Terrigenous supplies variability over the past 22 000 yr in the southern South China Sea slope: relation to sea level and monsoon rainfall changes

Abstract:

Variations in grain size and clay mineral composition have been investigated along core CG2 recovered from the southern South China Sea (SCS) in order to assess the impact of sea level changes and East Asian monsoon rainfall intensity on erosion and weathering during the last 22.0 ka BP. Our results indicate that illite, chlorite and smectite are mainly from the Mekong River controlled by East Asian monsoon rainfall intensity, while the gradual decrease in kaolinite starting around 22.0 ka BP are mainly result from differential sedimentation processes regulated by sea level rise, and relative increased proportion of kaolinite and higher δ¹⁸O_seawater values during the last 9.0 ka BP may reflect the higher influence of the tropical Indonesian Islands sources due to the reopen of southern straits. An abrupt increase of mass accumulation rate (MAR), mean grain size and its other populations between 17.6 and 16.9 ka BP provides evidence that increased monsoon rainfall affected by a southward shift of the Intertropical Convergence Zone (ITCZ) results in strong physical erosion on the exposed Sunda Shelf, coeval with the Heinrich Event 1 (H1) time interval in the North Atlantic region, underscoring the imprints of northern hemisphere forcing on tropical SCS climate. The smectite/(illite+chlorite) ratio combined with the δ¹⁸O_seawater records suggests a tight control of the erosion of the Mekong River drainage basin by the monsoon rainfall during the last deglaciation. The summer monsoon optimum (15.0-9.0 ka BP) lead to a higher proportion of weathered sediments originating from the low and middle reach of the Mekong River, whereas a reduction of the summer monsoon (9.0 ka BP until the present-day) induce lower inputs of detrital material from the lower relative to the upper reach of the Mekong River. Owing to the special background of the Mekong River basin, a rapid response of erosion processes of the Mekong River basin to the monsoon rainfall intensity changes is possible.