Middle Pleistocene chronology of the sediment sequence from Rodderberg, Germany, Numerical dating versus wiggle matching: A reply

Abstract

Middle to Late Pleistocene glacial–interglacial cycles appear to closely follow Milankovich cyclicity. This cyclicity has been observed to exert a discernible influence on both marine and terrestrial environments (EPICA community members, 2004; Lisiecki and Raymo, 2005). While the marine realm provides quasi-continuous sediment records back into the Tertiary, terrestrial environmental archives are more complex, often fragmentary, and commonly provide evidence of one interglacial only (Hughes et al., 2020). Sequences comprising multiple interglacials in superposition are uncommon, with notable examples including crater lake (de Beaulieu et al., 2001; Rohrmüller et al., 2017; Stebich et al., 2020) and tectonic lake records (Donders et al., 2021). Moreover, the majority of Central European records was analysed with a purely palaeobotanical (pollen) approach and their chronologies are typically based on wiggle matching (cyclostratigraphy), employing the global marine stable isotope stack (LR04) as a reference (Lisiecki and Raymo, 2005). Despite the prevailing consensus that pollen records offer primary regional, rather than local, signals, there is the possibility that they may be incomplete or influenced by site-specific conditions. This potential limitation renders interregional correlation a challenging endeavour. The presence of regional variations in vegetation patterns, in conjunction with the absence of independent dating methodologies, further complicates stratigraphic classifications. These factors give rise to debates and controversial discussions surrounding the nature of Middle Pleistocene environmental variability.

This assertion is particularly pertinent in the context of the discourse surrounding the palynologically defined Holsteinian interglacial in Central Europe. The initial correlation of this interglacial was with marine isotope stage (MIS) 7 (Caspers et al., 1995). However, subsequent studies moved it further back in time, to MIS 9 (Geyh and Müller, 2005; Litt et al., 2007), and finally to MIS 11 (Nitychoruk et al., 2006; Koutsodendris et al., 2013; Lauer et al., 2020; Fernández Arias et al., 2023; Schläfli et al., 2023). However, the temporal position of glacial advances in Central Europe and especially of intervening interglacials (cf., Van Beirendonck and Verbruggen, 2025a) remains a scientific controversy. Furthermore, direct numerical dating of the Holsteinian is yet limited to a very few case studies. Thus, ‘the lesson being that simple, one-to-one, uncritical correlations with terrestrial, and in particular with the marine isotope sequences, hold many potentially serious pitfalls for the unwary’. This is even more complicated because ‘the fact that 100 ka glacial–interglacial cycles produced glaciations of very different magnitudes in different places around the globe poses problems when relying on a global indicator of glacial change, as is often the case when using the marine isotope record’ (both quotations are from Hughes et al., 2020, p. 178). Similar phenomena are observed for interglacial conditions, albeit to a potentially greater extent. As most terrestrial records are from a stratigraphic context lacking superposition of warm stages from the Holsteinian to the well-datable Eemian, stratigraphic classifications overall remain uncertain (Stebich et al., 2020).

In light of this framing, it was a fundamental aim of our project to establish an independent numerical time control for the 72.8 m-long sediment record from Rodderberg (ROD11), with the objective of circumventing the challenges associated with wiggle matching. In summary of Middle Pleistocene palaeoenvironmental studies, Van Beirendonck and Verbruggen (2025a) propose a correlation of the oldest interglacial of ROD11 with MIS 7e, based exclusively on the pollen record, which they associate to MIS 7e and not to MIS 11, which is in contrast to Schläfli et al. (2023) and Zolitschka et al. (2024). Following the availability of a suite of proxy data for ROD11 on the PANGAEA database, Van Beirendonck and Verbruggen (2025b, this issue) see support for their interpretation that all organic-rich (interglacial) sediment sections recorded by ROD11 should be related to MIS 7, while Schläfli et al. (2023) argued against this option. The following section will provide a justification for the chronological framework established by Zolitschka et al. (2024), which is based on independent geochronological data.

Luminescence dating is a numerical method that allows for the direct determination of the time of sediment deposition. This method has been established as a common technique for the past 25 years. However, its application has been primarily constrained to sediments with a maximum age of 150 ka, as the luminescence signal exhibits saturation effects at a certain age. This phenomenon is associated with the specific properties of the natural dosimeter employed (quartz or feldspar), which varies according to specific mineralogical properties of the sample as well as with the dose rate, that is, the level of radioactivity at which the luminescence signal is generated. In this context, it is important to note that luminescence dating comprises a series of different techniques that are semi-independent from each other. Quartz is typically the mineral of choice due to its rapid reset by daylight during sediment transport with an apparent stability over time. However, it has been demonstrated that quartz typically exhibits a relatively low characteristic saturation dose, thus constraining its utilisation to approximately 150 ka (cf., Murray et al., 2021). Consequently, it may lead to underestimation of the true age of deposition when approaching the Eemian (Lowick et al., 2010). In contrast, the luminescence signal from feldspar has a higher saturation dose and thus a higher dating limit. However, this signal is less sensitive to daylight exposure (i.e., it is more difficult to reset) and associated with stability issues. Recent developments in dating protocols can overcome the problem of signal stability by using subsequent stimulation at progressively higher temperatures (Buylaert et al., 2009; Li and Li, 2011).

