孢粉稳定碳同位素研究进展

AN OVERVIEW OF CARBON STABLE ISOTOPE ANALYSIS OF POLLEN

由于植物的光合作用途径不同,C3和C4植物的δ13C值有明显不同的分布范围,这可以用来研究环境的变化。花粉的碳同位素分析结果显示其δ13C值与植物的δ13C值变化规律一致,也能很好地反映植物的光合作用途径,进而间接反映其所处的气候环境条件。相比于植物体,化石花粉颗粒组成成分较单一,而其生成时间短,因而其碳同位素或许能较好地反映植物开花期时的环境条件,如雨水、大气CO2等的性质,为更精确的环境重建提供可能。初步的研究结果也显示花粉的δ13C值与开花期的温度存在线性关系。高精度结果的取得依赖于实验技术和仪器设备的改进,研究表明传统孢粉的醋酸酐处理过程中存在碳同位素污染,采用浓H2SO4浸泡可达到相同的去除纤维素的目的而避免污染。另外,通过对质谱仪的改进——装配旋转镍丝等进样系统(SWiM-IRMS),使实验所需的测试量大大减少,对单粒花粉进行测定也已经成为可能,但实验的精度还待进一步提高。

Different in photosynthetic ways, plants have their C3 and C4 distinct in ranges of δ^13C values, which may be used to indicate modern or ancient environmental and ecological conditions. The measured δ^13C values from modern pollen grains show that they are consistent with the values from other tissues of the plants, and therefore may also indicate the photosynthetic pathways and the climatic conditions of the plants. Compared to the plant tissues, chemical compositions of pollen grains are less variable and are formed in a relatively short period, which means they may have higher temporal resolution in reflecting climatic conditions and features of rain-water and CO2 during the blooming period. The preliminary research shows that the pollen δ^13C value is nearly linear with the mean temperature of the flowering time. However, more exploring work is needed to confirm this conclusion. More precise reconstruction based on pollen δ^13C value also relies on improved experimental technology because some conditional methods may not be applied. Some research has shown that the acetylation methods of pollen preparation may introduce contamination of carbon isotope. A substitution method is the non-carbon containing acid extraction technique to isolate sporopollenin. A spooling-wire microcombustion device interfaced with an isotope-ratio mass spectrometer （SWiM-IRMS） may allow the analysis of δ^13C from a single pollen grain, although the error range is still large.