南极罗斯海氧化还原敏感元素沉积地球化学特征及其古海洋意义

王家凯, 李铁刚, 熊志方, 常凤鸣, 秦秉斌, 王琳淼, 贾奇

王家凯, 李铁刚, 熊志方, 常凤鸣, 秦秉斌, 王琳淼, 贾奇. 南极罗斯海氧化还原敏感元素沉积地球化学特征及其古海洋意义[J]. 海洋地质与第四纪地质, 2018, 38(5): 112-121. DOI: 10.16562/j.cnki.0256-1492.2018.05.011
引用本文: 王家凯, 李铁刚, 熊志方, 常凤鸣, 秦秉斌, 王琳淼, 贾奇. 南极罗斯海氧化还原敏感元素沉积地球化学特征及其古海洋意义[J]. 海洋地质与第四纪地质, 2018, 38(5): 112-121. DOI: 10.16562/j.cnki.0256-1492.2018.05.011
WANG Jiakai, LI Tiegang, XIONG Zhifang, CHANG Fengming, QIN Bingbin, WANG Linmiao, JIA Qi. Sedimentary gochemical characteristics of the Redox-sensitive elements in Ross Sea, Antarctica: Implications for paleoceanography[J]. Marine Geology & Quaternary Geology, 2018, 38(5): 112-121. DOI: 10.16562/j.cnki.0256-1492.2018.05.011
Citation: WANG Jiakai, LI Tiegang, XIONG Zhifang, CHANG Fengming, QIN Bingbin, WANG Linmiao, JIA Qi. Sedimentary gochemical characteristics of the Redox-sensitive elements in Ross Sea, Antarctica: Implications for paleoceanography[J]. Marine Geology & Quaternary Geology, 2018, 38(5): 112-121. DOI: 10.16562/j.cnki.0256-1492.2018.05.011

南极罗斯海氧化还原敏感元素沉积地球化学特征及其古海洋意义

基金项目: 

青岛海洋科学与技术国家实验室鳌山科技创新计划项目 2016ASKJ13

“MIS 6期以来罗斯海深层水流通性的变化及其对大气CO2冰期旋回的影响” 41606216

国家自然科学基金项目“80万年来热带西太平洋上层水体pH和pCO2演变及影响机理” 41230959

详细信息
    作者简介:

    王家凯(1992—),男,硕士研究生,主要从事地球化学与古海洋研究,E-mail:wangjiakai15@mails.ucas.cn

    通讯作者:

    李铁刚(1965—),男,研究员,主要从事古海洋与古环境研究,E-mail:tgli@fio.org.cn

  • 中图分类号: P734.2

Sedimentary gochemical characteristics of the Redox-sensitive elements in Ross Sea, Antarctica: Implications for paleoceanography

