2015, Vol.35
1 前言
晚上新世-更新世交界处(2.7～2.4Ma)是地质历史时期中的一个重要阶段，在北半球，喜马拉雅和阿尔卑斯山脉的隆起以及第四纪冰期的形成，极大地改变了全球的气候环境[1]。有研究认为在3.6Ma之后全球年均温逐渐下降，至2.7Ma时北极冰川开始扩增[2, 3, 4]。然而也有研究表明在2.7Ma前后存在一个短暂的间冰期，之后全球温度才急剧下降[5]。但是这一观点并未得到广泛的认同，例如: 太平洋氧同位素的记录显示在2.7Ma前后气候已经开始变冷，北半球冰川开始扩增[6]，并且日本海的海平面在2.7Ma前后下降也反映出当时温度有所降低[7]。因此这一间冰期是否存在仍需更多的研究证实，特别是利用不同技术手段得到的数据，以求从不同的角度证实这一事件的真实性。
利用稳定碳同位素(δ13C)恢复古气候是当今地球环境演变研究中的一个热点[8, 9]。但迄今为止，δ13C的研究多集中于利用黄土-古土壤[10, 11, 12, 13, 14]、 哺乳动物骨骼、 牙齿化石[15, 16]和湖泊相沉积中的δ13C值[17, 18, 19, 20, 21]来重建古环境，叶片化石同位素能否作为古气候重建的指标尚无定论。植物叶片化石中的δ13C值可以反映光合作用过程中碳固定的情况[22]，而其光合作用又受环境因子如光照、 温度、 水分等的影响，所以植物叶片化石的δ13C值可以用来指示古气候。另外，植物化石的δ13C值能够记录和反映地质历史时期植物的生理状况[23, 24]。在现生植物研究中，植物组织中的δ13C值是评估C3植物叶片中胞间平均CO2浓度的有效方法，而胞间平均CO2浓度与水分利用效率密切相关，因此，现代植物叶片中的δ13C值被看作是反映植物水分利用效率的可靠指标[23, 24]，而水分利用效率也直接受环境变化的影响。与古土壤与湖相沉积物的δ13C值不同，其反映的不是植物群落的变化情况而是特定物种的生理状态变化，因此可以从不同的层面反映古气候的变化。前人研究得到的欧洲中部、 日本新泻县和台湾木化石δ13C变化趋势与同时期有孔虫稳定碳同位素的变化趋势是一致的[25, 26]，这也进一步说明了植物组织的δ13C值可以记录气候的变化并且作为重建古气候的代理指标。但到目前为止，古生物学上测定植物化石δ13C用以恢复古气候环境的研究多集中于木化石[27, 28, 29]，而在其他植物组织化石上的研究较少。
叶片化石的形态性状能够反映气候特征及其演化趋势，同一种植物在不同的气候环境条件下，形态特征会发生相应的变化。反过来叶形态特征也就能够用于古气候的重建。由于大小和形状(叶相)在全球和区域尺度上也都与温度和湿度强相关，古植物学家很久以前就将这些叶片-气候相关性作为代理指标重建古气候[30, 31, 32]。这些方法的基本原理是将在现代植物群落中发现的规律应用于古植物群落中，从而反推古气候的演化。根据前人研究结果[33]可知在湿润条件下，植物的叶片面积会增大，并且在温暖条件下，植物叶片的长宽比会增加。由此，根据叶片化石面积和长宽比在地质历史时期中的变化可以推测出古气候变化的趋势。
水杉(Metasequoia glyptostroboides)是著名的活化石，目前仅存于我国华中一带。最早的水杉化石记录见于晚白垩世，之后的各地质时代均有水杉化石的报道。发现和报道的化石点有500个之多，范围几乎遍及整个北半球[34]。连续的化石记录和广泛的地史分布使得水杉化石及其稳定同位素中隐含了自晚白垩世以来的环境变化信息，使其成为研究全球变化的理想材料[34]。之前利用水杉化石重建古气候的研究多集中于利用其叶片气孔指数重建古大气CO2浓度[35, 36, 37]，重建出的古CO2结果与利用其他代理指标重建的结果较为一致[36, 38]，说明水杉化石可以有效的记录古气候的变化。例如，利用加拿大北极地区水杉叶片化石的气孔指数重建的始新世CO2浓度较高(约424ppmv)[36]，而美国[35]、 中国中新世(约310～334ppmv)和日本更新世的CO2浓度则较低(约280ppmv)(王雨晴等未发表数据)。过去也有研究分析过水杉叶片化石的δ13C值[35, 36, 37]，但其所用材料的时代集中于始新世和中新世，未曾有研究报道过晚上新世和更新世水杉叶片化石的δ13C用以指示古气候变化。
