第四纪研究  2018, Vol.38 Issue (1): 67-75   PDF    
青藏高原北部柴达木盆地中新世菌孢子变化及其意义初探
苗运法1, 吴福莉2,3, 方小敏2,3,4, 王梓莎1     
(1 中国科学院西北生态环境资源研究院, 沙漠与沙漠化重点实验室, 甘肃 兰州 730000;
2 中国科学院青藏高原研究所, 大陆碰撞与 高原隆升实验室, 北京 100101;
3 中国科学院青藏高原地球科学卓越创新中心, 北京 100101;
4 中国科学院大学, 北京 100049)
摘要:孢粉学-气候环境变化研究往往只注重高等植物孢子花粉(孢粉)的组合变化,对孢粉提取物中通常存在的一类低等植物孢子菌孢子的报道却很少,其类型和含量变化特征以及可能蕴含的古气候环境信息有待发掘。位于青藏高原北部的柴达木盆地西部(柴西)中新世KC-1孔是探讨晚新生代亚洲内陆气候环境变化的重要钻孔,本文首次建立了该钻孔中菌孢子的变化序列并尝试探讨其气候环境指示意义。结果表明:菌孢子类型较为单调,以单体型(single-celled)为主,其他类型比如双体型(double-celled)和多体型(multi-celled)含量很少;菌孢子浓度变化在18~5 Ma之间整体呈现增加趋势。推测这种增加的趋势可能与持续变干的气候有关,即在变干背景下,湖体盐度的增高增强了还原环境,从而有利于菌孢子的保存。其中,菌孢子浓度在14 Ma和10 Ma呈现两阶段增加的特征,前者与全球中中新世气候最佳期的结束对应;后者则与该地区进一步的变干以及青藏高原强烈的构造运动导致沉积环境更加有利于菌孢子的富集有关。综上,菌孢子的含量变化可能同样有利于解译过去气候环境变化,是孢粉学研究中不宜忽略的部分。
主题词青藏高原     中新世     菌孢子     环境变化    
中图分类号     P534.62+1;Q914.83                     文献标识码    A

0 引言

晚新生代亚洲内陆干旱环境演化及其驱动因素备受地学家关注。高质量的气候代用指标对重建该时期的干旱环境发展过程以及捕捉其间发生的重大气候环境事件,探讨其驱动机制具有重要科学意义。众多研究者已在青藏高原北部及其边缘区开展了大量晚新生代沉积研究工作,包括黄土高原典型风成黄土-红粘土堆积序列研究[1~4],一方面细化了晚新生代以来气候环境变化的基本特征,捕捉到多个重大气候环境事件[5~8],提出青藏高原阶段性强烈隆升可能是亚洲内陆干旱化和亚洲季风阶段性增强的主要因素[9~13],同时还可能与两极变冷存在较为显著的联系[14~18],为亚洲内陆地区气候环境变化研究提供了第一手资料。其中部分学者从对气候环境变化反映最敏感的有机指标之一——孢粉学的角度入手,建立了天山南北两侧[6, 19]、柴达木盆地[10~11, 16]、酒泉盆地[20]、天水盆地[21~22]和六盘山盆地[15]等盆地的孢粉图谱,获取在中新世较长时间尺度上的气候变化记录,然后提取出干旱化的指标,论述了亚洲干旱环境变化过程及其可能的驱动机制,这些研究表明孢粉学在过去气候环境变化及其驱动机制探讨等方面发挥了重要作用。

孢粉学(Palynology)是研究植物孢子、花粉(简称孢粉)形态、分类及其在各个领域中应用的科学,上述研究主要基于孢粉学传统的研究内容之一,即孢粉植物群组合对过去生态-气候环境变化的指示。事实上,在孢粉提取物中,除高等植物的孢粉颗粒以外,还含有其他的孢形(如藻类、菌孢子、沟鞭藻、疑源类等)、各类有机碎屑(动、植物残体、菌丝体等)以及炭屑等,富含更多的埋藏学、沉积学和气候环境变化等信息[23~27]。由于受传统孢粉学研究的限制,对沉积相中除高等植物的孢粉以外的各类成分研究鲜有涉及,这就从一定程度上影响了我们对这些成分的结果及其可能蕴含的环境信息(气候或构造)的系统性认识。本文正是基于该思路,首次尝试从柴达木盆地西部地区(柴西)获得的孢粉提取物中对菌孢子的特征进行探讨,以期为上述科学问题提供新证据。

