Thorium isotopes (228Th, 230Th, 232Th) and applications in reconstructing the Yangtze and Yellow River floods

In the past decades, the floods of the Yangtze and Yellow River introduced unexpected changes of the ecological community and sedimentary dynamics in the East China Sea (ECS). To reconstruct the flood events in the ECS, 228Th, 230Th and 232Th have been examined in a sediment core. The specific activities of three thorium isotopes have good positive relations with fine fractions (<63 μm), indicating that Th activity concentrations heavily depend upon the sediment grain size. The size-normalized activities of 228Th, 230Th and 232Th showed significant variations. Coincidences between the higher Th activities and historical floods of the Yangtze and Yellow River demonstrated that size-normalized Th recorded the two rivers' flood events. The activity ratios of thorium isotopes, i.e. 230Th/232Th and 228Th/232Th, also showed similar patterns to the historical river floods. In three periods (1740s, 1840–1860s and 1930–1960s), characterized by frequent floods, the thorium activity ratios were fairly low and close to the Yangtze and Yellow River estuary sediments, coinciding with the less oceanic 228Th and 230Th contributions during the flooding periods. Accordingly, these results support the size-normalized Th activity and thorium ratios as proxies of the river floods in coastal seas.