40Ar/39Ar and ESR/U-series dates for Guado San Nicola,Middle Pleistocene key site at the Lower/Middle Palaeolithic transition in Italy

The Middle Pleistocene archaeological site of Guado San Nicola was discovered in 2005 in a fossil fluvial terrace of the Volturno River, close to the village of Monteroduni, Molise, Italy. Palaeontological remains and lithic artefacts, including both handaxes and Levallois, discoid and opportunistic debitage, were recovered in fluvial and slope sediments rich in volcanoclastic materials. This site includes four distinct human occupation levels. In two of them both “shaping-façonnage” and “knapping-débitage” technologies are highlighted, placing this site at the Lower/Middle Palaeolithic transition.In the present study, geochronological analyses by ⁴⁰Ar/³⁹Ar on single-crystal and ESR/U-series on teeth were performed to precise the chronological framework of the occupations. The ⁴⁰Ar/³⁹Ar data obtained securely bracket the human occupation levels at the transition between the interglacial and glacial marine isotopic stages MIS 11 (i.e. 400 ± 9 ka) and MIS 10 (i.e. 345 ± 9ka). The weighted mean age obtained from ESR/U-series dating of six teeth (i.e. 364 ± 36 ka) is in very good agreement with the ⁴⁰Ar/³⁹Ar results. The radio-isotopic constraints we presented place the Guado San Nicola site as one of the earliest testimonies of Levallois debitage in Western Europe and confirm the potential and accuracy of paleo-dosimetric methods to date Middle Pleistocene sites.     

ESR/U-series dating of faunal remains from the paleoanthropological site of Biache-Saint-Vaast (Pas-de-Calais,France)

Discovered accidently during factory building construction works, the Middle Palaeolithic site of Biache-Saint-Vaast (Pas-de-Calais, France) was excavated from 1976 to 1982 by a team of Lille University directed by Alain Tuffreau. An abundant archaeological and paleontological material, including two human skulls, was there recovered from fossil alluvial deposits of the Scarpe River. In order to determine the ages of these remains, the ESR/U-series method was applied on bone and teeth. As the U-series data obtained of the main part of the analyzed tissues do not allow the use of the classical US-ESR model, the recently proposed AU-ESR model, taking into account if necessary U-leaching from some of the tissues, was used to calculate combined ESR/U-series ages for these samples. The obtained ages suggest a MIS7 attribution to the faunal remains and permit an age of ca 240 ka to be assigned for the human remains and associated archaeological material, in accordance with the stratigraphic data and the large mammal associations.     

New ESR/U-series dates of the lowest Acheuleo-Yabrudian levels of Qesem cave

Qesem cave is a Middle Pleistocene site located close to Tel Aviv, Israel, assigned to the Acheuleo-Yabrudian Cultural Complex (AYCC) of the Lower Palaeolithic. The site provides rich assemblages of knapped flint, animal remains and some human teeth making it of particular interest. Its location in the Levantine corridor confers a major interest to the understanding of human dynamics during the Middle Pleistocene.A series of 6 herbivorous teeth from the base of the 11 m archaeological sequence of AYCC Qesem Cave was analysed by combined ESR/U-series method; this complements previous dating series carried out on teeth and on heated flints on higher parts of the sequence (Mercier et al., 2013; Falguères et al., 2016) The teeth were measured according a regular protocol for which each tissue was analysed by inductively coupled plasma-quadrupole mass spectrometry (ICP-Q-MS) for U-series using a combination of a Thermo iCAP-RQ mass spectrometer coupled to a Cetac Aridus III desolvator system, and ESR analyses were implemented on enamel tissue. All these data were combined in order to yield modelled ages.The goal was to find out whether there was a time gap between the carbonates layers dated to 420 ka (Gopher et al., 2010) and archaeological layers of the AYCC found throughout the cave's sequence. The new results yield ages ranging between 220 and 430 ka confirming a great antiquity and a long duration of AYCC in the Levant.     