1 柴达木盆地及KC-1孔概况

柴达木盆地位于亚洲内陆腹地,属于青藏高原最北的部分。该盆地介于北纬35°00′~39°20′、东经90°16′~99°16′之间。盆地略呈三角形,西高东低,西宽东窄,北西西-南东东方向延伸,东西长约800 km,南北宽约300 km,面积约12×104 km2。盆地四周高山环绕,南面是东昆仑山脉,北为祁连山脉,西到阿尔金山脉,为封闭的内陆盆地(图 1)。盆地中沉积了巨厚的新生代地层,本文研究选取了位于柴西地区的KC-1孔(38°03′N,91°45′E;海拔2820 m),该孔为原地矿部20世纪80年代中期在实行矿产资源调查时所钻取的一口深井(3435 m),岩芯下部以灰色泥岩、砂质泥岩为主,向上逐渐过渡到浅灰色砂质泥岩、粉砂岩为主,向上盐分逐步增多。根据地震资料、岩性特征以及介形化石组合,该钻孔2705 m处为上、下油砂山组的界限,700 m处为上油砂山组与狮子沟组的界限,通过与柴达木盆地东部具有精确年代控制的怀头他拉剖面界限年龄对比,得到KC-1钻孔的年代为18 Ma到5 Ma[16],所获取的碎屑样品中富含孢粉,为解析柴西地区生态、气候和环境变化提供了基本证据[16~17, 28~29];同时,提取物中还发现了较多的菌孢子,为本研究提供良好素材。

图 1 柴达木盆地地理位置及其构造等基本特征 KC-1孔为本研究涉及的钻孔,其沉积年代为约18~5 Ma[16];南海的IODP U1433孔为发表的准同期菌孢子序列[24] Fig. 1 The geographical location and tectonic features of the Qaidam Basin. The KC-1 core is involved in this study with the sediment age of ca. 18~5 Ma[16], and contemporaneous sediments of the IODP U1433 from the South China Sea is also shown here for its fungal study[24]
2 实验方法与结果

在KC-1孔中共采集了58个样品,按照孢粉的标准流程进行提取[16]。称取60 g左右的样品,先用10%的HCl溶液处理除去碳酸盐成分;用39%的HF溶液除去硅酸盐成分;然后用超声波振荡过孔径10 μm的尼龙筛进行过滤;最后用试管富集净化后,用甘油保存制成活动片进行鉴定统计。所用显微镜鉴定和拍照使用Leica DM4000及其成像系统,所有样品均保存在中国科学院寒区旱区环境与工程研究所。本次主要针对样品中的菌孢子形态类型和含量变化进行统计计算,并与已发表的孢粉结果[16]进行对比,探讨菌孢子对气候环境变化的可能指示,存在的潜在问题以及运用前景。

菌孢子的统计按照细胞的个数进行分类,同时统计出外加石松(Lycopodium clavatum)的数量以换算菌孢子的浓度,浓度计算公式为:W=A×n/B/G。其中W为菌孢子浓度(粒/g),A为统计的菌孢子个数,n为外加石松孢子数(约27600粒/片),B为统计的外加石松孢子数,G为样品质量(g)。该方法同样适应孢粉浓度[28]计算。

分析结果表明,整个钻孔的菌孢子类型较为单调(图 2),基本上以单体型(single-celled)为主,类型简单,一般为1~2种,其他类型比如双体型(double-celled)尚可见到,但多体型(multi-celled)很少见到。整体上,从剖面下部至上部形态类型的变化呈现出增加的趋势(图 3a);菌孢子总浓度也呈现整体增加的趋势,并在约14 Ma和10 Ma呈现较为明显的阶段性变化特征,由平均约4粒/g增加到约8粒/g,再增加到约15粒/g(图 3b);同时,我们还计算了孢粉总浓度[28](图 3c)、菌孢子与孢粉浓度比值(图 3d)和孢粉组合中干旱分子含量变化[16](图 3e)。发现其均在约14 Ma和10 Ma处具有较为明显的变化,具体表现为,孢粉总浓度在约14 Ma出现低值(约500粒/g)、在约10 Ma开始出现高值(约1200粒/g)后并持续升高(图 3c)、菌孢子与孢粉浓度比值在约14 Ma和约10 Ma前后也出现相应的跳跃式变化(图 3d)。

图 2 柴达木盆地KC-1孔部分代表性菌孢子形态 1~7为单体型(single-celled type);8~10为双体型(double-celled type) Fig. 2 Typical morphologies of the fungal spores from the KC-1 core, western Qaidam Basin

