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Mass spectrometric U-series dating of the Chaoxian hominin site at Yinshan, eastern China
Quaternary International 211 (2010) 24–28
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Quaternary International journal homepage: www.elsevier.com/locate/quaint
Mass spectrometric U-series dating of the Chaoxian hominin site at Yinshan, eastern China Guanjun Shen a, *, Yingshan Fang b, James L. Bischoff c, Yue-xing Feng d, Jian-xin Zhao d a
College of Geographical Sciences, Nanjing Normal University, Nanjing, Jiangsu 210046, China Nanjing Museum, Nanjing, Jiangsu 210016, China c US Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, USA d Radiogenic Isotope Laboratory, Centre for Microscopy and Microanalysis, University of Queensland, Brisbane, QLD 4072, Australia b
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 11 March 2009
The fossils of Chaoxian hominin, widely accepted as representing archaic Homo sapiens in eastern China, were recovered from the middle or slightly higher levels of Layer 2 deposits of a collapsed cave at Yinshan, Anhui Province. Results of mass spectrometric U-series dating of intercalated speleothem calcites are presented. Based mainly on four broadly coeval calcite samples, the hominin fossils should be bracketed in the range of 310–360 ka or somewhat older. These ages are much older than the previous estimate at 160–200 ka based on the U-series dating of fossil teeth and bones, and may be cited as supporting evidence for an earlier H. erectus–archaic H. sapiens interface in China. Ó 2009 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction The Chaoxian hominin fossils were recovered from a collapsed limestone cave (31320 5500 N, 117 5105100 E) at Yinshan (Silvery Hill), a small hill after which the nearby village is named. The site is located about 7 km south of Chaohu City in Anhui Province, eastern China (Fig. 1). In 1981 mammalian fossils were first discovered by a group of geologists when carrying out fieldwork in the region. After being informed of the discovery by an amateur, researchers from the Institute of Vertebrate Paleontology and Paleoanthropology, Academia Sinica and the Institute of Archaeology of Anhui Province organized two seasons of excavation in 1982 and 1983 respectively. Hominin fossils including an incomplete occipital, a fragmentary maxillary and three isolated teeth as well as an abundance of mammalian fossils were unearthed (Xu et al., 1984, 1986; Wu and Poirier, 1995; Bailey and Liu, 2010). The site is composed of two groups of fossiliferous deposits at Locus A and Locus B respectively, situated at about the same level but separated by a limestone ridge 2–4 m thick. Based on associated mammalian fossils of 11 species including Hyaena brevirostris licenti, Megantereon sp., Tetralophodon sp. and Proboscidipparion sp., the Locus A deposits were attributed to the Early Pleistocene. With a faunal assemblage of 13 species including Hyaena brevirostris sinensis, Stegodon, Sus xiaozhu and Megaloceros pachyostus, the
* Corresponding author. Tel.: þ86 135 12501968; fax: þ86 25 85891347. E-mail address: [email protected] (G. Shen). 1040-6182/$ – see front matter Ó 2009 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2009.02.020
Locus B deposits were attributed to the Middle Pleistocene, approximately equivalent to Layers 1–4 of the Zhoukoudian Locality 1 or somewhat younger (Xu et al., 1984, 1986). No trace of cultural remains was reported. The hominin fossils were retrieved from heavily consolidated Layer 2 at Locus B. Though bearing some morphological features indicative of Homo erectus, Chaoxian hominin has been widely accepted as a type representative of archaic H. sapiens in eastern China (Xu et al., 1984, 1986; Wu and Poirier, 1995). Its precise dating may contribute to addressing issues concerning the mode of Middle Pleistocene human evolution in East Asia. The first effort to date the Chaoxian hominin site was made by Chen et al. (1987). Among the nine fossil teeth and bones analysed, four gave concordant 230Th/234U and 231Pa/235U dates. Based on these results a time range of 160–200 ka was proposed. As well, ESR dating of a deer tooth from the hominin fossil-bearing deposits was performed, giving a slightly older linear uranium uptake (LU) age of 212 30 ka (Liang et al., 1995). In Chen et al. (1987), almost equivalent dates of 150–190 ka were assigned to the nearby site of Hexian hominin, widely accepted as a later representative of H. erectus (Wu and Dong, 1982). The scenario of an apparent contemporaneity between the two fossil specimens prompted hypotheses concerning the mode of Middle Pleistocene human evolution in China (Chen and Zhang, 1991). The hominin fossil-bearing deposits are intercalated with localized speleothem formations. From a fragmented flowstone layer considered as ‘‘capping flowstone’’ three calcite samples were taken. Tripled alpha spectrometric 230Th/234U determinations were
G. Shen et al. / Quaternary International 211 (2010) 24–28
25
stratified, consolidated, containing a few mammalian fossils, 0.2–0.4 m thick. Layer 4 Sand, yellowish, stratified, slightly consolidated, containing a few fossil debris and rodent teeth, an exposed section ca. 80 cm thick, total thickness unknown.
Fig. 1. Map showing the location of Yinshan, the burial site of Chaoxian hominid and the neighbouring Hexian Homo erectus locality at Longtandong, which is discussed in the text.
