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地球科学进展, 2018, 33(9): 983-993
doi: 10.11867/j.issn.1001-8166.2018.09.0983
蚯蚓方解石颗粒——一种新的古气候信息记录载体
Earthworm Calcite Granule —A New Proxy for Paleoenvironmental Reconstruction
宗秀兰1,2,, 宋友桂1,*,, 李越1,2
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,陕西 西安 710061
2.中国科学院大学,北京 100049
Zong Xiulan1,2,, Song Yougui1,*,, Li Yue1,2
1.State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an 710061, China
2.University of the Chinese Academy of Sciences, Beijing 100049, China
 引用本文:
宗秀兰, 宋友桂, 李越. 蚯蚓方解石颗粒——一种新的古气候信息记录载体[J]. 地球科学进展, 2018, 33(9): 983-993, doi:10.11867/j.issn.1001-8166.2018.09.0983
Zong Xiulan, Song Yougui, Li Yue. Earthworm Calcite Granule —A New Proxy for Paleoenvironmental Reconstruction[J]. Advances in Earth Science, 2018, 33(9): 983-993, doi:10.11867/j.issn.1001-8166.2018.09.0983

摘要:

近年的研究表明在第四纪土壤和沉积物中广泛存在由蚯蚓特殊腺体产生的方解石颗粒 (ECG)。它们一方面可为放射性碳测年提供有效的测年材料,另一方面能够反映其形成时的温度和降水特征。ECG作为潜在的古环境变化信息的新载体,成为古气候研究领域的新方向。在前人研究工作的基础上,分析了ECG的研究现状,介绍了ECG的产生机理及特征,详细阐述了ECG的碳、氧同位素反映古气候、古环境变化的原理与机制,并介绍了其在年代学、古气候环境重建中的应用,探讨了其作为新指标的可靠性及存在的问题,展望了该领域的研究方向和趋势,希望能对未来的黄土古气候研究提供参考。

关键词: 蚯蚓方解石颗粒 ; 碳、氧同位素 ; 年代学 ; 古气候重建

Abstract:

Earthworm calcite granules (ECG), generally produced in Morren's glands of the earthworm Lumbricus terrestris and Lumbricus rubellus, are commonly preserved in Quaternary soils and sediments well. These granules can not only provide radio-carbon dating (Carbon-14) with the efficacious materials, but accurately record a wealth of climatic and environmental information on temperature and precipitation. For instance, researchers from France reconstructed the paleotemperature and paleoprecipitation during the last glacial in west part of Europe by taking advantage of δ18O and δ13C signal contained in ECG. Additionally, scientists from Germany and France carried out radiocarbon dating of ECG from two different loess-paleosol sequences, and the results showed consistency with the dating results of other materials (such as charcoal, bone, plant calcified root cells, etc.). Therefore, this new bio-indicator has been confirmed as a proxy for paleoenvironmental and paleoclimatic reconstruction, hopefully becoming the golden key to understanding the paleoclimate change. This paper, based on the previous literatures, reviewed the present research status of ECG in paleoclimatology, mainly consisting of five aspects: (i) ECGs' production mechanism and their characteristics; (ii) The theoretical foundation of stable carbon and oxygen isotopes in terrestrial fossil earthworm calcite granules for paleoclimatic reconstruction; (iii) The pre-treatment of ECG samples; (iv) Current applications in chronology and paleoclimatology of earthworm calcite granules;(v) Major problems at present regarding paleoclimatic explanation and radiocarbon-14 dating of ECG. Finally, we proposed the future research and development direction in this field, which is expected to make a reference to the future researches.

Key words: Earthworm calcite granules ; Carbon and oxygen isotopes ; Chronology ; Paleoclimatology.
1 引 言

陆生蚯蚓属于环节动物门寡毛纲后寡毛目,全球已记录的陆栖蚯蚓有12科,181属,约4 000种,生物量巨大,是典型的大型土壤动物之一[1,2,3]。最近,Versteegh等[4]发现蚯蚓的粪便中微小的方解石颗粒 (Earthworm Calcite Granule, ECG) 会记录下周围环境的信息,因此可以作为了解古气候变化的“金钥匙”。ECG在古气候研究领域已引起国际学者的广泛关注,相关成果已在PNASGeochimica et Cosmochimica Acta等国际一流刊物上发表[4,5,6,7],成为古气候研究中新的国际前沿交叉学科研究方向。Prud' Homme等[5,6] 利用末次冰期黄土中ECG 的氧同位素(δ18O)和碳同位素(δ13C)重建了土壤及大气的温度和降水量,证实ECG可以作为一个新的古气候指标。同时,ECG也可以为年代学研究提供很好的放射性碳测年材料,其结果与其他一些测年材料 (如木炭、骨骼、植物钙化根系细胞等) 的测年结果可以很好地对比[7,8],如Moine 等[7]对德国莱茵河谷地Nussloch黄土序列中的ECG进行放射性碳同位素测年,结果显示该土壤序列形成于47~20 ka BP,古土壤发育时期与格陵兰冰芯氧同位素记录的间冰阶可以很好地对应。

