Mound-shaped structures
The SBP profiles and bathymetric map (Fig. 6) show mounds at water depths of 70–100 m off eastern Miyako-jima. The mound distribution found in this area is limited to the eastern side of the island. In this paper, we defined the mound-shaped structures as transparent mounds rising from the base of the surface sediment with internal reflectors. The mounds are about 100 m wide and about 3 m or more in height. The mounds’ depths obtained in these surveys have gaps of several meters due to differences in measurement methods and correction data, but their relative width and height are very similar. The mound tops reached a maximum of 8 m above the seafloor. Sugisaki et al. (2019) reported that coarse- to medium-grained modern carbonate sediments are widely distributed throughout the area and that they collected coarse bioclastic sand and algae-covered carbonate block samples from the mounds using a grab sampler during the GK18-1 cruise. These results are consistent with Facies 1 and 2 in our SBP profiles. The mound surfaces were well indurated, and only algal carbonate blocks that encrusted the well-indurated surface were recovered in pictures taken by a submarine camera mounted on the grab sampler.
The mound-shaped structures’ strong reflectivity on the surface, indicating a hard bottom, (Facies 1) may suggest the presence of erupted rocks such as submarine volcanoes, limestone of remnant mounds (karst landforms), or submerged reefs in the study area. Sato (2016) reported that no gravity or magnetic anomalies were distributed around the mound-shaped structures. These results indicate that the mound-shaped structures are not made of volcanic rocks and basement rock, which would be accompanied by magnetic and gravity anomalies, but rather should be considered to be made of well indurated sedimentary rocks. We cannot state decisively that the mound-shaped structures are submerged reefs or limestone such as remnant mounds (karst landforms) at this time and subsurface sampling, such as drilling, will be required to clarify the origin of the mound-shaped structures. However, the sizes and shapes of the mound-shaped structures in the SBP profile images are topographically similar to those of pinnacles observed above the inner break of the island shelf off the central and southern Ryukyu Island Arc (Hori and Kayanne, 2000; Fig. 3a), although the depths of their distribution differ from those in Hori and Kayanne (2000).
Similar mound-shaped structures having acoustically chaotic properties under the seafloor were reported on the shelf off Irabu-Jima, which is located west of Miyako-jima (Obata and Tsuji, 1992). Sasaki et al. (2006) dated samples collected from an isolated mound-shaped structure and confirmed that it was a lowstand coral reef. The lowstand coral reef interpreted from a seismic reflection survey by Obata and Tsuji (1992) and dated by Sasaki et al. (2006) is a mound buried under the seafloor. It is a little larger than the mounds observed in this study area. However, the difference in size is possibly due to the resolution of the equipment used to recognize the subsurface geology, and we consider the mounds to be of similar sizes at the resolution of the seismic profiling survey, with widths on the order of several hundred meters and heights on the order of ten meters. The mound-shaped structures are on a similar scale in size and shape to submerged reefs (about 15 m in height: Arai et al., 2012) in the Last Glacial Maximum known around the study area. Consequently, we conclude that such mound-shaped structures in this study are possibly submerged coral reefs and reefal carbonate rocks that developed in the Last Glacial Period. Since the Last Glacial Maximum, the sea level has risen in a non-linear manner (Fairbanks, 1989). The depth (70–100 m) of the tops of the mound-shaped structures matched periods of slow to rapid sea-level rise (Hori and Kayanne, 2000; Fig. 4a), which could point to the mounds having grown during a period of slow sea-level rise and having been drowned during a period of rapid rise. In such cases, reef back-stepping and the ages of drowned reefs can be interpreted as successive cycles of reef-growth and drowning in low latitude areas of the world (Blanchon and Shaw, 1991; Khanna et al., 2017; Rovere et al. 2018). In considering the mound-shaped structures around Miyako-jima, it seems possible that reefs developed during the Last Glacial Period, then drowned due to sea-level rise outpacing reef growth.
