The precise date of the Laacher See eruption (LSE), central Europe’s largest Late Pleistocene volcanic event that occurred around 13,000 years ago, is still unknown. Here, we outline the potential of combined high-resolution dendrochronological, wood anatomical and radiocarbon (14C) measurements, to refine the age of this major Plinian eruption. Based on excavated, subfossil trees that were killed during the explosive LSE and buried under its pyroclastic deposits, we describe how a firm date of the eruption might be achieved, and how the resulting temporal precision would further advance our understanding of the environmental and societal impacts of this event. Moreover, we discuss the relevance of an accurate LSE date for improving the synchronization of European terrestrial and lacustrine Late Glacial to Holocene archives.

The Rhine River (Germany) - the largest river in Western Europe - was dammed by pyroclastic material multiple times during the major Plinian Laacher See Eruption (12.900 BP). Dams formed both upstream and downstream of the broad tectonic Lower Neuwied Basin (LNB) which interrupts the narrow Rhine canyon. Here we document upstream damming of the Rhine River at the entrance to the LNB close to the present city of Koblenz due to overloading with tephra fall into the Rhine and its major tributaries, the Moselle and the Lahn. The dam was formed repeatedly during rapid pumice tephra fall events and became breached during breaks in eruptive activity, causing extensive, high-energy flooding throughout the entire basin. The ephemeral Koblenz dams differed significantly from "normal" volcanically-induced darns by being composed principally of washed-together pumice lasts and some driftwood. The porous nature of pumice and its ability to absorb water were crucial factors. Thus, a large volume percentage of the tephra that had fallen into the Rhine floated submerged within the upper part of the water column or swam at the surface. Moreover, the absorption of the river water by the pumice clasts increased the sediment:water ratio of the two-phase flow considerably. We here present a model of dam formation resembling the formation of ice jams. We visualize the Koblenz dams to have been elongate, partly floating and partly grounded, permeable plugs several kilometers long and rising no higher than the flood plain. Damming was most plausibly initiated in the LNB within the area of maximum tephra loading and propagated upstream in a chain reaction similar to the formation of traffic jams. A major dam was finally accumulated at the bottleneck entrance to the LNB, a site combining several favorable conditions: the upstream multi-channel Rhine was confined to a single channel, change of flow direction by 125 degrees, extremely low gradient (0.19%.) starting already 24 km upstream of the bottleneck, constant decrease of flow velocity over several kilometers towards the bottleneck and the Moselle River - largest tributary of the Rhine within the LNB and an important conveyor of additional tephra masses - entered the Rhine only 700 m upstream of the bottleneck. We assume that the Koblenz dams could only have formed and been stabilized by an extremely long "foot region" that extended many kilometers downstream and that was possibly connected to one or several low-rise secondary jams/dams. The backwater of Lake Brohl that was dammed by pyrodastic flows 7 km downstream of the LNB about halfway through the eruption extended upstream into the LNB during the second Plinian stage of the Laacher See Eruption and was probably a major factor contributing to the formation and large size of Koblenz Dam No.4. The Koblenz dams were probably not completely sealed most of the time. This way, the major pre-eruptive Rhine channel received some water. An equilibrium condition was established that allowed the dams to remain stable as long as tephra fell into the Rhine relatively continuously. (C) 2020 Elsevier B.V. All rights reserved.