New ESR and U-series dating at Caune de l'Arago,France: A key-site for European Middle Pleistocene

The Caune de l'Arago, located at Tautavel in the southern part of France, is one of the best documented Middle Pleistocene sites allowing a good understanding of human evolution in Europe. Since its discovery in 1829, the cave yielded more than 140 human remains associated with abundant lithic industries and thousands of faunal remains in a 10 m thick stratigraphical sequence divided in three complexes (Lumley et al., 2014). The Lower stratigraphic complex is only known from cores while the Middle and Upper ones can be divided into about 17 main archaeological levels indexed from the bottom to the top: from level Q to level A. Since 1981, a number of dates were done using practically all the methods available for dating Quaternary period up to 700 ka. U-series dates performed on the upper stalagmitic floor yielded a minimum age of 400 ka for human remains found in the underneath level G (Falguères et al., 2004). This age range confirmed the direct non-destructive gamma-ray age published more than 30 years before (Yokoyama and Nguyen, 1981). Recently, a methodological work was published on herbivorous teeth coming from different parts of the G level highlighting the difficulties to get reliable radiometric dates on a level so rich in bones and partly weathered by guano deposits or issues associated with carbonate accumulation (Han et al., 2010).This study presents new ESR/U-Series dates obtained on herbivorous teeth coming from the lowest excavated layers (Q–P levels) which yield an age range for the entire thick archaeological infilling. The new results suggest that the sedimentological levels containing lithic and faunal artefacts began to be deposited at 550 ka in the P–Q levels and a younger age of 350–400 ka for the F level. The P–Q levels which have yielded bifaces are one of the oldest evidence of the Acheulian culture in the southern part of Western Europe while the dates obtained on G level suggest a contemporaneity with la Sima de los Huesos and TD10 Dolina level at Atapuerca.     

ESR/U-series dating of Eemian human occupations of Northern France

Palaeolithic sites associated with the Eemian Interglacial (MIS 5e) are very rare in NW Europe, and especially in Northern France, where their preservation is restricted to very specific geological contexts, in association with carbonated tufa (Caours) or peat deposits (Waziers). In order to check the reliability of ESR/U-series method to date teeth recovered from archaeological levels in such specific geological environments, teeth were sampled on these two Middle Palaeolithic sites and systematic in situ dosimetry was performed using portable gamma spectrometer. The ESR/U-series ages obtained on the Caours site are very homogeneous allowing the calculation of a mean age equal to 125 ± 11 ka, in agreement with the geological age, mammal's association and other available geochronological data (U-series on tufa carbonate, TL on burnt flints, OSL on sediments), despite a relatively heterogeneous dosimetric environment (gamma dose rate ranging between ca 200–450 μGy/a). At Waziers, reducing (water logging) environment linked to the peat leads to very specific U-series data of the analysed teeth (U content lower than 0.1 ppm in all the dental tissues, evidence of leaching in some tissues), but the mean ESR/U-series age, 129 ± 4 ka, is also in agreement with the available geological and palaeoenvironmental data indicating that the dated teeth were associated with Late Glacial deposits prior to the climatic interglacial optimum. These two case studies then confirm the reliability of ESR/U-series method to date with good reliability and accuracy the archaeological levels linked to such relatively short climatic events (ca 10 ka).     

Dating results on sedimentary quartz,bones and teeth from the Middle Pleistocene archaeological site of Coudoulous I (Lot,SW France): A comparative study between TT-OSL and ESR/U-series methods

This study presents palaeodosimetric results from the Middle Pleistocene archaeological site of Coudoulous I (Lot, SW France). Nine sedimentary quartz samples (41–60 μm) have been analyzed using a multiple aliquot protocol based on the measurement of the TT-OSL signal. In addition, 7 teeth and 7 bones have been dated by combining the ESR method with U-series analyses. Both methods gave consistent age results allowing correlation of the Early Middle Paleolithic Human occupation of the site to MIS 6 and part of the Lower Paleolithic tools to MIS 7. Beyond the establishment of a radiometric chronology for the Coudoulous I sequence, this study focuses on the information extracted from the intercomparison of the methods. Our data suggest that 1) the TT-OSL signal is stable over at least the last 230 ka (considering the age range of the studied samples), 2) there are not significant problems of incomplete bleaching leading support to the applicability of the TT-OSL technique for sedimentary deposits associated with karstic contexts. This approach highlights the interest of combining luminescence and ESR/U-series methods to discuss the reliability of the dating results.     

