The age of the Lower Paleolithic occupation at Schöningen

Journal of Human Evolution xxx (2015) 1e11

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€ ningen The age of the Lower Palaeolithic occupation at Scho Daniel Richter a, b, *, Matthias Krbetschek c, d, y a

Department of Human Evolution, Max Planck Institute for Evolutionary Anthropology, Deutscher Platz 6, 04103 Leipzig, Germany €tsstr. 30, 95447 Bayreuth, Germany LS Geomorphologie, University of Bayreuth, Universita c € €r, Institut für Angewandte Physik, TU Bergakademie Freiberg, Sachsische Akademie der Wissenschaften, Forschungsstelle Geochronologie Quarta Leipziger Straße 23, 09596 Freiberg, Germany d Freiberg Instruments GmbH, Delfterstr. 6, 09599 Freiberg, Germany b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 June 2013 Accepted 8 June 2015 Available online xxx

Thermoluminescence (TL) data are presented for eight samples of heated flint collected at the archae€ningen 13/I-1 (Cycle I), for which a Holsteinian age is suggested by palynology of ological site of Scho stratigraphically similar positions within a cyclic sedimentological model for the Quaternary sequence of € ningen. Although the fire responsible for the zeroing of the TL-signal cannot be unequivocally Scho attributed to human activities, any time difference between a natural fire and the human occupation is negligible for a site of this antiquity. The weighted mean age of 321 ± 16 ka places the last heating of the flints nominally in the age range of Marine Isotope Stages (MIS) 10 to 8. By inference this data would suggest an attribution of the Holsteinian to MIS 9 and may also serve as a maximum age estimate for the € ningen 13/II-4 (Cycle II). Considering the chronometric data available and following an spear site of Scho € ningen can be considered as alternative sedimentological model the age of these two sites at Scho belonging to the same climatic cycle. This suggests an attribution to MIS 9, and by inference provides an age estimate of 337e300 ka for the oldest spears in human history. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Thermoluminescence TL Chronometric dating Flint Spear horizon

1. Introduction Chronostratigraphy, for which stratigraphic information is combined with age estimates (Richter, 2015), provides the ultimate framework for the interpretation of human evolution, human behaviour, and geological processes of the past. While relative age information is sufficient as a first approximation, chronostratigraphical information on a defined time scale and/or with fixed points in time is necessary to understand sequences of €ningen, with its relatively processes. The Quaternary site of Scho large number of sites and large scale excavations, is situated within a long sedimentary sequence, which reflects environmental changes and thus provides a unique opportunity to investigate hominin land use. The interpretation of climate and landscape change, and how these factors influence hominin behaviour, requires specific knowledge of the time depth involved in such processes. However, the data necessary for placing the numerous and detailed litho- and biostratigraphical descriptions

* Corresponding author. E-mail address: [email protected] (D. Richter). y Deceased October 15th, 2012.

on such a detailed time scale are largely unavailable for €ningen. Previous studies (Heijnis and Urban, 1995; Geyh and Scho Krbetschek, 2012; Sierralta et al., 2012; Urban and Sierralta, 2012) obtained a limited number of numerical ages, and therefore the chronostratigraphy is mostly based on models of sedimentation and biostratigraphical correlations (e.g., Urban, 2007a), which has led to controversy regarding the age of the famous €ningen 13/II-4 (e.g., Jo €ris and Baales, spears from the site of Scho 2003). Here we provide thermoluminescence (TL) data for the €ningen, stratigraphically lowermost human occupation at Scho which sets a maximum age for the level with the famous €ningen spears and provides an anchor point for the stratigScho € ningen. raphies in Scho € ningen and the archaeological 2. The stratigraphy of Scho sites €ningen is located about 100 km southeast of Hannover in Scho the northern foreland of the Harz Mountains and at the southeastern edge of the Triassic limestone ridge called the Elm (Thieme, 1997). The area belongs to the northern region of the 70 km long sub-herzynic basin between Helmstedt and Staßfurt.

http://dx.doi.org/10.1016/j.jhevol.2015.06.003 0047-2484/© 2015 Elsevier Ltd. All rights reserved.

€ ningen, Journal of Human Please cite this article in press as: Richter, D., Krbetschek, M., The age of the Lower Palaeolithic occupation at Scho Evolution (2015), http://dx.doi.org/10.1016/j.jhevol.2015.06.003

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D. Richter, M. Krbetschek / Journal of Human Evolution xxx (2015) 1e11

The preservation of such a long Middle Pleistocene to Holocene sedimentary sequence is rare because of the erosion caused by repeating ice advances at various times. Sediments deposited under warm climatological conditions alternate with evidence of glacia€nen) of the major advances during Elsterian and tions (Grundmora Saalian (Drenthe) glacial times. Since 1983, archaeological excavations have been conducted in €ningen open-air lignite mine almost continuthe area of the Scho ously, exposing archaeological sites dating from the Lower Palaeolithic to the Iron Age (Thieme and Maier, 1995; Serangeli et al., 2012). Preservation of organic material is often exceptional, with plant remains, wood, small and large fauna, molluscs, fish, amphibians, and insects (Thieme, 2007b).

differences in these types of records, correlations are not straightforward. The recent stacking of oxygen isotope data developed by Lisiecki and Raymo (2005) functions as a representative model of approximate age estimates for marine isotope stages (MIS) and provides ages of 424e374 ka for MIS 11, 337e300 ka for MIS 9, 243e191 ka for MIS 7, and 130 to ~116 ka for MIS 5e (Table 1). However, differences between the marine and the terrestrial record are known (e.g., the base of the last interglacial [Eemian] appears to be about 6 ka younger than the base of MIS 5; Shackleton et al., 2003; Sier et al., 2011), and therefore such marine frameworks can only serve as references.

