Rare earth element and Sr-Nd isotopic characteristics of the sediments in Antarctic Cosmonaut Sea and their provenance significances since the late Middle-Holocene
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摘要: 开展海洋沉积物物源研究,可以更好地认识其搬运机理与区域及全球物质循环的过程。通过对南极宇航员海ANT36-C4-05岩芯沉积物的稀土元素及Sr-Nd同位素的测试分析开展了沉积物物源研究,结果显示ANT36-C4-05岩芯沉积物的稀土元素平均含量相对较高,轻、重稀土分馏明显,87Sr/86Sr平均值相对偏高,εNd(0)平均值明显偏负。沉积物的稀土元素及Sr-Nd同位素特征指示中全新世晚期以来,宇航员海沉积物主要来自于东南极普里兹湾周边陆地及恩德比地一带的高级变质岩,这些碎屑物质一方面在冰−海作用下进入宇航员海,其中南极沿岸流与南极陆坡流在其运输过程中发挥了重要作用;同时,其还可能依靠发源于南极内陆高原的下降风完成从源区到研究区的输送过程。物源端元混合模型的结果表明,岩芯沉积物绝大部分(大于70%)来自于普里兹湾地区的变质岩,来自恩德比地物质的贡献相对较少。两个源区对宇航员海沉积物的贡献存在明显差异,这与地区之间冰川、洋流及风力的差异有关:与恩德比地相比,中山站一带的冰川规模较大,运动速度较快,其对基岩的侵蚀与搬运能力更强;发源于普里兹湾底层的南极底层水可能在普里兹湾−宇航员海的物质输送过程中发挥了一定的作用;普里兹湾地区是南极下降风较为强盛的地区之一,它可以将更多的风化碎屑吹向下风向的宇航员海。这些因素综合导致了宇航员海ANT36-C4-05岩芯沉积物主要来自于普里兹湾的特征。Abstract: Carrying out researches on marine sediments’ source can help to better understand the mechanism of material transport and the process of regional and global material circulation. A sediment provenance study has been conducted by analyzing the rare earth element and Sr-Nd isotopes of the ANT36-C4-05 core sediment in the Cosmonaut Sea, Antarctic. The results show that the average content of rare earth element (REE) in the sediment is relatively high, with an obvious fractionation between light REE (LREE) and heavy REE (HREE); and the average 87Sr/86Sr value of the sediment is relatively high, while the average εNd(0) value is significantly negative. The REE and Sr-Nd isotopic characteristics indicate that since the late Middle-Holocene, the sediment is mainly originated from high-grade metamorphic rocks in the vicinity of the Prydz Bay and Enderby Land, East Antarctic. These detrital materials are partly introduced into the Cosmonaut Sea under the ice-sea interaction, with the Antarctic Coastal Current and Antarctic Slope Current playing important roles in the transportation; simultaneously, it may also rely on katabatic winds originating from the Antarctic interior plateau to complete the transport process from source areas to the study area. The results of the provenance end member mixing model show that the sediment is mainly originated from metamorphic rocks in the Prydz Bay area (>70%), while the contribution from Enderby Land is relatively small. The significant difference in the contribution of two source areas is related to the differences in glaciers, ocean currents, and wind forces between two areas: compared to Enderby Land, the glacier in the Prydz Bay area is larger in scale and moves at a faster speed, with a stronger erosive and transport capacity for the bedrock; in the meanwhile, the Antarctic Bottom Water originating from the bottom of the Prydz Bay area may play an important role during the westward transportation; and the Prydz Bay area is one of the stronger wind force regions of katabatic winds, which can contribute more bedrock debris to downwind areas, for example the Cosmonaut Sea. These factors result in the characteristic of the ANT36-C4-05 core sediment in the Cosmonaut Sea which is mainly from the Prydz Bay area.
