Sedimentary Dosimetry for the Saradj-Chuko Grotto: A Cave in a Lava Tube in the North-Central Caucasus, Russia.

Karst caves host most European Paleolithic sites. Near the Eurasian-Arabian Plate convergence in the Caucasus' Lower Chegem Formation, Saradj-Chuko Grotto (SCG), a lava tube, contains 16 geoarchaeologically distinct horizons yielding modern to laminar obsidian-rich Middle Paleolithic (MP) assemblages. Since electron spin resonance (ESR) can date MP teeth with 2-5% uncertainty, 40 sediment samples were analyzed by neutron activation analysis to measure volumetrically averaged sedimentary dose rates. SCG's rhyolitic ignimbrite walls produce very acidic clay-rich conglomeratic silts that retain 16-24 wt% water today. In Layers 6A-6B, the most prolific MP layers, strongly decalcified bones hinder species identification, but large ungulates inhabited deciduous interglacial forests. Unlike in karst caves, most SCG's layers had sedimentary U concentrations > 4 ppm and Th, > 12 ppm, but Layer 6B2 exceeded 20.8 ppm U, and Layer 7, > 5 ppm Th. Such high concentrations emit dose rates averaging ~ 1.9-3.7 mGy/y, but locally up to 4.1-5.0 mGy/y. Within Layer 6, dose rate variations reflect bone occurrence, necessitating that several samples must be geochemically analyzed around each tooth to ensure age accuracy. Coupled with dentinal dose rates up to 3.7-4.5 mGy/y, SCG's maximum datable ages likely averages ~ 500-800 ka.


Introduction
ESR (electron spin resonance) can establish absolute (chronometric) dates for fossils, like teeth and molluscs, which range from <10 ka to >4 Ma, depending on the radiation dose rates that the fossils experience in their depositional history. Like the other trapped charge dating methods, like thermoluminescence (TL), optically stimulated luminescence (OSL), and radioluminescence (RL), ESR measures the intensity of a radiation-induced signal recorded in the datable mineral compared with the rate at which that radiation irradiates that mineral when deposited in sediment's can date the hydroxyapatite in tooth enamel, carbonate minerals in molluscs and other invertebrates, and quartz in fault gouge and quartz-rich sediment. Used mainly in Quaternary geology and archaeology, ESR can date teeth and molluscs from some late Pliocene sites. Using teeth, ESR has been used to date many archaeological, human paleontological sites, and mammalian paleontological sites (see details in [1][2][3][4][5][6]).
In the hydroxyapatite in vertebrate tooth enamel, ESR dating uses the HAP signal at g = 2.0018 ± 0.0002, which has a mean signal lifetime, τ HAP ≈ 10 19 y ( [3]; for all variables and their definitions, see Table A1). If a tooth experiences a low total radiation dose rate, D Σ (t), the minimum and maximum datable ESR age limits will be relative old, whereas a high D Σ (t) translates into much younger minimum and maximum age limits. By comparing the radiation doses accumulated in tooth enamel, A Σ , with D Σ (t) affecting the enamel, ESR dating can calculate an ESR age: (D int (t) + D sed (t) + D cos (t))dt (2) where A Σ = the total accumulated dose in the sample, D Σ (t) = the total dose rate, D int (t) = the internal dose rate from within the tooth: U, its daughters, and other radioisotopes, D sed (t) = the external dose rate from sedimentary U, Th, K, and other radioisotopes, D cos (t) = the external dose rate due to cosmic radiation, t 1 = the sample's age, t 0 = today [7].
Ideally, 2-8 subsamples of enamel from each tooth are analyzed. For subsample, the accumulated dose, A Σ , is determined with the additive dose method, in which 12-15 aliquots of pristine, powdered, well homogenized enamel receives added radiation doses with the highest added dose~10 times higher than the accumulated dose measured in the 1-2 natural aliquot that receive no added irradiation. After measuring the ESR peak heights for both the naturally and artificially irradiated aliquots, the peak heights are plotted vs. their added doses to yield a growth curve whose x-intercept gives A Σ (for example, see [6]).
In a tooth, the internal dose rate, D int (t), depends on the U concentration in the dental tissues, and the U uptake rate at which U was absorbed into the enamel, dentine, dental cementum, and any immediately adjacent bone. In most environments, however, neither the external radiation dose rates, D sed (t) nor D cos (t), stay constant with time. For example, D cos (t) drops as sediment or water accumulates above the tooth, but D cos (t) rises again, if erosion exposes buried teeth. Meanwhile, D sed (t) varies as deposition, erosion, cementation, or sedimentary diagenesis alter the sedimentary composition, its water concentration, and its thickness around a tooth. For most Middle Paleolithic teeth found in caves, however, low enamel and dentinal U concentrations often make D int (t) low, while D cos (t) tends to be small compared to D sed (t). Together, these factors result in dating limits that range from~10-20 ka to~3-4 Ma and ESR ages whose accuracy and precision depend critically depend upon the accuracy and precision in the D sed (t) measurements (see references in [1][2][3][4][5][6]8]).
In other depositional settings, however, the ESR age's dating limits, accuracy, and precision can be dramatically different. This study examines how the sedimentary radioactivity in a Paleolithic site found in a lava tube affects the ESR dating analyses by examining the dosimetry on Saradj-Chuko Grotto (SCG), a newly discovered Middle Paleolithic (MP) site occurring in a lava tube in the north-central Caucasus in Kabardino-Balkaria ( Figure 1; Table 1).

