| 其他摘要 | Currently, researches on those plaeoenvironment and palaeoclimate information manifested by speleothem (especially for stalagmite) are hotspots worldwide. However, the most concerning question is the sensitivity of how those information responses to the environmental change. The variations of stalagmite’s microbanding thickness and fluorescence intensity are mainly caused by the change of organic carbon sediments in soil layers above ceilings of those cave systems. Those organic carbon are eroded by rainfalls and then coming into caves and formed microbandings in a stalagmite, therefore, the variations of a stalagmite’s microbanding thickness and fluorescence intensity could embody the information of early environment (i.e. vegetation) and climate (i.e. temperature or precipitation), hence can be taken as substitute indexes for plaeoenvironment and palaeoclimate after being corrected by modern environment and climate indexes. There are few researches focused on the role of organic carbon in the evolution process of caves. This paper discussed the responses of dissolved organic carbon (hereinafter DOC) and fluorescence intensity (hereinafter FI) to environment and climate, by systematically analyzing the variations of DOC and FI in four cave systems in Guizhou province, detailedly as Liangfeng Cave, Qixing Cave, Jiangjun Cave and Xiniu Cave. The following conclusions are drawn from the analysis.
1.It is found that there are higher concentration of DOC in drop water during Spring and early Summer, and less in concentration and minimum variation occurred in the Fall, by analyzing the monthly, seasonal and annually variations of DOC concentration in the four cave systems. Meanwhile, the time for the occurrences of peaks of DOC are different at those drop water collection points, some in Spring while others in the early Summer. This maybe resulted from the advance or lagging of precipitation, or the different distance where water travel in the cave ceilings. On the other hand, there are different DOC peaks at different times for few points, and this maybe because of the response sensitivity of those points to the rainfall, or the advance or lagging caused by different travel paths. Then it could be concluded that the different water dynamical conditions caused the differences in travel paths for supply source of drop water, hence to multi-peaks of DOC occurred at the same water collection point. Moreover, seasonal higher concentration DOC will produce a thicker microbinding because of DOC participating in the forming of a speleothem. furthermore, those points sensitive to the precipitation maybe produce microbandings with different thickness which are not only relative to seasonal DOC variations, but also the precipitation mode.
2.There are both vertical and horizontal variations of DOC concentration in those cave systems by analyzing the spatial variations of DOC and FI. Moreover, there are spatial variations of DOC concentration among those points in the same cave systems, while the variations among different cave systems is insignificant, and this maybe due to the differences of paths for the DOC traveling into a cave system. Furthermore, the spatial variation of DOC in soil water could reflect the source of soil organic carbon which maybe mainly comes from the upper 50cm surface soil layer.
3.There are three types of fluorescence spectrum according to the analysis of three-dimensional fluorescence spectrum, namely, there are fulvic-like, protein-like and a combination of UV fulvic-like, humic-like and protein-like. There is no peak of humic-like fluorescence spectrum in the four cave systems; however, there is no similar finding in other researches. Moreover, it is found that wavelength of the fluorescence spectrum varies little as a forest vegetation background changes by analyzing their correlation, while the FI is more stable with a consecutive soil coverage.
4.It could seen that the variation of fluorescence spectrum’s wavelength could reflect the differences of organic carbon matters entering into drop water, moreover, the spatially asymmetry of organic carbon and the differences of vegetations leading to the variation of the wavelength. There is a good linear relativity between the DOC and FI both in soil water and drop water in cave systems; this could imply that the DOC in drop water is transferring from soil water. Meanwhile, there maybe series of changes in the traveling paths during this transferring process, which made the concentration of DOC in drop water is lower at a magnitude than that of soil water. However, the linear relativity of DOC and FI is not significant in Jiangjun Cave systems, this maybe because of its FI is affected both by environmental factors and the inner-filter effect. Hence it could be concluded that, only taking those surface environment factors into consideration, then it could safely drawn a reasonable conclusion for the spatially variation of DOC and FI.
5.The variation tendency of FI in drop water in cave systems could manifest that, on the one hand, the change of their surface environment, on the other hand, the filtering effect of fluorescence matters by the different of hydrodynamic conditions, and this is especially true for Jiangjun Cave system which has a less significant variation in DOC concentration. As for pool waters in the four cave systems, there are seasonal water inputs in Liangfeng Cave and Xiniu Cave while there are constant drop water inputs in Qixing Cave and Jiangjun Cave, and the DOC and FI in pool waters are obviously higher than that of those drop waters in the former three cave systems while it is the contrary case in Jiangjun Cave system which maybe less affected by evaporation inside caves. Therefore, those speleothems inside cave systems are affected not only by surface environment factors, but also by the environmental changes inside those cave systems which are less significant comparing to the surface ones.
6.It could be concluded that the variation of DOC concentration is not sensitive to the change of vegetation types, by analyzing the DOC in the four cave systems. When the vegetation type varies in a series as follows, original karst forest, brushwood and grass slope, shrub-grasslands and thorn-grassland, although the DOC concentration in soil water varies significantly , the variation in drop water in cave systems is not evident which nearly keep at a same level. Moreover, the variation of DOC in drop water in those cave systems could not reflect the change of vegetation, the reason may lies in that there is a long period for the updating of organic carbon in soil and the organic carbon may dissolved in its traveling paths, what’s more, those organic carbon with different source may blend at a different degree, hence made the variation of DOC in soil water could not reflect the variation of DOC entering the system at that year.
7.The variation of DOC concentration would response both to precipitation and temperature. When the precipitation is a effective one, the infiltration will dissolve organic matters in the soil water, which will then traveling into cave systems as drop water, therefore the DOC concentration in drop water would increase, hence the FI. However, the precipitation would also dilute the DOC concentration, and this maybe the reason for the decrease of the concentration in some month. Moreover, the DOC concentration of drop water would reach a peak before July, but the concentration varies little in August, September and October, the reason maybe lies in that the temperatures are higher and the precipitations are less in this period, which would restrain the activity of decomposing of organic carbon by microbe. Therefore, it is possible to inverse the change of climate by analyzing the microbandings thickness of speleothem. At least, it maybe takes as a secondary index.
8.It could be concluded that DOC and FI could be affected by many factors including pH, HCO3-, SIC, PPCO2, the thickness of the cave ceiling and the dissolution of carbonate rock, etc. It is found that, the thickness of the cave ceiling and the concentrations of Ca2+ and Mg2+ affected the DOC and FI significantly, while the affection by pH value which varies in a small scale is insignificant. Meanwhile, the correlation analysis between DOC , FI and those HCO3-、SIC and PPCO2 shows an insignificant relation, which means the latter are minor factors that affect the variation of DOC and FI. Furthermore, it could be drawn that, when carrying out a research in deducing environmental change in cave systems by analyzing the records of speleothem, a fully understanding of the hydrological and geochemical process inside the cave ceiling where speleothem formed would be necessary for acquiring a complete and accurate information. |
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