carried out on the best (ACY-2) of the three samples judging by purity and density, their weighted mean (310þ45 32 ka, 1s) was proposed to represent the minimum age of the hominin fossils. Four calcite, three fossil and one secondary phosphate samples were analyzed and gave compatible age results. This indicates a much older Chaoxian hominin than previously estimated (Shen et al., 1994). To further constrain the chronological position of Chaoxian hominin, fieldwork was organized in March 2006 and April 2008. Stratigraphic studies of the extant deposits were carried out after clearing off a few centimetres of surficial deposits. Additional samples were collected from the speleothem formations recently exposed due to natural weathering processes or revealed during the cross-section clearance. The samples were analyzed using thermal ionization mass spectrometry (TIMS), which greatly improves the analytical precision with calcite samples of sub-gram size. The results obtained are presented here, with a brief discussion on their possible implications on the mode of Middle Pleistocene human evolution in China. 2. Samples in stratigraphic position On the basis of former descriptions (Xu et al., 1984, 1986; Shen et al., 1994), the extant cross-section was re-examined and described as follows (Fig. 2, numbers from top to bottom): Layer 1 Loose grey loam between big limestone blocks, containing a few mammalian fossils. Layer 2 Sandy clay supported limestone breccia, with a few marl and sandstone gravels, with big limestone blocks (>50 cm each side) mostly at the uppermost levels, brownish except for a purplish, more sandy, 30–50 cm thick sub-layer at central lowermost position, strongly consolidated by carbonate, rich in mammalian fossils but mainly at middle–upper levels, 0.5–2.3 m thick. Layer 3 Clayey sand, with occasional small weathered gravels, generally yellowish but with brownish and purplish patches,
Efforts were made to collect pure, well-crystallized calcite samples from well-defined stratigraphic context, which is key to a successful dating using speleothem formation as a temporal marker. Altogether 12 calcite samples were taken (including two from the previous study, Shen et al., 1994). Their positions on the cross-section are shown in Fig. 2. As the key sample of the previous study, ACY-2 was re-analyzed for better precision. During the 2006 and 2008 fieldwork, three more samples at about the same level were collected. At the left upper–middle part of Layer 2, a localized flowstone with an expanse of about 10 20 cm, 2–5 cm thick, with clear laminae and natural form was revealed. This flowstone is mostly composed of muddy calcite, but a central sub-layer, 0.2–0.8 cm thick, is quite pure and dense, from which the sample CYS-2 was collected. About 40 cm to its lower right side, CYS-8 was taken from a disc-like calcite formation, ca. 12 cm in diameter and ca. 5 cm thick. About 80 cm to the upper right side of CYS-8, CYS-3-1 was taken from a small stalagmite-like formation, 3.0–3.5 cm in diameter and ca. 7 cm high, quite pure and dense. Judging by the fine but semicircular laminae, it is an in situ formation from seeping groundwater rather than, as it appears, a stalagmite precipitated from dripping water. Five samples were collected from two speleothem formations at the boundary between Layers 2 and 3. CYS-4-2 was taken from a pure and dense sub-layer of a stalagmite-like formation, which is developed upon consolidated deposits and is ca. 12 cm in diameter and ca. 20 cm high. Just above CYS-4-2 a small disc-like drip-stone was taken as CYS-4-1. CYS-5-1, CYS-5-2 and CYS-5-3 were taken from upper, middle and lower sub-layers of a localized flowstonelike formation, which is ca. 25 cm wide and ca. 20 cm thick, quite pure and dense. This should be the same formation dated as ACY-5 and ACY-6 in the previous study (>448 ka and 376þ211 66 ka respectively, 1s), but mistaken for a fallen stalagmite at that time. Careful re-study of its form and laminae lead to its re-definition as an in situ flowstone-like formation. CYS-7 and ACY-13 were taken from translucent calcite crystal bundles between limestone fragments overlying and in the middle of Layer 2 respectively. ACY-8 was taken from calcite crystal aggregates wrapping a cervid mandible at an uppermost level of Layer 2 in the previous study with alpha spectrometry (weighted mean of doubled analyses 215þ15 13 ka, 1s). Without any depositional laminae, these calcite formations should have posteriorly precipitated in small cavities from percolating groundwater. The hominin fossils were discovered in the course of wellorganized excavations. In spite of this, no record about their precise provenience is now available. Judging from Fig. 3 of Xu et al., (1984, copied here as attached figure B of Fig. 2), the hominin occipital was retrieved from the middle section of Layer 2; while Fig. 1 of Xu et al. (1986, attached figure A3 of Fig. 2) shows that the hominin maxillary was recovered from a slightly higher position. Recently, we (or the first author) discussed this issue with the excavator, who assured us of the exactness of the published figures. Through close observation, the previously published cross-sections (A2, A3 and B of Fig. 2) correlate quite well with the extant depositional sequence (Fig. 2). This renders support to an absence of significant facies change of the Locus B deposits, which in turn justifies the representativeness of the extant cross-section for the excavated hominin fossil-bearing deposits. So, it seems reasonable to allocate on the preserved cross-section an area lower than CYS-3-1, about the
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G. Shen et al. / Quaternary International 211 (2010) 24–28
Fig. 2. Sketch showing the depositional sequence of the Chaoxian hominid site (Locus B). Here the encircled fragmentary maxillary denotes the most probable provenience of the hominid fossils. Below the main figure, the attached figures (A1,2,3) are adapted from Fig. 1 of Xu et al. (1986). A1 gives the plane figure of hominin fossil-bearing Locus B, but without nearby Locus A which locates 2–4 m to the lower left. A2 and A3, corresponding to lines a–a0 and b–b0 in A1, respectively, give cross-sections revealed during the 1983 excavation, while the extant cross-section corresponds to line d–d0 . The provenience of the hominin maxillary is shown in A3. The attached figure (B) is Fig. 3 of Xu et al., (1984), demonstrating the cross-section revealed during the 1982 excavation and the hominin occipital’s burial position. During the season, mainly Locus A deposits were excavated, only a small section and upper Layers 1 and 2 of Locus B (line c–c0 in A1) were explored. A higher Locus B sequence and a lower Locus A sequence as shown in the figure is only a seemingly reasonable deduction from faunal correlation. The two depositional sequences are situated at about the same level.
