAA procedure, combusted to CO, and reduced to graphitized carbon as described before. The 14C background induced by the target preparation was 0.6 pmC corresponding to an apparent age of 41 ka. Our results for the AT samples shown in Table 1 favor recent AMS measurements by Nakamura 191, but are in disagreement with decay counting measurements by Kigoshi and others ranging from 21 to 23 ka BP.
Acknowledgements This work was supported by the Deutsche Forschungsgemeinschaft.
W. Kretschmer et al./Nucl.
Instr. and Meth. in Phys. Res. B 123 (1997) 455-459
References [l] W. Kretschmer, G. Anton, M. Bergmann, E. Fmckh, B. Kowalzik, M. Klein, M. Leigart, S. Metz, G. Morgenroth and 1. Piringer, these Proceedings CAMS-7). Nucl. Instr. and Meth. B 123 (1997) 93. [2] K. Faegri and J. Iversen, Textbook of Pollen Analysis (Wiley, 1989). [3] T.A. Brown, G.W. Farwell, P.M. Grootes and F.H. Schmidt, Radiocarbon 34 ( 1992) 550. [4] E. Wagner, Technical University of Munich, diploma thesis, 1991.
4.59
[5] H. Kister and R.D. Low, University of Munich, private communication. [6] B. Kowalzik, University of Erlangen-Nuremberg, diploma thesis, 1996. [7] S. Merz, University of Erlangen-Nuremberg, diploma thesis, 19%. [8] H. Machida and F. Arai, Atlas of Tephra in and around Japan (Univ. of Tokyo Press, 1992). [9] M. Okuno, T. Nakamura, H. Moriwaki and T. Kobayashi, Contribution to the Int. Conf. on Geochemistry, Geochronology and Isotope Geology, Tokyo, 1996; T. Nakamura, private communication, 1996.
XII. ENVIRONMENTAL/PALEOCLIMATIC
STUDIES
and Methods in Physics Research B 123 (1997)455-459
Beam interactions with Materials 6 Atoms
ELSEVIER
14Cdating of sediment samples W. Kretschmer a** , G. Anton a, M. Bergmann a, E. Finckh a, B. Kowalzik ‘, M. Klein ‘, M. Leigart a, S. Merz a, G. Morgenroth a, I. Piringer a, H. fister b, R.D. Low b,l, T. Nakamura ’ a Physikulisches Institut. Universitd Erlangen-Niimberg. D-91058 Erlangen. Germany b InstitutJiir Vor- und Frchgeschichte. Universitiit Miinchen, D-80992 Miinchen. Germany ’ Dating and Materials Research Center, Nagoya University, Chikusu. Nugoya 464, Jupan
Abstract We report on the 14C dating of sediment samples from Bavaria using the Erlangen AMS facility. The absolute time calibration of different sediment profiles together with pollen analyses should establish a better chronology of climate and vegetation during the Holocene in Bavaria. The methods to prepare AMS samples from both bulk sediments and pollen grains are discussed. Finally we report on the dating of volcanic sediment samples from Southern Kyushu (Japan) which is interesting for the eruptive history of the Sakurajima volcano and its predecessor.
1. Introduction Since the last glacial period the temperature in Southem Germany has increased by about 10°C and vegetation has developed from only few species to a huge variety. In that time period of increased warming both animals and men have influenced the vegetation considerably. To investigate the evolution of biosphere in the Holocene the research program “change of geo- and biosphere during the last 15000 years” has been started in Germany. As part of this program we perform 14C dating of sediment profiles from Bavaria using the Erlangen AMS facility. In this paper we discuss methods for sample preparation and report on first results for two continuous sediment cores covering the time interval mentioned above. Both cores have been investigated by pollen analysis which gives some information on the vegetation history of the region they have been taken.
2. Methods The sediment cores have been taken by H. Kiister from bog sites, which developed from ancient lakes and fens. The cores were cut into pieces of half a meter and stored frozen to temperatures below 0°C to avoid the interaction with microorganisms containing “modem” carbon. For
* Corresponding author. Tel. + 49 913 1 857075, fax + 49 9 I3 1 15249, e-mail: [email protected]. ’ Present address: Center for Ancient Studies, Minneapolis, USA. 0168-583X/97/$17.00
the 14C measurements the samples which are extracted from the inner part of the core, have to be treated chemically to convert them to graphitized carbon (see Fig. I). In a chemical pretreatment two major components which may obscure the 14C results are removed: 1) carbonates arising from the erosion of limestone are removed by heating the material with HCI and 2) humic acid which may be younger due to its high mobility is removed by heating it together with NaOH. The second procedure is repeated until the brown color of the solution vanishes. This AAA pretreatment is finished by another heating in HCI, washing in distilled water and drying the remaining material in an oven. The residue is put into a quartz tube together with copper oxide and silver wool. After evacuation the tube is sealed by melting. The conversion to carbon dioxide is performed by heating the sealed tube to 900°C for about an hour. After a slow cooling the tube is cracked in the reduction facility, where graphitization takes place by heating an appropriate mixture of hydrogen and carbon dioxide at 625°C with iron powder of IO pm as a catalyst. Details of the reduction procedure are described in Ref. [I]. One problem with the radiocarbon dating of peat samples is the “hard water effect”, since the plants and possibly algae on which the peat is based can assimilate CO, both from the air and from the water. Depending on the site and on the pH value of the moor old carbonates from the lower stratum may be dissolved in the water with the consequence that the dating of the bulk sediments can be obscured by this effect. To obtain more reliable results the dating of macrofossils and pollen grains is being performed in addition to that of the bulk sediments. For the
