About 868 cal. yr BP tsunami event at the northern South China Sea revealed from offshore sediments

The northern coast of the South China Sea (SCS) is a densely populated and economically important area. Despite the absence of any tsunamigenic events in the last century in this region, their occurrence on a much longer timescale remains largely unknown. Given the catastrophic consequences a potential tsunami event could bring, we aim to bridge this research gap by conducting high-resolution sedimentological, geochemical, and geochronological analyses on two well-preserved offshore sedimentary columns, B01 and D02, from the northern SCS, at depths of −27 and − 46 m, respectively. Using statistical methods, we identified two anomalous sediment units, each ∼30 to 40 cm thick, in both columns. These units are dominated by coarse-grained, poorly sorted sediments with a normally graded sequence. Each unit is rich in biogenic debris, devoid of parallel or cross-bedding, and has a unique set of elemental and isotopic signatures. They contain allochthonous biological species and record a disordered age-depth pattern. Such observations are indicative of the sudden input of marine components, suggesting an instantaneous high-energy event. The median calibrated ages, measured by accelerator mass spectrometry (AMS), from shell and foraminifera in the anomalous units, return the mixed ages of 4454–842 cal. yr BP and 2098–840 cal. yr BP, respectively. Shell and foraminifera above and below the anomalous units define AMS 14C ages of 951–779 cal. yr BP, with a median age of ∼868 cal. yr BP. Our data collectively point to a tsunami event at ∼868 cal. yr BP, equivalent to the North-Song Dynasty of ∼1000 years ago. This age pattern is consistent with reported tsunami events in and around the SCS, also compatible with historical accounts describing a “tidal overflow in Eastern Guangdong and Eastern Fujian at AD1067–1068” in Chinese documents. Combined with available data, the ca 868 cal. yr BP tsunami event in the SCS likely originated from the joint interaction of earthquakes induced by the Manila Subduction Zone and the Littoral Fault Zone, along with related landslide activities within the northern SCS.