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Key words:
- REE /
- Sr-Nd isotopes /
- Cosmonaut Sea /
- dust /
- provenance end member
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1. 引言
沉积物物源分析是基于沉积物的化学、同位素、矿物学等特征追溯其来源的研究[1]。开展物源研究,可以明确沉积物的搬运机理,对于古环境恢复,特别是对于区域构造演变、气候变化、海洋环流和冰川活动等的再现都有十分重要的意义[2-3]。
地处南半球高纬度地区的南大洋是地球上与冰川关系最为紧密的海区之一,该地不仅是太平洋、大西洋与印度洋物质交换的重要地带,其独特的洋流体系与大气系统同时还促进了南极大陆与外界物质之间的相互联系,在南大洋开展物源研究不仅可以加深对南极地区“源−汇”系统的认识,其对于认识区域之间的相互响应同样具有重要的意义[4- 5]。
研究指出,南大洋各海区沉积物的来源具有鲜明的差异:从罗斯海往东延伸至威德尔海一带的沉积物主要来源于沿岸陆地古生代−中生代的火成岩,其中南极半岛海域的沉积物还会受到新生代火成岩的影响[6-7];从威德尔海东界往东至罗斯海西界的海洋沉积物则主要来源于沿岸陆地太古代−中元古代的基岩,变质岩成分是该地沉积物的主要来源,同时还有火成岩(如花岗岩)成分的影响 [8-10]。与其他海区相比,地处南大洋印度洋扇区的宇航员海是目前南大洋调查较少的区域之一,其底部地形崎岖不平,在冰川与洋流的共同作用下形成了一个由半圆形、细长型结构以及多道海底冲沟等构成的大型沉积序列,反映了该地区沉积物的来源相对复杂[11-12]。目前,关于该地沉积物物源的认识相对单一,前人在该地物源的研究多集中于物源区的探讨上,较少能够定量分析不同源区对该地沉积物的贡献值,尤其是中全新世晚期以来,南极地区几次冷、暖事件的发生对该地沉积物成分的变化带来了一定的影响,因此关于区域物源体系的认识尚未健全[6, 9, 13]。
稀土元素(Rare Earth Element, REE)的活动性较弱(保守)且溶解度较低,能够较好地保留沉积物的物源特征信息,是常用的物源分析指标之一[14]。Sr同位素(如87Sr/86Sr)不会随温度、压力和微生物的作用而发生分馏[15],Nd同位素指标(143Nd/144Nd、εNd(0))能够反映物质在地壳的停留时间,从而判断其母岩类型及沉积物的来源[15],Sr-Nd同位素结合物源端元混合模型可以定量分析不同源区的物质对沉积物的贡献[16]。
基于以上,本文拟通过宇航员海中部的ANT36-C4-05站位岩芯沉积物的REE及Sr-Nd同位素等指标,结合前人对本站沉积物建立的年代框架[13]进行沉积物物源特征的分析,重建中全新世晚期以来宇航员海沉积物的来源及其运移机理,并结合物源端元混合模型探讨不同物源端元对宇航员海沉积物的贡献。
2. 区域背景
宇航员海位于东南极地区,南靠恩德比地(Enderby Land),北至60°S,西部与里瑟−拉森海(Riiser-Larsen Sea, RLS)以冈纳斯洋脊(Gunnerus Ridge)为界,东邻合作海(Cooperation Sea)(图1a)。该地区的地质构造较为稳定,位于恩德比地的纳皮尔杂岩体(Napier Complex)与雷纳杂岩体(Rayner Complex)是该地区主要的地质基底,该地区的基岩以形成于太古代−元古代时期的高级变质岩为主,这种基岩从该地往东一直延伸至普里兹湾沿岸陆地皆有分布[17]。
图 1 ANT36-C4-05站位(a)、区域环流(b)及下降风(c)[26]示意图恩德比地样品区引自文献[27-31];普里兹湾样品区引自文献[32];中山站引自文献[32-33];罗斯海样品区引自文献[33-35];南极半岛东北部海域样品区引自文献[36];Vostok冰芯引自文献[37-38];Dome C冰芯引自文献[38-39];Taylor冰川引自文献[40];西南极火成岩引自文献[41- 42];横贯南极山脉(Transantarctic Mountain, TAM)粉尘样品引自文献[43]Figure 1. Sketch map of Station ANT36-C4-05 (a), the regional current system (b) and the katabatic winds (c) [26]Enderby Land sample area cited from references [27-31]; Prydz Bay sample area cited from reference [32]; Zhongshan Station cited from references [32-33]; Ross Sea sample area cited from references [33-35]; Antarctic Peninsula northeasten sea sample area cited from