图 3 柴达木盆地KC-1孔菌孢子形态类型和浓度的变化 (a)菌孢子形态类型数量;(b)菌孢子总浓度;(c)孢粉总浓度[28];(d)菌孢子总浓度/孢粉总浓度(粗实线为3点平滑结果);(e)孢粉组合中的干旱成分含量[16] Fig. 3 The morphological types and concentration changes of fungal spores of the KC-1 core, Qaidam Basin. (a) Morphological numbers of the fungal spore; (b) Total fungal spores concentrations; (c) Total pollen concentrations[28]; (d) Total fungal spores concentrations/total pollen concentrations; (e) Percentages of the xerophic taxa in the pollen assemblages[16]
3 讨论

KC-1孔孢粉结果较好地揭示了该地18~5 Ma期间的气候环境演变过程[16],结合同期柴达木盆地其他地区和研究揭示的气候环境变化框架下,探讨KC-1孔菌孢子变化可能的气候环境意义以及对未来开展菌孢子变化研究的启示。

众所周知,孢粉是反映气候环境变化中最直接和敏感的指标之一,一直被视为探讨气候环境变化研究的重要方法[30~37]。柴达木盆地西北部KC-1孔的孢粉结果显示[16]:18~5 Ma以来组合可以较为明显的分为两大类,即盆地的干旱类型和山地的针叶林类型,其含量发生有规律的变化。具体特征为盆地内部经历持续干旱,耐旱的麻黄科(Ephedraceae)、藜科(Chenopodiaceae)和菊科(Compositae)含量总和增加明显。山地针叶林早期以雪松(Cedrus)、松属(Pinus)、云杉属(Picea)为主,后期过渡到云杉属和松属为主,最后仅以云杉属为主,反映了山地气候的变干,截水(降水)能力下降。喜热成分百分含量与全球气候变冷的趋势一致,无论是中中新世气候最佳期(18~14 Ma)还是之后的持续变冷(14 Ma以后),二者都能较好地吻合[16]。在柴达木盆地偏中部的位置上,孢粉组合在5.3~3.0 Ma之间主要为针叶类型的松属和云杉属(含冷杉属)为主,并与蒿属(Artemisia)、藜科以及麻黄科的高含量交替出现为主要特征[11];在3 Ma以来孢粉类型主要以蒿属、藜科、菊科和禾本科(Gramineae)等草本植物为主,针叶林等乔木已被草本植物取代,盆地内部变得更干,开始发育多期次的盐类沉积[10, 38~39]。这些记录综合起来,表明柴达木盆地在18 Ma以来存在变干的整体趋势[29],并能与全球变冷的深海氧同位素记录相耦合[40]。最近依据介形类化石组合、稳定同位素以及岩石化学风化记录认为在该盆地中北部在13.3 Ma存在明显的变干过程[41],依据湖相和成壤碳酸盐同位素记录,在盆地东北部的12 Ma存在较为显著变干[42],同样在盆地东北部,依据哺乳动物化石牙釉质样本的稳定同位素结果,干旱主要发生在早上新世之后[43]。尽管不同的指标记录的干旱化事件略有差异,但综合在一起仍能描绘出持续变干的趋势[44~45]

KC-1孔菌孢子的形态类型数量变化,呈现出一种大致增加的趋势(图 3a)。如果依据与上述结论简单的耦合对比,可以得出菌孢子的增加代表了干旱环境的加强。但这需要小心论证。菌类保守估计大约有10多万种,分布十分广泛[46]。菌类植物可分为细菌门、粘菌门和真菌门三类。目前对其自然地理分布种类和自然各要素之间的关系研究较少。通过实验室对菌类生长的温度和湿度调控分析,发现菌类对温度和湿度没有明显依赖[47];然而通过对全球范围内土壤中菌类的研究发现,尽管其全球分布范围内似乎没有明显变化,但在干旱区种类还是明显偏少[48],这与对建筑物内菌类研究的发现,即对水分和温度具有较好的依赖性较为一致[49~53]。因此,我们倾向于菌类植物对气候环境的依赖,更倾向于偏好的生境状态。如果根据这一简单对比,可能就会得到菌孢子的数量增加对应着环境的变好。显然,这与孢粉组合揭示的环境变干不符。这种变化可能另有原因。同时,菌孢子的浓度变化,亦呈现出一种大致增加的趋势(图 3b)。所以菌孢子类型和含量的增加可能都不能指示水热条件的改善。

