第 四 紀 研 究(The Quaternary Research) 38 (3) p. 237-243 June 1999

Varved Lacustrine in Japan: Recent Progress

Hitoshi Fukusawa*

Annually laminated (varved) lacustrine sediments provide a potentially continu- ous, high-resolution records of the last glacial and Holocene paleoenvironments. In 1991-1993, long sequences of laminated lacustrine sediments were successfully taken from two in southwestern Japan, Suigetsu and LakeTougouike. Based on lamina counting and AMS 14C dating, we clarified that these laminated sediments were varves. mineral composition in these varved sediments would be reflected by eolian dust concentrations, transported from Chinese Loess Plateau, and by precipitations around both lakes. Also, formation of iron sulfides and carbonates in varves would be caused by cyclic changes of sea-water invasion, originated from sea-level changes. Annual to decadal oscillations of dust and iron mineral concentra- tions since the last glacial were detected in varved sequences of both lake sediments. These detailed sedimentological analysis of these sediments revealed varve chronol- ogy, process of varve formation and annual to decadal changes of sea-level and climate. Some of the climatic changes may correlate with abrupt changes ( and Heinrich events) observed in Greenland ice cores and marine sediments.

Key Words: varved lacustrine sediments, climate, sea-level, Asian monsoon, Younger Dryas, Heinrich events

located along the coast of the Sea of Japan and I. Introduction brackish water lakes since the Post Glacial. In The aim of this article is to review the nature our recent works, many cores includ- and formation of Japanese lacustrine varves ing continuously and/or partly varve se- with reference to global climatic changes. quences were also taken from other brackish Varves of Japan provide annual chronology water lakes of Japan. since the last interglacial with detailed records The annual character of was con- of paleoenvironmental changes which are com- firmed by observation of bio-clastic micro- parable to that of Greenland ice cores such as structures under optical and scanning electron GRIP and GISP 2. microscopes (Fukusawa, 1995; Kitagawa et al., Our research group took light to dark grey 1995; Fukusawa et al., in press) (Fig. 1) and laminated clay sediments from Lake Suigetsu high-resolution accelerator mass spectrometry (35°35'N,135°53'E) in 1991-1993 and Lake (AMS) 14C measurements of macrofossil sam- Tougouike (35°28'N,135°55'E) in 1992. Lake ples from the upper section (34.96m below the Suigetsu is 10 km around the perimeter and top sediments) of a 75m long core from Lake covers 4.3km2. It is a typical kettle-type lake, Suigetsu (Kitagawa and van der Plicht, 1998 a, nearly flat at the center, ca. 33.5m deep. Lake b) and from a 39.97m long core from Lake Tougouike which capture most of the allo- Tougouike (Fukusawa, 1998; Fukusawa et al., chthonous materials is presently ca. 2m deep; 1999). Presently the varve chronologies from nearly flat and 4.1km2 in area. Both lakes are Lake Suigetsu and Lake Tougouike extend to Recieved January 31, 1999. Accepted March 7, 1999. * Department of Geography , Graduate School of Science, Tokyo Metropolitan University. Minami-Osawa 1-1, Hachioji,192-0397, Japan. 238 Hitoshi Fukusawa June 1999