0 1997 Elsevier Science B.V. A11rights reserved
PII SOl68-583X(96)00603-9
XII. ENVlRONMENTAL/PALEOCLIMATIC
STUDIES
456
W. Kretschmer er al./Nucl.
AAA-treatment
Insrr. and Meth. in Phys. Res. B 123 (1997) 4.55-459
Oxidation and reduction
conversionto CO, with
lnHCIat80T (dissolving of carbonale)
CuO at 903 “C
V
I--
reductionto carbon
repeated treatment with 1 n NaOH at 80 “C
with H, and iron at 625 “C
___..--
I
solid carbonfor sputtertargets
1 n HCI at 80 “C I-
extraction of the pollen grains from the sediment we follow a procedure described in Refs. [2,3] shown schematically in Fig. 2. The material is heated in 1N NaOH at 80°C for the deflocculation and for the removal of humic acids, then it is sieved with a 100 pm nylon mesh. The remaining material is treated like the bulk sediments described above, the filtrate is repeatedly heated with NaOH followed by HCl to remove carbonates. For the removal of silicates we use cold 10% HF and hot 1N HCl, the cellulose is removed with H,SO, and lastly for the deflocculation of amorphous organic material a treatment with NaOCl is performed. After each step the dissolved material is separated from the pollen by sieving with a 20 pm nylon mesh. The efficiency of this separation method is finally checked under the microscope. The conversion to sputter targets is done in the usual manner described above. But due to the small sample size the reduction is performed in a newly built reduction apparatus with a volume of only 3.8 cmj described in Ref. [l]. The resulting material contains 50-500 pg carbon. Since samples of this size usually have more modem carbon background [3] both calibration- and graphite- samples with comparable carbon content are used for the AMS measurements.
3. Sediment samples from Bavaria wash with distilled water and dry in an oven
Fig. 1. Sample preparation of bulk sediments for AMS measurements.
The two sediment cores from Bavaria discussed in this paper are taken from bog sites close to lake Stamberg and to lake Chiemsee, respectively, both located in a short distance north of the Alps. Both cores have been roughly
Procedure of Dollen extraction
-1 I
l--E;
sieving at 1OOpm I
c1OOpm
Fig. 2. Extraction of pollen grains from sediment samples.
W. Kretschmer et al./Nucl.
457
Instr. and Meth. in Phys. Res. B 123 (1997) 455-459
I
-,--,1,-~-,-r.-,-.-,~~-~,-,-~,-~~,
75
100
125
150
175
200
225
250
275
300
325
350
375
400
425
450
475
‘0
120% 80 ,s LO; 75
100
150
200
250 depth
Fig. ‘3. Absolutetime
scale of the Leutstetten
.2mo~:;;.:.:.::;:..:~~.:::~:::::;:~ co
%?a
cm
300
350
(cm)
sediment core (top) compared
to the resuks of a
ea
700
LOO pOkn
9ca
06 5 a adySiS
(bottom).
,003
depth [cm]
80
6
60
2
40
% 0 g = E_
20 400
560
660
760 depth
Fig. 4. Absolute time scale of the Eggs&t
800
900
0 Id00
(cm)
sediment core (top) compared
to the results of a pollen analysis (bottom).