ESR Dating in Archaeological and Human Paleontological Sites
ESR dating is often used to date the fossils in caves sensu lato, especially those containing Neanderthals, other early hominins, their tool assemblages, and/or vertebrate fossils. Although open-air sites do exist that have yielded Pleistocene hominin and other vertebrate fossils, the higher propensity for open-air sites to experience erosion before, during, and after deposition means that such sites tend to lack thick stratigraphical sequences, when compared to those in caves. Lava tubes, especially the smaller tubes, tend to collapse as they undergo diagenesis (for example, see [9]), likely making it harder to find them. For Neanderthals and their tool assemblages, the majority of the multi-layer stratified sites occur in caves or abris (rock shelters). Saradj-Chuko Grotto is one of the few lava tube caves in Europe that has yielded Mousterian assemblages, whereas hundreds of karst caves in Europe, western Asia, and the Mediterranean coast of northern Africa have yielded Neanderthals, or earlier hominins, and their characteristic assemblages. In Iceland, Hawaii, and other volcanic islands, several lava tube caves have yielded recent archaeological finds (for example, see [10]). In Africa, several caves in granite, ash beds, or lava tubes have yielded hominin fossils or tool assemblages, especially those near the East African Rift. These caves include Kitum [11], Leviathan [12], the Mau Mau Caves, and Makubike [13]. Again, however, many more important multi-layered sites yielding hominins, their artefacts, or Quaternary or Pliocene vertebrates have occurred in karst caves within Africa. Many important archaological and vertebrate sites known in Eastern Asia or India also occur in karst caves, although caves in lava tubes or igneous units are also known (for example,

ESR Dating in Archaeological and Human Paleontological Sites
ESR dating is often used to date the fossils in caves sensu lato, especially those containing Neanderthals, other early hominins, their tool assemblages, and/or vertebrate fossils. Although open-air sites do exist that have yielded Pleistocene hominin and other vertebrate fossils, the higher propensity for open-air sites to experience erosion before, during, and after deposition means that such sites tend to lack thick stratigraphical sequences, when compared to those in caves. Lava tubes, especially the smaller tubes, tend to collapse as they undergo diagenesis (for example, see [9]), likely making it harder to find them. For Neanderthals and their tool assemblages, the majority of the multi-layer stratified sites occur in caves or abris (rock shelters). Saradj-Chuko Grotto is one of the few lava tube caves in Europe that has yielded Mousterian assemblages, whereas hundreds of karst caves in Europe, western Asia, and the Mediterranean coast of northern Africa have yielded Neanderthals, or earlier hominins, and their characteristic assemblages. In Iceland, Hawaii, and other volcanic islands, several lava tube caves have yielded recent archaeological finds (for example, see [10]). In Africa, several caves in granite, ash beds, or lava tubes have yielded hominin fossils or tool assemblages, especially those near the East African Rift. These caves include Kitum [11], Leviathan [12], the Mau Mau Caves, and Makubike [13]. Again, however, many more important multi-layered sites yielding hominins, their artefacts, or Quaternary or Pliocene vertebrates have occurred in karst caves within Africa. Many important archaological and vertebrate sites known in Eastern Asia or India also occur in karst caves, although caves in lava tubes or igneous units are also known (for example, see [14]). Few studies have examined the sedimentary dosimetry in lava tubes, unlike that in karst caves.

Determining the External Dose Rates, D ext (t)
Since Rink [1], Skinner [2], Blackwell [5], and Blackwell et al. [6] among other authors, have discussed the theory and application of ESR dating in detail, we will only discus the theory underlying the sedimentary dose rates herein. Radiation, α, β, and γ, from the sediment surrounding the dating sample generates D sed (t). In limestone caves, sediment may derive from aeolian and fluvial processes, mass wasting, seepage through cracks, stoping of the roof, non-human and human biogenic sources (Figure 2a). In a lava tube, like SCG, the sediment will lack or contain few, mostly carbonate deposits, while the more acidic sediment may destroy bone and carbonate fossils (Figure 2b). Thus, geological and archaeological sites have inhomogeneous ("lumpy") sediment sitting within sequences of several very thin horizons with mineralogically and biologically distinct geochemistries. This generates inhomogeneous radiation dose fields that must be measured precisely and accurately to generate reliable ESR dates.  Humans may contribute manuported minerals or fossils used as tools and ritual objects, but their fires also can add ash, burnt or charred wood and bone. Animals, including humans, accumulate their excretia, food debris, and their degradation products. As well as the dust and silt trapped on their feet, and in their pelts, feathers, or clothing, animals contribute their hunted or foraged prey carcasses. Plants add pollen. From the cave walls, roof, and the cliff face above the mouth come physical, chemical and biological weathering that contribute mass wasting products, including éboulis, as the cave stopes upward. Diagenetic processes and cementation can also add new authigenic minerals: (a) In karst systems, carbonate deposits, like stalagmites, stalactites, flowstone, and tufa, as well as authigenic carbonate cements usually form a significant sedimentary component. (b) In a grotto within a lava tube, carbonate sediment and cements will likely be insignificant compared to the clay generated by degradation of the igneous minerals or volcanic glasses.

