(1. Institute of Geology, Seismological Bureau of China. Institute of Marine Geology, Ministry of Land and Resources. Vertebrate paleontology Institute of Paleoanthropology, Chinese Academy of Sciences)

Dali people are an ancient type of early Homo sapiens, which is of great significance in the study of human origin. Electron spin resonance (ESR) dating technique was used to directly determine the ages of three shell samples in Dali people's horizon. At the same time, the age of uranium series is analyzed. According to the ESR dating results of shells, Dali people may be 250ka older than they are now.

Dali people; Shells; Electron spin resonance; age determination

1 Introduction

1978 In the spring, Comrade Liu Shuntang of Shaanxi Provincial Water Conservancy and Electric Power Bureau found a relatively complete human skull fossil in Tianshuigou (east longitude109 44', north latitude 34 52') in his hometown of Jiefang Village, Dali County, Shaanxi Province, which belongs to an older type of early Homo sapiens, and dali man represents a new subspecies-Homo sapiens. Because there are few complete skulls of early Homo sapiens found in the world, the discovery of Dali people not only fills a gap in the history of ancient human development in China, but also has great significance in the study of human origin.

Geomorphologically, the Dali fossil site is located in Jialiang, Heyang Loess Plateau on the left bank of Luohe, where there are two loess plateaus. The second platform consists of the third terrace of Luohe River and its overlying loess-paleosol sequence. S2, L2, S 1 and L 1 are developed in the overlying loess-paleosol sequence. Dali fossils are exposed in the river gravel layer about 8m below S2, which is the third layer of Dali 7806A archaeological site. This layer is a variegated gravel layer with complex gravel composition and high grinding degree. It is mainly composed of gravel with a diameter of about 0.5cm and fine sand with lenses, and shells and bone fossils are buried. This layer is 80 cm thick.

Due to the limitation of Dali people's time and dating sample materials, there is little research on Dali people's chronology. According to the uranium series age of bovine teeth and bone fossils, Chen Tiemei and others believe that the horizon age of Dali human fossils and animal fossils is 654.38+0.8 million ~ 230,000 a; Zhong made thermoluminescence dating and sporopollen analysis on the section where Dali people lived. According to the thermoluminescence age, the age of Dali people is not more than 300 thousand years. However, they also pointed out that the stratigraphic sequence, chronological data sequence and sporopollen sequence on the whole profile are not completely consistent. So far, the research level of Dali people's chronology is obviously not commensurate with the important position of Dali people in paleoanthropology. In this paper, the electron spin resonance (ESR) dating technique is used to re-determine the age of Dali people.

2 Processing and testing of dating samples

Sample shells were collected from the third floor of the 7806A archaeological site in Dali, and they were mainly mussels, ancient snails and mussels. Mytilus species generally live in deep water that does not dry up in winter, and the water quality is clear and transparent in rivers or streams. The bottom is hard, the upper sediment is thin, and the bottom is sand.

The shells used for dating are three big mussels (Lamprotula antiqua Odtiqua Odhner), which are named Dali- 1, Dali-2 and Dali-3 respectively. Among them, the thickest outer side of the shell is about 18mm, and the inner and outer sides are removed by 5 ~ 6 mm to remove the influence of A and β, and the intermediate sample is used for dating. By thin-section microscope observation, the shell is still composed of aragonite minerals and has not been converted into calcite.

Gently crush, grind and sieve the sample in the middle of the shell to obtain the component of150 ~ 200μ m. Each sample is divided into 8 ~ 9 small samples. 200 mg per sample. Except one sample used to measure the naturally accumulated ESR signal intensity, all the other samples were irradiated by artificial radioactive sources (60Co, γ source). The irradiation doses were 200, 400, 700, 1000, 1300, 1600 and 2000Gy, respectively.

The 1 measurement of the three samples was completed in China Institute of Metrology, and the Dali-2 sample was compared and tested in the Institute of Marine Geology of the Ministry of Land and Resources. The number in the table 1 is Dali-2a. The test conditions are: modulation amplitude 0. 1mT, microwave power 2mW, time conversion constant 8 1.92ms, conversion time 5 1. 12s, and magnification 5× 105. The measured signal is g=2.00 10.

Table 1 ESR dating results and analysis data table of shell samples 1 shell ESR results and data.

3 Reliability of aging results

3. 1 ESR dating status of shell

Ikeya et al., first applied ESR technology to shell dating, and then many researchers did more in-depth and extensive research, with many successful examples. However, because uranium in shells is in an open system and aragonite is transformed into calcite, the reliability of ESR age of shells is relatively poor (Figure 1).

The ESR signal spectrum of aragonite mollusk shells is complex, with five peaks. G =2.0058, 2.0032, 2.0020, 2.00 12 and 1.9976, respectively. Because different ESR signal peaks have different thermal stability and lifetime, sometimes different equivalent dose (ED) values can be obtained by selecting different peaks for the same sample. Therefore, different scholars have different choices about the peak. At present, most Shell ESR surveyors use 2.00 10 peak, which has relatively high thermal stability and long service life, equivalent to the C peak of coral aragonite ESR, and many ages over 500ka have been obtained (Figure 2).

