Lithological and palynological evidence of late Quaternary depositional environments in the subaqueous Yangtze delta, China Zhanghua Wang a, ⁎, Hao Xu b , Qing Zhan b , Yoshiki Saito c , Zhongfa He d , Jianlei Xie d , Xiao Li d , Yonghong Dong b a State Key Laboratory for Estuarine and Coastal Science, East China Normal University, Shanghai 200062, China b Department of Geography, East China Normal University, Shanghai 200062, China c Geological Survey of Japan, AIST, Central 7, Higashi 1-1-1, Tsukuba, Ibaraki 305-8567, Japan d Shanghai Geological Survey, Shanghai 200072, China abstract article info Article history: Received 15 April 2009 Available online 2 April 2010 Keywords: Depositional system Sedimentation rate Sea-level fluctuation East Asia monsoon AMS 14 C ages of post-glacial core sediments from the subaqueous Yangtze delta, along with sedimentary structures and distributions of grain size, pollen spores, and dinoflagellate cysts, show an estuarine depositional system from 13 to 8.4 cal ka BP and a deltaic system from 5.9 cal ka BP to the present. The estuarine system consists of intertidal to subtidal flat, estuarine, and estuarine-front facies, characterized by sand–mud couplets and a high sedimentation rate. The deltaic system includes nearshore shelf and prodelta mud featured by lower sedimentation rate, markedly fewer coastal wetland herbaceous pollens, and more dinoflagellate cysts. We explain the extremely high sedimentation rate during 9.2–8.4 cal ka BP at the study site as a result of rapid sea-level rise, high sediment load due to the unstable monsoonal climate, and subaqueous decrease of elevation from inner to outer estuary. A depositional hiatus occurred during 8.2– 5.9 cal ka BP, the transition from estuarine to deltaic system, caused possibly by a shortage of sediment supply resulting from delta initiation in paleo-incised Yangtze valley and strong tidal or storm-related reworking in offshore areas. The subsequent development of deltaic system at the study site indicates accelerated progradation of Yangtze delta post-5.9 cal ka BP. © 2009 University of Washington. Published by Elsevier Inc. All rights reserved. Introduction The subaqueous Yangtze River delta is one of the most dynamic geomorphologic units in the marginal seas of the western Pacific Ocean. The huge catchment of the Yangtze River rises to an altitude of over 5000 m and has supplied plentiful sediments to the estuary to form a mega-delta on the eastern China coast during the Holocene (Fig. 1). In recent decades, numerous studies have investigated envi- ronmental changes during the Holocene deposition of the Yangtze delta. These have included studies of the climate- and sea-level- controlled geomorphologic and sedimentary evolution of the delta plain (Stanley and Chen, 1996; Stanley et al., 1999; Chen et al., 2005; Wang et al., 2006; Atahan et al., 2007; Itzstein-Davey et al., 2007; Chen et al., 2008), changes in the sedimentary environment and stratigraphy of the paleo-incised Yangtze valley (Li et al., 2000; Hori et al., 2001a,b; Hori et al., 2002a,b), and changes in the vegetation and climate (Liu et al., 1992; Yi et al., 2003; Yi and Saito, 2004; Yi et al., 2006). Previous work has revealed only a thin early Holocene sequence on the delta plain and has shown that the major accumulation of sedi- ments there occurred after ca. 8 cal ka BP (Stanley et al., 1999). The Holocene sedimentary sequence is more complete (∼ 50 m thick) in the paleo-incised Yangtze valley than on the delta plain (Li et al., 2000; Hori et al., 2001a). The evolution of the depositional environment in the paleo-incised valley was closely related to sea-level changes (Hori et al., 2001a, 2002a,b; Hori and Saito, 2007). A large decrease of the sediment accumulation rate in the paleo-incised valley at ca. 8.5– 9.0 cal ka BP is attributed to a sea-level jump at that time (Hori and Saito, 2007). Comparison of borehole data from the delta plain and the paleo-incised valley shows some differences in the sedimentary record. For example, the early Holocene climate optimum evident in borehole ZX-1 from the delta plain (Chen et al., 2005) was not reported in boreholes HQ98 or CM97 from the paleo-incised valley (Yi et al., 2006). To date, studies of the subaqueous Yangtze delta are few compared to those of the paleo-incised Yangtze River valley. Only a few boreholes were drilled into late Quaternary sediments during the 1980s, and few dating results have been reported in previous work (Chen et al., 2000). Some recent, better age-controlled boreholes have been drilled in the distal area of the delta where Holocene sediment thickness is generally less than 10 m (Chen et al., 2003; Wang et al., 2005a). Seismic profiling has revealed that the Holocene sequence in the subaqueous delta is thick, but it failed to define the complete Holocene sequence because of strong attenuation of seismic signals by natural gas reservoirs in the sequence (Liu et al., 2007). Quaternary Research 73 (2010) 550–562 ⁎ Corresponding author. Fax: +86 21 62232416. E-mail address: zhwang@geo.ecnu.edu.cn (Z. Wang). 0033-5894/$ – see front matter © 2009 University of Washington. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.yqres.2009.11.001 Contents lists available at ScienceDirect Quaternary Research journal homepage: www.elsevier.com/locate/yqres