李 強，劉 禹,，Takeshi Nakatsuka，宋慧明，王麗麗，他維媛

1.中國科學院地球環(huán)境研究所 黃土與第四紀地質國家重點實驗室，西安 710061

2.全球變化研究協(xié)同創(chuàng)新中心，北京 100875

3.西安交通大學 人居環(huán)境與建筑工程學院，西安 710049

4.Research Institute for Humanity and Nature,Kyoto 603-8047,Japan

5.中國科學院地理科學與資源研究所，北京 100101

西藏亞東地區(qū)鐵杉樹輪樣本穩(wěn)定氧同位素的氣候響應

李 強1,2，劉 禹1,2,3，Takeshi Nakatsuka4，宋慧明1,2，王麗麗5，他維媛1

1.中國科學院地球環(huán)境研究所 黃土與第四紀地質國家重點實驗室，西安 710061

2.全球變化研究協(xié)同創(chuàng)新中心，北京 100875

3.西安交通大學 人居環(huán)境與建筑工程學院，西安 710049

4.Research Institute for Humanity and Nature,Kyoto 603-8047,Japan

5.中國科學院地理科學與資源研究所，北京 100101

為調查樹輪穩(wěn)定氧同位素在西藏南部的氣候應用潛力，選擇了西藏亞東縣三個鐵杉（Tsuga chinesis Pritz.）樣本進行分析。其中兩個樣本表現(xiàn)出長期的穩(wěn)定氧同位素下降趨勢，而另一個表現(xiàn)出增高趨勢，且三者相關性很低。當對三條穩(wěn)定氧同位素序列進行一階差處理之后，它們的相關關系有了非常顯著的提高，表現(xiàn)出很好的一致性（p＜ 0.0001）。無論是原始還是一階差樹輪穩(wěn)定氧同位素序列，它們都與夏季相對濕度和降水反相關，而與溫度正相關，然而方差解釋量未能達到古氣候重建的最低要求。周期分析發(fā)現(xiàn)亞東樹輪穩(wěn)定氧同位素序列僅存在4.657年的顯著周期，可能與ENSO活動有關，進一步研究顯示1978年之前上年冬天到當年夏天的赤道中東太平洋海表溫度對亞東樹輪穩(wěn)定氧同位素有顯著影響（p＜ 0.01），而在1978年之后則無任何影響，20世紀70年代末期的全球氣候突變改變了ENSO對該地樹輪穩(wěn)定氧同位素的影響。

西藏亞東；鐵杉；樹輪穩(wěn)定氧同位素；氣候響應；海表溫度

樹木年輪氣候學歷經近百年的發(fā)展，無論是理論思想還是技術路線都有了較大的突破，我國的樹輪氣候學研究也獲得了一些成就（劉禹，2010；劉禹等 ，2010，2012a，2012b；Lu et al，2012；王亞峰和梁爾源，2012；喻樹龍等，2012；張慧等，2012；包光等，2013；張艷華等，2013）。近些年，更多的新技術應用到樹木年輪氣候學中，由傳統(tǒng)單一的輪寬指標演變?yōu)橥凰亍⒚芏鹊戎笜耍ㄠ嵶限钡龋?014）。其中，樹輪穩(wěn)定氧同位素由于生理機制較為明了，樹齡效應較低，復本性好等優(yōu)點，在近幾年來得到長足發(fā)展。

為了驗證樹輪穩(wěn)定氧同位素在西藏南部地區(qū)進行氣候變化研究的前景，使用采自西藏亞東縣的鐵杉（Tsuga chinesis Pritz.）樣本，進行穩(wěn)定氧同位素分析，用來測試不同樹輪樣本間的個體差異特點，以及對當?shù)貧夂虻捻憫潭?。如果它們的穩(wěn)定氧同位素序列間存在較好關系，且對氣候反應較為敏感，那么，今后在該地區(qū)根據(jù)經典樹輪氣候學標準采樣，可獲得更為可靠的古氣候重建數(shù)據(jù)。