尽管在第四纪土壤和沉积物中已发现大量与蚯蚓相关的遗迹,但是对其产生机制与古气候环境意义仍需要深入的理解和认识[9]。目前关于ECG的研究主要在欧洲黄土,我国黄土高原黄土尚未见报道,作为过去全球变化的三大支柱之一的中国黄土为研究ECG形成机制与古气候环境意义提供了得天独厚的条件。因此,本文在查阅大量相关文献的基础上,介绍了ECG的产生机理及特征,阐明了颗粒中碳、氧同位素指示古气候环境变化的机理,列举了一些ECG在年代学和古气候学中的应用,分析了其作为新的古气候指标的可靠性及存在的问题,展望了该领域的研究方向和趋势,希望能为黄土高原和中亚黄土古气候研究提供参考。

2 蚯蚓方解石颗粒的产生机理及特征

相关研究者[10,11,12,13,14]在对各种第四纪钙质沉积物进行矿物筛分过程中发现了与蚯蚓活动有关的直径为0.5~1.5 mm的ECG 。这些颗粒由位于蚯蚓食管两侧3对体节的特定腺体 (钙腺,Calciferous glands, 图1a) 产生[10~12,15],这一过程被称为ECG的生物矿化过程:在蚯蚓钙腺中分泌含有无定形碳酸钙 (Amorphous Calcium Carbonate, ACC) 的胶质“乳状”流体,这种流体在食管囊中与酶发生反应从而固化,演变成肉眼可见的碳酸钙晶体,最终通过肠道排泄到土壤中[16,17,18]。但蚯蚓排泄方解石颗粒的原因仍然存在争议,目前的研究[11,17~19]认为,该过程用以排出蚯蚓体内过量的钙或控制体内二氧化碳(CO2)的量,以及中和缓冲肠道pH值。

体内能够发生上述生物矿化作用的蚯蚓种属 (主要为正正蚓和红正蚓) 被称为生物矿化者 (biomineralisers)[11],所产生的方解石颗粒主要分布于土壤表层,具有如下特征[6~7,15]:①一般呈卵形,粒径为0.01~2.5 mm,具有由聚合亮晶组成的辐射状构造 (图1b~d);②化学成分为结晶较好的低镁方解石,对成岩蚀变作用有抵抗性;③颗粒物质来源主要靠生物体进食;④形成于蚯蚓的钙腺。另外,少量颗粒也可以ACC、球霰石和文石的形式出现[10~14,16,20,21]

3 蚯蚓方解石颗粒指示古气候环境变化的理论基础

蚯蚓在其生命活动时期与周围环境存在同位素平衡,因而可以记录其生存环境中的气候要素[9,19,22]


图1

蚯蚓方解石颗粒 (ECG) 在蚯蚓体内的产生部位及内外部形貌特征(据参考文献[6]修改)
(a) 蚯蚓体内钙腺的位置[11,12];(b) 扫描电镜下的正正蚓方解石颗粒;(c) 蚯蚓方解石颗粒的显微镜下照片;(d) 蚯蚓方解石颗粒的剖面图

Fig.1

Position where the Earthworm Calcite Granule (ECG) is produced within the body of earthworm and morphology features and interior structure of the ECG (modified after reference[6])
(a) The position of calciferous glands in earthworm[11,12]; (b) Scanning Electron Microscopy of an ECG produced by Lumbricus terrestris; (c) Micrograph of one ECG; (d) Thin section of an ECG

3.1 碳同位素 (δ13C)

