Microstratigraphy reveals cycles of occupation and abandonment at the mid Holocene coastal site of Thach Lac, northern-central Vietnam

Shell mounds are an important component of the archaeology of coastal regions in northern Vietnam. Understanding cultural dynamics and settlement patterns within seemingly homogenous layers of shell accumulation is difficult based on field survey and excavation records alone. Here, we use microstratigraphic and microfacies analysis to decipher the complex stratigraphy and thereby reconstruct the occupation history of Thach Lac, a mid Holocene (c. 5000–4100 cal BP) coastal shell-bearing site in Ha Tinh Province, northern-central Vietnam. Our microstratigraphic approach utilises micromorphology, the microscopic study of undisturbed stratigraphic blocks, aided by compositional analyses including Fourier Transform Infrared (FTIR) spectroscopy, particle size analysis and phytolith concentrations. Our results establish that Thach Lac was occupied by three distinct cultures, known as the Quynh Van, Thach Lac and Bau Tro, across a millennium of significant sea level fluctuations in the mid Holocene. During the Quynh Van (5000–4850 cal BP) and Thach Lac (4850–4600 cal BP) occupations, repeated transient shell foraging activities took place across the period of maximum Holocene transgression. As sea levels stabilised to present levels, the site was removed from coastal sedimentary processes and a major subsistence and settlement shift was detected in the later Thach Lac and ensuing Bau Tro (4450–4100 cal BP) phases, based on the change from shell to animal bone deposition and evidence for extended, semi-sedentary occupation. A microstratigraphic approach to shell mound formation at Thach Lac enables high-resolution integration of cultural and environmental sedimentary records, thus affording exceptional insights into prehistoric settlement adaptations and resilience to dynamic coastal environments.