Kikai Island Research Articles

Rhodolith-bearing gravelly carbonate sediments on the shelf off Kikai Island, Kagoshima Prefecture

Rhodolith-bearing gravelly carbonate sediments on the shelf off Kikai Island, Kagoshima Prefecture

琉球列島喜界島の完新世サンゴ礁段丘における海面安定期と優占造礁サンゴについて

Coral reef terraces are one of the best recorders of biological response to environmental change events (e.g., sea-level changes). Kikai Island provides a rare opportunity to show biological and ecological frameworks (e.g., competition, coexistence, and succession) during a recent geological period. The island is fringed by raised Holocene raised reef terraces, which formed as a result of periodic tectonic uplifts. This study aims to characterize the spatial and temporal changes of corals at this island during the Holocene. The analysis is based on topographical and biological data obtained for the three sites (Shidooke, Kadon, and Nakugama reefs). Three raised reef terraces (Terrace II, III, and IV) grew from 7300 to 4500 years ago (during 2800 years), from 4500 to 2900 years ago (during 1600 years), and from 2900 to 1800 years ago (during 1100 years), respectively. Terrace II and III were uplifted 1-2 m around 4500 years ago and around 2900 years ago. Terrace IV was uplifted 1-2 m around 1800 years ago. The modern reef has been composed of corals for 1800 years. Sixteen coral genera and 53 species were recorded from the reef terraces. Terrace III and IV were dominated by four coral species (A. digitifera, A. robusta, G. retiformis, and F. stelligera), but Terrace II was predominantly composed of A. digitifera and A. robusta. These biological and ecological variations between the terraces represent a growth strategy responding to differences of reef growth time and/or insolation.

喜界島における低密度時期のアリモドキゾウムシ雄成虫の個体数推定

The sweet potato weevil, Cylas formicarius (Fabricius) is a major insect pest of the sweet potato, Ipomoea batatas (L.) Lam. throughout the tropical and subtropical regions of the world. We estimated the entire adult male population of C. formicarius at its low-density period on Kikai Island, Kagoshima Pref., Japan. The population of adult males at the high-density period in September was about 5 times larger than that at its low-density period in May, both of which were estimated by Yamamura' s method. Using this calculation in combination with an estimate of the maximal population size (<4×106) by Sugimoto et al. in 1994, the total number of male weevils at their low-density period can be assumed to be less than 8×105.

奄美群島へのデイゴヒメコバチ（ハチ目：ヒメコバチ科）の侵入

Quadrastichus erythrinae Kim, 2004 (Hymenoptera: Eulophidae) is known in tropical and subtropical regions to cause serious damage to various species of Erythrina (Fabaceae) by inducing stem, petiole, and leaf galls. In Japan, the existence of galls on Erythrina variegata Linnaeus was first recorded in Okinawa Prefecture in 2005, but galls were not found in that year on Amami Island, Kagoshima Prefecture. In our field survey from December 2006 to February 2007, we found that 65 (16.8%) of 323 trees of E. variegata bore galls on Amami Island and 118 (85.5%) of 138 trees of E. variegata and E. crista-galli Linnaeus on Tokunoshima Island, which is located about 49 km south of Amami Island. Because Erythrina trees were more heavily damaged on Tokunoshima Island, Q. erythrinae seems to have invaded the island earlier than Amami Island. In addition to possible multiple invasions, Q. erythrinae may have spread quickly through both islands by low-density dispersal. Up to 2007, no galls had been found on Kikai Island located east of Amami Island.

Intensified mid-Holocene Asian monsoon recorded in corals from Kikai Island, subtropical northwestern Pacific

Coupled records of Sr/Ca and oxygen isotope ratios (δ 18O) of coral skeletons have been used to produce quantitative estimates of paleo-sea surface temperature (SST) and δ 18O of surface seawater that can in some cases be converted to sea surface salinity (SSS). Two fossil corals from Kikai Island in the subtropical northwestern Pacific, a location affected by East Asian summer and winter monsoons, were analyzed to investigate differences between mid-Holocene and present-day SST and SSS. At 6180 cal yr BP, SSTs were roughly the same as today, both in summer and winter; δ 18O seawater and SSS values were higher both in summer (+ 0.5‰, +1.1 psu) and in winter (+ 0.2‰, + 0.6 psu) than modern values. At 7010 cal yr BP, SSTs were slightly cooler both in summer and winter (−0.8 and −0.6 °C), whereas δ 18O seawater and SSS had higher values in summer (+ 0.3‰, + 0.6 psu) and in winter (+ 0.8‰, + 1.9 psu) than present-day values. These results are consistent with other marine records for the mid-Holocene of the low and midlatitudes in the northwestern Pacific. Such regional conditions indicate that the East Asian summer and winter monsoons were more intense in the mid-Holocene, which was likely a function of the mid-Holocene insolation regime.

