10Be burial dating of the hominin site Bailong Cave in Hubei Province, central China

Quaternary International 389 (2015) 235e240

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Initial 26Al/10Be burial dating of the hominin site Bailong Cave in Hubei Province, central China Xuebin Liu a, Guanjun Shen a, *, Hua Tu a, Chengqiu Lu b, Darryl E. Granger c a

College of Geographical Sciences, Nanjing Normal University, Nanjing 210023, China Archeological and Cultural Relics Institute of Hubei Province, Wuhan 430077, China c Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN 47907-1397, USA b

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 12 November 2014

With the discovery of Homo erectus teeth, stone artifacts and a rich fauna, Bailong Cave in northern Hubei Province is an important hominin/archaeological site in China. However, due to the lack of suitable dating methods, the previous age estimates all come from biostratigraphic correlations. Here we report the first application of a radio-isotopic dating method to the site. Three 26Al/10Be measurements of two quartz samples give a weighted mean burial age of 0.76 ± 0.06 million years (Ma, 1s). Taking into account possible bias of the dating method, stratigraphic order and evidence for rapid sedimentation, the cultural deposits of the site should be somewhat younger than the above date. This is consistent with previous biostratigraphic age estimates at around the Early/Middle Pleistocene transition, and possibly indicates an earlier human coexistence with StegodoneAiluropoda fauna than previously estimated. © 2014 Elsevier Ltd and INQUA. All rights reserved.

Keywords: Bailong Cave China 26 Al/10Be burial dating Early/Middle Pleistocene transition Homo erectus StegodoneAiluropoda fauna

1. Introduction The hominin/archaeological site Bailong Cave is located at Shenwuling Village, ca. 10 km east of the county town of Yunxi in northern Hubei Province, central China (32
590 4000 N, 110
3103400 E) (Fig. 1). In June 1976 local farmers found a number of mammalian fossils in the cave deposits. Informed of the finds by a worker of the county Administration of Cultural Relics, specialists from Hubei Provincial Museum in Wuhan and Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences in Beijing identified two hominin teeth among the fossil discoveries. A joint team formed by researchers from the above two institutions and Wuhan University subsequently organized two excavations in 1977 and 1982, respectively. In situ recoveries include five human teeth, dozens of artifact-like stone objects and a rich collection of mammalian fossils (Qunli, 1983). In AprileMay 2007, Wu et al. (2009) studied the relics from the previous excavations housed in the county administration, and identified 15 artifacts out of a total of 41 stone objects. In SeptembereOctober of the same year and then in MarcheApril 2008 Wu et al. (2009) carried out a small scale excavation, recovering some twenty more stone artifacts and a number of mammalian fossils.

* Corresponding author. E-mail address: [email protected] (G. Shen). http://dx.doi.org/10.1016/j.quaint.2014.10.028 1040-6182/© 2014 Elsevier Ltd and INQUA. All rights reserved.

Based on morphometric correlations, Qunli (1983) assigned the hominin teeth from Bailong Cave to Homo erectus, which was accepted by Dong (1989), Wu and Poirier (1995) and Wu and Wu (1999). More detailed description and identification of these teeth was given by Wu and Poirier (1995), who identified two right upper P2, a lower left P1, a right upper M2, a right lower M1 or M2 and a right lower M3. Mammalian fossils recovered from the site represent 29 taxa, including typical members of the StegodoneAiluropoda fauna such as Ailuropoda wulingshanensis, Stegodon sp., Pochycrocuta licenti, Cuon javannicus, Megantereon sp., Sivapanthera pleistocenicus and Leptobos brevicornis etc. Based on the faunal composition, Qunli (1983) attributed the site to the Middle Pleistocene. Han and Xu (1989) and Wu and Poirier (1995) agreed with this timescale. But Li and Feng (2004) put more emphasis on the presence of P. licenti, a characteristic species of the Early Pleistocene, and suggested that the site should more likely be late Early Pleistocene in time. Based on the presence of Early Pleistocene species such as C. javannicus, P. licenti, A. wulingshanensis, S. pleistocenicus, Megantereon sp., Sus peii, Cervavitus fenqii, Cervus elegans, Cervus yunnanensis and L. brevicornis, Wu et al. (2009) proposed that the site should be no younger than an early stage of the Middle Pleistocene. Bailong Cave is one of the few cave sites in China chronologically assigned to around the turn of Early and Middle Pleistocene and from where hominin fossils and lithic artefacts were unearthed in