本研究所用材料均采集自日本中部地区，该地区晚上新世-更新世古植物群落的变化已经有详细报道[39-44]，可以推测出该阶段气候变化，但是依然需要更多的证据支持，得到一个更加真实、 准确的气候演化历史。本研究选取来自晚上新世至更新世5个化石点的水杉叶片化石为研究材料，测量其稳定碳同位素、 叶片大小和叶片长宽比作为古气候变化的指示，综合这3种不同性状指标推测晚上新世至更新世气候变化的趋势以检验2.7Ma左右的间冰期是否存在。
2 试验材料本研究所用水杉叶片化石材料均采于日本(图1)。根据叶片形态和其在小枝上对生的特点，化石材料被鉴定为水杉属植物，且其所属年代也已经过古地磁和钙质纳米浮游生物地层测年准确定年(表1)。八王子市、 东近江市和十日町市化石点水杉叶片化石的凭证标本存放于中国云南省昆明市中国科学院昆明植物研究所标本馆(KUN)； 泉南郡和生驹市化石点水杉叶片化石的凭证标本存放于日本千叶县千叶大学园艺学院研究生部。
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图 1 本研究水杉叶片化石的5个采样点， 晚上新世与更新世化石点分别用黑色和灰色图标区分 Fig. 1 Five localities where fossilized Metasequoia were obtained in Central Japan. Different colors correspond to the different ages of the localities: black for Late Pliocene and grey for Pleistocene |
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表 1 本研究所用材料信息表 Tab.1 Metasequoia samples used for reconstructing paleo-CO2 |
在实验室中，首先对待测水杉叶片化石材料进行预处理。具体过程依次如下:1)10％～25％的稀盐酸浸泡约2个小时； 2)40％的氢氟酸浸泡约12个小时； 3)10％～25％的稀盐酸浸泡至少1个小时。之后，用去离子水清洗至其pH值接近7，得到干净的化石叶片。
3.2 准备同位素材料为了保证结果的准确性，每个化石点视材料数量的不同，准备1～5份样品(表2)，每份样品包含5～10片经预处理后的化石叶片(净重大于0.05g)。用研钵将化石材料磨成粉末后，放入离心管内置于50℃烘箱内烘干(24小时)，然后寄送至德国(GeoZentrum Nordbayern，Friedrich Alexander Universität Erlangen Nürnberg)利用与ThermoFinnigan Delta V Plus射频质谱仪相连的元素分析仪(CE1110)进行化石叶片稳定碳同位素分析。同位素结果采用V-PDB标准，分析误差小于0.07‰(1个标准差)。由于生驹市的材料数量限制，该化石点只准备了1份稳定碳同位素测量材料，故没有方差数据。
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表 2 本研究所用材料的样本量 Tab.2 Sample size of this study |
利用数码相机对化石材料进行拍照之后，照片利用ImageJ(1.43μ，Wayne Rasband，http://rsb.info.nih. gov/ij/)图片处理软件对化石叶片进行长度和宽度的测量。每个化石点选取5片水杉叶片化石(生驹市除外)进行测量(表2)，化石叶片面积(mm2)=化石叶片长度(mm)×化石叶片宽度(mm)。由于生驹市化石点化石叶片较为破碎，故无法获得叶片面积及叶片长宽比的数据。
4 试验结果八王子市化石点和东近江市化石点水杉叶片化石的δ13C明显低于其他3个点，其中八王子市化石点的水杉叶片化石的δ13C为所有化石点中最低(-29.05‰)。而泉南郡化石点的水杉叶片化石的δ13C为本研究5个化石点中最高(-28.14‰)。东近江市、 十日町市和生驹市化石点水杉化石叶片的δ13C分别为-28.95‰、-28.39‰和-28.58‰(表3和图2)。
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表 3 本研究所测不同地质时期5个化石点水杉叶片化石的δ13C(A)、 叶片面积(B)和叶片长宽比值(C)(平均值±标准差) Tab.3 Leaf size， δ13C and leaf length/leaf width of fossilized Metasequoia during Late Pliocene to Pleistocene |
