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植物微体遗存分析在第四纪环境研究中的应用:综述与展望

2016-03-21张继效

地球环境学报 2016年3期
关键词:孢粉硅藻遗存

张继效,徐 海

(1.中国科学院地球环境研究所 黄土与第四纪地质国家重点实验室,西安710061;2. 中国科学院大学,北京100049;3.西安交通大学 人居环境与建筑工程学院 环境科学与技术系,西安710049)

植物微体遗存分析在第四纪环境研究中的应用:综述与展望

张继效1,2,徐 海1,3

(1.中国科学院地球环境研究所 黄土与第四纪地质国家重点实验室,西安710061;2. 中国科学院大学,北京100049;3.西安交通大学 人居环境与建筑工程学院 环境科学与技术系,西安710049)

以孢粉、植硅体等为代表的植物微体遗存由于其分布广泛,容易保存,可反映母体植物类型的优点,在第四纪环境研究中得到了广泛应用。本文介绍了植物微体遗存的概念、常见类型,以及它们的提取方法和原理,综述了利用植物微体遗存重建古环境的传统方法和近年来发展的几种古植被与古气候定量重建方法,最后简述了植物微体遗存在年代测定、稳定同位素分析研究上的应用进展与实例。文末指出了现有研究方法的问题与不足,并对今后的多代用指标的综合研究提出了展望。

植物微体遗存;孢粉;植硅体;提取方法;定量气候重建;稳定同位素

第四纪是地球历史上至关重要的时期,在这一时期全球经历了频繁而迅速的气候变化,与此同时人类活动也逐渐增强,因此第四纪环境变化成为多年来研究的热点之一。在长期的研究过程中,多种技术方法被应用于第四纪古环境重建,例如极地冰芯中氧同位素的研究(Johnsen et al,2001)、海洋沉积物中有孔虫(Di Stefano et al,2012)和微量元素的研究(Müller and Knies,2013)、动物遗存的研究(陈少坤等,2013)、黄土沉积物粒度(昝金波等,2014)和古地磁的研究(刘平等,2008),等等。地球上有几十万种植物,它们分布于各种生态条件下,不同的气候、地形、土壤类型对应着不同的植物组合。例如在热带海滩上常分布有红树林,亚热带季风区多分布有常绿阔叶林,温带半干旱区域常见植被类型为草原,而温带森林中潮湿的区域常分布有沼泽植被(武吉华等,2004)。反过来,研究地层中保存的植物残体或遗存可以推测出一个地区的古环境。因此,植物是第四纪研究中不可缺少的一部分。然而,植物体在死亡之后,通常很难完整地在地层中保存下来,单纯依赖种子、叶片、树干等大的植物残体进行环境研究具有极大的局限性。相比之下,植物的孢粉、植硅体、碳屑、淀粉粒等由于其个体微小、数量大、易保存的优点,在古环境研究中的应用更为广泛,在本文中将它们统称为植物微体遗存(Scaife,1987;Horrocks and Lawlor,2006)。此外,真菌孢子、浮游藻类在第四纪研究中同样发挥着重要作用,也可将它们纳入植物微体遗存的范畴。由此可见植物微体遗存类型众多,在第四纪科学中对其进行综合性研究可起到对照和补充的作用,使结论更为可靠。然而,目前学者们关于植物微体遗存的综述性文章多局限于某种或少数几种类型(杨晓燕等,2006;王伟铭,2009;唐领余等,2013),不够系统和全面。为了系统性地总结植物微体遗存领域的研究成果,为相关人士提供参考,在前人和本文作者研究的基础上,本文对其关键类型、提取方法和在第四纪环境研究中的应用实例进行了评述与分析,并对今后的多代用指标的综合研究提出了展望。

1 植物微体遗存的关键类型

1.1 孢粉

孢粉是植物微体遗存中研究最为深入的一类。蕨类植物在繁殖过程中产生大量的孢子,种子植物在繁殖过程中产生大量的花粉,两者合称孢粉。孢粉外壁的主要组成物质是孢粉素,这是一类十分稳定的有机物,对酸、碱、高温等有一定的耐受性,因而可在地层中长期保存(王开发和徐馨,1988)。不同类型的孢粉在形态上有明显差别,如蕨类孢子通常具有单裂缝或三裂缝,裸子植物中的松科花粉通常具有双气囊结构。被子植物的花粉具有复杂的孔、沟构造和多变的表面纹饰,如桃金娘科花粉具有三合沟,桑科花粉具有三孔,禾本科花粉为球形具单孔(图1)。以上特征使得鉴定孢粉母体植物的科属乃至种一级成为可能。通过分析地层样品中孢粉的类型和组成比,可推测出历史时期植被组成,从而进一步分析出降水和温度等古环境信息。