same level as or slightly lower than CYS-2 and CYS-8, as the most probable provenience of the hominin fossils. That the hominin fossils came from the middle or slightly higher levels of Layer 2 was also confirmed by two elderly local villagers who witnessed the discoveries as field workers. Moreover, this is in agreement with field observations that the fossils are mainly concentrated in middle–upper levels of Layer 2. 3. Dating results and interpretation The purest possible portions of a sample were selected out, reduced into small grains, cleaned ultrasonically and then hand-picked to avoid porous and less pure pieces. U-series age determinations were performed with TIMS at the University of Queensland (Zhao et al., 2001). The isotopic data and calculated ages are presented in Table 1. ACY-2 gives a result of 351þ16 14 ka, which is consistent within counting statistics with the previous alpha spectrometric determinations. This is also consistent with the results of CYS-2 þ9 (359þ14 12 ka) and CYS-8 (3528 ). As the latter two samples were assuredly taken from the upper–middle section of Layer 2, so the previous recognition that ACY-2 was taken from ‘‘capping flowstone’’ is put into question. The ‘‘capping flowstone’’ no longer exists, due either to natural weathering or to vandalism. However, the underlying limestone block remains, which was then considered as above Layer 2. On its surface are patches of remnant
consolidated deposits, similar to Layer 2 deposits in appearance. This indicates the existence of overlying deposits, which were most probably removed during previous excavations. So instead of being on the surface, the limestone block should be originally embedded in Layer 2, as was the flowstone partly overlying it and taken as ACY-2. With their stratigraphic positions as discussed above, the hominin fossils should be broadly contemporaneous or somewhat older than the three samples. The paucity of fossils in lower section of Layer 2 supports the judgment that the hominin fossils should not be much older than 360 ka. Compared with the above three samples, CYS-3-1 yields a slightly younger date of 309 5 ka. This is in line with the statigraphic order and may possibly define a minimum age for the hominin fossils. On the other hand, the difference between CYS-3-1 and the three samples discussed above may imply a relatively slow sedimentation rate. If so, climate fluctuations and changes in geochemical regime should have left imprints in the deposits. However, no sign of bedding can be identified for middle–upper horizons of Layer 2. This issue must remain open to later sedimentological study of the site. All the five samples at the Layers 2–3 boundary yield dates near the upper limit of mass spectrometric U-series dating, in agreement with the previous alpha spectrometric dates on ACY-5 and ACY-6. From weighted means of the five U–Th isotopic ratios an age of 598þN 93 ka is derived. With an infinite upper bound, this result can hardly define a maximum age limit for the hominin fossils. On the
G. Shen et al. / Quaternary International 211 (2010) 24–28 Table 1 Mass spectrometric U–Th isotopic ratios and
230
27
Th age results. Th/232Th
234
0.4054 0.0003 0.3006 0.0002 0.2775 0.0002
33.40 585.1 16.72
1.0027 0.0015 1.0151 0.0013 1.0737 0.0015
0.8313 0.0033 0.8259 0.0034 0.8984 0.0057
194 2 189 2 234 5
CYS-3-1 ACY-2
0.3814 0.0003 0.4118 0.0006
410.6 83.70
1.0668 0.0012 1.0677 0.0026
0.9599 0.0025 0.9838 0.0049
309 5 359þ16 14
CYS-2
2.8089 0.0072
876.7
1.1119 0.0017
0.9929 0.0052
351þ14 12
CYS-8
1.3826 0.0011
80.03
1.0970 0.0012
0.9895 0.0033
352þ9 8
CYS-4-1 CYS-4-2
0.1952 0.0001 0.2000 0.0001
58.16 135.3
1.0439 0.0016 1.0367 0.0017
1.0176 0.0049 1.0125 0.0042
>572 651þN 106
CYS-5-1 CYS-5-2 CYS-5-3
0.1986 0.0001 0.2529 0.0003 0.3126 0.0004
78.66 103.8 277.2
1.0707 0.0014 1.0602 0.0018 1.0610 0.0015
1.0219 0.0036 1.0240 0.0035 1.0120 0.0035
571þ97 52 >604 496þ42 31
1.0035 0.0015
0.9991 0.0036
659þN 110
Sample
U (ppm)
ACY-8 CYS-7 ACY-13
HU-1_020
690.5 0.8
230
32235
U/238U
230
230
Th/234U
230
Th age (ka)
230
Th age (ka, corrected)
191 2 228 5
All isotopic ratios shown are in radioactivity with 2s errors. Half-lives of Th and U are 75,380 and 244,600 a, respectively. For samples with 230Th/232Th ratio less than 50, corrected 230Th ages are given, assuming the initial 230Th/232Th atomic ratio of 4.4 2.2 106. This is the value for a material at secular equilibrium and with a bulk earth 232 Th/238U value of 3.8. The errors are arbitrarily assumed to be 50%. HU-1 is a uraninite solution used as an international standard for secular equilibrium of 238U–234U–230Th series.
other hand, its lower bound points definitively to a time gap of at least 140 ka between the two speleothem horizons. Further efforts should be made to search for additional calcite samples along the Layer 2 cross-section for a better understanding of the rate of sedimentation and for a tighter age constraint of the hominin fossils. For the moment, it is suggested that the above time gap is mainly occupied by the Layers 2–3 boundary or possibly by the boundary between the brownish and purplish deposits in Layer 2. Being taken from secondary calcite crystals, CYS-7, ACY-8 and ACY-13 should mark the minimum age of the horizon of their formation. So the result for CYS-7 indicates that the Layer 1 deposits between big limestone blocks and consequently the underlying Layer 2 deposits should be formed no later than 191 2 ka. This is further supported by ACY-8 which dates to 189 2 ka. So the hominin fossils should be older than 190 ka even if their provenience corresponds to the uppermost level of the extant cross-section. Considering its position, no further age constraint can be inferred from the result for ACY-13 (228 5 ka). In conclusion, while the hominin fossils cannot be younger than 190 ka, their best age estimate resides in the range of 310–360 ka or somewhat older. This is in general consistency with the previous study using alpha spectrometry (Shen et al., 1994). However, the discrepancy between the U-series dating of calcite samples and the U-series and ESR dating of fossil materials (Chen et al., 1987; Liang et al., 1995) cannot be accounted for by counting statistics. 