Changes in bottom water oxygen level of the Arabian Sea and the driving factors since the Last Glacial Period
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摘要:
末次冰期以来阿拉伯海水体氧含量变化在时空上具有显著的差异。目前对其空间变化规律及主导因素尚缺乏系统的研究,尤其缺乏对千年尺度上深层水氧含量变化过程及其控制因素的综合分析。本文基于阿拉伯海中部深水区WIND-CJ06-6与WIND-CJ06-13两个岩芯的XRF岩芯扫描结果,结合前人已发表的指示阿拉伯海水体氧含量变化数据,重建了末次冰期以来千年尺度阿拉伯海不同海域和深度的水体氧含量变化历史并分析了其驱动因素。阿拉伯海水深小于1 500 m的水体在千年尺度上的氧含量变化受到表层初级生产力和中层水流通性的共同控制,但在不同时期主导因素不同;在B/A(Bølling–Ållerød)到YD(Younger Dryas)期间,阿拉伯海西北部表层生产力显著高于同时期其他海域,导致了中层水体的氧含量在西北部降低而在其他海域增高的空间差异。阿拉伯海水深大于1 500 m的水体氧含量在末次冰期以来整体上受北大西洋深层水(NADW)强弱的控制,在LGM(Last Glacial Maximum)到HS1(Heinrich stadial 1)阶段则受到南大洋通风增强的影响,水体氧含量显著升高。
Abstract:Variations in the oxygen content of water column in the Arabian Sea since the Last Glacial Period have significant differences in space and time. However, regarding the spatial variation patterns and dominating factors, systematic studies are scarce, especially on the mechanism of changes in oxygen content in deep water and the controlling factors on a millennial scale. Based on XRF core scanning results from two cores, WIND-CJ06-6 and WIND-CJ06-13, in the central deep water of the Arabian Sea and previously published data, we reconstructed the processes and analyzed the drivers of the variations in oxygen content in the Arabian Sea in different areas and depths on millennial scale since the Last Glacial Period. Results show that the variations in oxygen content in the Arabian Sea in water depths less than
1500 m on the millennial scale are controlled jointly by the surface primary productivity and mesopelagic water fluxes, and the dominant factors varied in different periods. Surface productivity in the northwestern part of the Arabian Sea was significantly higher than that in the rest of the sea during the transition period from B/A (Bølling-Ållerød) to YD (Younger Dryas) events, resulting in spatial difference: the oxygen content in the intermediate water was high in the NW Arabian Sea but low in the rest of the sea. The oxygen content in water column in the Arabian Sea at depths greater than1500 m was mainly controlled by the strength of the North Atlantic Deep Water (NADW) since the Last Glacial Maximum (LGM), and the oxygen content in water was significantly increased due to enhanced ventilation in the Southern Ocean from the LGM to the HS1 (Heinrich Stadial 1) stage.-