1.2 植硅体

某些高等植物能形成硅质的植物微体遗存,死后在土壤中保存下来。以禾本科植物为例,其在生长发育过程中会吸收大量硅质,这些硅质沉积在细胞中成为晶体,它们被称作植硅体。植物死后细胞腐烂但植硅体却继承了原有细胞的形态(Piperno,2014)。不同种类植物会形成类似或不同形态的植硅体,因而一定程度上可通过植硅体形态推测其母体植物类型(图2)。例如棕榈科植物发育有特异类型的刺球型植硅体,竹亚科植物常发育有长鞍型植硅体,水稻发育有特征扇型植硅体,而莎草科植物发育有多边帽型植硅体(王永吉和吕厚远,1993)。植硅体的分类特征不如孢粉明显,但在很大程度上仍然可以反映出环境变化趋势。在温暖湿润气候条件下,植物产生较多的扇型、正方型、长方型等植硅体,而在寒冷干燥气候条件下,植物产生较多的尖型、刺棒型植硅体,根据不同植硅体形的比例可重建环境变化(王伟铭等,2003)。

图1 部分植物的孢粉形态1.蕨类三缝孢子;2.松属花粉;3.蒿属花粉;4.禾本科花粉;5.桃金娘科花粉;6.桑科花粉。根据张继效(2015)改绘。Fig.1 Sporo-pollen morphology of some plants 1. Ferns, trilete spore; 2.Pinussp.; 3.Artemisiasp.; 4. Poaceae; 5. Myrtaceae; 6. Moraceae. Redraw according to Zhang (2015).

1.3 淀粉粒

淀粉是一种高分子有机物,它以淀粉粒的形式广泛地存在于植物的根、茎及种子等器官中。不同植物的淀粉粒在形态、类型、大小、层纹和脐点等方面各有特征。可作为鉴定植物种类的依据之一(Matsushima,2015)。目前对淀粉粒形态的研究主要集中于栽培植物,因而其应用也主要集中于和人类活动密切相关的考古遗址中。如在距今约8000年的浙江跨湖桥遗址中发现稻属、薏米等植物的淀粉粒,揭示了古人的食物组成(杨晓燕和蒋乐平,2010)。对中国北方古代陶器碎片中淀粉粒的研究显示,距今11000年陶器已经被用于煮食谷物(Yang et al,2014)。在巴拿马热带雨林中提取的淀粉粒揭示了早期人类对块茎作物的栽培(Piperno and Holst,1998;Piperno et al,2000)。随着对淀粉粒研究的深入开展,其应用有望推广至第四纪环境研究的其他领域(Lentfer et al,2002)。

图2 部分植物的特异植硅体形态1. 水稻扇型植硅体;2. 蕨类植物;3. 木本植物导管;4. 棕榈科植物;5. 木本植物;6. 莎草科植物。根据张继效(2015)改绘,其中栽培稻植硅体图片由中国科学技术大学科技史与科技考古系罗武宏博士提供。Fig.2 Specifi c phytolith morphology of some plants 1.Oryza sativa; 2. Ferns; 3. Catheter of woody plants; 4. Palmae; 5. Woody plants; 6. Cyperaceae. Redraw according to Zhang (2015), phytolith photo ofOryza sativais provided by Dr. Luo Wuhong, Department of the History of Science and Scientifi c Archelogy, University of Science and Technology of China.

1.4 其他植物微体遗存

在海洋、湖泊、湿地等的沉积物中,一般会有大量的微体藻类出现,其种类复杂多样。在各种微体藻类中,硅藻的分类研究较为细致,在古环境研究中的应用也最为普遍。例如云南云龙天池岩芯中浮游硅藻和底栖硅藻的比值变化反映了湖泊水位和西南季风的变迁(张东涛,2012)。轮藻的钙质藏卵器也是植物微体的一种,例如西藏台错古湖第四纪地层沉积有大量的轮藻藏卵器,据此可分析出该湖区的环境变化(刘俊英和王海雷,2011)。沟鞭藻类常多存在于海洋,少量存在于淡水或咸水湖泊中,其种属变化可指示海平面变化等(D'Costa et al,2008)。盘星藻类在淡水湖泊中分布广泛,其含量变化与区域降水呈一定的相关性(张华等,2004;Weckström et al,2010)。

