INTRODUCTION

Peat is formed when the decomposition of plant mass production was very slowly or was not occurred due to environmental condition such as wet and cold temperature. In Korea, peatland was mainly distributed in west coastal alluvial plain, Pyeongtaek, Kyungi-Do (Cho, 2006).

In the past research from the pollen data, it was expected that Pinus increased in 6,700~4,500 years ago and after that period (before 4,500~1,400 years) Pinus decreased but Quercus increased in Korea. Also recent vegetation of Korean penisula was affected by human agricultural activity (Park and Kong, 2001; Kim, 2004).

When terrestrial plants die they are decompose via decomposer at moderate environment condition. Some parts of dead plants are not destroyed in the water or swamp in which air was not supplied insufficiently. As a result these plant parts may be preserved as fossils. Along the shoreline of the Korean peninsula, there were approximately 40 peatlands in Korea (Jo, 2006).

There were many studied to estimated the ancient environment condition using pollens or spores analysis of peatlands (Oh, 1971; 1976; Kang and Lee, 1983; Park, 1993; Kang, 1996; Yoon and Jo, 1996; Yoon, 1997; Lee, 2008). Especially, Choi (1998) wrote the essay regarding the vegetation history of post-glacial age of Korea based on the pollen analysis.

The existing studies about the peatland were focusing mainly vegetation changes of geologic age by analysing of the pollen data. But there are limits to elucidate the detailed environmental information about the era through pollen analysis. Therefore, it needs to include various methodological approach including macrophyte fossil records such as hydrophye indicators for complementing the shortcomings of pollen analysis in order to estimating the previous environments condition such as paleoclimate (Lee, 2008).

This study was designed to target the peatland discovered in Pyeongtaek and to interpret the past environmental condition, paleoclimate. There were many large plant fossils showed especially in indicators such as hydrophyte, softwood or hardwood in this studied area, which can be distinguished by the naked eye. We classified them into seeds, leaves and stems. Then we identified the plant fossil species and count the frequency of its appearance and date the carbon age, and then compared the characteristics.

MATERIALS AND METHODS

1.Studied area

The peatland, which is subject of this study, was located in Hwayang-ri, Hyeondeok-myeon, Pyeongtaek-city, Gyeonggi-do, Korea (N36° 57′ 30″, E126° 55′ 45″) (Figure 1). Low hills in the Hyeondeok-myeon were made from weathered gneiss of Precambrian and there is little exposure of bedrock surface.

Between these low hills, small valleys developed an arborescent shape and it is linked on the southern and western to Pyeongtaek area. Pyeongtaek area is influenced by sea water to the inland and the island that is linked to the Jinwi stream and the Daeban stream by difference of the tide. Therefore, over time has developed an intertidal zone sedimentary layer with silty clay from the tide changes in the small valleys of Pyeongtaek area. From a long time ago, Trapa was discovered in Anjung and Pyeongtaek. Recently, there was found rice seed that showed evidence of cultivation, so archeologists was interested in this area.

This hill is covered with a red-brown topsoil layer which is gneiss weathered and colluvial soil has developed from the weathering soil. These areas were cultivated fields, this was discovered when we completed an earth surface investigation.

2.RESULTS

We built a long trench along the Daeban stream from Hwayang-ri, Hyeondeok-myeon, Pyeongtaek-city. The standard was the area where traps was found which is identified as A-1 and A-2 from north to south and we found the peat at a point of under 200 cm. We built an extending trench sized 80 m×2 m at the Hwangsan-ri, Hyeonkeok-myeon. We identified this area as B-1 and B-2. We also found peat in this area at a point of under 150cm. We extracted a large sample size at both the A-1 and B-2 areas (10 cm vertical space). We extracted 17 samples at A-1 and 10 samples at B-2, each sample was 500 g.