土壤碳酸盐的δ13C值是其与土壤孔隙中的CO2进行同位素交换平衡后的结果,土壤孔隙中的CO2由土壤微生物降解有机质产生的CO2和从大气扩散进入土壤中的CO2组成。其中被降解的有机质来源于与大气CO2有交换作用的植物,因此,土壤碳酸盐的碳同位素组成受到当地生态系统中C3和C4植物比例的控制,即土壤碳酸盐的δ13C值与植被类型有关[23,24]。植物通过不同的代谢途径吸收大气中的CO2,这些代谢途径取决于植物生长的气候和环境。植物最常见的光合作用模式是C3卡尔文·本森 (Calvin Benson) 循环,出现于温和湿润气候条件中。C3 (Calvin型) 植物δ13C值变化范围为-34‰~-22‰,平均值为-27‰[25] (图2)。热带干旱—半干旱环境中C4 (Cranz型) 植物的光合作用通过海奇·史莱克 (Hatch Slack) 循环进行。该途径中与13C相关的分馏较少,致使有机质中δ13C相比于C3植物更偏正,其δ13C变化范围为-18‰~-8‰,平均值为-13‰[25] (图2)。除此之外,还有景天酸代谢型(Crassulacean Acid Metabolism, CAM)植物,为植物细胞采用景天科酸代谢途径的多肉植物。CAM植物在地球上相对较少 (约5%),存在于类似荒漠的环境中,其固定大气中CO2的方式有2种:一是通过磷酸核酮糖羧化酶 (C3途径),二是通过磷酸烯醇式丙酮酸 (类似C4途径)。因而CAM植物的δ13C值应该在C3和C4植物的δ13C值范围内变化[25] (图2)。因此,土壤有机质和土壤碳酸盐的δ13C值的变化范围一定程度上能反映C3,C4或CAM植物比例的变化[23,25]。ECG中的碳主要来源于凋落物 (死亡和分解的植物枯枝落叶) 和土壤有机质 (蚯蚓进食),少部分来源于腐殖质和吸入大气中的C O2[26,27],所以其δ13C的变化主要取决于被蚯蚓摄入的绿色植物的光合作用类型。尽管δ13C在ECG与植物之间存在一定的分馏值[6],但是前者δ13C值仍然能用于记录气候环境状况的变化。


图2

蚯蚓方解石颗粒 δ13C值与植被类型关系示意图

Fig.2

Schematic diagram showing the relationships between δ13C values of earthworm calcite granule and vegetation types

3.2 氧同位素

在与周围环境达到氧同位素平衡的状态下,生物体沉淀产生的碳酸钙是良好的古气候信息载体,其氧同位素组成可以记录古温度或周围水体氧同位素的变化[28,29,30]。与影响蚯蚓方解石碳同位素的因素相比,影响氧同位素的因素复杂得多。一方面,周围土壤的温度可以影响方解石颗粒氧同位素比值的变化[22];另一方面,土壤水分的氧同位素 (δ18Ow) 组成能够控制碳酸钙颗粒中的氧同位素组成[4]

Versteegh等[4]在现代条件下对蚯蚓进行培育实验,结果发现,当土壤持水能力较低时,受蒸发作用的影响,温度越高,土壤水分的δ18O (δ18Ow) 更偏正,此时ECG的δ18O (δ18Oc) 也会偏正,反之亦然。同时Versteegh等[4]进一步通过计算无机方解石与水介质之间的同位素分馏值发现,相比于前人的结果[31],氧同位素富集了(1.51±0.12)‰。因此,Versteegh等[4]对蚯蚓方解石的氧同位素组成 (δ18Oc)、周围大气来源水的氧同位素组成 (δ18Ow) 以及土壤温度重新建立了一个经验分馏公式。当ECG与外界环境达到平衡时,它们与土壤水分之间存在特定的氧同位素分馏值[22,30,32] (图3),因此根据上述经验公式可以定量重建土壤温度的变化。而且进一步的研究表明,由于这些方解石颗粒是由蚯蚓以相对较快的速度单个分泌产生,所以它们能够反映大气来源的渗流水中氧同位素的年内变化,从而用以重建温暖季节的古温度[22]。δ18Ow值决定了ECG的δ18O值,而δ18Ow值的变化取决于降水量和温度。据统计数据[33,34],在中、高纬度地区,δ18Ow值与平均气温呈线性关系,由此能够利用ECG来重建温度的变化[22]


图3

蚯蚓方解石颗粒δ18O反映大气来源渗流水氧同位素示意图

Fig.3

Schematic diagram showing links of δ18O values of earthworm calcite granule to oxygen isotope of vadose water with atmospheric sources

4 样品分析与预处理

ECG同位素分析关键在于同位素分析前样品的预处理,尤其ECG中有机质的去除最为重要,因为样品所含有机质严重污染方解石颗粒中碳、氧同位素组成,所以在对颗粒样品分析前必须进行去除有机质的预处理。

ECG有机质的去除方法如下[7]:① 在双目显微镜下挑选出大的、最干净的颗粒;② 用蒸馏水在超声波清洗仪中清洗10 min,以去除其表层的污染物,如黏土、铁氧化物、有机矿质涂层等;③ 将清洗的颗粒放在玛瑙研钵中轻轻研磨,将研磨好的样品置于0.01 mol/L的HNO3溶液中,浸泡30 min以除去其表面的杂质、氧化剩余的有机残留物;④ 用超纯水冲洗,冲洗完毕后用巴斯德吸管吸取剩余的水;⑤最后将样品烘干。

