An 8000-Year Record of Extreme Floods in the Yangtze Catchment from the Subaqueous Yangtze Delta

Thick massive mud deposits in the subaqueous Yangtze Delta offer the potential to record flood activities in the catchment, including a number of extreme floods. The borehole YD0903 (30°53′54.3″N, 122°54′33.0″E) was collected in a water depth of 36 m, with the top 22-m layer composed of massive muds. They were deposited about 8000 cal yr BP in an offshore to prodelta setting. Paleoflood reconstruction using XRF scanning elemental Zr/Rb ratios shows four mega-stages of extreme flood activities: (1) a highly active stage from 8000 to 5800 cal. yr B.P. with 5 extreme flood periods (7900~7600, 7440~7210, 6780~6590, 6430~6240, 6100~5840 cal yr BP); (2) a quiescent stage form 5800 to 3900 cal yr BP with 2 extreme flood periods (4900~4640, 4380~4240 cal yr BP); (3) a relatively quiescent stage from 3900 to 2000 cal yr BP with 3 extreme flood periods (3900~3700, 3460~3240, 3000~2800 cal yr BP) featuring extraordinarily large flood events; (4) a relatively active stage since 2000 cal yr BP with relatively weak flood events. Long-term variation in flood events is considered to be linked to the damping strength of the East Asian Summer Monsoon. Shorter-term periodicities of flood events in the Yangtze catchment are potentially related to solar variations over ~200 yr and ~100 yr periods, and to the Pacific Decadal Oscillation with a period of ~40 yr.