€ningen and its 2.1. The Quaternary stratigraphy of Scho chronostratigraphical interpretation

€ningen (Table 1) are up to The Pleistocene sediments at Scho 60 m thick (Urban et al., 1991b; Elsner, 2003), with the majority belonging to an Elsterian glaciation age sequence of melt water and €ne) at glaci-lacustrine deposits, with subglacial till (Grundmora its base. This till occasionally occurs in places twice as distinct stratigraphical separate layers as Lower and Upper Elsterian €ne’ (Urban et al., 1988). The Elsterian is covered by ‘Grundmora interglacial sediments (Hartmann, 1988; Elsner, 2003), which in turn are unconformable overlain by glacial deposits attributed to €ne) and meltthe Saalian glacial with a subglacial till (Grundmora water deposits. This is covered by redeposited loess and soils (Mania, 2006), a soft interglacial (Eemian) travertine (Urban, 1995b), and a loess-soil sequence of early Weichselian glacial age (Thieme et al., 1987; Meyer et al., 1995), followed by interglacial Holocene organogenic sediments (Thieme et al., 1987). Such long stratigraphies are rare, and they provide the opportunity for comparison of terrestrial records with marine sequences, where they may shed light on, for example, the position of the

Most chronostratigraphical approaches are relative and therefore can be ambiguous when tie points (e.g., marker horizons) for correlations are ill-defined, absent, or poorly documented. Sedimentology combined with palynology and relative stratigraphy often correlate terrestrial sequences with marine or ice core records or other cyclic systems. However, it is often necessary to provide anchor points by chronometric dating methods. The controversial € ris and Baales, 2003) for the Scho €ningen age models (e.g., Jo sequence are primarily based on geological observations (Mania, 2007), as well as biostratigraphic results, which both provide only relative age frameworks (Table 1). Such terrestrial frameworks are often linked to isotopic sequences from marine records or ice cores, often implying that climate changes are responsible for observed changes in terrestrial records and the resultant terrestrial framework. However, given the

€ningen 2.2. The Quaternary sediments at Scho

Table 1 € ningen (modified after Lang and Winsemann [2012]). The star symbol Summarized and simplified view of the different chronostratigraphic models for the sequences at Scho €ningen 13/I-1 and the diamond for Scho €ningen 13/II-4 (spear horizon). Note the inferred attributions to MIS for the channel model represents the respective position of Scho employing the inherent fundamental assumption of climatic cycles after Mania (1995). The first column represents the general North German Quaternary sequence from Litt et al. (2007).

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Holsteinian interglacial in the isotope chronostratigraphies (MIS 9 or MIS 11), a topic which is hotly debated (e.g., Desprat et al., 2005; Geyh and Müller, 2006; Nitychoruk et al., 2006; Koutsodendris, 2011; Geyh and Krbetschek, 2012; Stephan et al., 2012). Notable €ningen and also differences in the pollen assemblages within Scho in comparisons with other stratigraphies do not appear to be a unique feature because differences in pollen sequences attributed to the Holsteinian are recorded elsewhere (e.g., De Beaulieu et al., 2001). The differing views on the attribution of pollen assemblages to the Holsteinian may derive from the interpretation and definition of pollen data, and hence the unequivocal attribution to a single pollen stage. Given the recent observations on variations within Holsteinian pollen sequences (Koutsodendris et al., 2010, 2011) and sediments it appears that the palynological record is complex and that changes in climate might have produced several similar pollen assemblages in the time range in question. Regional variation and/or differential preservation also complicate matters. However, isotope signals appear to provide a possible basis for the inclusion of several interglacial-like events in the time range of MIS 11 and 9. Chronometric dating of sequences, or, as is the case here, of single specific entities within sequences, helps to establish chronostratigraphic frameworks and answer questions about the age of specific sites. 2.3. The palynological biostratigraphic models € ningen A biostratigraphical model of the formation of the Scho sediments (Urban et al., 1988, 2011, 2015; Urban, 1995b, 2007a; Urban and Sierralta, 2012) is mainly based on palynological investigations providing a relative chronological sequence within a series of sedimentological cycles (Table 1). Liminic-telmatic sediments in adepression of Elsterian age were attributed to the late Holsteinian and early Saalian glaciation in the northern part of the €ningen mine, unconformable located above erosional interScho glacial Holsteinian gravels (Urban et al., 1991a). From the same area, a series of peat is unconformable overlain by Saalian moraines (Drenthe-Stadial) and attributed to a post-Holsteinian interglacial, € ningen Interglacial (Urban et al., 1991a; locally known as Scho Urban, 1995b, 2007a). Urban (2007a) describes parallels to the € mnitz or Wacken interglacials, while Litt et al. (2007) favours the Do € mnitz interglacial (Table 1). The Scho €ningen Interglacial is bioDo stratigraphically located above the Holsteinian interglacial and therefore might correlate with Oxygen Isotope Stage (OIS) 7 (Urban, 2007a). In the southern part of the mine an additional interglacial was identified and named Reinsdorf, being palynologically distinct €ningen Interfrom the Holsteinian Interglacial as well as the Scho glacial, and located stratigraphically lower from the latter (Urban et al., 1991b; Urban, 1995a,b, 1999, 2007a). While Urban (2007a) €ningen Interglacials as part of the regards the Reinsdorf and Scho Saalian complex sensu lato, Litt et al. (2007) interpret the Reinsdorf as part of the Holsteinian (Table 1), which corresponds to its lithostratigraphical position (Urban et al., 2011). The Eemian inter€ ningen by its later glacial is palynologically represented at Scho phases, and early Weichselian Interstadials are present as well (Urban, 1995a). The mammalian biostratigraphy (Van Kolfschoten, 2012, 2014) indirectly confirms the attribution of the glacial sediments to the Elsterian and Saalian. The mammals from the deposits lying directly above the Elsterian (Holsteinian according to palynology, cycle I or channel I in a sedimentological model described below) are not discriminating for either exclusive glacial or interglacial conditions. The following package of sediments (Reinsdorf/cycle II/channel II) contains species younger than Boxgrove, Miesenheim I, Mauer, or Mosbach, but which are older than Maas de re or Weimar-Ehringsdorf. A correlation with tricht-Belve