The Rodderberg record comprises four organic-rich sections in superposition that have been interpreted as reflecting interglacial conditions. We applied different luminescence dating approaches to 40 samples along the entire profile of ROD11 using fine-grained quartz (4–11 µm, 19 dates) and polymineral fractions (67 dates). In the latter case, the signal is dominated by feldspar emissions. During the initial study carried out at the Leibniz-Institute of Applied Geophysics (LIAG), Zhang et al. (2024) used a multi-technique approach in combination with extensive experimental investigations characterising the physical luminescence properties of the material investigated. The applied techniques comprise quartz optically stimulated luminescence (OSL; Huntley et al., 1985), post-IR infrared stimulated luminescence measured at 225°C (pIRIR₂₂₅; Buylaert et al., 2009), pulsed pIRIR₁₅₀ (Schmidt et al., 2011), and pIRIR₂₅₀ derived from the multi-elevated temperature (MET) pIRIR protocol after Li and Li (2011). Ages determined for quartz and polymineral fractions show excellent agreement back to 45 ka. Beyond this age, quartz dates start underestimating without subsequent increase beyond ca. 120 ka. In contrast, the ages determined for the polymineral fraction exhibit an increase to approximately 250 ka, followed by substantial scatter and no further increase in ages. This lack of increasing age with depth, and the fact that equivalent dose values approach 2D₀ (a value indicative of reaching saturation), is interpreted as approaching the upper dating limit (saturation) with the applied dating procedures for ROD11 (Zhang et al., 2024). The oldest minimum age of approximately 250 ka at 37.5 m marks the onset of organic-rich (interglacial) sediments after a long period with inorganic sediments. This is consistent with the onset of MIS 7e at 243 ka, as identified for deep-sea records (Lisiecki and Raymo, 2005). Hence, the remaining 35 m of sediment below this age must be older than 250 ka.

Concurrently with the study at LIAG, the laboratory at the University of Freiburg (UFR) has tested a slightly different version of the MET pIRIR approach and applied it to fluvial and aeolian sediments from the River Rhine region (Schwahn et al., 2023; Gegg et al., 2024, 2025). Given the temporal extent of these studies reaching back to approximately 500 ka, it was considered appropriate to apply this approach to core ROD11 (Zolitschka et al., 2024). For samples from the upper part of the sequence (LUM4607), the MET pIRIR ages determined by UFR (35 ± 2 ka) and LIAG (34 ± 2 ka) are in excellent agreement within uncertainty, thus demonstrating the absence of any systematic interlaboratory problems, for example, calibration of irradiation sources. Further downcore, the ages determined by UFR are about 20% higher compared to those determined by LIAG. This offset of ages is not in conflict with the findings of Zhang et al. (2024), who interpreted their results as minimum estimates, that is, as old as the reported age or older. Zolitschka et al. (2024) concluded that this offset reflects luminescence physics, that is, the MET pIRIR signal measured by UFR might be more stable. It is important to note that UFR used a machine of a different manufacturer than LIAG, with differences in heat flux (Schmidt et al., 2018) and a narrower detection window. The latter may contribute to a better isolation of the presumably most stable feldspar signal emission at 410 nm (cf., Krbetschek et al., 1997). As there appears no indication of any partial bleaching of luminescence ages for the entire sediment sequence of ROD11, and it is not expected to have any significant effect on lacustrine sediments for this time range (e.g., Juschus et al., 2007; Lukas et al., 2012; Roberts et al., 2018), the age of approximately 330 ka for the base of the record is considered a reliable minimum estimate.