目前,国际上对ECG中的碳、氧同位素的分析采用方解石颗粒与纯H3PO4 (100%,预先在105 ℃下蒸馏3天,蒸馏完毕后保存在氩气中) 在恒温下反应,收集纯化反应产生的CO2气体供质谱仪测量同位素组成[6,7]

5 蚯蚓方解石颗粒指示古气候与古环境的应用
5.1 年代学上的应用

年代学研究是古气候环境重建的重要科学问题之一,建立可靠的年代学框架是古环境研究与记录对比的重要前提。虽然目前有关ECG测年的研究工作较少,但诸多因素使其在精确测年方面具有明显的优势[7]:这些颗粒主要产生于土壤层上部0.2~0.4 m的在温暖季节未冻结的土层中,其中的碳主要来自凋落物和土壤有机质 (蚯蚓进食),因此很少受土壤下层老碳和死碳的污染[26];方解石颗粒大量存在于冻原潜育土和极地与北方棕壤中,根据野外观察和薄片分析,这2种土壤中都缺少蚯蚓引起的生物扰动的证据。因此ECG已成为新的14C测年材料。

Pustovoytov等[8]初步将ECG用于黄土的定年当中。他们选取了位于德国西南部Schattenhausen的黄土—古土壤序列,对其中的钙化根系细胞、软体动物碳酸盐壳体和ECG同时进行了放射性14C测年,并对比了三者的测年结果 (图4)。其中软体动物壳体的放射性碳年龄为 (23 990±160) a BP,ECG的放射性碳年龄为 (24 510±190) a BP,二者都属于晚更新世,而钙化根系细胞得出的年龄偏年轻,可能受到成岩蚀变作用的影响,其结果落到了全新世范围内。


图4

Schattenhausen黄土—古土壤序列碳酸盐14C年龄[8]

Fig.4

14C age of carbonate from the loess-paleosol sequence at Schattenhausen[8]

Moine等[7] 对德国莱茵河谷Nussloch黄土—古土壤序列中的ECG放射性14C定年研究 (图5)显示,该黄土—古土壤序列所有土壤层的连续放射性碳年龄为47~20 ka,在36~13 ka cal. BP测年误差为0.6%~2.5% (100~800 a),44~38 ka cal. BP为1.9%~3.7% (700~1 500 a)。根据14C年龄计算得出中普伦尼冰期黄土堆积速率为0.19 mm/a,早期和晚期普伦尼冰期堆积速率分别为0.33和1.12 mm/a,这与基于释光年龄所估计的沉积速率较为一致, 并且基于方解石颗粒的14C年龄,黄土剖面也明显地记录了约23 ka和30 ka的2次主要的沉积和环境动力的改变事件。更重要的是,Nussloch黄土剖面的地层序列与格陵兰冰芯气候指标表现出很好的对应关系,在测年误差范围内,每个冻原潜育土和北极—北方棕壤都可对应到每个格陵兰间冰阶。因此,ECG至少为欧洲黄土建立很好的年代学框架。


图5

Nussloch P4-P8黄土—古土壤序列中ECG的放射性年龄及其与格陵兰间冰期(Greenland Interstadials, GI)的关系[7]
图中红色线条代表被弃用的年龄数据;灰色阴影部分表示放射性14C年龄误差在2σ范围内

Fig.5

Radiocarbon chronology of all soil horizons from the Nussloch P4-P8 composite loess profile based on earthworm granules and correlations with Greenland Interstadials[7]
Discarded ages are red-contoured; The gray shading indicates 2σ ranges of 14C ages

在年代学研究中,除了使用放射性14C测年,还可以用铀系测年法对ECG进行测年[4,7,9]。例如,Versteegh等[4]通过对英国威尔特郡Silbury Hill遗址中ECG进行U-Th不平衡定年,确定了该遗址形成年龄为(4 670±440 ) a BP,这与碳酸盐颗粒的铀系年龄较一致,表明它们与土壤的形成处于同一时期,确定了该遗址属于新石器时代。

ECG可以作为多种不同测年方法的测试材料,如放射性14C测年、氨基酸外消旋法及铀系法测年等,通过使用这些不同的方法来相互交叉验证,有助于第四纪沉积物可靠年代学框架的建立。尽管缺乏大量的有关蚯蚓方解石测年的数据,但放射性碳“蚯蚓时钟”仍然是年代学上的一大进步,拓展了我们对黄土记录的认识,从而能更好地理解末次盛冰期以来的气候变化过程及其对区域古环境的影响。

5.2 古气候环境重建

Versteegh等[4]将蚯蚓放置在不同温度环境下,然后对其粪便进行同位素检测,结果显示,粪便中的微小方解石颗粒能够“记忆”空气的温度。这种“记忆”有助于研究古气候变化规律。