喜界島におけるアリモドキゾウムシの分布と発生消長

喜界島におけるアリモドキゾウムシの分布と発生消長

Uranium-series Age of the Highest Marine Terrace of the Upper Pleistocene on Kikai Island, Central Ryukyus, Japan

The uplifted Pleistocene marine terraces in Kikai Island have been investigated using radiometric dating since the 1960s. The highest terrace was called the Hyakunodai Terrace, which is divided into three blocks by faults, assigned in this study as Block C, Block B and Block A in ascending order of height. Previous studies indicate that Block A was formed during Marine Isotope Stage (MIS) 5e and that MIS 5c corals were distributed on the cliff between Blocks A and B based on Uranium-series dating. We carried out careful fieldwork and report more precise α-spectrometric U-series ages of Blocks A and B. Fossil corals showed eight Mid- to Late Pleistocene dates ranging from 154.8±6.4 to 97.7±3.0 ky (2σ) and three Middle Pleistocene ages (>450 ky). Mid- to Late Pleistocene dates are divided into three age groups, 154.8±6.4-142.7±5.8 ky, 122.1±3.8 ky and 108.2±3.2-97.7±3.0 ky, corresponding to MIS 6, 5e and 5c, respectively. While all age groups were obtained from Block A, Block B only contains MIS 5c and Middle Pleistocene corals. The MIS 5c outcrop at Block A contains clear reef structure (bindstone) and the coral assemblages indicate depositional depth ranging from 5 to 15 m. These facts suggest that the highest terrace was formed during MIS 5c. Exposure of MIS 5e or 6 corals is possibly due to erosional effect during MIS 5c transgression.

奄美大島，喜界島における冬季のアリモドキゾウムシの交尾と産卵

We investigated the winter-time mating and oviposition of Cylas formicarius (Fabricius) on Amami-Oshima Island and Kikai Island, Japan. No wild females were found to develop their ovarian follicles or have mature eggs from late November to late January. The ratio of mated females was as low as about 30% from late December to late January. Females began to develop their ovarian follicle after late February, thereby leading to an increased mating ratio. As for the wild males, they seemed to be able to mate even during winter. However, since none of the females seemed to release sex pheromones at this time, winter-time mating hardly occurred in the field. The number of eggs laid by the wild weevil was very low from December to February and then gradually increased in late February. Those females that had emerged during the winter began to lay eggs in late March.

Application of cosmogenic 10Be to dating soils on the raised coral reef terraces of Kikai Island, southwest Japan

10Be concentrations of six soils on the raised coral reef terraces of Kikai Island, southwest Japan range from 0.80 to 7.17×10 9 atoms g −1. The annual deposition rate of 10Be from the atmosphere to Kikai Island from 2000 to 2002 ranges from 2.0 to 3.5×10 6 atoms cm −2 year −1, with a mean of 2.8±0.7×10 6 atoms cm −2 year −1. Minimum absolute ages, calculated from the inventory of meteoric 10Be in the soils and the annual deposition rate of 10Be, are: Initial Rendzina-like soil, ≥8 ka; Rendzina-like soil, ≥18 ka; Brown Rendzina-like soil, ≥56 ka; Terra fusca-like soil, ≥127 ka; Terra rossa-like soil, ≥136 ka; and Intergrade between Terra rossa-like and Red-Yellow soil, ≥102 ka. Therefore, it was concluded that soil formation during the first ∼60,000 years would be as Rendzina-like soils and more than ∼100,000 years would be required for Red-Yellow soils to develop on raised coral reef terraces under subtropical humid climatic conditions.

