Humidity changes in southern Tunisia during the Late Pleistocene inferred from U-Th dating of mollusc
Humidity changes in southern Tunisia during the Late Pleistocene inferred from U-Th dating of mollusc
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DOI：
10.1016/s0883-2927(03)00043-x
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发表时间：
2003-11-01
影响因子：
3.4
通讯作者：
Mamou, A
中科院分区：
文献类型：
--
作者：
Causse, C;Ghaleb, B;Mamou, A
钙质沉积物主要由软体动物贝壳堆积物组成，已通过 U/Th 不平衡法测年，标志着突尼斯南部大乔茨地区古湖高地的海岸线。研究的 5 个地点由类海洋生物化石的不连续堆积组成，例如：Cerastoderma glaucum、Melania tuberculata、Melanopsis praemorsa、Cerithium rupestre。这里报告了 39 个贝壳样品的 U/Th 等时线图和年龄频率直方图。壳的 U 含量和 U-234/U-238 活度比 (AR) 的有限变化支持了关于该元素的地球化学系统闭合的假设。值得注意的是，在 Chott Fejej 或 Chott Jerid 收集的贝壳的 U-234/U-238 AR 聚集在不同的值周围，反映了大陆 Intercalaire (CI) 或 Complexe Terminal (CT) 含水层的地下水补给可能不同。此外，在 Wadi el Akarit 附近收集的水显示 U-234/U-238 AR 值与观察到的贝壳值相当。对其中一些样品的等分试样进行的 C-14 测定表明年龄分布在 18 至 34 ka BP 之间。这 39 个贝壳样本的 U/Th 数据表明，这些湖泊的 4 次不同的洪水事件发生在大约 30、95-100、130-150 和 180-200 ka。对于以 30 ka 为中心的事件，年龄的频率直方图显示了一个多峰年龄组，可以代表多个潮湿脉冲的存在，而不是一个独特的事件。此外，当记录了非常大的古湖泊时，将δ(13)C和δ(18)O与更古老的湿润更新世阶段的δ(13)C和δ(18)O进行比较，表明这些年轻的碳酸盐壳与真正的高位湖泊无关。有人认为，它们代表了一个不太潮湿的气候条件时期，碳酸盐在小水塘中积累，其中可能发生了密集的生物活动。应该指出的是，这个时期比现在要短。 (C) 2003 Elsevier Ltd. 保留所有权利。
Calcareous deposits, mainly consisting of mollusc shell accumulations, which have been dated by the U/Th disequilibrium method, mark the shorelines of paleolake highstands in the Great Chotts Area of Southern Tunisia. The 5 sites' studied consist of discontinuous accumulations of fossils of marine-like organisms e.g.: Cerastoderma glaucum, Melania tuberculata, Melanopsis praemorsa, Cerithium rupestre. U/Th isochron plots and age frequency histograms for 39 shell samples are reported here. Limited variations for U content and U-234/U-238 activity ratios (AR) of shells support the hypothesis of closure of the geochemical system with respect to this element. It is remarkable that U-234/U-238 AR of shells collected in Chott Fejej or Chott Jerid are clustered around different values, reflecting probably different groundwater recharge from the Continental Intercalaire (CI) or Complexe Terminal (CT) aquifers. Furthermore waters collected near Wadi el Akarit show U-234/U-238 AR values comparable to those observed for shells. C-14 determinations made on aliquots of some of these samples suggested an age distribution between 18 and 34 ka BP. The U/Th data of these 39 shell samples imply that 4 distinct flood episodes of these lakes occurred at about 30, 95-100, 130-150 and 180-200 ka. For the episode centred around 30 ka, the frequency histogram of ages shows a multimodal age group that could represent the existence of several humid pulses rather than a unique event. Moreover, the comparison of delta(13)C and delta(18)O with those of older humid Pleistocene phases, when very large palaeolakes have been recorded, suggests that these young carbonate shells are not related to a true highstand lake. It is suggested that they represent a period of less humid climatic conditions with carbonate accumulation in minor water ponds in which intensive biological activity could have taken place. It should be noted that this period was less and than the present. (C) 2003 Elsevier Ltd. All rights reserved.