Keywords:
- oxygen content /
- surface productivity /
- intermediate water /
- deep water /
- Last Glacial Period /
- Arabian Sea
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全球变暖趋势的增强,海洋缺氧现象的加剧,对海洋生态系统和人类社会经济生活产生了巨大的威胁。因此,有必要深入研究驱动水体氧含量变化的各种因素及其影响机制。现代阿拉伯海西部受到南亚夏季风的驱动,发育强烈的沿岸上升流,高的表层生产力加上较弱的中层水通风作用,使阿拉伯海在水深小于1 500 m 的中层水发育了显著的最小含氧带(OMZ),这是现代全球海洋三大OMZ之一[1]。另外,阿拉伯海三面环陆,深层水来源于南大洋的绕极深层水(CDW),其组成和通风强度对阿拉伯海深层水体的氧含量起到较强的影响作用。基于海洋沉积物多指标研究发现,阿拉伯海中层和深层水氧化状态在第四纪曾发生过显著变化[2-5]。
阿拉伯海中层水氧含量受到表层生产力和中层水通风的共同控制,体现在水体的氧气供应量和消耗量的相对变化。当表层生产力升高时,相应的中层水有机质呼吸耗氧量增加,水体的氧含量降低。因此,表层生产力的变化是控制中层水氧含量的重要因素之一,而阿拉伯海表层生产力同印度季风的变化有关[4,6-7],阿拉伯海西部受到由南亚夏季风驱动的沿岸上升流的影响,夏季风期间表层生产力较高[7-8]。冬季风强盛阶段,阿拉伯海西北部受到自欧亚大陆而来的干冷东北季风的影响,海表温度降低,表层水体的垂直混合增强,使真光层的营养盐含量上升,表层生产力增高[9]。阿拉伯海东部表层生产力同样在冬季风盛行期间出现最大值,不过其原因是在冬季风期间,由孟加拉湾向阿拉伯海输入的淡盐水减少,水体层化减弱,最终导致较高的表层生产力[10]。此外,南部来源富氧中层水的强度对于阿拉伯海中层水的氧含量变化也有重要的控制作用[11-12]。
有关末次冰期以来阿拉伯海不同海域中层水氧含量变化的研究较多。阿拉伯海东部和北部研究显示,在末次盛冰期(LGM)期间受到高的表层生产力以及较弱的中层水通风影响,中层水的氧含量偏低;在北半球冷事件(HS1和YD)期间,受到较低表层生产力以及富氧南极中层水增强的影响,中层水氧含量相对于暖事件(早全新世和B/A)期间偏高[11,13]。阿拉伯海西北和西南部氮同位素的研究表明,受到南亚夏季风驱动的高表层生产力影响,末次冰期以来在北半球暖期,中层水体中反硝化作用较强且同现代水平相近,氧含量偏低;相反,在LGM以及北半球冷事件期间中层水中的反硝化作用较弱,含氧量偏高 [14-15]。最近,底栖有孔虫壳体孔隙度的研究也显示阿拉伯海东北部LGM期间中层水氧含量高于现代,但是并非受到生产力的控制,而可能是南部来源富氧的中层水输入强度的变化主导以上过程[12,16]。总之,末次冰期以来阿拉伯海中层水氧含量在千年尺度上的变化具有显著的空间差异,且驱动因素可能不同。
前人对阿拉伯海深层水的氧含量变化进行了大量的研究,由于深水区域沉积物沉积速率较低,时间分辨率不高,上述工作主要侧重于冰期-间冰期时间尺度上的对比研究。整体上,阿拉伯海深层水氧含量的变化趋势与中层水氧含量变化趋势相反,阿拉伯海深层水在LGM时期氧含量比早全新世期间偏低,可能指示了LGM时期深层水通风减弱,CDW中氧气含量降低;冰消期深海流通性恢复,深层水体的氧含量相对增加[17-20]。近期通过底栖有孔虫壳体孔隙度重建的氧含量历史记录也得到了与上述相同的结论[12]。
综上,目前针对LGM以来阿拉伯海中层水以及深层水含氧量的研究相对分散,仅个别研究对末次冰期以来阿拉伯海整体氧含量变化及其控制因素进行了总结[21],但对千年尺度上的空间变化规律和主导因素并没有进行深入分析,尤其缺乏对深层水氧含量千年尺度上变化的综合研究。本研究选取阿拉伯海中部深水区两个站位的沉积物岩芯样品进行底层水含氧量测试分析,同时收集已发表的阿拉伯海中层水和深层水水体氧含量变化数据,重点分析30 ka以来阿拉伯海不同海域和深度水体氧含量的时空变化规律,并探讨其控制因素。
1. 材料和方法
1.1 研究区概况