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Fig. 1. Map showing geographical location of Bailong Cave.

association with an abundance of mammalian fossils. Its precise dating is thus important for addressing the issues concerning the mode of human evolution in the region. However, so far the only age estimates for the site come from biostratigraphic correlations. No trial of radiometric dating has ever been carried out. If the biostratigraphic age estimates stand, Bailong Cave should be near to or slightly older than the upper limit of mass spectrometric U-series dating (~600 ka). It is this dating method that has been widely applied to MiddleeLate Pleistocene cave sites in China (e.g. Shen et al., 2001; Shen et al., 2013). The validity of U-series dating of pure and dense cave calcites has been well demonstrated (e.g. Ludwig and Renne, 2000; Richards and Dorale, 2003; Shen, 2004). But unfortunately, neither flowstone nor other types of calcite formations have ever been found intercalated in the deposits of this cavern. The chronology of African paleoanthropological sites has been established primarily by using K/Ar (40Ar/39Ar) dating of volcanic tuffs, the reliability of which has been well confirmed (e.g. Clark et al., 1994; McDougall et al., 2005; Beyene et al., 2013). However, 40Ar/39Ar dating is seldom applicable to Chinese hominin sites due to the lack of contemporaneous volcanic activity. This is also the case for Bailong Cave. Cosmogenic 26Al/10Be burial dating, which has been developing in the past dozen years, offers a means to date detrital sediments. This method is expected to fill in the blank of radio-isotopic dating of this site. When carrying out fieldwork at the site in 2009, two quartzose samples were collected. Here we report initial results of 26Al/10Be burial dating performed on these samples, and discuss the implications to models of human evolution in the region. 2. The dating method Burial dating with 26Al and 10Be in quartz is based on the buildup of the two nuclides by exposure to secondary cosmic rays, followed by radioactive decay after burial within a cave or sedimentary deposit. For a recent overview of this dating method and its applications in archaeology and paleoanthropology, the readers are referred to Granger (2014). The basic principles of this dating method are briefly outlined below.

Secondary cosmic-ray particles traverse earth's atmosphere and penetrate a few meters into soil or rock to react with nuclei within minerals, leading to in situ production of cosmogenic nuclides, including 10Be and 26Al (Lal and Arnold, 1985; Granger and Muzikar, 2001; Granger, 2006). The production rates of 26Al and 10Be in quartz depend on various parameters including latitude, altitude, and depth below the ground surface (Granger et al., 2013). Although their production rates change, the 26Al/10Be production rate ratio of ~6.75 keeps nearly constant (Argento et al., 2013). The concentrations of 26Al and 10Be depend on the mineral's exposure time near the ground surface, which is in turn controlled by the erosion rate of the host rock. For a rock that is eroding steadily, the 26Al and 10Be concentrations are given approximately by:

Ni ¼ Pi =ð1=ti þ rE=LÞ

(1)

where Ni refers to the concentration of cosmogenic nuclide i, ti its radioactive mean-life, r the density of the eroding rock or soil, E the erosion rate, and L the penetration length of secondary cosmic rays (~160 g cm2). If quartz minerals from the ground surface are rapidly buried by at least 5e10 m of overburden, then the production of cosmogenic nuclides drastically slows or even ceases. The inherited 26Al and 10 Be then decay, while there remains a continued slow buildup of cosmogenic nuclides by muon-induced reactions. The timedependent concentration of each nuclide is given by:

