Early to mid-Holocene sedimentary evolution on the southeastern coast of Hangzhou Bay, East China, in response to sea-level change

Muddy coasts downdrift of large river mouths trap fine-grained sediments supplied from these rivers and have provided important settlement sites for prehistoric people, although the evolutionary history of such environments has not been well described. In this study, we integrate multi-proxy analyses (lithology, particle size, sedimentary structure, bioturbation, microfossil, and AMS 14C) on two new sediment cores (CC and ZF) from the Ningbo Plain, southeast of Hangzhou Bay, east China, with results from previous studies to explore the early to mid-Holocene sedimentary history of the palaeo-Ningbo Bay in response to post-glacial sea-level rise. The results show that the early to mid-Holocene stratigraphy in core CC was characterized by a muddy embayment succession before 7.3 cal kyr B.P. and an aggradational tidal flat to coastal marsh succession after that time. Rapid sediment accumulation occurred before ca. 7.7 cal kyr B.P. and at 7.3–7.2 cal kyr B.P., whereas periods of reduced sediment supply are recorded between 7.7 and 7.3 cal kyr B.P. and after 7.2 cal kyr B.P. At site ZF, in contrast, only sediments of coastal freshwater marsh overlay the last glacial stiff muds after ca. 7.5 cal kyr B.P. We suggest that the sheltered embayment favoured trapping of the fine-grained sediments during the early to mid-Holocene, while geometric changes of the bay in response to a phase of accelerated sea-level rise at ca. 7.7 cal kyr B.P. resulted in a reduced sedimentation rate and increased tidal energy. Abundant, large, and diverse (0.5–1.0 cm wide, 5–24 cm long) vertical burrows are present in core CC during 8.9–7.9 cal kyr B.P. and the trace fossils declined abruptly at 7.9 cal kyr B.P. We suggest this may have been induced by the exploitation of seafood by Neolithic people settled in the sheltered embayment, as supported by archaeological remains found at the Neolithic site. Rapid progradation of tidal flats and coastal marsh occurred since ca. 7.2 cal kyr B.P. in the palaeo-bay, which may reflect a response to a stable sea level after that time. The development of a thick early to mid-Holocene sediment sequence in the palaeo-Ningbo Bay and its relationship to sea-level changes indicate that such muddy embayments downdrift of large river mouths should be considered important in reconstructing the Holocene depositional system of the adjacent coast.