阿拉伯海位于印度洋西北部,为典型的边缘海。现代阿拉伯海受到南亚季风的影响,在西部产生较强的上升流,高表层生产力导致常年发育OMZ(一般定义为O2<20 μmol/kg),其深度范围为200~1 500 m,该深度水体主要受到阿拉伯海中层水的影响[22]。现代阿拉伯海中层水(约200~1 500 m)主要由3部分组成:通过曼德海峡流出的红海水(Red Sea Water,RSW)(海槛深度130 m),从霍尔木兹海峡流出的波斯湾水(Persian Gulf Water,PGW)(海槛深度50 m)和自南部流入的由至少3种来源混合的跨赤道流。跨赤道流表层百米的水体形成于副热带环流体系(30~40°S),高盐富氧;该水团以深主要由形成于40~60°S的亚南极模态水(Subantarctic Mode Water,SAMW)和南极中层水(Antarctic Intermediate Water,AAIW)构成,其盐度较低但在形成时含氧量较高。这些中层水团携带着西太平洋来源印尼中层水(Indonesian Intermediate Water,IIW)向北流入阿拉伯海(图1a、c)[23-26]。研究显示,现代阿拉伯海中层水的通风状态主要受到AAIW的影响,其他水团(包括PGW、RSW、IIW)的影响相对较小[7]。
图 1 区域水文和研究站位a:印度洋表层洋流(黑色实线指示夏季表层流,黑色虚线指示冬季表层流。SC:索马里洋流。SMC:夏季风环流;WMC:冬季风环流;WICC:西印度沿岸流;EICC:东印度沿岸流)、中层水(棕色虚线)以及深层水(紫色实线)示意图(灰色虚线框指示图b范围)改自[36-37];b:站位分布(红色三角形为本次研究站位,黑点为收集站位);c:现代阿拉伯海水体氧含量剖面图,数据来源于World Ocean Atlas 2018[38]。Figure 1. Regional hydrography and research stationsa: Indian Ocean surface currents (solid black lines indicate summer surface currents, dashed black lines were winter surface currents. SC: Somali Current. SMC: summer monsoon circulation; WMC: winter monsoon circulation; WICC: West Indian Coastal Current; EICC: East India Coastal Current), intermediate water (brown dashed line), and deep water (purple solid line) (gray dashed box indicating range in Fig.1b) adapted from [36-37]; b: station distribution (red triangles are the current study stations and black dots are collected stations); c: Modern Arabian Sea water column oxygen content profiles with data from World Ocean Atlas 2018[38].在OMZ以下,阿拉伯海深层水主要来源于北大西洋和南大洋。北大西洋深层水(North Atlantic Deep Water, NADW)向南流动并在南极绕极流中同南极底层水(AABW)混合形成CDW[27-28]。CDW通过3条路径向北流入印度洋,包括西部的深层西边界流(Deep Western Boundary Currents, DWBC),以及从中部和东部沿东经90°海岭向北的分支(图1a)[29] 。影响阿拉伯海的主要为DWBC和中部分支,其中DWBC通过索马里海盆和欧文断裂带进入阿拉伯海[27,30-31]。CDW主要由两种水团组成,包括2 000~3 800 m深度的上层绕极深层水(UCDW)和3 800 m以下的下层绕极深层水(LCDW)[29,32-33]。LCDW主要由AABW构成,高盐富氧[34-35];UCDW主要由老化的改性NADW构成,同时受到太平洋深层水(Pacific Deep Ocean, PDW)和印度洋深层水(Indian Deep Water, IDW)的影响,相对于LCDW其盐度和氧含量偏低[39-41]。
1.2 数据和方法
