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