Webster et al. (2018) reported the reverse of this process, where seaward migration of coral reefs in the Great Barrier Reef was caused by a rapid sea-level fall during the Last Glacial Maximum. In addition, Fujita et al. (2019) reported that the development of fringing reefs with shallow back-reef lagoons and the timing of reef-flat and back-reef formation is consistent with the two stepwise falls in sea level during the Last Glacial Maximum. In the SBP profiles of this survey area, the bases of the mound-shaped structures are found shallower than 100 m deep, and strong reflection surfaces that indicate a hard and uneven bottom are recognized continuously with the surfaces of the mound-shaped structures. In this study, we cannot confirm the base of mound-shaped structures, we recognized the base (maximum depth) of surface of mounds is about 100 meters. It is possible that the mound-shaped structures and the related reef were formed before the Last Glacial Maximum and then a hard, sub-exposed bottom surfaced due to sea-level fall during the Last Glacial Maximum. Subsequently, stratified deposits after the Last Glacial Maximum overlay the mounds. The Pleistocene limestone karst forming similar topography eroded during subaerial exposure, but we can conclude that at least carbonate rocks exist at this depth. However, since we do not have a sample at present, we cannot say definitively that these are reef that developed during the sea-level fall. It will be necessary to collect samples in order to discuss their formation age and mechanism.
Developments east of the Miyako-jima reef
The mound-shaped structures examined in this study are very similar to those located off southern Okinawa-jima (Arai et al., 2012), although the base of the mounds off southern Okinawa formed at a water depth of 140 m, which is deeper than found in this study, thus indicating the possibility of tectonic subsidence in the southern Okinawa-jima region. Here, southern Okinawa-jima is subducting, related to the subduction of the Kerama gap (Arai et al., 2018b). In the survey area around Miyako-jima, on the other hand, the environment, in terms of tectonic movement, is considered to be relatively more stable than around southern Okinawa-jima. We consider the tectonically stable condition of the study area and the depth of the mound-shaped structures as being indicative of this structure having basically formed in the Last Glacial Maximum, possibly with aerial exposure off Miyako-jima.
The mound-shaped structures found in this survey are distributed in a limited area only offshore to the east of Miyako-jima. Recent coral reefs around Miyako-jima also develop commonly in the eastern part. In addition, Coral Reefs of Japan (2004) reported that recent corals are distributed in the eastern and northern areas off Miyako-jima. The eastern and northern areas have gentle slopes and shelves more than 20 km wide. On the other hand, the shelf off the southern slope of Miyako-jima is about 2 km wide and is steeper than the eastern and northern slopes (Fig. 2). The mound-shaped structures examined in our study are submerged coral reefs or reefal carbonate rock located only 5 km northeast of the modern Tsufutsuwa Reef. That reef, which is 6–7 km away from Miyako-jima, is about 0.8 km wide by 2.5 km long and extends northwest-southeast (Coral Reefs of Japan, 2004). The bathymetric contours from the modern Tsufutsuwa Reef to the area where the mound-shaped structures are located shows continuous deepening toward the northeast (Fig. 2), and it is possible that the modern reefs have grown and migrated continuously since the deglaciation period. The migration of the reefs’ position may have been caused by reef growth attempting to keep pace with the rising sea level during the early stages of deglaciation in the area of the mound-shaped structures.
We infer that the consolidated hard substrate of the mound-shaped structures influenced the development of the modern coral reef during deglaciation and shown that such back-stepping indicates sea-level rises. In the future, it will be necessary to conduct additional SBP and bathymetric mapping investigations in the study area, including the region around Tsufutsuwa Reef, using small boats because conventional research vessels are too large to operate in such shallow nearshore areas. The water at the base of the mounds was 100 m deep and the top of the mounds were approximately 70–100 m deep (Figs. 3 and 4). The position and depth of the mound-shaped structures show affected active reef growth and the resultant reef movement’s sensitivity to glacial sea-level changes in the Central and Southern Ryukyu Islands. Although we have no absolute chronology for the appearance of these mound-shaped structures, their depths indicate that coral reef development began and finished during sea-level fluctuations during the Last Glacial Maximum. If reef growth or drownings are indicators of rapid sea-level rises or falls, we believe that similarities in the structure and the top and bottom depths of such mounds can be used to provide a new approach to investigating local tectonics. With that in mind, additional research on this study area, including direct sampling using techniques such as submarine drilling, is required.