本研究基于阿拉伯海中部不同水深的两个岩芯:其中WIND-CJ06-6(CJ06-6)长3.6 m,水深3 680 m;WIND-CJ06-13(CJ06-13)长1.9 m,水深3 909 m。上述两个岩芯主要由黄棕色钙质软泥和灰色黏土组成。由于XRF岩芯扫描具有测试方便、无损和分辨率高等优点,通过该方法所获的Mn/Ti数据在重建底层水体氧含量变化的研究中已经得到应用[42],所以本文对研究的两根岩芯进行XRF扫描,通过对Mn/Ti比值变化的研究来重建末次冰期以来阿拉伯海深水区底层水体的氧化还原状态变化历史。利用Itrax Core Scanner XRF 岩芯扫描仪对CJ06-6以及CJ06-13两个岩芯进行扫描:高压发生器采用Mo管,X荧光射线分辨率为1 mm,电压30 kv,电流30 mA,曝光时间100 s。通过XRF扫描得到岩芯沉积物中Mn、Ti等元素的相对含量,其结果为每秒钟的计数(counts per second, cps)。两个岩芯中各选取5个层位挑取10 mg的浮游有孔虫Trilobatus sacculifer壳体进行AMS14C测试,测试由美国Beta实验室完成,测试结果利用Calib8.2.0和Marine 20校正到日历年[43],其中ΔR=42 ± 54 a。测试结果没有发现年龄倒转,表明岩芯沉积连续(表1)。
表 1 WIND-CJ06-6 和 WIND-CJ06-13孔AMS14C测年及日历年校正Table 1. AMS14C dating and calendar year correction for cores WIND-CJ06-6 and WIND-CJ06-13站位名称 深度/cm AMS14C 年龄/aBP 日历年龄/cal.aBP CJ06-6 4~5 8 900 ± 30 9 360(9 155~9 523) 24~25 11 770 ± 30 13 047(12 847~13 228) 44~45 17 220 ± 50 19 836(19 538~20 129) 64~65 24 420 ± 90 27 668(27 369~27 924) 84~85 30 360 ± 160 34 016(33 636~34 358) CJ06-13 3~4 4 040 ± 30 3 826(3 611~4 046) 23~24 11 910 ± 30 13 190(13 011~13 378) 43~44 20 180 ± 40 23 279(23 008~23 626) 63~64 27 920 ± 60 31 103(30 921~31 295) 83~84 33 610 ± 120 37 338(36 907~37 852) 此外,收集了阿拉伯海不同海域和深度已发表的30 ka以来的15N同位素、氧化还原敏感元素、底栖有孔虫种属组合以及表面孔隙度等共计25个沉积物岩芯数据来重建水体氧含量的变化过程(表2)。
表 2 研究站位汇总Table 2. Information of research stations站位 位置 水深/m 指标 来源 CJ06-13 14.54°N、65.8°E 3 909 Mn/Ti 本文 CJ06-6 16.3°N、65.8°E 3 680 Mn/Ti 本文 TN047/6GGC 17.38°N、58.8°E 3 652 有孔虫孔隙度 [44] SK304A/05 5.92°N、79.6°E 3 408 Mo/Ti [19] 3101G 6°N、74°E 2 680 Mn/Al [45] SK185-20 10°N、71.83°E 2 523 Uau [17] SK117/GC08 15.5°N、71.03°E 2 500 Mo [20] MD900963 5.05°N、73.88°E 2 446 Uau [46] SK129/CR05 9.33°N、71.98°E 2 300 U/Th [18] RC27-42 16.5°N、59.8°E 2 020 有孔虫孔隙度 [47] RC27-61 16.65°N、59.52°E 1 893 有孔虫孔隙度 [48] AAS9/21 14.51°N、72.65°E 1 807 U/Th [49] GeoB3004 14.61°N、52.92°E 1 803 有孔虫组合 [50] 3104G 12.9°N、71.9°E 1 680 Mn/Al [45] NIOP905 10.77°N、51.95°E 1 586 N同位素 [14] TN041/2PG 17.7°N、57.83°E 1 428 有孔虫孔隙度 [12] MD76-131 15.53°N、72.57°E 1 230 有孔虫组合 [51] NIOP455 23.56°N、65.95°E 1 002 Mn/Al [52] SK17 15.25°N、72.97°E 840 有孔虫组合 [53] MD04-2876 24.84°N、64°E 828 N同位素 [54] RC27-23 17.99°N、57.59°E 820 N同位素 [55] ODP723 18.05°N、57.61°E 808 N同位素 [15] SO90-111KL 23.1°N、66.49°E 774 N同位素 [56] TN041-8PG/JPC 17.81°N、57.51°E 761 有孔虫孔隙度 [57] RC27-14 18.25°N、57.66°E 596 N同位素 [55] NIOP478 24.21°N、65.66°E 565 Mn/Al [54] NIOP484 19.5°N、58.43°E 516 Mn/Al [54] 为了对比千年尺度上阿拉伯海水体含氧量的变化,本研究借鉴前人的研究方法[21],仅对所有站位的水体氧含量替代指标的变化趋势分阶段进行分析,而不考虑不同指标对水体氧含量的敏感程度。