A biomization method, which objectively assigns individual pollen assemblages to biomes (Prentice et al. , 1996), was tested using modern pollen data from Japan and applied to fossil pollen data to reconstruct palaeovegetation patterns 6000 and 18,000 14 C yr bp Biomization started with the assignment of 135 pollen taxa to plant functional types (PFTs), and nine possible biomes were defined by specific combinations of PFTs.Biomes were correctly assigned to 54% of the 94 modern sites. Incorrect assignments occur near the altitudinal limits of individual biomes, where pollen transport from lower altitudes blurs the local pollen signals or continuous changes in species composition characterizes the range limits of biomes. As a result, the reconstructed changes in the altitudinal limits of biomes at 6000 and 18,000 14 C yr bp are likely to be conservative estimates of the actual changes.The biome distribution at 6000 14 C yr bp was rather similar to today, suggesting that changes in the bioclimate of Japan have been small since the mid-Holocene.At 18,000 14 C yr bp the Japanese lowlands were covered by taiga and cool mixed forests. The southward expansion of these forests and the absence of broadleaved evergreen/warm mixed forests reflect a pronounced year-round cooling.

The East Asian monsoon is responsible for transferring huge amounts of heat and moisture between the land and the adjacent ocean. Signifi cant changes in its capacity to do this will have direct impacts on regional climatic gradients and global atmospheric circulation. Determining the mechanisms that force long-term variation in monsoon behavior is therefore important for understanding global climate change. Competing theories vary in the degree of importance attached to glacial forcing, other orbital rhythms, and internal feedback mechanisms as primary drivers of change. There is, however, no convincing explanation as to why different proxy records from closely neighboring regions are tuned to different orbital rhythms. Here we present quantitative climatic reconstructions for the past 450 k.y. based on a long pollen record from Lake Biwa in Japan. The data suggest that continental and oceanic air mass temperatures respond predominantly to the 100 k.y. orbital rhythm, whereas the land-ocean temperature gradient and monsoon vigor oscillate mainly at the 23 k.y. insolation cycle. We suggest that the mechanisms for this behavior lie in the differential response of land and ocean to solar forcing, and conclude that the 100 k.y. signal dominates monsoon intensity only when the amplitude of solar forcing falls below a threshold level.

No abstract

No abstract

No abstract