Long-term eustatic cyclicity in the Paleogene: a critical assessment

Global sea level has changed cyclically throughout Earth's history due to a variety of mechanisms that operate on a variety of timescales. Here we attempt to place constraints on the “actual” number of sea-level cycles that can be interpreted directly from the eustatic reconstructions. We apply an interpretative algorithm to Paleogene sea level records and identify three orders of eustatic cycles longer than 1 Ma. However, the three-ordered cyclicity might not represent cycles of global eustatic change. First, only cycles of the highest of the established orders (with a timescale of 10 s of Ma) are coherent among different sea-level records. Second, the interpreted cycles are not supported by the compilation of the regional maximum flooding surfaces. Third, climatic history alone cannot explain the eustatic changes. Fourth, the interpreted cyclicity differs significantly from what is known about the tectonic control of eustasy. Fifth, there may be other orders higher than those established. The problem is rooted in (1) the fact that eustatic curves might not necessarily reflect global events (e.g., fluctuations shown on these curves may be artifacts related to regional tectonic activity) and (2) the possible weakness of Paleogene (especially Eocene) eustatic cyclicity and its significant “overprint” by regional tectonic activity. Our attempted analysis claims for significant improvement of the available eustatic reconstructions. Unfortunately, the regional stratigraphical data remain still insufficient to develop any alternative eustatic curve that can be further interpreted to understand the number of “actual” cycle orders.