glacial streams (de Geer, 1912). However, most terms of "varve" are now used to describe structures formed by different process in dif- ferent environments. O'Sullivan (1983) shows that annually laminated sediments may be biochemical or bioclastic deposits in origin and contain several kinds of laminae at non-glacial region. The group of these laminations are identified as "non-glacial varve" (O'Sullivan, 1983). O'Sullivan (1983) divided laminations of varves into four types. Lamination of auto- chthonous sediments is produced by interac- tion of the climate and internal lake processes, and is either ferrogenic, calcareous, biogenic or clastic laminations (Boygle, 1993). Calcareous varve cannot be recognized in most of Japa- nese lake, because the nature of shows low level of pH due to infux of acidic water from the surrounding area of many volcanoes and springs. In Holocene varves sediments of Lake Suigetsu and Lake Tou- gouike, we could identified two kind of types as following biogenic and ferro-biogenic com- posite laminations (Fig. 1). We have identified two types of structures in the Holocene varve sediments from Lake Suigetsu and Lake Tougouike as well as other Japanese lakes; biogenic and ferro-biogenic lamination. Based on investigations of bio- Fig. 1 Varved lacustrine sediment with inter- clastic composition and micro sedimentary calated tephra and flood layers from Lake structures, an annual layer is divided into three Tougouike, southwestern Japan units (I, II, and III) (Fukusawa, 1998) (Fig. 2). From Fukusawa (1998). The Unit I is the light grey and white layer consisting predominately of diatom frustules ca. 40,000 and 35,000 years before the present without ferric and terrigenous materials. The (BP, relative to 1950), respectively. Unit II is dark grey layers consisting of large These varved sequences permit us to recon- crystals of iron sulfides such as pyrite (FeS2) or struct high-resolution paleoenvironmental re- iron carbonates such as siderite (FeCO3) formed cords since the Last Interglacial. Here, we in- in anoxic water conditions as well as remains troduce 1) process of varve formations with of diatom frustules and clay-size terrigenous reference to speeds of environmental changes materials. The Unit III is grey layer consist- and 2) annual to decadal climatic and sea-level ing of small size diatom frustules and terrige- changes of Japan, possibly related to global nous materials, and contain organic remains climatic change. such as fallen leaves from the surrounding area of lake. II. Structure and formation of varved The monomictic and meromictic lakes in the lacustrine sediments in Japan southwestern Japan such as Lake Suigetsu and A classical varve consists of rhythmic alter- Lake Tougouike show seasonal changes in bio- nations of two laminae deposited annually by chemical and physical conditions. In spring 1999年6月 Varved lacustrine sediments in Japan: Recent progress 239

Fig. 2 Photomicrograph showing seasonal changes in non-glacial varve of Lake Tougouike, southwestern Japan Fig. 3 Sketches of thin sections and sequential From Fukusawa (1998). changes of varve structures from Lake Suigetsu, southwestern Japan From Fukusawa et al. (in press). and autumn, light grey-white layers of diatom frustules (Unit I and Unit III) are formed by phytoplankton blooming relating to overturns III. Abrupt climatic changes in Japan of lake water. Although dark laminas in Eu- detected from varves ropean lake sediments are closely related to anoxic condition during ice covering season, During the late glacial period, we identified the anoxic condition in the Japanese lake en- four types of lamina in the varved lacustrine courages the dark-grey precipitation of iron sediments from Lake Suigetsu and Lake sulfides or carbonates from the bottom water Tougouike; (1) clastic varve, (2) clastic varve in summer. including resting spores, (3) bio-clastic varve An interesting finding is the appearance of including resting spores, (4) biogenic varve the crowding layers of resting spores of (Fukusawa et al., in press) (Fig. 3). The forma- Chaetoceros spp. or Chrysophyte cysts in tion of four types of lamina can be relevant to winter deposition, probably relating to ex- the change in environment and climate in this tremely cold winter or ice cover in the some- regions. time of Holocene. The population of resting Type (1) shows clastic lamination consisting spores can be related to free NO3- (Kuwata, of alternations between light grey coarser 1989) by no overturns of lake waters. terrigenous lamina intercalating siderite (FeCO3) crystals and fine terrigenous lamina. 240 Hitoshi Fukusawa June 1999