Species-area curve for land snails on Kikai Island in geological time

Abstract Historical changes in the coastline of Kikai Island of the Ryukyu Islands in the southeast part of Japan were estimated by using a numerical simulation based on a glacio-hydro-isostasy model. Temporal changes in the area of the island during the last 40 Kyr were compared with temporal changes in species diversity in fossil land snails of the island. The species number in the past was theoretically estimated by the area of Kikai Island in the past and a species-area relationship among the modern land snail fauna of the Ryukyu Islands. The theoretical species numbers are very close to the actual ones. This suggests that the change in island area is the main cause of the change in species diversity in Kikai Island. In addition, we discuss causes other than the area, such as island elevation, distance to the nearest large island, climate change, human activity, and imperfection of fossil data. We also discuss the change in Fisher's alpha and body size against the change in the area.

Seasonal Radiocarbon Variation of Surface Seawater Recorded in A Coral from Kikai Island, Subtropical Northwestern Pacific

A coral radiocarbon (Δ14C) investigation with a high time-resolution is crucial for reconstructing secular and seasonal Δ14C changes in the surface seawater which potentially reflect ocean circulations and dynamic ocean-atmosphere interactions. The Δ14C values of a modern coral (Porites sp.) from Kikai Island, southern Japan, in the subtropical northwestern Pacific, were determined for the period of 1991-1998 at a monthly resolution. A coral Δ14C time series for the 8 yr indicated seasonal cycles superimposed on a secular decreasing trend of 3.8 per yr. The seasonal amplitude of the coral Δ14C was about 18 on the average, and the minimum Δ14C was observed in late spring and summer. The Δ14C changes were tentatively explained by horizontal oceanic advections around Kikai Island or over the wide range of the equatorial and sub-equatorial Pacific.

Interstadial coral reef terraces and relative sea-level changes during marine oxygen isotope stages 3–4, Kikai Island, central Ryukyus, Japan

Coral reef terraces are one of the best recorders of relative sea-level changes during the last glacial cycle. Thus far, knowledge of relative sea-level record based on coral reefs during the marine Oxygen Isotope Stage (OIS) 3 has been limited to studies of the Huon Peninsula, Papua New Guinea. High-precision a α-spectrometric 230Th/ 234U dating demonstrated an offlapping sequence of five coral reef complexes, ages of which are ∼66, ∼64, ∼62, ∼55 and ∼52 ka, in the northern part of Kikai Island, central Ryukyus of Japan. Interstadial reefs, characterized by deepening-upward sequences of coral assemblages, recorded three hemicycles from transgression to highstand at ∼52, ∼62, and ∼66 ka, during which these reefs were drowned. These highstands in the relative sea-level record can be correlated with the eustatic record reconstructed from the Huon reef terraces and with the interstadials 14, 18, and 19 of the GISP 2 oxygen isotope record. This consistency confirms the Huon sea-level record of OIS 3 and implies that the eustatic sea level responded to the millennial-scale climate changes even during the glacial period of OIS 4.

Holocene sea-level change and tectonic uplift deduced from raised reef terraces, Kikai-jima, Ryukyu Islands, Japan

Kikai-jima (Kikai Island) is surrounded by four Holocene raised coral reef terraces, which are thought to be an offlapping sequence of reef deposits caused by combined effects of seismic uplift and Holocene sea-level change. Many studies in this region have investigated Holocene sea-level changes and reef growth, but there are relatively few in which reliable sea-level indicators are given. We have found that Pocillopora verrucosa, one of the most abundant coral species on the upper-reef slopes of fringing reefs in the Ryukyus, has its peak abundance at a depth of 1.5 m. Therefore, this species is considered ideal for the analysis of relative sea-level change and can be used as a dipstick for the Holocene reef deposits in this area. Based on the distribution of P. verrucosa on the four Holocene raised terraces, we calculate relative paleo-mean sea levels to be 10.8–11.1 and 8.5–8.9 m for Terrace I, 5.0–5.3 m for Terrace II, 4.0–4.3 m for Terrace III and 1.9–2.5 m for Terrace IV. These results, combined with hitherto known and newly measured radiometric dates (103 total), clearly show that the four terraces formed in response to repeated seismic uplifts at 6.3, 4.1, 3.1 and 1.4 ka, and that sea level was higher than present between 7.0 and 6.3 ka.

