Vol. 44 Supp. SCIENCE IN CHINA (Series B) August 2001 Progradation of the Changjiang River delta since the mid-Holocene Hori Kazuaki 1 , Saito Yoshiki 2 , ZHAO Quanhong () 3 , WANG Pinxian () 3 & LI Congxian (Ϋ) 3 1. Department of Earth and Planetary Science, University of Tokyo, Hongo 7-3-1, Bunkyou-ku, Tokyo 113-0033, Ja- pan; 2. Marine Geology Department, Geological Survey of Japan, Higashi 1-1-3, Tsukuba 305-8567, Japan; 3. Laboratory of Marine Geology, Tongji University, Shanghai 200092, China Corresponding author: Kazuaki Hori (e-mail: khori@gsj.go.jp) Present address: c/o Dr. Yoshiki Saito, Marine Geology Department, Geological Survey of Japan, Higashi 1-1-3, Tsukuba, 305-8567, Japan Received March 27, 2000 Abstract Subaqueous deltaic deposits with approximately 30 radiocarbon ages show that the Changjiang River delta was strongly affected by tides and that the delta progradation rate after 2 kaBP was almost double the rate before 2 kaBP. This change in the progradation rate correlates well with the active extension of the subaerial delta plain shown by previous work. Widespread human activities, such as farming, deforestation, and dike construction, probably resulted in an increase in sediment discharge to the river-mouth area. Keywords: Changjiang, Yangtze, sediment discharge, delta progradation, AMS radiocarbon dating, human impact. The Changjiang River, one of the largest rivers in the world, has formed a large delta at its mouth since about 6—7 kaBP, when sea level reached or approached its present position. Previous work has described the active extension of the subaerial delta plain after about 2 kaBP [1—3] . Most of these studies, however, were based only on paleogeographic maps and on land chenier studies. Thus, the subaqueous deltaic deposits constituting the main part of the delta system are poorly known. In this study, we investigated the subaqueous deltaic deposits and delta front progradation by analyzing samples from three boreholes. The three boreholes (CM97, JS98, and HQ98) were drilled in the present subaerial delta plain in 1997—1998. The cores were split, described, and photographed. X-radiographs were taken us- ing slab samples from all split cores. Five-cm-thick samples were taken every 20 cm for the de- termination of sand and mud content. Approximately thirty radiocarbon ages were measured on molluscan shells and plant materials by the Accelerator Mass Spectrometry (AMS) method through Beta Analytic Inc. The deltaic deposits, 25—30 m thick, were classified into five facies: prodelta, delta front, lower intertidal to subtidal flat, upper intertidal flat, and surface soil, in ascending order based on sediment facies analyses [4] . The prodelta facies was characterized by dark gray silt to clay with occasional thin shell beds and thin coarse-silt layers. The delta front facies consisted mainly of