石笋初始234 U/238 U值的冰量周期特征及其环境意义 ——以湖北三宝洞为例

石笋氧同位素记录具有明显的2万年周期，其他记录中广泛存在的10万年周期是否在石笋中有所表现目前还鲜有报道。通过对湖北三宝洞20支石笋的铀同位素数据的分析研究发现，石笋初始234U/238U值在序列连续性较好的640.3~299.6 ka B.P.时间段有强烈的10万年周期特征。在间冰期和冰期时，初始234U/238U值分别呈增大和减小状态。初始234U/238U值的10万年周期与全球冰量、黄土磁化率、黄土平均粒度和大气CO₂变化有良好的对应关系。这些对应关系表明全球冰量、大气CO₂对喀斯特区地球化学元素富集和迁移作用有重要影响。石笋氧同位素的显著岁差周期独立于石笋微量元素、高纬冰量和全球温室气体变化，暗示了太阳辐射变化对中低纬水汽环流的直接影响。石笋初始234U/238U与氧同位素、太阳辐射在冰消期时的对应变化支持北半球太阳辐射能量变化对冰期-间冰期旋回的调控作用。

Ice Volume Cycle Characteristics and the Environmental Significance of the Initial 234U/238U Ratio Inferred from Stalagmites: A case study from Sanbao Cave,Hubei

Obvious 23 000-year-long cycles have been identified in stalagmite oxygen isotope records, but 100 000-year cycles are also widely recorded in other geological records. Based on the analysis of isotopic data for 20 stalagmites from Sanbao Cave, Hubei, it is found that the initial 234U/238U ratio has a strong cycle of 100 000 years in the period 640.3-299.6 ka B.P. The values of (234U/238U)₀ increases during interglacial periods and decreases in glacial periods. The 100 000-year cycle of (234U/238U)₀ corresponds closely with changes in global ice volume, loess grain size and atmospheric CO₂ concentration. This coherence indicates that the changes of global ice volume and CO₂ concentration had important effects on the enrichment and migration of geochemical elements in karst areas. These observations indicate that the oxygen isotope record for the precession cycle in stalagmites is independent of the influences of high-northern-latitude ice volume and global greenhouse gases, suggesting a direct effect of solar radiation change on low-latitude water circulation. Corresponding changes of (234U/238U)₀, oxygen isotope and insolation in terminations support the hypothesis that the northern hemisphere summer insolation modulate the glacial to interglacial cycle.