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Why is the Mississippian-Pennsylvanian boundary in North America different from the Lower-Upper Carboniferous boundary in Europe?

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Carboniferous paleogeography — Carboniferous Period Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the late Carboniferous Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.

The distinction between the North American and European subdivisions of the Carboniferous Period arises from the historical development of geologic time scales, which were often based on locally defined rock successions rather than a single, global standard.1

In North America, the Mississippian and Pennsylvanian subsystems were established based on distinct regional strata—specifically, limestone-dominated sequences in the Mississippi valley and coal-bearing rocks in Pennsylvania. Conversely, European geologists subdivided the Carboniferous into the Dinantian and Silesian subsystems.12

These systems were developed independently, and when they were later correlated, it was discovered that they did not align perfectly. The boundary between the Dinantian and Silesian in Europe is positioned lower in the stratigraphic record than the internationally accepted boundary between the Mississippian and Pennsylvanian in North America. Consequently, the North American Mississippian–Pennsylvanian boundary is considered slightly younger than the Lower–Upper Carboniferous boundary used in Europe.21