4. Implications for Middle Pleistocene human evolution Concerning the phylogenetic relationship between H. erectus and archaic H. sapiens, Chinese anthropologists and some of their international colleagues have long believed that in East Asia as in other regions of the old continent the two hominin species were temporally and morphologically related as ancestor and descendant. However, this classical approach has been challenged in recent years. For example, Rightmire (1998) proposed that African and European Middle Pleistocene hominins constitute a new species named H. heidelbergensis, which may give birth to modern H. sapiens in Africa and to H. neanderthalensis in Europe. It is also suggested that contemporaneous hominin finds in China such as Yunxian, Dali, Jinniushan and Maba may be regarded as eastern representatives of H. heidelbergensis. As another example, based on apparent contemporaneity between Skull No. 5 of Peking Man (late H. erectus) and Jinniushan and Dali hominins (archaic H. sapiens) in
234
northern China and between Hexian and Chaoxian hominins in eastern China (Chen et al., 1987; Chen and Yuan, 1988), Chen and Zhang (1991) proposed a possible overlapping existence in China of the two hominin species. In addition, while the appearance of archaic H. sapiens dates to 600 ka in Africa, the H. erectus–archaic H. sapiens interface in China has been assigned to a far later time around 200 ka. This scenario lead to a hypothetical theory about a slower evolution rate of Asian H. erectus compared with their African counterparts (Clark et al., 1994). The above hypotheses have been largely established on the basis of the previous temporal framework for Chinese sites, of which the U-series dating of fossil bones has been prominent. Bones are known to be susceptible to post-burial U migration, leading generally to underestimated dates in a cave setting (e.g. Bischoff et al., 2003; Rae et al., 1989; Shen, 2007). On the other hand, the excellent reliability of the U-series dating of pure, compact and well-crystallized speleothem calcites has been well demonstrated (e.g. Edwards et al., 1997; Ludwig and Renne, 2000; Richards and Dorale, 2003). In the past w10 years with the U-series dating of intercalated calcite formations, the chronology of several key Middle Pleistocene sites has been studied, including Localities 1, 4 and 15 at Zhoukoudian (Shen et al., 2001,2004a,b), Nanjing hominin site (Zhao et al., 2001) and Sima de los Huesos (Bischoff et al., 2003). A general tendency is evident that the new and more robust age results are much older than the ones derived from fossil bones. The site of Chaoxian hominin may be cited as one more example of such a tendency. The results of this paper, with a best age estimate at 310–360 ka or somewhat older for Chaoxian hominin, are of importance in providing a benchmark for Middle Pleistocene human evolution and in indicating a H. erectus–archaic H. sapiens interface in China much earlier than 200 ka. Based on morphological and biostratigraphical studies Wu (1989) placed Chaoxian hominin in the middle of an age sequence for important finds of Chinese archaic H. sapiens, which stands as Dali, Jinniushan, Chaoxian, Xujiayao and Maba. If proved, an even earlier H. erectus–archaic H. sapiens interface in the region may be inferred. Adjustments should be made to the models of human evolution to accommodate the new chronological evidence. Acknowledgements This project was jointly supported by National Natural Science Foundation of China (40373031) and Leakey Foundation (2005/
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2006 general grant). TIMS U-series analyses were supported by ARC Project LP0453664. We would like to thank the government of Chaohu City for assistance in fieldwork, Drs Wu Liu and Christopher Norton for helpful discussions, the first author’s graduate students, Hong Wang, Bin Gao and Li Zhang for assistance in sample collection and preparation, and three anonymous reviewers for constructive criticisms. References Bailey, S.E., Liu, W., 2010. A comparative dental metrical and morphological analysis of a Middle Pleistocene hominin maxilla from Chaoxian (Chaohu), China. Quaternary International 211, 14–23. Bischoff, J.L., Shamp, D.D., Aramburu, A., Arsuaga, J.L., Carbonell, E., Bermudez de Castro, J.M., 2003. The Sima de los Huesos hominids date to beyond U/Th equilibrium (>350 kyr) and perhaps to 400–500 kyr: new radiometric dates. Journal of Archaeological Science 30, 275–280. Chen, T.M., Yuan, S.X., Gao, S.J., Hu, Y.Q., 1987. Uranium series dating of fossil bones from Hexian and Chaoxian human fossil sites. Acta Anthropologica Sinica 6 (3), 249–254 (in Chinese with English abstract). Chen, T.M., Yuan, S.X., 1988. Uranium series dating of bones and teeth from Chinese paleolithic sites. Archaeometry 30, 59–76. Chen, T.M., Zhang, Y.Y., 1991. Palaeolithic chronology and possible coexistence of Homo erectus and Homo sapiens in China. World Archaeology 23 (2), 147–154. Clark, J.D., de Heinzelin, J., Schick, K.D., Hart, W.K., White, T.D., WoldeGabriel, G., Walter, R.C., Suwa, G., Asfaw, B., Vrba, E., Haile-Selassie, Y., 1994. African Homo erectus: old radiometric ages and young Oldowan assemblages in the Middle Awash Valley, Ethiopia. Science 264, 1907–1910. Edwards, R.L., Cheng, H., Murrell, M.T., Goldstein, S.J., 1997. Protactinium-231 dating of carbonates by thermal lionization mass spectrometry: implications for Quaternary climate change. Science 276, 782–786. Liang, R.Y., Xu, Y.H., Peng, Z.C., Koshimizu, S., 1995. ESR dating of tooth samples from the paleoanthropological site of Yinshan, Chaoxian, Anhui. Nuclear Techniques 18 (8), 509–512 (in Chinese with English abstract). Ludwig, K.R., Renne, P.R., 2000. Geochronology on the Paleoanthropological time scale. Evolutionary Anthropology 9 (2), 101–110.