此处,我们将孢粉浓度的变化列出来[28](图 3c),可以看出其在18~10 Ma之间,基本上呈现降低的趋势,与变干导致的生物量减少有很大的关系。在10 Ma之后开始又逐渐增加,明显与干旱分子含量增加代表的变干趋势相反,被解释为当时存在的一期强烈构造运动导致盆地开始朝着逐渐富集孢粉的方向发展以及湖体的快速缩小,这种变化和缩小的速度超过了生物量降低的速率[28]。当把菌孢子总浓度与孢粉总浓度进行比较时(图 3d),发现它们的浓度具有相似性,但在18~10 Ma期间菌孢子与孢粉浓度的变化具有差异,即逐渐增多,这可能表明菌孢子与孢粉的保存能力不同或产量不同,但这需要进一步的论证。

因此,菌孢子类型和浓度增加的趋势与持续变干的气候环境有关不是直接的因果关系,而是由于变干的环境,可能导致湖体含盐的增高进而增强了还原环境,从而有利于菌孢子的保存有关。其中,菌孢子浓度在14 Ma和10 Ma呈现两阶段增加的特征,在14 Ma的升高与全球中中新世气候最佳期(Middle Miocene Climatic Optimum, 简称MMCO)的结束[40]对应。尽管有利于环境增强的直接证据还很少,但KC-1孔高分辨率的氯离子浓度在该时期的持续增加(注:未发表资料)无疑是最好的证据,代表沉积环境朝着更加还原的方向发展,使得菌孢子更易保存下来;在10 Ma的升高则与气候进一步变干以及青藏高原强烈的构造运动导致沉积环境更加有利于菌孢子的富集有关[28]。这一推测主要参考孢粉浓度变化[28],其在约10 Ma之前表现为逐渐减少趋势,可以用全球变冷导致生物量的降低这一原理[24]解释;但在约10 Ma之后,孢粉浓度陡然强烈增加,与干旱分子百分含量[16]、同位素[54]和盐类转换[55]等记录的整个盆地仍在持续变干的记录相矛盾。构造被考虑为重要的因素,因为强烈的构造本身不仅能够直接影响气候环流系统,还会通过改变沉积体系,干扰甚至改变气候环境代用指标结果[56],引起解释的复杂性。在柴达木盆地,大空间上的强烈构造挤压促使盆地沉降中心向东南方向迁移[57~58],小空间尺度上在柴西主要表现为数条褶皱变形的形成,引起局地沉积环境的快速变化(图 1),比如旧湖的瓦解和新湖的形成[59~60]。KC-1孔正好处于这一区域中心,必然会受到构造的强烈影响,迫使孢粉浓度在10 Ma之后朝着强烈富集的方向发展[28]。菌孢子作为生命体之一,亦势必会被强烈影响,出现浓度的升高。

截至目前,国际上生成连续记录的中新世菌孢子变化序列很少,仅在我国的南海深钻中有所报道[24](图 4),为对比提供了很好机会。这里,我们把KC-1孔(图 4a4c)与同期南海IODP U1433孔(图 4b4d)[24]的菌孢子形态类型和浓度变化进行对比,发现二者无论在形态类型还是浓度变化方面均具有大概一致的变化趋势。

图 4 柴达木盆地KC-1孔(a,c)与同期南海IODP U1433孔 (b,d)[24]的菌孢子浓度与类型变化对比
MMCO:中中新世气候最佳期,18~14 Ma
Fig. 4 Comparison of the morphological types and concentration changes of fungal spores between the KC-1 core (this study), Qaidam Basin (a, c) and IODP U1433 core, South China Sea (b, d)[24]. MMCO:middle Miocene Climatic Optimum, 18~14 Ma

在菌孢子类型方面,KC-1孔的数量明显较少,可能说明菌类生长对湿度的依赖,即西北内陆地区由于气候干旱,使得菌类植物生存不如南海的优越,导致其类型整体偏少,这与全球范围内土壤层的菌类在干旱区的种类偏少相一致[48]。在浓度方面,KC-1孔的数量依旧很低,可能说明菌类在柴达木盆地的产量偏低,又或者柴达木盆地流域相对于南海而言,范围太小有关,这都需要进一步的论证。因此,对于亚洲内陆地区与南海的菌孢子在类型和浓度变化上的相似,是一种巧合还是具有内在联系,也是将来我们需要进一步探讨的问题。比如,从构造角度看,南海在8 Ma前后类似于现代湄公河贯通,有利于携带陆源物质到钻孔附近沉积,从而使得菌孢子类型和浓度增加[24, 61]。如前面介绍,柴达木盆地西部地区在10 Ma前后出现强烈的构造运动和气候进一步变干导致的湖体萎缩,使得孢粉浓度开始出现增加趋势[28]。因此,这种相似性是否与大空间的构造运动准同期性具有耦合联系?从气候角度看,如果由气候变化驱动,则是否可能代表在大空间尺度上气候变化的内在联系性,比如水汽/温度同步变化效应等?从菌孢子保存的角度,是否能够说明保存下来被识别的孢子形态和浓度变化与菌孢子的抗腐蚀性有关?究竟与何种因素驱动或者三者皆而有之,需要我们进一步对更多地区更为详细的菌孢子形态类型和浓度变化研究,以期为上述科学问题提供更具说服力的证据。