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British fauna attributed to MIS 9 is considered as more plausible, especially as the water voles do not match the British (MIS 11) Holsteinian (Van Kolfschoten, 2014). 2.4. The sedimentological chronostratigraphic models While the attributions of the glacial sediments to the Elsterian and Saalian glaciations appear to be unequivocal, several competing sedimentological models attempt to explain the depositional history of the observed sequences (Table 1). The lens shaped interglacial sediments were interpreted as fillings of kettle holes (Elsner, 1987), tectono-climatologically driven channels reflecting the glacial series (Mania, 1995, 2006), or as river delta/ lake deposits in a tunnel-valley (Lang and Winsemann, 2012; Lang et al., 2012, 2015). According to Mania (1995, 2006), six major channels formed due to fluvial erosion during late glacials and due to subrosion during the subsequent interglacial (Table 1). Formation occurred parallel to the Offleben salt dome, which caused a shifting of the channel positions due to its uprising and subsequently resulted in partial overlap of channels (Mania, 1995). The channels are €ningen IeVI from old to younger (from West to numbered Scho East) and correspond to the cycles in Urban (2007a,b). They unconformable overlie the Elsterian glacial deposits. The model is linked to the biostratigraphical interpretation of the sequence by invoking climatological (glacial) cycles as cause of channel formation (Mania, 1995). This provides a more or less fixed time frame within a hemispheric system but also requires a good match of observed erosion and cyclic patterns with this system, which are not always warranted. The interglacial fillings of channel I (Holsteinian), channel II (Reinsdorf Interglacial), and channel III € ningen Interglacial) consist of fine-grained organogenic (Scho sediments and peat (Mania, 1995, 2006; Urban, 1995a, 2007a). Channel IV is located above the Saalian glacial deposits, and its filling consists of redeposited loess with palaeosoils, which are interpreted as periglacial formations, occurring between Drenthe and Warthe interstadials (Urban, 2007a). The uncomformable overlying channel V is filled with weakly consolidated secondary carbonates, peat, and soils, which are placed in the Eemian Interglacial, and followed by 6 m loess deposits in which the Late Glacial/Holocene channel VI is cut in (Urban, 1995a, 2007a,b; Mania, 2006). Correlations with the sedimentary sequence of the Ascherslebener See (Mania, 1967), the travertine terraces of Bilzingsleben (Mania, 1999; Mania and Mania, 2005), and the site of Neumark-Nord (Mania, 1996, 2004; Mania and Mania, 2008) provide a climato-stratigraphical framework within the OIS system (Mania, 2006). A contrasting model (Table 1) explains the interglacial sedi€ningen as a river-delta system, where the mentation at Scho observed cyclic drying (Verlandungsfolgen) as well as the unconformable overlapping structures are related to lake level changes and a delta sedimentation at the crossroad of a lake and the mouth of a river (Lang and Winsemann, 2012, Lang et al., 2012, 2015). €ningen is located in According to this model, the succession at Scho the sediment trap of a tunnel valley, and the interglacial sediments consist of lateral and vertical stacks of lacustrine deltaic deposits. While the observed large scale changes in sea levels are linked to climate change (Lang and Winsemann, 2012) the causality of observed sedimentology to changes in climate is negated and rather related to the nature of depositional regimes of fluvial deltas and lake level changes (Lang and Winsemann, 2012). However, there are no indications as to why climate changes should invoke only major lake level changes. The palynological evidence, however, demonstrates that major changes took place at these particular times. The inherent variability of delta sedimentation

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D. Richter, M. Krbetschek / Journal of Human Evolution xxx (2015) 1e11

precludes any correlation with chronostratigraphy, except the evidenced glacial deposits of the Elsterian and Saalian glaciations. The model by Lang and Winsemann (2012) does not require climatological driven changes, and hence allows for short time spans between stratigraphical units. But it neither contradicts the palynological model. Both sedimentological models require considerable fluvial activity, for which there is currently little evidence, as the present day creek ‘Missaue’ is rather small and it is questionable whether it was ever large enough for such a degree of activity. 2.5. Chronostratigraphy and numerical ages The different views, interpretations, and models for the € ningen sequence lead to rather different age estimates Scho (Table 1). Neither model can be entirely refuted, and numerical dating is required for falsification of models or the attribution of sedimentological layers or units to marine isotope stages (MIS) or any other reference. At present, only a few numerical ages are available for the sequence. The majority are 230Th/U-disequilibrium dates on peat. While Ueseries ages in general have to be checked and corrected for detrital Th, U-series dates on peat must be viewed with caution because peat is an open system. The presence of a plateau age for several U-series age determinations along the profile of the centre of a thick layer of peat might indicate quasiclosed system behaviour and ages therefore considered as more reliable (Geyh, 2008). €ningen sequence was attempted The first dating of the Scho with alphaspectrometric 230Th/U-dating, which resulted in un€ningen Interglacial (channel/ corrected dates on peat from the Scho cycle III) of 180 and 227 ka (data from Sierralta et al., 2012), later assigned to 320 ka (Heijnis and Urban, 1995), and should be viewed with caution. In a more recent study with Thermal Ionization Mass Spectrometry (TIMS), open system behaviour was detected for most samples (Urban et al., 2011; Sierralta et al., 2012). No age results are therefore given for these samples, but the ages for the uppermost five samples from the lower peat at € ningen 13/II (Reinsdorf interglacial, cycle II, level 2c5.2a) were Scho considered to be reliable. A weighted mean (n ¼ 5 @ 2s) of corrected ages of 294 ± 5 ka (Urban et al., 2011) was considered as 300 ± 18 ka in Urban et al. (2011) and a weighted mean (n ¼ 4 @ 2s) of corrected ages of 290 ± 5 ka for apparently the same samples was given in Sierralta et al. (2012). Inconsistencies in these data sets require a re-analysis before final conclusions are drawn (M. Geyh, pers. comm. 2013), but it appears that the peat, €ningen 13/I-4, and by inference the spears from the overlying Scho are around 300 ka in age. There are few alternatives to date the sequence because of the lack of datable material, (e.g., neither tephra for 39Ar/40Ar dating, nor secondary carbonates for U-series), and it appears that only dosimetric dating methods (luminescence and Electron Spin Resonance) are suitable. A thermoluminescence (TL) estimate of the last heating of a single piece of flint from the archaeological site € ningen 13/I-1 (channel/cycle I, layer 1) gave an age of of Scho 470 ± 60 ka (Richter and Thieme, 2012). Such singular dating is not providing a precise and good age estimate (Richter, 2007). Furthermore, the reanalysis of this data revealed an analytical inconsistency in the calculated proportional alpha dose-rate being larger than the respective iteratively (Mercier, 1991) calculated alpha induced TL. This should not be the case because TL-signal growth with aeradiation is linear over the entire datable age range, in contrast to the signal from b or g radiation (Mercier, 1992). This unexplained contradiction in the analysis, which was not observed in the data presented here, has to lead to the rejection of this age estimate.

