Temporal changes in ostracod assemblages during the past 10,000 years associated with the evolution of the Red River delta system, northeastern Vietnam

Abstract

Abstract 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 BI and BII. The environment of Biofacies A is interpreted as open marine, those of Biofacies BI and BII 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 sediments of the early Holocene transgression are characterized by ostracod Biofacies BI. 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 BI, BII, 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 paleoenvironmental changes.