XII. ENVIRONMENTAL/PALEOCLIMATIC
STUDIES
458
W. Kretschmer et al./Nucl.
Instr. and Meth. in Phys. Res. B 123 (1997) 455-459
predated by pollen analysis [4,5], but due to local variations of vegetation history this method has an estimated error of up to 2000 years. The first site, Leutstetten, is a bog close to the river Wiirm, the northern outflow of lake Stamberg. In the last glacial period this region was covered by glaciers extending northward from the Alps and terminated by a final moraine up to this position. In the late glacial period of increased warming the glaciers retreated, leaving lake Stamberg extending some kilometers more to the north and to the south than today. The formation of the bog originates from a decreasing lake-water level but due to drastic temperature changes the level of lake Stamberg could have changed considerably [5]. The corresponding periods of flooding and drying would in turn result in an irregular sedimentation rate of the core consisting of peat in the upper layer of 1.60 m and mainly of lake marl in the lower part. The result of our AMS measurement, calibrated age versus depth, for the Leutstetten sediment core [6] is shown in the upper part of Fig. 3. This is compared with a pollen distribution for beech, hazel and pine for the same profile, shown in the lower part of Fig. 3. The sediment core shows periods of slow and fast growth rate reflecting the above mentioned changes of the lake water level. Especially between 8000 BC and 12000 BC the increase in sedimentation is extremely slow indicating a rather dry period for this site. The pollen distribution shows a sharp increase for pine pollen at a depth of 3.90 m corresponding to an age of about 12000 BC. With increased warming hazel (after 7000 BC) and later beech (after 3000 BC) grew more frequently close to this place. The second site, the Eggst;itt moor, is located northwest of lake Chiemsee, in a kettle hole with no in- or outflow of water. Therefore a rather regular growth mte. of sedimentation is expected. The bog is now covered by a 3.50 m thick layer of water followed by a 0.50 m thick layer of humus, the sediment core thus beginning at 4 m below ground level. The results of both our AMS measurement [7] and the pollen analysis are shown in Fig. 4. For this core three fractions have been dated separately: pollen grains with a diameter between 20 pm and 100 pm, residues with a size larger than 100 pm and bulk sediments corresponding to the remaining material after the AAA treatment. Except for the sample taken at the deepest level the results agree within the experimental uncertainties supporting the relia-
Table 1 14Cage for charred wood samples from the Aim tephm Sample
‘k age yr BP
Statistical error
A: Osumi pumice fall B: Ito ignimbrite C: Tarumizu pumice fall
24423 24900 25 197
+ 128 f 133 f133
bility of the age determination. For the sample at a depth of 9.70 m, slightly above the moraine material, the bulk sediment is 1400 years older than the corresponding pollen grains, which might be an indication for the hard water effect. The pollen distribution shows a sharp rise for pine at 9.60 m corresponding to an age of about 12000 BC. The increase of hazel pollen and the simultaneous decrease of pine pollen at 8.80 m indicates that the pine population has been replaced by the hazel at about 8000 BC in the region around lake Chiemsee. The rise of beech pollen at 6.40 m corresponds to the appearance of beech tree at about 3500 BC. ‘“C dating with a simultaneous pollen analysis of sediment cores from bog sites represent a great potential for the deduction of vegetation history since last glacial period. For this discussion we have chosen three species of plants typical for the area. The pollen distributions of the other plants, e.g. grass, herbs and all sorts of trees, also analyzed in this sediment core, will be discussed in subsequent publications. Although the pollen record reflects largely the vegetation according to local conditions we have deduced as a common factor that at both sites the time sequence is very similar for these three species.
4. Volcanic sediment samples The Sakurajima volcano, a post caldera cone of the Aira caldera located in southern Kyushu, is one of the most active volcanoes in Japan. Tephra layers from large scale eruptions of this volcano and its predecessor provide excellent time markers in this region and have been investigated extensively [8,9]. The Aira tephra (AT), the product of the latest large scale ignimbrite eruption from the Aira caldera, is subject to a controversy concerning its age ranging from 21 to 25 ka BP. Therefore three charred wood samples from this AT layer, collected at different locations, have been dated using the Erlangen AMS facility. The wood samples and the graphite used for background determination were both treated with the AAA procedure, combusted to CO, and reduced to graphitized carbon as described before. The 14C background induced by the target preparation was 0.6 pmC corresponding to an apparent age of 41 ka. Our results for the AT samples shown in Table 1 favor recent AMS measurements by Nakamura 191, but are in disagreement with decay counting measurements by Kigoshi and others ranging from 21 to 23 ka BP.
Acknowledgements This work was supported by the Deutsche Forschungsgemeinschaft.
W. Kretschmer et al./Nucl.
Instr. and Meth. in Phys. Res. B 123 (1997) 455-459
References [l] W. Kretschmer, G. Anton, M. Bergmann, E. Fmckh, B. Kowalzik, M. Klein, M. Leigart, S. Metz, G. Morgenroth and 1. Piringer, these Proceedings CAMS-7). Nucl. Instr. and Meth. B 123 (1997) 93. [2] K. Faegri and J. Iversen, Textbook of Pollen Analysis (Wiley, 1989). [3] T.A. Brown, G.W. Farwell, P.M. Grootes and F.H. Schmidt, Radiocarbon 34 ( 1992) 550. [4] E. Wagner, Technical University of Munich, diploma thesis, 1991.
4.59
[5] H. Kister and R.D. Low, University of Munich, private communication. [6] B. Kowalzik, University of Erlangen-Nuremberg, diploma thesis, 1996. [7] S. Merz, University of Erlangen-Nuremberg, diploma thesis, 19%. [8] H. Machida and F. Arai, Atlas of Tephra in and around Japan (Univ. of Tokyo Press, 1992). [9] M. Okuno, T. Nakamura, H. Moriwaki and T. Kobayashi, Contribution to the Int. Conf. on Geochemistry, Geochronology and Isotope Geology, Tokyo, 1996; T. Nakamura, private communication, 1996.
XII. ENVIRONMENTAL/PALEOCLIMATIC
STUDIES