Sub sea surface temperatures in the Nordic Seas during the LGM by planktic foraminiferal Mg/Ca temperature reconstructions
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摘要: 对我国第五次北极科考在北欧海所采集的两根岩芯样品进行了冰筏碎屑(Ice-Rafted Debris, IRD)丰度、AMS 14C测年、有孔虫丰度统计、浮游有孔虫Neogloboquadrina pachyderma (sin.)(Nps)稳定氧碳同位素及其Mg/Ca重建的次表层海水古温度等指标分析,建立了20ka以来的年代框架。结果表明,在末次盛冰期(20.0~17.5kaBP),次表层温度整体较低(~3℃),钙质生产力下降,冰筏碎屑输入增加;在冰消期(17.5~11.7kaBP)的Heinrich Stadial 1(HS1)事件中较轻的δ18O和δ13C指示大量淡水输入,水体分层加剧,向北输送的北大西洋水聚集在次表层,导致次表层水温逐渐升高。从Bølling-Allerød (B/A)事件开始,次表层水温度达到4.5℃,表明北大西洋水流入增强。早全新世(11.7~8.2kaBP)早期次表层温度达到6.5℃,钙质生产力升高,冰筏碎屑输入降低;在中全新世(8.2~4.2kaBP)早期(8.2~5.6kaBP),钙质生产力逐渐升高反映通风作用增强,导致营养盐供应增加;6.6~5.6kaBP,明显降低的次表层温度(~4 ℃)反映夏季太阳辐射量降低以及大西洋水流入减弱;5.6~4.2kaBP期间次表层水变暖导致δ18O偏轻,而δ13C轻值反映生产力降低。晚全新世(4.2~0.8kaBP)的新冰期(4.2~3.0kaBP),次表层温度逐渐降低,Nps-δ13C偏轻反映生产力下降,Nps-δ18O偏轻以及IRD增加反映冰融水排放。3.0kaBP以来,生产力上升,次表层水体温度不断上升,可能是向北输送的北大西洋水增强。Abstract: In order to reconstruct the changes in (sub)surface water mass since 20.0kaBP, multiproxy investigations, including Ice Rafted Debris (IRD) abundance, AMS14C dating, foraminiferal abundance, stable oxygen and carbon isotopes of planktonic foraminifera Neogloboquadrina. pachyderma (sin.) (Nps), and (sub)surface temperature derived from Nps Mg/Ca ratios, have been carried out for two cores collected from the Nordic Seas during the Fifth Chinese National Arctic Expedition. The LGM (20.0~17.5kaBP) is characterized by low subsurface water temperature(~3℃), poor calcium productivity, and enhanced IRD input. During the deglaciation (17.5~11.7 kaBP). Nps-δ18O and-δ13C depletions suggested a freshwater event during Heinrich Stadial 1(HS1). Stronger water stratification and north Atlantic water gathering in the subsurface layer caused the increase in subsurface temperature. At the onset of Bølling-Allerød (B/A) event, the subsurface water temperature (4.5℃) indicated an increased advection of Atlantic water. The early Holocene (11.7~8.2kaBP) was characterized by higher subsurface temperature (6.5℃), bioproductivity, and lower IRD input. During the middle Holocene (8.2~4.2kaBP), the gradual increase in bioproductivity during 8.2~5.6kaBP indicated the enhanced ventilation, which led to an increase in nutrient supply. Lower subsurface temperature (~4℃) during the 6.6~5.6kaBP may suggest the decrease in summer solar radiation and weakening of Atlantic water advection. During 5.6~4.2kaBP, the depletion of Nps-δ13C and δ18O values indicated poor bioproductivity and subsurface water warming, respectively. During the late Holocene (4.2~0.8kaBP), the Neoglaciation (4.2~3.0kaBP), was characterized by low subsurface temperature and poor bioproductivity. The light values of Nps-δ18O and the increase in IRD reflected a meltwater event. Since 3kaBP, the continuous increasing of subsurface temperature and bioproductivity may be explained by the increased advection of Atlantic water.
