中国黄土高原沉积物稳定碳同位素指标在古植被环境研究中的进展

重建古植被、古气候演化历史一直是地质学领域的研究热点，恢复晚新生代以来古植被环境变化不仅可以加深气候驱动机制的理解，而且可以更准确地预测未来气候变化趋势。黄土沉积中不同载体（如土壤总有机碳、次生碳酸盐、正构烷烃和黑碳等）的稳定碳同位素组成响应植被和气候变化，被广泛应用于古植被环境研究。然而，黄土沉积中不同碳同位素组成的影响因素复杂等又在一定程度上限制了指标的准确性及不同指标间的相互印证。分析中国黄土高原22个典型沉积剖面约2500个样品的总有机碳稳定碳同位素（δ¹³C_TOC）与磁化率数据表明，二者具有复杂的时间、空间特征。约100ka以来，绝大部分剖面δ¹³C_TOC与磁化率的相关性较好，指示夏季风强盛时C₄植物增多。但在更长时间尺度上，要充分考虑δ¹³C_TOC分馏和区域气候差异的影响。空间演化上，现代季风-非季风区分界线附近的剖面δ¹³C_TOC与磁化率的相关性极高（R²>0.7，p < 0.001），可能指示该区域植被对季风强弱变化十分敏感。综合对比灵台、蓝田、白水等6个剖面的次生碳酸盐碳同位素记录（δ¹³C_PC），将该指标用于黄土高原地区古植被比例重建时，首先要修正大气CO₂的影响，也可利用黄土序列δ¹³C_PC记录来进行重建古大气pCO₂等新的研究。目前，正构烷烃单体碳同位素（δ¹³C_n-alkanes）相关记录较少，研究发现该指标可较好地指示植被演化，渭南剖面的δ¹³C_n-alkanes记录呈现出与古温度、古降水等气候指标较好的一致性，但样品提取困难也阻碍了δ¹³C_n-alkanes的深入研究。黑碳稳定碳同位素（δ¹³C_BC）更多反映的是燃烧植被信息，在以草本和灌木植被类型为主的区域与δ¹³C_TOC记录更为一致，可以较为准确地指示生长植被类型的变化。

A review for the stable carbon isotope proxies in the paleovegetation researches in the loess-paleosol deposits on the Chinese Loess Plateau

The Chinese Loess Plateau is located in the Northern Hemisphere mid-high latitudes and its continuous loess-paleosol deposit is widely distributed, providing reliable records of vegetation and climate changes since the Late Cenozoic. Not only can reconstructing the evolutionary history of palaeovegetation and paleoclimate on the Chinese Loess Plateau deepen the understanding of the forcing mechanisms of global climate change, but also predicts future climate change. The stable carbon isotope of different proxies(such as soil organic carbon, pedogenic carbonate, n-alkanes, black carbon, etc.) in loess deposits controlled by vegetation and climate changes, are widely used to evaluate the relative abundance of C3 versus C4 plants. To some degree, the external sources of loess deposits and the complex factors of carbon isotopes(δ13C) restrict the accuracy of the indicators and the mutual verification of different proxies. In this article, δ13C records from 33 loess-paleosol sequences on the Chinese Loess Plateau are collected to summarize current researches of four different proxies above. It is found that the stable carbon isotope of total organic carbon(δ13CTOC) and the magnetic susceptibility have complex spatio-temporal characteristics through analyzing about 2500 samples from 22 typical loess-paleosol profiles on the Chinese Loess Plateau. First of all, the change of δ13CTOC does not necessarily mean the change of relative abundance of C3 versus C4 vegetation, and the correlation between δ13CTOC and magnetic susceptibility varies in different regions, such as near the boundary between monsoon and non-monsoon region where the δ13CTOC correlates with magnetic susceptibility very well(R2>0.7, p < 0.001), which might indicate that vegetation types are very sensitive to the changes of monsoon intensity in this area. The correlation between δ13CTOC and magnetic susceptibility on the same profile on a short time scale(about 100ka) is much higher than that of a million-year time scale. Therefore, total organic carbon degradation and the responding differences between δ13CTOC and magnetic susceptibility to climate changes on the long-term scale should be fully considered. Correcting the impact of atmospheric CO2 is of primary importance when the pedogenic carbonate carbon isotope(δ13CPC) is used to reconstruct the proportion of paleovegetation on the Loess Plateau. The previous δ13CPC records might also be applied for atmospheric pCO2 reconstructions. There are few loess-paleosol records of n-alkane carbon isotope(δ13Cn-alkanes) on the Chinese Loess Plateau so far. It is found that δ13Cn-alkanes records of Weinan section are in good agreement with the tetraether-derived paleotemperature and aridity records. However, the extracting of n-alkanes is difficult which hinders its further study. The stable black carbon isotope(δ13CBC) is an ideal proxy of burning vegetation, and it is more consistent with the δ13CTOC records in some specific areas where are dominated by herbaceous and shrub plants.