The lamination formed mainly from allochtho- in the other sediment section. The increase of nous matter often arose in oligotrophic lakes terrestrial-origin organic remains in sediments where other sources of suspended matter were seems to correspond to the reduction of forest minute (Boygle, 1993). This formation mecha- by the climatic cooling. The turbidites may nism is similar to that of classical varves (de be episodically occurred by the strong precipi- Geer, 1912). Type (2) is clastic laminations tation, related to intensive Asian summer mon- with siderite intercalations including resting soon. spores. Type (3) is characterized in alterna- The abrupt cooling event at about 11,320 tions between diatomaceous lamina including years BP by AMS 14C dating (Kitagawa and siderite crystals and terrigenous lamina. It is van der Plicht, 1998b) may correspond to about formed by weak blooming of diatom prior to 100 years long cooling event from 11,300 to siderite precipitation in summer. Existence of 11,400 years ago recorded in the Greenland ice diatom-poor lamina in type (3) suggests that core (GISP 2) (Stuiver et a1., 1995). The possi- absence of diatom blooming occurs in three ble teleconnection of the abrupt cooling events seasons except summer due to the colder cli- in the East Asia and the Greenland is re- mate than the Holocene climates. Type (4) is cognized and suggests that abrupt climatic biogenic lamination observed in Holocene changes did not only occur at high latitudes varves. It is formed by seasonal changes in such as North Atlantic around the Greenland, diatom productivity and/or seasonal break- but also at low latitudes. down of the stratified water column. IV. Abrupt changes of the Asian Supply of nutrients such as NO3- and PO42- sonsoon and the Westerlies caused blooming of diatom continuously oc- activities reconstructed curs in Lake Suigetsu, because many organic from sediment fluxes materials originating from forest vegetations and soils are decomposed and then flow into The climate in the East Asia is influenced by lake water. In supply of oversaturated nutri- Asian summer monsoon and the Westerlies. ents, the appearance of biogenic varves sug- The summer monsoon is a steady air flow of gests that the climate was relatively warm be- moistures onto the continents, whereas the cause diatom production inferred from the con- Westerlies is a air flow of cold, dry out of centration of diatom frustules is controlled by central Asia. We have researched annual to the surface water temperature. decadal changes of Asian summer monsoon An example of the sequential changes of and the Westerlies activities over the last varved sediment from Lake Suigetsu shows in 35,000 years by using varved sediments of Lake Figure 3 (Fukusawa et al., in press). Figure 3 Tougouike (Fukusawa, 1998). The climate of shows 8cm long section from 13.96m in depth Japan is moist and mild with the rainy season of SGP 3 piston core of Lake Suigetsu. The associating with Asian summer monsoon. section yields 85 years varves of about 11,320 Fallout of modern eolian dust, such as illite and years BP (Kitagawa and van der Plicht, 1998b). quartz grains, can be recognized from late The sequential change of this section can be autumn to early spring when the weather of interpreted climatologically as follows: 1) inner area of Asian Continents is cold and dry. abrupt cooling term of 9 years suggested by We investigated illite crystallinity as an indica- the disappearance of biogenic varves, 2) ex- tor of Asian summer monsoon intensity and tremely cold term of 23 years suggested by quartz flux as an indicator of dry-wet climate appearance of clastic varves and 3) abrupt at the inner area of Asian Continent (Fuku- warming term of 19 years suggested by the sawa,1998). Because ordering of illite crystals shift from clastic varves to biogenic varves. become poor by decomposition due to moisture The frequency of terrestrial-origin organic input (Chamley, 1989), and quartz and illite are remains increases in cooling periods and very resistant minerals against weathering and turbiditic muddy layers in warming periods. early diagenetic alterations. Such similar sequential changes are also seen Sediment flux (mg/cm2yrs) is calculated by 1999年6月 Varved lacustrine sediments in Japan : Recent progress 241 using values of content (weight % transfer to inner Asia was more rapidly than that of GISP mg/g), bulk density (g/cm3) and sedimentary 2 ice core with time lag of about 350 years, but rate (mm/yrs) in the sedimentary column. We warming was more slowly. The latter facts calculated accurate and high-resolution sedi- suggest as follows. It is possible that one of mentation rate, based on varve countings trigger of global climatic changes was located (Fukusawa, 1998; Fukusawa et al., 1999) (Fig. at low latitudes rather than high latitudes 4). Figure 4 shows quartz flux (mg/cm2yrs) around Greenland ice sheet because of exist- changes detected in varves of Lake Tougouike ence of low latitude climatic changes prior to and δ18 O changes of GISP 2 during the post same changes at high latitudes. glacial period from 20,000 to 10,000 years ago V. Sea-level changes and Asian (Taylor et al., 1993; Stuiver et al., 1995). Both records are roughly similar, but not in detail. summer monsoon The decreasing of illite crystallinity by Asian We investigated annual to decadal sea-level summer monsoon strengthen prodded increas- changes and relations among sea-level, Asian ing of quartz flux reflecting dry and cold cli- summer monsoon intensity and flood events mate in the inner area of Asia. Also, abrupt showing in Figure 1 since the last glacial by increasing of quartz flux occurred in the varve using varves of Lake Tougouike (Fukusawa, sequence was more quickly than increasing of 1998; Fukusawa et al., 1999) (Fig. 5). Iron min- δ18 O changes of ice core in GISP 2, but the eral composition is used as a of sea-level decreasing rate was more slowly. These facts changes. Because pyrite is only produced by indicate that the weakening of Asian summer sulfate reduction consuming SO42- of sea water monsoon followed abrupt onset of dry and cold origin under anoxic condition, but siderite only climate in inner area of Asian Continent and forms under anoxic fresh water condition ex- suggest that cooling response of climate of cluding Ca2+. Figure 5 includes δ18 O changes of GISP 2. We can see 8.2ka cooling event (Alley et al., 1997), Younger Dryas cooling event and Heinrich events from H1 to H3 (Heinrich, 1988) in the most of paleoenvironmental records from Lake Tougouike. All the flood events are only recognized in Holocene period except one horizon of the . These evidences show that climatic and sea- level changes of the East Asia since the last glacial are possibly related to Dansgaard- Oeschger Cycle (Johnsen et al., 1992; Dans- gaard et al., 1993) and Bond Cycle (Bond et al., 1993, 1997), because abrupt climatic changes excluding sea-levels seems to be synchronized through quickly teleconnection due to atmos- pheric circulation such as Asian monsoon and the Westerlies activities. In particular, abrupt climatic and sea-level changes during Holocene (Fukusawa, 1998; Fukusawa et al., 1999) are characterized as follows. Many abrupt warm- Fig. 4 Possible synchronization of isotopic records ing and cooling events occurred in annual to from Greenland Ice Core (GISP 2) (Stuiver et al., 1995) (upper) and quartz flux (mg/cm2yr) decadal scales during Holocene. Annual in non-glacial varves of Lake Tougouike abrupt changes of sea-level episodically oc- (lower) curred with rising and falling rates more than a From Fukusawa et al. (1999). rate of 15cm/year by our recent research 242 Hitoshi Fukusawa June 1999