Ni ðtÞ ¼ Ni;inh e

tt

i

Zt þ

Pi;pb ðt 0 Þe

tt0

i

dt 0

(2)

0

where Ni,inh refers to the decaying inherited concentration prior to burial, Pi,pb the production rate after burial, and t0 a dummy variable of integration. For deeply buried rocks or for rocks with a long pre-burial exposure time, the first term in equation (2) dominates and we are left with a simple relationship:

X. Liu et al. / Quaternary International 389 (2015) 235e240

  N26 =N10 ¼ ðN26 =N10 Þinh exp  t=tbur

(3)

where tbur ¼ 1/(1/t10  1/t26), an apparent mean-life of 2.08 Ma for the 26Al/10Be ratio. This type of simple burial dating often works well for cave deposits in the range of ~0.3e5 Ma. For less deeply buried rocks where the second (integral) term in equation (2) is important, the ages can still be calculated by either solving for the postburial production explicitly using known production rate profiles (e.g. Balco et al., 2013), or by using isochron methods on multiple clasts (e.g. Balco and Rovey, 2008; Erlanger et al., 2012). In the particular case of Bailong Cave postburial production is minor but not negligible, so we account for postburial production and calculate model burial ages. 26 Al/10Be burial dating was first applied to sediments in caves for deriving river incision rates (Granger et al., 1997). It was later applied to hominin sites including Sterkfontein (Partridge et al., 2003), Wonderwerk Cave (Chazan et al., 2008) and the Rietputs Formation (Gibbon et al., 2009) in South Africa, Sima del Elefante in Spain (Carbonell et al., 2008), Attirampakkam in India (Pappu et al., 2011) and Zhoukoudian Locality 1 (Shen et al., 2009) and Xihoudu (Kong et al., 2013) in China. Owing to its well-founded basis in physics and to its independence from other dating methods, 26Al/10Be burial dating is now a standard tool of geochronology widely applied in long-term landscape and human evolution. However, as in the case of other radiometric dating methods, the reliability of 26Al/10Be burial dating depends on whether individual samples conform to a set of assumed preconditions. Here, the most important one is the “simple steady-state erosion” of quartz minerals. This model

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requires that the quartz minerals have experienced only one exposure-burial cycle in the past 10 Ma or so. If sediments have a prior burial history, then the resulting ages should be considered as maximum. 3. The cave, stratigraphy and samples for dating Bailong Cave is found in the northwestern margin of Wudang Geological Uplift in the Qinling Orogenic Belt. The cave is developed in lacustrine limestone of the Pliocene Shaping Group, which includes conglomerate, algal limestone and biological frame limestone (Li et al., 2012). The horizontal cave corridor is ~18 m in length, with a northeast facing entrance which is ca. 550 m above sea level and ~10 m above the valley floor. The ~8 m long outer section of the cave has been completely dug out in the course of previous excavations. After the failure of its side wall, which happened only a few years ago, this part of the cave now looks more like a rock shelter. Cultural deposits, as well as extant cross-sections from more recent excavations, are now preserved in the cave's inner section (Fig. 2A). No publication can be found for the excavations in 1977 and 1982. Wu et al. (2009) studied the stratigraphy of the site, and divided the 2.4 m-thick depositional sequence into eight layers (Fig. 2C). The uppermost Layer 1 is ~50 cm thick and composed of yellowish-brown clay with a few fragmentary fossils. Its loosely structured upper part may have been anthropogenically disturbed over recent times, while the lower part with horizontal laminae remained undisturbed. The underlying Layer 2, ~50 cm thick and composed of brownish red clay, is the only fossil and lithic artifactbearing unit of the site. Further underlying are Layers 3e7,

Fig. 2. (A) Plan map of Bailong Cave, the dashed line marks the collapsed side wall. Region 1 and Region 2 are the excavation areas in 2008 and 2009, respectively. (B) A crosssection showing the position of the cave inside the hill, corresponding to line BeB0 in Fig. 2A. From this figure, we obtain that the samples BLD-1 and BLD-2 are shielded in the vertical direction by an 8.86 m-thick limestone layer and by clayey deposits in thicknesses of 2.0 m and 2.8 m, respectively. (C) A cross-section depicting the western wall of Region 1 which is ~2.65 m thick and showing the positions of the samples for dating. Fig. 2(C) is adapted from Fig. 4 of Wu et al. (2009), but with minor difference in thicknesses of the depositional layers.