  • 摘要: 大洋深部氧化还原环境与深部水体流通状况以及表层水体生产力密切相关。表层生产力与深部流通性变化影响着有机碳-呼吸CO2的转化及其在海洋-大气中的转移,最终与大气CO2分压(pCO2)变化密切相关。故探明大洋深部氧化还原环境的变化对于解决大气pCO2冰期旋回机制具有重要意义。本次研究以中国第31和32次南极科考获得的南极罗斯海柱状岩心ANT31-R23及表层样为研究材料。通过元素钙、钛,以及氧化还原敏感元素(RSE)锰、钼、镍、钴、镉的测试分析,以表层样中RSE与Ti的比值作为判断ANT31-R23孔中相应RSE富集程度的背景值。结果显示,Mn在沉积期均表现出富集,表明罗斯海深部在该孔沉积期为氧化环境。根据Mn在不同层位出现的富集峰识别出4次强氧化脉冲事件,可能由南大洋底层水流通性增强和/或生产力降低导致。4次氧化脉冲事件层位中Mo、Ni、Co的明显富集,是由于锰(氢)氧化物对其捕获或吸附所致。此外,推测分析认为罗斯海对冰期大气pCO2降低似乎没有明显贡献,但很可能对冰消期大气pCO2迅速升高起重要作用。然而这些有关南极罗斯海深部氧化还原环境与大气pCO2变化之间关联的推测,有待后续该孔精确年代模式的构建,方可进一步验证。
    Abstract: Redox conditions of deep ocean are supposed closely related to deep ocean circulation and surface water production. Facts prove that surface water production and deep water circulation may strongly influence the formation of respiration carbon and its migration from ocean interior to atmosphere, which is closely related to the rise of atmospheric pCO2. Hence, verifying the redox environment evolution of the ocean could help us clarify the mechanism of variation in atmospheric pCO2 in glacial-interglacial cycles. Samples from core ANT31-R23 and the surface sediment of central Ross Sea, which were taken by R/V Xuelong in the 31st and 32th Chinese National Antarctic Research Expedition, are used as research materials in this study. Both the major and minor elements are analyzed, including calcium, titanium and the elements sensitive to paleo-redox environment of deposition, so-called Redox-sensitive elements (RSE), such as manganese, molybdenum, nickel, cobalt and cadmium. RSEs normalized by Ti are adopted as background values to estimate if the RSEs are enriched or depleted. The result shows that enrichment of Mn occurs in the entire core indicating an oxidizing condition. Four strong oxidation pulse events are identified based on Mn peaks in different depths, which may be related to stronger circulation conditions and/or lower surface water production in the Southern Ocean during late Quaternary. The layers enriched by Mo, Co and Ni in addition to Mn, are resulted from absorption, capture or scavenge by Mn-oxyhydroxides. These results suggest that the Ross Sea does not have significant contribution to the reducing of atmospheric pCO2 during glaciation. The strong oxidation pulse events, however, may play an important role in elevating atmospheric pCO2 during deglaciation. Nevertheless, the detailed processes of this mechanism will be effectively revealed by follow-up work after the establishment of accurate chronology framework.
  • 查明大洋深部的氧化还原环境对于理解深部水体流通性与表层水体生产力变化具有重要意义[1-3]。水体氧化还原沉积环境通常与深部流通性密切相关[4],而流通性强弱进一步影响大洋深部碳的释放与“收押”[3, 5];例如,冰消期时南大洋深层水流通性增强可导致大气CO2分压(pCO2)迅速升高[6-8]。同时,水体氧化还原沉积环境也与表层水体生产力水平变化密切相关[9, 10],而表层水体生产力的高低能调控大洋深部氧气与有机碳反应生成呼吸CO2的含量[3, 4, 11],进而影响大气pCO2变化;例如,冰期时极高的生产力导致更多的呼吸CO2在成层化增强条件下聚集在南大洋深部[12],从而降低大气pCO2。由于深部流通性与表层生产力都与大气pCO2含量密切相关,因而探索连接这两个过程的深部氧化还原环境变化对于理解大气pCO2冰期-间冰期变化的驱动机制非常重要。

    水体的氧化还原环境根据氧含量水平从有到无可分为4类:氧化(Oxic)、亚氧化(Suboxic)、缺氧(Anoxic)和硫化缺氧(Sulfidic Anoxic或Euxinic)[13]。在排除成岩作用的影响条件下,沉积物中氧化还原敏感元素(Redox-sensitive element, RSE)的富集可以反映底层水及其水-岩界面氧含量水平的高低,因而可作为深部水体氧化还原环境的指示剂[14, 15]。不同RSE对水体氧化还原条件的敏感程度不同,因而会在不同的氧化还原环境下富集在沉积物中[13]。例如,Mn在氧化条件下以形成Mn(Ⅳ)的形式变为Mn(氢)氧化物而富集,在还原或亚氧化条件下以离子态Mn(Ⅱ)溶解在水体中[14, 16]。U在氧化条件下以溶解态U(Ⅵ)形式存在,在温和还原(缺氧)条件下则以还原态U(Ⅳ)的形式变为U3O72-或U3O84-被颗粒物质“清扫”进入沉积物中产生富集[17, 18];而Mo、Ni、Co等在氧化的水体环境下以游离态的形式存在[19, 20],在强烈还原(硫化缺氧)环境下发生富集,富集的形式有以形成溶固体的方式进入黄铁矿,或变成还原态后被金属颗粒与富硫有机质捕获[21-24]。由于不同RSE的富集能响应不同的水体氧化还原条件,因此,联合运用多个RSE而非单个RSE可更加有效地反演深部水体的氧化还原沉积环境[13]

