Palaeoenvironment During the Initial Jomon Period
The Iyai rock shelter site was predominantly used from around 10,000 to 6,500 years ago, spanning the Initial and Early Jomon periods. The thick ash deposit in the terrace in front of the rock shelter was formed between 10,000 and 9,000 years ago, and marks a period of substantial human activity at the Iyai site.
The Earth’s environment undergoes repeated warm interglacial and cold glacial periods in cycles of approximately 100,000 years. The most recent glacial period ended around 11,000 years ago, initiating a phase of rapid warming, marking a pivotal change in the environment of the Japanese archipelago (Fig. 1).
Around 10,000 years ago, rising sea levels began penetrating deeper into valleys formed during the Last Glacial Maximum in the Kanto region. By approximately 7,000 to 6,000 years ago, the sea level stabilized at approximately 2-3 meters above the present level, extending Tokyo Bay (Oku-Tokyo Bay) into the lowlands in the southeast of current Gunma Prefecture. This expansion led to a rapid and significant rise in fish and shellfish populations. Additionally, around 10,000 years ago, the Tsushima current, a northeastward-flowing branch of the Kuroshio along Japan’s west coast, began flowing into the Sea of Japan. This current likely contributed to the existing snowy environment in the neighboring mountainous regions. These changes in climate, topography, and biomes had a profound impact on the lives of people during the Initial Jomon period.
Before roughly 15,000 years ago, the forests of central Japan were primarily dominated by Pinaceae conifers found in today’s subalpine zone, including spruce, butterfly gorse, larch, Japanese red cypress, and Japanese white birch. However, the distribution of these conifers declined rapidly after the 15,000-year mark, giving way to deciduous hardwoods such as birch, beech, and oak. Concurrently, the lowlands along the Pacific coast in the southern Kanto Region witnessed an upsurge in evergreen broad-leaved trees like tabunoki, oak, and cypress. While evergreen broad-leaved woodlands existed in the southern part of the Boso Peninsula 10,000 years ago, during the Early Jomon and Early Initial Jomon periods, deciduous broad-leaved woodlands, primarily featuring oak trees, were interspersed with evergreen broad-leaved woodlands from the north of Tokyo Bay to Oku-Tokyo Bay. Sediments from archaeological sites in Central Japan from the latter part of the Initial Jomon period onwards show a relatively high proportion of chestnut pollen. A relatively high proportion of chestnut pollen can be found in sediments from archaeological sites in Central Japan from the latter part of the Initial Jomon period onwards, but chestnut is not dominant in naturally occurring forests, so it is highly likely that they were artificially maintained. Chestnuts likely played a crucial role in supporting the Jomon people’s lifestyle, producing a large amount of edible hard fruit and frequently used as fuel, building material, and tools due to their rapid growth.
The palaeoenvironment during the Initial Jomon period in the Joshinetsu Mountains (ranging from 200-1500 meters above sea level), where the Iyai rock shelter site is situated, was traced through pollen analyses from Lake Nojiri (660 meters above sea level), Kayanodaira (1550 meters above sea level) in northeastern Nagano Prefecture, and the Unoki peat layer site (200 meters above sea level) in Tsunan Town, Niigata Prefecture. This analysis indicated the prevalence of cool-temperate deciduous broad-leaved forests dominated by beech and Japanese oak, alongside trees like Japanese walnut, chestnut, and horse chestnut, that produce hard fruits suitable for consumption. Pollen from Japanese Asa trees, also found in the Unoki peat layer, indicated their use as a food source. Compared to evergreen coniferous forests, deciduous broadleaved forests provided a more favourable photoperiod on the forest floor, resulting in faster decomposition of fallen leaves and branches, and better soil nutrients. Consequently, the higher density of plants in the shrub and herbaceous layers allowed the utilization of fruit-bearing shrubs and various wild plants and root crops, depending on the season. Although this environment hindered hunting visibility compared to coniferous forests, it provided easy access to food plants for herbivores like deer, cerow, and wild boars.
In depth borehole investigations of marsh sediments have been carried out in the Nonokai Marsh (northeastern Nagano Prefecture, altitude approx. 1020 m) and Komatsubara Marsh (southern Niigata Prefecture, altitude 1350-1600 m) in order to investigate the specific palaeoenvironment contemporary to the period when the Iyai rock shelter was in use. Furthermore, dating of the sediments, pollen analysis, and analysis of larger plant remains, such as seeds and fruits, has been carried out. In the Nonokai Marsh, large spores of the creeping fern moss cedar orchid (Fig. 2-1), which is found in the present-day alpine zone, are abundant in the strata of the coldest period of the Last Glacial Maximum, suggesting that the forest boundary may have declined to around 1000 m above sea level. In addition, the results of pollen and large plant remains analyses from Komatsubara Marsh and the results of pollen analyses from various parts of central Japan reported so far suggest that coniferous forests in the present-day subalpine zone (altitude of about 1500-2500 m) were poorly defined and a grassland-like environment prevailed until the mid-Jomon period or later. Changes in the elevation of the forest boundary since the Last Glacial Maximum have important implications for the distribution of herbivores and human behaviour such as hunting, the quarrying stone tools, and migration, and therefore further studies should be carried out in conjunction with pollen analysis and analysis of large plant remains.
In-depth borehole investigations of marsh sediments have been carried out in the Nonokai Marsh (northeastern Nagano Prefecture, altitude approx. 1020 m) and Komatsubara Marsh (southern Niigata Prefecture, altitude 1350-1600 m) in order to investigate the specific palaeoenvironment contemporary to the period when the Iyai rock shelter was in use. Additionally, dating of sediments, pollen analysis, and scrutiny of larger plant remains such as seeds and fruits were carried out. In the Nonokai Marsh, significant quantities of large spores from the creeping fern moss cedar orchid (see Fig. 2-1), a species found in today’s alpine zones, were abundant in strata from the coldest period of the Last Glacial Maximum. This abundance suggests a potential decline in the forest boundary to approximately 1000 meters above sea level. Furthermore, analyses of pollen and large plant remains from Komatsubara Marsh, along with previous pollen analyses across various regions of central Japan, suggest that coniferous forests in the present-day subalpine zone (ranging from about 1500 to 2500 meters altitude) were indistinct, indicating the prevalence of a grassland-like environment until the mid-Jomon period or even later. Changes in the elevation of the forest boundary since the Last Glacial Maximum hold significant implications for the distribution of herbivores and human behaviours like hunting, stone tool quarrying, and migration. Consequently, further studies should be conducted in conjunction with pollen analysis and examination of large plant remains to deepen our understanding of this evolution.