reference [36];Vostok Ice Core cited from references [37-38];Dome C Ice Core cited from references [38-39];Taylor Glacial cited from reference [40]; West Antarctic volcanoes cited from references [41-42]; Transantarctic Mountain (TAM) dust samples cited from reference [43]该区域的洋流体系主要由南极绕极流(Antarctic Circumpolar Current, ACC)、南极沿岸流(Antarctic Coastal Current, ACoC)、南极陆坡流(Antarctic Slope Current, ASC)与威德尔环流(Weddell Gyre)构成(图1b)。该区域的ACC活跃于64°S以北的地带,其北界是ACC南部锋(southern ACC Front, sACCF),南界为ACC南部边界(Southern Boundary of ACC, SB of ACC)[18]。ACoC与ASC则是围绕南极大陆边缘自东向西流动,前者对表层与陆架地区影响较大,后者则是主要影响陆坡地区[19-20]。当ACoC与ASC遇到冈纳斯洋脊时会发生转向,前者在洋脊东部35°~45°E会形成一个顺时针旋转的环流,后者则是会沿洋脊走向前进,在越过洋脊后又重新向南沿陆坡向西前进[19, 21]。威德尔环流在该地的影响主要集中在66°S以北,30°~40°E的区域,它可以将威德尔海的冷水输送至此地[18, 20]。
雷纳冰川(Rayner Glacier)是该地主要的冰川系统,其主要分布在恩德比地的东部,从北部的凯西湾(Casey Bay)入海[22](图1b)。宇航员海海冰的范围变化极大(图1b),夏季的海冰范围在69°S左右,冬季的海冰可北扩至60°S[23-24]。
与南极其他地区相似,下降风是影响宇航员海的主要大气系统。下降风的形成与南极内陆高原表层寒冷气团的分布有关,内陆高原与沿海地区存在明显的气压梯度差,使得风可以从南极内陆高原吹向北部的南大洋,在这过程中受地转偏向力、地表摩擦力等的影响,其方向会向西偏转[25](图1c)。在下降风的影响下,南极沿海地区可以接收到来自内陆地区及沿途的风化碎屑物质。
3. 样品与方法
3.1 样品
本文的样品是在中国第36次南极科学考察中获取的ANT36-C4-05岩芯沉积物(64.69°S,44.94°E,水深3 871 m),样品长22 cm,沉积物为橄榄灰色黏土软泥,岩性较均一,为黏土质粉砂。
3.2 实验方法
3.2.1 REE及富集因子的计算
样品REE的测试方法为酸溶解法,实验在南京聚谱检测科技有限公司完成,间隔1 cm取样,共完成了22个样品的测试分析,具体步骤可见胡栟铫等[13]已发表的成果。在原始数据的基础上计算富集因子(Enrichment Factor, EF)以消除粒度对REE的影响,以便更好地分析REE的特征及其指示意义[44],公式如下:
$$ {\rm{EF}}=(X/{\rm{Al}})_{测试值}/(X/{\rm{Al}})_{标准值} \text{,} $$ (1) 式中,X代表具体的REE值,标准值为Taylor[45]统计的地壳元素丰度值。
3.2.2 Sr-Nd同位素
将经高纯盐酸淋洗且研磨后的样品(0.05 g)置于坩埚中,加入HNO3、HF和HClO4进行融样,用聚乙烯比色管定容,过离子交换柱分离Sr与Nd。使用英国Nu公司生产的Nu NPII型多接收器电感耦合等离子质谱仪(HR-MC-ICP-MS)进行Sr-Nd同位素比值的测试。Sr与Nd的全流程空白值均小于样品含量的0.3%,可忽略不计。实验在南京聚谱检测科技有限公司完成,间隔1 cm取样,共完成了22个样品的测试分析。
Sr同位素的测定采用钢研纳克检测技术股份有限公司生产的GSB G62031-90标液(1 000×10−6 Sr)来监控分析质量,87Sr/86Sr的长期测试范围为0.707 610~0.707 630,测试值为0.707 627±7×10−6,在其可控范围内,标准化值86Sr/88Sr为0.119 4[46]。Nd同位素的测定则采用该公司生产的GSB G62050-9(1 000×10−6 Nd)标液来监控分析质量,143Nd/144Nd的长期测试范围为0.512 200~0.512 210,测试值为0.512 205±6×10−6,在其可控范围内,标准化值146Nd/144Nd=0.712 9[47],εNd(0)值的计算公式如下:
$$ \rm \varepsilon Nd(0)=[(^{143}Nd/^{144}Nd)_{测试值}/(^{143}Nd/^{144}Nd)_{CHUR}-1]\times 10^4 \text{,} $$ (2) 式中,CHUR表示球粒陨石均一储库(Chondritic Uniform Reservoir),其143Nd/144Nd值为0.512 638[48]。
