19. Neogene Palynology of Holes 794a, 795a, and 797b in the Sea of Japan: Stratigraphic and Paleoenvironmental Implications of the Preliminary Results

This initial survey of pollen from 192 samples from Hole 794A, supplemented by 189 samples from Hole 795 and 797B, suggests that marine pollen assemblages from the southwestern Sea of Japan provide a consistent Neogene pollen stratigraphy and a solid basis for regional paleoenvironmental reconstructions. Late Miocene vegetation inferred from these pollen data, a mix of conifer and broad-leaf elements with now-extinct Tertiary types well represented, appears similar to Aniai-type floras of Japan. During the late Miocene through early Pliocene, as Tertiary types declined, conifers (including the Sequoia/Cryptomeria group) became more prominent than broad-leaf elements, and herbs played an increasing role in the vegetation. Middle Pliocene pollen assemblages imply significant changes in forest composition. In a 500,000-yr interval centered at ~4 m.y., Tertiary and warm-temperate deciduous types re-expanded and were comparable to or greater than middle-late Miocene levels. Temperate and cold-temperate conifers {Picea, Abies, Tsuga) were minimal. Subsequently, Tertiary and deciduous forest components (including Quercus) decreased, Picea, Tsuga, and Abies were again prominent, and herbs formed an increasingly larger part of the vegetation. Between ~3 m.y. and -2.5 m.y., conifers, except for Cryptomeria types, were prominent, Quercus continued to decline, and other broad-leaf trees were minor. Over the last 2 Ma, the very large and frequent changes in forest composition inferred from pollen in the Sea of Japan correspond to forest dynamics inferred from changes in pollen and floral assemblages throughout Japan. Given present vegetation/climate relationships, broad trends in Neogene climate inferred from these preliminary pollen data include decreasing temperatures, increasing seasonality in temperatures and precipitation, and increasing amplitude and frequency of climatic change. Two significant events, centered at ~9 m.y. and ~4 m.y., punctuate the gradual deterioration of the equable warm, humid subtropical/warm temperate late Miocene and early Pliocene climates. The first indication of cold-temperate conditions comparable to those of Pleistocene glacial intervals occurs ~3 m.y. Subsequently, regional climates oscillated rapidly between temperate and cold-temperate regimes that supported conifer and mixed broad-leaf forests; however, climatic extremes were apparently never great enough to displace warm-temperate and temperate forests from Honshu nor to produce arctic climates on the west coast of Japan. INTRODUCTION Analysis of plant microfossils in sediments deposited in the ocean may seem anomalous and even esoteric; however, pollen records obtained from deep-sea sediments provide useful and unique paleoclimatic and stratigraphic information: lengthy, continuous, chronostratigraphically controlled terrestrial paleoenvironmental records that are, by their nature, directly correlative with marine paleoenvironmental records derived from the same sediment samples. As these preliminary results of pollen analysis of Neogene sediments from Sites 794, 795, and 797 in the Sea of Japan suggest, marine pollen stratigraphy can provide a means of correlating marine sediments and of correlating marine and terrestrial stratigraphic and climatic records. The source of pollen in the Ocean Drilling Program cores from the Sea of Japan is primarily the nearby vegetation of the Japanese archipelago (Heusser, 1989). The "natural" vegetation of Honshu and Hokkaido, Japan, can be divided into three major latitudinal/altitudinal forest groups: (1) warm-temperate evergreen broad-leaf, (2) temperate evergreen conifer and deciduous broad-leaf, and (3) subalpine (subboreal) conifer forests (Numata, 1974; Franklin et al., 1979; Igarashi, 1986). Mixing between elements of the cool-temperate and warm-temperate forests commonly makes distinction between these two forest zones tenuous, and the composition of all these groups varies in response to local climatic and edaphic factors. The compoPisciotto, K. A., Ingle, J. C, Jr., von Breymann, M. T., Barron, J., et al., 1992. Proc. ODP, Sci. Results, 127/128, Pt. 1: College Station, TX (Ocean Drilling Program). 2 Lamont-Doherty Geological, Observatory of Columbia University, Palisades, NY 10964, U.S.A. sition of natural lowland vegetation, which has been severely disturbed for more than 2000 yr, is essentially unknown. Warm-temperate forests, which extend to ~40°N on the Pacific coast, include Castanopsis (Chinquapin), Quercus (oak), and conifers such as Cryptomeria japonica (Japanese cedar) and Sciadopitys verticillata (Umbrella pine). Pinus thunbergii (pine) is essentially restricted to the coast and lowlands, whereas Tsuga sieboldii (hemlock), and/or Abies firma (fir) occur in higher elevations and in the northern parts of Honshu (Ohwi, 1984). Characteristic genera of cool-temperate forests, which form the primary part of the Japanese flora, include Quercus, Fagus (beech), Tsuga, and Abies, as well as Cryptomeria japonica. On Honshu, at elevations between -1000 and 2400 m, subalpine forests with Abies, Betula (birch), Picea (spruce), and Tsuga diversifolia develop (Franklin, 1979; Saito, 1979). Moors and alpine dwarf scrub with Betula ermanii and Pinus pumila occur above -2400 m. In the north, on Hokkaido, broad-leaf deciduous forests merge with forests dominated by Picea jezoensis and Picea glehnii, Abies sachalinensis, B. ermanii, P. pumila, and Quercus mongolica. Coastal vegetation is characterized by extensive Sphagnum bogs, Alnus (alder) swamps, and mixed stands of conifers and hardwoods (Numata, 1974; Watanabe, 1979). Distribution of Japanese vegetation is said to be primarily controlled by temperature because precipitation is relatively abundant (1000 to >4000 mm/yr) throughout the archipelago. Like other mid-latitude oceanic climates, the climate of temperate Japan has a small annual mean temperature variation. Average January and July temperatures range from 5.3°C to 25°C in the warm-temperate zone, from -1.0°C to 22.4°C in the cool-temperate zone, and from -6.2°C to 17.8°C in the subarctic zone. Summer and winter isotherms curve northward on both sides of Honshu, reflecting the year-round