€ningen 13/I-1 2.6. The Palaeolithic site of Scho Palaeolithic remains occur throughout the Pleistocene stratig€ ningen, either in isolation or in association with raphy at Scho €ningen are faunal remains. All of the major Palaeolithic sites at Scho stratigraphically located between the Elsterian deposits and the Saalian till (Thieme, 2007a; Serangeli et al., 2012). Many of those provide excellent preservation of wood remains, which are sometimes modified (Thieme, 1999), including the famous spears from €ningen 13/II-4 (Thieme, 1997; where the first arabic number in Scho the site denomination refers to an area, the roman number to the channel/cycle, and the last arabic number to sediment layers within). € ningen is labelled as The oldest archaeological site at Scho €ningen 13/I-1 and is stratigraphically located above Elsterian Scho deposits. It is attributed to the earliest part of the Holsteinian complex because of its location in sediments of the channel I system (Thieme, 2007b). Two different strata were excavated, with the €ningen 13/I-2) containing faunal remains only upper one (Scho €ningen 13/I-1) (mainly bison) and only the lower one (Scho providing archaeological material, which was embedded in and on top of a gravel deposit. This sediment contains abundant pieces of flint, which were transported by glacial activities from the Baltic Sea, many of which are frost shattered. Separation of artefact from natural material is thus difficult (e.g., Pasda, 2012), especially as the € ningen 13/I-1 are not very diagnostic and flint artefacts from Scho consist mostly of small flakes and some notched flake tools, which are considered to be Lower Palaeolithic from a technological and typological perspective (Richter and Thieme, 2012). Some flints exhibit macroscopic signs of severe heating to high temperature like potlids, crazing, and craquelation. However, such macroscopic features could be attributed to weathering as well (e.g., Richter, 2007). Large faunal remains of proboscidaea, Equus mosbachensis cf, Bos primigenius, Bos/Bison and Cervus elaphus suggest an open landscape (Thieme, 2007b; van Kolfschoten, 2012, 2014). €ningen 13/I-1 is the stratigraphically lowermost site at Scho €ningen, and, regardless of the chronostratigraphical model, Scho one of the oldest Palaeolithic sites in Germany. Establishing its age is therefore important not only for pinpointing the stratigraphical sequence but also for providing an age estimate for the archaeology and, ultimately, a maximum age for the overlying spear horizon of €ningen 13/II-4. Scho 3. Thermoluminescence dating Here we present thermoluminescence dating results on eight €ningen 13/I-1 to help establish the age of heated flints from Scho this stratigraphically important site. While chronometric information is needed for the geological stratigraphy, and hence placing the archaeology in a chronostratigraphic context, it is also desirable to obtain dates for prehistoric events which can be related to human activities, like lighting of a fire which caused damage to flint. However, unequivocal attribution to human activities can be made only on the basis of evident, and sometimes latent (e.g., AlpersonAfil et al., 2007), structures. The analysis of the site is ongoing and no specific spatial patterns or evident structures were observed during the excavation (H. Thieme, pers. comm.). The thermal alterations of flints from the find layer could therefore have been due to either past human activities, or the heating could have been caused by a natural fire. However, if a natural fire was the cause, we would expect to see a large quantity of flints with traces of fire, not just a fraction. The nature of the deposit (sandy/gravel) and its location at a lake shore are not consistent with environments being frequently exposed to fire, and the occurrence of a natural fire is therefore unlikely, especially as sediment penetration depths of