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图 2 水杉叶片化石的δ13C、 叶片面积和叶片长宽比自晚上新世至更新世的变化趋势 水平误差棒为对应指标的方差， 垂直误差棒为各化石点的定年范围 Fig. 2 Evolutionary trends of δ13C， leaf size and leaf length/leaf width of fossilized Metasequoia needles during Late Pliocene to Pleistocene. Horizontal error bars: standard deviation of δ13C， leaf size and leaf length/leaf width of each locality， and vertical error bars: standard deviation of materials' ages |
八王子市化石点的水杉叶片化石的叶面积(7.45mm2)明显高于其他3个点，其中南泉郡化石点的水杉叶片化石的叶面积为所有化石点中最小(5.34mm2)。东近江市和十日町市化石点水杉叶片化石的面积分别为5.79mm2和5.77mm2(表3和图2)。
水杉叶片化石的长宽比在晚上新世持续上升，晚上新世的东近江市化石点水杉叶片化石的长宽比(5.31)高于其他3个点，而南泉郡化石点的水杉叶片化石的长宽比为所有化石点中最低(3.76)。八王子市和十日町市化石点水杉叶片化石的长宽比分别为4.22和4.40(表3和图2)。
5 讨论植物叶片δ13C值的变化能够反映植物的生理变化，并且与植物生长的外界环境差异，尤其是水分和温度的差异紧密相关[55, 56, 57, 58]。植物叶片δ13C值代表植物叶片在生长过程中吸收的13C与12C的比值，其大小直接反映植物的长期水分利用效率，进而指示植物生长的外界环境状况[55, 59]。生长环境适宜时，植物能够吸收和同化更多的CO2，由于含12C的CO2被优先吸收和同化，因此光合产物中13C与12C的比值(δ13C值)较低； 反之，植物在较恶劣的生长条件下，叶片δ13C值较高[56, 60]。过去很多研究表明，植物的δ13C值与其生长地的降水状况显著相关。例如，Yang等[61]指出植物δ13C值与降水量呈负相关； Francey和Farquhar[62]证明，降雨量越大，红松叶子的δ13C值越低(负值越大)； 陈拓和马健[63]对阜康典型荒漠中C3植物δ13C值与环境间相关关系的分析结果也表明: 降水可以改变叶片稳定碳同位素值的大小: 降水越多，叶片δ13C越负。由于降水影响土壤含水量，所以植物叶片δ13C值与土壤含水量之间也存在显著负相关。青藏高原北部植物叶片δ13C值随土壤含水量和土壤温度的变化而变化，土壤含水量越高，植物叶片δ13C值越小[64]。已经发表的大量研究都一致性地表明C3植物碳同位素值随降雨或水分的增加而降低[65]。另外，Farquhar等[56]和Andreeva等[66]也指出: 生长于湿润和半湿润条件下植物的δ13C值较低，这可能是由于降水增多导致空气湿度、 土壤含水量增加，植物叶片气孔导度增大，蒸腾速率增强，从而降低δ13C值。植物叶片δ13C值受温度影响也较明显[67, 68]。研究表明[69]，中国北方多种C3植物的δ13C值与温度呈负相关。青藏高原现生禾本科植物的δ13C值随海拔的增加而升高，温度是引起该δ13C值随海拔高度变化的主要因素之一[70]。Yang等[71]指出现代水杉叶片的δ13C值为-28.93‰±0.15‰。该值略高于本研究中八王子市和东近江市化石点(2.6～2.7Ma)水杉化石叶片的δ13C值(表2)，说明与现代水杉相比，晚上新世时期水杉的光合作用产物中固定的13C更少，说明其生长的环境较现代更为适宜，光合作用更为活跃。植物的δ13C值与降水和温度的直接关系也表明晚上新世的气候较现在更为温暖湿润。而与现在水杉叶片的δ13C值相比，本研究中南泉郡(3.0～2.8Ma)、 十日町市(1.85Ma)和生驹市(0.95Ma)化石点水杉化石叶片的δ13C(表2)较高(更接近正值)，意味着这些时期的水杉植物叶片中δ13C较高，说明光合作用较弱，生长环境(温度和水分)较现代恶劣，即，当时的气候较现在气候更为寒冷干燥。水杉叶片化石δ13C值表明: 研究区在2.7～2.6Ma期间气候较为温暖湿润，可能为一个间冰期； 而这一时期相邻阶段的气候则相对较为寒冷干燥。
