Submarine sinkholes are unique and important geomorphological features with a typical cavity structure that are of great scientific value. Submarine sinkholes were discovered for the first time in the isolated Ganquan carbonate platform on the Xisha Islands, the northwestern South China Sea. Based on high-resolution multibeam bathymetric data and seismic profile data, we identified 37 submarine sinkholes at water depths ranging from 550 to 1267 m. They are subcircular to circular negative-relief features, and most of them are V- or compound V-shaped in the cross-section. Their average diameters range from 57 to 667 m, and the depth of the depression ranges from 2.5 to 241 m. By comparing submarine sinkholes in the Ganquan platform with those in other carbonate platforms worldwide, we can infer that the Ganquan platform submarine sinkholes are the largest sinkholes developed on an isolated carbonate platform. Remotely operated vehicle (ROV) “Haima 2” images revealed that the inner walls of submarine sinkholes are characterized by stalactite-like structures, possible dikes, flow marks, and corroded holes, which are typical karstic landscape features. The temperature within submarine sinkholes is 2 °C higher than that of the open ocean at the same water depth. Based on the results of the shallow formation profile and multichannel seismic profiles, we propose that the submarine sinkholes in the Ganquan platform probably formed via the dissolution of the carbonate platform via acidic hydrothermal fluids that originated from magmatic activity and migrated along faults.

This study presents the first application of a deep-towed transmitter–receiver marine controlled-source electromagnetic (TTR-MCSEM) system for gas hydrate exploration in the Shenhu area of the South China Sea. High-resolution electromagnetic data were acquired along a 13 km transect using dynamic source–receiver offsets and a 500 A transmitter. The results reveal the following: (1) unprecedented near-seafloor resolution (20~100 m) for the precise delineation of hydrate-bearing caprock, surpassing conventional ocean-bottom electromagnetic systems; (2) laterally continuous high-resistivity anomalies (~10 Ω·m) extending from the base of the gas hydrate stability zone to the seafloor, which correlate with seismic bottom-simulating reflector (BSR) distributions and suggest heterogeneous hydrate saturation; and (3) fault-controlled fluid migration pathways that supply hydrate reservoirs and lead to seabed methane seepage at structural highs. Through 2D inversion, we show that the inverted resistivity values (~10 Ω·m) are slightly higher than those obtained from resistivity logs (~5 Ω·m). Saturation values derived from inverted resistivity exhibit remarkable consistency with well-log-based measurements. The high efficiency of the system confirms its potential for the transformative quantitative assessment of hydrate systems, seafloor massive sulfides, and marine geohazards.

A large number of sand waves are developed on the seafloor off the coast of Dongfang, Hainan Island. This paper analyzes the geophysical characteristics of active submarine sand waves synthetically by using multi-beam echo-sounding(MBES), side-scan sonar(SSS), sub-bottom profiler (SBP), and single-channel seismic(SCS) data, and discusses the distribution characteristics, migration mechanism, activity, and morphologic evolution characteristics of sand waves. The results show and confirm that there are significant spatial differences in the distribution and scale of submarine sand waves, large and medium-sized sand waves mainly develop on the sand ridge, small sand waves mainly develop on both sides of the sand ridge, and near-symmetrical sand waves develop in the pit and trough area. Submarine sand waves were not developed in the southwest of the study area. Constrained by tidal current and Coriolis force, the migration direction of the sand wave on the west side of the submarine sand ridge is mainly north (slightly east), while that on the east side is mainly south (slightly west). Due to the topography, the migration of near-symmetric sand waves may stop or the migration direction has changed. The signatures of strong active submarine sand waves mainly include: (a) Asymmetric "ridge tip trough gentle" shape; (b) Superimposed small sand waves and sand ripples development on submarine sand waves; (c)Sub-bottom profile has transparent layer; (d)Reflection on the steep slope is blank; (e) Internal oblique progradation configuration. The analysis shows that the activity of the sand wave is closely related to its morphology, including three evolution stages low activity, high activity, and motionless.