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Submerged tropical karst features were discovered in Nagura Bay on Ishigaki Island in the southern Ryukyu Islands, Japan. The coastal seafloor at depths shallower than similar to 130 m has been subjected to repeated and alternating subaerial erosion and sedimentation during periods of Quaternary sea-level lowstands. We conducted a broadband multibeam survey in the central area of Nagura Bay (1.85 x 2.7 km) and visualized the high-resolution bathymetric results over a depth range of 1.6-58.5 m. Various types of humid tropical karst landforms were found to coexist within the bay, including fluviokarst, doline karst, cockpit karst, polygonal karst, uvalas, and mega-dolines. Although these submerged karst landforms are covered by thick postglacial reef and reef sediments, their shapes and sizes are distinct from those associated with coral reef geomorphology. The submerged landscape of Nagura Bay likely formed during multiple glacial and interglacial periods. According to our bathymetric results and the aerial photographs of the coastal area, this submerged karst landscape appears to have developed throughout Nagura Bay (i.e., over an area of approximately 6 x 5 km) and represents the largest submerged karst in Japan.

To evaluate the relationships between coral calcification, thermal stress, and sedimentation and eutrophication linked to human impact (hereafter referred to as “land development”) by river discharge, we analyzed growth characteristics in the context of a paleoenvironment that was reconstructed from geochemical signals in modern and fossil (1.2 cal kyr BP and 3.5 cal kyr BP, respectively) massive Porites corals from Nagura Bay (“Nagura”) and from modern Porites corals from the estuary of the Todoroki River, Shiraho Reef (“Todoroki”). Both sites are on Ishigaki Island, Japan, and Nagura is located approximately 12 km west of Todoroki. At Nagura, the individual corals provide time windows of 13 (modern), 10 (1.2 cal kyr BP), and 38 yr in length (3.5 cal kyr BP). Here, we present the coral annual calcification for Nagura and Todoroki, and (bi) monthly resolved records of Sr/Ca (a proxy of sea surface temperature (SST)) and Ba/Ca (a proxy of sedimentation and nutrients related to land development) for Nagura. At Nagura, the winter SST was cooler by 2.8°C in the 1.2 cal kyr BP, and the annual and winter SSTs in the 3.5 cal kyr BP were cooler by 2.6°C and 4.6°C, respectively. The annual periodicity of Ba/Ca in modern coral is linked to river discharge and is associated with land development including sugar cane cultivation. Modern coral calcification also has declined with SST warming and increasing Ba/Ca peaks in winter. However, calcification of fossil corals does not appear to have been influenced by variations in Sr/Ca and Ba/Ca. Modern coral growth characteristics at Nagura and Todoroki indicate that coral growth is both spatially and temporally influenced by river discharge and land development. At Nagura, our findings suggest that land development induces negative thermal sensitivity for calcification in winter due to sugar cane harvest, which is a specifically modern phenomenon.

Ryukyu Islands are an active island arc on the island shelf in high latitude between the Ryukyu Trench and the Okinawa Trough. They are surrounded mainly by fringing reefs, except for barrier reefs in the Sekisei Lagoon between Ishigaki and Iriomote Islands in the southern Ryukyus and off the eastern coast of Kume Island in the central Ryukyus. In order to clarify development of the Sekisei Barrier Reef, drilling of 3 cores through the Holocene reefs was carried out to reach the basement rock of the Pleistocene Ryukyu Limestone with about 20m in depth. Two cores (B-1, B-2) are drilled through the barrier reefs, and one core (B-3) through the patch reef in the inner lagoon. The Holocene reef in the Sekisei Lagoon commenced to have formed about 7630 ^14C yrs BP, which is almost same as those at the reef in the western Kume Island and at the Gushichan reef in the southern Okinawa Island in the central Ryukyus, although depth of the basal reefs in these areas is about 10m. Mean upward growth velocity of reefal sediment in each core shows 3.6 m/ 1000 yrs (B-1), 4.7 m/ 1000 yrs (B-2), and 8.9 m/ 1000 yrs (B-3). Inner reef at the B-3 point started to develop on the deepest basement and the catch-up type reef grew rapidly upward mainly with branching corals. Upward development of the reef finally reached the sea level about 5600 ^14C yrs BP, and formed the patch reef which was the first reef-phase in the Sekisei Lagoon. The outer reefs at the B-1 and B-2 points grew upward with almost same grow rate of reefal sediments as those in the Ryukyus and other fringing reefs in the world. Upward development of the outer reefs was late to reach the sea level about 3720～3620 ^14C yrs BP, and formed barrier reefs showing the second reef-phase in the Sekisei Lagoon.