We observed after sealing the sample, moving the sample and storage not to evaporate the water. The samples were extracted onto a 4 stage standard sieve (KSIC4545: 2.0 mm, 0.5 mm, 0.35 mm, 0.29 mm) and we analyzed the occupancy rate and frequency of the trait of dead plants at each level. The following is the order for analysis of the plant leaves.

3.The Trait of Sample

At the A-1, A-2 area, the topsoil is organic silty clay and the color is dark yellowish brown (7.5 YR 3/4). From the depth of the topsoil, we found peat which has plentiful fiber until a depth of 210 cm. Underneath the peat, there is a silty clay layer with organic and clay gray silt and sandy soil which has very little organic material. At the B-1, B-2 area, the topsoil is also clay and has a yellow brown color. Peat was discovered in sample No. 3~9 (a.s.l. 5.8~4.9 cm). Dead plant material was found more often according to the depth and there were many kinds of plant fossils discovered in sample No. 5~9 (a.s.l. 4.9~5.5 cm). Gray clay was discovered under the peat. In comparing area A to area B, it was discovered that area B had more decomposition and it was discovered plant fossils species were not various, Therefore, the study and detail analysis about the large plant fossil was only A-1 (Figure 2).

RESULTS AND DISCUSSION

1.Age determination

Samples NO. 1, 3, 5, 8, 9 and 11 from the upper peatland contained xerophytes and samples NO. 14 and 17 included hydrophyte (Figure 2). The upper age was 4,540 ± 40 yr C¹⁴ B.P., the lower was 6,970 ± 40 yr C¹⁴ B.P. (Table 1).

2.Plant fossils of peat

1)Seed

According to the results of 17 samples at the A-1 area, there were 7 species such as Alnus japonica, Hydrocaryaceae Gramineae, Polygonaceae and two kinds of unknown. Except, more than 90 % was non-arboreal plant, A. japonica 2.9 %, Hydrocaraceae 27.0 %, Cyperaceae 31.6 %, Gramineae 18.4 %, Polygonaceae 5.7 % and two kinds of unknown was 14.4 %. The reason why there were many Hydrocaryaceae in sample 12~17 (a.s.l. 3.03~3.71 cm) is when they were accumulated marshland they were formed because of the sea level rising and after that the seas level settled (Table 2, Figure 3). We can assume that this area had an oceanic climate with warmer and more humidity than now so the marsh land environment developed and after that the climate changed and became dry.

2)Plant leaves

Five species were found after separation and restoration of leaves in the peatland. Cyperaceae and Gramineae were discovered in the sample 1~7 (a.s.l. 5.0~4.18 m) which are not easy to classify. Most of them in sample 8~11 (a.s.l. 4.18~3.71 m) were discovered as flustered or aggregated. There were 3 kinds of trees; A. japonica, Fraxinus and Fagaceae in sample 13~16 (a.s.l. 3.59~3.13 m). Cyperaceae and Gramineae were 39.1 % without Pinaceae. A. japonica 34.8 %, Fagaceae 14.5 %, Oleaceae 11.6 % and this frequency is similar to the frequency of A. japonica and Fagaceae stems (Table 2, Figure 3). Therefore, these two species are the two major species in the peatland.

3)Plant stem (shoot)

We counted the sample stem attached with twig, tree bark and root from fossil record layer. Fagaceae, A. japonica were increased and decreased in all layers. Notable, Pinacease barks were discovered in sample 2~4 (a.s.l. 4.87~4.53 m) and this means that there were artificial factors, such as lumbering by people. Twigs were discovered in the older layer. Bark and root appeared in sample 3~10 (a.s.l. 4.76~3.83 m) which is the top later. The result of trunk sample, A. japonica 44.3 %, Oleaceae 10.3 %, Fagaceae 26.8 %, Pinaceae 6.2 % (Table 2, Figure 3). This is similar to the previous study on peat in Hyeonnhwari (Kang, 1996). Therefore, A. japonica and Fagaceae increase and decrease repeatedly. A. japonica grows well in a wet environment and Fagaceae grows in most Korean forests, so we can say that the dry environment and wet environment were repeated.

