Morphological characters and multi-element isotopic signatures of carbonates from Chinese loess–paleosol sequences
Morphological characters and multi-element isotopic signatures of carbonates from Chinese loess–paleosol sequences
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DOI：
10.1016/j.gca.2008.06.024
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发表时间：
2008-09
影响因子：
5
通讯作者：
Xuefen Sheng;Jun Chen;J. Ji;Tianhu Chen;Gaojun Li;H. Teng
中科院分区：
文献类型：
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作者：
Xuefen Sheng;Jun Chen;J. Ji;Tianhu Chen;Gaojun Li;H. Teng
通过对中国西北黄土-古土壤序列中碳酸盐矿物的形态特征和多元素同位素组成的研究，探讨了碳酸盐矿物的成因。在层序内从全新世至0.9Ma的不同剖面上采集样品，并从土壤中分离出粒度>45和<2μm的颗粒，用扫描电镜和透射电镜对样品进行了分析。结果表明，大于45μm的颗粒呈近似球形，而小于2μm的颗粒则以直径为30- 50 nm、长度为0.3 ~ 2μm的纳米棒为主，表明粗粒颗粒为碎屑成因，细粒颗粒为自生特征。碳酸盐矿物的多元素同位素组成证实了这一点。矿物和生物源样品的δ 13 C和δ 18 O值的比较表明，<2μm的组分具有与共存的陆生蜗牛壳相似的组成。两个粒级之间的其他差异还表现在206 Pb/204 Pb、207 Pb/204 Pb、208 Pb/204 Pb和87 Sr/86 Sr比值上。这些结果表明，黄土高原土壤剖面中的碳酸盐细颗粒可用于重建更高分辨率的δ 13 C和δ 18 O时间序列，以阐明黄土高原更新世的古气候变化。在中国黄土中发现的纳米棒方解石，以及以前在亚洲尘埃中发现的类似矿物形式，强烈表明这些高活性CaCO 3形式可能会改变运输过程中的气溶胶性质的可能性。
Morphological characters and multi-element isotopic compositions of carbonates from the loess–paleosol sequences in Northwestern China are examined to explore the origin of the minerals. Samples are collected from various sections ranging from Holocene to 0.9Ma within the sequences and fractions with grain sizes >45 and <2μm are separated from the bulk soil and examined by SEM and TEM. The results show that the grains >45μm exhibit an almost perfect spherical shape while those <2μm are dominated by nano-rods having diameters of 30–50nm and lengths of 0.3–2μm, presumably indicating the detrital origin of the coarse fractions and the authigenic characters of the fine ones. Such implications are corroborated by the multi-elemental isotopic compositions of the carbonate minerals. A comparison of the δ13C and δ18O values between minerals and biologically originated samples indicates that the <2μm fractions have a similar composition to those of coexisting land snail shells. Additional differences between the two size fractions also manifest in the ratios of206Pb/204Pb,207Pb/204Pb,208Pb/204Pb, and87Sr/86Sr. These results suggest the utility of fine carbonate particles in the soil profile in reconstructing a potentially higher resolution δ13C and δ18O time series to elucidate the paleoclimatic fluctuation in the Chinese Loess Plateau during Pleistocene. The discovery of the nano-rod calcite in Chinese loess, together with previous findings of the similar mineral form in Asian dust, strongly suggests the possibility that these highly reactive CaCO3form may alter the aerosol properties during transport.