Chronology of the Upper Pleistocene loess sequence of Havrincourt (France) and associated Palaeolithic occupations: A Bayesian approach from pedostratigraphy,OSL, radiocarbon,TL and ESR/U-series data

In connection with the future Seine-North Europe Canal (Seine-Scheldt), a large-scale rescue archaeological survey was conducted at Havrincourt (northern France) between 2008 and 2011. The discovery of several levels of Palaeolithic flint artefacts embedded in a relatively thick loess sequence (ca 6–7 m) preserved on a gentle slope facing North-East, resulted in a 6000 m² excavation. This opened the opportunity for a detailed pedosedimentary and interdisciplinary geochronological survey (¹⁴C, optically stimulated luminescence, thermoluminescence, ESR/U-series) that has allowed us date the sequence reliably. On the basis of these results we propose this sequence as a new pedostratigraphic and archaeological reference sequence for northern France. We present here the optical dating of the sequence performed on fine (4–11 μm) quartz grains extracted from 17 samples. The luminescence characteristics of these extracts indicate that the single-aliquot regenerative dose optically stimulated luminescence (SAR-OSL) procedure that was applied is well suited. A consistent set of optical ages was obtained for the loess deposited up to around 70 ka ago. Independent age control (pedostratigraphy, ¹⁴C, ESR/U-series dates) allowed us to apply a Bayesian approach to build a chronometric model. This in turn enabled a regional chronostratigraphic framework to be built, to constrain the correlations with neighboring regions (northern France and Belgium) and to calculate a precise age for the four Palaeolithic levels discovered, including a unique occupation related to the early Upper Palaeolithic which was previously unknown in the area.     

Testing mathematical uranium migration models in combined ESR/U-series dating of fossil teeth from open-air sites

Several mathematical models have been suggested to describe uranium migration kinetics for ESR dating of fossil tooth. In the present study, different U-uptake models were used to calculate ages for fossil teeth from two open-air Paleolithic sites in Nihewan Basin – Donggutuo and Hougou and compared with independent age control of the sites. For Donggutuo site, the results show that the ages of three teeth calculated by EU and AU models were much younger than magnetostratigraphic age. At Hougou site, two teeth from the top of Nihewan lacustrine deposits give the consistent EU and AU ages within errors but younger than sediment OSL age. This study indicates that mathematical modeling could not always give convincing results for the open-air sites, which may experience dramatic hydrological variations and high erosion rates.     

Coupled ESR and U-series dating of early Pleistocene Gigantopithecus faunas at Mohui and Sanhe Caves,Guangxi, southern China

Several caves of the Guangxi Zhuang Autonomous Region, southern China, have delivered Gigantopithecus blacki remains, an extinct Pleistocene giant ape, in association with abundant mammalian faunas. To determine their geological ages, fossil teeth from Mohui and Sanhe Caves were dated using the coupled ESR/U-series method. The teeth from Mohui Cave gave age estimates of 1.69 ± 0.22 Ma and 1.29 ± 0.11 Ma. The Sanhe Cave samples had age estimates ranging from 910 ± 200 ka to 600 ± 150 ka with error weighted mean ages of 890 ± 130 ka and 720 ± 90 ka for the layers 5 and 4, respectively. Our results and previous paleomagnestism data place the Gigantopithecus fauna at Mohui Cave between Olduvai and Jaramillo subchrons and suggest that it was coeval with Chuifeng, Longgupo and Liucheng assemblages. The Sanhe fauna is younger, of late early Pleistocene age, and can be dated to the period between Jaramillo subchron and B/M boundary.     

ESR dating in Song Terus cave (East Java, Indonesia)

Electron spin resonance (ESR) dating on teeth has been used to determine the antiquity of the human presence in the area of Gunung Sewu (East Java, Indonesia). The Song Terus cave is one of the archaeological sites located in this area, which has yielded an important collection of fossil bones and prehistoric lithic industry. ESR dating shows that this cave belongs to a karstic system which has been in place since Middle Pleistocene. The results of coupled ESR/Uranium-series dating indicates closed system for uranium in tooth tissues yielding ages of between 216 and 392 ka for the oldest archaeological levels, confirming the possibility of finding Middle Pleistocene human occupations in such karstic sites in Java.     