Saradj-Chuko Grotto (SCG), South Russia
Lying at 934 m amsl in the central Caucasus Mt., south Russia, Saradj-Chuko Grotto sits ~ 35 m above the Saradj-Chuko River, which is a tributary of the Kishpek River that is, in turn, a tributary of the Baksan River within the Terek River Basin in Kabardino-Balkaria. Some 70-80 km NE of Mount Elbrus, Europe's tallest mountain, the grotto also sits 4 km south of Zayukovo and 20 km SE of Nalchik, the capital of the Kabardino-Balkaria Republic (Figure 1; [17][18][19]).
The cave formed in rhyolitic ignimbrites and tuff deposited in the Lower Chegem Formation, which erupted in the extension zone between the Laba-Malka monoclinal uplift and the Terek-Caspian Trough, in the continental convergence zone between the Saudi Arabian and Eurasian Plates ( Figure 3). These Lower Chegem volcaniclastic deposits mainly comprise rhyolitic and liparitic tuffs, andesites, basalts, and rhyolitic ignimbrites ranging from a few tens to almost 500 m thick in the junction zone, although some extrusive domes, like Mt. Elbrus and Kazbek, also exist ( Figure 3; [17][18][19]). Discovered in 2016, Saradj-Chuko Grotto contains 11 distinct geoarchaeological horizons ranging from modern, Medieval, Roman to Middle Paleolithic deposits (Figure 4, 5). Averaging 5-6 cm thick, Layer 1 contains a grey sandy silt that dips to the NE. With few animal fossils and slag, mixed Medieval ceramic and several obsidian pieces suggest a bioturbated deposit. Layers 1A-C comprise yellow, sandy silt units mixed with charcoal and ash [17][18][19].
Dipping to the west, Layer 2 ranges from 11 to 24 cm of yellow, sandy silt ( Figure 4, 5). Although Layer 2 yielded a few mammal bones, it lacks artefacts. Also dipping to the west, Layer 3 averages 12-17 cm thick, with yellow sandy silt interspersed with three gruss horizons. Layer 3 yielded a few mammal fossils and obsidian artefacts, one of which was a Middle Paleolithic bifacial tool. Comprising 14-30 cm of grey-brown silt dipping to the west, Layer 4 also yielded obsidian artefacts and several mammal bones. Actually a tectonic fissure, Layer 5 produced flakes, but few fauna in a dark brown-black sandy silt [17][18][19]. Humans may contribute manuported minerals or fossils used as tools and ritual objects, but their fires also can add ash, burnt or charred wood and bone. Animals, including humans, accumulate their excretia, food debris, and their degradation products. As well as the dust and silt trapped on their feet, and in their pelts, feathers, or clothing, animals contribute their hunted or foraged prey carcasses. Plants add pollen. From the cave walls, roof, and the cliff face above the mouth come physical, chemical and biological weathering that contribute mass wasting products, including éboulis, as the cave stopes upward. Diagenetic processes and cementation can also add new authigenic minerals: (A) In karst systems, carbonate deposits, like stalagmites, stalactites, flowstone, and tufa, as well as authigenic carbonate cements usually form a significant sedimentary component. (B) In a grotto within a lava tube, carbonate sediment and cements will likely be insignificant compared to the clay generated by degradation of the igneous minerals or volcanic glasses.
To determine a reliable D sed (t) experienced by a tooth, the individual dose rate, D sed,i,j (t), for each mineral j, in each layer or horizon i within the sphere of influence that generates the total dose rate reaching the tooth must be measured. For the β radiation component in sediment, that sphere of influence has a radius of~3 mm in sediment, but the γ radiation sphere's radius averages 30 cm in sediment. The radius for the sphere of influence represents the distance over which the radiation attenuates in sediment or its intensity drops below the level at which it produces a measurable effect in the sample. Radiation sources closer to the sample will produce a stronger effect on the sample than sources further away: This effect is modelled as a function of the distance squared between the source and the sample. Therefore, to assess the sedimentary β radiation dose rate, D sed,β (t), the fine sediment immediately attached to, or surrounding, and any other larger clasts, like éboulis, bone, or flint, within 3 mm of the dating sample is usually analyzed by neutron activation analysis (NAA) or other radiogeochemical analyses (i.e., fission track, X-ray fluorescence, TL dosimetry, β or γ spectrometry), while for the sedimentary γ radiation dose rate, D sed,γ (t), D sed,γ,i (t) measurements for all the layers or horizons within 30 cm of the dating sample must be calculated. Thus, since most archaeological and paleontological sites have inhomogeneous ("lumpy") sediment, finding the most representative value for D sed (t) requires collecting multiple samples immediately around the sample for sedimentary dosimetry up to 30 cm away from the tooth [5]. Brennan et al. [15] discussed the issues in lumpy sites. In most sites, using γ spectrometry and TL dosimetry usually requires that the dosimetry be completed before excavation removes any sediment. Because that sedimentary inhomogeneity produces significant variation in D sed (t) laterally and vertically for any tooth to be dated, γ spectrometry and TL dosimetry measurements often lack the precision and accuracy needed to get the highest reliability, if not supplemented by geochemical analyses.
