The impact of the Eocene-Oligocene transition on a midlatitude fluvial system (Almazán Basin, Spain)

The EOT (Eocene-Oligocene Transition) stands out among the greatest reorganizations of the global climate during the Cenozoic era, marking a shift from the Late Eocene “greenhouse” state to “icehouse” conditions on the Oligocene (34-33.5 Ma). While its temporal framework is well established and its global characteristics are increasingly understood, further research is required to assess the regional imprints of this major climatic shift. In this regard, records from epicontinental basins are particularly valuable, as they provide excellent archives to complement global perspectives of environmental change.

This study aims to shed light on how the signals produced by the EOT were transmitted and archived, focusing on changes in the weathering products, sediment production, and water discharge in fluvial systems. To achieve this, we investigated the Paleogene record of the Almazán Basin in central Spain. The EOT is identified within an outstanding exposed fluvial system (Gómara Fm.), supported by a well-defined chronostratigraphic framework.

We applied a multiproxy approach that includes sedimentology, rock magnetism, geochemistry and paleohydraulic estimates to selected stratigraphic intervals encompassing the EOT. New demagnetized samples enhanced the resolution of the EOT. Magnetic properties were measured on discrete samples, across the EOT in order to understand changes in weathering conditions. In addition, we measured paleochannel parameters to provide paleohydraulic estimates enabling assessments of shifts in slope and water discharge.

All analyses and measurements were integrated within a comprehensive stratigraphic framework. We will discuss the meaning of the observed shifts and the changes in magnitude. The findings are discussed in the context of environmental changes associated with the EOT, highlighting their implications for understanding regional responses to this critical climatic transition.