The site of the Karasuma Deep Drilling at the southern part of Lake Biwa, the largest in Japan, was located west of the earlier main fluvial valley recognized above the basement topography under the thick sediment of the lake bottom. Depositional environments during about 1.7-1.9 Ma at the site, analyzed from the facies of the lowermost sequence of the cores, indicate the existence of an alluvial fan and/or talus cone in a feeder valley. Then, the place changed to a shallow, narrow lake filled with floodwater from the main valley lake, which formed as a seismic naturally dammed lake. Furthermore, it gradually changed to a deeper, wider lake with fluctuating lake levels. The estimation indicates that older records may exist in the deposits filling the main valley above the basement surface.

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The Lake Biwa sediment, with its clayey deposit 250 m thick covering the past 430,000 years, is suitable for orbital-scale palaeoclimate reconstruction because of abundant tephras as well as quick and stable sediment accumulation. A fossil pollen record from the 250 m muds (Miyoshi et al., 1999) contains five glacialinterglacial cycles correlating with MIS 1 to MIS 12, based on the 780 kyr pollen stratigraphy constructed by connecting it with an older pollen record obtained from the Choshi district, east-central Japan.
This article demonstrates two important findings recovered from the Lake Biwa pollen record. One is about age control, showing that the pollen ratio between temperate conifers (Cryptomeria, Sciadopitys and T-C-C) and boreal conifers (Abies, Picea, Tsuga and Pinus) correlates with marine δ¹⁸O records during the past 430 kyr. Coupled with climate preferences of these pollen taxa, we propose that the temperate/boreal conifer pollen ratio serves as an indicator for the glacial-interglacial cycle, at least in central Japan, providing a time control method for terrestrial deposits of middle Pleistocene age.
Another finding from Lake Biwa pollen data is the result of an application of modern analogue technique (MAT), which can quantitatively extract palaeoclimate information from fossil pollen data. The significance of MAT application is that it provides a new finding concerning a monsoon circulation mechanism. The reconstructed palaeotemperature variations correlate well with the above-mentioned temperate/boreal conifer ratio, and show a significant 100 kyr cycle similarly to marine δ¹⁸O. By contrast, the reconstructed palaeoprecipitation variations (particularly Psum) are dominated by a strong 19-23 kyr periodicity, revealing that the summer monsoon variations are almost free from the glacial-interglacial cycle. These observations can be explained as follows. Despite the significant influence of the glacial-interglacial cycle on terrestrial temperatures, it affects the Pacific air mass and the Siberian air mass almost equally, and so is cancelled out from the land-ocean temperature difference (i.e., Tvar). By contrast, insolation forcing affects the marine and continental air masses differentially, and so it is never can-celled out from the land-ocean temperature difference, providing a strong 19-23 kyr periodicity for monsoon variations. This mechanism is probably shared by monsoon regions of different continents.
The paleoclimate reconstruction resulted in new findings, but also some new problems. This article describes two current problems and some resolutions in the pollen-based palaeoclimate reconstruction from Lake Biwa sediment.

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Sediment cores recovered from Lake Biwa since 1965 have provided important information on paleoenvironemental change during Quaternary time. After reviewing research on Lake Biwa sediments, we describe the detailed stratigraphy during the last 50,000 years using core sediments obtained in 2007 and 2008. Analysis of six multiple site piston-cores and two drilled cores provides a detailed stratigraphy of the lake sediments. Description of lithology and measurement of magnetic susceptibility reveal the occurrence of volcanic ashes, which were visually observed or microscopically found in clayey sediments. Most volcanic ash layers are observed in multiple site cores and also in earlier cores, establishing correlations and age assignments for the core sediments. The age-depth curves deduced from published tephra ages and newly obtained carbon 14 data show significance differences in sedimentation rate at multiple sites in Lake Biwa.

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Diatom cell size inferred from fossil valve size is discussed from the viewpoints of taxonomy, ecology, and paleoenvironmental sciences. Diatoms decrease their cell size during vegetative growth periods and restore it in sexual reproduction with relation to environmental factors which affect their life cycle. Cell size changes observed in Lake Biwa cores suggest ways to understand ecological responses in the diatom life cycle strategy. Cell size of Stephanodiscus suzukii from the last deglaciation is larger than that from the middle Holocene. Frequency of sexual reproduction of Aulacoseira nipponica in the late last glacial is inferred to be ten times as large as that in the Holocene. These examples should be interpreted as actual responses of diatoms to environmental change, and as probable proxies for paleoenvironments in the future.

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In the postglacial age, the development of human culture and civilization has influenced the local and global environment. An important factor in this regard is the use of fire. Large anthropogenic fires produce huge numbers of particles, some of which are preserved in sediments. Although the presence of charcoal fragments in sediment is not necessarily indicative of anthropogenic fire, such fires may be recognized based on related evidence such as the pollen record and the history of human settlement in the region of interest. Spheroidal carbonaceous particles (SCPs), which are produced by the combustion of fossil fuels, have been detected in sediments deposited since the 19th century. These particles often represent a kind of human activity, indicating the use of fire in the past. The different types of particles derived from combustion reflect different uses of fire. These particles may play a significant role in clarifying when and how humans used fire to influence the local or global environment, and in establishing the timing of such events.

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In this paper, time and spatial scales for palaeoecological studies are reviewed. Various natural phenomena involved in changes of ecosystems have their own time and spatial scales. We should take these scales into account based on the purpose of the palaeoecological study. For example, high-resolution pollen records can reveal vegetation responses to millennial-scale climate oscillation such as Dansgaad-Oeschger cycles and Heinrich events of the last glacial age. The sizes of depositional basins are also related to the scale of reconstructed vegetation. Large lakes such as Lake Biwa have pollen source areas from regional vegetation. Pollen records from small forest hollows can reveal local vegetation changes. Palaeoecological study using a combination of different sizes of sedimentary basin can indicate the spatial pattern of past vegetation.

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