Uranium series

The general term "Uranium series" comprises several closely related dating methods, which are based on the radiometric disequilibrium within the radioactive decay series arising from the two uranium isotopes 238U and 235U (Ivanovich and Harmon 1992). Also the terms decay series, disequilibrium, or uranium-thorium methods of dating are occasionally applied. In paleoanthropology, it is essentially the 230Th/234U method, which is most important for calcareous remains up to about 400 ka old. Of particular interest for 230Th-234U dating are secondary carbonates of caves and springs and to a lesser degree also fossil teeth and bones. Occasionally also the 231Pa/235U method is applied up to 150 ka.

The uranium isotope 238U, which constitutes 99.3% of natural uranium, is radioactive and decays over a chain of intermediate radioactive daughter nuclides to the stable lead isotope 206Pb. In closed systems, equilibrium develops among all radioactive nuclides within the decay chain. In the state of radioactive equilibrium, all radioactive nuclides possess equal activity (O Eq. 6). In nature, most minerals and unweathered rocks represent closed systems, in which radioactive equilibrium persists simply as a result of their geologically high ages. If such a system is disturbed it will take sometime, practically five half-lifes, until the daughter nearly returns to equilibrium with its parent nuclide. For the system 230Th/234U, in which 234U decays by a-emission to 230Th, disequilibrium arises through geochemical fractionation. Uranium is readily dissolved in groundwater from where it is taken up by secondary carbonates as well as dental/bone tissues. Thorium, on the other hand, stays adsorbed to mineral particles and thus is not dissolved in groundwater. Consequently, fresh calcareous deposits incorporate uranium but not thorium, i.e., (230Th/234U)t

=0 = 0. During the following 400 ka, corresponding roughly to five times the 230Th half-life of 75.6 ka, the 230Th/234U system gradually builds up to equilibrium. The time-dependent increase of 230Th/234U enables the determination of the time t elapsed since the event of disturbance.

The essential requirements for dating are that the initial abundance of the daughter is, unless known, negligibly small and that after the disturbance the radioactive system remains closed. Complications in 230Th-234U dating have various sources: (1) During the incorporation, 234U is not necessarily in equilibrium with its radioactive predecessor 238U, since in groundwater the activity ratio is 234U/238U > 1 so that the 234U/238U ratio must be determined and taken into consideration for age calculation. (2) The requirement that at deposition the sediment is free of 230Th is frequently not fulfilled since thorium, and consequently also 230Th, may be present. Negligible 230Th contamination is indicated by 230Th/232Th

> 20. (3) The system may experience a secondary opening as a result the geochemical mobility of the uranium.

Originally, the activity of the Th and U isotopes was analyzed by a-spectrometry. The introduction of thermoionization mass spectrometric (TIMS) by Edwards et al. (1986/87), which requires less than 1 g of sample material and provides high age precision, brought a great impetus for 230Th-234U dating. Under favorable circumstances age precision better than 1% can be obtained. The uranium content should be more than 0.1 mg/g.

When sampling speleothems (calcareous flowstones formed on the cave floor), dense and pure carbonates should be collected in order to minimize the dangers of open system behavior and 230Th contamination by detrital components. In the Tongtianyan Cave, Guangxi, China, one of the few fossils of H. sapiens in China was discovered. Several speleothem layers, intercalated in the cave sediments, were dated by 230Th/234U. The age results of 13.5 ± 2.3, 63.1 ± 2, and 148 ± 4 ka for three layers from top to bottom, respectively, follow the stratigraphic order (Shen et al. 2002). Although the stratigraphic position of the hominid find is to some extent uncertain, it is certainly below the second layer. This renders this finding as one of the earliest in East Asia, indicating that the Moderns arrived there before 63 ka. Depending on the measurement precision, the 230Th/234U system in speleothems older than 350 ka often shows equilibrium so that only a terminus ante quem can be given for the age. This situation was met in the fossil hominid sites known as the Sima de los Huesos, Spain, and the Caune de l'Arago, France. At Sima de los Huesos, the numerous human individuals, which are considered as evolutionary ancestors of the Neanderthal, are overlain by a speleothem that is in equilibrium and thus older than 350 ka (Bischoff et al. 2002). Analogously, the pre-Neanderthals found in the Middle Pleistocene unit III at Caune de l'Arago must be older than 350 ka (Falgueres et al. 2004). At the famous Zhoukoudian site near Peking, with the occurrence of H. erectus, commonly known as Peking Man, speleothems that are intercalated in the fossiliferous sediments were 230Th/234U-dated. In most samples, the 230Th/234U system was close to equilibrium. However, high-precision analysis of these nuclides enabled to push the dating method to its upper limits, which is at 500-600 ka age range with the present instrumentation (Shen et al. 2001). The results indicate that the youngest member of H. erectus at this site is >400 ka and the oldest members from the lower layers significantly older than 500 ka.

The 230Th/232Th-234U/232Th-isochron technique permits determining the initial 230Th/232Th ratio. Schwarcz (1989) used this technique for the dating of the sinter crust on the cranium ofthe classic Neanderthal at Monte Circeo, central Italy. The encrustation on the cranium consisted of a brighter inner and a dark-brown outer layer. The outer layer provided 16 ka. From the inner layer, containing detrital contamination, several subsamples of different U/Th ratio were obtained through fractionated leaching. The slope of the straight line (isochron) through the data points in the 230Th/232Th-234U/232Th diagram gives an age of

Precise 230Th-234U dating of travertine, a calcareous spring-sinter, is hampered by the common detrital 230Th contamination and by open system behavior. Applying a microsampling technique, in which 100 mg were selectively drilled from the micrite/spar phases, Mallick and Frank (2002) dated successfully travertine from various Thuringian sites, among them Weimar-Ehringsdorf with pre-

Neanderthal remnants. The results assign these finds firmly to oxygen isotope stage 9.

Buried teeth absorb uranium from the groundwater. The knowledge of the time-function of the uranium uptake is crucial for the 230Th/234U-age evaluation. Since the exact temporal development of uptake is unknown, one has to rely on models such as early uptake (EU) or linear uptake (LU) (cf. ESR). For instance, in order to strengthen dating of the long coexistence of early Moderns and Neanderthals in the Levantine corridor (cf. luminescence), McDermott et al. (1993) applied mass spectrometric 230Th-234U dating to mammal teeth from the Middle Pleistocene sites Tabun, Qafzeh and Skhul, Israel. They showed that at Tabun, both types of humans were approximately coeval some 100 ± 5ka. The analyses were carried out on 50-100 mg subsamples of dentine and enamel of the same tooth, whereby it is important that the ages agree to each other, although both subsamples differ strongly in U content and in porosity, indicating EU and closed-system behavior. The mode of uranium uptake can be constrained when

230Th/234U

is combined with ESR-dating (Grün et al. 1988), which in the meantime became the common practice. Falgueres et al. (1997) reported coupled 230Th/234U and ESR-dating on horse teeth from Acheulian and Mousterian levels at Micoque, France. The 230Th/234U ages of enamel and dentine range widely from 150 to >350 ka. However, when combined with ESR-dating, consistent ages between 300 and 350 were obtained.

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