【摘要】 为调查树轮稳定氧同位素在西藏南部的气候应用潜力,选择了西藏亚东县三个铁杉(Tsuga chinesis Pritz.)样本进行分析。其中两个样本表现出长期的稳定氧同位素下降趋势,而另一个表现出增高趋势,且三者相关性很低。当对三条稳定氧同位素序列进行一阶差处理之后,它们的相关关系有了非常显著的提高,表现出很好的一致性(p<0.0001)。无论是原始还是一阶差树轮稳定氧同位素序列,它们都与夏季相对湿度和降水反相关,而与温度正相关,然而方差解释量未能达到古气候重建的最低要求。周期分析发现亚东树轮稳定氧同位素序列仅存在4.657年的显著周期,可能与ENSO活动有关,进一步研究显示1978年之前上年冬天到当年夏天的赤道中东太平洋海表温度对亚东树轮稳定氧同位素有显著影响(p<0.01),而在1978年之后则无任何影响,20世纪70年代末期的全球气候突变改变了ENSO对该地树轮稳定氧同位素的影响。更多还原

【Abstract】 Background, aim, and scope Dendroclimatology is one of the important approaches in paleoclimate studies because of tree-ring’s high temporal resolution and accuracy of dating. Traditional dendrcoalimatology employed tree-ring width to investigate climate variations. To obtain more climate signals, other tree-ring parameters such as density, stable isotope ratios are employed during recently years. Tree-ring stable oxygen isotope ratios have the advantage that the physiological controls are well understood and relatively simple in comparison to many factors controlling tree-ring width. The tree-ring stable oxygen isotope ratios may keep the memory of past changes in the stable oxygen isotope ratio of precipitation because tree roots absorb soil water that are from precipitation. In addition, relative humidity is known as the other factor impacting the tree-ring stable oxygen isotope ratios by affecting the enrichment of stable oxygen isotope ratios in leaf water. Generally, it is a necessary step to remove age-related trend in tree-ring widths studies. However, time series of tree-ring stable oxygen isotope are not necessary to carry out the detrending process because of rarely juvenile effect, consequently tree-ring stable oxygen isotope series could preserve more low-frequency climate signals. In Europe and North America, tree-ring stable oxygen isotope study has developed over the past half century. Comparing with tree-ring widths studies, tree-ring stable oxygen isotope studies are very rare in China, especially in some ecologically fragile regions such as Tibetan Plateau, where climate variations are very important to global climate change. To investigate the climate potential of tree-ring stable oxygen isotope in southern Tibetan Plateau, we employed three tree-ring samples of growing Tsuga chinesis Pritz from Yadong County, Tibetan Plateau to carry out the analysis of climate responses in this study. Materials and methods There are missing rings or false rings in most trees in Tibetan Plateau because of critical hydrological conditions. In order to obtain the exact calendar year of the samples, we performed cross dating using the Skeleton Plot method. Annual tree-ring width was measured using a LINTAB system that has a precision of 0.01 mm. Quality control was carried out using the COFECHA program. Tree-ring cores, 11 C, 13 C and 20 C, were selected to carry out cellulose stable oxygen isotope analysis in this study. The pith year of the samples are unclear because all samples may be fractured during store. Annual wood material of the three cores without any missing rings, were separated from each tree-ring core. Most of the rings is very narrow, and there is indistinct boundary between earlywood and latewood. To avoid separation errors, we used whole annual rings to do the isotopic analyses. We usesd a razor blade to separate annual sample carefully under a binocular microscope. The wood material of the annual ring was put to a labeled small bottle. Then the cellulose of annual ring was extracted from annual wood material by organic solvent and sodium hydroxide. About 0.13 — 0.17 mg of homogeneous cellulose was loaded into a silver capsule,（in duplicate for each sample） and then determined the cellulose stable oxygen isotope ratio with a continuous flow system with a pyrolysis-type elemental analyzer（Finnigan TC/EA） and an isotope ratio mass spectrometer（Thermo Delta V Advantage） in Research Institute for Humanity and Nature, Japan. We calculated cellulose stable oxygen isotope ratio by a comparison with an isotope ratio that was predetermined using commercial cellulose（Merck KGa A, Darmstadt, Germany） which was inserted frequently during the measuring process. The oxygen isotope ratios were expressed as δ¹⁸O, which represents the per mil deviation relative to the Vienna Standard Mean Ocean Water（VSMOW）. The analytical accuracy for repeated measurements of the commercial cellulose was 0.2‰（1σ）. Results The cellulose value of 11 C, 13 C and 20 C were（29.00 ± 1.82）‰,（26.71 ± 1.33）‰ and（23.66 ± 1.60）‰, respectively. The results revealed that two of three trees presented a decreasing trend in stable oxygen isotope ratios. The other one showed a slight increasing trend in stable oxygen isotope ratios. The correlations among three tree-ring stable oxygen isotope series were from 0.230 to 0.467. DiscussionAlthough the correlations among three orignial tree-ring stable oxygen isotope series are low. We found there may exist similarity in high-frequency signals of three orignial tree-ring stable oxygen isotope series. To emphasize high-frequency signals, we used year-to-year data to investigate the correlations. Their yearto-year difference data represented a strong common variance（p < 0.0001）. The correlations improved to 0.585 — 0.787. Both stable oxygen isotope series and their year-to-year difference series were negatively correlated with local summer relative humidity/precipitation, positively correlated with summer temperature. However, the results did not meet the requirement of quantitatively climate reconstruction because lower explained variance of observed climate records（< 40%）. Spectrum analysis suggested that there was a significant cycle at 4.657 year, which probably related to ENSO variations. Conslusions Three tree-ring stable oxygen isotopes series from Yadong Country, Tibetan Plateau represented common high-frequency signals, but did not show similar variations in their original time series. All year-to-year times series were negatively correlated with summer relative humidity and precipitation, positively correlated with summer temperature. Spatial correlation between gridded Sea Surface Temperature（SST） and mean tree-ring stable oxygen isotope demonstrated that central-eastern tropical SST of previous winter to current summer impacted on tree-ring stable oxygen isotope before late 1970 s, on the contrary there were any significant correlation after late 1970 s. Recommendations and perspectives The tree-ring stable oxygen isotope in southern Tibetan Plateau is significant in climate study, which not only could present past climate variations, but also could supply the evidence of ENSO’s influence. More such studies in southern Tibetan Plateau are needed.更多还原