Older than expected: fluvial aggradation of the Rhine's main terrace at Kärlich dated around 1.5Ma by electron spin resonance

The Mülheim–Kärlich clay pit (Kärlich hereinafter) is located along the Middle Rhine Valley (MRV), about 10 km north-westward of the city of Koblenz (Fig. 1a). Owing to its > 30 m thick Quaternary sequence (Fig. 1b), it perhaps represents the most significant sedimentary profile along this entire valley reach. The sequence provides key information on long-term landscape evolution in the MRV, including successive fluvial aggradation phases related to the so-called main terraces of the Rhine and the Moselle (e.g. Boenigk and Frechen, 2006, 1998), and is also relevant for its archaeological and palaeoenvironmental record (e.g. Gaudzinski et al., 1996). Despite this importance, the quality of chronological constraints on the whole Quaternary sequence is far from being uniform (Fig. 1b). dating of three tephra layers interspersed in loess and loess-like sediments yields ages ranging from ∼ 0.47 to ∼ 0.36 Ma (Van Den Bogaard et al., 1989; Gallant et al., 2014; Fig. 1b) for the upper part of the Middle Pleistocene sequence. Whilst intermediate units can be tentatively correlated to Marine Isotope Stages (MISs) 16–12 based on biostratigraphy and on the composition of volcanic mineral assemblages (Boenigk and Frechen, 1998), there is no numerical age control on the fluvial sequence at the base (Fig. 1b). It is merely constrained by questionable palaeomagnetic data suggesting a minimum age of 0.77 Ma (all geomagnetic boundaries are from Channell et al., 2020), corresponding to the Brunhes–Matuyama boundary (e.g. Boenigk and Frechen, 1998). Altogether, the lack of a reliable chronological framework for the fluvial units is highly problematic since Rhine deposits at Kärlich were often used as a reference site for a key terrace level in the Quaternary evolution of the river, the so-called main terraces (Boenigk and Frechen, 2006). This study thus aims to fill this gap by applying electron spin resonance (ESR) dating of quartz grains from two sediment samples of the Rhine's main terrace. This new age control further refines the whole Kärlich chronostratigraphy and numerically constrains, for the first time, the aggradation time of key terrace deposits along the MRV.