1 材料與方法

1.1 樣本信息及當?shù)貧夂?/p>

亞東縣位于西藏自治區(qū)南部、喜馬拉雅山脈中段南麓，西接印度、東連不丹，是西藏地區(qū)的邊境縣之一。全縣海拔2800 — 3400 m，縣域內大部分地區(qū)為天然森林。樹木年輪樣本于2004年5月采自亞東縣的亞東林場（27°24′N，88°56′E）（圖1）。使用5 mm生長錐在每棵樹上采集樣芯3個，其中，A、B兩芯由合作單位進行分析，C芯由中國科學院地球環(huán)境研究所樹木年輪實驗室進行穩(wěn)定同位素分析。所采樹種為鐵杉（Tsuga chinesis Pritz.），采樣海拔高度2840 m，坡度40°，坡向東偏北30°。由于C芯樣本混合放置，且大部分樣本都斷裂，選擇有樹皮的樣本，然而，由于這些斷裂樣本的起始生長年仍然未知，這就有可能導致有些樣本處于幼齡期間，而有些樣本處于成熟穩(wěn)定期。最終選擇3個生長輪較為清晰的樣本（11C、13C和20C）進行更進一步的實驗以測量它們的穩(wěn)定氧同位素變化，研究時段取最近50年，以便與氣象資料進行對比。

氣象資料取自亞東縣帕里氣象站，氣象測量數(shù)據(jù)始于1956年。降雨量、溫度和相對濕度都在夏季達到最大（圖2）。從1956年到2003年，年總降水量426 mm，年平均溫度0.1℃，年平均相對濕度68%。

1.2 纖維素樣本測量

每年樣本在顯微鏡下用刀片切成薄片，將樣本放置在U型帶濾芯的玻璃管中進行化學反應。根據(jù)Sohn and Reiff（1942）描述的纖維素提取方法，通過NaOH和NaClO的一系列化學反應，得到纖維素樣本。為了使樣本混合均勻，用超聲波細胞粉碎機對每個纖維素樣本進行均質化。最后，經過冷凍干燥的樣本就成為最終提取到的纖維素。需要說明的是，其中幾年由于樣本較窄，提取出的纖維素樣本不夠質譜測量的量，因此存在幾個缺年值。

圖1 采樣點位置（黑色三角形）Fig.1 The location of sampling site (black triangle)

圖2 帕里氣象站平均月降水、溫度和相對濕度（1956 — 2003）Fig.2 Mean annual precipitation,temperature and relative humidity during 1956 — 2003 at Pali meteorological station

將130—180 μg的纖維素樣本放入銀囊中，壓制成立方體后在TC/EA-Delta V Advantage穩(wěn)定同位素質譜儀中進行測量，纖維素在高溫下裂解成CO氣體，后者被送入同位素質譜儀中測量穩(wěn)定氧同位素（δ18O）比率，測量精度通過重復測量標準物質得到，即小于0.2‰。

2 穩(wěn)定氧同位素結果

在研究時段（1954 — 2003年）內，樣本11C、13C和20C序列的δ18O值范圍分別為（29.00 ± 1.82）‰，（26.71 ± 1.33）‰和（23.66 ± 1.60）‰。然而，三個序列呈現(xiàn)出不同的長期趨勢（圖3a），例如，11C和13C的δ18O有一個長期下降的趨勢，而20C表現(xiàn)出一個輕微上升的趨勢。它們的原始序列相關很低，相關系數(shù)范圍0.230 — 0.467，僅11C和13C的相關系數(shù)達到顯著（p= 0.001）。由于缺乏足夠的信息，判斷11C和13C或許處于幼齡階段，表現(xiàn)出明顯的δ18O幼齡效應。

然而，這三個樣本具有明顯一致的高頻特征，例如，1958年和1966年都為高δ18O年份，而1989 年和2000年都為低δ18O年份。為了調查序列間的高頻變化是否有共同特征，對每個序列都進行了一階差計算，如圖3b，三個δ18O序列展現(xiàn)出了極高的一致性，相關系數(shù)范圍也增加至0.585 — 0.708，相關全部達到顯著水平（p＜0.0001）。三個隨機樹輪樣本δ18O的原始序列雖然存在較大差異，但是經過一階差計算之后，發(fā)現(xiàn)它們重合度很高，具有一致的變化。

圖3 三個樹輪樣本氧同位素的原始序列（a）和一階差序列（b）變化以及樣本間相關系數(shù)Fig.3 Three tree-ring stable oxygen isotope series (a) and their year-to-year difference series (b); tables show the correlations among them

3 氣候響應分析

為了與一階差的氣象響應關系進行對比，也調查了三個原始δ18O序列對當?shù)貧夂虻捻憫P系，選擇附近的帕里氣象站溫度、降水和相對濕度數(shù)據(jù)進行逐月相關分析，分析時段從上年4月至當年10月，結果如圖4所示。另外，也分析了三個δ18O序列的算數(shù)平均序列與氣象資料的關系（圖4d）。

