Clay mineral assemblages and sedimentary environment evolution over the past 13 kyr in the Gulf of Tonkin, northwestern South China Sea

High-resolution clay mineralogy, X-ray fluorescence (XRF) scanning element geochemistry, and detrital grain size distribution were analyzed on a gravity core GC44-5 (18°44.227‧N/107°11.764‧E, water depth 67 m) in the central Gulf of Tonkin, northwestern South China Sea to determine detrital sediment source variations and to reconstruct sedimentary environment evolution since the last deglaciation. Seven AMS-14C datings of carbonate shells and plant debris show the continuous normal sequence and an age of about 13 kyr BP at the core's bottom. Our results indicate that the Red River system has been the main source of detrital fine-grained sediments to the central gulf since 13 kyr BP. Clay mineral assemblages exhibit relatively lower smectite contents (26-45%) and higher illite and chlorite contents (total 35-56%) during the glaciation and early deglaciation times (13-11 kyr BP), and relatively higher and stable smectite contents (50-67%) and lower illite and chlorite contents (total 23-40%) during the Holocene. The sedimentary environment in the central gulf evolved from coastal marsh and floodplain during the glacial time (13.0-12.6 kyr BP) to tidal-influenced transitional coastal zone during the deglaciation (12.6-10 kyr BP), and to shallow marine environment since 10 kyr BP. The clay mineral assemblage variations are controlled mainly by the sea level rise and the sediment discharge of the Red River.