第四纪研究  2020, Vol.40 Issue (1): 157-164   PDF    
末次间冰期以来兴凯湖黑碳的沉积记录及其环境意义
孙伟伟, 沈吉     
( 中国科学院南京地理与湖泊研究所, 湖泊与环境国家重点实验室, 江苏 南京 210008)
摘要:在我国东北地区,黑碳记录较少,尤其是长时间尺度记录,区域火灾如何响应于气候变化还存在争议。本研究通过对兴凯湖XK08-A2钻孔约336 cm岩芯,共136个沉积物样品的黑碳含量进行了分析,并结合石英光释光和加速器质谱14C测年结果,探讨了末次间冰期以来东北地区火灾历史与古环境的联系。兴凯湖沉积物黑碳含量的平均值在末次间冰期、末次冰期以及全新世分别为0.51±0.10 ‰、0.68±0.15 ‰和0.71±0.20 ‰。兴凯湖沉积物中黑碳含量从下向上呈增加趋势,表明末次间冰期区域火灾的发生频次较低、规模较小,末次冰期火灾的频次升高、规模越来越大,全新世阶段火灾的频次和规模与末次冰期相当。这些变化与轨道尺度东亚夏季风进退以及人类活动密切相关。从末次间冰期到末次冰期,东亚夏季风变化是东北地区火灾发生的主要控制因素。强盛的东亚夏季风带来充沛的降水,促进了低洼地区湿地沼泽的发育,缩短了火风险期,致使发生大规模火灾的概率较低。自然背景下季风降水主导我国东北地区火灾的发生,与黄土高原和西南地区一致,但与欧美地区受温度影响存在显著的空间差异。全新世在人类活动的影响下,虽然东亚夏季风增强,但是区域大规模火灾发生的概率已达到末次冰期的水平。
关键词黑碳    火灾    东亚夏季风    气候变化    人类活动    湖泊沉积    
中图分类号     P942;P532                     文献标识码    A

0 引言

黑碳或称火成碳,是生物质和化石燃料不完全燃烧的产物,包括木炭、炭屑、石墨碳以及烟炱等一系列不同碳化程度的含碳物质[1~3]。黑碳化学性质稳定,具有很强的抗氧化能力和耐分解能力,能够长期保存在土壤、海洋和湖泊等沉积物中,不仅在全球的碳循环过程中具有重要的作用,而且是古火灾信息的良好记录[1~4]。黑碳具有很强的吸光特性,能够降低大气能见度、改变地球的辐射平衡以及云层凝结核的吸湿能力,加剧全球的气候变暖和增加区域极端降水事件发生的频率[5~6]。由于黑碳在气候调节与古环境重建中扮演着重要的角色,黑碳的生物地球化学过程已成为全球变化研究的热点问题之一[7~9]

由于黑碳是一系列不同碳化程度的含碳物质共同体,各种组分之间难以明确区分,对黑碳的定量分析还没有一种标准的测量方法[8, 10~12]。目前黑碳的测量方法一般是依据黑碳的部分特征而进行的,只测量了一部分黑碳的含量。例如显微镜记数法以黑碳的光学特征为基础,常用于木炭和炭屑的测量[13~15];热光反射法依据黑碳的热稳定性高于有机碳,分步氧化有机碳和黑碳,进行黑碳的定量[16];而化学氧化法以黑碳比其他有机组分有更强的化学稳定性为前提,使用氧化剂提取沉积物中的黑碳[17~18]。其中,化学氧化法提取的黑碳可以进行稳定碳同位素(δ 13CBC)分析,进一步提供黑碳来源的信息[19~27]

我国东北地区位于北半球的中高纬度,是东亚夏季风与高纬极地气候系统相互作用的交汇部位,对全球变化响应敏感,是研究第四纪环境演变过程的理想区域[28~29]。然而东北地区黑碳的研究还比较少,主要集中在晚全新世,使得人们对区域火灾与气候变化关系的认识存在很大的分歧。月亮湖炭屑研究表明,冰消期阶段随着气候变暖与东亚夏季风增强,当地火灾发生的频次增加,规模变大[30];而金川和勤得利泥炭的炭屑研究则表明,火灾的发生与东亚夏季风减弱,气候变干有关[13, 31]。在中世纪暖期,东北地区火灾的发生频率与强度要高于小冰期,可能是人类毁林垦荒与气候变化共同作用的结果[32~34]。因此,认识自然条件下东北地区火灾的发生过程,需要更多长时间尺度的记录,以减少人类活动产生的不确定性。本研究选择兴凯湖为研究对象,通过末次间冰期以来沉积物中黑碳含量的研究,探讨该区域冰期-间冰期尺度火灾对气候变化和人类活动的响应过程。