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natural fires are low (e.g. Richter, 2007). In any case, the time difference between a natural and an anthropogenic fire and/or occupation seems to be negligible for a site of this antiquity because neither artefacts nor natural pieces show signs of prolonged surface exposure. In fact, both find categories have relatively sharp edges (H. Thieme, pers. comm.) and therefore any time difference between deposition and heating, if present, must have been negligible. 3.1. Principle of thermoluminescence dating Thermoluminescence (TL) dating of heated flint artefacts is a well-established method for dating Middle Palaeolithic archaeological sites (e.g., Valladas et al., 1988; Mercier et al., 1991; Richter et al., 2010b). In contrast to many other methods, the age of a past human activity d the lighting of a fire d can be directly determined using TL dating. In a simplified model, TL dating is based on the accumulation of electrons in excited states in the crystal lattice caused by omnipresent ionizing radiation. The resulting metastable states are quantitatively measured by thermoluminescence analysis and provide the radiation dose the sample has received since the last zeroing (heating, i.e. fire), when excited states were all eliminated before accumulation to the present state. Only such metastable stages are considered, which have a life-time of at least an order of magnitude longer than the age to be determined. If the rates of all ionizing radiation sources (dose-rate) at the sample position are known, an age can be calculated by dividing the radiation dose by the radiation dose-rate. Dosimetric dating presumes constant dose-rates over the entire burial time. In flint TL dating, dose-rates are comprised of internal (a and b) radiation from the inner part of a sample, which is considered stable, and external (g and cosmic) dose-rates, where the g-component might have varied through time (e.g., Richter, 2007). The stability of the internal dose-rates is not contested over the time range of interest because all parts of the sample are discarded that show macroscopical traces of geochemical alterations d like patination d which could indicate changes in isotopic composition. Spatially clustered distributions of radioelements can lead to inhomogeneous dosimetric fields within solid samples (hot spots), which results in erroneous ages if not corrected for (e.g., Tribolo et al., 2006). However, variations of magnitudes significant for age determination appear to be linked to visible zonations in flint (Schmidt et al., 2013) and thus can be easily detected and discarded from analysis. The constancy of the external g-dose-rate from the surrounding sediment can be verified for the recent past (e.g., by High Purity Germanium [HpGe] g-ray spectrometry), but ancient (repeated) occurrences of disturbances (disequilibria) between the members of the uranium decay chain, which give rise to the g-dose-rate, cannot be excluded by such analysis. However, the failure to detect disequilibrium by HpGe g-ray spectrometry is usually interpreted as an indication that the decay chains have always been in secular equilibrium, and the external g-dose-rate is thus assumed to have been stable over the entire burial period. HpGe g-ray spectrometry does allow the analysis of only the small particles of sediments, but sediment layers in archaeological sites often contain larger pieces of rocks. HpGe g-ray spectrometry on sediment samples in the laboratory may therefore not be representative of the actual radiation conditions at the sampling spot (Schwarcz, 1994). Therefore the external g-dose of such ‘lumpy’ sites is preferentially measured with dosemeters buried at least 30 cm deep into the profiles (Richter, 2007). Flint samples are removed from their context by excavation, and the external g-dose-rate therefore cannot be measured at the

5

sample's exact position. Therefore measurements in positions considered as representative of the samples' immediate environment (geometry within a sphere of ~60 cm diameter) have to be taken. In order to estimate the external g-dose, several positions next to the samples' locations are measured and the average result is used for the age calculation. As a result of this approach, the g-dose-rate used for the age calculation is not necessarily identical to the g-dose-rate the sample was actually exposed to during burial. Therefore, the resulting individual TL ages may be too young or too old, which leads to an increased variation in age results because a mean external g-dose-rate for all the samples has to be used. This also explains the large variability of TL age results often observed for a given layer, which is a priori assumed to represent a more or less single event of a time length that cannot be resolved by the chronometric dating method. However, the positions for dosemeters are chosen quasi-randomly and their individual geometries are unknown because they are inserted 30 cm deep into the sediment profiles. This applies to the independently and randomly chosen flint samples for dating as well. A precise and reliable TL age can be obtained only from a large number of samples and where the average external g-doserate was obtained with a large number of dosemeters. This implies an assumption that the heating took place within a reasonably short time interval or at the same time for all samples. The latter is generally assumed for Palaeolithic sites, which commonly represent palimpsests that accumulated in time periods too short to be of significant relevance for the interpretation. It is therefore necessary to date several samples from a Palaeolithic layer with TL to provide a good age estimate of the last heating and thus the occupation of a site. Alternatively, the dosimetric environment can be reconstructed for individual rin, 2012), provided that samples (Guibert et al., 1998; Gue detailed documentation is available or that dose-rates were rin and Mercier, constantly monitored during excavation (Gue 2012), which is often not feasible. €ningen 13/I-1 3.2. Sample preparation and dosimetry at Scho Heated flint samples were identified by macroscopic inspection €ningen 13/I-1 by H. Thieme, using the of the inventory of Scho criteria described in Richter (2007), and samples were prepared accordingly (see Supplementary Online Material [SOM] for details). For the external dose only the g-component needs to be taken into account in heated flint TL-dating, because the sample was stripped of the parts affected by a and b radiation from the sediment. The preservation of organic material suggests that most of €ningen was located below the water table for the sequence of Scho most of its burial time. Water reduces the g-dose-rate (attenuation) significantly. Therefore, any dosimetric measurement of the sediments needs to be corrected for complete water saturation of the sediments, and the water capacity of the sediment has to be estimated in order to determine the effective external g-dose-rate. Several sediment samples in steel tubes of about 10 cm in diameter and 20 cm in length were taken from the profiles to measure the sediment moisture. The tubes, which were completely filled with sediment, were slowly filled up with water during the course of one day, the surplus water removed and then weighed. After several weeks of storage at 70  C the dry weights were determined and the ratios of the weights provided the water saturation content after subtraction of the weight of the steel tubes (Table 2). Results were similar for the various samples, and water saturation was only slightly higher than the moisture at sampling. The present day ‘as is’ g-doses as measured in situ by portable g-ray spectrometry or dosemeters (or in the laboratory by HpGe g-ray spectrometry of dried sediment material; SOM Table 1) were corrected for the

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The g-dose-rate results from the a-Al2O3:C dosemeters are given in Table 3 (two additional ones were lost and one failed the analysis). The mining activities lowered the water table and the sediments are therefore not water saturated any more. To correct for water saturation conditions, which prevailed for almost the entire burial time, average factors from Table 2 were employed for dose-rates in age calculation (Table 3). The internal dose-rate of the flint samples may have been affected by water saturation, given their location in a watersaturated environment. Therefore, the water saturation of three geological, and therefore certainly unheated, flint samples from similar geological origin was experimentally determined by immersion in water for one month. The internal dose-rates were calculated from the specific activities of the radioisotopes (U, Th, and K) of the flint samples (Table 4; SOM).

