罗布泊沉积物多相态Mg同位素的分离提取及其古气候指示意义

Multi-phase Mg isotope extraction from Lop Nor sediments and implications for paleoclimate

晚中新世地球气候格局和生态系统转换的原因是学界长期关注并富有争议的科学问题。近期发现中高纬度海表温度在7~5.4 Ma期间发生了一次剧烈降温事件,支持大气CO_(2)下降驱动假说,但一直缺乏同时期陆地明显降温的证据。Mg是主要的造岩元素之一,在表生地质过程中会发生显著的同位素分馏,是古气候领域的新兴代用指标。本研究选取塔里木盆地的罗布泊深钻岩心,进行了不同深度沉积物的多相态Mg同位素(δ^(26)Mg)的淋滤提取,并与Rb/Sr、CIA等传统化学风化指标进行对比,探讨其古气候指示意义。结果显示,沉积物岩心中碳酸盐相、硅酸盐相和全岩样品之间存在显著的δ^(26)Mg组成差异,其平均值分别为(−2.37±0.47)‰、(−0.20±0.33)‰和(−0.80±0.24)‰(2SD,n=7)。沉积物碳酸盐相的δ^(26)Mg在化学淋滤提取过程中易受硅酸盐相污染,且难以区分湖泊自生碳酸盐和外来碎屑碳酸盐,记录的环境气候信号较为复杂。相反,分离提取实验结果表明,不同深度、不同岩性的罗布泊沉积岩心样品淋滤后的残余相基本由硅酸盐相组成,与Rb/Sr、CIA等传统化学风化指标相比,其δ^(26)Mg组成与晚中新世以来海表温度等其他气候记录有更好的一致性。研究结果表明,罗布泊沉积物硅酸盐相δ^(26)Mg可以指示其物源区化学风化强度的变化。因此,通过测定气候敏感区湖泊沉积物中硅酸盐相δ^(26)Mg记录,可以为探讨中国西部晚中新世以来的气候变化机制提供新的制约。

The mechanism by which the Earth’s landscape and terrestrial ecosystem transformed buring the Late Miocene remains elusive.The Late Miocene cooling event was marked by a synchronous large-magnitude decrease in the sea surface temperature(SST)in the mid-to high-latitude oceans of both hemispheres.However,reports on the continental records of this cooling event are rare.As one of the main rock-forming elements,Mg isotopes fractionate significantly during low-temperature surface geological processes and thus may record paleoclimate conditions.In this study,the Mg isotopes of different phases extracted from a drill core in Lop Nor,Ls2,were measured to investigate their potential as paleoclimate proxy.Traditional chemical weathering indicators such as Rb/Sr ratio and CIA were also measured for comparison.Our results show that there are significant distinctions ofδ^(26)Mg values among the carbonate phase,silicate phase,and whole sediments,with average values of(−2.37±0.47)‰,(−0.20±0.33)‰and(−0.80±0.24)‰,respectively(2SD,n=7).Theδ^(26)Mg value of carbonate-phase extraction may be polluted by the silicate phase and cannot represent theδ^(26)Mg of the carbonate phase of the sediments precisely.In contrast,silicatephase extraction eliminated contamination from non-silicate phases and thus can reflect the Mg isotopic information of the silicate phase accurately.Compared with traditional chemical weathering indexes,the silicate-phaseδ^(26)Mg of the sediments showed better correlation with global SST records and was more reliable for indicating the chemical weathering intensity of Lop Nor.Our study implies that the silicate-phaseδ^(26)Mg of lake sediments from climate-sensitive areas in Western China can provide potential constraints on the mechanism of climate change.