如上所述,蚯蚓排出的方解石颗粒中的碳、氧同位素组成与其生存时期的植被碳同位素、大气降水同位素、温度等要素之间有良好的对应关系,可作为估算年、季降雨量和温度变化的指标[4,5,6,7]。Prud'Homme等[22]将取自德国莱茵河谷地区的Nussloch黄土—古土壤序列中的ECG作为一种新的生物气候指标,利用收集的国际原子能机构 (International Atomic Energy Agency, IAEA) 高纬度气象台站气候数据以及Versteegh等[4]与Zhang等[35,36]建立的经验公式,建立起ECG中δ18O值、大气和土壤温度与水循环之间的转换函数,重建的末次冰期时期空气和土壤的绝对温度为10~(12±4)℃ (图6)。由于蚯蚓存活的时期是由季节决定的,一般在春、秋季活动最频繁[37],所以该结果反映了一年中温暖时期的气温,而不是年平均温度。


图6

德国Nussloch P8剖面中ECG的δ18O值推算的土壤和空气温度[22]

Fig.6

Soil and air temperatures inferred from the δ18O values of earthworm calcite granules for profile P8 at Nussloch, Germany[22]

Kohn[38]从480个地点收集了C3植物的现代碳同位素组成与年平均降水量数据。虽然该研究目前仍存在争议[39,40],但是这种方法确实为根据ECG的δ13C值来估算古降水量提供了思路。Rey等[41]基于该数据[38]建立了植物叶片Δ13Cleaf和年平均降水量 (log (MAP+300)) 之间线性回归方程。最近,Prud'Homme[6]等对Rey等[41]公式的均方根误差进行计算,并在计算Δ13Cleaf时,使用了Vostok冰芯中大气CO2的δ13Catm[42,43],基于校正公式,根据Nussloch P8剖面中ECG的δ13C值估算出冻原潜育土层 (普伦尼冰期早期) 的古降水量为 26 9-163+226~46 0-209+289mm,其平均值为36 7-183+254mm;北方/极地棕色土层 (普伦尼冰期中期) 古降水量为30 5-166+230mm[6] (图7)。由于ECG中包含的碳同位素组成能够反映同时期的植被覆盖类型信息,故可将其作为黄土序列研究中重建第四纪年平均降水量的新的、有价值的代用指标。


图7

德国Nussloch P8剖面中ECG的δ13C值恢复的古降水量[6]

Fig.7

Paleo-precipitation reflected by δ13C values of earthworm calcite granules from the Nussloch profile P8, Germany[6]

对单个ECG进行实验分析,得出蚯蚓摄取碳与颗粒积累碳之间的碳同位素分馏值约为11.7‰[26]。该值包含由ACC与蚯蚓产生的方解石颗粒之间的碳同位素分馏值 (ε方解石-ACC = -1.2‰)[19]。Prud'Homme等[6]假定该同位素分馏值不随时间变化,由此获得德国Nussloch黄土剖面中ECG的δ13C值:冻原潜育土层在-15.4‰~-10.3‰变化;棕色土壤在-14.9‰~-9.5‰变化。基于蚯蚓摄取碳与颗粒积累碳之间的碳同位素分馏值,估算出冻原潜育土层中植被的平均δ13C值为 (-24.3±0.9)‰,棕色土壤层中植被的平均δ13C值为(-24.1±0.9)‰。计算出的这些不同层位的平均碳同位素分馏值(δ13Cpl)与前人的研究结果一致,表明该地区土壤有机质主要来源于C3植物,代表温和湿润气候[19,44~46]

6 ECG研究中存在的问题

ECG在黄土古环境研究中有独特的优势,它具有时空分布广泛,抵抗成岩蚀变作用,遭受较少的生物扰动作用等特点,能够提供较准确的古环境信息[7,9,22]。但是也存在以下问题:

(1)样品的可及性。在对颗粒进行14C定年时,同一层位中颗粒的数量需足够多,才能确保测年结果的相对准确[4]。但由于蚯蚓对气候环境变化极为敏感,其生长和繁殖与生活环境关系密切,因此ECG存在差异性保存的问题,其主要存在于古土壤 (冻原潜育土或棕色土) 中,在黄土中含量极少甚至缺失[5,7]。在干旱半干旱区的黄土高原和中亚地区中的黄土中是否存在ECG仍需要做大量的调研工作。

(2)古气候恢复的不确定性。在利用颗粒中的δ18O重建不同地区古温度时,要确保有能够用于重建古温度的土壤水分δ18O值。由于湿润气团的起源和轨迹不同,Tair18Omw关系会随时间在不同地区发生变化[22],因此必须根据全球海洋不同δ18O值对现有的温度与δ18O值之间的关系进行校正。在使用ECG碳同位素恢复古降水量时,要先确定颗粒中碳的来源[22]。虽然ECG中的碳主要来自混合的凋落物和土壤有机质 (蚯蚓进食),但大气CO2可能对颗粒中碳组成也有一定贡献,尤其对栖息于土壤表面的蚯蚓物种[33],同时蚯蚓进食时土壤本身的碳来源(原生或次生)也有影响,所以在不确定ECG形成时碳不同来源贡献时,使用方解石颗粒碳同位素恢复的古降水量具有不确定性。