    南极罗斯海是南大洋边缘海中深入南极洲的大海湾,也是位于全球大洋最南端的边缘海[25],其深部水体的生成强弱深刻影响着南大洋深部流通性[26]。罗斯海季节性海冰和陆架冰能影响南极底层水(Antarctic Bottom Water,AABW)的形成,成为南大洋深部水体的重要来源,对南大洋深部流通性变化至关重要[27-29],继而影响全球气候变化。罗斯海海冰与陆架冰冰量增长以及海冰持续时间增强时,罗斯海AABW与高盐陆架水(High Salt Shelf Water,HSSW,SW中盐度较高的底层水团)的产量会迅速增加[30, 31]。通过该方式生成的AABW与HSSW具有高盐高氧的特征[27, 32],可强烈影响研究区深部水体的流通性和氧气含量[33, 34],从而表明研究区适合开展水体氧化还原环境的古海洋学研究。本文首先利用罗斯海典型的表层样品,确定RSE的岩屑背景值。以此为基础,判断罗斯海柱状岩心中部分RSE的(不)富集程度,分析RSE的地球化学属性特征与相互关联机制,确定岩心沉积期的深部氧化还原环境,并试图分析研究区氧化还原环境对大气pCO2演化机制的启示。本文研究结果对于解决大气pCO2冰期旋回的驱动机制有重要意义。

    研究区位于南极锋(Polar Front,PF)以南,西邻维多利亚地(Victoria Land),南邻世界上最大的漂浮冰架—罗斯冰架(Ross Ice Shelf),东接阿蒙森海(Amundsen Sea)。研究区位于罗斯海巴雷尼岛(Balleny Island)附近,海底地形由大陆架迅速过渡到大陆斜坡,地形崎岖海脊海槽相间分布。研究区内的主要水团从上到下依次为南极表层水(Antarctic Surface Water,AASW)、SW、绕极深层水(Circumpolar Deep water,CDW)和AABW,其中深部环流以CDW和AABW为主,陆架区环流主要以陆架水(Shelf Water,SW)为主(图 1A)。AASW是冰融水形成的低温(冰点附近)低盐表层水[29],罗斯海AASW沿冰架边缘向西流动,沿Victoria land边缘向北流动最终汇入南极环极流。CDW是环南极相对温暖(1~2℃)且富含营养物质的深层水团[35]。AABW是形成于南极大陆架,位于环南极海底盆地底层的低温、高盐、极富溶解氧气的水团,由AASW、SW和CDW混合后下沉到深部形成[32]。研究区AABW则由上述低温、低盐的AASW和温暖高盐的CDW以及源自罗斯海陆架的SW一起混合形成的[32]。由于较低的水温可导致较高的氧气溶解度,研究区水体氧含量整体偏高[36],变化范围在4~8mL/L,氧气含量剖面呈“三明治”状对称分布,表层与深部水体氧含量较高,中层水体氧含量较低(图 1B)。

    图  1  南极罗斯海地理位置、水体和氧含量剖面
    图A中标示了ANT31-R23钻孔及表层样位置。图中简称:AASW为南极表层水,CDW为绕极底层水,SW为陆架水,AABW为南极底层水;图B中黑色虚线分别代表PF:极锋,SAF:亚南极锋
    Figure  1.  Basic information of geography, ocean currents and water column oxygen profile in Ross Sea, Antarctic