3.Sedimentary environment

This studied area was low hills of 5.1 m in height. There were very diverse and plentiful plants fossil in the peatland. When it comes to the analysis of see, non-arboreal plant were more than 90 % of the results, and arboreal plant were only 3 % of the results. When it comes to the analysis of the leaves, the results were 60 % trees and 40 % herbs. When it comes to the analysis of stems, the results were 80 % trees and 20 % herbs. According to radiocarbon dating, in the lower part of peat 6,970 ± 40~6,070 ± 40 yr. B.P. we can estimate that the peatland were formed in a marsh land environment, because there were many Trapa and aquatic plants. At 5,800 ± 40~4,540 ± 40 yr. B.P. deposits flowed from upper streams, so Gramineae and Cyperceae were increased and A. japonica and Fagaceae were increased and then decreased, so we can estimate that there was a cool environment with a forest.

4.Environmental changes

The vegetation environment in the coastal accumulation layer plain responds sensitively to sea level variation. The variation of dead plant is the indicator of sea level variation and climate variation so we can discover the relationship between the west coast later half of post glacier age vegetation development.

Fossil zone III (a.s.l. 3.03~3.71 m) was 6,970 ± 40~6,070 ± 40 yr. B.P. and showed Hydrocaryaceae which is a kind of aquatic plant at that time. Therefore, we are able to assume that according to the rising sea level, a fresh water low swamp or moorland was formed and then the sea level reached a stable status. There were arboreal plant leaves A. japonica trunk and many Trapa and aquatic plants. Trapa was found only in the zone 3 period so we can infer that it was warmer than now and an oceanic climate with a swamp or marshland.

Fossil zone II (a.s.l. 3.72~4.18 m) was 6,070 ± 40~5,800 ± 40 yr. B.P. During this period there was a serious dissolving process of peat in the peatland. There was a large breaking down of the plans which make peat. So we can assume that after forming peat, the surface of the under ground water was lowered and then dissolved the formed peat. The variation of this plants at this period, twigs from the Fagaceae, Gramineae and Cyperaceae was increased and most of them were herbs and A. japonica and Fagaceae plant appeared increasing and decreasing many times. A. japonica grows well in a swampy environment and Quercus habitat most of the Korean forests, so we can conclude that swamp and dry land were repeated many times since there were not big variations of the sea level the latter period of postglacial age.

Fossil zone I (a.s.l. 4.19~5.0 m) was 5,800 ± 40~4,540 ± 40 yr. B.P. The era was very dry. Cyperaceae, Gramineae and A. japonica increased. Jo et al. (1994) assumed that transgression at the postglacial age reached Ilsan Kawagigok in 6,000 yr. B.P. and that the sea level was comparatively stable. They said 5,000~3,000 yr. B.P., the sea level was stable, the peat was formed inland and A. Japonica were thick.

The weather would have been wet, and there would have been many herbs which means that people interfered with nature. A. japonica and Fagaceae increased and decreased which means that a warm and wet swampy and dry environments were formed repeatedly.

Also in this study, before and after 6,000 yr. B.P. peatland were accumulated in a silt layer so the period had a stable sea level. According to the decomposition of peat and dead plant in 6,970 ± 40~6,560 ± 40 yr. B.P., we can infer that the sea level was lowered. Before and after 5,560 yr. B.P. There were a variation of frequency of A. japonica and Fagaceae. We can infer that there were many periods of the sea level rising and lowering.

Also, there were freshwater diatoms such as Aulacoseira distance, Aulacoseira granulata, Cyclotella meneghiniana, Cymbella turmida, Eunotia pectinalis, Eunotia praerupta, Fragilaria brevistriata, Fragilaria construens, Navicula lanceolata in the same surveyed area as our study site, which dominated the algal community (Ryu et al., 2003; 2005; 2007). This report was coincided with our result in the period of fossil zone III and assumed that this area were experienced with the flooding of heavy precipitations and thus had a changeable accumulated environment condition.