Variable Early Eocene continental hydroclimate in Central Europe?

Predicted future climate scenarios share similar characteristics with the Eocene ‘greenhouse’ period. However, short-term Early Eocene terrestrial climate variability is still poorly constrained mainly due to the rarity of adequately resolved climate archives. This lack of information restricts not only the evaluation of past continental climate conditions but additionally limits regional climate modelling efforts but also the validation of model outputs. Here, we present highly-resolved biomarker-based (bacterial membrane lipid and leaf wax) paleoclimate data from the UNESCO World Heritage Site Messel Fossil Pit (Germany) that cover an interval of ca. 640 ka. The drilled Messel paleolake succession, characterized by finely laminated and frequently varved black pelites (referred to as ‘oil shale’) represent a regional climate and environmental archive from the latest Early to Middle Eocene (~48.0-47.4 Ma) of western Central Europe. Downcore mean annual air temperature (MAAT) reconstructions inferred from bacterial-derived branched glycerol dialkyl glycerol tetraethers (brGDGTs) show a long-term cooling trend and range from 14 to 22°C. High-resolution sampling within the basal and middle core interval reveal several short-term negative temperature excursions of 4-5°C, respectively. Moreover, we measured compound-specific δ²H and δ¹³C of excellently preserved odd carbon numbered mid- and long-chain leaf wax n-alkanes in order to estimate past regional hydroclimatic conditions. δ²H values of terrestrial long- and aquatic mid-chain n-alkanes show exceptional variations of up to 45‰ and 60‰, respectively. In contrast, δ¹³C values of long-chain n-alkanes are within 5‰ (-28‰ to -33‰) while mid-chain δ¹³C values vary by 11‰, ranging between -26‰ and -37‰. Our results indicate that the Early to Middle Eocene temperature history of central western Europe, particularly on short geological timescales was much more variable than previously assumed. We recognize two abrupt shifts in MAAT that coincide with lower δ²H values and therefore may point to either wetter climate conditions or changed atmospheric moisture trajectories. We emphasize that the long-term decline in estimated MAAT towards the top of the Messel section has to our best knowledge not been quantified from any time-equivalent terrestrial archive in Central Europe, but resembles Early Eocene cooling patterns well-documented from the global oceans.