3.2.3 物源端元混合模型
Mills等 [16]基于热液沉积物来源的研究提出了一组以同位素比值及元素含量为参数的二端元混合方程,它能够通过同位素指标定量地分析两个端元对于沉积物的贡献程度:
$$ {R_{\rm{M}}} = ({R_{\rm{A}}} \times {X_{\rm{A}}} \times {f_{\rm{A}}} + {R_{\rm{B}}} \times {X_{\rm{B}}} \times {f_{\rm{B}}})/({X_{\rm{A}}} \times {f_{\rm{A}}} + {X_{\rm{B}}} \times {f_{\rm{B}}})\text{,} $$ (3) $$ 1 = {f_{\rm{A}}} + {f_{\rm{B}}} \text{,} $$ (4) 式中,RM代表混合物的同位素比值;RA、RB分别代表端元A和端元B的同位素比值;XA、XB分别代表端元A和端元B的相应元素含量;fA、fB分别代表端元A和端元B的混合比例。
4. 结果
4.1 年代框架
本文ANT36-C4-05岩芯沉积物年代框架的建立参考文献[13]的方法,主要包括4个有效的有机碳测年数据,选择1 120为ΔR(a)值,结合沉积物210Pbex比活度的变化趋势,将2 233 a作为本站沉积物的老碳年龄。校正后的日历年龄显示,沉积物底部的年龄为6 500 cal a BP,顶部年龄则为0 cal a BP,总体经历了中全新世晚期(6 500~4 200 cal a BP)与晚全新世(4 200~0 cal a BP)两个阶段,绝大部分处于晚全新世。沉积物的沉积速率大致呈现出“两段式”的变化特征:6 500~5 200 cal a BP的沉积速率为8.46 cm/ka,5 200~0 cal a BP的沉积速率为2.13 cm/ka,其总体经历了由快转慢的过程[13]。
4.2 REE
如表1所示,沉积物REE含量(ΣREE)平均值为216.47×10−6,轻稀土含量(ΣLREE)平均值为201.27×10−6,重稀土含量(ΣHREE)平均值为15.52×10−6;ΣLREE/ΣHREE的平均水平为12.97,沉积物轻稀土明显富集。沉积物δCe在1.06~1.08内变化,平均值为1.07,呈现正异常的特征。沉积物REE指标的标准差总体要小于0.17;变异系数整体偏小,在0.01~0.03内变化。以上指标表明,中全新世晚期以来,除ΣLREE与ΣREE外,ANT36-C4-05沉积物的REE指标总体变化较小。
表 1 沉积物稀土元素(REE)指标统计学特征Table 1. Statistical characteristics of rare earth element (REE) proxies in the sediment站位 指标 ΣLREE ΣHREE ΣREE ΣLREE/ΣHREE δCe ANT36-C4-05 最小值 186.06×10−6 14.29×10−6 200.06×10−6 12.67 1.06 最大值 210.41×10−6 16.15×10−6 225.88×10−6 13.33 1.08 平均值 201.27×10−6 15.52×10−6 216.47×10−6 12.97 1.07 标准差 5.81×10−6 0.46×10−6 6.21×10−6 0.17 0.01 变异系数 0.03 0.03 0.03 0.01 0.01 注:δCe=CeN/(La×Pr)N1/2,N表示数据经球粒陨石REE含量[49]标准化处理;LREE表示轻稀土元素,HREE表示重稀土元素。 4.3 Sr-Nd同位素
如表2所示,ANT36-C4-05沉积物的87Sr/86Sr值在0.737 6~0.739 9内变化,平均值为0.738 4;143Nd/144Nd值在0.511 5~0.511 6内变化,平均值0.511 55;εNd(0)值则位于–21.76~–20.67的范围内,平均水平为–21.26;87Sr/86Sr值与143Nd/144Nd值的标准差与变异系数整体偏小,位于0~0.001内,εNd(0)值的则稍大,分别为0.27、–0.01。沉积物Sr-Nd同位素指标的统计学特征表明,中全新世晚期以来,ANT36-C4-05沉积物的Sr-Nd同位素指标的变化较小,总体较为稳定。
表 2 沉积物Sr-Nd同位素指标统计学特征Table 2. Statistical characteristics of Sr-Nd isotopic proxies in the sediment站位 指标 87Sr/86Sr 143Nd/144Nd εNd(0) ANT36-C4-05 最小值 0.737 6 0.511 5 −21.76 最大值 0.739 9 0.511 6 −20.67 平均值 0.738 4 0.511 55 −21.26 标准差 0.000 6 <0.000 1 0.27 变异系数 0.000 8 <0.000 1 −0.01 5. 讨论
5.1 沉积物REE的物源意义
