Temporal changes in ostracod assemblages during the past 10,000 years associated with the evolution of the Red River delta system, northeastern Vietnam Gengo Tanaka a, ⁎, Toshifumi Komatsu b , Yoshiki Saito c , Duc Phong Nguyen d , Quang Lan Vu e a Gunma Museum of Natural History, 1674–1, Kamikuroiwa, Tomioka, Gunma 370–2345, Japan b Graduate school of Science and Technology, Kumamoto University, Kumamoto 860–8555, Japan c Geological Survey of Japan, AIST, Central 7, Higashi 1-1-1, Tsukuba 305–8567, Japan d Department of Paleontology & Stratigraphy, Vietnam Institute of Geosciences and Mineral Resources (VIGMR), Hanoi, Vietnam e Northern Mapping Division, Department of Geology and Minerals of Vietnam, Ai Mo 1, Gia Lam, Hanoi, Vietnam abstract article info Article history: Received 24 February 2011 Received in revised form 13 August 2011 Accepted 15 August 2011 Available online xxxx Keywords: Climatic event Core Holocene Ostracoda Red River delta Vietnam Eighty-five species belonging to 46 genera of Ostracoda were recovered from 169 samples in three sediment cores (VN, NB, and GA) from the Red River delta plain, northeastern Vietnam. From a Q-mode cluster analysis, we recognized three biofacies (Biofacies A, B, and C). Biofacies B is subdivided into Biofacies B I and B II . The environment of Biofacies A is interpreted as open marine, those of Biofacies B I and B II as marine to brackish water and brackish water, respectively, and that of Biofacies C as oligohaline to mesohaline brackish water. The Holocene sediments consist of estuarine sediments deposited under relatively high sea levels following transgression, deposited during 11–8.5 cal kyr BP, overlain by deltaic sediments deposited under relatively low sea levels following regression, and the ostracod assemblages reflect these changes. The estuarine sedi- ments of the early Holocene transgression are characterized by ostracod Biofacies B I . An erosional surface marks an abrupt change to the shelf environment, accompanied by sediment starvation. The interval around this surface, identified as the maximum flooding surface, is characterized by ostracod Biofacies A. During the subsequent delta progradation, the sedimentary environment changed from a prodelta to delta front, and then to a delta plain, and the corresponding ostracod assemblages changed from Biofacies A to B I ,B II , and then C in ascending order. Taken together, our results show that the temporal changes in ostracod assemblages since 10 cal kyr BP are closely related to the evolution of the Red River delta system and can serve as good indicators of paleoenvir- onmental changes. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Holocene environmental changes and the corresponding changes in marine organisms make the Indochinese Peninsula one of the most in- teresting places in the world from the viewpoint of biodiversity. Near river mouths, coastal environmental changes from estuarine to deltaic environments resulted from rapid shoreline migration and sediment de- livery changes during the Holocene and led to notable changes in com- munities of marine and brackish water organisms. A delta is discrete geomorphologic feature that forms where a river enters an ocean, a semi-enclosed sea, a lake, or a lagoon (Elliott, 1986). Most modern ma- rine deltas were initiated when the rate of the postglacial sea-level rise decelerated during the early Holocene (Stanley and Warne, 1994). Along Asian coasts, the sediments of large-river deltas (megadeltas) formed during the Holocene record delta evolution and human impacts on coastal landforms (e.g., Rao et al., 1990; Saito et al., 2001; Tanabe et al., 2003a). We studied the temporal changes in ostracod assemblages in sed- iments of the Red River delta, northeastern Vietnam, during the last 10,000 years and investigated the relationship between these changes and the evolution of the delta, as recorded in the sedimentary facies of three sediment cores obtained from the lower delta plain (Tanabe et al., 2006)(Fig. 1). Tanabe et al. (2006) previously described the sedi- ment facies and radiocarbon chronologies of the cores, and Li et al. (2006) have reported on the Holocene pollen assemblages in two of the cores. 2. Regional setting The Red River delta, located on the western coast of the Gulf of Tonkin, Vietnam, in the South China Sea, is the fourth largest in terms of delta plain area in Southeast Asia, after the Mekong, Irrawaddy, and Chao Phraya deltas. The delta formed about 8.5–9 cal kyr BP near the present site of Hanoi, and it subsequently prograded ~100 km seaward, forming a huge subaerial delta plain with an area of 10,300 km 2 (Tanabe et al., Marine Micropaleontology 81 (2011) 77–87 ⁎ Corresponding author. E-mail address: tanaka@gmnh.pref.gunma.jp (G. Tanaka). 0377-8398/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.marmicro.2011.08.001 Contents lists available at SciVerse ScienceDirect Marine Micropaleontology journal homepage: www.elsevier.com/locate/marmicro