千年尺度事件的结构特征和区域响应是理解和认识冰期气候变化及动力机制的关键。本文基于陕西汉中宁强县地洞河一支长417 mm石笋DDH1的11个U-Th年代和417个氧同位素(δ18O)数据,重建了末次冰期(64.8~33.2 ka B.P.)亚洲季风演化历史。石笋δ18O记录在64.7±0.2 ka B.P.和59.3±0.1 ka B.P.快速负偏,分别指示了DO18事件的开始和H6事件的结束。在DO18事件结束过程,石笋δ18O记录在64.0~60.4 ka B.P.持续正偏,振幅达2.9 ‰,反应季风逐渐减弱,但未出现明显转型阶段。其总体形态与大西洋Cariaco盆地岩芯反射率、南极冰芯的记录较为一致,但是不同于格陵兰冰芯的记录。因地洞河所处青藏高原、秦岭叠加影响的区域,纬度和地形的双重影响使得其石笋δ18O记录较其他地区偏负,振荡幅度更大。因地形阻挡了高纬信号,使得低纬甚至南半球的信号突现出来,ITCZ的移动、南半球温度和水汽潜热的释放作用在这一区域尤为显著。
The iron stable isotope compositions (δ56Fe) and iron valence states of ultrahigh‐pressure eclogites from Bixiling in the Dabie orogen belt, China, were measured to trace the changes of geochemical conditions during vertical transportation of earth materials, for example, oxygen fugacity. The bulk Fe3+/ΣFe ratios of retrograde eclogites, determined by Mössbauer spectroscopy, are consistently higher than those of fresh eclogites, suggesting oxidation during retrograde metamorphism and fluid infiltration. The studied eclogites (five samples) display limited mid‐ocean ridge basalts (MORB)‐like (~0.10‰) δ56Fe values, which are indistinguishable from their protoliths, that is, gabbro cumulates formed through differentiation of mantle‐derived basaltic magma. This suggests that Fe isotope fractionation during continental subduction is limited. Garnet separates display limited δ56Fe variation ranging from ?0.08 ± 0.07 ‰ to 0.02 ± 0.07‰, whereas coexisting omphacite displays a large variation of δ56Fe values from 0.15 ± 0.07‰ to 0.47 ± 0.07‰. Omphacite also has highly variable Fe3+/ΣFe ratios from 0.367 ± 0.025 to 0.598 ± 0.024, indicating modification after peak metamorphism. Omphacite from retrograde eclogites has elevated Fe3+/ΣFe ratios (0.54–0.60) compared to that from fresh eclogites (~0.37), whereas garnet displays a narrow range of ferric iron content with Fe3+/ΣFe ratios from 0.039 ± 0.013 to 0.065 ± 0.022. The homogenous δ56Fe values and Fe3+/ΣFe ratios of garnet suggest that it survived the retrograde metamorphism and preserved its Fe‐isotopic features and ferric contents of peak metamorphism. Because of similar diffusion rates of Fe and Mg in garnet and omphacite, and constant Δ26Mgomphacite‐garnet values (1.14 ± 0.04‰), equilibrium iron isotope fractionation between garnet and omphacite was probably achieved during peak metamorphism. Elevated Fe3+/ΣFe ratios of omphacite from retrograde eclogites and variant Δ56Feomphacite‐garnet values of the studied eclogites (0.13 ± 0.10‰ to 0.48 ± 0.10‰) indicate that oxidized geofluid infiltration resulted in the elevation of δ56Fe values of omphacite during retrograde metamorphism. 相似文献