Submerged karst landforms observed by multibeam bathymetric survey in Nagura Bay, Ishigaki Island, southwestern Japan

Hironobu Kan,Kensaku Urata,Masayuki Nagao,Nobuyuki Hori,Kazuhiko Fujita,Yusuke Yokoyama,Yosuke Nakashima,Tomoya Ohashi,Kazuhisa Goto,Atsushi Suzuki

doi:10.1016/j.geomorph.2014.07.032

Abstract

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 ~130m 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 (1.85×2.7km) and visualized the high-resolution bathymetric results over a depth range of 1.6–58.5m. 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 6×5km) and represents the largest submerged karst in Japan.

Highlights

Evidence suggests that coastal regions shallower than ~130 m have been subjected to repeated alternations between subaerial and marine conditions during the frequent glacio-eustatic sea-level changes that occurred throughout the Quaternary, with extensive environmental change occurring between glacial and interglacial periods (e.g., Yokoyama et al, 2000; Camoin et al, 2004; Fujita et al, 2010)
The seafloor morphology of Nagura Bay resembles that associated with terrestrial karst landforms and may be attributed to a submerged karst that developed during a sea level lowstand
There may be some uncertainty attached to applying terminology related to subaerial geomorphology to the seafloor landforms, as suggested by Taviani (1984), we tentatively apply the terminology of terrestrial karst landforms to describe the submarine landforms of Nagura Bay because no terminology exists to describe either the landforms found in the coral reef or the geomorphology of the seafloor in this context

Summary

Introduction

Evidence suggests that coastal regions shallower than ~130 m have been subjected to repeated alternations between subaerial and marine conditions during the frequent glacio-eustatic sea-level changes that occurred throughout the Quaternary, with extensive environmental change occurring between glacial and interglacial periods (e.g., Yokoyama et al, 2000; Camoin et al, 2004; Fujita et al, 2010). The geomorphology of shallow coastal regions has been modulated by repeated subaerial and submarine processes during glacio-eustatic sea-level change. Terrestrial landforms, few previous studies have dealt with shallow seafloor landforms, which represent former terrestrial landscapes modified by present marine processes, from a geomorphological perspective. In this regard, karst geomorphology is no exception, Taviani (1984) provided some discussion regarding karst-like features on the ocean bottom from various perspectives.

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Conclusion