关键词: 原地宇生核素; 石英; 26Al/10Be埋藏测年; 早期人类遗址

Abstract: This paper introduces the principles of a recently established dating method based on the radioactive decay of 26 Al and 10 Be. These two nuclides are produced in situ in quartz at a known atomic ratio of about 6.8∶1 by secondary cosmic ray particles that penetrate rocks near the ground surface. Their concentrations depend on the mineral’s exposure time to cosmic radiation, which in turn depends on the erosion rate of the host rock. If quartz grains, after a long period of exposure near the surface, are suddenly shielded from cosmic rays by being washed into a cave or buried beneath sediment that is at least 10 meters thick , then the production of cosmogenic nuclides drastically slows and the inherited radionuclides decay exponentially over time. Because 26 Al decays approximately twice as fast as 10 Be, the 26 Al/10 Be ratio also decreases exponentially over time with an effective half-life of 1.52Ma. This technique offers a means to date burial events in the range of 0.3—5Ma.
One of the first applications of the burial dating in China is to Zhoukoudian Locality 1, chosen for its great importance in paleoanthropology. The timescale of this site has long been studied, but has remained debated due to the lack of suitable dating methods. Although a range of 230 —500 ka has been commonly accepted, previous studies based on mass spectrometric U-series dating of speleothem calcite indicated much older dates of 400ka for the upper sediments and ≥600ka for the middle and lower parts of Layer 5, and suggested that early members of the Peking Man family could range up to 800ka. The availability of a new dating method offers an opportunity to resolve this controversy by directly dating quartz grains and quartzite artefacts from the lower fossiliferous sediments. Six meaningful results for samples from Layers 7 —10 give a weighted mean of 770 ±80ka, redering strong support to an earlier and longer human presence at the site than once estimated.
As for all the dating methods based on the decay of radioactive isotopes, some implicit prerequisites have to be introduced for the application of 26 Al/10 Be burial dating, the most important one being the “simple steady-state erosion model ” of quartz minerals. Besides, we meet with difficulties in finding coarse quartz grains at quite a number of cave sites in southern China and in low initial cosmogenic nuclides due to high erosion rates. In spite of these, 26 Al/10 Be burial dating is a radioisotopic method well-founded in physics and chemistry. It is independent of other dating methods and its timescale fills a “blank period” in radiometric dating in China. Moreover, this method should be widely applicable because it requires only quartz, a mineral that is often available at hominid sites. It is foreseeable that in the coming years this dating method will contribute substantially to the establishment of a reliable timescale for early hominid evolution in China.

Key words: Cosmogenic nuclides; Quartz; Burial dating; Early hominid sites