    重力柱状岩心ANT31-R23由中国第31次南极考察航次在罗斯海西北部取得,取样站位168°11.139′E、66°13.786′S,水深2967m,岩心总长825cm。为了获得研究区元素的碎屑背景值,选取了8个代表性的表层沉积物。这些表层沉积物由中国第32次南极考察航次通过箱式取样器在罗斯海湾内取得,取样站位范围为74°~77°S、175°~180°E(图 1),样品多为黏土质粉砂,可塑性弱无粘性,含粒径2~10cm不等的漂砾。ANT31-R23孔沉积连续,无明显生物扰动,主要为橄榄色黏土与棕色黏土。其中0~10cm样品缺失;169~179cm处浅黄棕色与深灰棕色黏土相夹,层理明显;374cm处有浅橄榄灰色平整的层理;422~443cm为浅黄棕色与深灰棕色黏土互层。对岩心按照4cm间隔取样,总共分得203个样品。

    ANT31-R23孔样品的全岩组分以及表层样品的碎屑组分用于元素地球化学分析。岩心和表层沉积物样品在-44℃条件下真空冻干48h后,用玛瑙钵研磨至200μm备用;为获得表层样品的碎屑组分,取上述干样2g置于烧杯中,加入100mL 10%的H2O2,在60℃水浴下加热4h除去有机质。样品离心清洗后,用400mL 0.1mol/L的盐酸常温浸泡24h,移出上层废液后移入聚乙烯离心管离心清洗,烘干后碾碎。岩心全岩样品与表层碎屑组分样品用氢氟酸、硝酸、高氯酸消解并蒸发至干燥状态,用王水溶解后,移至聚乙烯试管中,定容,摇匀。取部分澄清溶液,用硝酸(3+97)稀释至1000倍后,上机测定。Ti、Ca、Mn使用IRIS Intrepid XSP Ⅱ型电感耦合等离子体发射光谱仪(ICP-OES)测定;Mo、Ni、Co、Cd使用X Series Ⅱ型电感耦合等离子体质谱仪(ICP-MS)测定。本测试在中国地质科学院地球物理地球化学勘察研究所进行,分析的准确度选择中国岩石与沉积物标准物质(GBW07303a、GBW07304a、GBW07312和GBW07450)评估,Ti在各自置信值的±10%范围内,而其他元素在各自置信值的±5%范围内。分析精度用多个样品的重复性分析结果来评价,对于上述所有元素除Cd外,分析精度均优于±5%(RSD);而对于Cd,则优于±10%(RSD)。

    ANT31-R23孔Ca和Ti与Mn、Mo、Ni、Co和Cd之间没有明显的负相关(图 2),表明生源碳酸钙及陆源碎屑组分没有对沉积物组成产生稀释作用。考虑到生物硅可能对沉积物产生潜在的稀释,由于目前该孔缺乏生物硅含量数据,保险起见我们使用Ti标准化后的元素比值而非单个元素含量作为后述讨论的指标。全柱Mn/Ti总体上变化不大,一般稳定在0.14的平均水平,只在42~102、162~178、226~266和422~482cm出现明显的宽幅波峰(图 2c)。Mo/Ti、Ni/Ti和Co/Ti的变化趋势基本与Mn/Ti一致,总体上分别稳定在1.40×10-4、5.46×10-3和3.23×10-4的平均水平。唯一不同的是Co/Ti在42~102、162~178、226~266和422~482cm出现峰值,而Mo/Ti、Ni/Ti只在42~102、226~266和422~482cm出现峰值(图 2d-f)。Cd/Ti与上述元素的变化趋势明显不同,从岩心底部的9.17×10-6逐渐增加到顶部的18.45×10-6(图 2g)。变异系数(标准偏差/平均值)是对数据离散程度的直接反映,碎屑组分元素含量变异系数越小则元素分异不明显,适合作为背景值。表层沉积物碎屑组分元素含量经Ti标准化后,显示除Cd、Mo外,其他元素的变化幅度较小,变异系数均不超过20%(表 1),这表明罗斯海碎屑物质组成比较稳定。研究表明,研究区碎屑物质与本研究使用的表层样碎屑组分同源,均来自于罗斯海南部横贯南极山脉[37],因而所选表层沉积物碎屑组分适合作为判断ANT31-R23孔相关元素富集程度的标准。整体来看全柱Mn、Mo、Ni和Co的变化具有相似性,Mn富集的上述层位几乎都伴随着Mo、Co、Ni的富集。