單月或月份組合的最高相關系數(shù)為上年7月到當年6月降水與13C，其相關系數(shù)為? 0.51（n= 49，p＜ 0.001）?？傮w上看，三個序列均與當年夏季降水和相對濕度負相關，而與夏季溫度正相關，相關模式與大部分樹輪氧同位素研究相同。

再來看δ18O序列的一階差變化與氣象資料一階差的響應關系（圖5），雖然三個δ18O序列經過一階差處理后表現(xiàn)出較高的相關性，但是它們與當?shù)貧夂蛸Y料的關系并未見提高。最高相關出現(xiàn)在2月相對濕度與20C（r= 0.54，n= 49，p＜ 0.001）。其余整體相關與原始序列相似，即與夏季降水和相對濕度負相關，與夏季溫度正相關。

圖4 帕里觀測降水（P）、相對濕度（RH）和溫度（T）與11C（a），13C（b），20C（c）和三芯平均（d）氧同位素序列的相關系數(shù)Fig.4 Correlations between observed precipitation (P),relative humidity (RH) and temperature (T ) and measured tree-ring δ18O of 11C (a),13C (b),20C (c) and mean of them (d) at Pali

從以上結果可以發(fā)現(xiàn)，不管是原始序列還是一階差序列，它們與當?shù)氐臍庀笥涗涬m有相關，但是相關較低，均未能達到方差解釋量超過40%這一氣候重建的要求。以下幾方面可能導致較低的氣候響應。其一，采樣點與氣象站距離較遠，兩地的環(huán)境不同，海拔高度不同。例如，若氣象站建立在高山之上，而樣本采集自河邊，其相關關系自然會減弱。再次，原始δ18O數(shù)據(jù)存在與生長相關的趨勢，而這些趨勢與氣候變化本身無關，當一階差之后，氣象數(shù)據(jù)和δ18O數(shù)據(jù)均缺少低頻信號，一些氣候的長期變化信號被抹去。最后，樹輪δ18O序列有較為明顯的缺失值。

4 對海表溫度的響應

為了調查亞東樹輪穩(wěn)定氧同位素受哪些大尺度氣候因素影響，利用周期分析探討平均原始序列的顯著周期。結果顯示，在95%置信水平上，僅存在4.657年的周期（圖6）。雖然樣本總長度只有50年，發(fā)現(xiàn)這樣的短周期也是可靠的。

本研究發(fā)現(xiàn)的4.657年周期很有可能是ENSO周期，因為ENSO的顯著周期是2 — 7年。為此，用整序列（及其一階差序列）與印度洋-太平洋海表溫度（SST）進行空間相關，結果顯示樹輪δ18O與SST并無明顯關系（未在本文展示相關圖）。更進一步分析，發(fā)現(xiàn)三個樹輪δ18O序列的一階差雖然相關很高，但在1980年以后它們的振幅相差很大，而在1980年之前三者的振幅基本重合（圖3b）。無論在陸地還是海洋，20世紀70年代末期是一個氣候上公認的突變點（Gong and Ho， 2002；Wu et al，2005）。將樹輪δ18O平均序列分為兩個時段，即1978年以前和以后，與印度洋-太平洋海表溫度（SST）進行空間相關分析。原始樹輪δ18O序列中并未發(fā)現(xiàn)與SST有非常顯著的相關關系，但是卻在一階差數(shù)據(jù)中發(fā)現(xiàn)了非常好的關系（圖7），其原因可能是本研究的樹輪δ18O和ENSO均具有顯著的高頻周期。如圖7a所示，在ENSO盛期（上年冬天，12 — 2月）赤道中-東太平洋SST對1955 — 1978年的亞東樹輪δ18O有最顯著的影響，接下來的月份具有持續(xù)影響(圖7b — d)，直到當年夏天（圖7e）。結果表明在20世紀70年代末期氣候突變前，ENSO對亞東樹輪δ18O有顯著影響（p＜ 0.01），而在突變點之后，則不受ENSO影響（圖7f — j）。

圖5 帕里觀測降水（P），相對濕度（RH）和溫度（T）與11C（a），13C（b），20C（c）和三芯平均 （d） 氧同位素一階差序列的相關系數(shù)Fig.5 Correlations between observed precipitation (P),relative humidity (RH) and temperature (T ) and measured tree-ring δ18O of 11C (a),13C (b),20C (c) and mean of them (d); all data are processed by year-to-year difference