图3 部分植物的特异淀粉粒形态1.小麦;2.水稻;3.粟;4.莲藕;5.山药;6.麻栎。根据杨玉璋等(2015)改绘,比例尺均为 20 μm。Fig.3 Specifi c starch grains morphology of some plants 1.Triticum aestivum; 2.Oryza sativa; 3.Setaria italica; 4.Nelumbo mucifer; 5.Dioscorea opposita; 6.Quercus acutissima. Redraw according to Yang et al (2015), Scale bar: 20 μm.

在地层孢粉的分析过程中常见有黑色团块, 其中大部分是碳屑。碳屑是植物体不完全燃烧的产物,它可以保存原来植物的某些结构,也可以是无结构的球形体。在一次火灾后,少量的碳屑会随风传播,其余大部分由雨水等携带并沉积下来。碳屑在沉积物中可保存数千万年不变,成为火灾历史的最好记录(Whitlock and Larsen,2001)。

孢粉的分析过程中通常还能见到真菌的孢子,它们在成分和形态上与通常意义上的孢粉具有明显差别,也具有一定的环境指示意义(Davis and Shafer,2006)。例如Glomus在土表以下生长,其在湖相沉积中的出现可以用来指示该流域的土壤侵蚀(郝秀东和翁成郁,2015)。然而,由于对现生真菌孢子的形态研究极其有限,在第四纪环境研究中它的应用受到很大的局限。

此外,在一些现生植物体内还存在草酸钙结晶,其成因与植硅体类似,可将其称为“植钙体”,具有一定的分类学意义(Franceschi and Nakata,2005;李秀丽等,2012)。目前对现生植钙体形态的研究还较少,尚难应用于第四纪环境研究,随着今后基础研究的展开,有望将植钙体应用于茶树栽培历史(Zhang et al,2014)、藜科植物区分(Cuadra and Hermann,2013)以及古环境研究(Moore et al,2000)等领域。

图4展示了一些其他植物微体遗存的代表形态。由于各种植物微体遗存的化学成分不同,它们在地层中的保存性也有差异。孢粉适于在还原性、偏酸性地层中保存,在氧化性、碱性环境中易受破坏(Tian et al,2009;Lebreton et al,2010),而植硅体在氧化性地层中也可较好保存。可见,不同岩性的样品适合采用不同的分析手段。此外不同禾本科植物的花粉形态相似,除非借助扫描电子显微镜等手段,否则难以区分(Mander et al,2013),而在植硅体形态上则有明显差别,两者起到互补作用。因此,综合多种分析手段可更全面有效地获取古环境信息。

2 主要植物微体遗存的提取方法

植物微体遗存分散在地层沉积物中,个体微小,与各种无机矿物和有机杂质混合在一起,要对它们进行研究就必须将其从沉积物中进行分离提纯。按照不同类型植物微体遗存化学成分的不同,提取方法也有相应的区别。

图4 部分其他植物微体遗存形态1.轮藻藏卵器(李莎,2012);2.盘星藻(Lenarczyk et al,2015);3.沟鞭藻(Bringué et al,2016);4.真菌孢子(Cugny et al,2010);5.碳屑(陆岸青等,2009)。轮藻藏卵器的比例尺为100 μm,其余比例尺为10 μm。Fig.4 Morphology of some other plant microfossils 1. Charophytes oogonia (Li, 2012); 2.Pediastrum(Lenarczyk et al, 2015); 3. Dinofl agellate (Bringué et al, 2016); 4. fungal spore (Cugny et al, 2010); 5. charcoals (Lu et al,2009). Scale bar: Charophytes oogonia, 100 μm; others, 10 μm.

