Lake Messel, a high resolution archive for early Middle Eocene climate variability.

The oil shale of the Middle Eocene Lake Messel is worldwide known for the exceptionally well preserved fossil assemblage. A continuous core from the center of the basin proved that the finely laminated oil shale was deposited in a meromictic maar lake which formed due to a phreatomagmatic eruption dated as 47.8 +- 0.2 Ma. The new drill core included a complete reference section of the Middle Eocene lake deposits representing a unique climate archive for the Middle Eocene in Central Europe. The classical “Messel oil-shale” of the Middle Messel Formation is characterized by a continuous succession of finely laminated bituminous claystones, representing long-term stable meromictic conditions. They show a very fine light and dark lamination, which is the product of annual algal blooms of the coccal green alga Tetraedron minimum. Therefore, the oil shale is annually laminated and made up of light spring and summer layers caused by algal blooms that were superimposed on the terrigenous background sedimentation, as represented by the dark autumn and winter layers. Oil shale laminae counting estimate an average sedimentation rate of 0.14 mm/yr but short-term fluctuations in varve thickness can be attributed to an Eocene ENSO. High resolution palynological analysis of the oil shale of the Middle Messel Formation, which represents a time interval of 640 kyr, provides now an insight into the dynamics of a paratropical climax vegetation during the Middle Eocene greenhouse climate. The analysis of the microflora shows that the vegetation surrounding Lake Messel did not change substantially in qualitative composition, but a change from a more humid climate with relatively high water levels in the lake to less humid conditions and lower water levels may be reflected by the quantitative assemblage composition towards the top of the section. In addition to the long-term changes in vegetation short-term fluctuations in the frequency of individual taxa are recognizable. Assuming annual lamination, time series analyses of palynological data suggest that pollen assemblages vary with periods within the range of eccentricity, obliquity, long precession and short precession. This implies that orbital control of climate change was sufficient to impose quantitative changes in the composition of terrestrial vegetation though no taxonomic turnover occurred. According to the cyclicity of the palynological data and with the availability of the astronomical solutions of La2004 and Va2003, it is now possible to implement an astronomical tuning of the 640 kyr record of the Middle Messel Formation. The tuning of pollen data to the La2004 Earth’s orbital solution was done by aligning data and target at the obliquity frequency. In this way, the Middle Messel Formation can be astronomically dated between 46.6 and 47.3 Ma