圖6 平均原始樹輪δ18O序列的周期分析（藍線為95%顯著性水平）Fig.6 Spectrum analysis of mean tree-ring δ18O series (Blue line indicated 95% con fi dence level)

圖7 三芯平均氧同位素一階差序列與海表溫度（SST）的相關系數(shù)Fig.7 Spatial correlation patterns between mean year-to-year difference δ18O series and gridded SST datasets

5 結論

本研究使用來自西藏亞東縣的鐵杉樹輪樣本，調查它們的δ18O變化序列。結果顯示測量的三條δ18O序列存在趨勢不一致、序列間相關較差的現(xiàn)象。然而，經過一階差去除低頻信號之后，三條δ18O序列的顯著性有非常明顯的提高（p＜ 0.0001），其變化趨勢也一致，說明原始序列或許存在幼齡效應。原始和一階差δ18O序列均與夏季相對濕度和降水正相關，而與夏季溫度負相關，雖然相關系數(shù)顯著，但仍然未達到氣候重建的要求。周期分析發(fā)現(xiàn)平均樹輪δ18O序列僅存在4.657年的顯著周期，可能與ENSO活動有關。通過與太平洋-印度洋SST格點數(shù)據(jù)空間相關，發(fā)現(xiàn)ENSO在20世紀70年代末期的氣候突變之前對亞東樹輪δ18O影響非常明顯（p＜ 0.01），影響時段從上年冬天開始，到當年夏天結束，而在氣候突變之后，則沒有影響，這也可能是三條樹輪δ18O序列在1980年之后振幅出現(xiàn)大幅不一致的原因。

致謝：野外采樣得到王雷等人的支持，在此一并感謝！

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Climate responses of tree-ring stable oxygen isotopes from Yadong County,Tibetan Plateau

LI Qiang1,2,LIU Yu1,2,3,Takeshi Nakatsuka4,SONG Huiming1,2,WANG Lili5,TA Weiyuan1
1.State Key Laboratory of Loess and Quaternary Geology,Institute of Earth Environment,Chinese Academy of Sciences,Xi’an 710061,China
2.Joint Center for Global Change Studies (JCGCS),Beijing 100875,China
3.School of Human Settlements and Civil Engineering,Xi’an Jiaotong University,Xi’an 710049,China
4.Research Institute for Humanity and Nature,Kita-ku,Kyoto 603-8047,Japan
5.Institute of Geographic Sciences and Natural Resources Research,Chinese Academy of Sciences,Beijing 100101,China

Background,aim,and scopeDendroclimatology 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 oxygenisotope 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 methodsThere 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,11C,13C and 20C,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 fl ow 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 KGaA,Darmstadt,Germany) which was inserted frequently during the measuring process.The oxygen isotope ratios were expressed asδ18O,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).ResultsThe cellulose value of 11C,13C and 20C 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 lowerexplained 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.ConslusionsThree 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 1970s,on the contrary there were any significant correlation after late 1970s.Recommendations and perspectivesThe tree-ring stable oxygen isotope in southern Tibetan Plateau is signi fi cant in climate study,which not only could present past climate variations,but also could supply the evidence of ENSO’s in fl uence.More such studies in southern Tibetan Plateau are needed.

SKLLQG; Western doctor of CAS; Youth Innovation Promotion Association,CAS (2017451)

LIU Yu,E-mail:liuyu@loess.llqg.ac.cn

Yadong County;Tsuga chinesis Pritz.; tree-ring stable oxygen isotope; climatic response; sea surface temperature

2016-11-30；錄用日期2016-12-28

Received Date:2016-11-30;Accepted Date2016-12-28

黃土與第四紀地質國家重點實驗室開放基金；中國科學院西部博士；中國科學院青年創(chuàng)新促進會（2017451）

劉 禹，E-mail:liuyu@loess.llqg.ac.cn

李 強,劉 禹,Takeshi Nakatsuka,等.2017.西藏亞東地區(qū)鐵杉樹輪樣本穩(wěn)定氧同位素的氣候響應[J].地球環(huán) 境學報,8(1):6 – 14.

: Li Q,Liu Y,Takeshi Nakatsuka,et al.2017.Climate responses of tree-ring stable oxygen isotopes from Yadong County,Tibetan Plateau [J].Journal of Earth Environment,8(1):6 – 14.

10.7515/JEE201701002