    图  2  南极罗斯海ANT31-R23孔Ti、Ca以及Ti标准化的RSE含量深度剖面
    灰色条带指示Ti标准化的RSE富集层位;黑色三角代表相关Ti标准化元素的岩屑背景值
    Figure  2.  Major and minor element contents variation with depth of core ANT31-R23

    RSE的富集与否是分析铁锰(氢)氧化物与其他RSE关联过程以及重建氧化还原环境的重要基础。RSE的富集程度可用富集因子(Enrichment Factor,EF)来定量表征:

    $$ \text{E}{{\text{F}}_{\text{X}}}=(\text{X}/\text{T}{{\text{i}}_{\text{样品}}})/(\text{X}/\text{T}{{\text{i}}_{\text{背景}}}) $$

    其中X表示特定RSE,X/Ti的背景值为上述罗斯海表层样品X/Ti的平均值(表 1)。如果元素EF大于1则认为该元素富集,如果元素EF小于1则认为该元素亏损[13]。在氧化的海洋环境中,Mn以高度不溶的Mn(Ⅲ)和Mn(Ⅳ)氢氧化物形式(后者为主体)存在[16, 20]。在亚氧化至缺氧的环境中,Mn氢氧化物被还原到Mn(Ⅱ)(Mn2+或MnCl+),其在沉积物中能向上或向下扩散[38-40]。另外,向下扩散的Mn(Ⅱ)使孔隙水中Mn2+浓度超过相应矿物的溶解度时,就以Mn碳酸盐和Mn-Ca碳酸盐的形式沉淀下来[39, 40]。ANT31-R23孔全柱Mn都是富集的,除42~102、162~178、226~266和422~482cm处外,其他层位富集系数为1.2~8.5(图 3a),平均为1.6;在42~102、162~178、226~266和422~482cm处富集程度极高,富集系数为1.8~26.4(图 3a),平均为7.0。通过背景值可以看出ANT31-R23孔自生钙含量极低(图 2b),推测海相自生碳酸钙含量极低。故Mn的富集不是以碳酸盐形式存在,而是由于Mn在氧化环境中不稳定,并会最终被氧化成Mn(Ⅳ),从而形成Mn的(氢)氧化物在沉积物中明显富集所导致的[16, 20]

    表  1  罗斯海研究区表层沉积物碎屑组分常微量元素比率
    Table  1.  Major and minor element ratio in detrital components of surface sediments in study area of Ross Sea
    站号Mn/TiMo/Ti (×10-4)Ni/Ti (×10-4)Co/Ti (×10-4)Cd/Ti (×10-6)Ca (%)Ti (%)
    RB02B0.080.8442.8122.212.650.930.31
    RB03B0.100.6340.2920.716.361.060.29
    RB05B0.081.2437.6719.77.940.870.34
    RB06B0.100.5646.1723.912.251.000.29
    RB07B0.091.3543.6923.08.390.850.40
    RB08B0.081.5442.3822.440.750.810.44
    RB11B0.100.7145.4823.114.061.200.33
    RB16B0.101.0642.8821.732.241.110.27
    平均值0.090.9942.6722.118.080.980.33
    标准偏差0.010.362.721.3611.910.140.06
    变异系数7.11%36.59%6.38%6.18%65.90%13.95%17.75%
    下载: 导出CSV 
    | 显示表格
    图  3  南极罗斯海ANT31-R23孔不同层位RSE富集因子盒须图
    盒须图中盒的中线(水平线)、上端和下端分别代表数据的中值、75%处的值和25%处的值;须(垂直线)的上端和下端分别代表数据的极大值和极小值
    Figure  3.  Box-and-whisker plots of enrichment factors of RSE in different depths of Core ANT31-R23 in Ross Sea, Antarctic