沉积物REE的EF值可以大致指示其来源,若EF值接近1则指示地壳来源,大于10则被认为是非地壳来源[50]。ANT36-C4-05沉积物REE的EF值总体位于1~2内,少部分小于1(表3),表明其总体为地壳来源。沉积物的δCe同样具有物源指示意义,δCe正异常(大于1)指示沉积物中的碳酸盐含量较低,海洋自生成分较低[51],ANT36-C4-05沉积物的δCe具有正异常特征,表明其主要来自于陆地。同时,沉积物的REE含量相对较高,LREE相对富集,ΣLREE/ΣHREE比值高,指示其母岩具有长英质成分[52]。
表 3 沉积物稀土元素的富集因子(EF)值统计Table 3. Enrichment factors (EF) of the rare earth element in the sedimentLa Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu EF 2.107 2.147 1.628 1.766 1.382 1.351 1.226 1.116 1.752 0.858 1.015 0.913 0.937 0.840 球粒陨石中的REE组分被认为未发生分异作用,经球粒陨石REE值标准化后的REE配分模式可以更好地展示样品REE的分异特征[53]。结果显示(图2),ANT36-C4-05沉积物的REE标准化配分模式明显高于上陆壳(Upper Continental Crust, UCC)[54]与包括罗斯海 [34]、南极半岛[36]等在内的西南极地区沉积物及火成岩的水平,而低于罗斯岛火成岩[35]的水平;与东南极地区沉积物及岩石的水平相比,ANT36-C4-05沉积物的REE标准化配分模式要略低于恩德比地变质岩的水平[27],尤其是后者的重稀土含量相对偏高,沉积物总体与普里兹湾沉积物[32]、中山站片麻岩[32-33]的REE标准化配分模式较为相似,这暗示了ANT36-C4-05沉积物与东南极地区的物质具有相似的组成,沉积物可能受到这些地区物质的影响。
轻、重稀土分馏特征显示(图3a),ANT36-C4-05沉积物的(La/Sm)N值要高于西南极地区沉积物与火成岩的水平[34-36],而与恩德比地变质岩[27]、普里兹湾沉积物[32]及中山站片麻岩[32-33]有重叠的范围;沉积物的(Gd/Yb)N值要高于南极半岛沉积物与火成岩[36]的水平,而与恩德比地变质岩[27]、普里兹湾地区[32-33]及罗斯海物质[33-34]的水平大致持平;两个指标都指示了ANT36-C4-05沉积物的轻、重稀土分馏程度要低于罗斯岛火成岩[35]的水平。与REE配分模式一致,ANT36-C4-05沉积物的轻、重稀土分馏程度与恩德比地变质岩及普里兹湾地区物质的水平相近,尤其是与后者的水平最为相近。
结合沉积物的Sm/Nd值进一步分析其物源特征(图3b)。ANT36-C4-05沉积物的Sm/Nd值明显低于西南极物质[34, 36]的水平,与东南极物质[27, 32-33]的水平接近;除罗斯岛的火成岩[35]外,沉积物的ΣREE值与东南极物质[27, 32-33]的水平总体较为相近,其明显要高于西南极物质[34-35]的水平。西南极地区地处有火山活动的地壳裂谷上,火山活动较多,尤其是南极半岛地区,分布有晚第四纪形成的火成岩,沉积物母岩相对年轻,其Sm/Nd值相对较大[34, 55-56];而东南极地区的地壳相对稳定,火山活动较少,基岩大多为古老的岩体,其Sm/Nd值较小[34, 55]。沉积物的REE特征指示了ANT36-C4-05沉积物与恩德比地变质岩及普里兹湾地区的物质具有相近的组分,它们是沉积物的潜在源区。
研究指出,恩德比地−普里兹湾沿岸一带的变质岩以片麻岩与麻粒岩为主[8, 17]。其中,普里兹湾两侧陆地的基岩露头主要分布着富含石榴子石、辉石、黑云母等大颗粒重矿物的片麻岩,这种片麻岩不仅是普里兹湾沉积物的重要来源,它还是有效的REE聚集体[57-58]。ANT36-C4-05沉积物的REE含量相对较高,很大可能是受到了这种片麻岩物质输入的影响。而恩德比地的变质岩由沉积在火山环境中的较古老的上地壳成分组成,随后被长英质侵入体所侵入[59],沉积物中的长英质成分很大可能源自于此。
东南极地区的古老变质岩在当地冰川的剥蚀与搬运下,其碎屑物质可以随扩张的冰川或冰流从陆地进入到北部的南大洋中[60-61]。中全新世晚期以来,东南极地区的日照量有所增加,地区气候回暖,南大洋进入季节性海冰主导阶段[62-63]。ANT36-C4-05站位地处南大洋冬、夏两季海冰带之间,其沉积速率在很长一段时间内保持较低的水平,表明其受海冰的影响较为明显[24]。当地区海冰消融时,夹杂在其中的普里兹湾周边陆地及恩德比地的变质岩碎屑在ACoC以及ASC的输送下,最终可以在本站位沉降下来[18, 20, 64]。ANT36-C4-05站位沉积物位于接近4 000 m水深的开阔海区,离南极陆架与陆坡有一定的距离,而位于该站位西部的冈纳斯洋脊在宇航员海接收源区物质的过程中发挥了重要作用。在冈纳斯洋脊的阻挡下,ACoC的方向会发生偏转,此时向西前进的洋流不仅方向会发生偏转从而在洋脊东部形成一个顺时针的环流[19],其流速也会加强,对沉积体的侵蚀也更加强烈[11, 65],其携带的物质最终可以在洋脊东侧的海域中沉降下来。当位于ACoC下层的ASC到达冈纳斯洋脊时,理应也会像ACoC一样发生偏转而形成顺时针运动的环流,使得携带的物质最终沉降在ANT36-C4-05站位附近。Stewart等[66]指出,ASC在750~1 250 m的陆坡地带具有很强的活动性,该深度以下其流量与流速逐步减弱,同时纵向上的局地环流也有所减弱,这种接近静止的环境有利于物质的沉降。
