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How did Gondwanan ice sheets cause cycles of sea-level rise and fall during the Carboniferous?

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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.

During the Carboniferous Period, the expansion and contraction of widespread continental ice sheets in Gondwana acted as a primary driver for fluctuations in global sea level.1

As these massive ice sheets grew, they sequestered significant volumes of water, causing sea levels to drop and exposing continental shelves. Conversely, during warmer intervals, the melting of these glaciers released water back into the oceans, leading to sea-level rise and the subsequent flooding of coastal areas.234

This rhythmic oscillation between glacial and interglacial conditions produced cyclic patterns of marine and nonmarine sedimentation known as cyclothems. These repeated sequences—which often include layers of limestone, shale, and coal—reflect the alternating periods of marine incursion and terrestrial deposition that characterized the later part of the Carboniferous, particularly during the Pennsylvanian Subperiod.15