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Keywords:
- paleoceanography /
- sub SST reconstruction /
- Last Glacial Maximum (LGM) /
- Nordic Sea
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北欧海包括格陵兰海、挪威海和冰岛海,是北大西洋深层水(Northern Atlantic Deep Water,NADW)的源区之一[1]。北欧海表层海洋环境的变化,如冰融水增多,会改变表层海水盐度,这导致水体分层加强[2, 3],促使NADW的生成减弱,进而导致大西洋经向反转流(Atlantic meridional overturning circulation,AMOC)减弱,最终影响全球气候变化[4]。因此对北欧海的古海洋研究,能够为我们了解全球气候变化提供有利信息。
末次盛冰期(Last Glacial Maximum,LGM)以来,挪威-格陵兰海(Norwegian-Greenland Sea,NGS)短时间尺度的气候波动剧烈而频繁[5-7],AMOC的位置和现代也有很大的不同。有证据表明,LGM时期,AMOC的位置比现代更加浅,更加靠南,较暖的大西洋水聚集在比现代更南的区域[3, 8-11]。末次冰消期包含HS1(Heinrich Stadial 1)事件,B/A(Bølling-Allerød)事件,YD(Younger Dryas)事件。HS1事件时,劳伦冰盖融化输出大量淡水,AMOC减弱,最终对全球气候变化产生了深远影响[8, 12-14]。全新世以来的气候演化可分为早全新世(11.7~8.2kaBP)、中全新世(8.2~4.2kaBP)以及晚全新世(4.2~0.8kaBP)3个阶段[15]。与冰期-间冰期气候变化幅度相比,全新世气候相对稳定[16]。太阳辐射量变化[17]、大气模式的变化,如北大西洋涛动(North Atlantic Oscillation, NAO)以及大西洋水流入北欧海的变化[18],成为影响北欧海气候的主要因素。北大西洋涛动是冰岛上空的冰岛低压和位于大西洋中低纬度的亚速尔高压之间的气压变化,是控制北大西洋地区的主要气候模式。当NAO指数较高处于正相位时,西风强劲,北大西洋水向北流动增强。当NAO指数较低处于负相位时,西风减弱,北大西洋水向北传输减弱[18]。
前人基于不同的指标对末次盛冰期以来的北大西洋区域的古海洋演化进行了大量研究,包括浮游有孔虫Neogloboquadrina pachyderma (sin.)(Nps)稳定氧碳同位素、IRD丰度以及浮游有孔虫丰度和转换函数等[19-21]。但是这些指标无法直接提供关于该海区次表层海水温度演化的信息。首先,利用种属组合转换函数包含了不同水层生活的种属,不能直接反映地质历史时期的次表层温度变化[22, 23]。此外,有孔虫氧碳同位素指标同时反映了海水温度和盐度(如冰融水输入)的信号[24]。因此,还需要用古海温的其他指标来反映北欧海温度的变化。研究表明,Nps的Mg/ Ca比值与水温线性相关[25]。一些学者尝试利用Nps的Mg/Ca值重建了弗拉姆海峡全新世以来的海水次表层温度(sub Sea Surface Temperature,sSST)变化历史[26-29]。研究表明,基于次表层水体中的浮游有孔虫Nps(生活在水深50~200m)[25, 30]计算的sSSTMg/Ca不仅反映了太阳辐射量的变化,而且也反映了北大西洋水向北传输的强弱变化[26-29, 31]。
1. 区域地质背景
北欧海区域受到两支表层洋流的影响(图 1a)。一支是温暖高盐的北大西洋流(North Atlantic Current,NAC, T>2℃,S>35psu,图 1b)[32],NAC在向北冰洋运输过程中,其热量主要散失在挪威海和巴伦支海[4, 33]。流入北冰洋的大西洋水聚集在弗拉姆海峡的东部,占据了上部700m的水体[34-35]。另一支是寒冷、低盐度的极地水(T<0℃,S<34.4psu)[36],它沿着格陵兰岛大陆架边缘向南流,形成东格陵兰洋流(East Greenland Current,EGC),将北冰洋水输入北欧海。
图 1 北欧海区域洋流及水深150m温度分布特征和研究岩芯位置(a)及现代北欧海区域4个站位的温度和盐度垂直分布特征(b)(1955—2012年平均)白色圆点是本文的研究站位,黄色圆点是引用站位[21, 26]。图中红色箭头和蓝色箭头的洋流分别指示北大西洋暖流和东格陵兰海寒流。灰色虚线分别指示冬季和夏季多年平均海冰界限(1981—2010)((National Snow and Ice Data Center; http://nsidc.org/data/seaice/)。图中灰色块为缺失数据区域。图中NAC:北大西洋流;EGC:东格陵兰流。b的数据来源于WOA13,由ODV绘制(http://odv.awi.de/en/data/ocean/world_ocean_atlas_2013/)Figure 1. (a) Nordic Seas water temperature at 150 m depth. White and yellow dots denote the studied sites and the reference sites[21, 26]. Red arrows indicate North Atlantic Current (NAC), blue arrows indicate East Greenland Current (EGC). Gray dashed lines show average sea ice limits (1981-2010) for summer and winter (National Snow and Ice Data Center; http://nsidc.org/data/seaice/). The gray block in the figure is the missing data area. (b) Water mass vertical profiles(annual, 1955-2012)from locations close to the sediment cores shown in (a). Datas were obtained from WOA13 and plotted by ODV(http://odv.awi.de/en/data/ocean/world_ocean_atlas_2013/)北欧海区域(图 1b)上部0~50m水体为混合层(温度最高达到7℃,盐度最低达到34.4psu),向北温度和盐度降低。50~700m水体为大西洋暖水,温度为1~6℃,盐度大于34.9~35.15psu(图 1b)。700~1100m以下水体分层减弱,为大西洋中层水(温度为-0.5~2℃,盐度为34.9~35psu)。1100m以下水体混合均匀,为深层水,温盐性质稳定(温度<0℃,盐度为34.9psu)。研究区域的主要浮游有孔虫种属是Nps,生活于约50~200m的次表层水中[25, 30, 37]。
