炭素14年代を測定し，暦年較正した結果によると，北部九州の弥生前期の板付Ⅰ式は前780年頃に始まる。南四国も前8世紀のうち，板付Ⅱa式併行期に始まる中部瀬戸内の前期は前7世紀，近畿の前期は前7～6世紀に始まる。すなわち，弥生前期は西周末頃に併行する時期に始まり，前380～350年の間，戦国中期に終わる。弥生前・中期の展開を考古学的に追究するうえで，青銅器の年代は重要な意味をもっている。日本出土の青銅器のうち年代がはっきりしている最古例は，福岡県今川遺跡出土の遼寧式銅剣の鋒と茎を銅鏃と銅鑿に再加工した例であって，板付Ⅰ式に属する。同様の例は朝鮮半島では忠清南道松菊里遺跡などから出土しているので，ほぼ同時期と考えてよいだろう。松菊里式の較正年代は前8世紀であるので，板付Ⅰ式の炭素年代とも整合する。青銅器鋳造の開始を証明する根拠は鋳型の出土である。現在知られている資料では，近畿では和歌山県堅田遺跡から銅鉇の鋳型が前期末の土器とともに見つかっている。北部九州では，福岡県庄原遺跡の銅鉇の鋳型が中期初めないし前半の土器と出土している。また，中期初めの甕棺墓に副葬してあった銅戈に朝鮮半島の銅戈と区別できる北部九州独特の型式が知られているので，中期初めには青銅器の鋳造が始まっていたとみられる。弥生前期の存続期間が著しく延びたので，北部九州の中期初めと近畿の前期末とが実年代では一部重なっていないかどうかの検討が必要である。銅鐸は愛知県朝日遺跡から最古型式の銅鐸の鋳型が中期初めの土器とともに見つかっている。同時期の石川県八日市地方遺跡出土の木製竪杵のみに知られている独特の羽状文を身に施しているので，北陸の集団も関与して銅鐸が創出されたことは確かである。朝日遺跡から出土した銅鐸鋳型だけでは，最古の銅鐸の鋳造が中期初めに濃尾平野で始まったとまでは断定できないとしても，きわめて重要な手がかりが得られたことはまちがいない。

According to the radiocarbon dates, as calibrated to calendar ages, the Itazuke I phase of the Early Yayoi in northern Kyushu began around 780 BC. The Early Yayoi of the Setouchi area, which parallels the Itazuke Il-a phase of northern Kyushu, began sometime between the 8ᵗʰ and 7ᵗʰ centuries BC, while the beginning of the Early Yayoi would date in the Kinki region to the 7ᵗʰ and 6ᵗʰ centuries BC. In other words, the Early Yayoi period began about the time when the Western Zhou ended, and lasted until about the middle of the Waring State Period sometime between 380 and 350 BC.For archaeological investigation of the Early and Middle Yayoi development, the ages of bronze implements have been particularly important. The earliest well-dated bronze artifacts recovered from Japan are parts of Liaoning bronze daggers that were made into an arrowhead and a chisel, recovered from Itazuke I level of the Imagawa site in Fukuoka Prefecture. Since similar artifacts have been excavated from the Songguk-ri site in Chungcheonnamdo in South Korea, it is likely that the Japanese and Korean samples are about the same age. The Songguk-ri pahse has been radiocarbon dated to the 8th century BC, which coincides with the ¹⁴C age of Itazuke I phase in Kyushu.Moulds for bronze casting provide clear evidence of bronze production. The currently available examples include a mould for spear-shaped planes, recovered from the Katada site in Wakayama Prefecture, which was found in association with the pottery dating to the end of the Early Yayoi period. In northern Kyushu, a mould for planes was found at the Shobara site in Fukuoka Prefecture, associated with the first half of the Middle Yayoi period. By the ebeginning of the Middle Yayoi period, the bronze halberds placed in burial jars as gave offerings are of the style quite distinct from those found in the Korean Peninsula, suggesting that the bronze were being manufacture in the Japanese archipelago. Since the Early Yayoi period now covers a much longer span of time, it is necessary to examine whether the end of Early Yayoi in Kinki in fact overlapped in time with the beginning of Middle Yayoi in northern Kyushu.As to dotaku (bronze bell) production, a mould for the oldest type was recovered from the Asahi site in Aichi Prefecture, in association with the pottery of the beginning of the Middle Yayoi period. While the presence of the decorative motifs on the body of the bell that are characteristics of the regions west of Kinki seem to suggest that the beginning of dotaku production has some connection with the Kinki region, it does not explain why the decorative motifs of the Early Yayoi of Kinki should appear on a Middle Yayoi bronze bell. The mould for the bronze bells from the Asahi site could not be the definitive evidence to suggest that bronze bell production began in the Nobi Plain.

source:https://www.rekihaku.ac.jp/outline/publication/ronbun/ronbun6/index.html#no137