7 结论与展望

ECG包含丰富的环境信息,在古气候环境的重建中有很大的优势,具有进一步推广发展成新的古气候指标的潜力。但由于蚯蚓的丰度受以下因素影响:① 气候,包括气温、降水量;② 植被组成及密度;③ 土壤参数,如湿度、温度、pH、土壤通气性、氮气浓度和氧含量等[3,47~49],ECG中汇集了多种环境信息,因此,需要使用多种方法交叉验证,比如,ECG可用于放射性14C测年、氨基酸外消旋法及铀系测年,通过对比可以建立可靠的年代学框架;通过比较ECG丰度与其他陆生软体动物、沉积矿物学参数 (粒度指数、CaCO3、总有机碳 (Total Organic Carbon, TOC)、磁化率等) 的变化,可以反映不同层位的沉积学特征[5,50,51]。到目前为止,ECG作为一种新的古环境指标,仅用于欧洲的个别黄土—古土壤序列,在其他地区未见相关报道。中国是世界黄土的主要分布区,其黄土—古土壤序列记录的古气候变化能与冰芯和深海氧同位素记录媲美,并称为古气候研究的三大载体之一。尤其是黄土高原黄土古气候已成为过去全球变化研究的典型案例,因此,研究特定层位或特征时段高分辨率的ECG化石既可验证ECG的年代可靠性与确认其古气候意义,同时也将促进中国黄土古气候学与过去全球变化的研究。

在中国,蚯蚓的种类也很丰富,而且作为ECG的主要生产者,正正蚓(Lumbricus terrestris)和红正蚓(Lumbricus rubellus)在我国东北及西北地区都有分布[2,52,53],因此运用ECG来提取生物环境信息在我国亦具有巨大的发展潜力。但目前我国对现代不同属种蚯蚓的生态学研究较少,蚯蚓与气候环境关系的研究也相对薄弱。因此将来可以开展不同区域的ECG化石记录的研究,探索不同时间及空间尺度上ECG的古气候、古环境意义。随着ECG在古气候和古环境研究中的应用和发展,其所具有的优势会逐渐凸显,有望推动第四纪科学、古气候学、古生态学、过去全球变化等研究的发展。

今后的研究中,应重视以下2个问题:

(1) 加强现代不同属种的蚯蚓生态学研究,厘清ECG碳同位素与蚯蚓生长时期植被碳同位素组成的定量对应关系,以及不同区域背景下,气候环境中控制ECG碳、氧同位素组成的因素,这样才能了解ECG中 δ18O、δ13C与温度、降水量和植被等指标之间的定量关系,更好地建立相应的定量指标体系。

(2) 结合优化的数学模型和统计方法,充分挖掘ECG碳、氧同位素数据在古气候、古环境重建中的应用价值。

The authors have declared that no competing interests exist.

