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Human–environment interactions across historical periods in China have been well reconstructed over the past decade; however, few studies have focused on southern China, particularly Hainan Island. This study investigated two sediment cores (cores X1 and X2) collected from the continental shelf off Hainan Island in the northern South China Sea. Combined with geochemical compositions (Al, Ni, Cu, Zn, Cd, and Pb), Sr-Nd isotopes, grain size, and AMS 14C ages, our results showed that Cd and Cu in core X1 and Cd and Pb in core X2 have been mainly influenced by anthropogenic activities since ~4.0 kyr BP. The enrichment factors showed obvious increases starting at ~4.0 kyr BP and were in good accordance with the appearance of abundant archeological sites on Hainan Island. Significant climatic changes (i.e. decreases in the East Asian summer monsoon and increases in El Niño–Southern Oscillation activities) at ~4.0 kyr BP likely promoted more intense, localized human activities, which in turn enhanced heavy metal accumulation in the continental shelf sediments. These results provide new insights into the history of human activities on Hainan Island and indicate that the geochemical compositions of continental shelf sediments have the potential for broadening our knowledge of the human activities and cultural development in tropical areas. [ABSTRACT FROM AUTHOR]

Clay mineralogy of 54 fluvial samples collected from 20 major rivers on Hainan Island are investigated in order to determine compositional changes of clay minerals and to assess the weathering processes. The clay mineral assemblages consist dominantly of kaolinite (31–66%), with a lesser abundance of chlorite (22–44%) and illite (4–33%), and a trace amount of smectite (0–15%). Fluvial sediments from the east and northwest of Hainan Island are characterized by a higher kaolinite content and illite chemical index and poorer illite crystallinity than those from southwest Hainan. Only minor smectite (mean of 7%) occurs in the sediments from west Hainan; smectite is total lacking in east Hainan. Compared with the adjacent basins, Hainan Island is characterized by moderate to intensive chemical weathering with strong hydrolysis. Our results suggest that rainfall is the principal factor controlling the intensity of chemical weathering on Hainan Island, with more intense chemical weathering occurring in eastern and northwestern Hainan. Another practical implication of this study is that it provides a “missing” end member (Hainan Island) in the provenance discrimination study focused on the northern South China Sea (SCS). Hainan fine-grained sediments likely play an important role in providing clay minerals to the northern SCS carried by the South China Sea Warm Current (SCSWC) during the summer. [ABSTRACT FROM AUTHOR]

• Sediments on the continental shelf of southeastern Hainan Island are mainly derived from Hainan. • During the period of significant sea level rise, extreme cold events had a considerable impact on this area. • The sedimentary record during the sea-level highstand was mainly controlled by climate change. • An increase in chemical weathering could be attributed to a rise in typhoon intensity or frequency. The South China Sea, which connects the Indian Ocean and the Pacific Ocean, is an important part of the Indo-Pacific Warm Pool. This study examined the provenance and evolution of the sedimentary environment on the southeastern Hainan Island continental shelf in the northwestern South China Sea through AMS 14C dating, grain size analysis, and clay mineral analysis of sediments from Core QZ4. The clay mineral analysis indicated that the terrigenous detritus originated from Hainan Island, while the grain size data identified three endmembers: EM1 and EM2, corresponding to the fine-grained material transported by the oceanic current system, and EM3, corresponding to the coarse-grained material input by rivers from proximal sources. The sedimentary environment in the study area underwent three stages: foreshore (11.2–9.2 ka BP), nearshore (9.2–6.2 ka BP), and shallow shelf (6.2 ka BP to present). From 11.2 to 6.2 ka BP, the sedimentary environment was mainly controlled by sea level changes, although extreme cold events (such as the 9.4 ka BP and the 8.2 ka BP event) had a significant effect on the southeastern continental shelf of Hainan Island. From 6.2 ka BP to the present, the sedimentary record has mainly been influenced by climate change, and an increase in riverine contribution and the enhancement of chemical weathering may have been caused by rising typhoon intensity or frequency and possibly by an increase in human activities. These findings will be valuable for conducting high-resolution research on typhoons by analyzing sediments on the eastern Hainan Island shelf and neighboring areas. [ABSTRACT FROM AUTHOR]