中国第五次北极考察期间在挪威海采取的两个岩芯(ARC5-AT06和ARC5-BB04)(图 1a)拥有连续的有孔虫Nps记录。在本项研究中,我们分析测试了这两个岩芯样品中IRD含量、浮游有孔虫丰度、Nps的氧碳同位素和Mg/Ca值。在结合前人的研究基础上,试图重建挪威海LGM晚期以来次表层海水温度的变化历史,并综合分析研究区LGM晚期以来的古海洋演化模式。
2. 材料与方法
2.1 材料来源
本次研究的材料来源于2012年中国第五次北极科学考察在挪威海取得的两个沉积物多管样ARC5-AT06(岩芯长30cm)和ARC5-BB04(岩芯长37cm)(表 1),对两根多管样以1cm间隔进行采样,共计67个样品。对比站位来源于弗拉姆海峡的MSM5/5-712-2[26]以及格陵兰海南部的PS1878站位[21]。
2.2 实验方法
本次研究对挪威海的ARC5-AT06和ARC5-BB04两个岩芯样品做的分析包括:粗组分含量分析、浮游有孔虫丰度统计、浮游有孔虫Nps稳定氧碳同位素、Mg/Ca测定,以及Nps的AMS14C测年,以上测试除了AMS14C测年是在美国加州大学尔湾分校的地球系统科学系放射性碳实验室(Radi-ocarbon Laboratory of Earth System Science Department, University of California Irvine)和BETA实验室完成之外,其余分析在同济大学海洋地质国家重点实验室完成。
粗组分含量分析:取干样大约10~15g,干样用水泡开,经孔径63μm的筛子用水流进行冲样,获得>63μm的屑样,烘干后称重并记录。取>63μm屑样再依次通过孔径150和250μm的筛子进行干筛并称重。计算获得>63μm、>150μm和>250μm的不同粗组分的百分含量。
浮游有孔虫和IRD丰度统计:丰度统计是在实体显微镜下鉴定,统计>150μm的浮游有孔虫的个体数量,计算其丰度。统计>250μm中的IRD的数量,计算其丰度。
浮游有孔虫Nps的δ18O和δ13C测定:>150μm的样品中挑出浮游有孔虫Nps完整个体20枚左右,进行稳定氧碳同位素测定。先将有孔虫用酒精及超声清洗,然后将处理好的有孔虫壳体用Finnigan MAT253型稳定同位素质谱仪测定氧碳同位素的比值δ18O和δ13C。测试精度通过国际标样(NBS19)来检测控制,分析检测的标准偏差δ18O为0.07‰,δ13C为0.04‰。
浮游有孔虫Nps的Mg/Ca测定:选取完整、相对干净的有孔虫Nps壳体(150~250μm)50个。首先,将有孔虫在显微镜下检验,剔除杂质,称重后压碎每个壳体房室,然后进行预处理,具体操作过程遵循海洋地质国家重点实验室超净实验室有孔虫ICP-AES/MS测试清洗过程[38]。
有孔虫壳体中记录的Mg/Ca比值主要受周围海水的温度控制[39-41]。有孔虫壳体对于Mg的吸收的热力学控制表明,温度和Mg的吸收量之间的关系呈正指数关系[42],在狭窄的温度范围内有近似的线性关系[25]。次表层水体温度(sSSTMg/Ca)可通过Nps的Mg/Ca比值依据以下公式计算得到[25]:
$$ {\rm{Mg/Ca}}({\rm{mmol/mol}}) = 0.13( \pm 0.037)T\left( {℃} \right) + 0.35( \pm 0.17) $$ (1) 该方程基于来自北欧海表层样的Nps的Mg/Ca和δ44Ca指标的交叉校正,在高于大约3℃的时候是一个可靠的估算次表层温度的公式[25]。
AMS14C测年:从>150μm的屑样中挑出Nps的完整个体约8mg,送到美国加州大学尔湾分校的地球系统科学系放射性碳实验室和BETA实验室进行AMS14C测年。14C测年结果通过Calib7.0.4程序校正为日历年龄[43],使用400年碳储库年龄以及Marine13校正曲线[44](表 2)。
表 2 挪威海AT06和BB04岩芯Nps-AMS14C测年数据及校正Table 2. Calibration of Nps-AMS 14C dating of Core AT06 and BB04样品编号 深度/cm AMS14C年龄/aBP 碳储库校正/aBP 日历年龄/aBP UCIT33499 AT06/0-1 1365±15 965±15 857±5 UCIT32780 AT06/2-3 1965±15 1565±15 1473±4 UCIT32781 AT06/6-7 2310±15 1910±15 1856±11 BETA-407693 AT06/11-12 2710±30 2310±30 2337±13 UCIT32782 AT06/14-15 2845±15 2445±15 2488±29 UCIT32715 BB04/2-3 3335±15 2935±15 3100±18 UCIT32716 BB04/6-7 5320±15 4920±15 5631±11 UCIT32717 BB04/10-11 8980±20 8580±20 9540±4 UCIT32718 BB04/15-16 11495±30 11095±30 12982±60 BETA-407694 BB04/20-21 13420±40 13020±40 15593±116 UCIT32719 BB04/23-24 14960±40 14560±40 17744±91 UCIT32720 BB04/29-30 15925±35 15525±35 18784±50 UCIT32721 BB04/35-36 16695±45 16295±45 19668±94 3. 结果
3.1 地层年代框架