Since water absorbs radiation, it reduces D sed (t), making it essential to correct D sed,γ,i (t) measurements, regardless of how they were measured, if the water content varies by >5 wt% [5]. In newly excavated sites, the modern water concentration, [W sed (0)], should be measured for each layer or horizon. Nonetheless, [W sed (0)] measurements do not guarantee that [W sed (t)] has been constant over time, because paleoclimatic and hydrological changes may have altered the time-averaged sedimentary water concentrations W sed (t) in the past. Therefore, geomorphological, mineralogical and geochemical data should be used to model in order to find better D sed (t) and D cos (t).
To find the tooth's time and volumetrically averaged dose rate, D sed,γ (t) derived from the γ radiation sources can then be calculated by integrating all D sed,γ,i (t) over the 30 cm sphere and over time [6,8,15,16]. is This includes correcting D sed,γ,i (t) for any absorption or leaching of radioactive elements from the various sedimentary components [16]. Analagously, D sed,β (t) is integrated over the 3 mm sphere and over time.

Saradj-Chuko Grotto (SCG), South Russia
Lying at 934 m amsl in the central Caucasus Mt., south Russia, Saradj-Chuko Grotto sits~35 m above the Saradj-Chuko River, which is a tributary of the Kishpek River that is, in turn, a tributary of the Baksan River within the Terek River Basin in Kabardino-Balkaria. Some 70-80 km NE of Mount Elbrus, Europe's tallest mountain, the grotto also sits 4 km south of Zayukovo and 20 km SE of Nalchik, the capital of the Kabardino-Balkaria Republic ( Figure 1; [17][18][19]).
The cave formed in rhyolitic ignimbrites and tuff deposited in the Lower Chegem Formation, which erupted in the extension zone between the Laba-Malka monoclinal uplift and the Terek-Caspian Trough, in the continental convergence zone between the Saudi Arabian and Eurasian Plates ( Figure 3). These Lower Chegem volcaniclastic deposits mainly comprise rhyolitic and liparitic tuffs, andesites, basalts, and rhyolitic ignimbrites ranging from a few tens to almost 500 m thick in the junction zone, although some extrusive domes, like Mt. Elbrus and Kazbek, also exist ( Figure 3; [17][18][19]). Discovered in 2016, Saradj-Chuko Grotto contains 11 distinct geoarchaeological horizons ranging from modern, Medieval, Roman to Middle Paleolithic deposits (Figures 4 and 5). Averaging 5-6 cm thick, Layer 1 contains a grey sandy silt that dips to the NE. With few animal fossils and slag, mixed Medieval ceramic and several obsidian pieces suggest a bioturbated deposit. Layers 1A-C comprise yellow, sandy silt units mixed with charcoal and ash [17][18][19].
Dipping to the west, Layer 2 ranges from 11 to 24 cm of yellow, sandy silt (Figures 4 and 5). Although Layer 2 yielded a few mammal bones, it lacks artefacts. Also dipping to the west, Layer 3 averages 12-17 cm thick, with yellow sandy silt interspersed with three gruss horizons. Layer 3 yielded a few mammal fossils and obsidian artefacts, one of which was a Middle Paleolithic bifacial tool. Comprising 14-30 cm of grey-brown silt dipping to the west, Layer 4 also yielded obsidian artefacts and several mammal bones. Actually a tectonic fissure, Layer 5 produced flakes, but few fauna in a dark brown-black sandy silt [17][18][19].
Averaging 30-40 cm thick dipping to the west, Layer 6A comprises gray sandy silt with some tuff éboulis likely fallen from the roof (Figures 4 and 5). During the 2017 excavations, Layer 6A yielded 72 artefacts. Of these, most were made on obsidian, with only two on flint, and one on silicified limestone. With no formal tools or cores, eight flakes, 46 shatters, and 18 chips were identified, probably attributable to the Middle Paleolithic. Most of the obsidian tools and chips were made on Zayukovo (Baksan) obsidian [17][18][19]. artefacts. Of these, most were made on obsidian, with only two on flint, and one on silicified limestone. With no formal tools or cores, eight flakes, 46 shatters, and 18 chips were identified, probably attributable to the Middle Paleolithic. Most of the obsidian tools and chips were made on Zayukovo (Baksan) obsidian [17][18][19].
Undoubtedly, hominins knapped local obsidian in SCG, but any flint artefacts had been imported ready to use. High quality grey, black, and pink flint was also available along the Baksan, Chegem, and Kamenka River valleys ( Figure 3). The flint may have come from local sources, such as HanaHaku and Shtaucukua 5-7 km to the northwest, and from Kamenka to the southeast. Inhabitants rejuvenated flint artefacts at the site. In Layer 6B, the few retouched tools included simple side-scrapers, rare diagonal and transverse sidescrapers, convergent tools, and Mousterian points. Neanderthals likely made this Levallois-laminar Mousterian industry [17][18][19].
In Layers 6B and 7, pollen analyses found many tree and shrub grains, including Alnus glutinosa, Ulmus campestris, and Castanea sativa, with rare Compositae, Poaceaea, and Apiacae. These layers were deposited during an interglacial period, most likely Marine Isotope Stage (MIS) 5 when hazel-hornbeam-oak forests with oriental beech, chestnuts, and walnuts surrounded the cave. Sula and Strobus suggest that the minimum winter temperature locally reached −9 °C, while precipitation averaged 80-150 cm/y. These mixed deciduous and coniferous forests provided hominins with complex dietary choices [17][18][19].