Rae, A., Hedges, R., Ivanovich, M., 1989. Further studies for uranium-series dating of fossil bones. Applied Geochemistry 4, 331–337. Richards, D.A., Dorale, J.A., 2003. Uranium-series chronology and environmental applications of speleothems. Reviews in Mineralogy and Geochemistry 52, 407–460. Rightmire, G.P., 1998. Human evolution in the Middle Pleistocene: the role of Homo heidelbergensis. Evolutionary Anthropology 6, 218–227. Shen, G.J., 2007. Discussions on the reliability of U-series dating of fossil bones from karstic caves. Quaternary Sciences 27 (4), 539–545 (in Chinese with English abstract). Shen, G.J., Cheng, H., Edwards, R.L., 2004a. Mass spectrometric U-series dating of New Cave at Zhoukoudian, China. Journal of Archaeological Science 31, 337–342. Shen, G.J., Gao, X., Zhao, J.X., Collerson, K.D., 2004b. U-series dating of Locality 15 at Zhoukoudian, China: implications for hominid evolution. Quaternary Research 62 (2), 208–213. Shen, G.J., Fang, Y.S., Jin, L.H., 1994. Re-examination of the chronological position of Chaoxian Man. Acta Anthropologica Sinica 13 (3), 249–256 (in Chinese with English abstract). Shen, G.J., Ku, T.L., Cheng, H., Edwards, R.L., Yuan, Z.X., Wang, Q., 2001. Highprecision U-series dating of Locality 1 at Zhoukoudian, China. Journal of Human Evolution 41, 679–688. Wu, R.K., Dong, X.R., 1982. Preliminary study of Homo erectus remains from Hexian, Anhui. Acta Anthropologica Sinica 1 (1), 2–13 (in Chinese with English abstract). Wu, X.Z., 1989. Early Homo sapiens in China. In: Wu, R.K., Wu, X.Z., Zhang, S.S. (Eds.), Early Humankind in China. Science Press, Beijing, pp. 24–41 (in Chinese). Wu, X.Z., Poirier, F.E., 1995. Human Evolution in China. Oxford University Press, New York. 134–136. Xu, C.H., Zhang, Y.Y., Chen, C.D., Fang, D.S., 1984. Human occipital bone and mammalian fossils from Chaoxian, Anhui. Acta Anthropologica Sinica 3 (3), 202–209 (in Chinese with English abstract). Xu, C.H., Zhang, Y.Y., Fang, D.S., 1986. Human fossil newly discovered at Chaoxian, Anhui. Acta Anthropologica Sinica 5 (4), 305–310 (in Chinese with English abstract). Zhao, J.X., Hu, K., Collerson, K.D., Xu, H.K., 2001. Thermal ionisation mass spectrometry U-series dating of a hominid site near Nanjing, China. Geology 29 (1), 27–30.
Contents lists available at ScienceDirect
Quaternary International journal homepage: www.elsevier.com/locate/quaint
Mass spectrometric U-series dating of the Chaoxian hominin site at Yinshan, eastern China Guanjun Shen a, *, Yingshan Fang b, James L. Bischoff c, Yue-xing Feng d, Jian-xin Zhao d a
College of Geographical Sciences, Nanjing Normal University, Nanjing, Jiangsu 210046, China Nanjing Museum, Nanjing, Jiangsu 210016, China c US Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, USA d Radiogenic Isotope Laboratory, Centre for Microscopy and Microanalysis, University of Queensland, Brisbane, QLD 4072, Australia b
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 11 March 2009
The fossils of Chaoxian hominin, widely accepted as representing archaic Homo sapiens in eastern China, were recovered from the middle or slightly higher levels of Layer 2 deposits of a collapsed cave at Yinshan, Anhui Province. Results of mass spectrometric U-series dating of intercalated speleothem calcites are presented. Based mainly on four broadly coeval calcite samples, the hominin fossils should be bracketed in the range of 310–360 ka or somewhat older. These ages are much older than the previous estimate at 160–200 ka based on the U-series dating of fossil teeth and bones, and may be cited as supporting evidence for an earlier H. erectus–archaic H. sapiens interface in China. Ó 2009 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction The Chaoxian hominin fossils were recovered from a collapsed limestone cave (31320 5500 N, 117 5105100 E) at Yinshan (Silvery Hill), a small hill after which the nearby village is named. The site is located about 7 km south of Chaohu City in Anhui Province, eastern China (Fig. 1). In 1981 mammalian fossils were first discovered by a group of geologists when carrying out fieldwork in the region. After being informed of the discovery by an amateur, researchers from the Institute of Vertebrate Paleontology and Paleoanthropology, Academia Sinica and the Institute of Archaeology of Anhui Province organized two seasons of excavation in 1982 and 1983 respectively. Hominin fossils including an incomplete occipital, a fragmentary maxillary and three isolated teeth as well as an abundance of mammalian fossils were unearthed (Xu et al., 1984, 1986; Wu and Poirier, 1995; Bailey and Liu, 2010). The site is composed of two groups of fossiliferous deposits at Locus A and Locus B respectively, situated at about the same level but separated by a limestone ridge 2–4 m thick. Based on associated mammalian fossils of 11 species including Hyaena brevirostris licenti, Megantereon sp., Tetralophodon sp. and Proboscidipparion sp., the Locus A deposits were attributed to the Early Pleistocene. With a faunal assemblage of 13 species including Hyaena brevirostris sinensis, Stegodon, Sus xiaozhu and Megaloceros pachyostus, the
* Corresponding author. Tel.: þ86 135 12501968; fax: þ86 25 85891347. E-mail address: [email protected] (G. Shen). 1040-6182/$ – see front matter Ó 2009 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2009.02.020
Locus B deposits were attributed to the Middle Pleistocene, approximately equivalent to Layers 1–4 of the Zhoukoudian Locality 1 or somewhat younger (Xu et al., 1984, 1986). No trace of cultural remains was reported. The hominin fossils were retrieved from heavily consolidated Layer 2 at Locus B. Though bearing some morphological features indicative of Homo erectus, Chaoxian hominin has been widely accepted as a type representative of archaic H. sapiens in eastern China (Xu et al., 1984, 1986; Wu and Poirier, 1995). Its precise dating may contribute to addressing issues concerning the mode of Middle Pleistocene human evolution in East Asia. The first effort to date the Chaoxian hominin site was made by Chen et al. (1987). Among the nine fossil teeth and bones analysed, four gave concordant 230Th/234U and 231Pa/235U dates. Based on these results a time range of 160–200 ka was proposed. As well, ESR dating of a deer tooth from the hominin fossil-bearing deposits was performed, giving a slightly older linear uranium uptake (LU) age of 212 30 ka (Liang et al., 1995). In Chen et al. (1987), almost equivalent dates of 150–190 ka were assigned to the nearby site of Hexian hominin, widely accepted as a later representative of H. erectus (Wu and Dong, 1982). The scenario of an apparent contemporaneity between the two fossil specimens prompted hypotheses concerning the mode of Middle Pleistocene human evolution in China (Chen and Zhang, 1991). The hominin fossil-bearing deposits are intercalated with localized speleothem formations. From a fragmented flowstone layer considered as ‘‘capping flowstone’’ three calcite samples were taken. Tripled alpha spectrometric 230Th/234U determinations were
G. Shen et al. / Quaternary International 211 (2010) 24–28
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stratified, consolidated, containing a few mammalian fossils, 0.2–0.4 m thick. Layer 4 Sand, yellowish, stratified, slightly consolidated, containing a few fossil debris and rodent teeth, an exposed section ca. 80 cm thick, total thickness unknown.