AT06岩芯年龄-深度关系如图 2(a)所示。岩芯顶部年龄经校正为0.8kaBP,底部年龄根据相邻年龄和沉积速率外推为3.2kaBP。沉积速率自3.2kaBP以来(19.8cm/ka)逐渐降低,在1.5~0.8kaBP降到最低(3.2cm/ka)。
图 2 挪威海AT06岩芯(a)和BB04岩芯(b)深度-年龄模式及其沉积速率图中数字标注的菱形点是AMS 14C校正的年龄数据,圆点是外推的底部和顶部的年龄。图中虚线是外推出的年龄以及沉积速率Figure 2. Age model and sedimentation rate (cm/ka) for Core AT06 and BB04Diamond dots are the AMS 14C ages, marked with numbers. Dots are the age of the bottom and top. Dashed lines show ages and sedimentation rates, which are extrapolatedBB04岩芯(图 2(b))顶部和底部年龄根据相邻年龄和沉积速率外推所得,分别是1.8和19.8kaBP。自19.8kaBP以来,该岩芯较高的沉积速率出现在19.8~17.7kaBP。17.7~1.8kaBP沉积速率较低,波动范围保持在2cm/ka以内。
3.2 有孔虫和IRD丰度变化
BB04岩芯浮游有孔虫(planktic foraminifer, PF)丰度的变化范围为427~18190枚/g,平均值为3452枚/g。末次盛冰期(LGM)晚期(19.8~17.5kaBP),浮游有孔虫的丰度较低,平均值为628枚/g。HS1期间(17.5~14.7kaBP),浮游有孔虫丰度稍有增加,平均值是1726枚/g。B/A和YD期间(14.7~11.7kaBP),浮游有孔虫丰度较低,平均值为1776枚/g。全新世(11.7~1.8kaBP),浮游有孔虫的丰度明显上升,平均值为7797枚/g。在此期间,底栖的瓷质壳有孔虫两玦虫(Pyrgo depressa)丰度也显著上升,平均值为64枚/g。在AT06岩芯中,全新世晚期(3.2~0.8kaBP)浮游有孔虫丰度的平均值为548枚/g(图 3)。
图 3 挪威海AT06和BB04岩芯约20.0kaBP以来古海洋与古气候替代指标的变化包括IRD以及两玦虫丰度(虚线)、浮游有孔虫丰度(PF)、浮游有孔虫Nps氧碳同位素、浮游有孔虫Nps的Mg/Ca数据以及基于Nps的Mg/Ca换算的古温度数据。还引用了格陵兰冰芯(GISP2 ice-core)氧同位素记录[45];黑色三角标注的分别是BB04站位50 m和200 m的温度(1955—2012年7—9月的平均温度)。数据来源于WOA13(http://odv.awi.de/en/data/ocean/world_ocean_atlas_2013/)Figure 3. AT06 and BB04 proxy records versus agetotal abundance of IRD and Pyrgo depressa, total abundance of planktic foraminifera(PF), stable carbon and oxygen isotope ratios of planktic foraminifer Nps, reconstructed sSST of planktic foraminifer N. pachyderma (sin). Also plotted is the oxygen isotope record from the GISP2 ice-core [45]. Present water temperatures at 50m and 200m(Average temperature from July to September, 1955-2012) are shown under black triangle on X-axis. Temperature data were obtained from WOA13(http://odv.awi.de/en/data/ocean/world_ocean_atlas_2013/)BB04岩芯的IRD丰度的变化范围为92~568枚/g,平均值为211枚/g。末次盛冰期(LGM)晚期(19.8~17.5kaBP),丰度相对较高,平均值为243枚/g。HS1期间(17.5~14.7kaBP),IRD出现明显峰值,为510枚/g(在15.5kaBP)。B/A和YD期间(14.7~11.7kaBP),IRD丰度下降,平均值为195枚/g。全新世以来(11.7~1.8 ka),IRD丰度较低,平均值为160枚/g(图 3)。
3.3 有孔虫稳定氧碳同位素
BB04岩芯的Nps-δ18O变化范围为2.6‰~4.9‰,平均值为3.8‰。LGM晚期(19.8~17.5kaBP),Nps-δ18O较重,平均值为4.7‰。HS1期间(17.5~14.7kaBP),Nps-δ18O大幅变轻,平均值为3.5‰,在17kaBP附近有较轻的峰值(3.2‰)。B/A和YD期间(14.7~11.7kaBP),Nps-δ18O偏重,平均值为4.2‰。全新世以来(11.7~1.8kaBP),Nps-δ18O较轻,平均值3.0‰。AT06岩芯在全新世晚期(3.2~0.8kaBP)的Nps-δ18O变化范围为2.3‰~3‰,平均值为2.5‰(图 3)。
BB04岩芯的Nps-δ13C变化范围为-0.2‰~0.8‰,平均值为0.19‰。LGM晚期Nps-δ13C较轻,平均值为0.07‰。HS1期间,Nps-δ13C明显偏轻,平均值为0.04‰,在17ka附近有较轻的峰值(-0.24‰)。B/A和YD期间,平均值为0.28‰。全新世以来,Nps-δ13C较重,平均值为0.31‰。全新世晚期AT06岩芯的Nps-δ13C的变化范围为-0.05‰~0.7‰,平均值为0.38‰(图 3)。
3.4 次表层海水温度重建