Radiometric dates for the Middle Palaeolithic sequence of Payre (Ardèche, France)

The archaeological site of Payre (France) yielded Middle Palaeolithic layers with Neanderthal remains, which are partly bracketed by two stalagmitic flowstones. To obtain a precise chronological framework for the human occupation and the faunal remains of this site, several dating methods were used: uranium-series (U-series by alpha spectrometry and TIMS) and electron spin resonance (ESR) on stalagmitic flowstones, combined ESR/U-series on teeth and bones, and thermoluminescence (TL) on burnt flints. Most of the ages obtained for levels H to C range from 300 to 140 ka. The age-spread obtained for the anthropic (burnt flints) and faunal remains (bones and teeth) suggests that the dated levels represent several human occupation periods, near the transition from MIS8 to MIS7 (levels G–F) and from MIS6 to MIS5 (levels D–E).     

Applying a Bayesian approach for refining the chronostratigraphy of the Yumidong site in the Three Gorges region,central China

Yumidong (Corn Cave) is a newly discovered Paleolithic site in the Three Gorges region of central China. Numerous Paleolithic artifacts have been excavated from the sedimentary deposits of the cave in association with faunal remains attributed to the Middle-Late Pleistocene Ailuropoda-Stegodon fauna of southern China. To establish the chronology of the sedimentary sequence (>5 m thick), ¹⁴C dating was applied to bone and charcoal samples (n = 6); the U-series method was used to date in situ precipitated speleothems (n = 12), transported speleothem samples (n = 6) and 18 subsamples of a fossil tooth; and the coupled ESR/U-series method was used to date fossil teeth (n = 6). The derived dates were combined using a hierarchical Bayesian approach to generate a unified chronostratigraphy for the Yumidong sequence. In our Bayesian analyses, the ¹⁴C and coupled ESR/U-series dates were considered to provide direct age estimates for the target layers, while the U-series dates of the in situ precipitated speleothems and fossil tooth were used as minimum age constraints and those of the transported speleothem fragments as maximum age constraints. The Bayesian analyses provided robust time intervals for the archeological layers: L2-Upper (14–23 ka), L2-Lower (27–63 ka), L3 (106–171 ka), L4 (140–192 ka), L10 (157–229 ka), L11 (181–256 ka), and L12 (214–274 ka) with a probability of 95%, allowing the establishment of a ∼300 ka long geological and archeological history for the Yumidong site and placing it as a reference site for Paleolithic cultural evolution in the Three Gorges region from the late Middle Pleistocene to Late Pleistocene.     

ESR chronology of the fluvial sequence of Cueva del Silo (Sierra de Atapuerca,Spain)

The Cueva del Silo is part of the lower karst level of the Cueva Mayor-Cueva del Silo karst system (Sierra de Atapuerca, Burgos, Spain), whose evolution has preserved an impressive archeo-paleoanthropological sequence since the Early Pleistocene. Cueva del Silo is remarkable for the presence of fluvial deposits that record the entry of the Arlanzón River water in the cave system, providing a key sequence to investigate the fluvio-karstic relationships that give rise to this endokarst system. In order to provide a chronological framework to these fluvial deposits, six sediment samples were dated by Electron Spin Resonance and 16 samples by paleomagnetism, collected from two outcrops: Galería de las Arenas and Sala del Caos. Our results provide maximum mean age of around 1600 ka and a minimum age of 916 ± 136 ka for the deposits in Sala del Caos. The younger date from Sala del Caos might represent the last fluvial input from Arlanzón River in the lower karst level. In contrast, ESR ages estimates of 1268 ± 133 ka and 1262 ± 108 ka were obtained for Galería de las Arenas sequence, which could indicate re-sedimentation processes from the intermediate karst level where similar ages were published. The annual dose might be wrongly assessed due to the re-sedimentation processes inside the karst, hence, ESR ages for these facies should be treated with extreme caution.     

Radiometric dating of Meipu hominin site in China by coupled ESR/U-series and cosmogenic 26Al/10Be burial dating methods

The Three Gorges and Western Hubei area in the geographic central part of China was a potential migration corridor for early hominin and mammals linking South and North China during the Pleistocene period. Some key early hominin sites are known in this region where limestone cave and fissure sites are numerous but difficult to date as beyond the dating range of OSL and mass spectrometry U-series method. Here, we report radiometric dating study for such a hominin site, Meipu (Hubei Province), by coupled ESR and U-series dating of nine fossil teeth and cosmogenic ²⁶Al/¹⁰Be burial dating of one quartz sediment. The burial age calculated by simple burial model (573 ± 266 ka) gives a minimum age constraint of the sediment. The fossil dating provided two main age groups at 541 ± 48 ka and 849 ± 39 ka, the older age group is in agreement with the U-series age (>630 ka) of the flowstone overlying the fossil layer and the paleomagnetic data which placed the Brunhes-Matuyama boundary in the fossil layer. The reason of this age difference is probably caused by the U-content discrepancy in the enamel of the dated fossil samples. This study exhibits the limitation of ESR/U-series fossil dating and the importance of using multiple dating approach when it is possible in order to identify the problematic ages.     