Fig. 1. Map showing the location of Yinshan, the burial site of Chaoxian hominid and the neighbouring Hexian Homo erectus locality at Longtandong, which is discussed in the text.
carried out on the best (ACY-2) of the three samples judging by purity and density, their weighted mean (310þ45 32 ka, 1s) was proposed to represent the minimum age of the hominin fossils. Four calcite, three fossil and one secondary phosphate samples were analyzed and gave compatible age results. This indicates a much older Chaoxian hominin than previously estimated (Shen et al., 1994). To further constrain the chronological position of Chaoxian hominin, fieldwork was organized in March 2006 and April 2008. Stratigraphic studies of the extant deposits were carried out after clearing off a few centimetres of surficial deposits. Additional samples were collected from the speleothem formations recently exposed due to natural weathering processes or revealed during the cross-section clearance. The samples were analyzed using thermal ionization mass spectrometry (TIMS), which greatly improves the analytical precision with calcite samples of sub-gram size. The results obtained are presented here, with a brief discussion on their possible implications on the mode of Middle Pleistocene human evolution in China. 2. Samples in stratigraphic position On the basis of former descriptions (Xu et al., 1984, 1986; Shen et al., 1994), the extant cross-section was re-examined and described as follows (Fig. 2, numbers from top to bottom): Layer 1 Loose grey loam between big limestone blocks, containing a few mammalian fossils. Layer 2 Sandy clay supported limestone breccia, with a few marl and sandstone gravels, with big limestone blocks (>50 cm each side) mostly at the uppermost levels, brownish except for a purplish, more sandy, 30–50 cm thick sub-layer at central lowermost position, strongly consolidated by carbonate, rich in mammalian fossils but mainly at middle–upper levels, 0.5–2.3 m thick. Layer 3 Clayey sand, with occasional small weathered gravels, generally yellowish but with brownish and purplish patches,
Efforts were made to collect pure, well-crystallized calcite samples from well-defined stratigraphic context, which is key to a successful dating using speleothem formation as a temporal marker. Altogether 12 calcite samples were taken (including two from the previous study, Shen et al., 1994). Their positions on the cross-section are shown in Fig. 2. As the key sample of the previous study, ACY-2 was re-analyzed for better precision. During the 2006 and 2008 fieldwork, three more samples at about the same level were collected. At the left upper–middle part of Layer 2, a localized flowstone with an expanse of about 10 20 cm, 2–5 cm thick, with clear laminae and natural form was revealed. This flowstone is mostly composed of muddy calcite, but a central sub-layer, 0.2–0.8 cm thick, is quite pure and dense, from which the sample CYS-2 was collected. About 40 cm to its lower right side, CYS-8 was taken from a disc-like calcite formation, ca. 12 cm in diameter and ca. 5 cm thick. About 80 cm to the upper right side of CYS-8, CYS-3-1 was taken from a small stalagmite-like formation, 3.0–3.5 cm in diameter and ca. 7 cm high, quite pure and dense. Judging by the fine but semicircular laminae, it is an in situ formation from seeping groundwater rather than, as it appears, a stalagmite precipitated from dripping water. Five samples were collected from two speleothem formations at the boundary between Layers 2 and 3. CYS-4-2 was taken from a pure and dense sub-layer of a stalagmite-like formation, which is developed upon consolidated deposits and is ca. 12 cm in diameter and ca. 20 cm high. Just above CYS-4-2 a small disc-like drip-stone was taken as CYS-4-1. CYS-5-1, CYS-5-2 and CYS-5-3 were taken from upper, middle and lower sub-layers of a localized flowstonelike formation, which is ca. 25 cm wide and ca. 20 cm thick, quite pure and dense. This should be the same formation dated as ACY-5 and ACY-6 in the previous study (>448 ka and 376þ211 66 ka respectively, 1s), but mistaken for a fallen stalagmite at that time. Careful re-study of its form and laminae lead to its re-definition as an in situ flowstone-like formation. CYS-7 and ACY-13 were taken from translucent calcite crystal bundles between limestone fragments overlying and in the middle of Layer 2 respectively. ACY-8 was taken from calcite crystal aggregates wrapping a cervid mandible at an uppermost level of Layer 2 in the previous study with alpha spectrometry (weighted mean of doubled analyses 215þ15 13 ka, 1s). Without any depositional laminae, these calcite formations should have posteriorly precipitated in small cavities from percolating groundwater. The hominin fossils were discovered in the course of wellorganized excavations. In spite of this, no record about their precise provenience is now available. Judging from Fig. 3 of Xu et al., (1984, copied here as attached figure B of Fig. 2), the hominin occipital was retrieved from the middle section of Layer 2; while Fig. 1 of Xu et al. (1986, attached figure A3 of Fig. 2) shows that the hominin maxillary was recovered from a slightly higher position. Recently, we (or the first author) discussed this issue with the excavator, who assured us of the exactness of the published figures. Through close observation, the previously published cross-sections (A2, A3 and B of Fig. 2) correlate quite well with the extant depositional sequence (Fig. 2). This renders support to an absence of significant facies change of the Locus B deposits, which in turn justifies the representativeness of the extant cross-section for the excavated hominin fossil-bearing deposits. So, it seems reasonable to allocate on the preserved cross-section an area lower than CYS-3-1, about the
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G. Shen et al. / Quaternary International 211 (2010) 24–28
Fig. 2. Sketch showing the depositional sequence of the Chaoxian hominid site (Locus B). Here the encircled fragmentary maxillary denotes the most probable provenience of the hominid fossils. Below the main figure, the attached figures (A1,2,3) are adapted from Fig. 1 of Xu et al. (1986). A1 gives the plane figure of hominin fossil-bearing Locus B, but without nearby Locus A which locates 2–4 m to the lower left. A2 and A3, corresponding to lines a–a0 and b–b0 in A1, respectively, give cross-sections revealed during the 1983 excavation, while the extant cross-section corresponds to line d–d0 . The provenience of the hominin maxillary is shown in A3. The attached figure (B) is Fig. 3 of Xu et al., (1984), demonstrating the cross-section revealed during the 1982 excavation and the hominin occipital’s burial position. During the season, mainly Locus A deposits were excavated, only a small section and upper Layers 1 and 2 of Locus B (line c–c0 in A1) were explored. A higher Locus B sequence and a lower Locus A sequence as shown in the figure is only a seemingly reasonable deduction from faunal correlation. The two depositional sequences are situated at about the same level.