    Mo在有氧气存在的海水中属于保守型元素,虽然是类营养盐元素具有许多生物必需微量元素的特征[19],但通常不会在浮游植物的躯体中积累也不会被水体中的颗粒物质“清扫”。其在硫化缺氧条件下会被缓慢的还原,继而被富金属元素颗粒物质与盒须图中盒的中线(水平线)、上端和下端分别代表数据的中值、75%处的值和25%处的值;须(垂直线)的上端和下端分别代表数据的极大值和极小值富硫有机质“捕获”,并最终进入沉积物产生富集[23, 24]。EFMo除42~102、226~266和422~482cm外,其他层位接近于1.4,而在42~102、226~266和422~482cm处,富集程度非常高,为1.6~16.7,平均为3.9(图 3b),Co和Ni的自生富集发生于静海相还原环境中,该环境条件下元素或作为固溶体相被摄入到Fe硫化物中或作为独立的硫化物沉淀下来[14, 41],除42~102、162~178、226~266和422~482cm外,Co在其他层位没有明显富集,在42~102、162~178、226~266和422~482cm处,EFCo的范围为1.4~6.0(图 3c),平均为2.2。除42~102、226~266和422~482cm外,Ni在其他层位没有明显富集,在42~102、226~266和422~482cm处,EFNi的范围为1.3~5.0(图 3d),平均为1.5。Cd只有一个价态Cd(Ⅱ),与Mo相似Cd也具有类营养盐行为[42, 43],海水中的Cd主要通过与有机质结合而进入水-岩界面[44],在有微量可溶硫化物存在的条件下会以CdS的形式保存下来产生自生富集[21]。EFCd的变化范围为0.5~1.2,平均为0.7,表明Cd在全柱中都没有富集。

    在42~102、162~178、226~266和422~482cm处,我们注意到在氧化条件下富集的Mn和在还原条件下富集的Mo、Co和Ni都出现了高度富集的情况。Mn、Ni、Co的富集均有可能与热液活动相关[45],同时热液活动也能向海水输入大量Fe、Mn等物质,导致区域氧化还原环境变化造成一些元素的富集[46],然而研究区未见有晚第四纪以来热液活动的报道。研究表明在氧化环境中,沉积速率较慢的大洋深部沉积物上部的游离态Mn可再矿化形成微结核,并在此过程中发生Mn、Ni和Co的自生富集[47, 48]。综合来看,上述情况的出现归因于氧化条件下锰(氢)氧化物对Mo、Co和Ni的捕获或吸附较为合理。Mo、Co和Ni可以在水体中和沉积物表面被锰(氢)氧化物“捕获”或被其“清扫”出水体,在海底沉积物中富集或形成自生微结核产生富集[47, 49, 50]。本研究中Mo、Co和Ni与Mn之间的相关系数极高,分别为0.76、0.80和0.66(图 4),极好地支持上述对Mn、Mo、Co和Ni同时富集的解释。

    图  4  南极罗斯海ANT31-R23孔Mn与Mo、Co、Ni的相关性
    Figure  4.  The correlation figure of Mn relative to Mo、Co、Ni