Within Layer 6B, the very fragile bones ranged from whitish grey to yellowish-orangish red. In some, the original bone shape ("ghosts") could be discerned when first found, but they had been decalcified enough that they disintegrated into tiny flakes, spots, or dust if disturbed. Most intact bones had been broken or cracked before deposition. Thus, taxonomically or anatomically (typing) identifying many bones was impossible. Some bones, however, had been heated, charred, or calcined, hinting the animals had likely been hunted by Neanderthals [17][18][19].

Method
Since this study does not focus on the ESR dating age analyses, only a brief description of the tooth's preparation for ESR dating will suffice here. All samples were prepared using standard protocols for a Class 10,000 clean lab. To reduce cross-sample contamination, all glass-and plasticware were soaked in 6 M HCl(aq) and rinsed ≥ 15 times with doubly distilled, deionized water to remove all Cl − ions [21].
After measuring their thicknesses with a CD-4C digital caliper, the tooth was split into four subsamples with a diamond-tipped Dremel drill, and each subsample cleaned of any attached sediment or dentine, which were removed and saved for NAA. After measuring enamel thicknesses in 30-50 places with a Mitutoyo IP-C112E micrometer, 20 µm were shaved off both enamel sides to remove the externally α-dosed enamel and thicknesses were remeasured. After powdering the enamel to 200-400 mesh (38-76 µm) in an agate mortar and pestle, it was split into 13-16 identical aliquots, each weighing 20.0 ± 0.1 mg [21]. At 20 cm thick, Layer 6B contains dark brown silt with ignimbrite pebbles and tuff (Figures 4 and 5). Some organic matter contained secondary iron hydroxides, while some bones had decomposed to decalcified phosphate clusters. In 2017, Layer 6B yielded 1591 lithic artefacts, 98% of which were made on obsidian, again mainly Zayukovo obsidian [17][18][19].
Undoubtedly, hominins knapped local obsidian in SCG, but any flint artefacts had been imported ready to use. High quality grey, black, and pink flint was also available along the Baksan, Chegem, and Kamenka River valleys (Figure 3). The flint may have come from local sources, such as HanaHaku and Shtaucukua 5-7 km to the northwest, and from Kamenka to the southeast. Inhabitants rejuvenated flint artefacts at the site. In Layer 6B, the few retouched tools included simple side-scrapers, rare diagonal and transverse sidescrapers, convergent tools, and Mousterian points. Neanderthals likely made this Levallois-laminar Mousterian industry [17][18][19].
In Layers 6B and 7, pollen analyses found many tree and shrub grains, including Alnus glutinosa, Ulmus campestris, and Castanea sativa, with rare Compositae, Poaceaea, and Apiacae. These layers were deposited during an interglacial period, most likely Marine Isotope Stage (MIS) 5 when hazel-hornbeam-oak forests with oriental beech, chestnuts, and walnuts surrounded the cave. Sula and Strobus suggest that the minimum winter temperature locally reached −9 • C, while precipitation averaged 80-150 cm/y. These mixed deciduous and coniferous forests provided hominins with complex dietary choices [17][18][19].
Within Layer 6B, the very fragile bones ranged from whitish grey to yellowish-orangish red. In some, the original bone shape ("ghosts") could be discerned when first found, but they had been decalcified enough that they disintegrated into tiny flakes, spots, or dust if disturbed. Most intact bones had been broken or cracked before deposition. Thus, taxonomically or anatomically (typing) identifying many bones was impossible. Some bones, however, had been heated, charred, or calcined, hinting the animals had likely been hunted by Neanderthals [17][18][19].

Method
Since this study does not focus on the ESR dating age analyses, only a brief description of the tooth's preparation for ESR dating will suffice here. All samples were prepared using standard protocols for a Class 10,000 clean lab. To reduce cross-sample contamination, all glass-and plasticware were soaked in 6 M HCl(aq) and rinsed ≥15 times with doubly distilled, deionized water to remove all Cl − ions [21].
After measuring their thicknesses with a CD-4C digital caliper, the tooth was split into four subsamples with a diamond-tipped Dremel drill, and each subsample cleaned of any attached sediment or dentine, which were removed and saved for NAA. After measuring enamel thicknesses in 30-50 places with a Mitutoyo IP-C112E micrometer, 20 µm were shaved off both enamel sides to remove the externally α-dosed enamel and thicknesses were remeasured. After powdering the enamel to 200-400 mesh (38-76 µm) in an agate mortar and pestle, it was split into 13-16 identical aliquots, each weighing 20.0 ± 0.1 mg [21].
Immediately after collection in the field ( Figure 5), sediment samples were doubly sealed in ziplock bags. Samples with sharp gravel were triply bagged. Before powdering the sediment for NAA,~30 g of sediment was sampled for [W sed (0)] immediately after opening the bags. To calculate [W sed (0)], the sediment mass was measured and gently heated at~50 • C for several days until no mass changes were recorded to find the mass loss.