植物叶片的大小和形态指标也可以反映植物生长的外界环境状况以及植物对其所处环境变化的生理响应[72]。温度和降水是影响植物生长和植物叶片形态性状变化的主要因素，较低的温度和降水往往对应着较小的叶片面积[33, 73]。大多数植物叶片面积随海拔的升高显著降低，这是对较高海拔地区较低的温度和降水的典型响应[74, 75, 76]。在全球范围内，年均温和年降水量与植物叶片面积之间的正相关关系已经多次被证实[77-79]。Jacobs[80]对靠近赤道的30个植物群落植物叶片形态特征与气候因素格局的关系研究也表明这一点，并且叶片长宽比与年均降水量呈显著正相关。较低的温度和降水能够直接限制叶片的扩展，导致较小、 较短的叶片[81, 82]；另外，低温增加水的粘性并减小酶的活性，可能导致严重的生理水分亏缺和较低的光合作用[75]。降低叶片大小能够增加叶片与外界环境接触的物理边界层导度，增加叶片的物理对流散热，减少植物在缺水环境下的蒸腾散热，进而减少叶片水分丧失[83]。对天山北坡不同种杨树功能性状的分析表明，胡杨单叶面积较其他种类最小，这即是该种为减少蒸发量，保存其植物体内水分，对外界缺水环境的适应[84]。王力[85]对现生水杉同一植株内叶片大小和形态变化的研究表明: 由同一水杉植株的下部至上部，单枝中部叶片的长度和长宽比有减小的趋势。造成这种现象的原因主要可能是植株上部可利用的水分较植株下部少。这也进一步印证了前人的结论[77, 78, 79]: 叶片的大小和长宽比与水分可获得性呈正相关，干旱的情况下叶片倾向于变小、 变短。应用此规律于本研究结果(表3和图2)，在2.7～2.6Ma期间较大的叶片长宽比和面积说明该阶段较其相邻时期更加的温暖湿润，这也与δ13C值所推测的结果一致。研究区δ13C值和水杉叶片形态演化的研究表明: 晚上新世-更新世交界处是一个较为温暖湿润的阶段，该阶段在约2.7Ma存在一个间冰期，而晚上新世之后，气候开始变冷变干。
这一结果也得到了古植被演变和古气候重建结果的证实。上新世-更新世交界处，大阪组植物群包含了大量的喜温类群，并且几乎没有适应低温的针叶类群，说明当时的温度可能较为温暖[39]。利用植物叶片化石气孔指数重建出的晚上新世-更新世CO2结果显示在2.7Ma时出现了一个峰值[86]，而且这一短暂温暖的时期不仅仅在中纬度地区有报道，高纬度地区俄罗斯埃利格格特根湖(Lake Elgygytgyn)的孢粉、 生物硅等研究也表明了这样一个温暖时期的存在[87]。之后，植物大化石记录显示全球气候变冷。例如，晚上新世后，日本中部植物群落中的许多中国中南部地区的特有种类逐渐灭绝，而在现代寒冷地区和亚高山森林中占优势的物种开始出现或增加[40-42]； 晚上新世柳杉属在大阪组的出现也说明当时气候较为寒冷[43]； 2.2Ma时，睡菜属植物(Menyanthes)在新泻县出现，表明该时期温度降低[88]； 1.7Ma时，东海组(Tokai Group)植物群的化石记录中包含了富士山云杉(Picea maximowiczii)，鱼鳞云杉(Picea jezoensis)，和王桦(Betula maximowicziana)等寒温性及亚北极的树种，也表明该时期气候已经变冷[40, 44]。之后，约1.7～1.2Ma时，东海组植物群的优势种变为了分布于现代寒带林的富士山云杉，红松(Pinus koraiensis)和睡菜，进一步证明在这一时期为冰期的寒冷气候[40]。
本研究利用采集于日本中部地区5个化石点的水杉叶片化石的δ13C、 叶片面积以及叶片长宽比作为古气候的替代性指标，定性估测了晚上新世-更新世期间的气候变化，认为在2.7Ma左右存在一个间冰期，并且这一结果支持前人根据孢粉学和化石叶片气孔指数的推测出的结果[86, 87]。并且日本中部地区晚上新世-更新世期间植物群落的变化也可以从侧面印证这一结论:2.7Ma前后，日本中部地区存有许多温带性的物种，而之后这些物种逐渐灭绝，由更加耐寒的物种取代。
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Abstract
