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

Abstract 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, 228 Th, 230 Th and 232 Th have been examined in a sediment core. The specific activities of three thorium isotopes have good positive relations with fine fractions ( 228 Th, 230 Th and 232 Th 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. 230 Th/ 232 Th and 228 Th/ 232 Th, 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 228 Th and 230 Th 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.