Fig. 5 Diagrams showing relationship among illite crystallinity (upper left), pyrite-siderite contents (upper right), thickness of flood layers (lower right) in non-glacial varves of Lake Tougouike, and oxygen isotope ratio (lower left) of Greenland ice core (GISP 2) during the last 35,000 yrs YD indicates Younger Dryas period, and also H1-H3 indicate Heinrich events (Vidal et al ., 1997) From Fukusawa et al. (1999).

(Fukusawa et al., in press) Acknowledgments VI. Concluding remarks Thanks are due to Prof. H. Kitagawa, Prof. Y. The Japanese varve chronology has an im- Yasuda, Prof. S. Iwata, Prof. K. Takemura, Prof. portant role to reconstruct annual to decadal A. Hayashida and Prof. M. Okamura for contin- paleo-environmental changes since the last in- uous supports and encouragements to my terglacial in the East Asia. Varved sediments recent studies of non-glacial varves in Japan allow a detailed history to be constructed, espe- during the last 5 years. cially speeds of climatic and sea-level changes. Applications of varve chronology will be References widely available for the high resolution recon- Alley, R. B., Mayewski, P. A., Sowers, T., Stuiver, M., structions in relation to climatic changes, veg- Taylor, K. C., Clark, P. U. (1997) Holocene climatic etation history, sediment flux, monitoring of instability : A prominent, widespread event 8200 contemporary environmental process (Boggle, yr ago. Geology, 25, 483-486. 1993) and detection of natural disasters includ- Bond, G., Broecker, W., Johnsen, S., McManus, J., ing volcanic eruptions, earthquakes and floods Labeyrie, L., Jouzel, J. and Bonani, G. (1993) Correla- etc. It seems that varves are natural time- tions between climate records from North Atlantic sediments and Greenland ice. Nature, 365, 143-147. keepers and well preserved archives of global Bond, G., Showers, W., Cheseby, M., Lotti, R., Almasi, and local paleoenvironments in Quaternary re- P., deMenocal, P., Priore, P., Cullen, H., Hajdas, I. and search. Bonani, G. (1997) A pervasive millennial-scale cycle 1999年6月 Varved lacustrine sediments in Japan: Recent progress 243

