Non-destructive elemental analysis of prehistoric potsherds in the southern Ryukyu Islands, Japan: Consideration of the pottery surface processing technique in the boundary region between the Japanese Jomon and Neolithic Taiwan

The southern Ryukyu islands are at the southwest end of the Japanese Archipelago, and can be regarded as boundary region between Japan and Taiwan. Some artifacts with a presumed relationship with Neolithic Taiwan were found, dated to 4,800-3,300 BP. "Shimotabaru-type" pottery in particular could indicate cultural propagation from Taiwan due to similar pottery shapes. Other factors, like manufacturing techniques, have not been discussed much. Interestingly, some of this pottery had a characteristic heterogenous surface structure, which showed the possibility of some surface processing techniques like the slips or paint that is an indicative connection between the Southern Ryukyu Islands and Taiwan. Therefore, in this study, we analyzed this structure on Shimotabaru pottery using nondestructive methods such as X-ray fluorescence microscopy. Chemical analysis revealed that this structure was mainly composed of phosphorus and calcium, which differed from the Neolithic "red-coating" in Taiwan or other surface processing found in surrounding regions. Additionally, we also compared the chemical composition of the potsherd structure with the organic remains and soil associated with the potsherds, and concluded that it was not artificial surface processing but was caused by post-depositional alternation of many faunal remains. To our knowledge, this report is the first case in this region showing a distinct color change in the pottery surface because of coexisting faunal remains. Our results indicated that the heterogenous surface structure of Shimotabaru pottery was not kind of surface treatment but incidental and a result of the post-depositional environment. This result showed the surface processing such as slip or paint had not been developed in the Shimotabaru phase and supported an inconstant and weak cultural relationship the southern Ryukyu islands and surrounding regions, which in turn caused a re-estimation of the previous cultural relationship hypothesis.