Glacial advances and ESR chronology of the Pochengzi Glaciation,Tianshan Mountains,China

The Pochengzi Glaciation is a typical glaciation in Quaternary in the Tianshan Mountains. The glacial landforms comprise several integrated end moraines, like a fan spreading from the north to the south at the mouth of the Muzhaerte River valley and on the piedmont on the southeastern slope of the Tumur Peak, the largest center of modern glaciation in the Tianshan Mountains. The landforms recorded a complex history of the ancient glacier change and contained considerable information of the glacial landscape evolution, and dating these landforms helps us understand the temporal and spatial shifts of the past cryosphere in this valley and reconstruct the paleoenvironment in this region. Electron spin resonance (ESR) dating of the glacial tills in the upper stratum from a well-exposed section, end moraines, and associated outwashes was carried out using Ge centers in quartz grains, which are sensitive to the sunlight and grinding. The results could be divided into three clusters, 13.6–25.3, 39.5–40.4 and 64.2–71.7 ka. Based on the principle of geomorphology and stratigraphy and the available paleoen- vironmental data from northwestern China, the end moraines were determined to deposit in the Last Glaciation. The landforms and the three clusters of ages demonstrate that at least three large glacial advances occurred during the Pochengzi Glaciation, which are corresponding to marine oxygen isotope stage 4 (MIS4), MIS3b and MIS2. The landforms also indicate that the gla- ciers were compound valley glacier in MIS2 and MIS3b and piedmont glacier in MIS4, and the ancient Muzhaerte glacier were 94, 95 and 99 km at their maximum extensions in these three glacial advances.     

Chronology of Upper Pleistocene sequences at Sidi Messaoud (wadi Noun, southwestern Morocco) based on C, optical and U-series dating

The Sidi Messaoud site provides a 40 m thick sedimentary infilling of great importance for the study of environmental changes and landscape evolution during the Upper Pleistocene in the Western Sahara. In this study, chronological results obtained by the ¹⁴C, U-series and optical dating techniques are compared. Each individual age, depositional environment and post-deposition history of the various samples are evaluated, in order to explain the methodological reasons for discordances and concordances. Stratigraphic and palaeoenvironmental implications for the last 120 ka history in southwestern Morocco are also inferred: the wadi Noun gorges had been incised before MIS 5, and later, during MIS 3, the valley has been filled in with a mean sedimentation rate of 1 m/ka.     

Element enrichment and U-series isotopic characteristics of the hydrothermal sulfides at Jade site in the Okinawa Trough

The geochemical and U-series isotopic characteristics of hydrothermal sulfide samples from the Jade site (127°04.5′E, 27°15′N, water depth 1300–1450 m) at Jade site in the Okinawa Trough were analyzed. In the hydrothermal sulfide samples bearing sulfate (samples HOK1 and HOK2), the LREEs are relatively enriched. All the hydrothermal sulfide samples except HOK1 belong to Zn-rich hydrothermal sulfide. In comparison with Zn-rich hydrothermal sulfides from other fields, the contents of Zn, Pb, Ag, Cd, Au and Hg are higher, the contents of Fe, Al, Cr, Co, Ni, Sr, Te, Cs, Ti and U lower, and the ²¹⁰Pb radioactivity ratios and ²¹⁰Pb/Pb ratios very low. In the hydrothermal sulfide mainly composed of sphalerite, the correlations between rare elements Hf and U, and Hf and Mn as well as that between dispersive elements Ga and Zn, are strongly positive; also the contents of Au and Ag are related to Fe-sulfide, because the low temperature promotes enrichment of Au and Ag. Meanwhile, the positive correlations between Fe and Bi and between Zn and Cd are not affected by the change of mineral assemblage. Based on the ²¹⁰Pb/Pb ratios of hydrothermal sulfide samples (3.99×10⁻⁵-5.42×10⁻⁵), their U isotopic composition (²³⁸U content 1.15–2.53 ppm, ²³⁸U activity 1.07–1.87 dpm/g, ²³⁴U activity 1.15–2.09 dpm/g and ²³⁴U/²³⁸U ratio 1.07–1.14) and their ²³²Th and ²³⁰Th contents are at base level, and the chronological age of hydrothermal sulfide at Jade site in the Okinawa Trough is between 200 and 2000 yr. Supported by the Pilot Project of Knowledge Innovation Project, Chinese Academy of Sciences (Grant No. KZCX2-YW-211), National Natural Science Foundation of China (Grant Nos. 40830849, 40176020), and Special Foundation for the Eleventh Five Plan of COMRA (Grant No. DYXM-115-02-1-03)     