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composed mostly of clayey deposits with various colors and 15e35 cm in thickness. Sandy or gravelly sandy sub-layers can be found intercalated in the mostly clayey deposits of Layers 4 and 6. At the bottom Layer 8 consists of sand supported quartz gravels, mostly 1e3 cm in diameter, with a few clay lenses. All these layers are successively and conformably superposed, except for a depositional hiatus between Layers 7 and 8. No systematic study on sedimentology and sediment provenance of the site has been reported. Wu et al. (2009) described that the stone artifacts are mainly made of vein quartz, which is not found in bedrock near the cave. We found that the pebbles inside the cave are also mainly composed of vein quartz. Similar vein quartz pebbles can be found within the alluvial sediments on the valley floor outside the cave, but not within the bedrock. We were informed by local people that such pebbles must come from a peak ~2 km further up the valley. Based on the descriptions as above we believe that the sandy and gravelly deposits of Layer 8 may have been washed into the cave by a stream or during floods, and that

acid, precipitated as hydroxides, and transformed to oxides in a furnace at 1100
C. BeO was mixed with niobium and Al2O3 with silver powder for 10Be/9Be and 26Al/27Al measurements by accelerator mass spectrometer (AMS) at PRIME Lab, Purdue University. The concentrations of 26Al and 10Be were derived from the measured isotopic ratios, the known spike of 9Be and the measured 27 Al content. Age results were calculated by iteratively solving Equations (1) and (3) in section 2, using 10Be and 26Al production rates at the ground surface prior to burial of 6.10 and 41.26 atoms/g/ year, respectively, for a latitude of 32
590 4000 N and an elevation of 0.55 km, following Stone (2000) and revised to match the AMS standards of Nishiizumi et al. (2007). Table 1 presents the 26Al and 10Be concentrations and corresponding minimum burial ages. The three measurements yield age results 0.65 ± 0.11, 0.67 ± 0.10 and 0.81 ± 0.09 (1s uncertainties are quoted here and throughout the paper), respectively, being consistent within ±1s error range and with a weighted mean of 0.71 ± 0.06 Ma.

Table 1 Cosmogenic nuclide concentrations and burial ages of quartz samples from Bailong Cave. Sample

[26Al]1 (106 at/g)

[26Al]2 (106 at/g)

[10Be] (106 at/g)

26

Al/10Be

BLD-1(Gra) BLD-1(Quz) BLD-2 Weighted Mean

0.399 ± 0.040 0.395 ± 0.040 0.386 ± 0.032

0.394 ± 0.018 0.425 ± 0.011 0.362 ± 0.008

0.0805 ± 0.0028 0.0869 ± 0.0035 0.0801 ± 0.0032

4.91 ± 0.29 4.87 ± 0.29 4.54 ± 0.21

Simple burial age (Ma) 0.648 0.665 0.808 0.714

± ± ± ±

0.110 0.095 0.093 0.057

Corrected age (Ma) 0.690 0.705 0.859 0.761

± ± ± ±

0.119 0.101 0.096 0.060

[26Al]1 values were initially measured with relative counting errors of ~10%. Recently with the introduction of a gas-filled magnet (Fifield et al., 2007) to the AMS at PRIME Lab, a much improved precision of ~2% for 26Al/27Al can now be realized. [26Al]2 values were obtained by re-measuring the remaining Al2O3 from the same samples. The weighted mean of the two measurements, 0.395 ± 0.016, 0.423 ± 0.010 and 0.364 ± 0.008 (106 at/g), were used to calculate the 26Al/10Be ratios and then the age results. Mean-lives of 10 Be and 26Al are 2.01 ± 0.02 Ma and 1.02 ± 0.02 Ma, respectively. The uncertainties given here denote statistical error of AMS measurement.