BB04岩芯中Nps的Mg/Ca平均值是0.867mmol/mol(n=36),平均温度是3.97℃。根据公式(1)表层样品Mg/Ca值换算温度为7.8℃,明显高于有孔虫生活水深(50~200m)现代多年夏季平均值(3.31~5.1℃)。研究表明,有孔虫的Mg/Ca异常高值可能和壳体表面方解石的二次胶结有关[25, 26],镜下观察表层样品中Nps壳体呈明显黄—白色,推测壳体受到污染,导致温度异常高值,因此该数据被剔除。LGM晚期,sSSTMg/Ca较低,变化范围为2.7~3.9℃,平均温度为3.2℃。HS1期间,温度开始回升,变化范围为3.1~4.0℃,平均温度为约3.7℃。早全新世(11.7~8.2kaBP)温度相对较高,变化范围为4.2~6.5℃,平均温度为5.3℃,在11ka有整个记录的最大值(6.5℃)。中全新世(8.2~4.2kaBP),sSSTMg/Ca有所下降,变化范围为3.8~5.6℃,平均温度为4.8℃。晚全新世(4.2~0.8kaBP),主要由AT06岩芯重建。该岩芯基于Nps的Mg/Ca平均值是0.859mmol/mol(n=17,17cm之下无有孔虫),平均温度是3.91℃。sSSTMg/Ca逐渐升高,变化范围为2.4~6.2℃,在~1.5ka达到峰值(6.2℃)(图 3)。
4. 讨论
4.1 末次盛冰期(20.0~17.5kaBP)
LGM时期的低温对应了较低的浮游有孔虫丰度和Nps-δ13C低值,反映较低的表层生产力[46, 47](图 4h)。LGM时期,研究区域被海冰覆盖[2],降低了浮游植物的生产力,进一步导致浮游有孔虫丰度的减小[23, 48-49]。前人对于高纬度有孔虫丰度的数据模拟结果表明,温度以及浮游植物生产力都是影响有孔虫钙质生产力的重要因素[50-53]。
图 4 挪威海BB04和AT06站位与格陵兰中央海PS1878站位[21]各项古气候指标的对比(a)浮游有孔虫Nps的Mg/Ca数据重建的次表层古温度记录;(b)利用LOVECLIM模型建立的格陵兰岛东南部AMOC的强度变化(实线)[60]以及利用TraCE-21模型建立的60°N以北区域的AMOC的强度变化(虚线)[65];(c, d)浮游有孔虫Nps-δ13C; (e, f)浮游有孔虫Nps-δ18O; (g, h)浮游有孔虫丰度;(i, j)IRD丰度Figure 4. Proxy records for Core AT06 and BB04 compared with Core PS1878 records (Telesiński et al, 2014)(a) reconstructed sSST of planktic foraminifer Nps for Core AT06 and BB04; (b) AMOC strength of southeast Greenland as simulated in the model LOVECLIM (solid line)[60], AMOC strength of north of 60°N as simulated in the model TraCE-21(dashed line)[65]; (c, d) stable carbon isotope ratios of planktic foraminifer Nps; (e, f) stable oxygen isotope ratios of planktic foraminifer Nps; (g, h) total abundance of planktic foraminifera; (i, j) IRD abundancesPS1878站位和BB04站位的Nps-δ18O在LGM晚期偏重(图 4e, f),是该地区LGM时期Nps-δ18O的典型特征[21, 54, 55]。研究表明,Nps-δ18O值的变化与海水温度(温度降低1℃相当于Nps-δ18O值偏重0.26‰)[24]以及北大西洋水输入有关[21, 54, 56]。LGM时期,AMOC的位置比现代更加靠南,较暖的大西洋水聚集在比现代更南的区域[3, 8-11]。因此,挪威海BB04站位在LGM晚期的sSSTMg/Ca较低(约3℃),导致了这时期Nps-δ18O偏重[21, 56]。
格陵兰海南部的PS1878站位[21]和挪威海BB04站位的IRD含量在LGM晚期都较高(图 4i, j),表明较多的冰山排泄。挪威海BB04站位的IRD高峰(18.3kaBP)反映该期间冰山排泄增加。古环境记录及数值模拟结果[2-3, 23]表明,LGM时期北大西洋的水体部分开放[57],表层流动性增加导致冰山输入增加,IRD沉积增加。
4.2 末次冰消期(17.5~11.7kaBP)
在HS1事件期间,BB04站位IRD丰度显著上升(图 4j),伴随着Nps-δ18O和δ13C显著偏轻(图 4e, f),指示了HS1事件期间该地区发生了冰融水事件[8, 12-14]。同时,次表层温度逐渐上升(图 4a),推测与北大西洋暖水在向北输送的过程中在次表层聚集有关。数值模拟以及海洋记录表明(图 4b),HS1融冰事件中AMOC显著减弱[8, 13, 59, 60],次表层水体中温暖的北大西洋水聚集以及海平面升高,引起了附近斯堪的纳维亚冰盖(Fennoscandia Ice Sheet,FIS)前缘崩塌[61],冰筏碎屑物输入挪威海。这些结论与BB04站位中该时期出现的冰筏碎屑高峰和sSSTMg/Ca升高相符(图 4a, j)。格陵兰海南部PS1878的IRD并没有显著上升(图 4i),表明和挪威海不同的冰融水来源,可能来自于附近的格陵兰冰盖[21]。
陆地冰盖中含有较轻δ18O的冰融水注入导致Nps-δ18O偏轻。低密度的表层水增强了上层水体的分层,使得海气交换减弱,生物生产力较低,进而导致了Nps-δ13C偏轻[8, 13, 59, 60]。这些指标变化与弗拉姆海峡西部[19, 56]、格陵兰海南部[21]以及东挪威海[62]的沉积物记录一致。
B/A事件中,虽然温度升高,但Nps-δ18O值较HS1事件偏重(图 4f),指示冰融水的影响减弱。同时,逐渐偏重的Nps-δ13C值表明水体通风作用增强,生产力提高。这与该时期AMOC增强有关[59-60](图 4b)。次表层水体温度在B/A早期较高(约4.5℃),反映了HS1之后冰融水影响减弱,温暖的北大西洋水流入增强的信号[63]。
YD事件中,北大西洋再次受到冰融水注入的影响,AMOC减弱[59-60, 64]。由于本研究站位中,该时期样品分辨率较低(图 4),各指标未发现明显的变化特征。
4.3 全新世
4.3.1 早全新世(11.7~8.2kaBP)