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Dating phases of pedogenesis, soil carbonate deposition or even the burial of whole soil profiles using 14 C is a valuable goal in archaeology and pedology, but one that has been consistently hampered by the presence of old carbon skewing the measurements to produce apparent dates older than the true formation date. Calcite produced by earthworms could be a useful alternative source of datable carbon. Since earthworms both inhabit and ingest soils with an old carbon content, however, the granules could yield a 14 C date older than the date of their formation. In this study, by examining granules from two sites of known-age stratigraphy, we show that the radiocarbon date derived from the granules' calcite closely reflects their true formation date, opening up the possibility of using the granules either individually or as distributions of dates to understand soil processes and date sealed archaeological layers.
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A suite of divalent metal (Ca, Cd, Ba) carbonates was synthesized over the temperature range 10-40°C by the classical method of slowly bubbling N through a bicarbonate solution. It was discovered that carbonates could be precipitated reproducibly in or out of isotopic equilibrium with the environmental solution by varying the concentrations of bicarbonate and cation. Precipitation rate had little or no influence on the isotopic composition of the product. Relatively high initial concentrations of up to 25 mM in both bicarbonate and cation were prepared by adding solid metal chlorides to solutions of NaHC0 . On the basis of results of equilibrium experiments and a new determination of the acid fractionation factor, a new expression is proposed for the oxygen isotope fractionation between calcite and water at low temperatures: 10001nα(Calcite-H O) = 18.03(10 T ) - 32.42 where α is the fractionation factor, and T is in kelvins. Combining new data for low-temperature precipitations and the high-temperature equilibrium fractionations published by O'Neil et al. (1969) results in a revised expression for the oxygen isotope fractionation between octavite (CdCO ) and water from 0° to 500°C: 10001nα(CdC0P sbnd H O) = 2.7 (10 T ) - 3.96 The ability to produce nonequilibrium carbonates allowed assessment to be made, for the first time, of the temperature dependence of nonequilibrium stable isotope fractionations in mineral systems. The temperature coefficients of a(carbonate-water) for nonequilibrium divalent metal carbonates are greater than those for equilibrium carbonates, a finding that may bear on the interpretation of analyses of biogenic carbonates forming out of isotopic equilibrium in nature. New determinations of acid fractionation factors (10001nα) at 25°C for calcite (10.44 - 0.10), aragonite (11.01 ± 0.01), and witherite (10.57 - 0.16) are mildly to strongly different from those published by Sharma and Clayton (1965) and point to a control on this fractionation by some physical property of the mineral. Reproducible values for octavite (CdC0 ) varied from 11.18 to 13.60 depending on the conditions of preparation of the carbonate. These new values need to be considered in determinations of absolute 80 60 ratios of international reference standards and in relating analyses of carbonates to those of waters, silicates, and oxides.
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The relationship between temperature and O
(18) content relative to that for a Cretaceous belemnite of the Pee Dee formation previously reported (Epstein, Buchsbaum, Lowenstam, and Urey, 1951) has been re-determined using modified procedures for removing organic matter from shells, and is found to be t(°C) = 16.5 - 4.3δ + 0.14δ
2 where δ is the difference in per mil of the O
(18) to O
(16) ratio between the sample and reference gas. The new relationship agrees with that determined by McCrea (1950) for inorganically precipitated calcium carbonate. Carbonate-carbon dioxide exchange experiments were done to determine the direct and indirect effects of organic matter in the shell on the mass spectrometer analyses.
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DOI:10.1029/2004RG000157 URL
[1] The presence of seasonal snow cover during the cold season of the annual air temperature cycle has significant influence on the ground thermal regime in cold regions. Snow has high albedo and emissivity that cool the snow surface, high absorptivity that tends to warm the snow surface, low thermal conductivity so that a snow layer acts as an insulator, and high latent heat due to snowmelt that is a heat sink. The overall impact of snow cover on the ground thermal regime depends on the timing, duration, accumulation, and melting processes of seasonal snow cover; density, structure, and thickness of seasonal snow cover; and interactions of snow cover with micrometeorological conditions, local microrelief, vegetation, and the geographical locations. Over different timescales either the cooling or warming impact of seasonal snow cover may dominate. In the continuous permafrost regions, impact of seasonal snow cover can result in an increase of the mean annual ground and permafrost surface temperature by several degrees, whereas in discontinuous and sporadic permafrost regions the absence of seasonal snow cover may be a key factor for permafrost development. In seasonally frozen ground regions, snow cover can substantially reduce the seasonal freezing depth. However, the influence of seasonal snow cover on seasonally frozen ground has received relatively little attention, and further study is needed. Ground surface temperatures, reconstructed from deep borehole temperature gradients, have increased by up to 4C in the past centuries and have been widely used as evidence of paleoclimate change. However, changes in air temperature alone cannot account for the changes in ground temperatures. Changes in seasonal snow conditions might have significantly contributed to the ground surface temperature increase. The influence of seasonal snow cover on soil temperature, soil freezing and thawing processes, and permafrost has considerable impact on carbon exchange between the atmosphere and the ground and on the hydrological cycle in cold regions/cold seasons.