During the Plio-Pleistocene boundary (2.7~2.4Ma), the glaciation of the Northern Hemisphere started.Previous research reported the expansion of northern hemispheric continental ice sheet and the climate deterioration began at around 2.7Ma.On the contrary, other studies argued that 2.7Ma was an interglacial period.Therefore, more evidences are necessary to clarify the debate.Here we use organic carbon isotope(δ13C), leaf size and leaf length/leaf width ratio of fossil Metasequoia needles as the proxy of paleoenvironment to assess if 2.7Ma ago was an interglacial period or not.
The fossil Metasequoia needles were collected from five localities in central Japan:Sennan, Hachioji, Higashiomi, Tokamachi and Ikoma.The ages of these materials varies from 3.00Ma to 0.95Ma(Late Pliocene to Pleistocene).For each locality, we prepared 1~5 samples for δ13C analysis, and 5 leaves for leaf size and leaf length/leaf width ratio measurements.These three characters have been shown to be mainly affected by temperature and precipitation of the growth environment.δ13C values are negatively correlated with temperature and precipitation whereas leaf size and leaf length/leaf width ratio are positively correlated with temperature and precipitation.
The results of this research show that: the lowest δ13C value(-29.05 ‰)was measured for the period around 2.6~2.7Ma(Late Pliocene), indicating a warm and humid environment.The leaf area and leaf length/width ratio of fossil Metasequoia needles during 2.6~2.7Ma(Late Pliocene)are also larger(7.45mm2 and 4.22 respectively), supporting that 2.6~2.7Ma is a warm and humid period.Thus, our results indicate that there is an interglacial period at the Plio-Pleistocene boundary(around 2.7Ma).After that period, the climate deteriorated, as the environment became cooler and dryer.This climate change is corroborated by the mega fossil records, as the succession of floral assemblages during Pliocene to Pleistocene also indicated a similar trend.