Constraining Holocene lake-level highstands on the Tibetan Plateau by 10Be exposure dating: a case study at Tangra Yumco, southern Tibet
Constraining Holocene lake-level highstands on the Tibetan Plateau by 10Be exposure dating: a case study at Tangra Yumco, southern Tibet
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DOI：
10.1016/j.quascirev.2013.09.016
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发表时间：
2013-12
影响因子：
4
通讯作者：
E. Rades;R. Hetzel;Qiang Xu;L. Ding
中科院分区：
文献类型：
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作者：
E. Rades;R. Hetzel;Qiang Xu;L. Ding
青藏高原上的许多湖泊都被古海岸线所包围，这些海岸线记录了以前的湖泊水位高点，从而记录了过去的气候变化。为这些海岸线建立准确的年表对于将湖泊水位波动与通常从湖泊沉积记录中推断的古环境历史相关联至关重要。在这里，我们应用表面暴露测年，以限制在西藏最大的湖泊之一-坦格拉Yumco的两个湖平面高的年龄-其中有发达的古海岸线位于高达185米以上的当前湖泊。对于采样，我们集中在波浪切割的基岩阶地，这些阶地的宽度变化很大，因为它们被雕刻成不同可蚀性的岩石。3个不同地点的最高斯特拉特阶地（湖上180-185 m）样品的~（10）Be年龄在7.39 ± 0.19 ka和7.87 ± 0.27 ka之间（内部误差），这将全新世早期湖平面最高阶地的结束限制在7.6 ± 0.6 ka（外部误差）。10 Be年龄的一致性表明，基岩侵蚀波攻击减少了继承的10 Be组件到可以忽略不计的金额，因为否则样本将显示相当大的年龄分散。两个来自低阶地（距湖面140-145 m）的基岩样品和一个来自同一阶地上的海滩脊的混合碎屑样品的~（10）Be年龄在4.04 ± 0.14 ~ 4.50 ± 0.15 ka之间。这些年龄记录表明，在17.6 ~ 14.3ka之间，湖平面下降了1.40 m。花岗岩巨石从两个调查阶地产生明显的10 Be年龄是106 ka和1027 ka的年龄比各自的阶地，表明这种样品类型是不可靠的，在这种设置。总的来说，我们的研究结果表明，基岩阶地暴露测年提供了一个有价值的工具，以重建青藏高原的湖泊水平的历史。
Many lakes on the Tibetan Plateau are surrounded by palaeo-shorelines that document former lake-level highstands and hence past changes in climate. Establishing accurate chronologies for these shorelines is crucial to correlate lake-level fluctuations with palaeo-environmental histories that are commonly inferred from lake sedimentary records. Here we apply surface exposure dating to constrain the age of two lake-level highstands at one of the largest Tibetan lakes – Tangra Yumco – which has well developed palaeo-shorelines located up to ∼185 m above the current lake. For sampling we focussed on wave-cut bedrock terraces that vary considerably in width, because they were carved into rocks of different erodibility. Samples from the highest strath terrace (∼180–185 m above the lake) at three different sites yield tightly clustered10Be ages between 7.39 ± 0.19 ka and 7.87 ± 0.27 ka (internal errors), which constrain the end of an early Holocene lake-level highstand at 7.6 ± 0.6 ka (external error). The consistency of the10Be ages indicates that bedrock erosion by wave attack reduced the inherited10Be component to negligible amounts, because otherwise samples would show considerable age scatter. Two bedrock samples from a lower terrace (∼140–145 m above the lake) and one amalgamated clast sample from a beach ridge on the same terrace yield10Be ages between 4.04 ± 0.14 ka and 4.50 ± 0.15 ka. These ages document that the lake level dropped by ∼40 m between ∼7.6 ka and ∼4.3 ka. Granite boulders from the two investigated terraces yield apparent10Be ages that are ∼6 ka and ∼27 ka older than the ages of the respective terraces, indicating that this sample type is not reliable in this setting. Overall, our results demonstrate that exposure dating of bedrock terraces provides a valuable tool to reconstruct lake-level histories on the Tibetan Plateau.