在早全新世(11.7~8.2ka),出现了全新世温度极暖期(Holocene Thermo Maximum-HTM)[28],对应于高纬度夏季日照辐射量峰值[17](图 5c)。在此期间,BB04站位sSSTMg/Ca也达到最高值6.5℃(图 5d)。本文通过Mg/Ca记录的早全新世次表层暖水与有孔虫转换函数的计算结果一致[31]。挪威海BB04站位中sSSTMg/Ca的较高值出现在11.8~10.0kaBP,在约11kaBP达到峰值(图 5d)。弗拉姆海峡712-2岩芯中的sSSTMg/Ca较高值出现在10.0~8.7kaBP,在约10kaBP达到峰值(图 5e)。挪威海次表层水体变暖比弗拉姆海峡早约1ka, 这个时间差异可能与两个站位的位置有关,更加靠北的弗拉姆海峡在此期间可能更持续地受到北冰洋排泄的冷水影响。研究表明,在11.8~10.0kaBP,弗拉姆海峡主要受到海冰、冰山以及冰融水影响,温度较低[20, 26, 66]。随着北大西洋暖水的流入增强,弗拉姆海峡的海冰和冰融水被北大西洋暖水替代,温度升高[67]。总体上,该时期北大西洋暖水流入增强[18, 31],挪威海可能存在一个厚而温暖的混合层,水体上层200m是温暖的北大西洋水[29, 31],受到北大西洋暖水影响,次表层水温度较高。
图 5 利用TraCE-21模型建立的60°N以北区域的AMOC的强度变化[65](a)与利用TraCE-21模型面积加权平均得出的60°N以北区域的年际温度异常[65](b)及70°N 7月太阳辐射量变化[17](c)及BB04和AT06站位重建的次表层温度(d)及MSM5/5-712-2站位重建的次表层温度[26](e)Figure 5. (a) AMOC strength of north of 60°N as simulated in the model TraCE-21[65]; (b) area-weighted mean annual temperature anomaly north of 60°N as simulated in the model TraCE-21[65]; (c) June mean insolation at 70°N[65]; (d) reconstructed sSST of planktic foraminifer Nps for core AT06 and BB04; (e) reconstructed sSST of planktic foraminifer Nps for core MSM5/5-712-2[26]BB04站位早全新世早期浮游有孔虫丰度逐渐升高(图 4h),这与格陵兰海南部PS1878站位(图 4g)和北欧海以及周边区域的其他记录一致[19, 26, 31, 68]。较高的夏季日照辐射量以及温暖的次表层水体温度,导致浮游植物爆发,提供充足的食物,促使浮游有孔虫生产力提高[29, 51]。同时,BB04站位中,底栖有孔虫两玦虫丰度大幅上升(图 3),也表明了早全新世该区域表层/次表层生产力较高[69]。
早全新世BB04站位的Nps-δ18O逐渐偏轻至3‰(图 4f),与挪威海其他岩芯中的记录一致[19, 68, 70]。该时期,格陵兰海南部PS1878站位Nps-δ18O变化不明显,仍然保持在约4‰(图 4e)。挪威海BB04站位Nps-δ18O值在早全新世明显较格陵兰海PS1878站位偏轻,这主要是受到温度的影响,格陵兰海一侧主要受到极地冷水影响,温度较低,Nps-δ18O偏重,挪威海一侧受北大西洋暖水影响更大,Nps-δ18O偏轻[71-72]。
全新世对应于BB04站位的IRD最低值,表明斯堪的纳维亚冰盖的消融已基本停止。而位于格陵兰海的PS1878站位在早全新世记录了大量筏冰碎屑(图 4i),说明在此期间,格陵兰冰盖持续消融,IRD输入增加[73],反映了北欧海东西两侧冰筏碎屑输入的不同。
4.3.2 中全新世(8.2~4.2kaBP)
挪威海BB04站位在8.2~5.6kaBP次表层水体温度降低,对应了该期间偏重的Nps-δ18O,其中较冷的持续时间为6.6~5.6kaBP(图 5d)。弗拉姆海峡的中全新世较冷的时期略有差异(7.9~6.6kaBP)[26]。虽然弗拉姆海峡更加靠近北部,太阳辐射量降低以及受到高纬度冷水影响,次表层水体变冷可能会更早,但不能解释在随后南面的BB04站位变冷的时候,更北的弗拉姆海峡反而升温。因此,这个时间差有可能是年龄控制所导致。另外,两个区域的温度演化趋势基本一致,也从侧面佐证可归因于年龄控制的差异。在此期间,各站位的IRD含量很低,可以认为融冰作用较弱。因此,研究区海水温度主要受太阳辐射和大西洋水的北向输送控制。在中全新世期间,太阳辐射量较早全新世减弱[17, 74],而NAO处于负相位[18, 75],导致大西洋暖水北向运输减弱[76]。
8.2~5.6kaBP期间Nps-δ13C偏重,与周围格陵兰海的PS1878站位以及北欧海其他岩芯的Nps-δ13C结果一致[19, 21, 68],同时对应了浮游有孔虫丰度逐渐升高(图 4h),表明该期间生产力逐渐升高。显然,这期间升高的生产力与温度无关,可能由于水体通风作用增强,表层营养盐供应增加,导致生产力升高。数值模拟的AMOC记录也表明该期间AMOC逐渐增强[65](图 5a)。
中全新世晚期(5.6~4.2kaBP)挪威海的Nps-δ18O值逐渐偏轻(图 4f)对应sSSTMg/Ca升高,表明该期间的温度升高导致Nps-δ18O值偏轻[24, 28, 77]。但该时间格陵兰海的Nps-δ18O值较重(图 4e), 推测原因是格陵兰海温度较低,导致Nps-δ18O值偏重。该期间挪威海BB04站位浮游有孔虫丰度下降(图 4h),对应Nps-δ13C偏轻,表明该期间生产力较低。
4.3.3 晚全新世(4.2~0.8kaBP)
BB04和AT06站位sSSTMg/Ca在4.2~3.0kaBP较低,基本对应于弗拉姆海峡712-2站位的sSSTMg/Ca低值(5.2~2.4kaBP)(图 5d, e),这一时期被称为新冰期[78],北欧海的不同区域中都有记录[21, 26, 38, 65]。新冰期出现的时间在不同地区存在差异[65]。一般认为,新冰期是对夏季太阳辐射量减弱的响应[67, 76],导致海水表层被北冰洋低盐水控制或被海冰覆盖[28, 79]。同时,该期间Nps-δ18O偏轻对应IRD增多,反映此时有冰融水注入。较低的浮游有孔虫丰度(期间只有一个异常高值)和Nps-δ13C轻值反映冷水环境下较低的生产力。
