Holocene maar lakes
- Figure Holocene Maar Lake
- Figure ELSA Team and Freeze Core
The west Eifel volcanic field shows 7 open maar lakes, which reach water depth below 20 m. This are Pulvermaar, Meerfelder Maar, Holzmaar, Weinfelder Maar, Gemündener Maar, Schalkenmehrener Maar und Ulmener Maar. 3 other maar structures are covered with shallow ponds from anthropogenic activity; this are Jungferweiher, Immerather Maar, Eichholzmaar. The ELSA-Project has focused its drilling on Holzmaar, Schalkenmehrener Maar and Ulmener Maar. Sediment cores are taken from a platform with an UWITEC drilling equipment, operated by Klaus Schwibus and Frank Dreher from Mainz University.
Cores are drilled in 2 m segments. The sediment is often compacted from the drilling processes, which explains that a core from 0-2 sediment depth is not entirely 2 m long. The next 2 m segment is then taken from the same borehole at 2-4 sediment depth. It remains an open question if some sediment loss occurred between individual segments. Respective proxy time series have to take these drilling disturbance into consideration. We thus cored for ELSA-20 always overlapping cores to check for sediment loss – and sampled the most complete core.
The ELSA-20 section from today back 1000 AD is perfectly varved in two freeze cores from Schalkenmehrener Maar. The uppermost meter is dated with 210Pb and 137Cs; the second meter shows the 1342 flood layer as a 10 cm thick layer. This layer is usually the thickest of all medieval flood layers, but in particular well distinguishable by a high amount of debris from plants, which were analysed in detail by Herbig & Sirocko (2013). This well visible flood layers is documented also in all cores from Holzmaar and used to continue the ELSA-20 record from SMfreeze into HM4. This core has a twin record drilled with a 1 m offset at the same location, to document that HM4 is indeed continuous. HM4 reveals the distinct Laacher See Tephra layer at 10.15 m and reached further back in time continuously, even back to the late glacial initial warming at 14,690 yr b2k, according to the ice core chronology. This warming is documented by an increase of Corg and Phosporous at the same level were first pollen of juniper and grasses occur.
The same environmental change is observed in the late Pleistocence sections of cores AU3 and AU4 and is thus used to extent the ELSA-20 record without any gap into the last glacial maximum times.
