季 刊 地 理 学 Vol. 47 (1995) pp. 1-12 Quarterly Journal of Geography Developmental Processes of Mangrove Habitat Related to Relative Sea-level Changes at the Mouth of the Urauchi River, Iriomote Island, Southwestern Japan Kiyoshi FUJIMOTO* and Yasuhiro OIINUKI** Abstract In order to clarify geomorphologically the developmental processes of the mangrove habitats of Iriomote Island, topographic and surface geological surveys were performed along four lines in and around the mangrove habitat at the mouth of the Urauchi River in northwestern Iriomote Island. As a result, it was learned that the mangrove habitat of Iriomote Island has migrated in response to relative sea level changes. It is especially worth noting that the mangrove habitat abruptly moved seaward by a relative sea level fall that occurred about 1,000 yrs BP. During the rising sea-level phase between 2, 000 yrs BP and 1, 000 yrs BP, the mangrove forests of the island retreated, landward, which indicates that they are possibly more sensitive to sea-level changes than mangrove forests of tropical areas such as Micronesia and the Philippines, where landward movement of mangroves during this period has not recognized. On the other hand, at the Rhizophora stylosa habitat situated in the inner mangrove forest, it was found that the mangroves have maintained their habitat and position by accumulating mangrove peat against relative sea level rise after 260 yrs BP. Keywords Mangrove habitat, Relative sea level changes, Radiocarbon dating, FeSZ contents analysis, Aerial photograph, Iriomote Island habitat has little sediment supply and no geomor- L Introduction phological barrier against coastal processes. Mangrove habitats are classified into three The frameworks for the estuary or delta type and types based on their geomorphological situations, backmarsh or lagoon type have been formed and i.e., estuary or delta type, backmarsh or lagoon maintained by fluvial processes and coastal type, and tidal fiat type (Miyagi et al., 1989). processes, respectively, within a changing envi- The tidal-flat type is found in the geomor- ronment of sea level (Thorn et al., 1975; Kikuchi phological environment that excludes estuaries, et al., 1978; Woodroffe et al., 1989, etc. ). On the deltas, backmarshes and lagoons. This type of other hand, the tidal fiat type has been formed and maintained by mangrove peat accumulation * Forestry and Forest Products Research Insti- with sea level changes. Namely, the existence tute ** Kyushu Research Center, Forestry and Forest of the mangrove itself is the most important fac- Products Research Institute. tor for the formation and maintenance of this type -1- Quarterly Journal of Geography 47-1 (1995) of habitat. Therefore, excessive deforestation itats are still largely unknown. Though Kikuchi on this type will induce severe loss of the habitat. et al. (1978, 1980) discussed geomorphologically Hence, the forests of this type must be managed the formative processes of the mangrove habitat carefully (Fujimoto and Miyagi, 1993). As at the mouth of the Nakama River, there have as mentioned above, understanding the formative yet been no discussions based specifically on the and maintainable mechanisms of mangrove hab- distribution of organic materials of mangrove itats are very important for sustainable use of origin in sediment confirmed through radiocarbon mangrove forests. Moreover, in order to predict dating. This study aims to clarify the formative the effect of sea level rise induced by global and maintainable mechanisms of the mangrove warming, it is necessary to clarify the formative habitat based on the concrete data of the distribu- processes of mangrove habitats with sea level tion of organic materials of mangrove origin and changes. radiocarbon dating at the mouth of the Urauchi In Japan, there are valuable mangrove forests River. that define the northern limit of mangrove distri- bution in the world. However, the formative and IL Study area maintainable mechanisms of the mangrove hab- Iriomote Island, one of the Yaeyama Islands of □ hills and terraces □ Pandanus tectorius □ evergreen forest ■ mangrove forest □ paddy field thickt except mangrove □ fresh-waterres-water marsmarsh ■ bare land □ settlmnt ■ beach Fig. 1. Map showing the vegetation and landuse on the alluvial plain of the Urauchi River, the location of profiles shown in Fig. 2 and Fig. 3 and areas covered by aerial photographs in Photo 1. -2- Fujimoto and Ohnuki; Developmental Processes of Mangrove Habitat on Iriomote Island, Southwestern Japan southwestern Japan, has a total mangrove forest area of about 400 ha (Nakasuga, 1979). Almost IIL Study method all of the mangrove forests are situated at the Topographic profile surveys using tilting level mouths of rivers. The mangrove forests of the and surface geological surveys using Hiller type Urauchi are about 100 ha in area, almost the same sampler were conducted along four lines, two of as at the Nakama. The mangrove forest at the which are located in the mangrove habitats. The mouth of the Urauchi is situated