The aforementioned points address the primary question posed by Van Beirendonck and Verbruggen (2025b, this issue), namely the rationale behind the exclusion of MIS 7e from consideration for the oldest interglacial of ROD11. This exclusion is based on numerical dating, which indicates that this interglacial should in fact be designated as older, that is, at least as MIS 9 (Zolitschka et al., 2024). In addition, the sedimentation rate of 0.23 mm a⁻¹ obtained for MIS 7 in this study is in close agreement with the 0.30 mm a⁻¹ recorded for MIS 7 at Neualbenreuth (Rohrmüller et al., 2017). Given that the crater basin at Neualbenreuth is half the size of that at Rodderberg, and its catchment area is at least three times larger (data from Fig. 2 of Rohrmüller et al., 2017), it can be deduced that higher sedimentation rates are to be expected. Moreover, considering the oldest interglacial of ROD11, as a component (MIS 7e) of MIS 7, would result in an almost quadrupled sedimentation rate of 0.89 mm a⁻¹ for MIS 7, rounded by Van Beirendonck and Verbruggen (2025b, this issue) to ~1 mm a⁻¹. This is almost threefold the value recorded for Neualbenreuth and would require further elucidation. The correlation of the upper three organic-rich sediment sections of ROD11 with the three peaks of MIS 7e-a of LR04 (Fig. 12 in Zolitschka et al., 2024) is illustrated with Fig. 1.

Our chronological interpretation is furthermore supported by tephrochronology (Zolitschka et al., 2024). A tephra layer (ROD11-T1) intercalating the sediments was deposited at the onset of the oldest interglacial and is related to the Rieden Tephra (RIT). In contrast to the interpretation by Van Beirendonck and Verbruggen (2025b, this issue), this tephra was not produced by the Rodderberg eruption. Instead, it exhibits a completely different geochemistry as illustrated by Figs. 6 and 8 of Zolitschka et al. (2024). Unfortunately, it has proven unfeasible to directly date single crystals of this tephra using the ⁴⁰Ar/³⁹Ar method and to determine the geochemistry of individual glass shards due to the fine-grained character of this volcanic ash (mean grainsize: 36 µm). However, this would be necessary for an unequivocal correlation to the RIT. Nevertheless, based on bulk geochemistry and assuming a certain amount of chemical alteration (cf. Fig. 8 of Zolitschka et al., 2024), there appears to be sufficient geochemical evidence to support the conclusion that ROD11-T1 corresponds to the RIT, one of three major explosive (Plinian) eruptions that occurred in the vicinity of the present-day Laacher See in the East Eifel Volcanic Field (EEVF; Fig. 2). Furthermore, the stratigraphic position of this tephra at the onset of an interglacial provides strong evidence for a correlation with MIS 11 (Fig. 2). The mean of ten published ⁴⁰Ar/³⁹Ar dates (Fuhrmann and Lippolt, 1986; Lippolt et al., 1986; van den Bogaard et al., 1987; van den Bogaard and Schmincke, 1990; van den Bogaard, 1995a) provides an age of 417 ± 16 ka for the RIT (Table S4 of Zolitschka et al. (2024). Another option to correlate the lower tephra from Rodderberg would be the Hüttenberg Tephra (HBT), ⁴⁰Ar/³⁹Ar dated to 215 ± 4 ka (van den Bogaard et al., 1989) and Table S4 of Zolitschka et al. (2024). However, it should be noted that this potential correlation refers to MIS 7c, which is in the middle of an interglacial and not at its beginning. It appears not appropriate to speculate about the source of ROD11-T1 as a yet unknown volcanic eruption, since the rare phonolithic volcanic eruptions of the EEVF are highly explosive; only three such eruptions have been documented for the past 500 ka (Schmincke, 2007). Plinian eruptions are known to produce substantial quantities of tephra that are both widespread and pumice-rich and thus readily apparent in the field.

Another chronological constraint is the age of the eruption of the Rodderberg volcanic complex, which is associated with a thermoluminescence (TL) age of 321 ± 37 ka (Paulick et al., 2009). The fact that tephra assigned to the Rodderberg intercalates loess in nearby outcrops (Remy, 1960; Bartels and Hard, 1973) links the eruption to glacial conditions. The interpretation of Paulick et al. (2009), as reproduced by Van Beirendonck and Verbruggen (2025b, this issue), links the Rodderberg eruption to the glacial stage of MIS 8 (300–243 ka). However, Paulick et al. (2009) clearly state that their TL ages have to be regarded as minimum estimates, because mid-term and long-term anomalous fading might not have been detected (Zöller and Blanchard, 2009). Hence, the minimum age of the eruption is much more likely MIS 10.

Thus, numerical dating has led to the rejection of the approach of Van Beirendonck and Verbruggen (2025, this issue), who suggested that all organic-rich (interglacial) sediments of ROD11 belong to MIS 7. Despite the compelling results of their wiggle-matching with LR04, our numerical age constraints link the younger interglacial deposits to MIS 7 and the older to MIS 11. At the same time, this approach dates the eruption of the Rodderberg volcanic complex to MIS 12, that is, an age older than 424 ka. More specifically, using mean sedimentation rates based on occasionally occurring annually laminated sediment layers during MIS 12, the age of the Rodderberg eruption was estimated to a minimum age of 439 ka (Zolitschka et al., 2024). It is evident that this correlation of organic sections to marine isotope stages unveils one of the unanticipated revelations presented by the ROD11 record: the absence of any discernible organic-rich sediments indicative of the intervening MIS 9. Consequently, a fundamental assumption initially postulated at the project's inception appears to have been erroneous: the sediment record of the Rodderberg crater probably did not archive a continuous sediment sequence.

