柳江现代智人化石地点的地层及年代

产现代智人化石的“柳江人洞”遗址现存8m多厚堆积层可分为4个单元,即上部钙板-粘土层;中上部切入下伏地层的楔状角砾堆积体;中部钙质胶结的含化石和角砾粉砂质粘土层;下部河流相砂层。覆盖楔状角砾堆积和含角砾粉砂质粘土层的第2钙板层年代大约为70kaB.P.。中部地层的上部有两个被楔状角砾堆积切断的钙板残层,年代分别约为150kaB.P.和220kaB.P.,表明在大约150kaB.P.之后该地点曾发生冲刷-再沉积事件。据前人报告,“柳江人”化石出土于楔状角砾堆积,其年代大约为150～70kaB.P.。另外,不能排除人类化石埋藏于中部地层的可能性,如是,其年代应早于150kaB.P.。无论上述两种可能性何者成立,“柳江人”化石的年代都不会晚于约70kaB.P.。这大大早于以前的估计,并使“柳江人“成为迄今已知东亚南部最早的具有现代人解剖特征的化石智人。与“柳江人”化石共生的哺乳动物群原先归属晚更新世,现应调整为晚更新世早期至中更新世晚期。

STRATIGRAPHY AND CHRONOLOGY OF DEPOSITS IN “LIUJIANG HOMINID CAVE”,GUANGXI,CHINA

Stratigraghic and chronological studies demonstrate that the >8-m-thick deposits of “Liujiang Hominid Cave”, from which a skull and a specific partial skeleton of modern Homo sapiens was found, can be divided into four main units, each corresponding to a specific depositional pattern. The upper unit with a total thickness of 0.6～0.9m, being formed during a period when seeping-water deposition was predominant, consists of silty clay sandwiched in flowstone layers. The middle unit is composed of ca. 6m thick fossiliferous, gravel-bearing and carbonate-cemented sandy clay, representing floodwater deposition with fallen blocks from walls and ceiling. The lower unit at the base of depositional sequence is composed of more than 2m thick fluvial sands with lenses of clay resulting from underground river deposition. A cone-shaped refilling breccia, the upper-middle unit, which reveals a flood washout and re-depositional event, cuts into the upper part of the middle unit. A total of twelve calcite samples were taken from the capping and 2 nd , 3 rd , 4 th and 5 th flowstone layers. A secondary calcite vein and a calcite fragment in the refilling breccia were also sampled. On these samples sixteen 230 Th/ 234 U age determinations were performed with conventional α spectrometry. Moreover, high-precision TIMS U-series dates were obtained on eight key samples. The results show that the five flowstone layers were formed ca. 16±1kaB.P., 68±1 ～62±1kaB.P., 153±2kaB.P., 223±7kaB.P. and 278±43kaB.P. respectively, while the calcite vein and the calcite fragment date to 111±6kaB.P. and 139±4kaB.P. respectively. Being an amateur recovery, the provenience of the Liujiang skeleton has been clouded with uncertainty. According to Youheng Li, the first geologist arrived at the site, the human and Ailuropoda melanoleucus skeletons were “embedded in the deposits of unconsolidated breccia”, while other mammalian fossils were in the “consolidated yellowish deposits”. If so, the association of the hominid fossils with the refilling breccia seems the most plausible. This would date them to 153～68kaB.P., and more likely to 139～111kaB.P. However, the possibility that the hominid fossils were included in the carbonate-cemented sandy clay of the middle unit cannot be excluded. In this case the hominid fossils would be older than 153kaB.P. as defined by the overlying 3 rd flowstone layer. Both of the two aforementioned scenarios demonstrate that this site should be much older than previous estimation. The Liujiang specimen documents thus the earliest modern humans in Southeast Asia, possibly contemporaneous with its Levantine and African counterparts. The mammalian fossils were derived from the consolidated sandy clay of the middle unit, being thus in the range of 280～153kaB.P., i.e. Late Middle Pleistocene in age instead of previously and commonly accepted Late Pleistocene.