图 1 兴凯湖及岩芯采样位置 Fig. 1 Map showing location of Lake Xingkai (a) and sampling site in the lake (b)
1 研究区概况

兴凯湖位于我国黑龙江省的东部边缘,是我国与俄罗斯的边界湖,地理范围大概在44°32′~45°21′N,131°58′~132°51′E,面积约4380 km2,是东北亚地区最大的淡水湖(图 1a)。兴凯湖的最大水深约为10 m,平均4.5 m,湖面的平均海拔高度约为69 m[35]。在大地构造上,兴凯湖位于兴凯-佳木斯地块,是敦化-密山断陷区自新生代持续下沉而形成的构造湖,其西、东南、东和北侧为山地丘陵包围。兴凯湖周围河网密集,由于地势由西南向东北倾斜,湖水在东北部由唯一出口松阿察河汇入乌苏里江。

兴凯湖流域的气候呈现出显著的温带季风气候特征。随着西太平洋副热带高压向北移动,夏季降水显著增加;而冬、春季受蒙古-西伯利亚高压的影响,气候寒冷干燥。附近的鸡西气象观测站的记录表明,研究区年平均温度约3 ℃,最冷月1月平均气温约-18 ℃,最热月7月平均气温约21 ℃;年降水量约540 mm,其中70 %集中在夏季[36]。在气候和地形的影响下,平原地区的自然植被主要为湿地沼生植物,但大部分湿地已经被改造成农田;山地植被主要为以红松为主的针阔混交林。

2 材料与方法

2008年8月,利用奥地利制造的UWITEC水上采样平台和活塞取芯设备进行了兴凯湖沉积岩芯的钻取工作。在水深7.0 m处采集了XK08-A1和XK08-A2两根柱状沉积岩芯,分别长308 cm和336 cm,采样位置为45°12′21.7″N,132°30′33.3″E(图 1b)。样品运回实验室后,在4 ℃的岩芯库低温保存。对采集的岩芯进行体积磁化率扫描后,沿着中轴线将其切开。XK08-A1岩芯在暗室里以5~10 cm间距分割,获得49个石英光释光(OSL)样品,以此来进行兴凯湖沉积物的年代学研究[37]。XK08-A2岩芯主要是青灰色的粘土质粉砂,在251~255 cm的沉积物中含有显著的粗砂颗粒。在64 cm深度挑出一个木质植物残体,用于加速器质谱(AMS) 14C测年,其结果为26.5 cal. ka B. P.,与XK08-A1同深度OSL样品测年结果相近[36]。通过与岩芯XK08-A1扫描磁化率结果进行地层比较,发现两根岩芯的磁化率变化具有很好的一致性。在气候相对温暖的全新世磁化率较低,而在相对寒冷的末次冰期磁化率值却偏高。这种模式与来自贝加尔湖长时间尺度的磁化率变化相同[38],可能是生物硅在温暖期更加丰富,稀释了磁性矿物的含量。通过比对,可以确定XK08-A2岩芯轨道尺度的年代框架:336~216 cm、216~27 cm和27~0 cm分别对应于深海氧同位素阶段(MIS)5、MIS 4~2和MIS 1(图 2)[36]。兴凯湖沉积岩芯的沉积速率在冰期要比间冰期高,可能是由于在千年-百年尺度东亚夏季风减弱期,湖泊水位快速下降后湖流对沉积物产生的冲蚀造成的[36~37]。在极端干旱事件结束之后,湖泊又开始形成新的沉积物。

图 2 兴凯湖岩芯XK08-A2与XK08-A1的地层对比与年代[36~37] Fig. 2 Stratigraphy correlation of cores XK08-A2 (a) and XK08-A1 (b)dated using the optically stimulated luminescence method (c)[36~37]