5.2 沉积物Sr-Nd同位素的物源意义
有学者基于沉积物的Sr-Nd同位素特征指出,除了常规的冰−海运输方式外,南极沿海地区还可以通过当地大气系统的输送作用接收到南极内陆地区的物质,这些被输送的物质以细颗粒的粉尘物质为主[67]。本文将ANT36-C4-05沉积物的Sr-Nd同位素指标与南极冰芯粉尘的进行对比(图4),发现ANT36-C4-05沉积物中的87Sr/86Sr值明显高于南极冰芯粉尘的水平(0.71~0.72),而εNd(0)值则是相反,其明显低于后者的水平(–9.99~4.50)[37-40]。已有研究指出,南半球中纬度地区的粉尘物质是南极大陆冰芯粉尘的主要来源,如南美洲南部与澳大利亚地区,尤其是末次盛冰期期间,这些地区的粉尘与南极冰芯粉尘的Sr-Nd同位素特征十分相似:87Sr/86Sr值较小,εNd(0)值较大[37-40, 68]。ANT36-C4-05沉积物的Sr-Nd同位素指标与南极冰芯粉尘的呈现出不同的特征,表明二者的来源不同,一定程度上指示了沉积物受包括南美洲南部及澳大利亚在内的南半球中纬度粉尘的影响要小。
与末次盛冰期相比,全新世期间,南极地区气候回暖,来自南半球中纬度的粉尘输入明显减少,南极沿岸地区一带受南极本土的粉尘影响更为明显[67]。与南极地区其他物质的Sr-Nd同位素特征相比(图4),ANT36-C4-05沉积物的87Sr/86Sr值明显高于西南极地区沉积物与火成岩的水平,而沉积物的εNd(0)值则明显要低[41- 42];ANT36-C4-05沉积物的εNd(0)值与横贯南极山脉(TAM)粉尘物质的水平相近,而沉积物的87Sr/86Sr值要高于其水平[43]。ANT36-C4-05沉积物的Sr-Nd同位素特征与西南极物质的水平具有不同的特征,尽管其εNd(0)值与TAM粉尘物质[43]的水平相近,但整体的Sr-Nd同位素特征不能很好地吻合,因此这些地区不能作为沉积物的潜在粉尘源区。
如图4所示,ANT36-C4-05沉积物的87Sr/86Sr值总体位于普里兹湾物质[33]以及恩德比地变质岩[28-31]的水平范围内。沉积物的87Sr/86Sr值越高指示其源头是风化时间较长的基岩[67],而恩德比地−普里兹湾沿岸一带的基岩露头受寒风与冰川造成的风化影响较大[22, 69],沉积物较高的87Sr/86Sr值应该与二者成分的输入有关。在εNd(0)值的特征方面,ANT36-C4-05沉积物与普里兹湾物质的水平(–18.28)较为接近[33],而恩德比地变质岩的水平却明显偏负(–50)[28-31]。Koffman等[67]指出,εNd(0)值偏负的程度越高,表明该物质主要来源于高度变质的母岩。恩德比地变质岩的形成受到高温与镁铁侵入体的影响,导致该地区基岩的变质程度较高[8, 17, 57, 69],与南极地区大部分物质的εNd(0)值相比,ANT36-C4-05沉积物εNd(0)值的偏负程度相对较高,这很大可能与恩德比地高度变质的基岩成分的输入有关。与REE特征相似,ANT36-C4-05沉积物的Sr-Nd同位素特征指示其与普里兹湾周边陆地及恩德比地的变质岩的有相似的特征,因此本文确定二者是ANT36-C4-05沉积物两个主要的物源端元。
Jamieson等[70]与Golledge等[71]指出,恩德比地−普里兹湾一带是东南极地区风化侵蚀程度及沉积通量较高的地区之一,这些被风化的基岩碎屑除了在冰川的搬运下进入南大洋,其还可以依靠南极地区的下降风完成从南极大陆到南大洋的运移过程[70]。发源于南极内陆高原的下降风在沿南极大陆表面的斜坡延伸方向完成从南极内陆向北部的南大洋的运移过程中,其不仅可以对沿途裸露的基岩进行侵蚀与剥蚀,还可以将沿途裸露的细粒基岩碎屑卷入其中,并随之吹向南大洋[72-73]。下降风在地转偏向力与地表摩擦力的影响下会向西偏转,而地处普里兹湾地区以及恩德比地纳皮尔杂岩体及雷纳杂岩体西北部的ANT36-C4-05站位恰好位于两个源区下降风的下风向,很大可能通过下降风接收到两个源区的粉尘物质。
5.3 不同物源端元的贡献
在以上基于沉积物的REE特征与Sr-Nd同位素特征的物源分析基础上,选择普里兹湾周边陆地及恩德比地的变质岩分别作为沉积物的物源端元,结合端元混合方程,计算各自的贡献值。结果显示(表4),中全新世晚期以来,ANT36-C4-05沉积物绝大部分来自普里兹湾周边陆地(中山站一带)的变质岩,恩德比地变质岩对ANT36-C4-05沉积物的贡献相对要少。