same level as or slightly lower than CYS-2 and CYS-8, as the most probable provenience of the hominin fossils. That the hominin fossils came from the middle or slightly higher levels of Layer 2 was also confirmed by two elderly local villagers who witnessed the discoveries as field workers. Moreover, this is in agreement with field observations that the fossils are mainly concentrated in middle–upper levels of Layer 2. 3. Dating results and interpretation The purest possible portions of a sample were selected out, reduced into small grains, cleaned ultrasonically and then hand-picked to avoid porous and less pure pieces. U-series age determinations were performed with TIMS at the University of Queensland (Zhao et al., 2001). The isotopic data and calculated ages are presented in Table 1. ACY-2 gives a result of 351þ16 14 ka, which is consistent within counting statistics with the previous alpha spectrometric determinations. This is also consistent with the results of CYS-2 þ9 (359þ14 12 ka) and CYS-8 (3528 ). As the latter two samples were assuredly taken from the upper–middle section of Layer 2, so the previous recognition that ACY-2 was taken from ‘‘capping flowstone’’ is put into question. The ‘‘capping flowstone’’ no longer exists, due either to natural weathering or to vandalism. However, the underlying limestone block remains, which was then considered as above Layer 2. On its surface are patches of remnant
consolidated deposits, similar to Layer 2 deposits in appearance. This indicates the existence of overlying deposits, which were most probably removed during previous excavations. So instead of being on the surface, the limestone block should be originally embedded in Layer 2, as was the flowstone partly overlying it and taken as ACY-2. With their stratigraphic positions as discussed above, the hominin fossils should be broadly contemporaneous or somewhat older than the three samples. The paucity of fossils in lower section of Layer 2 supports the judgment that the hominin fossils should not be much older than 360 ka. Compared with the above three samples, CYS-3-1 yields a slightly younger date of 309 5 ka. This is in line with the statigraphic order and may possibly define a minimum age for the hominin fossils. On the other hand, the difference between CYS-3-1 and the three samples discussed above may imply a relatively slow sedimentation rate. If so, climate fluctuations and changes in geochemical regime should have left imprints in the deposits. However, no sign of bedding can be identified for middle–upper horizons of Layer 2. This issue must remain open to later sedimentological study of the site. All the five samples at the Layers 2–3 boundary yield dates near the upper limit of mass spectrometric U-series dating, in agreement with the previous alpha spectrometric dates on ACY-5 and ACY-6. From weighted means of the five U–Th isotopic ratios an age of 598þN 93 ka is derived. With an infinite upper bound, this result can hardly define a maximum age limit for the hominin fossils. On the
G. Shen et al. / Quaternary International 211 (2010) 24–28 Table 1 Mass spectrometric U–Th isotopic ratios and
230
27
Th age results. Th/232Th
234
0.4054 0.0003 0.3006 0.0002 0.2775 0.0002
33.40 585.1 16.72
1.0027 0.0015 1.0151 0.0013 1.0737 0.0015
0.8313 0.0033 0.8259 0.0034 0.8984 0.0057
194 2 189 2 234 5
CYS-3-1 ACY-2
0.3814 0.0003 0.4118 0.0006
410.6 83.70
1.0668 0.0012 1.0677 0.0026
0.9599 0.0025 0.9838 0.0049
309 5 359þ16 14
CYS-2
2.8089 0.0072
876.7
1.1119 0.0017
0.9929 0.0052
351þ14 12
CYS-8
1.3826 0.0011
80.03
1.0970 0.0012
0.9895 0.0033
352þ9 8
CYS-4-1 CYS-4-2
0.1952 0.0001 0.2000 0.0001
58.16 135.3
1.0439 0.0016 1.0367 0.0017
1.0176 0.0049 1.0125 0.0042
>572 651þN 106
CYS-5-1 CYS-5-2 CYS-5-3
0.1986 0.0001 0.2529 0.0003 0.3126 0.0004
78.66 103.8 277.2
1.0707 0.0014 1.0602 0.0018 1.0610 0.0015
1.0219 0.0036 1.0240 0.0035 1.0120 0.0035
571þ97 52 >604 496þ42 31
1.0035 0.0015
0.9991 0.0036
659þN 110
Sample
U (ppm)
ACY-8 CYS-7 ACY-13
HU-1_020
690.5 0.8
230
32235
U/238U
230
230
Th/234U
230
Th age (ka)
230
Th age (ka, corrected)
191 2 228 5
All isotopic ratios shown are in radioactivity with 2s errors. Half-lives of Th and U are 75,380 and 244,600 a, respectively. For samples with 230Th/232Th ratio less than 50, corrected 230Th ages are given, assuming the initial 230Th/232Th atomic ratio of 4.4 2.2 106. This is the value for a material at secular equilibrium and with a bulk earth 232 Th/238U value of 3.8. The errors are arbitrarily assumed to be 50%. HU-1 is a uraninite solution used as an international standard for secular equilibrium of 238U–234U–230Th series.