孢粉、真菌孢子和沟鞭藻、盘星藻等藻类由较难分解的高分子有机物组成,大小相近,其提取方法基本一致,碳屑的提取也与之一致。在长期的研究过程中,多种分析手段被应用和改进。这些方法均基于样品本身的性质,如氢氟酸可以除去硅质,盐酸可以除去碳酸盐类,弱碱溶液可以去除腐殖酸等成分。孢粉的比重一般小于碎屑状盐类,所以使用适当比重的重液可以提取孢粉富集物(Nakagawa et al,1998)。绝大多数孢粉的直径在7—150 μm,因而利用不同孔径的筛网可过滤除去部分杂质(李育等,2007)。总结起来,孢粉的提取方法主要有氢氟酸筛选法(李春海和何翠玲,2004)、重液浮选法(李小强等,2006)、筛滤分析法(李小强等,1999),实际提取过程中,以上方法经常综合运用。本文综合以上研究方法,结合实验样品分析经验,给出了孢粉分析的流程(图5),由于第四纪样品类型繁多,成分、含量多变,每一步所需的样品量和反应时间都需要具体问题具体分析,不可完全套用模版。值得一提的是7—10 μm过筛这一步由于筛子的网眼孔径不一致,且筛网容易破损,常常会造成孢粉的丢失,经笔者尝试可用低速离心法代替。低速离心法的依据是物理学上的斯托克斯沉降公式(郑其良和钱志伟,1998),不同粒径的颗粒在液体中沉降速率不同,该公式表达式如下:

其中:υ是沉降速度,ρ是颗粒比重,ρ0是液体比重,η是液体的粘滞系数,g是沉降加速度,r是颗粒半径,κ是形状系数。在净水中,g等于重力加速度,而在离心机中,g= 4π2Rn2,其中R为离心机半径,n为转速。以7 μm大小的孢粉为例,在半径0.1 m的离心机中以5 转/s(即300转/min)转速的沉降而言:ρ=1800 kg·m-3,ρ0=1000 kg·m-3,η= 1.0×10-3kg·(m×s)-1,g= 98.7 m·s-2,r=7×10-6m,κ= 1。据此计算得出的孢粉沉降速度为5 cm·min-1,这意味着在高度10 cm的离心管中,300转/min的转速下,7 μm以上的孢粉在仅需2 min可完全沉降在管底,而杂质留在上层液体中。此过程重复多次,可除去大部分粒径小于孢粉的有机杂质,起到与筛网类似的效果。

植硅体主要成分为硅质,与硅藻等统称为生物硅,它们在沉积物中常常共存。提取植硅体的要点在于:用稀盐酸除去样品中的碳酸盐等可溶性矿物;用浓硝酸等氧化剂除去样品中的有机成分;用沉降法或微波消融法(Parr et al,2001)或过筛法除去样品中的粘土;用筛子除去大颗粒成分;用重液浮选出植硅体等(吴妍,2008)。与孢粉的提取类似,植硅体也可应用重液浮选法,但它们的比重较孢粉高,孢粉的比重约1.8,而植硅体的比重可达2.3(Banning,2000),因此需选用比重大于2.3的重液。植硅体的直径通常大于5 μm,对小于5 μm的杂质也可利用低速离心法除去。植硅体的沉降速率和孢粉对比有3个参数不同:ρ= 2290 kg·m-3,r= 5×10-6m,κ= 0.9,其余一致。据此计算,在半径0.1 m的离心机中以300转/min转速离心,得出的沉降速度为3.8 cm·min-1,这意味着在高度10 cm的离心管中,5 μm以上的孢粉或植硅体仅需约2.5 min可完全沉降在管底,而杂质留在上层液体中。重复该步骤可起到与沉降法类似的效果。综合前人研究成果,并结合本文实验经验,图6列出了植硅体提取的步骤。硅藻的化学成分与植硅体类似,但更容易破碎,在含量较丰富的样品中,使用盐酸和氧化剂处理即可,而对含量较低或需要做特殊用途(如同位素分析)的样品,则可采用与植硅体提取类似的方法(Parr et al,2004;刘娟等,2013)。

图5 第四纪样品中孢粉的提取流程Fig.5 The process of extraction of sporopollen in Quaternary samples

对植物钙质微体遗存如轮藻藏卵器的研究较少(García,1999;Soulié-Märsche,2008),有关其提取方法可参考李莎(2012)对松辽盆地第三纪样品的轮藻藏卵器研究。其基本提取方法为:用15%双氧水氧化有机物并松散样品;用孔径1000 μm和100 μm的筛子分别过滤掉大于和小于轮藻藏卵器粒径的杂质;烘干样品;在显微镜下挑选样品。淀粉粒目前的研究主要应用在考古地层、粪化石中,其中一种提取方法为:加蒸馏水分散样品;1500转/min 离心5 min;比重1.8的重液浮选样品;洗涤并干燥(葛威,2010)。