The Rhine - the largest river in Western Europe - was dammed during the Plinian Laacher See Eruption (LSE; 12,900 BP). Damming during the climactic Plinian episode of LSE occurred both upstream and downstream of the broad tectonic Lower Neuwied Basin (LNB) that interrupts the narrow Rhine canyon. We here document details of the upstream damming at the bottleneck entrance to the LNB near the present city of Koblenz. Our reconstruction is based on a high-resolution analysis and correlation of the complex intercalation of primary fallout tephra relics with fluvially reworked Laacher See Tephra in the LNB. Tephra units representing complete eruptive cycles repeatedly fell on drained ground in between one minor and 4 major flooding events - even at the base of side channels that had been active prior to the LSE and that had been flooded by a preceding flooding event. This demonstrates that flooding occurred generally during breaks and not during fallout events. The repeated formation and breach of a dam at the upstream entrance of the LNB (Koblenz Dam) consisting of fallout components and driftwood washed together convincingly explains the multiple repetition of the drainage of the channels in the LNB followed by large-magnitude flooding in rapid succession. The strongly pulsating nature of the LSE reflected in multiple interruptions of eruptive activity fundamentally controlled the damming and flooding dynamics. The Rhine became completely blocked during distinct fallout phases due to overloading with pumice that had fallen into the river and its major tributaries. The temporary dam collapsed during eruptive breaks. This is the first recorded example of tephra fallout damming a major watercourse. The extremely low gradient of the Rhine River allowed the repeated accumulation of large volumes of water in a long, multi-phase dammed-up lake (Lake Koblenz) that extended along the upstream course of the river for up to c. 30 km despite the low height (<10 m) of the dams. Each breach of Koblenz Dam caused extensive and wide-spread erosion and reworking of freshly deposited tephra throughout the entire LNB up to 7 km perpendicular to the major axial Rhine channel. The floods deposited striking, large-scale upper flow regime structures interpreted as in-phase wave draping, antidunes and chute-and-pool structures consisting largely of gravel-sized tephra components. Primary tephra sheets - several meters thick - became detached by undercurrents above impermeable boundaries and floated potentially along the full length of both active and abandoned channels. Large tephra bodies with the dimensions of a garage were lifted off by the flood waves and transported downstream for at least tens of meters. Damming of a major river while a large Plinian eruption is in full progress represents an extraordinary challenge for hazard mitigation. This is especially pertinent for an area close to, and downwind from, the vent and therefore simultaneously affected by massive Plinian fall such as in the LNB. (C) 2019 Elsevier B.V. All rights reserved.

The precise date of the Laacher See eruption (LSE), central Europe’s largest Late Pleistocene volcanic event that occurred around 12,900 years ago, is still unknown. Here, we outline the potential of combined high-resolution dendrochronological, wood anatomical and radiocarbon (14C) measurements, to refine the age of this major Plinian eruption. Based on excavated trees that were killed during the explosive LSE and buried under its pyroclastic deposits, we describe how a firm date of the eruption might be achieved, and how the resulting temporal precision would further advance our understanding of the environmental and societal impacts of this event. Moreover, we discuss the relevance of an accurate LSE date for improving the synchronization of European terrestrial and lacustrine Late Glacial to Holocene archives, and outline how the proposed, interdisciplinary dating approach can be applied to other large, yet undated, volcanic eruptions.

Laacher See Volcano (LSV), 40km south of Bonn, explosively erupted ca. 6.3km3 of chemically strongly zoned phonolite magma probably during spring, 12,900yearsago, the resulting eruption column having reached at least 20km in height. The bulk of the Laacher See Tephra (LST) was deposited east of the volcano within the Neuwied Basin. LST ashes form the most important stratigraphic marker in Lateglacial deposits over much of Central Europe. A minimum of 2Mt total S was calculated to have been released during the eruption by comparing pre- and posteruptive volatile concentrations in glass inclusions and pumice matrix glasses. Because a separate S-bearing vapor phase was probably present in the magma prior to eruption as indicated by the high melt-H2O contents close to saturation level, the actual amount of S released during the eruption, could have significantly exceeded 2Mt. The sulfuric acid aerosol layer resulting from the massive stratospheric S-input probably resided in the stratosphere for years and most likely had a significant impact on climate and thus the environment. Increased precipitation in central Europe and/or impairment of the vegetation cover for several years is suggested by several proxies, especially increased sediment supply into lakes. Proximal to the eruptive center, major environmental impacts include an initial blast that felled trees up to 4km away from vent. Fast deposition of huge tephra volumes led to the complete disruption of Rhine River within the lower Neuwied Basin and the damming up of a lake ca. 140km2 in areal extent. Sudden collapse of the temporary tephra dam caused a catastrophic flood wave downstream whose deposits are recognized as far north as Bonn. Reworking of unconsolidated tephra deposits over several 100km2 was widespread and extensive, generating abundant lahars and flood plain deposits. A braided-river system, established in Neuwied Basin, probably persisted up to several years. Recent reports for a second eruption of LSV are not confirmed.