5:15 PM - 6:45 PM
[SEM13-P07] Fundamental research on magnetic properties of sediments from the Nhon Thanh site in Vietnam
Keywords:Nhon Thanh site, Vietnam, Mekong delta, Magnetic property, Detrital remanent magnetization
The Nhon Thanh site in Can Tho City, southern Vietnam, is a settlement site located on a low plain area in the Mekong Delta. According to archaeological excavations conducted thus far, including the excavation in 2023, two cultural layers have been unearthed. These layers correspond to the early Oc Eo period (approximately the 1st-3rd centuries AD) and the typical Oc Eo period (4th to 7th centuries AD). Further research on the age and land use of this site is considered very important for understanding the culture of Funan, an early state in ancient southern Vietnam, and Oc Eo, a port city in Funan. In this study, to characterize the properties of the Detrital remanent magnetization (DRM) recorded by the sediments and to provide a basic understanding of the formation environment of each layer, (1) stepwise AF demagnetization measurements, (2) thermomagnetic analysis, and (3) IRM acquisition experiments were performed on sediment samples (n=3 per layer) collected from a total of 6 layers in two excavation trenches at the Nhon Thanh site. The archaeological age of the lowest layer of the trench is estimated to be the 1st CE, and the surface layer is a modern soil layer.
As a result of the stepwise AF demagnetization, the characteristic remanent magnetization components could be identified on the Zijderveld diagrams for all six layers of the sample. In particular, the MAD of the characteristic remanent magnetization from the third to the lowest layer is about 2 to 3 o, and it can be considered that the DRM has high stability. Archaeodirections with a certain degree of concentration (a95: 3.4-18.3 o) were obtained from the samples from the third to the lowest layer. Whether or not these archaeodirections are the true orientation of the Earth's magnetic field at the time of deposition of the soil layers needs to be investigated in more detail in the future. However, these orientation data are expected to be used as one of the reference indices for future chronological studies of sites associated with the Oc Eo culture.
The results of thermomagnetic analysis (in air) and IRM acquisition experiments showed the following characteristics in the bottom, middle and surface layer samples. The thermomagnetic curves of the bottom and middle layer samples showed a gradual decrease in magnetization from room temperature to 700 oC or showed a Curie point around 500-600 oC; the IRM acquisition curves showed a characteristic high coercivity component around 560-4000 mT. These features suggest the presence of goethite or maghemite of goethite origin and are consistent with archaeological evidence that the layer is associated with a canal water settlement or cultivated land. The thermomagnetic curves of the surface layer samples showed a Curie point around 250-350 oC; the IRM acquisition curves showed an outstanding coercivity component around 36 mT. These features suggest the presence of iron sulfide minerals such as pyrrhotite. The area around the Nhon Thanh site is currently being developed as a tourist plantation, and it is suggested that the generation of iron sulfides in the layer may have been caused by the modern fertilizers used there.
As a result of the stepwise AF demagnetization, the characteristic remanent magnetization components could be identified on the Zijderveld diagrams for all six layers of the sample. In particular, the MAD of the characteristic remanent magnetization from the third to the lowest layer is about 2 to 3 o, and it can be considered that the DRM has high stability. Archaeodirections with a certain degree of concentration (a95: 3.4-18.3 o) were obtained from the samples from the third to the lowest layer. Whether or not these archaeodirections are the true orientation of the Earth's magnetic field at the time of deposition of the soil layers needs to be investigated in more detail in the future. However, these orientation data are expected to be used as one of the reference indices for future chronological studies of sites associated with the Oc Eo culture.
The results of thermomagnetic analysis (in air) and IRM acquisition experiments showed the following characteristics in the bottom, middle and surface layer samples. The thermomagnetic curves of the bottom and middle layer samples showed a gradual decrease in magnetization from room temperature to 700 oC or showed a Curie point around 500-600 oC; the IRM acquisition curves showed a characteristic high coercivity component around 560-4000 mT. These features suggest the presence of goethite or maghemite of goethite origin and are consistent with archaeological evidence that the layer is associated with a canal water settlement or cultivated land. The thermomagnetic curves of the surface layer samples showed a Curie point around 250-350 oC; the IRM acquisition curves showed an outstanding coercivity component around 36 mT. These features suggest the presence of iron sulfide minerals such as pyrrhotite. The area around the Nhon Thanh site is currently being developed as a tourist plantation, and it is suggested that the generation of iron sulfides in the layer may have been caused by the modern fertilizers used there.