有时为加快实验流程,减少样品用量,可利用重液提取等方法在同一样品中同时获得多种植物微体遗存(Lentfer and Boyd,2000;Coil et al,2003)。

3 研究实例:基于植物微体遗存的植被与气候定量重建方法

传统的植物微体遗存分析方法是依据地层中主要植物微体遗存的种类和数量变化来重建古环境和古气候。典型的例子如唐领余等(2007)以孢粉分析为手段对黄土高原西部四万年来的环境研究,根据孢粉组合成分和植物生态特征,将地层剖面划分为若干个孢粉带,分析各个时期的植被状况。再如新疆巴里坤湖的植被和环境分析(陶士臣等,2010),也是通过分析湖泊地层沉积物中各类孢粉的百分含量变化来重建植被变化,并依据某些特征类型如蒿属和藜科花粉的比值来分析湿度变化。植硅体的研究方法与孢粉类似,如Lü and Liu(2005)把美国东南沿海沉积物中的植硅体组合作为重建该地区海岸带环境变化的重要依据, 并由此推测了Western湖中的沙层来源于飓风侵蚀。以硅藻为手段重建古气候也采用类似的方法,如Li et al(2015)研究了云南澄海钻孔中的硅藻,根据Cyclotella rhomboideo-elliptica等主要属种7.8 ka BP以来的相对数量变化建立组合带,并进一步探讨了其对应的亚洲季风波动。

图6 第四纪样品中植硅体的提取流程Fig.6 The process of extraction of phytolith and diatom in Quaternary samples

植硅体和硅藻等由于其移动性相对较差,多为原地保存,所以它们在沉积物中的含量变化通常反映了原有的植物状况。但对于孢粉而言,不同植物的孢粉在数量和传播距离上都是不同的。松树的花粉产量很高,而且容易远距离传播,所以地层中其含量常常较为丰富,但却不一定说明当时有大量松树存在,因为这些花粉也有可能是远距离传播来的;而榆树等植物则相反,即使地层中只有少量榆树花粉,也可以表明当时有榆树林的存在(许清海等,2007)。因此,传统上单纯依赖孢粉百分含量变化来重建古植被和古气候就有一定的局限性,通常只能定性地分析出温度和降水的相对变化,而无法得出确定的温度和降水值。为探讨孢粉与植被、气候的定量关系,空气孢粉和表土孢粉谱的研究正受到广泛重视,这些研究对了解孢粉散布、搬运和沉积机制,建立现代孢粉-植被-气候关系模型提供可靠的依据(杨振京和徐建明,2002)。例如,对云南亚热带南部表土的孢粉分析表明,木本、草本植物花粉基本代表了区域内乔木和草本植物特征, 而蕨类植物孢子则具有超代表性(潘韬等,2008)。对中国北方森林植被主要表土花粉类型的研究显示,几种主要花粉类型百分比与母体植物盖度的相关性有明显差异(李月丛等,2005)。Lü et al(2011)分析了青海青藏高原地区不同海拔高度的若干表土样品的孢粉类型和比例,将其与年均降水量、年均温度、海拔高度、相对湿度等指标进行对比,结果发现年均降水量是影响孢粉分布的最主要因素。对青海湖附近表土花粉的分析表明,风速和风向对青海湖花粉的散布与沉积影响巨大,湖泊的汇聚作用以及河流和湖流作用等也造成了花粉沉积的局部差异性(尚雪等,2009)。结合GIS等技术手段,现代植被孢粉数据库正在建立和完善(肖霞云等,2002;Gajewski,2008;Fyfe et al,2009;Zheng et al,2014)。