In addition to the above factors affecting the age reliability, there are many aspects such as the efficiency of A particles relative to β in the radioactive elements of the sample itself, that is, the estimation of an effective coefficient.

3.2 ESR age reliability evaluation of samples in this paper

These three shells showed an excellent peak of g=2.00 10, and the signal peak was used for equivalent dose calculation. There is no equivalent dose calculation for other signals. After γ -ray source irradiation, the sample was placed for 3 weeks before 1 measurement, and the second measurement was 30 days after 1 measurement. The ED values of the second measurement are consistent within the error range, so it can be considered that the unstable signal caused by irradiation has been removed before the measurement. However, the error of ED value is relatively large.

Fig. 65438 +0 ESR signal curve of Dali-2a shell aragonite recorded by ESR spectrometer 65438 +0 ESR spectrum of Dali-2a sample shell recorded by ESR spectrometer.

Curves 1, 2, 3, 4, 5, 6, 7 and 8 are ESR signal curves of irradiated samples at 2000, 1600, 1300, 1000, 700, 400, 200 and 0 Gy 600 GO, respectively.

Curves 2, 3.4, 5, 6.7 and 8 are ESR spectra of natural plus.

2000, 1600, 1300, 1000, 700, 400, 200 and 0Gy60Co irradiation.

Fig. 2 ESR signal growth curve and equivalent dose (ED) value of Dali-3 sample.

The detection signal is g=2.00 10, which is a common peak in ESR dating of shells, and its life is much longer than the estimated age of shells.

Although the measurement error of annual dose is not given in table 1. However, there may be some certainty and error in the annual dose. The Institute of High Energy Physics of Chinese Academy of Sciences used neutron activation analysis technology to determine the radioactive element content of shells. The contents of uranium and thorium in environmental sediments were determined by A-thick source counting method, and potassium was determined by flame photometer technology. A the effective coefficient is 0.0 1. Considering that the outer layer of 5 ~ 6 mm has been removed, the contribution of external A and β is not estimated. Therefore, the annual dose of the sample is provided by A, β and γ produced by radioactive elements in the sample itself (inside), γ produced by radioactive elements and cosmic rays in the surrounding sediments (150mgy/ka), and the water content of the sediments is 13%. Finally, the annual dose was calculated according to the relevant parameters provided by Aitken.

U-series dating of similar shells (mussels) collected from the same place was carried out. The 227-year-old of mussels was 1 measured by the 227/230' s method. There may be shells absorbing uranium in the later stage, which cannot be verified at present. Uranium age indicates that the actual age of shells may be older than that before 200ka.

In order to simplify the calculation, the early uranium addition model (table 1) is adopted in the annual dose calculation, and the ESR age obtained may be close to or smaller than the actual age.

3.3 Comparison with stratum age

The mussel shell samples used for dating were collected from the horizon where Dali people are located, that is, the third floor of Dali archaeological site 7806A. The section at point 7806A is loess-paleosol sequence, with L 1, S 1, L2 and S2 in the upper part and fluvial deposits in the lower part. Dali people are located in fluvial strata about 13m below S2 in loess-paleosol sequence. In the climatic strata, S2 in the Loess Plateau is equivalent to 7 stages of deep-sea oxygen isotope, and its lower age is about 247 kab p. Therefore, it is inferred that the age of Dali people should be greater than 250 kab p. The ESR age of shells in this paper is consistent with the age of stratigraphic sequence.

4 conclusion

With the help of ESR dating method, assuming that uranium is early absorbed (EU), the age value of shell samples collected from Dali people's horizon is about 250 ka, and the EU age value of ESR should be less than the real age of the samples, that is, the real age of Dali people's horizon should be greater than 250 kab.p., that is to say, the age of Dali people should be above 250 kab.p., but the reliability of ESR age of shells in the past is not very high. This ESR measurement, Therefore, the age of Dali people needs to be further studied.

Thanks to Mr. Wu Xinzhi and researcher from vertebrate paleontology Institute of Paleoanthropology, China Academy of Sciences, and researcher Lu from Institute of Geology, Seismological Bureau of China for their encouragement and guidance. Also participating in the sample collection were researcher Zhou Chunmao from Shaanxi Institute of Archaeology, Dr. Zhao Hua from xi Loess and Quaternary Laboratory of China Academy of Sciences, and teacher Wang Yuying from Institute of Zoology, Chinese Academy of Sciences, who identified the species of shellfish. Lin Chuanyong, a researcher at the Institute of Geology, Seismological Bureau of China, identified the mineral characteristics of shells. ESR measurement was assisted by Gao Juncheng, a researcher at China Institute of Metrology. Professor Shen of Nanjing Normal University has tested and analyzed the age of uranium series, and put forward many valuable suggestions for revising the first draft of this paper. I would like to express my heartfelt thanks here.

References (omitted)

(Journal of Anthropology, 200 1.2, Vol. 20,No. 1, pp. 34-38)