这些阶段分别为LGM—早全新世 (LGM到早全新世,下同),LGM—HS1,HS1—B/A,B/A—YD以及YD—早全新世。不同阶段分别对应的时间为:LGM:22~19 ka;HS1:17.5~15 ka;B/A:15-12.9 ka;YD:12.9~11.6 ka;早全新世:10~8 ka。
对各阶段氧含量在千年尺度上的变化进行分析时,仅统计气候期内至少存在两个数据点的站位。在分析统计结果时仅关注整体变化过程,即关注大多数站位的变化趋势,对于个别站位的不同趋势不进行深入分析。为了确保年龄框架的准确性,对于仅用氧同位素数据建立年代框架的站位(包括SK185-20、MD900963、SK129/CR05和ODP723),本文仅讨论其氧含量在冰期-间冰期尺度上的变化趋势。
1.3 水体氧含量指标
本次研究主要利用沉积物中的氧化敏感元素、N同位素、底栖有孔虫壳体孔隙度以及底栖有孔虫组合等指标来研究水体的氧含量变化。
海水的氧化还原状态控制着Mo、U、V、Mn和Fe等氧化还原敏感元素在海水和沉积物之间的分布和迁移,可以利用这些氧化还原敏感元素在沉积物中的富集状态来反映底层水体的氧化还原状态。总体上,Mn和Fe元素在氧化条件下易形成高价的氧化物/氢氧化物进入沉积物,在还原条件下则以溶解态存在于海水中[58]。Mo、V 和U等元素在氧化条件下以溶解态存在于海水中,而在还原状态下进入沉积物中[59]。因此在沉积物中,当Mn和Fe元素含量升高时指示上覆水体含氧量增加,而当Mo、V 和U等元素含量升高时则指示水体含氧量降低。在应用敏感元素指示氧化还原状态时,常使用Al、Ti等元素对敏感元素进行归一化,消除陆源输入的影响[59-60] 。
在缺氧的OMZ水体中,反硝化作用优先将水体中14NO3−去除,导致水体中15NO3−相对富集,浮游植物利用其生成的颗粒有机氮中δ15N偏高并记录在沉积物的有机质中,沉积物中高的δ15N值代表了上层水体中缺氧程度的增强[15,61]。由于这些富15NO3−的中层水体可能会随洋流从其他海域携带而来,从而影响本海域的δ15N信号,为了避免这种影响,氮同位素数据仅选取位于OMZ内部的站位 。
在低氧环境中,表生底栖有孔虫壳体的表面需要更大和/或更多的孔(即高孔隙度)以便于气体的交换,而在相对富氧的环境则主要利用壳口来进行呼吸作用[62]。当海水中氧含量低于100 mmol/kg时,底栖有孔虫Cibicidoides spp.壳体表面孔隙度同底层水氧含量(BWO)之间呈强负对数关系,该现象在阿拉伯海和全球其他海域均普遍存在[63]。
沉积物中底栖有孔虫的分布除了受到环境中食物多寡的影响外,溶解氧含量也起着重要的作用。低氧环境下喜氧的表生种底栖有孔虫含量减少,而内生种含量增加,从而使底栖有孔虫的群落结构发生变化[64-65]。所以,通过统计沉积物中底栖有孔虫的群落结构并分析优势种的生态环境偏好,可以定性反映底层水的氧化还原状态。
2. 结果
以阿拉伯海OMZ可以影响的最大深度为界,末次冰期以来阿拉伯海各站位氧含量变化趋势如图2所示。氧化还原敏感元素Mn/Ti指示,CJ06-6以及CJ06-13两站位末次冰期以来底层水的氧化状态变化基本一致,LGM到 B/A的过程中阿拉伯海深层水逐渐氧化,B/A到YD氧含量下降,此后到早全新世再次相对氧化(图2b),指示驱动两站位氧含量变化的因素可能相同,但在LGM到HS1的转变过程中CJ06-13站位氧化开始时间早于CJ06-6站位。
早全新世同LGM相比,阿拉伯海约1 500 m以浅的中层水体的氧含量降低,而OMZ以下的深层水体的氧含量相对增加(图3a)。LGM到HS1阶段,阿拉伯海中层和深层水体氧含量整体增加(图3b)。从HS1到B/A过渡期间氧含量变化情况与LGM到早全新世的变化相同,OMZ内部氧含量降低而OMZ以下深层水体氧含量增高(图3c)。从B/A到YD的转变过程中,水深超过3 000 m的站位氧含量相对降低;OMZ内部的变化出现差异,阿拉伯海西北部中层水体氧含量相对降低,而阿拉伯海东部、东北部中层水体氧含量增加(图3d)。YD到早全新世的转变过程中整个阿拉伯海的OMZ发生扩展,2 000 m以上水体整体呈现氧含量下降,2 000 m以下水体氧含量变化不大(图3e)。总体上,仅从LGM到HS1的转变过程中,阿拉伯海上层中层水体与深层水的氧含量相对增加,这与前人的研究结果相一致[21]。
