Tephra (volcanic ash layers)

  • ELSA-23-Tephra-Stack

We can distinguish 7 tephra layers by their distinct petrology during the last 60,000 years, which are visible in all cores. Thus the respective eruptions were strong and wind direction change during the weeks of a maar eruption distributed these tephra over the entire Eifel as cm thick layers. It is quite likely, that small eruptions can have produced additional ash layers, which form cm thick layers only in the immediate vicinity of the eruption, but only mm thick layers at distance of more than a few kilometer. These thin ash layers are difficult to identify and analyse geochemically and will be focus of future work.

At the moment we distinguish only the omnipresent 7 marker tephra layers with a sediment petrographic method developed by Förster & Sirocko (2016) to differentiate 10 different minerals from the bedrock and the eruption itself. Förster et al. (2020)  analysed some of these tephra mineralogically and started to extend the marker tephra record back to 130,000 yr b2k. The high resolution stratigraphy of ELSA-20 is now used to date these marker tephra with the best possible precision, in particular its relation relative to warming events.

The most important tephra is the Laacher See Tephra (LST) from the Laacher See eruption in the east Eifel volcanic field. This tephra is found in all lake records below the Holocene pollen and is characterized by sanidine grains of more than 40%. However, the pumice of the LST is easily transported by running water, and thus LST particles are often not from the direct airfall, but can be washed into the lake together with reddish and greyish sandstone fragments from the river catchment, that drain into the maar lake. We observe two pronounced events of reworked LST, one during the first part of the Younger Dryas, one in the early Holocene. The latter occurs exactly at the time for which the Ulmener Maar tephra was described by earlier authors. Unfortunately, we never found a distinct tephra layer with the mineral composition of the Ulmener maar tuff wall, however a flood layer with reworked LST pumice at around 11,000 yr b2k. This is true for the Ulmen Maar Tephra
at about 10,650 yr b2k.

We also do not observe any distinct tephra during the last glacial maximum, but many thin layers of volcanic minerals at the base of glacial turbidites and flood layers, which apparently transport volcanic debris into the maar during ephemeral meltwater events.

We observe a small tephra with the petrographical and geochemical composition of the Eltville Tephra – well known from many loess outcrops – at 24,700 yr b2k. This age, and all subsequent ages place the marker tephra of Förster & Sirocko (2016) not on the new ELSA-20 stratigraphy (Sirocko et al., 2021).

A very well distinguishable tephra occurred according to ELSA-20 than at 28,100 yr b2k, between the interstadials GI4 and GI3. The high pyroxene content associate it perfectly with the composition of volcanic minerals of the Wartgesberg eruption. The Wartgesberg Tephra (WBT) has often three distinct subtephra. We have not yet distinguished if these layer are from contemporaneous  smaller eruptions near to the Wartgesberg, or if they are all derived from the Wartgesberg complex. The same petrographic composition as the WBT is prominent in a tephra at 30,300 yr b2k, which cannot be related to a specific site of eruption.

LST ash layer in various maar sediment cores

A sanidine rich tephra at 40,370 yr b2k fits nicely the complex mineral composition of the Dreiser Weiher, where sanidines and olivine occur together in the same tuff layers around the site. The eruption mixed apparently olivines from the upper mantle with sandidines, formed in the lower crust, and bedrock fragments  from the upper crust. The tephra is thus visibly zonated with grey and dark layers.

A tephra with an age of 47,340 yr b2k is well comparable to the tuff around the Meerfelder Maar, but the mineral fingerprint is not very specific. It is however known from 14C analysis (Schaber & Sirocko, 2005) that this eruption has occurred  at an uncalibrated age of 43,000 14C BP, which fits quite well with the new ELSA-20 age of  47,340 yr b2k, and we thus assign this prominent marker tephra to the Meerfelder Maar.

The last marker tephra of the ELSA-20 tephra stack is the eruption from Auel, which we analyse at the depth of cores AU2, AU3, AU4, and which we find with the same petrographic composition in the nearby Dehner maar. Its age presents the base of the ELSA-20 stratigraphy (Sirocko et al., 2021) at 59,130 yr b2k, exactly during the transition from the MIS 4 stage to the GI17 interstadial.

The most distinct petrographic fingerprint for all tephra reveals 5% of leucite, which we only observe once during the entire last 130,000 years. Förster & Sirocko (2016) called it accordingly Leucite tephra. Leucites have been reported up to now only from the tuffwall around the Schalkenmehrener Maar, and the idiomorphic grains from the tuffwall and lake sediment are indeed identical. We attribute the Leucite Tephra accordingly to the Schalkenmehrener Maar Tephra and call it now SMT. This dates the eruption time of the Dauner Maar Group (Schalkenmehrener Maar, Gemündener Maar, Weinfelder Maar) to have occurred at around 65,000 yr b2k. This time interval is documented at the base of the Dehner maar cores and the HL cores from Hoher List (Sirocko et al., 2005). Annual resolution Corg records and pollen are used at the moment to extent the ELSA Tephra Stack – and the entire ELSA stratigraphy –  back to 130,000 years.

DMT in cores and outcrop
Geochemical Fingerprint
Core Correlation with ELSA-Tephra-Stack