以1 cm间距对XK08-A2沉积岩芯进行了分割,共获得336个样品。随后利用真空冷冻干燥机对XK08-A2的沉积物样品进行了冷冻干燥处理,密封保存,直至对样品进行实验分析。本研究在0~70 cm按照1 cm的间距,70~336 cm以4 cm的间距,选择了136个沉积物样品进行了黑碳含量的分析。黑碳的提取采用K2Cr2O7湿化学氧化法[17]。精确称取1.5 g左右的干样放入聚乙烯离心管,加入15 ml 3 mol/L的稀盐酸,充分振荡,在室温下反应24 h,以去除沉积物中可能存在的碳酸盐矿物;离心清洗后,再加入15 ml的浓度比为10 ︰ 1的氢氟酸与盐酸的混合液,以去除硅酸盐晶格中的碳酸盐矿物;再次离心清洗,加入10 ml的10 mol/L的盐酸以去除上步反应可能产生的CaF2沉淀;酸处理后的样品转移到玻璃试管中,加入0.2 mol/L的K2Cr2O7的硫酸溶液(硫酸浓度为2 mol/L),在55 ℃水浴锅中反应60 h,根据反应溶液的颜色,调节K2Cr2O7的硫酸溶液的使用量,以完全去除易降解的有机质和干酪根。处理后样品中所含的有机质即认为是难降解的黑碳,将剩余样品烘干后研磨均匀。称取20 mg左右的样品置于锡杯中,用于黑碳含量分析,测试仪器为中国科学院南京地理与湖泊研究所湖泊与环境国家重点实验室的Flash EA 1112型元素分析仪。仪器测试的参考标准样品为已知有机碳含量的湖泊沉积物标样,相对误差在± 5 %以内。黑碳含量转换为沉积物干样的重量千分数。

3 结果与讨论

兴凯湖XK08-A2岩芯沉积物的黑碳含量在0.35 ‰ ~1.51 ‰之间,平均0.65±0.17 ‰。在时间序列上,黑碳含量从下向上总体呈增加趋势(图 3)。地质时期黑碳主要来源于生物质燃烧,一次火灾黑碳的产量与可燃物的丰度及燃烧强度正相关[39]。燃烧过程中,在地形、风速以及燃烧产生的对流影响下,大于1000 μm的黑碳颗粒基本上原位沉积在燃烧点;大于125 μm的颗粒的运移距离不超过7 km;而细颗粒可以被搬运到上千公里以外的地方[40~41]。然而,仅有少量的黑碳能够随着气流升空进入高层大气,大部分滞留在原地或沉降在火烧地周围,之后随流水作用进入到湖泊或海洋沉积物,或者被其他陆相沉积所埋藏[42]。因此,沉积物中黑碳含量也可以反映区域的火灾事件发生的频次和规模[23, 43~44]

图 3 兴凯湖黑碳含量(a)与沉积物粒度重建的东亚夏季风指数[36] (b)以及黑碳δ 13CBC记录[27] (c)对比 Fig. 3 Comparison of the black carbon content (a) with other climate records: (b)the East Asian Summer Monsoon Index derived from Lake Xingkai sediment grain-size end members[36]; (c)δ 13CBC record from Lake Xingkai[27]

由于千年-百年尺度气候事件对兴凯湖沉积物存在一定的扰动,以及OSL年代在MIS 5~3期间存在较大的相对误差,限制了我们对东北地区千年-百年尺度火历史演化的认识。因此本文仅讨论轨道时间尺度区域火历史的变化过程及其与环境变化的关系。兴凯湖沉积物的黑碳含量表明该区域火灾频次与规模呈上升趋势。MIS 5沉积物中较低的黑碳含量(平均0.51±0.10 ‰)指示了该阶段火灾的发生频次较低,规模较小;沉积物中黑碳含量在MIS 4~2逐渐升高(平均0.68±0.15 ‰),表明区域火灾的频次逐渐增加,规模越来越大;在误差范围内,全新世阶段黑碳含量(平均0.71±0.20 ‰)与MIS 4~2沉积物黑碳含量接近,表明该阶段火灾的频次和规模与末次冰期相当(图 3)。在百年以上的长时间尺度上,自然火灾的发生与蔓延受到气候、植被和地形等因素的影响[45]。另外,人类活动可以通过森林砍伐、杂草清理和引燃等管理方式改变自然的火格局[46]

在1980~2010年间,黑龙江省75 %以上的火灾事件发生在3月至6月,其他月份火灾发生的频次显著降低[47]。春季随着气温的升高,降水缺乏而使东北地区有效湿度较低,植物凋落物基本干燥,遇到着火点容易迅速蔓延,形成大规模的火灾。而到6月之后受东亚夏季风的影响,有效湿度升高,地表可燃物减少,而且植物处于生长季节,火灾发生的频率显著减少。因此,东北地区火灾的发生与东亚夏季风季节性的进退存在紧密的联系。