随着全球孢粉、植被和气象数据库的逐渐完善,基于孢粉资料重建古气候的方法迅速向定量化方向发展,并涌现出了大量可靠的古气候定量重建方法(Birks et al,2010;秦锋和赵艳,2013),依据其原理的不同大致包括:指示种法,如共存分析法(Mosbrugger and Utescher,1997)和分布区叠加分析法(Overpeck et al,1985);多元函数法,如转换函数法(ter Braak and Juggins,1993);集合法,如现代类比法(Thompson et al,2008)和响应曲面法(Bartlein et al,1986)。在指示种法方面,Klotz et al(2003)根据松属(Pinus Linn)、云杉属(Picea Dietr.)、冷杉属(Abies Mill)、栎属(Quercus L.)、榛属(Corylus L.)等的共存组合变化推测了欧洲大陆北部阿尔卑斯前沿地在Eemian间冰期至Würmian冰期之间的冬季和夏季平均温度以及年降水量。在多元函数法方面,朱诚等(2008)运用线性回归方法,选取10种代表性花粉,根据表土数据确定每种花粉在线性方程中的系数,从而建立了神农架地区孢粉与年平均温度的线性函数,并进一步重建了该地区15.753 ka BP 以来的年平均温度序列。Park and Park(2015)利用在韩国济州岛汉拿山采集的表土孢粉建立起孢粉-温度函数,并通过该函数分析了哈农古玛珥湖在末次盛冰期的温度。在现代类比法方面,Tarasov et al(2011)整理了日本和俄罗斯远东地区的表土孢粉数据,将这些数据与日本Biwa湖的孢粉记录做对比,得出日本中部地区在冰期及间冰期时最热月和最冷月的平均温度、年均降水量等数值。黄康有等(2013)将神农架大九湖沼泽剖面孢粉数据和东亚表土孢粉数据库进行对比计算,选取其中相似度最高的样品,从而获得相应的古温度数据。在响应曲面法方面,许清海等(2003)重建了全新世以来岱海盆地的7月均温和年降水量,其原理是根据各物种花粉在不同气候条件下的丰度建立非线性的多项式函数,再根据地层里该物种花粉的丰度值推断气候参数值。由于孢粉和气候数据关系的复杂性,上述古气候定量重建方法适用范围也有差别,需根据不同的研究区域和研究时间段确定其中最合适的方法。

与孢粉相比,植硅体和硅藻等已采用的气候定量重建方法相对较少,一般采用线性回归方程。例如对东北泥炭地沉积植硅体的研究,首先根据聚类分析确定表土植硅体中与气候密切相关的形态类型,再结合气象资料建立年均温度、年均降水、年均湿度与这些植硅体类型的线性函数,然后依据线性函数绘制3000 a来榆树剖面的温度、降水、湿度变化曲线(张新荣等,2008)。Lü et al(2006,2007)首先利用典范对应分析和除趋势对应分析等数学手段建立了中国表土植硅体组合与年均温等指标的对应气候模型,进而重建了渭南黄土沉积136 ka BP以来的年均温度和年均降水数据。格陵兰西部海域对硅藻的典型相关分析显示硅藻分布与海冰密度有显著相关性,据此可构建两者的线性回归模型,从而重建长时间尺度的古海冰变化(蒋辉,2012)。Gomes et al(2014)在巴西Boqueirão湖的不同区域采集表层沉积物样品中,根据其中硅藻类型变化建立起硅藻-湖泊深度模型,并将其应用于湖泊古深度的重建。通过硅藻分析还可重建沉积物pH变化,如利用英国威尔士地区湖泊表层沉积物的现代硅藻组合可建立硅藻-pH转换函数,定量重建湖泊在历史时期的pH变化曲线(成小英和李世杰,2006)。

4 植物微体遗存与同位素分析的综合研究

从上述研究可以看到,孢粉等植物微体遗存重建古环境的原理是依据其大小、形态的不同确定所属植物种类,通过与现代植被的对比确定它们和气候因子的相关关系,进而从地层植物微体遗存的种类和数量变动曲线判断环境变化过程。但近年来,对植物微体遗存的研究已经超出形态范畴,拓展到其本身化学组成,如碳同位素等。

孢粉由有机质组成,含有丰富的碳元素,因而可用于碳同位素研究。某些沉积物中有机质含量很低,难以达到14C测年的要求,此时如能从沉积物中提取到足够量的孢粉浓缩物,借助AMS手段,同样可达到测年目的(Brown,1992;李宜垠等,2007)。例如Piotrowska et al(2004)通过贝加尔湖不同钻孔孢粉提取物的AMS14C数据讨论了湖区不同部位的沉积速率。再如李育等(2012)通过孢粉浓缩物的AMS14C 测年手段,讨论了甘肃省猪野泽全新世湖泊沉积物的再沉积作用,发现通过孢粉测出的年龄略偏老。植硅体在形成过程中,其内部常常会包埋有碳,且植硅体在沉积物中分布广泛,故也可用于测年。例如金和天等(2014)对浙江余姚田螺山遗址进行了植硅体测年分析,并与同层位的植物种子测年结果对比,发现结果相近,但前者年龄略偏老。Yin et al(2014)对现代植物中提取出的植硅体进行14C测年,结果显示植硅体在900℃以下提取出的有机碳测年结果较好,高于这一温度测出的温度偏老。因此不排除前述的测年结果偏老与测年流程存在错误有关。总体来看,利用孢粉和植硅体进行测年是可行的,但结果可能需要校正。