Dating of Soils on the Raised Coral Reef Terraces of Kikai Island in the Ryukyus, Southwest Japan: With Special Reference to the Age of Red-Yellow Soils.

Dating of Soils on the Raised Coral Reef Terraces of Kikai Island in the Ryukyus, Southwest Japan: With Special Reference to the Age of Red-Yellow Soils.

Mineralogical composition of iron oxides in red- and yellow-colored soils from Southern Japan and Yunnan, China

Six soil samples from the Bt2 horizon of red- and yellow-colored soils collected in Southern Japan and Yunnan, China, were used in this study. Detailed mineralogical studies by differential X-ray diffraction analysis showed the presence of hematite in association with goethite in the red-colored soils, whereas no hematite was detected in the yellow-colored soils. Among the samples studied, the increase in the hematite contents was as follows: Terra rossa-like/Red-Yellow soil in Kikai Island, Red soil in Toyota City, Lateritic Red soil in Minami-Daito Island and Lateritic Red soil in Yunnan, China. Since Lateritic Red soil and Lateritic Yellow soil are distributed adjacently on the Pleistocene raised coral reef terrace in Minami-Daito Island, having a fairly similar soil temperature regime, soil pH, soil organic matter and iron oxide contents, the difference in the soil color between the two soils may be caused by the difference in the hydrologic conditions.

Coral bleaching, geological ranges, and adaptation to high sea surface temperatures

Coral bleaching was described in Kikai-jima (Kikai Island) in October, 1998. Mortality of coral species belonging to the families Acroporidae and Pocilloporidae was the highest of all scleractinian corals in this island. In contrast, some species of the genera Psammocora and Pavona were never or seldom damaged. These results were similar to those observed in the other Indo-Pacific regions. Significant differences in mortality among families could be attributed to the their geological ranges. The species that were free from appreciable damages by bleaching belong to the families which first appeared in the geological periods when the climate was globally warm (greenhouse periods).

１９９５年奄美大島近海（喜界島近海）地震の地震活動とテクトニクス 奄美海台衝突域における変動地形

The Amami Plateau is a remnant arc situated in the northwestern part of the Philippine Sea Plate. The convergent rate of the Philippine Sea Plate relative to the Ryukyu Arc is 5.0 to 7.0cm/yr in a N50° W direction (Seno et al., 1993). The morphology of the continental slope of the collision area between the Amami Plateau and the Ryukyu Arc is characterized by a wide variety of tectonic landform.Since 1983 HDJ (Hydorographic Department of Japan) has conducted geophysical surveys by S/ V TAKUYO and S/ V MEIYO under the Continental Shelf Surveys Project and the Earthquake Prediction Project in the adjacent water of Japan. The area surveyed by these projects includes the northern part of Ryukyu forearc and the northwestern part of the Philippine Sea Plate. HDJ obtained SeaBeam swath bathymetric data, single channel seismic reflection profiles, and magnetic and gravity data in the collision area. These surveys reveal the process of the formation of the tectonic landforms caused by the collision between the plateau and the arc.The continental slope of the collision area is characterized by two tectonic landforms, ridges and minor ridges. Ridges are distributed on the topographic high including the Kikai Island, which are NE-SW trending straight ridges and valleys. The north and south ends of the topographic high are consistent with these of the Amami Plateau. Minor ridges are distributed on the gentle continental slope in the east of the topographic high with the ridges. These minor ridges are divided into two groups by the strike of the ridge axis, which are NE-SW and NW-SE trending ridges. The NE-SW trending ridges are distributed on three blocks in the gentle slope bordered by NW-SE trending ridges and valleys. A subducted seamount, which is estimated from the magnetic anomaly, lies below the central block, and the Kikai Seamount borders on the northeastern block on the trench.The collision between the Amami Plateau and the arc results in large tectonic effects to the Ryukyu Arc. In the collision area the continental slope is uplifted in two difference processes correspondent with two tectonic landforms, one is uplift caused by the buoyant subduction of the plateau, the other is uplift caused by the subduction of seamounts on the plateau.