兴凯湖沉积物的粒度在MIS 5比较粗,主要反映了该时期东亚夏季风增强,降水增多,对地表的侵蚀能力增强(图 3)[36]。这与来自黄土高原的黄土-古土壤序列、干旱-半干旱区古湖岸线的地质记录表明东亚夏季风在MIS 5总体比MIS 4~2强相一致[48~49]。兴凯湖δ 13CBC记录表明区域C4植物在MIS 5显著扩张,对应着我国北方地区温度的升高(图 3)[27]。例如,黄土高原渭南剖面的植硅体组合重建的温度表明该时期的平均温度要比目前高2 ℃[50]。强盛的东亚夏季风带来充沛的降水,一方面促进了陆生植被的生长,增加了区域的生物量,另一方面也促进了低洼地区湿地沼泽的发育。另外,东亚夏季风在季风强盛期可能提前影响东北地区,缩短火灾的风险期,减少火灾事件的发生频次。因此,虽然温度升高,生物量增加,但是由于降水的增加以及季风提前,区域大范围洼地被水体淹没而形成湿地沼泽,火灾的发生局限于高地,导致兴凯湖地区MIS 5发生的火灾频次较低,规模较小。而在MIS 4~2,兴凯湖地区火灾事件逐渐增加,规模增大,表明温度降低,气候变干,生物量减少的情况下却更加有利于火的发生与蔓延。主要是由于降水的减少增加了干燥度,加快了凋落物的干燥速度;区域内大面积的湿地干涸转变成陆地,在风力作用下,一旦发生火灾,将迅速蔓延扩散。

兴凯湖沉积物的黑碳含量研究表明,在人类活动可以忽略的自然背景下,东北地区轨道时间尺度上火灾的发生主要受东亚夏季风引起的干湿变化影响。这与亚洲季风区内已有的火历史研究记录基本一致。李家塬、灵台和渭南这3个黄土剖面最近两个冰期-间冰期循回的黑碳沉积通量研究显示,在干旱的冰期,火灾的频次、规模显著高于湿润的间冰期[51]。其中,李家塬剖面的黑碳记录还揭示了晚冰期以来黄土高原火的发生频次与规模在千年尺度冷干事件时期快速升高[52]。在人类活动影响较晚的云贵地区,腾冲青海的黑碳研究显示区域火灾在晚冰消期具有显著的千年尺度波动,在湿润期显著减少,而在干旱期明显增加[21]。而在西欧和北美由于冰期气候冷湿,这些地区苔原广布,缺少可燃的生物质,火灾主要发生在温暖的间冰期[44, 53]。这些地质记录表明我国季风区火灾对气候变化的响应和欧美地区具有显著的区域差异,在轨道时间尺度上,尽管冰期夏季风出现衰退,但温度仍然适合一些植物的生长,地表生物量提供的凋落物仍能够维持火灾的蔓延。

进入全新世以后,东亚夏季风再次增强,兴凯湖地区降水增加,季节性变化显著,湿地扩张[36, 48]。这些气候特征与MIS 5具有很好的相似性,但是兴凯湖地区火灾事件并没有减少,表明除了气候因素外,人类活动已经影响该区域火的格局。三江平原地区分布着众多的新石器时期的人类活动遗址,在新开流遗址的墓葬中发现了陶器碎片,在小南山遗址发现了火炕的遗存,这些遗址与松嫩平原、黑龙江流域中下游以及朝鲜境内的新石器遗址属于同一期[54]。文化层中炭屑的AMS 14C测年结果表明早在全新世之前人类已经掌握了用火,以此来进行烹饪、取暖和烧制陶器[55]

在晚全新世,区域内以渔猎和采集为主的生存方式逐渐被农业耕作所取代[31, 34, 56]。在黑龙江双鸭山遗址,发现了用于农业耕作的铁器遗存,同时也发现了炭化的大麻种子,其[14]年代约为2.2 cal. ka B. P.,表明秦汉时期人类已经在黑龙江地区进行农业耕作[34]。金川泥炭的花粉记录表明在约1.3 cal. ka B. P.,松的花粉明显减少,取而代之的农作物花粉显著增加[31]。镜泊湖沉积物的粒度、磁化率和花粉研究同样表明牡丹江地区农业活动加强,松、落叶栎和榆等木本植物花粉显著减少,禾本科、藜科和蒿属等草本花粉明显增加,并出现了比较严重的水土流失现象[56]。古人类通常采用刀耕火种的农业方式,在雨季砍伐森林和清理草原,到旱季焚烧形成农田[31]。人类活动导致火灾事件显著增加也被记录在兴凯湖地区的密山杨木和金川泥炭剖面中[31, 34]