To measure [U sed ], [Th sed ], and [K sed ], all associated sediment were powdered to ≤500 mesh and analyzed by neutron activation analysis (NAA). In JT5, 1-2 dentines/subsample and enamels/subsample were analyzed for their U concentrations, [U den ] and [U en ] respectively. After a 60.0 s irradiation and a 10.0 s delay, U was counted for 60.0 s in a delayed neutron counting system. Th and K were counted for 20.0 min in a γ counter. Th was counted after a 1.0 h irradiated and a 7.0 day delay. K had a 24-30 h delay after a 60.0 s irradiation. To ensure accuracy, all results were calibrated against NIST Standard 1633B [21].
Each D sed,i,j (t) and its error for each mineral, j, in Layer i, was calculated from [U sed ], [Th sed ], [K sed ], and [W sed (0)] [22], assuming no cosmic dose rate contribution, using both Rosy v. 1.4.2 [23] and Data [24]. Both these programs calculate D sed,i,j (t) as seen by the sample sitting in a sphere of homogeneous sediment corrected for radiation backscattering and for β and γ, but not α, attenuation, due to the sedimentary and tissue water concentrations, tissue density, and thicknesses from sediment and tissues. D sed (t) were calculated by using the program VolSed v. 2.2 that integrates all the applicable D sed,i,j (t) values within 3 mm for the β dosimetry or 30 cm for the γ dosimetry respectively over their applicable sedimentary units, weighted by their volume and distance from the sample. Distances between sedimentary components and a tooth was determined visually in the field, from the total station data, and from the site photographs. For the saturation simulations and modelling, D cos (t) were calculated assuming ramped box models [7] using Rosy v. 1.4.2, and integrated over time since the simulated deposition using the program AgeTimeAv v. 4.4. Comparisons between the other dosimetry methodologies, namely "instantaneous" γ and TL dosimetry, with the geochemical analyses described here, show that insignificant differences occurred between the three dosimetric methods (for example, see [5,6,25], and more references therein).

Results and Discussion
In Saradj-Chuko Grotto, water, U, Th, and K concentrations were measured for 16 different layers or horizons, from which D sed,β (t) and D sed,γ (t) were calculated (Table 2). To see the trends, means for the four concentrations and the two dose rates were plotted vs. depth ( Figure 6).   1 Abbreviations: D BG sed,β (t) = the bulk sedimentary dose rate from β sources; D BG sed,γ (t) = the bulk sedimentary dose rate from γ sources; Concentrations and 1 σ errors were calculated for the closest tooth; 2 Calculated using the nearest tooth's thicknesses and clastic sediment density, ρ sed = 2.66 ± 0.01 g/cm 3 ; enamel density, ρ en = 2.95 ± 0.02 g/cm 3 ; enamel density, ρ den = 2.85 ± 0.02 g/cm 3 ; 3 Calculated using cosmic dose rate, D cos (t) = 0.00 ± 0.00 µGy/y; 4 Values below detection limits: Assumed to be 0.00 ± 0.00 ppm for calculations. While both the highest D sed,β (t) and D sed,γ (t) within Layer 6 occurred in 6B2, Layers 6B1b and 6A both had very low D sed,β (t) and D sed,γ (t). Since both D sed,β (t) and D sed,γ (t) showed substantial variation and high uncertainties from one horizon to the next, the means, D BG sed,β,i, j (t) and D BG sed,γ,i, j (t), could not be used to calculate the specific volumetrically averaged dose rates, D sed,β,i (t) or D sed,γ,i (t), near each tooth. Instead, several samples with 30 cm of each tooth must be measured, which will increase the cost to date each tooth.

Sedimentary Geochemistry
Throughout the layers at SCG, the sediment retained high modern water concentrations, [W sed (0)] (Table 2; Figure 6a). Layers 3A and 2 had the lowest mean [W sed (0)] at 15.5 ± 0.9 and 15.6 ± 0.4 wt% respectively, while Layer 1 averaged 15.9 ± 0.2 wt%, and Layer 4, at 16.5 ± 0.3 wt%. Probably due to its higher sand concentrations and less clay, Layer 7 had a water concentration at 17.1 ± 0.3 wt%. Within Layer 6, water concentrations ranged from 18.9 ± 0.3 to 23.8 ± 2.4 wt%, with higher uncertainties than seen in other layers. Meanwhile, Layers 1B, 1C, and 6B3b all had >22 wt% [W sed (0)]. Within Layer 6, [W sed (0)] ranged from 18.9 ± 0.3 to 23.8 ± 2.4 wt%, with higher uncertainties than seen in other layers. In most karst cave sediment, [W sed (0)] tends to average 5-15 wt% [5,6,8,[25][26][27][28][29][30][31][32][33]. At [W sed (0)] >12-15 wt%, the sediment feels damp, which tends to discourage both long-term human inhabitation and cave bear hibernation. Undoubtedly, the high clay concentrations in most of SCG's sedimentary layers ensures that they stayed wet, which also promoted higher degradation of bone and other the organic remains, as well as the igneous minerals found in the ignimbrite constituting the cave rocks. Whether under different climate regimes, the sediment may not have stayed as wet as it is now. Nonetheless, wet sediment might have discouraged hominin and cave bear inhabitation in the grotto during all but the driest seasons and the coldest glacial periods. With the high bone dissolution, the dental seasonal analyses needed to answer this question have yet to be completed.
