黄土碳酸盐古气候意义及其研究展望

Research Progress on the Paleoclimate Significance of Loess Carbonate

碳酸盐是黄土中的主要矿物,是黄土—古土壤序列中含量变化最显著的矿物之一,其在表生环境下极易移动,并且它的迁移变化受大气降水、温度等古气候因素制约,在黄土高原风尘堆积序列的古气候研究中具有极其重要的意义,其变化能够较好地反映古季风的演变规律。黄土—古土壤中碳酸盐的含量变化和东亚夏季风降水的强度密切相关,碳酸盐矿物中白云石和方解石的存在与否及淋失深度对夏季风演变同样有很好的指示意义。稳定同位素δ^(18)O可以作为夏季风的代用指标,指示成壤时期环境中的古温度;δ^(13)C值表现出在黄土层中高,而在古土壤层中低,因此碳酸盐δ^(13)C值可能更多反映了植被发育程度。碳酸盐中Sr同位素能够反映风化作用的强弱,而去除碳酸盐后的Sr同位素则能更好地反映成壤作用的强度和判别风尘物质的来源。与其它季风气候替代性指标相比,酸溶相中的δ^(11)B值变化与降水有直接关系,可以很好地作为夏季风降水指标。非传统稳定同位素Li、Ba和Fe有应用于黄土研究中,但目前的研究表明它们对气候环境暂无较好的指示意义。

Carbonate is the main mineral in loess,with the most significant content changes in the loess-paleosoil sequence.It is easy to migration in the superficial environment,and its migration is restricted by paleoclimatic factors such as atmospheric precipitation and temperature.Carbonate plays an important role in the study of palaeoclimate,and the content variation is closely related to the intensity of East Asian summer monsoon.The existence of dolomite and calcite in carbonate minerals and the leaching depth are also very important indicators for the evolution of summer monsoon.Stable isotope δ^(18)O and δ^(13)C values can be used as proxy indexes of summer monsoon,indicating the evolution of palaeo-temperature and palaeo-vegetation types in the loess area.When the climate is warm and humid with large rainfall and prosperous vegetation,the value of δ^(13)C is light,and the value of δ^(18)O is high;When the climate is dry and cold,with low rainfall and sparse vegetation,the value of δ^(13)C is heavy and the value of δ^(18)O is low.The ^(87)Sr/^(86)Sr ratio in carbonate can reflect the intensity of weathering,while the ^(87)Sr/^(86)Sr ratio of acid-insoluable residues(removal of carbonate)reflects the intensity of pedogenesis and act as a tracer for dust origin.Compared with other monsoon climate substitution indexes,the change ofδ11 B values in the acid-soluble phase is directly related to precipitation,which can be used as an indicator of summer monsoon precipitation.The non-traditional stable isotopes Li,Ba and Fe have been used in loess research,but they have no good indications for the climate and environment.