    ANT31-R23孔Mn在全柱中都富集,表明在该孔沉积期内罗斯海深部都为氧化条件。尽管在42~102、162~178、226~266和422~482cm,Mo、Co和Ni都有不同程度的富集,但是这些层位的富集不是由于还原环境导致的富集,如上一节所述,是由于锰(氢)氧化物捕获的结果。除这4个层位外,Mo、Co和Ni都没有富集,表明该孔沉积期内罗斯海深部没有出现还原的环境。而Cd在全柱中不富集,也进一步支持沉积期内罗斯海深部没有出现还原的环境。前人研究结果显示,晚第四纪以来的冰期-间冰期旋回中,研究区深部水体氧气含量会发生改变,但始终未出现亚氧化甚至缺氧的条件[51]。前人的这些研究结果与我们的上述推论一致。

    我们认为42~102、162~178、226~266和422~482cm 4个层位Mn极度富集,表明该孔这4个沉积期时罗斯海深部氧含量快速剧烈升高,代表 4次大规模的强烈氧化脉冲事件。由于缺乏精确的年代模式,我们不能将这4次氧化脉冲事件与具体的海洋环境联系起来,并对成因做进一步分析。这些氧化脉冲事件可能的成因有(1)氧气通过水团的形成(如SW等盐卤水的生成速率加强),从垂向上对研究区底层水氧含量进行快速补充[31, 32, 52],此外该水团流通性增强伴随着罗斯海陆架区底流搬运作用的增强[53],或将大量的Mn、Ni、Co从大陆边缘带入深海沉积[54],或进一步促进自生Mn的富集;(2)通过CDW和/或AABW流通性增强,使底层水受到侧向上富氧水体补充,从而使其氧气含量快速升高[31, 55, 56];(3)表层生产力的剧烈降低导致深部水体中有机碳的呼吸作用减弱,使氧气得以快速保存[57]。这4次氧化脉冲事件的具体成因机制,有待年龄模式揭晓后确定。

    尽管目前没有确定ANT31-R23孔的精确年代,但是前期通过高分辨率元素(如Si、Ca、Al等)扫描的定性结果与南极冰心δD和风尘记录的对比,显示ANT31-R23孔沉积期可达150ka,这表明该孔的沉积年龄已跨过末次冰期。先前研究显示南大洋冰期时生产力水平较高,“生物泵”过程将大气中的CO2以有机碳的形式固定起来并带到深部[5],与氧气发生呼吸作用生成呼吸CO2,在成层化的条件下封存起来,从而促进大气pCO2的降低[12]。由于氧气的大量消耗,此时深部水体处于亚氧化条件。假如南极罗斯海冰期对大气pCO2降低起作用,则其深部应为亚氧化条件。然而ANT31-R23孔沉积期一直处于氧化环境,这初步表明罗斯海没有贡献于冰期大气pCO2的降低,南大洋的另外一些海区应该在冰期大气pCO2的降低起重要的作用[6, 58, 59]。然而我们注意到ANT31-R23孔在4个沉积期(42~102、162~178、226~266和422~482cm)发生非常明显的氧化脉冲事件,推测这些事件可能与冰消期南大洋流通性加强有关。冰消期时,南大洋流通性增强[6, 7],可导致氧气供应加强,深部应该处于氧化环境,同时增加的流通性导致深部更多的呼吸CO2随南大洋深部上涌增强排放到大气中,从而导致大气pCO2升高[3, 60]。从这个角度来看,罗斯海很可能在冰消期大气pCO2升高中扮演重要角色。

    当然,更确切的推断和细节过程的揭示有赖于该孔准确年代模式的建立以及生产力水平的重建。鉴于最近的研究显示,对于钙质微体生物缺乏的南大洋,生源Ba、特征硅藻和磁学性质曲线可以与全球氧同位素曲线(LR04)很好地对应,从而精确厘定年代模式[61-63]。我们正在测试该孔高分辨率的Ba及其他生源要素(生物Si和P等),以期为精确探讨罗斯海在大气pCO2冰期-间冰期变化中的作用机制奠定基础。

    (1) ANT31-R23孔Mn在全柱中富集,尤其在42~102、162~178、226~266和422~482cm处高度富集,除Mo在42~102、162~178、226~266和422~482cm处,以及Ni、Co在42~102、226~266和422~482cm处明显富集外,在其他层位不富集。Cd在全柱中都不富集;