4 结论

通过对位于青藏高原北部的柴西KC-1孔孢粉提取物中的菌孢子分析,初步获得了中新世18~5 Ma期间菌孢子变化记录,其形态以单体型为主,其他类型比如双体型和多体型含量很少;菌孢子浓度变化呈现整体增加趋势,推测可能与持续变干的气候有关,即在变干背景下,湖体盐度的增高增强了还原环境,有利于菌孢子保存。其中,具体的表现为在约14 Ma和10 Ma呈现阶段性的增加。这种变化趋势在气候上可与KC-1孔孢粉记录揭示的中新世持续变干耦合,即变干可以增强保存菌孢子的还原环境,从而有利于菌孢子的保存;在约14 Ma的增强可以与全球中中新世气候最佳期的结束相对应;在约10 Ma的增强可能与构造和气候共同作用下更加有利于菌孢子的富集有关。本文首次尝试探讨了在柴达木盆地建立的菌孢子组合变化可能蕴含的气候环境变化信息,说明其在气候变化重建中具有不可忽视的作用,未来将开展更为详尽的研究,为探讨其广泛应用提供更多的基础和证据。

致谢 感谢同行评审专家和编辑部老师提出的宝贵修改意见!

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Preliminary exploration of the fungal spores in Qaidam Basin, north Tibetan Plateau during the Miocene period
Miao Yunfa1, Wu Fuli2,3, Fang Xiaomin2,3,4, Wang Zisha1     
(1 Key Laboratory of Desert and Desertification, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou 730000, Gansu;
2 Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101;
3 CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101;
4 University of Chinese Academy of Sciences, Beijing 100049)

Abstract

The traditional researches of the palynology-climate and environmental changes tend to focus on the spores and pollen (sporopollen)of higher plant, while the fungal spores usually existing in the pollen extracts as lower plant spores are rarely reported, and the characteristics of its types and contents together with their information of the paleoclimate and paleoenvironment are undiscovered. The Miocene lacustrine-dominated KC-1 core, drilled to a depth of 3435 m in the western part of the Qaidam Basin (at 38°03'N, 91°45'E; elevation 2820 m a.s.l.), is important to explore the paleoclimate and paleoenvironmental changes of Inner Asia during the Late Cenozoic (ca. 18~5 million years), and the fungal spores discovered in this core will firstly establish the evolutionary series of the fungi in this area. A total of 58 samples from this core have been extracted followed the methods of palynological analysis. Samples of approximately ca. 60 g of sediment were treated with 10% HCl and 39% HF to remove carbonates and silica; separation of the palynomorphs from the residue was carried out using a 10-μm nylon sieve; finally, the palynomorphs were mounted in glycerin jelly. The statistics of the fungal spores were classified according to the number of cells, and the concentration was calculated using the weight methods (in dry weight)by initially adding a known number of Lycopodium clavatum to each sample. The results show that fungal spores are simple in type, mainly single-celled, while other types such as double-celled and multi-celled are rare; the change of fungal spore concentrations show an overall increase trend during 18~5 Ma. This increased trend may be related to the continuous aridification trend, that is, under the background of aridification, the increase of salinity in the lake can enhance the strength of the reducing environment, which is beneficial for the preservation of fungal spores. There are two rapid changes of the fungus spore concentrations at ca. 14 Ma and 10 Ma, from averaged ca.4 grains/g to ca. 8 grains/g and then ca.15 grains/g. The increase at ca.14 Ma is corresponding to the end of the global Mid-Miocene Climate Optimum; then the increase at ca.10 Ma is related to the further drying and the strong tectonic movements of the northern Tibetan Plateau, which are eventually helpful for the fungus spores enrichments. It can be seen that fungus spores are conductive to the analysis of paleoclimate and paleoenvironmental changes, and in future it will be very interesting to detailedly study the fungal in the palynofacies.
Key words: Tibetan Plateau     Miocene     fungal spores     paleoenvironment