末次冰期黄土中蜗牛壳体碳酸盐同位素组成与其环境指示意义

为了揭示蜗牛化石壳体碳酸盐（文石）稳定同位素组成的古气候和古生态环境指示意义，对采集于河南荥阳邙山末次冰期黄土剖面上部中的粉华蜗牛（Cathaica pulveratrix）化石壳体碳酸盐进行了碳、氧稳定同位素分析，同时还对全岩有机物质（SOM）碳同位素组成以及全岩磁化率和粒度等气候替代指标进行分析，结果显示：剖面中反映蜗牛食物碳同位素组成的壳体δ13CSSA的变化，与反映古植被碳同位素组成的全岩有机物质碳同位素组成（δ13CSOM）无显著的相关关系，但是壳体13C相对于SOM的富集程度（Δδ13CSSA-SOM）的变化与石笋氧同位素记录的末次冰期东亚夏季风强度演化同步一致；   壳体δ18OSSA的变化不但与黄土磁化率、粒度等气候替代指标变化具有显著相关性，同样也与末次冰期东亚夏季风强度演化同步一致。这些特征，一方面说明受季风环流控制的气候温湿程度变化左右蜗牛夏季活动的几率和食物的类型，干冷气候条件下，相对温湿夏季成为蜗牛活动主要时期，相对富集13C苔藓、菌类和植物可能是蜗牛的主要食物；   另一方面暗示蜗牛化石壳体碳酸盐稳定同位素组成能够指示气候温湿程度和生态环境的变化。

STABLE CARBON AND OXYGEN ISOTOPES IN LAND SNAIL CARBONATE SHELLS FROM A LAST GLACIAL LOESS SEQUENCE AND THEIR IMPLICATIONS OF ENVIRONMENTAL CHANGES

Compositions of carbon and oxygen isotopes in shell carbonate(aragonite)of land snails are correlated with vegetation and climate，and have the potential to indicate changes of paleoclimate. Fossil land snail carbonate shells are plentiful in the wind blown loess sequences in the Loess Plateau. However，their isotopes had not been broadly used to reconstruct paleo￣climatic history until present. In this paper，based on climatic proxy analyses，we approach paleo￣enviromental significances of ¹³C and ¹⁸O in carbonate shells of fossil land snails from the last glacial loess sequence at Mangshan，Xingyang，Central China(34.96°N，113.37°E).
Stable carbon and oxygen isotopic analyses have been carried out on fossil carbonate shells of the land snail(Cathaica pulveratrix)collected from the upper part(about 47～17kaB.P.)of the last glacial loess sequence at Mangshan，Xingyang，Central China. Magnetic susceptibility，grain size，carbonate content，and concentration and carbon isotope of total organic matter of bulk samples from the sequence are also measured to determine changes in paleoclimate and paleovegetation. The carbon isotope measurements on the samples from the loess section show that snail shell aragonite  δ¹³C_SSA varies in a range from -3.0‰ to -7.3‰ with an average of -5.5‰，and soil organic matter  δ¹³C_SOM in a range from -23.6‰ to -19.0‰ with an average of -22.0‰. Differences between the δ¹³C_SSAand δ¹³C_SOM for each of the snail shell samples are calculated，off the ¹³C enrichment of the shells relative to snail food for most of the samples. This feature implies that the  δ¹³C_SSAwould not be deduced precisely to the carbon isotope composition of paleo￣vegetation although the  δ¹³C_SSA and  δ¹³C_SOMdepends on abundance of ¹³C in snail vegetation food and local plants respectively. However，variations in ¹³C enrichment(Δδ¹³C_SSA-SOM)of snail shells relative to soil organic matter are close correlated to changes in paleomonsoon intensity recorded by stalagmite δ¹⁸O. Theδ¹⁸O_SSA of the snail shells from the loess sequence changes frequently within a range from -0.9‰ to -7.9‰ during late last glacial，which is parallel with variations in climatic proxies such as magnetic susceptibility and grain size，and also compatible with the record of the paleomonsoon. Increases of Δ¹³C_SSA-SOM and  δ¹⁸O_SSA with aridity lead us to deduce that plant growing length and snail active season would be controlled by temperature and humidity those depend on the monsoon intensity，and imply that the ¹³C and ¹⁸O in land snail carbonate shells can be used as a tool to reconstruct past climatic changes.