The SCG sediment also contained high K concentrations, [K sed ] (Table 2; Figure 6b). In both Layers 1B and 1C, [K sed ] exceeded 6.5 wt%, but in Layer 6B3c, [K sed ] averaged a low of 2.47 ± 0.24 ppm. Nonetheless, throughout SCG, [K sed ] at 2.5-4.0 wt% more than doubles the typical [K sed ] at 0.5-2.0 wt% seen in most karst cave sediment [5,6,8,[25][26][27][28][29][30][31][32][33]. Thus, the high [K sed ] also leads to higher D sed,β (t) (see below  [5,6,8,[25][26][27][28][29][30][31][32][33]. Hence, SCG's [Th sed ] ranged from 150% to >300% more than typically seen in karst cave sediment. Interestingly, within Layer 6, large variations in both [U sed ] and [Th sed ] occurred (Table 2i-o; Figure 6b). Due to the high [W sed (0)], and its igneous source rocks that weather into sediment with high acidity, the SCG sediment contained up to 40-50% clay in many horizons. That acidity contributed to high dissolution rates for bone and other organic tissues, as shown by the prevalence of the bone ghosts seen in many layers. Once exposed to U in the sediment, however, bone, dentine, and dental cementum begin to scavenge U rapidly [34][35][36][37][38][39]. Certainly, samples with higher bone concentrations had more [U sed ], coupled with somewhat lower [Th sed ]. These horizons also yielded the higher artefact numbers. Likely, in these horizons, the hominins contributed more bone and dental tissues to the sediment than in other horizons. Without a good proxy for the initial bone concentrations, however, estimating how much [U sed,os ] derived from U scavenging by bone and dental issues from the groundwater compared to U scavenged from [U sed,igrx ], including [U sed,éb ], after igneous rock dissolution is difficult. Nor can we estimate how high [U sed ] might have been for each horizon before the bones began to dissolve and release [U sed,os ] again to the sediment, from where it may have been subsequently leached. Within Layer 6 at Saradj-Chuko, having more bone in the sediment produced: (A) higher β sedimentary dose rates, D sed,β (t) (B) higher γ sedimentary dose rates, D sed,γ (t) Bone-rich samples had up to 0.32 mGy/y for D sed,β (t) and 1.0 mGy/y for D sed,β (t). If bone-rich samples were removed from the means for D sed,β (t)and D sed,β (t), the resulting D sed,β (t)and D sed,γ (t) had lower mean rates with significantly smaller uncertainties. Thus, knowing the precise locations for bone-rich sediment will increase the precision for the dose rates.

The Effects on the ESR Ages
With such high D sed (t) in several horizons and their high variations within Layer 6, the volumetrically averaged sedimentary dose rates, D sed (t), will need to be calculated using the individual D sed,i,j (t) for each layer and sedimentary component within the 3 mm and 30 cm spheres of influence around each tooth. Using the means, D BG sed,β,i,j (t) and D BG sed,γ,i,j (t), for each horizon will not provide, D sed (t) that will be accurate and precise enough to get the most reliable ESR ages. Thus, several individual sediment samples near each tooth must be tested by NAA. This could dramatically increase the costs for dating each tooth.
To test the dentinal dose rates, eight subsamples from JT5 were analyzed for its U concentrations, [U den ] ( Table 3). Not surprisingly, both [U inden ] and [U outden ] ranged from 136.12 to 162.38 ± 0.02 ppm, which produce D den (t) ranging from 3.751 ± 0.270 to 4.475 ± 0.322 mGy/y assuming an early U uptake model in the dentine. Again, [U den ] tend to range 100-150 ppm higher than comparable [U den ] values seen in dentine from karst caves [5,6,8,[25][26][27][28][29][30][31][32][33]38,39]. JT5 s D den (t) emits as much as~2.5-4.0 mGy/y, significantly more than in the comparable dentine seen in Middle Paleolithic teeth collected from karst caves. In humans, the lethal dose for 50% of people tested, LD 50 , is ≤4 Gy of radiation, although a dose as low as 0.25 Gy produces measurable effects in the body [40]. With a combined D sed (t) averaging from 1.9 to 3.7 mGy/y, but locally as high as 4.1-5.0 mGy/y, coupled with D den (t) as high as 3.7-4.5 mGy/y, hominins living in SCG received measurable effects after as few as~26 years at the highest dose rates, assuming that no areas in the cave have a higher dose rate. Although they would not likely accumulate lethal doses in a lifetime, especially if the wet sediment discouraged long-term inhabitation, the effects on mutation and cancer rates likely affected people who inhabited SCG for short times periodically over many years or those visiting frequently. Since both D sed (t) and D den (t) are so high, any tooth from SCG will have a much higher accumulated dose, A Σ , than a tooth of comparable age from a karst cave. Because the precision with which A Σ can be calculated drops as A Σ rises, the precision for the ESR ages for the SCG teeth also drops with the rising A Σ . For most teeth, full saturation of the HAP signal occurs at~13-22 kGy, but must be tested in each. At the highest D sed (t) and D den (t) seen in SCG, teeth might reach their full saturation dose, A sat , within 250 ka of being deposited. More realistically, given that neither all the horizons within the spheres of influence around each tooth are likely to emit the highest D sed (t) nor are all D den (t) will have the highest concentrations, the maximum dating age could be <500-800 ka. If the teeth date <200-250 ka (i.e., Marine Isotope Stage, MIS, 7) or even <360-420 ka (i.e., MIS 11), reliable ages should be calculable. Given that the assemblages in the oldest layers resemble closely the Mousterian, one would expect their teeth to post-date 200 ka. If, however, the cave contains teeth deposited older than MIS 15, their calculated ages might represent minimum ages, because their teeth might have reached their A sat . Should more archaeological layers occur below Layer 7, the potential for encountering teeth that might have reached saturation increases.
