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

Abstract:

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.