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Tephra deposits and especially Laacher See tephra (LST) deposits resulting from the Laacher See eruption (12.9 ka) are an important stratigraphic marker for the Allerød period in central Europe (van den Bogaard and Schmincke, 1995). Within the central German low mountain range (Rhenish Massif and eastern areas) the LST was found within soils (initial deposits, sheltered slope positions) and valleys (relocated deposits) (Bos and Urz, 2003; Hahn and Opp, 2005). The Niederweimar gravel quarry, located on the lower terrace in the middle reach of the Lahn River valley south of Marburg (Hesse, Germany), is known for its high-resolution stratigraphy of Quaternary gravel deposits and late glacial, as well as Holocene, floodplain fines (Lomax et al., 2018). This particular stratigraphy is mainly achieved by the up to 2 m thick LST deposits, which consist of pure LST beds and a multitude of fine LST bands (partly interbedded with black sands or interrupted by clay bands). The origin of the LST in the floodplain is attributed to an extensive deposition (aeolian, directly in the floodplain), as well as later fragmentation of the tephra deposits by surface erosion and renewed deposition of LST from the catchment area through changing river systems (Bos and Urz, 2003; Lomax et al., 2018). The surroundings of the gravel quarry are also rich in archaeological finds reaching more or less continuously from the Mesolithic (11.7 to 7.5 ka) to the Middle Ages (Bos and Urz, 2003; Lomax et al., 2018). Further well-summarized information about the situation within the Niederweimar gravel quarry can be found in Lomax et al. (2018) or on the website of the archaeological survey of Hesse (https://lfd.hessen.de/, last access: 21 March 2021).

The evidence of LST in the Lahn valley, as in other valley sediments, is often limited to gravel pits (other larger excavations). These pits and their profiles offer very good insights (e.g. detailed lithostratigraphic description of profiles), but they are always limited to a comparatively small spatial section of the entire floodplain (gravel pit area). Therefore, the objective of the presented study is to provide a spatial survey of LST deposits in the middle Lahn valley, covering the entire floodplain cross section. The following two questions form the focus of the spatial survey. (1) How is the lateral and vertical extension of the LST deposits within the Lahn valley floodplain? (2) Does the spatial distribution provide overarching information about the deposition dynamics of the LST? For this purpose, a transect-based survey with qualitative analysis of LST grains based on density separation and visual identification (stereomicroscope) was applied.