晚全新世晚期3.0kaBP以来,挪威海AT06站位sSSTMg/Ca逐渐升高,Nps-δ18O明显偏轻(图 5d),表明次表层水体变暖。McKay等(2018)根据数值模拟建立的60°N以北区域10.0kaBP以来的温度异常数据(图 5b),表明自3.0kaBP以来表层海水温度不断下降。同时,晚全新世的海冰扩张和表层水体的逐渐变冷在北欧海的其他指标中都有记录[79, 80],普遍认为这与北半球太阳辐射量的降低有关[17](图 5c)。然而,本研究中挪威海和弗拉姆海峡sSSTMg/Ca重建记录表明晚全新世温度的升高,与太阳辐射量的降低相反[28, 68, 81]。因此,在晚全新世整体气候变冷的过程中,海冰的扩张使得表层海水分层加剧,而sSSTMg/Ca的升高反映的是海水层化加剧的情况下北大西洋水在次表层的聚集。该时期NAO处于正相位[18, 75],导致大西洋暖水北向运输增加。在此期间,挪威海AT06站位有孔虫丰度逐渐上升,对应Nps-δ13C重值,反映生产力升高。
5. 结论
(1) 在末次盛冰期(20.0~17.5kaBP),挪威海区域温度较低,导致了较低的钙质生产力,水体部分开放,冰山输入以及冰筏碎屑含量增加。
(2) 末次冰消期(17.5~11.7kaBP)的HS1期间,发生融冰事件,导致淡水排出,表层淡水覆盖降低了水体盐度并且抑制了海水通风作用,钙质生产力降低,次表层聚集大西洋暖水,水体温度逐渐升高;B/A期间,北大西洋水流入增强,次表层温度达到4.5℃。
(3) 早全新世(11.7~8.2kaBP)早期,次表层水体温度达到最大值6.5℃,钙质生产力逐渐升高,冰筏碎屑输入降低。中全新世(8.2~4.2kaBP)早期(8.2~5.6kaBP)钙质生产力逐渐上升,反映了通风作用增强,导致营养盐供应增加;其中的6.6~5.6kaBP,明显降低的次表层温度反映了夏季太阳辐射量降低以及大西洋水流入减弱;5.6~4.2kaBP期间,次表层水体变暖导致Nps-δ18O偏轻,Nps-δ13C偏轻表明钙质生产力降低。晚全新世(4.2~0.8kaBP)的新冰期(4.2~3.0kaBP),次表层温度逐渐降低,Nps-δ13C偏轻表明钙质生产力下降,Nps-δ18O变轻以及IRD增加反映冰融水排放。3.0kaBP以来,生产力逐渐上升,次表层水温度逐渐升高,由于NAO处于正相位导致北大西洋水持续地流入北欧海,次表层聚集大西洋水。
致谢: 本项工作由国家自然科学基金(41776187)和中国北极考察专项(CHINARE2013-2016-03-02)资助,是国家海洋局极地办公室组织实施的“中国第五次北极科学考察项目”的一部分。感谢中国第五次北极科考队的全体科考队员和“雪龙”号全体船员为沉积物样品的采集所付出的艰辛努力;感谢李英搏为有孔虫丰度实验提供的帮助。 -
图 1 北欧海区域洋流及水深150m温度分布特征和研究岩芯位置(a)及现代北欧海区域4个站位的温度和盐度垂直分布特征(b)(1955—2012年平均)
白色圆点是本文的研究站位,黄色圆点是引用站位[21, 26]。图中红色箭头和蓝色箭头的洋流分别指示北大西洋暖流和东格陵兰海寒流。灰色虚线分别指示冬季和夏季多年平均海冰界限(1981—2010)((National Snow and Ice Data Center; http://nsidc.org/data/seaice/)。图中灰色块为缺失数据区域。图中NAC:北大西洋流;EGC:东格陵兰流。b的数据来源于WOA13,由ODV绘制(http://odv.awi.de/en/data/ocean/world_ocean_atlas_2013/)
Figure 1. (a) Nordic Seas water temperature at 150 m depth. White and yellow dots denote the studied sites and the reference sites[21, 26]. Red arrows indicate North Atlantic Current (NAC), blue arrows indicate East Greenland Current (EGC). Gray dashed lines show average sea ice limits (1981-2010) for summer and winter (National Snow and Ice Data Center; http://nsidc.org/data/seaice/). The gray block in the figure is the missing data area. (b) Water mass vertical profiles(annual, 1955-2012)from locations close to the sediment cores shown in (a). Datas were obtained from WOA13 and plotted by ODV(http://odv.awi.de/en/data/ocean/world_ocean_atlas_2013/)
图 2 挪威海AT06岩芯(a)和BB04岩芯(b)深度-年龄模式及其沉积速率
图中数字标注的菱形点是AMS 14C校正的年龄数据,圆点是外推的底部和顶部的年龄。图中虚线是外推出的年龄以及沉积速率
Figure 2. Age model and sedimentation rate (cm/ka) for Core AT06 and BB04
Diamond dots are the AMS 14C ages, marked with numbers. Dots are the age of the bottom and top. Dashed lines show ages and sedimentation rates, which are extrapolated
图 3 挪威海AT06和BB04岩芯约20.0kaBP以来古海洋与古气候替代指标的变化
包括IRD以及两玦虫丰度(虚线)、浮游有孔虫丰度(PF)、浮游有孔虫Nps氧碳同位素、浮游有孔虫Nps的Mg/Ca数据以及基于Nps的Mg/Ca换算的古温度数据。还引用了格陵兰冰芯(GISP2 ice-core)氧同位素记录[45];黑色三角标注的分别是BB04站位50 m和200 m的温度(1955—2012年7—9月的平均温度)。数据来源于WOA13(http://odv.awi.de/en/data/ocean/world_ocean_atlas_2013/)
Figure 3. AT06 and BB04 proxy records versus age