１９９５年奄美大島近海（喜界島近海）地震の地震活動とテクトニクス １９９５年奄美大島近海地震余震の喜界島における強震波形のモデリング

１９９５年奄美大島近海（喜界島近海）地震の地震活動とテクトニクス １９９５年奄美大島近海地震余震の喜界島における強震波形のモデリング

Early Pleistocene birth of the Okinawa Trough and Ryukyu Island Arc at the northwestern margin of the Pacific: evidence from Late Cenozoic planktonic foraminiferal zonation

Widespread development of the upper Pleistocene Ryukyu Limestone suggests the occurrence of prototypes of the Okinawa Trough and Ryukyu Island Arc, barkarc-basin and forearc at the northwestern margin of the Pacific, before its deposition. Prior to the deposition, however, bathyal sediments of the Shimajiri Group were distributed over the arc region. On southern Okinawa Island, this group is divided into 11 planktonic foraminiferal zones, some of which may be not globally valuable but regionally useful, ranging in age from late Pliocene to earliest Pleistocene. This zonal scheme is applied to zoning 421 outcrop samples and a geologic map is succesfully obtained. The map indicates the development of many faults that cut only the Shimajiri Group but not the overlying Ryukyu Limestone and majority of these faults run in perpendicular to the arc extension. Similar faults are also recognized in the lower Pliocene to lowermost Pleistocene Shimajiri Group on Miyako Island (southern Ryukyu Island Arc), in a submarine seismic profile, which was correlated with well stratigraphy, at the east of Miyako Island, and in the upper Pliocene to lowest Pleistocene Somachi Formation on Kikai Island (northermost central arc). The structures observed in the Shimajiri Group and its equivalent may have been formed by an upward warping of the arc region associated with subsidence of the trough region. Considering the pre-Shimajiri basement geology as a whole, therefore, it is concluded that the initial appearance of this backarc basin and forearc occurred after the deposition of the Shimajiri and before that of the Ryukyu Limestone during the late Pleistocene (∼ 1.7 −0.5 Ma).

南・北大東島のサンゴ礁段丘からみた第四紀後期の地殻変動

Daito Islands, ca. 150 km east of the Ryukyu Trench, are located on the northwestern part of the subducting Philippine Sea Plate, and are composed of two main islands, Kita (north) -Daito and Minami (sourth) -Daito. Both islands are well-known raised atolls and tectonic movements are expected to be different from the Ryukyu Islands on the Eurasian Plate.We investigated raised coastal morphology and geology at eight localities of these islands with special reference to the occurrence of fossil corals, and detailed maps and cross sections are drawn using Electric Distance Meter. Thirty three autochthonous corals are dated by 230Th/234U method and some corals are dated by ESR and non-destructive 226Ra/238U methods. Some allochthonous corals are also dated. 230Th/234U dates on autochthonous corals cluster in 113±6 to 133±6 ka in Kita-Daito and in 113±6 to 159±10 ka in Minami-Daito. Therefore, the dated autochthonous corals, except for one relatively older date, represent the sea level of the last interglacial maximum, corresponding to isotope stage 5e. The upper limit of occurrence of autochthonous corals is 8.1 m in Kita-Daito and 11 m in Minami-Daito. Elevation of the last interglacial shoreline judged by emerged coastal morphology is 10 m in Kita-Daito and 12 m in Minami Daito. No autochthonous corals younger than the last interglacial maximum exist in these islands. However, the occurrence of allochthonous corals dated at ca. 80-100 ka suggests the possible presence of corals corresponding to isotope stage 5c and 5a, below the present sea level.Average uplift rate in both islands is nearly the same, that is 0.03 m/ka at Kita Daito and 0.05 m/ka at Minami-Daito, however, it is significantly lower than 1.8 m/ka in Kikai Island and even 0.3 m/ka in Hateruma Island, both are located on the Eurasian Plate, being subducted by the Philippine Sea Plate. Sequence of vertical tectonic movement of Daito Islands can be estimated as follows : Daito Islands had been subsided resulting in the formation of atolls and lagoons underlain by more than 400 m thick coralline limestone (Sugiyama, 1934 and 1936), and then tectonic uplift, time of which is unknown because of dolomitization of calcareous sediments, has started in association of the northwestward migration of the Philippine Sea Plate and has resulted in the formation of upbulge which is represented by the emerged atolls, fringed by a series of fringing reefs. Uplift rate has been very slow at least since the last interglacial maximum, possibly implying that these islands are tending towards the new tectonic phase (stable to subsidence).