4 结论

本研究选取我国东北兴凯湖长336 cm的XK08-A2岩芯为研究对象,通过AMS 14C测年以及与高分辨率OSL测年的相邻钻孔的磁化率地层比对,建立了XK08-A2岩芯的年代框架。采用K2Cr2O7湿化学氧化法对136个湖泊沉积物样品进行了黑碳含量分析,并探讨了该区域冰期-间冰期尺度火灾对气候变化和人类活动的响应过程。结果表明,兴凯湖沉积物中黑碳含量在0.35 ‰ ~1.51 ‰之间,平均0.65±0.17 ‰,从下向上呈增加趋势。其中,MIS 5沉积物黑碳平均含量为0.51±0.10 ‰,相对较低,指示区域火灾的发生频次较低、规模较小,MIS 4~2沉积物黑碳平均含量为0.68±0.15 ‰,表明火灾的频次升高、规模越来越大,全新世阶段火灾的频次和规模相对于末次冰期没有显著的变化,黑碳平均含量为0.71±0.20 ‰。通过与兴凯湖沉积物粒度和δ 13CBC记录对比发现,从末次间冰期到末次冰期,兴凯湖地区火灾的发生主要受控于东亚夏季风强度的变化,温度对可燃生物质的影响较小。在轨道时间尺度上,我国东北地区火灾的发生主要影响因素与黄土高原和西南地区相似,但与欧美地区受温度影响存在显著的空间差异。全新世虽然东亚夏季风增强,但是在人类活动的影响下,区域火灾的频次和规模相比末次冰期相近。

致谢: 感谢审稿专家和编辑部杨美芳老师宝贵的修改意见,以及汪勇博士在野外采样过程中提供的帮助。

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A black carbon record from Lake Xingkai in Northeast China and implication for environmental reconstruction since the last interglacial period
Sun Weiwei, Shen Ji     
( State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, Jiangsu)

Abstract

Black carbon (BC) is pervasive in the environment but playing an important role in the global carbon cycling and paleoenvironmental reconstructions. There is few long-term BC record from Northeast China and the relationship between fire regime and climate is not well understood from this region, which hampers prediction of the response of fire to future climate change. In this study, we present a BC record from Lake Xingkai since the last interglacial period, in order to determine the main factor influencing regional fire regime.Two parallel and overlapping sediment cores XK08-A1 (308 cm in depth) and XK08-A2 (336 cm in depth) were collected near the China-Russia border (45°12'21.7"N, 132°30'33.3"E) using a UWITEC piston corer in the summer of 2008. In the laboratory, core correlation between XK08-A1 and XK08-A2 was carried out using surface scanning magnetic susceptibility. Core XK08-A1 was used for optically stimulated luminescence (OSL) dating, and a total of 49 OSL samples were obtained. The chronology of XK08-A2 is established mainly based on the correlation of the magnetic susceptibility with Core XK08-A1, where the depth of 336~216 cm is corresponded to the Marine Isotope Stage (MIS) 5 216~27 cm to the MIS 4~2, and 27~0 cm to the MIS 1.Samples at 1-cm intervals above 70 cm depth and 4-cm intervals below were used for the analysis of BC content, respectively, yielding a total of 136 samples. The BC in the freeze-dried bulk samples were extracted using the dichromate (K2Cr2O7) oxidation method. BC measurements were performed using a Flash EA 1112 elemental analyzer. The BC content is relatively low during the last interglacial period with a mean of 0.51±0.10 ‰; and relatively higher during the last glacial period with a mean of 0.68±0.15 ‰; it is slightly higher during the Holocene with a mean of 0.71±0.20 ‰. The results indicate that the scale of regional fires in the last interglacial period was relatively small, compared to the scale of fires in the last glacial period. Meanwhile, the scale of fires in the Holocene could exceed that in the last glacial period.These changes are closely related to the intensity of the East Asian summer monsoon and human activities. From the last interglacial period to the last glacial period, the intensity of East Asian summer monsoon is the main controlling factor of fire occurrence in Northeast China. The strengthened East Asian summer monsoon brought abundant rainfall, promoting the development of wetlands and marshes in low-lying areas, and resulting in a low probability of large-scale fires. The orbital time-scale fire regime in Northeast China is similar to that of the Chinese Loess Plateau and Southwest China under the natural background. However, there are significant spatial differences between monsoonal China and the continents of Europe and North America, where fire regime is more sensitive to temperature changes. The fire regime in Northeast China may have been influenced by human activity in the Early Holocene, and possibly related to the intensification of cereal cultivation in the Late Holocene.
Key words: black carbon    fire    East Asian summer monsoon    climate change    human activity    lacustrine sediment