除利用放射性同位素测年外,孢粉和植硅体等的稳定同位素研究也已展开。常见的植物依据光合作用途径的不同分为C3和C4植物,两者在同位素上的差异主要表现为δ13C的不同,孢粉和植硅体作为植物体的一部分,与植物整体的δ13C相近,故而可用于δ13C测定(Kelly et al,1991;边叶萍和翁成郁,2009)。有研究认为,孢粉的δ13C与温度之间存在正相关性(Loader and Hemming,2001;Hatté et al,2009),但近来也有学者对此提出疑问(King et al,2012);在孢粉的δ13C与年降水量方面,多数人认为两者呈负相关,但也有少数学者认为两者无关或呈正相关(Kohn,2010)。故而,孢粉和植硅体δ13C与温度、降水的关系还有待深入研究。此外,植硅体中的δ18O已被证实和生长时的土壤含水量有关,故而在重建古环境过程中有望提供年均降水和年均温信息(Alexandre et al,2012)。相比孢粉和植硅体,稳定同位素研究在硅藻上的应用更加广泛,尤其是在海洋沉积物中,如以δ13C指示海洋生产力,以δ15N指示硝酸盐利用程度,以δ30Si指示硅酸盐利用程度等,以δ18O指示海表温度等(李铁刚和熊志方,2010);在淡水湖泊中,目前研究较多的是硅藻的δ18O(Leng et al,2001)。

5 小结

孢粉、植硅体、硅藻等植物微体遗存分析在第四纪环境领域已经得到了较为较广泛的应用。它们在重建古植被、年均温、年均降水等古气候信息方面都有着独特的优势,在稳定同位素分析和测年等研究领域也有一定的交叉应用研究潜力。目前尚待解决和完善的问题有:现代植物微体形态(如孢粉形态分类)研究还不够细化,有些孢粉类型难以鉴定到种、属乃至科一级,这些局限性给地层孢粉样品的鉴定带来了挑战与不确定;现代植被类型和孢粉等的定量关系研究还不够全面,因而从地层中获取的古温度等信息不够精确,需要完善相关的数据库,同时也有赖于更好的数学模型的建立;目前的植物微体遗存研究多集中于孢粉、植硅体、硅藻等领域,尚有许多类型的如真菌孢子缺乏研究,而且将多种植物微体遗存结合起来的研究存在不足;对植物微体遗存的稳定同位素等的研究还比较少,有待进一步展开,其次,植物遗存分析与其他地球化学指标的综合分析研究也有待加强。

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Application of plant microfossils in Quaternary environmental research: a review and perspective

ZHANG Jixiao1,2, XU Hai1,3
(1. State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an 710061, China; 2. University of Chinese Academy of Sciences, Beijing 100049, China; 3. Department of Environment Science and Technology, School of Human Settlements and Civil Engineering, Xi'an Jiaotong University, Xi'an 710049, China)