total abundance of IRD and Pyrgo depressa, total abundance of planktic foraminifera(PF), stable carbon and oxygen isotope ratios of planktic foraminifer Nps, reconstructed sSST of planktic foraminifer N. pachyderma (sin). Also plotted is the oxygen isotope record from the GISP2 ice-core [45]. Present water temperatures at 50m and 200m(Average temperature from July to September, 1955-2012) are shown under black triangle on X-axis. Temperature data were obtained from WOA13(http://odv.awi.de/en/data/ocean/world_ocean_atlas_2013/)
图 4 挪威海BB04和AT06站位与格陵兰中央海PS1878站位[21]各项古气候指标的对比
(a)浮游有孔虫Nps的Mg/Ca数据重建的次表层古温度记录;(b)利用LOVECLIM模型建立的格陵兰岛东南部AMOC的强度变化(实线)[60]以及利用TraCE-21模型建立的60°N以北区域的AMOC的强度变化(虚线)[65];(c, d)浮游有孔虫Nps-δ13C; (e, f)浮游有孔虫Nps-δ18O; (g, h)浮游有孔虫丰度;(i, j)IRD丰度
Figure 4. Proxy records for Core AT06 and BB04 compared with Core PS1878 records (Telesiński et al, 2014)
(a) reconstructed sSST of planktic foraminifer Nps for Core AT06 and BB04; (b) AMOC strength of southeast Greenland as simulated in the model LOVECLIM (solid line)[60], AMOC strength of north of 60°N as simulated in the model TraCE-21(dashed line)[65]; (c, d) stable carbon isotope ratios of planktic foraminifer Nps; (e, f) stable oxygen isotope ratios of planktic foraminifer Nps; (g, h) total abundance of planktic foraminifera; (i, j) IRD abundances
图 5 利用TraCE-21模型建立的60°N以北区域的AMOC的强度变化[65](a)与利用TraCE-21模型面积加权平均得出的60°N以北区域的年际温度异常[65](b)及70°N 7月太阳辐射量变化[17](c)及BB04和AT06站位重建的次表层温度(d)及MSM5/5-712-2站位重建的次表层温度[26](e)
Figure 5. (a) AMOC strength of north of 60°N as simulated in the model TraCE-21[65]; (b) area-weighted mean annual temperature anomaly north of 60°N as simulated in the model TraCE-21[65]; (c) June mean insolation at 70°N[65]; (d) reconstructed sSST of planktic foraminifer Nps for core AT06 and BB04; (e) reconstructed sSST of planktic foraminifer Nps for core MSM5/5-712-2[26]
表 1 本文研究岩芯及对比岩芯信息
Table 1 Detail information of studied sites and reference sites
表 2 挪威海AT06和BB04岩芯Nps-AMS14C测年数据及校正
Table 2 Calibration of Nps-AMS 14C dating of Core AT06 and BB04
样品编号 深度/cm AMS14C年龄/aBP 碳储库校正/aBP 日历年龄/aBP UCIT33499 AT06/0-1 1365±15 965±15 857±5 UCIT32780 AT06/2-3 1965±15 1565±15 1473±4 UCIT32781 AT06/6-7 2310±15 1910±15 1856±11 BETA-407693 AT06/11-12 2710±30 2310±30 2337±13 UCIT32782 AT06/14-15 2845±15 2445±15 2488±29 UCIT32715 BB04/2-3 3335±15 2935±15 3100±18 UCIT32716 BB04/6-7 5320±15 4920±15 5631±11 UCIT32717 BB04/10-11 8980±20 8580±20 9540±4 UCIT32718 BB04/15-16 11495±30 11095±30 12982±60 BETA-407694 BB04/20-21 13420±40 13020±40 15593±116 UCIT32719 BB04/23-24 14960±40 14560±40 17744±91 UCIT32720 BB04/29-30 15925±35 15525±35 18784±50 UCIT32721 BB04/35-36 16695±45 16295±45 19668±94 -
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