图 3 阿拉伯海水体氧含量从LGM到早全新世不同阶段的变化a:LGM—早全新世,b:LGM—HS1,c:HS1—B/A,d:B/A—YD,e:YD—早全新世。其中蓝色填充代表氧含量降低,黄色填充代表氧含量增加,灰色填充代表无明显变化。正方形代表氮同位素数据、三角形为氧化还原敏感元素数据,圆点代表有孔虫数据。水平虚线代表现代OMZ的影响深度,竖直虚线代表阿拉伯海东西部的分界。各站位详细信息见表2。Figure 3. Variation of oxygen content in Arabian Sea water in different periods from LGM to Early Holocenea: LGM-Early Holocene, b: LGM-HS1, c: HS1-B/A, d: B/A-YD, e: YD-Early Holocene. Blue: decrease in oxygen content; yellow: increase in oxygen content; gray: ambiguous variation. Squares: nitrogen isotope data; triangles: redox-sensitive element data; dots: foraminiferal data. Dashed line: the depth of influence of the modern OMZ; vertical dotted line: the boundary between east and west of the Arabian Sea. Details of each station are shown in Table 2.3. 讨论
3.1 末次冰期以来阿拉伯海中层水氧含量变化及控制因素
前文已经提到,阿拉伯海中层水氧含量的变化主要受到表层生产力和中层水AAIW的影响,本文主要对比阿拉伯海不同海域的生产力和AAIW(图4)与中层水氧含量变化(图3)之间的关系,讨论影响阿拉伯海中层水体氧含量变化的因素。
图 4 末次冰期以来NADW、AAIW以及南亚夏季风强度与阿拉伯海OMZ影响区表层生产力变化的对比a:北大西洋GGC5岩芯沉积231Pa/230Th(棕色) [72]与ODP1063岩芯231Pa/230Th(绿色)指示NADW强度[73],b:南大西洋KNR159-36GGC岩芯εNd记录[67],c:印度东北部Mawmluh Cave 石笋δ 18O记录[71],d:阿拉伯海西部海域(WAS)岩芯NIOP905 Ba/Al 记录[14],e:阿拉伯海北部海域(NAS)NIOP464岩芯总有机碳(TOC)质量累积速率(MAR)[74],f:阿拉伯海东部海域(EAS)SK17岩芯富营养浮游有孔虫指数数据[53],g:阿拉伯海西北部海域(NWAS)MD00-2354岩芯初级生产力数据[9]。Figure 4. Comparison among NADW, AAIW, and South Asian in summer monsoon intensity with changes in surface productivity in the OMZ (Minimum Oxygen Zone) affected area of the Arabian Sea since the last glacial perioda: 231Pa/230Th (brown) in core GGC5 (McManus et al., 2004) and 231Pa/230Th in core ODP1063 (green) of North Atlantic Ocean, indicating NADW intensity[72], b: the εNd record of KNR159-36GGC core in South Atlantic Ocean [67], c: δ 18O record of stalagmite in Mawmluh Cave on northeast of Indian [71], d: Ba/Al record in core NIOP905 of Western Arabian Sea (WAS) [14], e: Total Organic Carbon (TOC) Mass Accumulation Rate (MAR) in core NIOP464 of the Northern Arabian Sea (NAS) [74], Eutrophic planktonic foraminiferal index. Data are from core SK17 in the eastern Arabian Sea (EAS) [53], g: Primary productivity data are from core MD00-2354 in the northwestern Arabian Sea (NWAS) [9].