表 4 沉积物物源端元计算结果Table 4. Results of the provenance end member mixing model of the sedimentSr同位素贡献率/% Nd同位素贡献率/% 恩德比地 29 17 中山站一带 71 83 本文使用REE标准化配分模式对物源端元模拟的结果进行检验,发现模拟结果的REE标准化配分模式总体位于恩德比地变质岩[27]与中山站一带变质岩[32-33]的水平范围内,其LREE明显富集,HREE含量较少,Ce具有正异常的特征,其整体特征与ANT36-C4-05沉积物的水平相近,表明该物源端元模拟结果可靠(图5)。
两个物源端元对沉积物的贡献存在较大的差异,这可能与这两个地区之间冰川、洋流与下降风搬运能力的差异有关。位于普里兹湾南部的兰伯特冰川−埃默里冰架系统(Lambert Glacier-Amery Ice Shelf system, LAS)是东南极地区规模较大的冰川系统,其上游由8个冰川盆地组成,从上游的冰川进入下游冰架的冰流不仅流量大,且流速快,对沿途基岩的侵蚀、搬运能力较强,同时得益于两侧冰流的注入,从上游一路往下到达陆架前缘之前的冰流可再次加速并一度以1 450 m/a的极高速度冲入南大洋[74-75]。与LAS相比,位于恩德比地纳皮尔杂岩体附近的雷纳冰川,其面积相对要小,尽管该地的冰流在进入下游冰架时的最大速度可达1 000 m/a并最终经冰架前端的海湾进入南大洋,其对基岩的侵蚀与搬运能力相对要弱[75-76]。尽管两地经冰川搬运入海的物质在ACoC与ASC的作用下可以到达ANT36-C4-05站位,但受冰流量有限的影响,雷纳冰川对沿途变质岩的侵蚀、搬运能力明显要弱于LAS,使得该地区的物质对沉积物的影响要弱于普里兹湾地区的物质。
从普里兹湾到宇航员海的洋流运输,除了ACoC与ASC外,可能还有南极底层水(Antarctic Bottom Water, AABW)的参与[18]。研究指出,普里兹湾70°E地区的陆架水与冰架水混合后会形成一种密度更高的水团,这种水团顺陆坡而下最终在底层发育成该地区的AABW[77]。AABW从陆坡底部向北扩展,在地转偏向力的作用下向西进入宇航员海,在宇航员海30°E与40°E的深层海域都发现了其踪迹[21, 78]。ANT36-C4-05沉积物位于40°E附近的海底,理论上可以受到这种AABW的影响,从而接收到来自普里兹湾地区的物质。普里兹湾西部的达恩利角冰间湖一带也有AABW的生成,它在普里兹湾−宇航员海的物质搬运过程中可能也发挥了一定的作用[79-80]。
位于普里兹湾以南的埃默里冰架地区,其表层流场相对密集,是南极地区下降风较强的区域之一,在从沿岸陡峭地区进入南大洋时的风速增强之前,普里兹湾地区的下降风在经过冰架沿岸的中山站时风速会先加强[26]。而恩德比地纳皮尔杂岩体一带的表层流场相对稀疏,尽管同样受下降风的影响可以将沿途小颗粒的变质岩碎屑吹向宇航员海,但其风速与风力要弱于前者,携带与搬运物质的能力相对要弱,对沉积物的贡献可能因此要弱于前者[26, 72-73]。
6. 结论
本文在对南极宇航员海ANT36-C4-05沉积物的REE与Sr-Nd同位素特征研究的基础上,探讨了研究区沉积物的物源特征及其“源−汇”过程,主要得出以下结论:
(1)ANT36-C4-05沉积物REE的EF值基本大于1,δCe为正异常,指示其具有明显的陆源属性。沉积物的REE标准化配分模式、分馏特征及ΣREE-Sm/Nd模式与西南极物质区别明显,与东南极地区沉积物与基岩露头的成分接近,其主要来自于研究区东南部恩德比地−普里兹湾周边陆地一带的高级变质岩。这些地区的变质岩碎屑在冰−海作用下从陆地进入南大洋,而后携带这些碎屑物质的ACoC与ASC受宇航员海西部冈纳斯洋脊的阻挡作用,其方向发生偏转,在洋脊东部形成顺时针旋转的环流,并在环流的作用下最终到达本站位附近沉降下来。
(2)ANT36-C4-05沉积物的87Sr/86Sr值要高于南极冰芯粉尘的水平,其εNd(0)值则是要低于冰芯粉尘的水平,表明二者的来源不同,沉积物受南半球中纬度潜在源区粉尘物质的影响较少。沉积物的Sr-Nd同位素特征与西南极物质的水平区别明显,总体位于普里兹湾周边陆地以及恩德比地变质岩的水平范围内,指示其可能受到两个源区风尘物质的影响。物源区的风化碎屑主要是依靠下降风完成从陆地到海洋的运输过程,期间受地转偏向力、地表摩擦力影响的下降风会向西偏转,从而可以将源区的物质输送到位于下风向方位的宇航员海,并最终在本站位沉降下来。
(3)物源端元模拟结果显示,ANT36-C4-05沉积物绝大部分来自于普里兹湾周边陆地的变质岩,少部分来自恩德比地的变质岩,这可能与地区之间冰川、洋流以及下降风搬运能力的差异有关。普里兹湾地区的LAS与恩德比地的雷纳冰川相比,前者的冰川面积、冰流量较大,冰流速更快,对当地基岩的侵蚀与搬运能力要强于后者;发育于普里兹湾70°E深层海域的AABW在地转偏向力的作用下可以向西进入宇航员海,其在普里兹湾地区的物质进入宇航员海的过程中发挥了一定的作用;同时,受地形地貌的影响,普里兹湾南部的埃默里冰架是南极地区下降风较强的区域之一,其携带与搬运沿途基岩碎屑的能力要强于恩德比地的下降风。
致谢:本文研究的样品由中国第36次南极科考队伍采集,在这里对全体科考队员表示诚挚的感谢,尤其是海洋地质与地球物理工作小组的队员们,同时还要感谢“雪龙2”破冰船上的全体船员与他们在采样时付出的努力!感谢国家海洋局极地考察办公室与中国极地研究中心在本次调查中提供的帮助!