Table 2 Moisture measurements in the laboratory and g-dose-rate correction factors for water saturation. Tube #

Correction Water Correction factor for Natural factor for ‘dry’ ‘present moisture’ saturation moisture (% dry weight) (% dry weight) g-dose-rate g-dose-rate

13 I-2 13 I-3 13 I-4 average

11 14 12 12

14 18 17 16

0.85 0.82 0.81 0.83

0.97 0.95 0.96 0.96

appropriate water saturation conditions (Table 2). Correction followed (Aitken, 1985) and not (Aitken and Xie, 1990) because the external g-dose has to be corrected for the size and shape of the flint sample (effective external g-dose-rate after Valladas, 1985). The in situ g-dose-rate was measured in the field with two portable g-ray spectrometers, both equipped with NaI crystals of different sizes. The Target NanoSpec has multi-channel capabilities, while the Harwell spectrometer can discriminate only four channels. The latter instrument therefore provides less resolution and is more prone to temperature-dependent drift. More reliable results for g-dose-rates of heterogeneous sediments can be obtained by dosemeters, which were placed into the sediment body for one year. The accuracy of using this type of material and this method was recently demonstrated by Richter et al. (2010a). A total of ten a-Al2O3:C dosemeters were implanted in sediment profiles at depths of about 40 cm in order to compensate for the sloping profiles and obtain a 4p-geometry.

3.3. Thermoluminescence measurements Seventeen flint samples with macroscopic signs of heating (Richter, 2007), mostly originating from the screening of the sediments and sampling for dosimetry, were available for TL analysis. The exterior as well as the interior material (after removal of the outer 2 mm rim) of each flint sample were investigated by the TL heating plateau test after (Aitken, 1985) to determine if the prehistoric heating was sufficient to fully erase the TL signal to allow dating analysis (Table 4). For the heating plateau test (Aitken, 1985) the TL signal is increased by an additional irradiation in the laboratory, and a constant ratio to the natural TL signal (heating plateau) over the peak temperature range between 350 and 390  C (Fig. 1) provides evidence for the past full erasure of the TL signal. Only ten samples passed both tests and were therefore sufficiently heated for TL-dating analysis, but two of these were too small for the method employed here (see SOM for measurement details). Because the luminescence signal of the samples is close to the onset of saturation, a technique similar to the ‘Australian Slide Method’ (Prescott et al., 1993) was employed to determine the dose the samples received since their prehistoric heating. This palaeodose was obtained by a multi-aliquot-additive-regeneration (MAAR) slide protocol (Valladas and Gillot, 1978; Mercier, 1991, 1992) on the 90e160 mm grain size fraction. In this protocol the additive and regenerated TL data are approximated by an exponential function (Fig. 2). While the mathematical function does not correspond to the physical processes, it is a sufficient approximation to describe the TL growth curves (signal increase as a function of dose) and was found to be empirically more appropriate compared to quadratic functions, which showed a lesser degree of homoth etie (or similarity, see below).

Table 3 Results of field measurements (in situ) and water saturation corrected g-dose-rates from a-Al2O3:C dosimeter and portable NaI g-ray spectrometer measurements.

Nutmaq-Harwell 4-channel Nutmaq-Harwell 4-channel NanoSpec 2  200 NaI(Tl) NanoSpec 2  200 NaI(Tl) dosimeter S-11 dosimeter S-12 dosimeter S-15 dosimeter S-16 dosimeter S-17 dosimeter S-19 dosimeter S-20 Dosimeter average a

Position

‘as is’ in situ g-dose rate (mGy a1)

Water saturation corrected g-dose rate (mGy a1)

± (%)

1

921

884

13

2

944

906

13

3

620

595

7

4

610

586

9

11 12 15 16 17 19 20

758 702 883 683 611 629 674

728 673 848 656 586 603 647 678

4 4 5 5 2 8 2 12a

Standard deviation of water saturated dose rates.

Table 4 Results (1-s) of neutron activation analysis and thermoluminescence measurement results.a Sample SCHON-3 SCHON-9 SCHON-10 SCHON-12 SCHON-13 SCHON-14 SCHON-15 SCHON-111

Heating plateau ( C)

DE-plateau ( C)

320e400 350e410 330e410 330e410 310e420 310e400 320e430 330e410

340e375 360e390 350e380 320e370 340e380 310e395 350e380 360e390

Palaeodose (Gy) 299 270 409 281 275 261 304 311

± ± ± ± ± ± ± ±

19 18 23 21 17 21 27 23

b-value (Gy cm2) 0.21 2.01 1.58 1.82 1.01 1.40 2.18 1.79

± ± ± ± ± ± ± ±

0.01 0.12 0.08 0.10 0.04 0.10 0.44 0.13

U (ppm) 1.22 0.75 3.22 0.90 0.13 0.49 0.96 0.58

± ± ± ± ± ± ± ±

0.03 0.03 0.05 0.03 0.02 0.02 0.03 0.06

Th (ppm) 0.11 0.13 0.14 0.10 0.06 0.05 0.12 0.12

± ± ± ± ± ± ± ±

0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

K (ppm)

Ḋgeff. ext (mGy a1)

Ḋcosmic (mGy a1)

± ± ± ± ± ± ± ±

631 641 646 647 644 647 641 630

82 82 82 82 82 82 82 82

404 404 497 395 200 245 420 427

8 8 3 8 5 6 8 21

Ḋaeff. (mGy a1) 4 27 87 28 3 12 36 19

± ± ± ± ± ± ± ±

1 2 4 2 1 1 7 2

Ḋbeff. (mGy a1) 213 145 513 165 36 92 177 122

± ± ± ± ± ± ± ±

4 4 3 5 2 3 4 9

a The effective external g-dose-rate takes the shape and weight of the sample into account (after Valladas, 1985) and the effective internal dose-rate the sensitivity of the sample to a-radiation.

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7

Figure 1. Natural and additive thermoluminescence glow curves (not shifted to a common peak temperature) and ratio (in grey) of 105 Gy additive TL-signal to natural TL-signal (heating plateau 350e390  C with no scale) of sample Schon-13. The DE-results over temperature (DE-plateau of 340e380  C) are indicated as black error bars in the top of the graph.