At a depth of +11 cm, JT5 sat within Layer 6B1a, just above the boundary with Layer 6B1b. For the β dosimetry, both Layers 6b1a and 6B1b fell within JT5 s sphere of influence, giving its preliminary D sed,β (t) = 955 ± 91 µGy/y. For the γ dosimetry, 2 cm of Layer 4 and 18 cm of Layer 6A overlay JT5, while 18 cm of Layer 6B2 lay below Layer 6B1b. After estimating and correcting for the amount of bone and rooffall around JT5, its preliminary D sed,γ (t) = 2000 ± 109 µGy/y. Final calculations must await excavation of nearby quadrants to assess the currently hidden sediment nearby and its inhomogeneous components. If JT5 was deposited during MIS 5 (i.e.,~74-128 ka), as the palynological analyses suggest [18], JT5 s accumulated dose, A Σ , would be expected to lie between 580 and 1080 Grays, assuming a linear U uptake model (LU; Figure 8a), which would allow an definite age determination for JT5, rather than a minimum age estimate. Assuming LU, JT5 would likely reach A sat at ages between 1.16 and 1.92 My after its initial deposition (Figure 8b). With the crystal damage rates produced by the high ionizing radiation fields bathing the teeth and the dentinal U concentrations (Table 3), however, the teeth might have uptaken U more frequently as the tooth aged. Thus, an RU model, with the uptake rate, p > 0, would better model the tooth's U uptake history. If so, JT5 s MIS 5 ages would be older, and it would likely reach its A sat if it had been deposited earlier in the Quaternary than would be generated by assuming LU. Assuming that an early U uptake model (EU, p = −1) describes the U uptake rate into the tooth, JT5 would have A Σ = 0.99-1.96 kGy if it dates to MIS 5. Under EU, JT5 would reach its saturation dose, A sat , after~665-1100 ky following its initial deposition. Using a linear U uptake model (LU, p = 0), JT5 would have A Σ = 0.58-1.06 kGy if it dates to MIS 5. Under LU, JT5 would reach its A sat at~1.16-1.92 My after its initial deposition. Assuming a recent U uptake (RU) model with an U uptake rate, p = 10, an age for JT5 dating to MIS 5 would have A Σ = 280-520 Gy. Assuming p = 10 (RU), JT5 would likely reach its A sat after~2.92-4.94 My. Assuming p = 20, a tooth from MIS 5 would have A Σ = 250-240 Gy, and reach its A sat after~3.59-5.90 My in the sediment. Thus, this analysis shows that JT5 will give definitive ESR ages regardless of its actual U uptake model. For teeth found in other layers with higher D sed,β (t) and D sed,γ (t), or for those found in thicker bone-rich horizons, however, their A Σ may approach the saturation dose, A sat , too closely to distinguish its A Σ from A sat . That would make it impossible to calculate anything other than a minimum age limit.

Conclusions
Unlike typical karst limestone caves, Saradj-Chuko Grotto nestles in rhyolitic ignimbrite. Due to its calcium carbonate, limestone buffers the sediment and its groundwater, but the rhyolitic ignimbrite at SCG lacks an effective geochemical buffer. Thus, in situ degradation of the ignimbrite has made the sediment very acidic. Thus, the acidic sediment contains high clay concentrations from the degraded silicates and obsidian in the lava. Acting as an aquatard, the clay also retains water well, producing [W sed (0)] higher than 16 wt% on average, but as high as~24 wt%. Since clays adsorb Th, the SCG sediment contains 10-20 ppm more [Th sed ] than those seen in most karst cave sediment. At SCG, [K sed ] concentrations also far exceeded those in karst caves. Dissolution of bones that scavenged [U sed ] from the local groundwater has produced [U sed ] up to~30 ppm, SCG's [U sed ] averaged 5-10 times higher than [U sed ] seen in most karst caves. In SCG, D sed,β (t) averaged from 0.727 ± 0.064 to 1.519 ± 0.142 mGy/y and D sed,γ (t) from 1.212 ± 0.016 to 2.987 ± 0.024 mGy/y. Namely, SCG's D sed,β (t) and D sed,γ (t) both exceed those seen in typical karst caves by 200-300%. In the fossils, the dentine and bone scavenged U from the uraniferous groundwater bathing the sediment, leading to high D den (t) measures. With high [U den ] near 130-160 ppm, D den (t) also contribute radiation at 2-4 times faster than typical seen in karst caves. Therefore, for some SCG teeth found in the horizons rich in bone where the highest D sed (t) occur, a viable maximum datable age may be as small as 0.25-0.8 Ma. Hominins living in SCG might have begun to experience medical effects from the high radiation rates within a few decades. People excavating in SCG should also be monitored with personal dosimeters.