in a lagoon-like other two lines were set in the freshwater marsh environment surrounded by hills (Fig. 1). Thus, behind the mangrove habitat (Fig. l) to clarify the this mangrove habitat is similar to the backmarsh distribution of earlier mangrove forests. A or lagoon type despite its geomorphological set- bench mark situated on the right bank of the ting. A Pandanus tectorius thicket is situated Urauchi near the Urauchi Bridge was used as the behind the mangrove forest, and a freshwater standard point for the leveling. marsh, part of which is used as paddy field, is The substratum of the mangrove forest usually located behind the Pandanus thicket. includes peat, humus and wood fragments. It is In the Yaeyama Islands, a tide station has been apparent that these organic materials are of established at Ishigaki Harbor. The mean spring mangrove origin. Therefore, the layer contain- tidal range at the station is 179 cm. Almost all of ing these organic materials was used as an indica- the mangroves on Iriomote Island grow in the tor of the existence of past mangrove forests. In upper half of the tidal range. In Iriomote Island, order to confirm the depositional environment of the greater part of the root systems of Rhizophora the layer in the sediment of the freshwater marsh stylosa and Bruguiera gymnorrhiza, which are the area, FeS2 content analysis was conducted. main species of the island, usually exist within 40 FeS2 is a kind of authigenic mineral and is cm from the surface (Komiyama et al., 1989 and formed by chemical changes in SO42- in the pore Ninomiya et al., 1989). Therefore, most of the water of the surface sediment caused by sulfur organic materials of mangrove origin are believed reductase (Berner, 1970). FeS2 content in sedi- to be deposited in the restricted environment ment is determined by the concentration of So42- between a level slightly lower than mean tide oxidation -reduction potential, organism content level and mean spring high tide level. for the reductase, and so on (Nakai et at., 1982). Emerged benches, beachrocks, and notches In the sediments of mangrove habitats, much FeS2 which formed during the late Holocene are found is formed because the SO42- concentration in sea along the coast of Iriomote Island. Therefore, water is a thousand times greater than in fresh the formative processes of mangrove habitat of water (Nakai et at., 1982), and there are much this area have likely been affected by relative greater levels of organisms in mangrove habitat. sea level changes. The methodology of FeS2 content analysis foll- owed Shiragami (1985). FeS2 content was in- dicated by the ratio of the sulfur weight in FeS2 to -3- Quarterly Journal of Geography 47-1 (1995) the sample weight. This is presented as FeSZ-S The recent movement of the mangrove forest content in this paper. was examined using aerial photographs in connec- Radiocarbon dating was performed on six sam- tion with the relative sea level changes observed ples obtained from these four lines by Kyoto at Ishigaki Harbor. Sangyo University and the Japan Radioisotope Association. The radiocarbon ages were calcu- IV. Results lated using Libbys value, 5568 years, as the half Fig. 2 shows vegetation, landform and geologi- life of 14C. Measurement error indicates 1 G. cal profiles in the mangrove habitat. The man- □ sand ■ peat mangrove ■ silt □ wood frag. oregin □ clay □ humus ■ shell fran Fig. 2. Vegetation, landform and geological profiles in the mangrove forest. A: scale for trees. -4- Fujimoto and Ohnuki; Developmental Processes of Mangrove Habitat on Iriomote Island, Southwestern Japan grove forest mainly consists of Rhizophora stylosa stylosa is found in relatively low areas between and Bruguiera gymnorrhiza. The latter grows on higher microlandforms. slightly higher ground, such as natural levees and Along profile a-a, the layer containing organic the banks of tidal creeks. On the other hand, R. materials of mangrove reaches no lower than 0.8 □ clay □humus □ silt ■ wood frag. ■ fine-medium sand □ stump ■ coarse sand ■ peat Fig. 3. Landform and geological profiles on the freshwater marsh behind the mangrove forest. Organic materials in these profiles include both mangrove and nonmangrove origin. -5- Quarterly Journal of Geography 47-1 (1995) m below the present mean sea level. This layer materials of mangrove is only 40cm thick except consists of sand or sandy loam except for the for along the tidal creek (around Loc. 19). At the inner part of the mangrove forest (around Loc. 11 river side (around Loc. 0), a layer similar to the to 20), where the ground level is relatively low and above is covered with a natural levee deposit. the sediment consists of a peaty deposit. The boundary is 0.2 m below the present mean sea Radiocarbon age indicating 26080 yrs BP level. Radiocarbon dating indicated an age of (KSU-2181) was obtained from the basal horizon 1,28070 yrs BP (N-6468) for wood fragments in of the peaty deposit (0.3 to 0.4m below the present the top horizon of the layer (0.2 to 0.3m below the mean sea level) at Loc.