Geomorphic development of an unvegetated shingle cay on the Ximen Reef in the southern South China Sea

Unvegetated shingle cay (USC) is a type of coral island typically located in the southern South China Sea (SCS). Currently, there is a lack of detailed understanding of its development process and dynamic mechanism. The geomorphic development of a representative USC at the Ximen Reef in the southern SCS was examined through a field survey and high-resolution imagery analysis. The USC consisted of shingle sediments composed of coral skeletal clastics. It exhibited a composite geomorphic structure with shingle ridges and a central depression. From 2000 to 2020, the USC showed highly dynamic changes such as an increase in fully emerged area, morphological variations, and southeastward migrations. The USC development model has four main stages: 1) a shingle bank representing the embryonic foundation of the USC, 2) a mound-shaped shingle cay representing the early growth of the USC, 3) a single shingle ridge-depression representing the formation of the composite structure, and 4) a multiple shingle ridges-depression representing the adjustment of the geomorphic structure. The USC has undergone all development stages multiple times over the past two decades. The centripetal refracted wave convergence, which is controlled by the elliptical reef shape, is the key dynamic factor influencing the USC development. Monsoons and tropic cyclones play a joint role in the morphological variations in the USC during the past two decades. The short-term development of the USC at the Ximen Reef helps us understand the long-term evolution of the USCs in the southern SCS under past and future climate changes.