Background, aim, and scopePlant microfossils, such as sporopollen, phytoliths, starch grains and charcoals, have been widely used in Quaternary environmental research due to their features of huge quantity, wide distribution and easy preservation. However, review articles on plant microfossils in China so far have not been systematic and comprehensive enough. In this paper, key types, extraction methods and application examples in Quaternary environmental research of plant microfossils are reviewed and analyzed, and the prospects on integrated multi-proxy studies are proposed.Materials and methodsFerns spores and pollen of seed plants are collectively referred to as sporopollen, and they can be identifi ed and categorized in different parent plants by comparing their morphologies. By analyzing the types and composition ratios of sporopollen in sedimentary strata, the vegetation composition, precipitation and temperature in the historical period may become available. Some plants such as grasses can form a type of silica-plant microfossils called phytolith, which can be preserved in the soil after its death. Different plants can form distinct or similar forms of phytolith, so phytolith can also be usedin Quaternary research. Similarly, starch grains of different plants have their own characteristics and are a basis for the identifi cation of plant species. So starch grains are expected to be used in Quaternary research, especially in archaeology. In the oceans, lakes, wetlands and other sediments, a large number of micro-algaes, such as diatoms, Charophytes oogonia, dinofl agellates,Pediastrummay appear, which are complex and diverse, and they can be categorized in the plant microfossils too. Generally, during the analytic process of sporopollen, charcoals and fungal spores can be found. Charcoals are commonly used in the study on the history of fi re. Fungal spores can implicate some certain environmental information. Furthermore, some plants contain calcium oxalate crystals, which are similar to phytoliths and can be referred to as “plant calcium body”, and now studies on them are relatively less, but with the deepening of the research, they could be expected to be used in Quaternary research. Plant microfossils is dispersed in sediments and mixed with organic and inorganic impurities, so they should be separated and purifi ed for the implementation of further research. Depending on the chemical composition of plant microfossils in different types, extraction method also has a corresponding difference. To sum up, the method to extract sporopollen mainly includes hydrofluoric acid screening method, heavy liquid flotation method and screening method, and in actual extraction process, all these methods are integratedly used. Phytoliths and diatoms are mainly composed of the silicon, and they often coexist in the sediments. To extract them, oxidants are used to remove organic components, while hydrochloric acid is used to remove the soluble carbonate minerals. Besides, sedimentation method, microwave ablation method or sieving method is used to remove clay during the extraction process of phytoliths. In physics, different sizes of particles have different sedimentation velocities in a liquid, which can be calculated by Stokes sedimentation equation. Based on this principle, the author of this article uses the low-speed centrifugation method to remove those impurities which are smaller than sporopollen or phytoliths, and finds it can replace the screening method. Sometimes, in order to speed up the extraction process and reduce the dosage of samples, heavy liquid extraction methods and so on are used to obtain various plant microfossils at the same time.ResultsTraditionally, plant microfossils are used in reconstructing paleoenvironment and paleoclimate by identifying their species and comparing relative quantity changes among major species in sediments. For example, the pollen ratio ofArtemisiaand Chenopodiaceae is often applied in analyzing humidity changes. Phytoliths and diatoms, due to their relatively poor mobility, especiallyin situconservation, their content changes in sediments usually refl ect the original plant condition. But for pollen, yielding quantity and propagation distance in different plants are different, so relying solely on pollen content change to rebuild paleovegetation and paleoclimate has certain limitations. To explore the quantitative relationship between pollen and vegetation or climate, surface pollen and air pollen spectrum research are widely carried out, thus providing a reliable basis for modern pollen-vegetationclimate relationship model. With the gradual replenish of the global pollen, vegetation and meteorological databases, a large number of reliable quantitative paleoclimate reconstruction methods are built, and according to their different principles they generally include: indicator species method, such as the coexistence analysis and areal overlay analysis method; multivariate function method, such as converting function method; assemblage method, such as modern analogy and response surface method.DiscussionIn recent years, the study on plant microfossils has transcended the limitation of morphology and expanded to their chemical composition, such as carbon isotope. For example, some sediments contain a low content on other organic matters but are rich in sporopollen, so sporopollen can be extracted for dating. Some studies show that theδ13C content of sporopollen is related to its temperature and annual precipitation. Therefore, it is expected to provide useful information during the process of reconstructing paleoenvironment. Furthermore,δ18O in phytolith has been verifi ed of its relation to soil moisture duringits growth andδ13C in diatoms of marine sediments can refl ect the ocean productivity.ConclusionsAll in all, plant microfossils have been widely used in Quaternary research, and they have unique advantages in providing useful information in rebuilding paleovegetation and paleoenvironment, and also have potential uses in dating and stable isotope analysis.Recommendations and perspectivesHowever, some problems still remain: the morphology of modern plant microfossils (such as pollen) requires further research; the quantitative relationship between vegetation and modern pollen etc. is not comprehensive enough; some types of plant microfossils (such as fungal spores) are lack of the research on them; stable isotope study of plant microfossils is still few, and comprehensive analysis of plant microfossils and other geochemical indicators should also be strengthened.

plant microfossils; sporopollen; phytolith; extraction methods; quantitative climate reconstruction; stable isotope

ZHANG Jixiao, E-mail: zhangjx@ieecas.cn

10.7515/JEE201603003

2015-12-02;录用日期:2015-12-25

Received Date:2015-12-02;Accepted Date:2015-12-25

国家自然科学基金项目(41473120);国家重点基础研究发展计划(973计划)(2013CB955903)

Foundation Item:National Natural Science Foundation of China (41473120); National Basic Research Program of China (973 Program) (2013CB955903)

张继效,E-mail: zhangjx@ieecas.cn

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