in north Atlantic Holocene and Glacial climates. Jouzel, J., Stauffer, B. and Steffensen, J.P. (1992) Science, 278, 1257-1266. Irregular glacial interstadials recorded in a new Boygle, J. (1993) The Swedish varve chronology-a Greenland ice core. Nature, 359, 311-313. review. Progress in Physical Geography, 17, 1-19. Kitagawa, H., Fukusawa, H., Nakamura, T., Okamura, Chamley, H. (1989) Clay sedimentology. 549p, Sprin- M., Takemura, K., Hayashida, A. and Yasuda, Y. ger-Verlag. (1995) AMS 14C dating of the varved sediments Dansgaard, W., Johnsen, S. J., Clausen, H. B., Dahl- from Lake Suigetsu, central Japan and atmospher- Jensen, D., Gundestrup, N., Hammer, C. U., FIivid- ic 14Cchange during the late Pleistocene. Radiocar- berg, C. S., Steffensen, J. P., Sveinbjornsdottir, A. E., bon, 35, 371-378. Jouzel, J. and Bond, G. (1993) Evidence for general Kitagawa, H. and van der Plicht, J. (1998 a) Atmos- instability of the past climate from 250-kyr ice core pheric radiocarbon calibration to 45,000yr B.P.: record. Nature, 364, 218-220. Late glacial fluctuations and cosmogenic isotope de Geer, G. (1912) A geochronology of the last 12,000 production. Science, 279, 1187-1190. years. Proc. 11th International Geological Congress Kitagawa, H. and van der Plicht, J. (1998b) A 40,000 (1910), Stockholm, 1, 241-253. year varve chronology from lake Suigetsu, Japan : Fukusawa, H. (1995) Ndn-glacial varved lake sedi- Extension of the radiocarbon calibration curve. ments as a natural timekeeper and detector on Radiocarbon, 40, 505-516. environmental changes. The Quat. Res. (Daiyonki- Kuwata, A. (1989) Formations of resting spores at Kenkyu), 34, 135-149. (J+E) local upwelling area. Monthly, 21, 588-592. (J) Fukusawa, H. (1998) Archives of annual environ- O'Sullivan, P. E. (1983) Annually-laminated lake sedi- mental changes since the last glacial. Kagaku, 68, ments and the study of Quaternary environmental 353-360. (J) changes-a review. Quat. Sci. Rev., 1, 245-313. Fukusawa, H., Yamada, K. and Kato, M. (1999) High Stuiver, M., Grootes, P.M. and Braziunas, T.F. (1995) resolution reconstruction of paleoenvironmental The LISP 2δ18 O climate record of the past 16,500 changes by using varved lake sediments and loess years and the role of the sun, ocean and volcanoes. -paleosol sequences in East Asia and Japan . Sci. Quat. Res., 44, 341-354. Rep. Natn. Mus. Japanese History-Proc. Intern. Taylor, K. C., Lamorey, G. W., Doyle, G. A., Alley, R. B., Symp. Terrestrial Environmental Changes and Natu- Grootes, P. M., Mayewski, P. A., White, J.W. C. and ral Disasters during the Last 10,000 Years, 81, 33-52. Barlow, L. K. (1993) The 'flickering switch' of late Fukusawa, H., Yamada, K., Kato, M. and Fujiwara, O. Pleistocene climate change. Nature, 361, 432-436. (in press) Speeds of climatic and sea-level fluctua- Vidal, L., Labeyrie, L., Cortijo, E., Arnold, M., tions from Late Glacial to Holocene estimated by Duplessy, J. C., Michel, E., Becque, S. and van Weer- using varves. Proc. Joint Meeting of Japan Earth and ing, T. C.E. (1997) Evidence for changes in the North Planetary Sciences, 55. Atlantic Deep Water linked to meltwater surges Heinrich, H. (1988) Origin and consequences of cyclic during the Heinrich events. Earth Planet. Sci. Lett., ice rafting in the Northeast Atlantic ocean during 146, 13-27. the past 130,000 years. Quat. Res., 29,142-152. Johnsen, S. J., Clausen, H. B., Dansgaard, W., Fuhrer, (J+E) in Japanese with English abstract, (J) in Japa- K., Gundestrup, N., Hammer, C. U., Iversen, P., nese