Figure 2. TL growth curves (exponential fit) of additive (alpha contribution subtracted; hatched line), regeneration (dark grey line), and shifted (light grey line) regeneration (scaled) TL-growth curves of the flint sample Schon-13. The constant TL-ratios (polygon) of the additive to shifted regeneration growth curve for the additive dose points is a measure of the similarity (‘homothetie’) of the two growth curves (Mercier, 1991). Glow curves were aligned to natural peak temperature prior to analysis.

The regeneration TL growth curve is subsequently scaled (following Sanzelle et al., 1996) and shifted along the dose axis until the best fit with the additive growth curve is obtained, which is defined as the minimized variation of the ratio of the growth curves within the measured dose ranges. A constant ratio of the additive TL-signal to the TL-signal of the shifted regenerated TL growth curve is a measure of the similarity (homoth etie) of the two dose response curves and is used as a verification of the appropriateness of the slide approach (after Mercier, 1991). The shift is the palaeodose (Table 4), and TL-age estimates (Table 5) are calculated following Aitken (1985).

4. Results 4.1. Dosimetry results HpGe g-ray spectrometry of the fine grained sediment particles indicates the recent absence of disequilibria of the U- as well as the Th-decay chain (SOM Table 1; SOM Fig. 1). Thus, the long-term stability of the external g-dose-rate from the surrounding sedi€ ningen 13/I-1 was assumed. ment of the flint samples from Scho The results from the different methods of measuring the external g-dose-rate vary considerably, by up to a factor of two. Not

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Table 5 Age results (1-s) and relative contribution of the effective g-external and internal dose-rate to the total dose-rate. Sample SCHON-3 SCHON-9 SCHON-10 SCHON-12 SCHON-13 SCHON-14 SCHON-15 SCHON-111 Weighted average

Ḋeff.

int.

(% total Ḋ) 23 19 45 21 5 13 23 16

Ḋgeff.

ext.

(% total Ḋ) 68 72 49 70 84 78 68 74

Age (ka) 321 302 308 304 359 313 325 365 321

± 39 ± 37 ± 31 ± 38 ± 50 ± 41 ± 40 ± 46 ± 16

surprisingly, the laboratory HpGe g-ray spectrometry on dry sediment provided the largest dose-rates (dry 1051 mGy a1 and watersaturation corrected 872 mGy a1) because it does not take into account neither the contribution of the larger sediment components (gravel and small boulders) nor the sediment layers above and below the find horizon. The result is therefore not representative of the actual g-dose-rate for the samples. The in-situ doserates measured with the portable equipment are considerably lower and yet still differ to some degree as well (Table 3). However, as they were recorded in different years, in different seasons, and in different positions, some variation is to be expected. The results of the Nutmaq-Harwell are considered to be less reliable than those of the NanoSpec due to the lower resolution and potential temperature drift problems of the former. The g-dose-rates measured by the dosemeters are considered to be the most reliable measurement of the external g-dose-rate because they provide data for different geometries of the g-doserate, which are caused by differences in gravel content within the layer and by its varying thickness, as well as differences in the gamma dose-rate from the adjacent layers. The water saturation corrected average value of 678 mGy a1 (Table 3), which was used in age calculation, lies within the range of the results of the portable equipment. As expected for such ‘lumpy’ sediment, some variation (sd 14%) is observed (Table 3) and accordingly an uncertainty of 15% was employed for age calculation. The water saturation of the geological flint samples was determined as <0.01%weight. While heated flint is probably more porous compared to unheated flint, it is assumed that the attenuation of ionizing radiation by additional water within the sample is neglirin et al. gible. The internal dose-rates were calculated after Gue (2011) from the specific radioisotope concentrations of the flint samples (Table 4), which were obtained by neutron activation analysis (NAA), and accounting for each sample's sensitivity to aradiation (Table 4). The cosmic dose-rate of 82 mGy a1 (57 78 445 N, 44 30 825 E at 108 m a.s.l. after Serangeli et al., 2012) was calculated with the software ‘KosmDL’ (P. Karelin, 1997 using data from Prescott and Stephan (1982), Barbouti and Rastin (1983), and Prescott and Hutton [1994]) for a sediment cover of 15 m prior to excavation (Thieme and Maier, 1995), which is assumed to have accumulated constantly after burial of the sample. However, the TL-age results are not significantly 2-s affected for varying sediment overburden between 10 and 50 m.

4.2. Thermoluminescence measurement results and TL-ages The samples show good heating plateaus of lengths between 30 and 85 K, and the natural TL-peak occurs at temperatures between 355 and 375  C at a heating rate of 5 K s1 (Table 4). The palaeodoses exhibit a large range (Table 4), which is a direct consequence

