https://en.wikipedia.org/wiki/Thraco-CimmerianThraco-Cimmerian is a historiographical and archaeological term, composed of the names of the Thracians and the Cimmerians. It refers to 8th to 7th century BC cultures that are linked in Eastern Central Europe and in the area west of the Black Sea.
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Archaeologically, Thraco-Cimmerian artifacts consist of grave goods and hoards. The artifacts labelled Thraco-Cimmerian all belong to a category of upper class, luxury objects, like weapons, horse tacks and jewelry, and they are recovered only from a small percentage of graves of the period. They are metal (usually bronze) items, particularly parts of horse tacks, found in a late Urnfield context, but without local Urnfield predecessors for their type. They appear rather to spread from the Koban culture of the Caucasus and northern Georgia, which together with the Srubna culture, blends into the 9th to 7th centuries pre-Scythian Chernogorovka and Novocherkassk cultures. By the 7th century, Thraco-Cimmerian objects are spread further west over most of Eastern and Central Europe, locations of finds reaching to Denmark and eastern Prussia in the north and to Lake Zürich in the west. Together with these bronze artifacts, earliest Iron items appear, ushering in the European Iron Age, corresponding to the Proto-Celtic expansion from the Hallstatt culture.
https://en.wikipedia.org/wiki/CimmeriansThe origin of the Cimmerians is unclear. They are mostly supposed to have been related to either Iranian or Thracian speaking groups which migrated under pressure of the Scythian expansion of the 9th to 8th century BC.[4][5]
According to Herodotus, the Cimmerians inhabited the region north of the Caucasus and the Black Sea during the 8th and 7th centuries BC (i.e. what is now Ukraine and Southern Russia), although they have not been identified with any specific archaeological culture in the region
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The supposed origin of the Cimmerians north of the Caucasus at the end of the Bronze Age loosely corresponds with the early Koban culture (Northern Caucasus, 12th to 4th centuries BC), but there is no compelling reason to associate this culture with the Cimmerians specifically
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The Biblical name "Gomer" has been linked by some to the Cimmerians.
from area of Koban culture north of Caucasus and Black sea Cimmerians have spread to areas of central and east Europe.. this settlement is known as Thraco-Cimmerian and lasted from 9th century till 7th century BC. In my opinion 9th to 7th century BC admixture to east Europe is good timeline for I2a din spread. It spread in R1a areas giving present mix of R1a and I2a in Slavic countries