the overlying clayey sediments with sandy or gravelly sandy sublayers were likely carried into the cave by slope wash. By close observation, we found that Layers 4 and 6 contain sufficient coarse quartz grains and a few vein quartz pebbles, from where samples for 26Al/10Be burial dating BLD-1 and BLD-2 were taken, respectively. However, we failed to find the 7e8 cm-thick gravelly-sandy lens inside cultural Layer 2 as reported by Wu et al. (2009), and were thus unsuccessful in collecting a sample for dating from this layer. 4. Experiment and results Several kilograms of quartzose deposits were collected and rinsed with water at a nearby pond to remove silt and clay. The raw samples thus obtained were taken back to laboratory for further treatment. HCl was added to dissolve carbonate and phosphate. The quartz-rich grains 0.2e0.9 mm in size were sieved out. It was found that BLD-1 contained quite a number of vein quartz pebbles ~1 cm in size. These pebbles were picked out, crushed to 0.2e0.9 mm and renamed as BLD-1(Gra), while its quartzose sand equivalent as BLD1(Quz). The samples were leached several times in hot 2% HF-2% HNO3 overnight with agitation. Then the quartz grains were separated from mafic minerals by magnetic separation and from heavier and lighter minerals with heavy liquids. Any colored impurity was removed by handpicking. The semi-prepared samples were further treated by repeated leaching overnight with 1% HF-1% HNO3 in an ultrasonic tank to remove meteoric 10Be. 50e100 g of purified quartz were dissolved in 5:1 HF/HNO3, and spiked with ~0.2 mg 9Be prepared from beryl. An aliquot was taken for Al determination by inductively coupled plasma optical emission spectrometry. After evaporation and fuming of HF in H2SO4, Al and Be were separated on ion-exchange columns in 0.4 M oxalic

These burial ages were calculated by assuming that the samples were buried sufficiently deeply to ignore post-burial production of cosmogenic nuclides, which cause the simple burial ages to be underestimated. However, being situated near the foot of a quite steep slope, the ceiling and the side wall of Bailong Cave do not quantitatively shield against cosmic rays. To estimate the postburial production rate due to muons, we need to know the depth of the samples from the slope surface. Using a laser range finder we outlined the cave's cross-section as shown in Fig. 2B. From this figure, it was found that the samples for dating were buried at depths of ~10 m in the vertical direction. The overburden becomes much thicker toward the direction to the peak, but becomes thinner toward the slope foot. As the gradient in production rates at such depths is quite gentle, rather than integrate over the angular distribution of incoming muons we deem it appropriate to take the mass depth at the vertical direction as an average value (Fig. 2B). We also consider that the samples' burial depth could have changed over time due to erosion of the hillslope. Chao et al. (2005) reported an erosion rate of 17 mm/a for a site in the suburbs of Beijing, and Xu et al. (2013) presented six measurements in the range of 17e50 mm/a for areas with different geographical and climatic settings in Guizhou Province. As it is difficult to judge which value is better suited for the study area, and as the correction for insufficient shielding is not so sensitive to the changing erosion rate in the above range, here we arbitrarily take the mathematical average of the seven measurements (32 mm/a) for Bailong Cave. Then by using the muonic contribution to in situ nuclide production reported by Braucher et al. (2013), the corrected burial ages are 0.69 ± 0.12, 0.70 ± 0.10, 0.86 ± 0.10 Ma, respectively, with a weighted mean of 0.76 ± 0.06 Ma. The burial ages here presented should be regarded as maxima considering the possibility that the samples may have experienced