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图 1 ANT36-C4-05站位(a)、区域环流(b)及下降风(c)[26]示意图
恩德比地样品区引自文献[27-31];普里兹湾样品区引自文献[32];中山站引自文献[32-33];罗斯海样品区引自文献[33-35];南极半岛东北部海域样品区引自文献[36];Vostok冰芯引自文献[37-38];Dome C冰芯引自文献[38-39];Taylor冰川引自文献[40];西南极火成岩引自文献[41- 42];横贯南极山脉(Transantarctic Mountain, TAM)粉尘样品引自文献[43]
Fig. 1 Sketch map of Station ANT36-C4-05 (a), the regional current system (b) and the katabatic winds (c) [26]
Enderby Land sample area cited from references [27-31]; Prydz Bay sample area cited from reference [32]; Zhongshan Station cited from references [32-33]; Ross Sea sample area cited from references [33-35]; Antarctic Peninsula northeasten sea sample area cited from reference [36];Vostok Ice Core cited from references [37-38];Dome C Ice Core cited from references [38-39];Taylor Glacial cited from reference [40]; West Antarctic volcanoes cited from references [41-42]; Transantarctic Mountain (TAM) dust samples cited from reference [43]
表 1 沉积物稀土元素(REE)指标统计学特征
Tab. 1 Statistical characteristics of rare earth element (REE) proxies in the sediment
站位 指标 ΣLREE ΣHREE ΣREE ΣLREE/ΣHREE δCe ANT36-C4-05 最小值 186.06×10−6 14.29×10−6 200.06×10−6 12.67 1.06 最大值 210.41×10−6 16.15×10−6 225.88×10−6 13.33 1.08 平均值 201.27×10−6 15.52×10−6 216.47×10−6 12.97 1.07 标准差 5.81×10−6 0.46×10−6 6.21×10−6 0.17 0.01 变异系数 0.03 0.03 0.03 0.01 0.01 注:δCe=CeN/(La×Pr)N1/2,N表示数据经球粒陨石REE含量[49]标准化处理;LREE表示轻稀土元素,HREE表示重稀土元素。 表 2 沉积物Sr-Nd同位素指标统计学特征
Tab. 2 Statistical characteristics of Sr-Nd isotopic proxies in the sediment
站位 指标 87Sr/86Sr 143Nd/144Nd εNd(0) ANT36-C4-05 最小值 0.737 6 0.511 5 −21.76 最大值 0.739 9 0.511 6 −20.67 平均值 0.738 4 0.511 55 −21.26 标准差 0.000 6 <0.000 1 0.27 变异系数 0.000 8 <0.000 1 −0.01 表 3 沉积物稀土元素的富集因子(EF)值统计
Tab. 3 Enrichment factors (EF) of the rare earth element in the sediment
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu EF 2.107 2.147 1.628 1.766 1.382 1.351 1.226 1.116 1.752 0.858 1.015 0.913 0.937 0.840 表 4 沉积物物源端元计算结果
Tab. 4 Results of the provenance end member mixing model of the sediment
Sr同位素贡献率/% Nd同位素贡献率/% 恩德比地 29 17 中山站一带 71 83 -
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