other hand, its lower bound points definitively to a time gap of at least 140 ka between the two speleothem horizons. Further efforts should be made to search for additional calcite samples along the Layer 2 cross-section for a better understanding of the rate of sedimentation and for a tighter age constraint of the hominin fossils. For the moment, it is suggested that the above time gap is mainly occupied by the Layers 2–3 boundary or possibly by the boundary between the brownish and purplish deposits in Layer 2. Being taken from secondary calcite crystals, CYS-7, ACY-8 and ACY-13 should mark the minimum age of the horizon of their formation. So the result for CYS-7 indicates that the Layer 1 deposits between big limestone blocks and consequently the underlying Layer 2 deposits should be formed no later than 191 2 ka. This is further supported by ACY-8 which dates to 189 2 ka. So the hominin fossils should be older than 190 ka even if their provenience corresponds to the uppermost level of the extant cross-section. Considering its position, no further age constraint can be inferred from the result for ACY-13 (228 5 ka). In conclusion, while the hominin fossils cannot be younger than 190 ka, their best age estimate resides in the range of 310–360 ka or somewhat older. This is in general consistency with the previous study using alpha spectrometry (Shen et al., 1994). However, the discrepancy between the U-series dating of calcite samples and the U-series and ESR dating of fossil materials (Chen et al., 1987; Liang et al., 1995) cannot be accounted for by counting statistics. 4. Implications for Middle Pleistocene human evolution Concerning the phylogenetic relationship between H. erectus and archaic H. sapiens, Chinese anthropologists and some of their international colleagues have long believed that in East Asia as in other regions of the old continent the two hominin species were temporally and morphologically related as ancestor and descendant. However, this classical approach has been challenged in recent years. For example, Rightmire (1998) proposed that African and European Middle Pleistocene hominins constitute a new species named H. heidelbergensis, which may give birth to modern H. sapiens in Africa and to H. neanderthalensis in Europe. It is also suggested that contemporaneous hominin finds in China such as Yunxian, Dali, Jinniushan and Maba may be regarded as eastern representatives of H. heidelbergensis. As another example, based on apparent contemporaneity between Skull No. 5 of Peking Man (late H. erectus) and Jinniushan and Dali hominins (archaic H. sapiens) in
234
northern China and between Hexian and Chaoxian hominins in eastern China (Chen et al., 1987; Chen and Yuan, 1988), Chen and Zhang (1991) proposed a possible overlapping existence in China of the two hominin species. In addition, while the appearance of archaic H. sapiens dates to 600 ka in Africa, the H. erectus–archaic H. sapiens interface in China has been assigned to a far later time around 200 ka. This scenario lead to a hypothetical theory about a slower evolution rate of Asian H. erectus compared with their African counterparts (Clark et al., 1994). The above hypotheses have been largely established on the basis of the previous temporal framework for Chinese sites, of which the U-series dating of fossil bones has been prominent. Bones are known to be susceptible to post-burial U migration, leading generally to underestimated dates in a cave setting (e.g. Bischoff et al., 2003; Rae et al., 1989; Shen, 2007). On the other hand, the excellent reliability of the U-series dating of pure, compact and well-crystallized speleothem calcites has been well demonstrated (e.g. Edwards et al., 1997; Ludwig and Renne, 2000; Richards and Dorale, 2003). In the past w10 years with the U-series dating of intercalated calcite formations, the chronology of several key Middle Pleistocene sites has been studied, including Localities 1, 4 and 15 at Zhoukoudian (Shen et al., 2001,2004a,b), Nanjing hominin site (Zhao et al., 2001) and Sima de los Huesos (Bischoff et al., 2003). A general tendency is evident that the new and more robust age results are much older than the ones derived from fossil bones. The site of Chaoxian hominin may be cited as one more example of such a tendency. The results of this paper, with a best age estimate at 310–360 ka or somewhat older for Chaoxian hominin, are of importance in providing a benchmark for Middle Pleistocene human evolution and in indicating a H. erectus–archaic H. sapiens interface in China much earlier than 200 ka. Based on morphological and biostratigraphical studies Wu (1989) placed Chaoxian hominin in the middle of an age sequence for important finds of Chinese archaic H. sapiens, which stands as Dali, Jinniushan, Chaoxian, Xujiayao and Maba. If proved, an even earlier H. erectus–archaic H. sapiens interface in the region may be inferred. Adjustments should be made to the models of human evolution to accommodate the new chronological evidence. Acknowledgements This project was jointly supported by National Natural Science Foundation of China (40373031) and Leakey Foundation (2005/
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2006 general grant). TIMS U-series analyses were supported by ARC Project LP0453664. We would like to thank the government of Chaohu City for assistance in fieldwork, Drs Wu Liu and Christopher Norton for helpful discussions, the first author’s graduate students, Hong Wang, Bin Gao and Li Zhang for assistance in sample collection and preparation, and three anonymous reviewers for constructive criticisms. References Bailey, S.E., Liu, W., 2010. A comparative dental metrical and morphological analysis of a Middle Pleistocene hominin maxilla from Chaoxian (Chaohu), China. Quaternary International 211, 14–23. Bischoff, J.L., Shamp, D.D., Aramburu, A., Arsuaga, J.L., Carbonell, E., Bermudez de Castro, J.M., 2003. The Sima de los Huesos hominids date to beyond U/Th equilibrium (>350 kyr) and perhaps to 400–500 kyr: new radiometric dates. Journal of Archaeological Science 30, 275–280. Chen, T.M., Yuan, S.X., Gao, S.J., Hu, Y.Q., 1987. Uranium series dating of fossil bones from Hexian and Chaoxian human fossil sites. Acta Anthropologica Sinica 6 (3), 249–254 (in Chinese with English abstract). Chen, T.M., Yuan, S.X., 1988. Uranium series dating of bones and teeth from Chinese paleolithic sites. Archaeometry 30, 59–76. Chen, T.M., Zhang, Y.Y., 1991. Palaeolithic chronology and possible coexistence of Homo erectus and Homo sapiens in China. World Archaeology 23 (2), 147–154. Clark, J.D., de Heinzelin, J., Schick, K.D., Hart, W.K., White, T.D., WoldeGabriel, G., Walter, R.C., Suwa, G., Asfaw, B., Vrba, E., Haile-Selassie, Y., 1994. African Homo erectus: old radiometric ages and young Oldowan assemblages in the Middle Awash Valley, Ethiopia. Science 264, 1907–1910. Edwards, R.L., Cheng, H., Murrell, M.T., Goldstein, S.J., 1997. Protactinium-231 dating of carbonates by thermal lionization mass spectrometry: implications for Quaternary climate change. Science 276, 782–786. Liang, R.Y., Xu, Y.H., Peng, Z.C., Koshimizu, S., 1995. ESR dating of tooth samples from the paleoanthropological site of Yinshan, Chaoxian, Anhui. Nuclear Techniques 18 (8), 509–512 (in Chinese with English abstract). Ludwig, K.R., Renne, P.R., 2000. Geochronology on the Paleoanthropological time scale. Evolutionary Anthropology 9 (2), 101–110.
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