X. Liu et al. / Quaternary International 389 (2015) 235e240

previous burial histories. However, on either sedimentological or analytical grounds, we see no evidence that this is the case. The close agreement between the ages of sand and pebble sub-samples of BLD-1 implies that they were all brought into the cave at the same time. Because they are likely from distinct sources, it seems unlikely that either would have a significant inherited burial signal. Moreover, as this is a small cave, there is no opportunity for reworking of older sediments within the cave itself. It should also be noted that the Pliocene Shaping limestone contains some sandy marl gravels (Wu et al., 2009; Li et al., 2012). However, as the vein quartz pebbles and sandy deposits were brought into the cave by slope wash, we consider it quite unlikely that the quartz minerals were eroded from within the cave itself. The samples for dating come from Layers 4 and 6, so they strictly provide a maximum age constraint to the overlying fossil and artifact-bearing Layer 2. However, the fossils are unlikely to be much younger. This is for the reason that there is no evidence of a depositional hiatus, and that the three measurements on two layers give consistent age results that corroborate the earlier biostratigraphic age estimate at around the Early/Middle Pleistocene transition (Wu et al., 2009). We therefore propose that the hominin site Bailong Cave should be younger than but possibly not far from 0.76 ± 0.06 Ma at 1s (±0.12 Ma at 2s). 5. Discussion The mode of human biological evolution during Middle Pleistocene is one of the most debated topics in paleoanthropology (e.g. Rightmire, 1996, 1998; Bae, 2010; Stringer, 2012). With the discovery of a number of hominin sites, China is one of the key regions in the world for clarifying the debated issues. However, because of the lack of suitable dating methods, numerical dating beyond the upper limit of mass spectrometric U-series dating, ~600 ka, is particularly difficult in China. The established timescales of numerous hominin fossil and stone artifact-bearing sites in this time period are mainly based on biostratigraphic correlations and part of them on paleomagnetic analyses. Reliable dates from radioisotopic dating methods have been nearly absent. The inaccuracy or even misplacement of the temporal position of relevant hominin fossils is among the major causes of the controversies. With the availability of 26Al/10Be burial dating, reliable radio-isotopic age constraints can be expected for the sites in China temporally situated in the aforementioned “blank period”. The application of this method to Bailong Cave marks one of the first steps forward in this direction. Whether humans were a member of the StegodoneAiluropoda fauna is one of the debated issues in paleoanthropology. After surveying a number of cave sites in southern China and peninsular Southeast Asia, Ciochon (2010) suggested that H. erectus should be divorced from the StegodoneAiluropoda fauna. Dennell (2009) concurred with this proposition, but considered that the late Middle Pleistocene Panxian Dadong in Guizhou Province, southwestern China, provides the first clear evidence for the relationship of archaic Homo sapiens with StegodoneAiluropoda fauna. The results of this paper potentially push the human/fauna coexistence to an earlier time, and here the human actor on the stage is a later representative of H. erectus. This paper provides an initial radio-isotopic age for Bailong Cave, but additional work is needed to refine the chronology of the site. Notably as the Brunhes-Matuyama magnetic reversal is within error range of the proposed date, paleomagnetic analyses may provide an effective means to narrow the uncertainties. Also, by washing dozens of kilograms of excavated deposits during a small scale excavation, we may possibly be able to obtain enough coarse quartz grains for a direct 26Al/10Be dating of the cultural Layer 2.

239

Finally, if additional samples were collected it might be possible to carry out isochron 26Al/10Be burial dating to circumvent possible geological uncertainties (Balco and Rovey, 2008; Erlanger et al., 2012). We are hopeful that in the coming years, the wide application of the burial dating method to this and other contemporary sites will contribute greatly to addressing the uncertainties surrounding the mode of EarlyeMiddle Pleistocene human evolution in East Asia. Acknowledgements This research was supported by National Natural Science Foundation of China (40873042 and 41273067) and PAPD of Jiangsu Higher Education Institutions. We would like to thank the government of Yunxi County for assistance in fieldwork. Dr. Wu Liu is thanked for helpful discussions, and the corresponding author's graduate students Lijuan Kong and Haixu Li are thanked for assistance in sample preparation. We also are grateful to two anonymous referees for constructive comments and suggestions. 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