VARIATION IN URANIUM ISOTOPES OF STALAGMITES FROM SANBAO CAVE, HUBEI PROVINCE: IMPLICATIONS FOR PALAEOCLIMATE
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摘要: 通过对湖北三宝洞17支石笋160个测年样品的铀同位素数据分析,发现过去18万年以来石笋238U值长尺度变化与其δ18O记录有一定的负相关关系,与西太平洋海表温度变化具有良好的正相关。石笋238U值在间冰期波动剧烈,而在冰期波动相对平缓,在冰期-间冰期转型时238U值"爆发式"增大。在轨道尺度上,高浓度238U对应于石笋平均生长速率的高值期,而低浓度值对应于生长速率的低值期。上述结果表明与外界气候变化相关的洞穴上覆土壤的成壤过程和岩溶水文变化可能是控制石笋238U含量变化的主要因素,石笋238U可以作为一个新的洞穴气候环境代用指标,但仍须谨慎对待。Abstract: A great number of oxygen isotope ratios and U-Th ages data were collected and studied by the authors for the stalagmites from the Sanbao cave, Hubei Province, China. Uranium-series dating of the speleothem suggests that the stalagmites are formed in the period from the penultimate glacial period up to the present. This paper deals with the relationship between uranium concentration in stalagmites and paleoclimate data induced from the 160 precise ICP-MS 230Th data collected from 17 stalagmites from the Sanbao Cave. We discovered that the variations in 238U show a negative relation with the δ18O records of the studied stalagmites, and a positive relation with the temperature changes in West Pacific Warm Pool. The concentration of 238U in the stalagmites fluctuated rapidly during the interglacial periods, slowly during the glacial periods, and increased sharply in the past two terminations. On the orbital scale, the higher the average growth rate, the higher the uranium concentration is, and vice versa. The results also show that the pedogenic processes of the soil profile above the cave and the complex soil-water-rock interaction are well related with the outer-cave climate changes, possibly responsible for the shifts of uranium concentration. However, the shift of the 234U/238U in stalagmites seems having no any relation with the external palaeoenvironment over the past 180000 years. Therefore, the uranium concentration in the stalagmite could only be used as a new proxy for climate change with caution.
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Keywords:
- stalagmites /
- U concentration /
- palaeoclimate /
- Sanbao cave
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