Lacustrine rhythmites from the Mulhouse Basin (Upper Rhine Graben, France): a sedimentary record of increased seasonal climatic contrast and sensitivity of the climate to orbital variations through the Eocene-Oligocene Transition

Abstract

The Eocene-Oligocene Transition (EOT) marks the passage from Eocene greenhouse to Oligocene icehouse conditions. It holds keys to our understanding of the behavior of climate systems under major pCO2 shifts. While the environmental impact of the EOT is rather homogenous in oceans, it is much more heterogeneous on continents. Although little to no changes are recorded in some regions, several EOT studies in western Eurasia suggest an increase in seasonal climatic contrast (e.g., higher amplitude of changes in mean temperature or precipitation), along with a higher sensitivity of the climate to orbital variations. However, these variations remain to be properly documented through changes in sedimentary facies and structures and forcing mechanisms. Here we investigate the depocenter of the Mulhouse Basin (Upper Rhine Graben; URG) revealing a prominent transition from massive mudstones to laminated sediments and varves, alongside the emergence of astronomically-forced mudstone-evaporite alternations. These changes are identified in the distal and proximal parts of the southern URG, where they consist of millimeter-thick mudstone-evaporite couplets and siliciclastic-carbonate couplets. The elemental composition and micro-facies analysis of the laminae show a recurrent depositional pattern consistent with a seasonal depositional process, which suggests that they are varves. We propose that the occurrence of varved sediments, together with the observed orbital cyclicity in the southern URG, reflects an increase in seasonal climatic contrast, and an increase in the sensitivity of climate to orbital variations across the EOT. We show that similar changes were noticed in the Rennes and Bourg-en-Bresse basins, and that of other western Eurasian records for similar climatic conditions. This work emphasizes the potential of high-resolution sedimentary structures to serve as markers of climate change across the EOT.