青藏高原东北部黄土次生碳酸盐氧同位素的古气候意义

Paleoclimatic Implications of Oxygen Isotope from Authigenic Carbonates in Loess Deposit of Northeastern Tibetan Plateau

黄土中含有较为丰富的碳酸盐,其中次生碳酸盐记录了成壤时期的气候和环境,可以用来重建古气候.测试了末次盛冰期和全新世早期青藏高原东北部乐都、中国东北和新疆巴音布鲁克黄土中次生碳酸盐的δ^(18)O值,结果显示高原东北部这两个时期的δ^(18)O值分别为-1.38‰和-5.58‰,比该地区现代季风气候条件下次生碳酸盐δ^(18)O理论值分别高5.74‰和1.54‰.温度的差异不足以导致次生碳酸盐δ^(18)O值如此幅度的变化.同时,末次盛冰期和全新世早期乐都地区黄土中次生碳酸盐的δ^(18)O值比同时期黄土高原和东北地区等东亚季风区明显偏正约2.0‰~6.2‰,但其与新疆等西风区次生碳酸盐的δ^(18)O相近.这些不同时期黄土中次生碳酸盐的δ^(18)O值的分布特征及空间差异,可能反映东亚夏季风夹带的水汽可能不是青藏高原东北部末次盛冰期和全新世早期降水最重要的来源,而西风降水和(或)局部水汽蒸发循环对该地区的降水可能有重要贡献.青藏高原东北部黄土次生碳酸盐的δ^(18)O值从末次盛冰期到全新世早期逐渐降低,这可能是由于气候由干冷向暖湿转变、有效湿度增加而导致的.有限的数据表明不同气候带(青藏高原东北部、东亚季风区和西风带)的黄土中次生碳酸盐氧同位素存在明显差异,它所代表的气候意义值得进一步的深入研究.

The carbonate in loess deposit records the pedogenic environment, especially the moisture information, and can be used to qualitatively and/or quantitatively reconstruct paleoclimate. The δ18O values of the pedogenic carbonate in loess of LGM and Early Holoeene, from Ledu （northeastern Tibetan plateau）, Northeast China and Bayanbulak basin in Xingjiang, were measured. It showns that the δ18 O values of the pedogenic carbonate in loess of LGM and Early Holocene from northeastern Tibetan plateau are -1.38 ‰ and -5.58‰, which are 5.74‰ and 1.54‰ respectively higher than the theoretical values of δ18O under the condition of modern monsoon climate. However, the temperature difference was not enough to cause this significant magnitude change of the δ18O values in the pedogenic carbonate. Compared with that in East Asian monsoon region including the Chinese Loess plateau and the Northeast China, the values of the δ18O of the pedogenic carbonate in loess from Ledu during LGM and the Early Holocene are ～2.0‰0-6.2‰ higher obviously, but similar to those in the westerly regions such as Xin-jiang. The distribution characteristics and spatial differences of the δ18O values of the pedogenic carbonate in loess in different periods may reflect that the East Asian summer monsoon was not the controlling factor of the precipitation during the LGM and early Holocene in the northeastern Tibetan plateau （NETP）, while the westerly rainfall and/or local vapor evaporation cycle had an important impact in this period. The climate changing from dry and cold to warm and humid, with higher temperature but less evaporation, could be the reason for the gradual decrease of the δ18O values of the pedogenic carbonate in the loess from LGM to Early Holocene in NETP. The 8180 values of the pedogenic carbonate in the loess from different climatic zones （NETP, the East Asian monsoon region and the westerly zone）, revealed by limited data in this study, are significantly different, with which climatic indication is worth further investigation.