On the interest and the limits of using combined ESR/U-series model in the case of very late uranium uptake

Before ESR/U-series models were applied to ungulate teeth, it is generally assumed that calculated ages are close to the “real” ages when they range between the values obtained with the EU and LU models. When the samples show signs of uranium-leaching, a realistic mathematical modelling of the age is not possible. On the other hand, some samples have undergone a very late U-uptake “affecting” directly the U-series ages and rendering the calculated ages younger.

We present results obtained on two open-air sites, which show the limits of such combined models in the case of late uranium incorporation. For the Middle Pleistocene site of Isernia la Pineta (Italy), the ESR dates are expected to fall within the limits imposed by the Ar–Ar results. The other site, the Lower Palaeolithic deposits of Ambrona (Spain), yielded two sets of samples: some at the top of the sequence with uranium uptake ranging between EU and LU limits and thus giving a good idea of the “real” age of the site; others at the base displaying a very late U-uptake, which render an age determination difficult.     

ESR dating of sedimentary quartz: Possibilities and limitations of the single-grain approach

Usually, multiple-grain aliquots are used for electron spin resonance (ESR) dating of sediments. However, this approach excludes the ability of detecting insufficient bleaching, as would be the case when measuring several single aliquots or single grains. In this paper, we present preliminary results of single-grain ESR dating experiments on three different sedimentary deposits (fluvial, fluvio-aeolian and desert aeolian), ranging in age between 100 ka and 2 Ma. Titanium-related impurity centres (Ti–Li and Ti–H) were measured in a Q-band ESR spectrometer for estimating equivalent doses (D_e) of individual quartz grains (0.5–1 mm). Both additive and regenerative dose methods were used. The resulting D_e plots show a large scatter in D_e—from 100 Gy to more than 600 Gy—for the fluvial sample, probably reflecting different bleaching histories of the grains. On the contrary, a clear plateau can be observed in D_e for sands with a strong aeolian component. Preliminary single-grain ESR ages are in relatively good agreement with independent estimates, the best results being obtained for the desert aeolian sample (200 ka). It is concluded that ESR has a promising potential for estimating large naturally accumulated doses of well-bleached sediments. Nevertheless, the method is challenged by several experimental difficulties, such as long measurement times, excessive errors, and presumably a complex interrelationship between Ti–Li and Ti–H components.     

OSL chronology of Quaternary terraced deposits outcropping between Mt. Etna volcano and the Catania Plain (Sicily,southern Italy)

In this paper we tested the applicability of the Optically Stimulated Luminescence (OSL) technique through Single-Aliquot Regenerative-dose (SAR) protocol, on single grain quartz extracted from alluvial–coastal sediments. Five samples were collected from deposits belonging to a flight of seven orders of coastal–alluvial terraces outcropping in the area between Mt. Etna volcano and the Catania Plain (Sicily, southern Italy), at the front of the Sicilian fold and thrust system. After various performance tests, we obtained OSL ages ranging between 240 ± 12 and 80 ± 4 ka, consistent with the normal evolutionary model of a terraced sequence, moving from the highest to the lowest elevation. Obtained data allowed us to determine a mean uplift rate of 1.2 mm/year during the last 240 ka, mostly related to regional uplift processes coupled with sea-level changes. Moreover, terraces belonging to the two highest orders are folded, forming a large anticline. According to our results, the frontal thrust of the Sicilian chain was active between 236 and 197 ka ago, even though seismological and geodetic data suggest current activity to the back.