南京直立人的U/Th和U/Pa年代

U/Th and U/Pa Dating of Nanjing Man

中国直立人化石的准确定年对于研究人类演化有着极为重要的意义。1993～1994年在南京汤山葫芦洞发现的两个直立人头盖骨化石和一枚牙化石被称为“南京直立人”。其中1号头盖骨化石之上的方解石钙板的U/Th年龄为53.3_(-1.2)^(+1.5)万年;但考虑到定年的准确性,则为53.3_(-3.0)^(+3.5)万年。其^(231)Pa/^(235)U活度比值为0.998±0.006。这表明“南京直立人”的年代应该大于50万年。与”南京直立人”伴生的动物牙化石U/Th年代为18.5～29.0万年;U/Pa年代为13.7～17.2万年。此外,对于同一颗牙化石,牙釉的年龄小于牙本质的年龄。同一样品的U/Pa年龄也显著小于其U/Th年龄。因此,牙化石的U摄取过程并不符合U早期摄取模式。多数牙化石分析点在^(234)U/^(238)U-^(230)Th/^(238)U图上落在位于U早期摄取和线性摄取模式曲线之间,指示牙化石的U摄取过程很可能介于上述两种模式之间。如果这一假设成立,那么牙化石的U/Th和U/Pa线性摄取模式年龄则为其年代的上限。因为不受U摄取过程^(234)U/^(238)U变化的影响,U/Pa线性摄取模式年龄比U/Th较为可靠。最小的U/Pa线性摄取模式年龄为1Ma,这是”南京直立人”上限年龄的估计。从定年结果看,”南京直立人”可能生活在海洋同位素(MIS)16阶段,但这不是最终结论。

Evidence from China is crucial in order to understand hominid evolution, with age dat- ing being critical to interpret the evidence. Homo erectus fossils, including two skulls and a tooth, were discovered interbedded with cave deposits in the suburbs of Nanjing in 1993 and 1994, and are referred to as 'Nanjing Man'. U/Th ages of the calcite flowstone overlying the Nanjing Man fossils are 533_(-12)^(+25) ka or 533_(-30)^(-35) ka, if considered accurately. The [^(231)pa/^(235)U] activity ratio of the flowstone is 0.998±0.006, indicating concordance of U/Th and U/Pa ages. A stratigraphically younger stalagmite gives an age progression consistent with the internal stratigraphy of the stalagmite: 465_(-35)^(+45); 398±15 ka; 350±10 ka; 260±8 ka and 160±2 ka. Our data indicate that Nanjing Man is older than 500 ka. We analyzed five animal teeth from the Nanjing site by TIMS. U/Th ages vary from 185 to 290 ka and U/Pa ages from 137 to 172 ka. For both U/Th and U/Pa ages, the enamel age is less than that of the dentine for the same tooth. In addition, the U/Pa age of each tooth subsample is less than its U/Th age by 24～40%. Thus, these teeth do not follow an early uranium uptake model. In a [^(234)U/^(238)U] vs. [^(230)Th/^(238)U] dia- gram, the tooth data plot in a field between early U uptake (EU) and linear U uptake (LU) curves. This indicates a U uptake history between the EU and LU model, which has been the general consensus on uptake history for these types of samples. In this case, both U/Th and U/ Pa LU model ages will be the upper limit of the samples. However, the U/Pa age may give a bet- ter estimation because it is not affected by the possible variation of input 234U/238U ratios. The minimum U/Pa LU model age is 1 Ma, which gives a constraint on the upper limit of Nanjing Man age. Although possible, it is not conclusive that Nanjing Man lives in marine 160 isotope stage 16 (MIS16).