    (2) ANT31-R23孔沉积期罗斯海深部均为氧化环境,且在42~102、162~178、226~266和422~482cm 4个沉积期发生明显的氧化脉冲事件,可能是由于富氧水体从垂向或侧向的补充以及生产力的迅速降低所致。其中,第一种情况即由SW等富氧盐卤水从垂向补充所导致的氧化脉冲事件,则同时可能会伴随大量的Mn从罗斯海陆架进入沉积物中,该过程或促进Mn自生富集程度增加,增强氧化脉冲事件的信号;

    (3) Mo、Ni、Co在42~102、162~178、226~266和422~482cm处出现不同程度的富集,是由于Mn(氢)氧化物对其捕获或吸附所致;

    (4) 罗斯海似乎对冰期大气pCO2降低没有明显的贡献,但很可能对冰消期大气pCO2迅速升高起重要作用。罗斯海在大气pCO2冰期旋回中具体的作用机制和细节过程有待ANT31-R23孔建立精确的年代模式之后才能更加有效地揭示。

    致谢: 感谢国家海洋局第一海洋研究所陈志华老师提供的罗斯海表层样,感谢中国科学院海洋所曾志刚研究员以及郭景腾、施江南博士对本文提出的意见与建议。
  • 图  1   南极罗斯海地理位置、水体和氧含量剖面

    图A中标示了ANT31-R23钻孔及表层样位置。图中简称:AASW为南极表层水,CDW为绕极底层水,SW为陆架水,AABW为南极底层水;图B中黑色虚线分别代表PF:极锋,SAF:亚南极锋

    Figure  1.   Basic information of geography, ocean currents and water column oxygen profile in Ross Sea, Antarctic

    图  2   南极罗斯海ANT31-R23孔Ti、Ca以及Ti标准化的RSE含量深度剖面

    灰色条带指示Ti标准化的RSE富集层位;黑色三角代表相关Ti标准化元素的岩屑背景值

    Figure  2.   Major and minor element contents variation with depth of core ANT31-R23

    图  3   南极罗斯海ANT31-R23孔不同层位RSE富集因子盒须图

    盒须图中盒的中线(水平线)、上端和下端分别代表数据的中值、75%处的值和25%处的值;须(垂直线)的上端和下端分别代表数据的极大值和极小值

    Figure  3.   Box-and-whisker plots of enrichment factors of RSE in different depths of Core ANT31-R23 in Ross Sea, Antarctic

    图  4   南极罗斯海ANT31-R23孔Mn与Mo、Co、Ni的相关性

    Figure  4.   The correlation figure of Mn relative to Mo、Co、Ni

    表  1   罗斯海研究区表层沉积物碎屑组分常微量元素比率

    Table  1   Major and minor element ratio in detrital components of surface sediments in study area of Ross Sea

    站号Mn/TiMo/Ti (×10-4)Ni/Ti (×10-4)Co/Ti (×10-4)Cd/Ti (×10-6)Ca (%)Ti (%)
    RB02B0.080.8442.8122.212.650.930.31
    RB03B0.100.6340.2920.716.361.060.29
    RB05B0.081.2437.6719.77.940.870.34
    RB06B0.100.5646.1723.912.251.000.29
    RB07B0.091.3543.6923.08.390.850.40
    RB08B0.081.5442.3822.440.750.810.44
    RB11B0.100.7145.4823.114.061.200.33
    RB16B0.101.0642.8821.732.241.110.27
    平均值0.090.9942.6722.118.080.980.33
    标准偏差0.010.362.721.3611.910.140.06
    变异系数7.11%36.59%6.38%6.18%65.90%13.95%17.75%
    下载: 导出CSV
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  • 收稿日期:  2017-12-26
  • 修回日期:  2018-02-05
  • 刊出日期:  2018-10-27

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