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DOI:10.1126/science.1217161 PMID:22461496 URL
The stable carbon isotope ratio of atmospheric CO2 (δ13Catm) is a key parameter in deciphering past carbon cycle changes. Here we present δ13Catm data for the past 24,000 years derived from three independent records from two Antarctic ice cores. We conclude that a pronounced 0.3 per mil decrease in δ13Catm during the early deglaciation can be best explained by upwelling of old, carbon-enriched waters in the Southern Ocean. Later in the deglaciation, regrowth of the terrestrial biosphere, changes in sea surface temperature, and ocean circulation governed the δ13Catm evolution. During the Last Glacial Maximum, δ13Catm and atmospheric CO2 concentration were essentially constant, which suggests that the carbon cycle was in dynamic equilibrium and that the net transfer of carbon to the deep ocean had occurred before then.
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[44]
Hatté Christine, Antoine Pierre, Fontugne Michel, et al.δ13C of loess organic matter as a potential proxy for paleoprecipitation[J]. Quaternary Research, 2001, 55(1):33-38.
DOI:10.1006/qres.2000.2191 URL
Paleoprecipitation reconstructions on the basis of pollen are well known, but they do not provide high temporal resolution for glacial periods. High-resolution paleoprecipitation reconstructions for the last glaciation based on the isotopic record organic matter in loess from Nussloch (Rhine Valley, Germany) are consistent with paleoprecipitation inferred from peat in the same area using an independant method. Thus, 13 C of loess organic matter can be used as a proxy for paleoprecipitation.
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[45]
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Antoine Pierre, Rousseau Denis-Didier, Moine Olivier, et al.Rapid and cyclic aeolian deposition during the last glacial in European loess: A high-resolution record from Nussloch, Germany[J]. Quaternary Science Reviews, 2009, 28(25):2 955-2 973.
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Moine Olivier, Rousseau Denis-Didier, Jolly Dominique, et al.Paleoclimatic reconstruction using mutual climatic range on terrestrial mollusks[J]. Quaternary Research, 2002, 57(1):162-172.
DOI:10.1006/qres.2001.2286 URL
Terrestrial mollusks, easily identified in Quaternary sediments, represent a reliable tool for quantitative estimates of environmental parameters. Our study, comparing the species distribution with meteorological parameters in Europe, shows that mean temperature of the coldest month and annual thermal magnitude are the most important forcing parameters. This survey allows us to adapt the mutual climatic range (MCR) method to terrestrial mollusk assemblages following two main steps. A set of assemblages from different European regions (northern Norway to southern France) is used to apply the method to present-day mollusks. The reconstructed values describe the latitudinal temperature gradient prevailing over Europe. However, the comparison between the reconstructed and the measured values indicates a shift, similar to that observed, with the same method applied to beetle assemblages. Thus, estimates must be calculated after the reconstruction is tuned with the observations. The results from the modern mollusk assemblages indicate that the MCR method can be safely applied to reconstructing temperatures from terrestrial mollusk assemblages in any worldwide Quaternary sequence. A trial application is made on Late Pleistocene assemblages from Achenheim (Alsace, France).
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Xu Qin.Geographical distribution of terrestrial earthworms in China[J]. Journal of Beijing Institute of Education (Social Science Edition), 1996,(3):54-61.
URL
前言中国陆栖蚯蚓地理分布状况的研究,起步于1930年。当时,陈义先生立志调查中国蚯蚓的资源与分布。26年之后,陈义在《中国蚯蚓》(科学出版社,1956)一书中首次论述了中国蚯蚓的地理分布问题。此后,一些学者先后对中国部分地区作了一些调查,但终未形成较全面的资料。19
[徐芹. 中国陆栖蚯蚓地理分布概述[J]. 北京教育学院学报: 社会科学版, 1996, (3):54-61.]
URL
前言中国陆栖蚯蚓地理分布状况的研究,起步于1930年。当时,陈义先生立志调查中国蚯蚓的资源与分布。26年之后,陈义在《中国蚯蚓》(科学出版社,1956)一书中首次论述了中国蚯蚓的地理分布问题。此后,一些学者先后对中国部分地区作了一些调查,但终未形成较全面的资料。19
[本文引用: 1]
[53]
Xu Qin.A study on the classification of terrestrial earthworms in China[J]. Journal of Beijing Institute of Education (Social Science Edition), 1999, (3):52-57.
URL
中国陆栖蚯蚓分类研究自1872年开始。至1929年前,外国学者记录了产于中国的陆栖蚯蚓4科6属28种2亚种。1929年,方炳文先生描述了产于广西凌云县九丈的异腺环毛蚓PheretimaaParglandlaris,揭开了国人研究蚯蚓分类的历史。1930年,著名蚯蚓分类学家陈义先生立志调查中国蚯蚓的资源,开始了大规模的蚯蚓分类研究工作。陈义先后描述了100多个新种(含新亚种),占我国已记录种类的40%以上。由于分类依据的不断更改与变动,以及计算技术在蚯蚓分类学上的应用,蚯蚓分类系统大规模的修订工作还在继续。
[徐芹. 中国陆栖蚯蚓分类研究史探讨[J]. 北京教育学院学报: 社会科学版, 1999,(3): 52-57.]
URL
中国陆栖蚯蚓分类研究自1872年开始。至1929年前,外国学者记录了产于中国的陆栖蚯蚓4科6属28种2亚种。1929年,方炳文先生描述了产于广西凌云县九丈的异腺环毛蚓PheretimaaParglandlaris,揭开了国人研究蚯蚓分类的历史。1930年,著名蚯蚓分类学家陈义先生立志调查中国蚯蚓的资源,开始了大规模的蚯蚓分类研究工作。陈义先后描述了100多个新种(含新亚种),占我国已记录种类的40%以上。由于分类依据的不断更改与变动,以及计算技术在蚯蚓分类学上的应用,蚯蚓分类系统大规模的修订工作还在继续。
[本文引用: 1]
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