湖北清江和尚洞洞穴滴水脂肪酸分布特征及其古生态意义

DISTRIBUTIONS OF FATTY ACIDS IN THE DRIP WATER AT HESHANG CAVE, HUBEI PROVINCE AND THEIR ECOLOGICAL IMPLICATIONS

对采自湖北省清江和尚洞滴水样品,采用XAD大孔径树脂吸附法及气相色谱-质谱联用仪(GC-MS)检测出种类丰富的脂肪酸,包括直链(nC12∶0～nC30∶0)与支链(iC14∶0～iC26∶0,及aC15∶0和aC17∶0)饱和脂肪酸,直链单不饱和脂肪酸(nC15∶1,nC16∶1,nC17∶1和nC18∶1),多不饱和脂肪酸(nC18∶2)及微量的支链不饱和脂肪酸(iC17∶1)。滴水脂肪酸分布特征显示有机质以微生物来源为主,兼有高等植物贡献。占优势的偶碳直链饱和脂肪酸(主要是nC16∶0,nC18∶0和nC14∶0),含量相对较高的单不饱和脂肪酸及多不饱和脂肪酸可能主要来源于地下水微藻。滴水中还检出含量较高的nC17∶1和nC15∶1及微量的iC17∶1和iC15∶1,它们很可能来自洞顶土壤厌氧层中的硫酸盐还原菌。洞顶土壤、滴水与现代石笋碳酸钙沉积脂肪酸分布对比揭示,石笋有机质主要来源于土壤,也有地下水微生物及洞穴原地微生物贡献。

The Heshang Cave is located at the south riverside of the Qingjiang River, Hubei Province, Southern China, where the mean annual precipitation and temperature are 1400mm and 16 - 17℃, respectively. The cave, developing within the Cambrian carbonate of the Sanyoudong Group, has a height about 205m, with 250m in length and 10 -30m in width. Dripping water, top soil, and modern stalagmite samples were collected for fatty acid analysis . Two dripping water samples were taken under the stalactites located at the former growing site of the HS-4 stalagmite and in the rear of the cave. In addition, the stalagmite samples were collected from the topmost section of HS-4, which was still growing when picked off. After crushing to 100 meshes, organic matter in soil and stalagmite was extracted using acid digest method. In contrast, the dripping water samples were firstly enriched by mixture absorbents of XAD-2 and XAD-7 （4 ： 1, V/V） and then eluted with dichloromethane. All extracted fatty acid compounds were transformed to silyl ester by reacting with N, O-bis （trimethylsilyl）trifluoroacetamide （BSTFA）, and finally were analyzed by gas chromatography-mass spectrometry （GC-MS）. The dripping water is found to be composed of straight acids （nC12 ： 0 - nC30 ： 0） , branched acids （iC14 ： 0 - iC26 ： 0, aC15 ： 0, and aC17 ： 0） , monounsaturated acids nC15 ： 1 ,nC16 ： 1, nC17 ： 1, nC18 ： 1 ,） , polyunsaturated acids （nC18：2）, and trace of branched monounsaturated acids （iC17 ： 1 ）. These compounds are proposed to be contributed mostly from microorganisms, not only from top soil, but also from groundwater. There are still some minor inputs from terrestrial higher plants. In detail, the dominant median chain length saturated acids （nC12 ： 0 - nC20 ： 0） and plenty of monounsaturated and polyunsaturated acids probably indicate a contribution from microalgae living in the percolating water. The presence of sulfate reducing bacteria in the overlying soil is revealed specifically by nC17 ： 1, nC15 ： 1 and iC17 ： 1. The similar distribution of fatty acids among top soil, dripping water, and modern stalagmite deposition suggests that the stalagmite organic matter principally originates from soil. In other words, organic matter contributed by soil organisms （e. g., sulfate reducing bacteria, cyanobacteria, and fungi）, as well as by groundwater microorganisms （e. g., microalgal） , is transported by percolating water and finally accumulated in the calcite deposition. However, discrepancies still exist among fatty acid distributions of soil, water, and stalagmite. For example, the content of polyunsaturated fatty acid is relatively low in stalagmite compared to soil and dripping water and the isomer acids are more enriched in calcite deposition. These may approach some ecological indications in the stalagmite. During the process of transportation, the content of polyunsaturated fatty acids （e. g., nC18 ： 2） decreases because of their unstable character and easy degradation by microorganism. This sensitivity to environmental temperature is consistent with the formerly published result of HS-2 fatty acid, which recorded a low temperature H1 event. Alternatively, the nC17 ： 1 and nC15 ： 1, biomarkers of soil sulfate reducing bacteria, can avoid degradation during their transportation and accumulate in the stalagmite calcite, which provides a potential clue to trace the sulfur cycle in geological history. Furthermore, the enrichment of isomer fatty acid in stalagmite can be used to reflect the fluctuation of microbes inhabiting on the stalagmite. This study of fatty acid of top soil, dripping water, and modern stalagmite deposition suggests that the microorganism activity is strong in this karst area. In addition, organic matter originated from sulfate reducing bacteria, cyanobacteria, fungi, microalgae and cave autochthonous microbes can be deposited in the stalagmite. Thereby, stalagmite has great potential to provide high-resolution paleoecological information.