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How can radiometric dating of volcanic ash help determine the age of Carboniferous fossils?

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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.

Radiometric dating of volcanic ash is a critical tool for determining the absolute age of Carboniferous fossils because ash layers often serve as key beds that can be precisely dated.1

When volcanic ash is deposited, it settles over vast areas in an instant of geologic time, allowing geologists to correlate these layers across different sedimentary environments. These ash deposits frequently contain unaltered mineral crystals, such as zircon, which are ideal for radiometric dating. By analyzing these crystals—often using advanced techniques like the uranium-lead method or argon-40/argon-39 dating—scientists can calculate a precise absolute age for the ash layer.123

Because these ash layers are often found in close proximity to fossil-bearing strata, the absolute age obtained from the ash provides a reliable temporal anchor for the fossils in the adjacent units. This method effectively confirms the relative ages established by faunal succession, providing a robust framework for dating the Carboniferous Period, which spanned from approximately 358.9 to 298.9 million years ago.14