of the differences in radioelement concentrations. Alpha sensitivities are in the expected range, except sample SCHON-3, which has an unusually low value (Table 4). However, the calculated age of this sample almost equals the average from all samples (Table 5). This is also the case for the sample exhibiting the highest concentration in uranium (SCHON-10), which, as a consequence, exhibits the largest palaeodose. The sample with one of the lowest content in uranium (SCHON-13) has, as expected, a relatively old age (Tables 4 and 5), but an age of similar antiquity is also obtained for SCHON-111, which has higher radionuclide concentrations. The sample with the second largest dependency on the external gamma dose-rate, which might be expected to be more prone to variation in g-dose-rate, provides an age close to the overall average. All this indicates that the external dose-rate estimate is appropriate, and we conclude that there are no apparent systematic trends in the data. Within the limitations of the methods used, the data therefore provide reliable age estimates (Table 5). The TL-ages, presented in Table 5 at 1-s, show some variability between 302 ± 37 ka and 365 ± 46 ka. The calculated ages are rather dependant on the external g-dose-rate, which contributes between 49 and 84% of the total dose-rate, and the internal dose, which is considered stable over burial time, accounts for only 5e43%. The sample with the lowest proportional contribution of the external g-dose-rate (SCHON-10) could be considered to be the most reliable (Richter, 2007) because this parameter is the least well known and could have undergone fluctuations. For sample SCHON-14 the external g-dose-rate is responsible for 78% of the total dose-rate, and the age is therefore highly dependent on the external dose-rate estimate. The age of 313 ± 41 ka, however, is almost identical to the mean from all samples and close to the most reliable age from SCHON-10. This might again indicate a good approximation of the external dose-rate and that the TL-dating overall appears appropriate. Statistical tests after Dixon (after Rorabacher, 1991) and Grubbs (Grubbs, 1969) show the absence of outliers. The data set belongs to a normal distribution which was tested by passing the ShapiroWilk-Test (software package Origin 8) and a Chi-square test. Therefore, a weighted average of 321 ± 16 ka (including the systematic uncertainties twice [after Sachs, (1992)]) was calculated as the best age estimate for the last heating of these samples under the assumption that the heating took place at the same time or within the limits of the method. 5. Discussion and conclusions At a level of 95% probability the weighted TL age of 321 ± 16 ka €ningen 13/I-1, provides an age range of 353 to 289 ka for Scho which nominally comprises marine isotope stages (MIS) 10 to MIS 8 (MIS after Lisiecki and Raymo, 2005). The pollen and sediment at stratigraphically similar locations in channel I within the channel system of Mania (1995, 2006) appear to indicate the onset of an interglacial climate, which is interpreted as Holsteinian (Urban et al., 1991a). This is in accordance with the mammalian record, where no species unequivocally attributable to either glacial or interglacial conditions were recovered from this cycle, but the €ningen same mammals occur in the following interglacial at Scho (Van Kolfschoten, 2014). Despite the mammals in summary indicating non-glacial conditions (Van Kolfschoten, 2014), it does not €ningen 13/I-1 has to be appear to be clear if the site of Scho necessarily placed within an interglacial, especially in a rigid isotopic scheme. Within the TL-dating age range a nominal correlation to an interglacial is possible only for MIS 9. Under the assumption that the attribution of this sediment to the Holsteinian is correct, MIS 9 has to be assigned to the Holsteinian by inference. However, the

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D. Richter, M. Krbetschek / Journal of Human Evolution xxx (2015) 1e11

€ ningen, as well as the general variable palynological data for Scho appearance of the Holsteinian in the Northern Central European Plain (e.g., De Beaulieu et al., 2001) appear to indicate a rather complex picture which does not allow for simple correlations. This is also evident by the frequent Central European discrepancy to the British record with correlations of the Holsteinian to MIS 9 and MIS 11 respectively (e.g., Geyh and Krbetschek, 2012 and references provided above). Especially in light of potential problems in pollen preservation, bias in deposition, and other factors in the sedimentation environments at a lake shore/partially lacustrine/ delta/fluvial/channel, i.e. independent of the sedimentological model, age estimates based on palynology do not seem to be unproblematic. While the lacustrine-deltaic sedimentation model (Lang et al., 2012) does not require a specific age, the TL result does not appear to be compatible with the channel model for the sequence €ningen proposed by Thieme and Mania (1993), Mania (1998, of Scho 2006). Additionally a disagreement of the presented dating result has to be stated with the mammalian record which correlates € ningen 13/I with MIS 11 (Van Kolfschoten, 2014), as well as Scho with the palynological model if the Holsteinian is correlated with MIS 11. However, considering the model put forward by Lang et al. €ningen 13/I and 13/II do not (2012, 2015), the sediments of Scho have to correspond to different MIS and the only discrepancy of the TL-dating result lies with the generally disputed correlation of the Holsteinian palynology to MIS 11. €ningen 13/I-1 are only The mean TL-dating result for Scho € ningen 13/II-2 (Urban slightly older than the U-series dating of Scho et al., 2011; Sierralta et al., 2012), and within uncertainties the age results have to be considered as statistically of the same age. Stra€ nintigraphy, however, provides evidence of the older age of Scho gen 13/I-1. These dating results clearly allow for an attribution of €ningen 13/I-1 and 13/II-2 to the same isotopic the sediments of Scho stage, in accordance with the sedimentation model by Lang et al. (2012, 2015) and also with the occurrence of the mammals from 13/1 in 13/II, which might indicate a short time span between sediment depositions. € ningen 13/I stratiBoth sedimentological models place Scho € ningen 13/II (Table 1). The TL age estimate of graphically below Scho €ningen 13/I-1 may 321 ± 16 ka for the Lower Palaeolithic site of Scho therefore, likewise the U-series dating, serve as a maximum age €ningen 13/II-4. The estimate for the overlying spear layer Scho evident interglacial character of the sediments therefore allows for the most plausible correlation of the sediments containing the spears to Marine Isotope Stage 9, and thus an age range of 337e300 ka. Acknowledgements This work was partially supported by the Forschungsstelle €ometrie der Heidelberger Akademie der Wissenschaften Archa located at the Max-Planck Institute for Nuclear Physics in Heidelberg and a 'PhD finishing grant' from the University of Tübingen. We thank G. Wagner for his support, guidance, and help during the work conducted at his lab in Heidelberg. We are grateful to E. Pernicka (Mannheim/Tübingen) and A. Gouveia (ITN) for providing the neutron activation results and G. Cardoso (ITN) for help in the lab. We are indebted to H. Thieme for providing the samples and his guidance through the site, and we gratefully acknowledge him and his excavation crew for their help with placing the dosemeters. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jhevol.2015.06.003.

9

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€ ningen, Journal of Human Please cite this article in press as: Richter, D., Krbetschek, M., The age of the Lower Palaeolithic occupation at Scho Evolution (2015), http://dx.doi.org/10.1016/j.jhevol.2015.06.003