In a closed basin, such as Rodderberg, the only potential cause to explain a discontinuous sediment record is erosion by wind. For aeolian activity to be considered as effective, however, there are several preconditions that must be fulfilled. These include the presence of strong winds, a lack of forest cover, and dryness, that is, a desiccated lake in the centre of the Rodderberg crater. It is evident that such environmental conditions are congruent with glacial climate conditions. Therefore, we propose that the interglacial sediments of MIS 9 were eroded under the glacial climate of MIS 8 characterised by low temperatures and dry conditions. It is very likely that erosion also affected sediments related to MIS 8 and MIS 10. During MIS 8 or the Early Saalian ice advance, the Fennoscandian Ice Shield was located ca. 60 km north of Rodderberg on the eastern banks of the River Rhine (Roskosch et al., 2015; Lang et al., 2018), resulting in the occurrence of strong katabatic winds from north-easterly directions. These prevailing cold and dry climatic conditions, in conjunction with the gradual accumulation of sediments over time, ultimately caused the filling of the crater basin. Consequently, the respective sediment surface became elevated above the groundwater level leading to desiccation of palaeolake Rodderberg and to the onset of substantial wind erosion.

Another question posed by Van Beirendonck and Verbruggen (2025b, this issue) pertains to the sediment composition of ROD11 and why the grainsize increases during interglacial periods instead of decreasing. This is yet another surprising aspect of the ROD11 record. It is consensus that during interglacial periods with elevated temperatures, more precipitation and a resulting forest cover, the influx of minerogenic matter into lakes should have been significantly diminished but not increased. Usually, this results in a substantial increase of organic deposition. However, the record from Rodderberg documents a slightly different pattern (Fig. 1). In order to facilitate a more profound comprehension of these deposits, we have to consider two different lacustrine depositional regimes: (1) minerogenic sediments characterised by a high abundance of carbonates and a mean grainsize of 27 µm, alternating with (2) organic sediments, characterised by a paucity of carbonates and a coarser mean grainsize of 90 µm. The minerogenic sediments of (1) represent aeolian deposits (loess) in combination with redeposition of loess from the inner crater walls (loess derivates) during cold and dry glacial periods. This aeolian material originates from the Lower Rhine Embayment, while the organic sediments of (2) consist of autochthonous and allochthonous organic matter and of minerogenic components from the small but steep catchment area of Rodderberg with a basaltic composition. This particular type of deposition has been observed to occur during warm and wet interglacial periods. To answer the question why coarser-grained minerogenic components were deposited at Rodderberg during interglacials, we hypothesise that this is related to the large amount of available fine-grained tephra particles on the inner crater walls. Very likely, surface runoff was the agent under a higher interglacial precipitation regime responsible for transporting these grains down the steep slopes and into the crater lake. This phenomenon did not occur during glacial periods, nor during the peak interglacial of MIS 11c, as substantial quantities of loess were deposited and replaced volcanic material as the dominant minerogenic sediment component, and a dense forest cover (arboreal pollen >90%) suppressed erosion, respectively.

Finally, Van Beirendonck and Verbruggen (2025b, this issue) pose the question why certain proxy parameters from ROD11 appear to effectively capture MIS 7 oscillations, thereby lending support to their interpretation of the chronology. Should a distinctly periodic Milankovich cyclicity regulate signals such as the marine LR04 and also terrestrial palaeoclimate proxies such as those of ROD11, it has to be expected that a range of solutions for covariation between these signals can be observed. In this instance, it is imperative to employ independent numerical dating techniques to substantiate any hypothesis.

In conclusion, the proposition advanced by Van Beirendonck and Verbruggen (2025, this issue) that all four organic-rich sections of the 72.8 m-long Rodderberg sediment record should be amalgamated into MIS 7 is rejected. Instead, Zolitschka et al. (2024) correlate these interglacial sediment sections with MIS 11 and MIS 7 with a lacking and likely eroded MIS 9. This interpretation is based on luminescence dating and tephrochronology. Nevertheless, it is acknowledged that the dating of Middle Pleistocene deposits remains challenging. In order to provide meaningful regional and continental correlations of environmental and climatic proxy parameters, further efforts are required to extend the limits of luminescence dating and to enhance the performance of ⁴⁰Ar/³⁹Ar dating techniques.