在冰期—间冰期尺度上,相对于LGM,早全新世期间阿拉伯海中层水氧含量整体降低(图3a)。阿拉伯海西部站位沉积记录显示早全新世期间表层生产力显著高于LGM(图4d)[4,14],表层生产力的显著增加可能主导了该阶段阿拉伯海西部中层水氧含量的变化。而阿拉伯海西北部、北部和东部的生产力记录显示早全新世期间表层生产力低于LGM期间(图4e-g)。随着冰消期海平面的上升,在冰期流量被限制的RSW和PGW得到恢复[66],以上综合因素都会导致早全新世期间中层水氧含量相对于LGM期间增加,但综合各氧含量指标显示早全新世期间中层水氧含量降低(图3a),所以应该还有其他因素存在并主导了以上过程。南大洋沉积物岩芯中的Nd同位素记录指示,在冰期和北半球冷期(包括LGM、HS1和YD)NADW生成速率减弱,富氧的AAIW生成速率增强,印度洋中层水向北通风增强[67]。相对于LGM,早全新世期间阿拉伯海中层水氧含量的整体降低可能受到富氧的AAIW流量减弱的影响更大,从而使阿拉伯海西北部、北部和东部表层生产力下降以及RSW、PGW流通性增强导致的中层水氧含量增加的信号被压制。此外,随着全新世全球温度升高导致水体氧溶解度降低,这也会导致早全新世期间中层水氧含量的降低[21]。
从LGM到HS1的转变过程中,阿拉伯海中层水氧含量整体增加(图3b)。该阶段阿拉伯海表层生产力下降(图4d-g),在HS1期间阿拉伯海表层生产力的下降可能归因于次表层水体营养盐减少[68-69]。另外,相对于LGM,HS1期间南部来源的中层水AAIW通风强度增加[67](图4b),以上两种因素可能共同导致了中层水氧含量的增加(图4)。
从HS1到B/A的转变过程中,向北移动的热带辐合带(ITCZ)驱动南亚夏季风增强(图4c)[70-71],阿拉伯海西部表层生产力增加(图3d)[14];而阿拉伯海西北部、北部和东部表层生产力下降(图4e-g)[9,47,53]。氧含量指标显示在这个过程中阿拉伯海中层水氧含量整体减小(图3c),这与阿拉伯海西部表层生产力的变化情况吻合,说明从HS1到B/A的转变过程中阿拉伯海西部中层水氧含量变化主要受到表层生产力的控制。但是,阿拉伯海西北部和东部表层生产力的变化与中层水氧含量的变化出现解耦(图4e-g)。随着从HS1向B/A转变,富氧的AAIW向北贡献减弱(图4b)[67],从而向阿拉伯海中层水中氧气输入减少,可能主导了HS1到B/A转变过程中阿拉伯海西北部、北部和东部中层水氧含量的降低。
有趣的是,从B/A到YD的转变过程中,阿拉伯海西北部中层水氧含量降低,而阿拉伯海西部、北部和东北部中层水体氧含量增加(图3d),这与表层生产力的变化情况相一致(图4d-g)。该阶段AAIW的强度有小幅度增加(图4b),说明在该时期阿拉伯海的表层生产力显著的空间变化差异主导了不同海域中层水氧含量变化。
从YD到早全新世的转变过程中,南亚夏季风增强(图4c),阿拉伯海西部表层生产力增加,而阿拉伯海西北部、北部表层生产力降低(图4d-g),综合氧含量变化数据显示中层水氧含量整体降低,指示了表层生产力可能主导了西部的OMZ变化。同时期南部来源的AAIW强度减弱(图4b),加上冬季风减弱引起的水体层化加强,共同导致了阿拉伯海其他海域中层水氧含量的减小。
总之,末次冰期以来阿拉伯海西部表层生产力的变化同中层水氧含量的变化相一致,即当表层生产力增加(降低)时,中层水氧含量降低(增高),暗示阿拉伯海西部中层水氧含量变化主要受到该海域表层生产力的控制。在北半球气候由冷向暖的转变过程中(包括LGM到早全新世、HS1到B/A、YD到早全新世),阿拉伯海西北部和北部中层水氧含量变化与表层生产力变化解耦但与中层水通风和氧气溶解度变化吻合,指示在转变过程中,中层水通风对中层水氧含量变化影响更强。在北半球气候由暖向冷的转变过程中(包括LGM到HS1,B/A到YD),阿拉伯海西北部、北部和东部表层生产力变化同中层水氧含量变化情况相一致,指示在这个过程中上述海域的表层生产力可能主导了中层水氧含量的变化,而中层水通风的影响被压制。
3.2 末次冰期以来阿拉伯海深层水氧含量变化及控制因素
海洋环流在调节全球热量收支和海洋碳库的过程中起着至关重要的作用[75-76]。阿拉伯海深层水仅来源于南大洋,因此是研究南部水源影响范围及其变化的理想海域。底层水体氧含量主要受到底层水循环和表层生产力的影响。由于海洋表层产生的有机质大部分在沉降过程中被再矿化,只有10%可以下沉到数百米,仅有1%可以沉降到4 000 m的水深[77],所以在分析影响阿拉伯海深层水体氧含量变化因素时,主要考虑底层水的通风和性质变化[78]。
阿拉伯海深层水受到CDW影响,在千年尺度上,CDW的强度变化受到NADW生成速率的影响,印度洋底层水通风年龄[79-80]以及北大西洋沉积物231Pa/230Th记录[72]均指示在北半球冷期(HS1、YD)时NADW生成速率降低(图4a),大西洋经向翻转流(AMOC)减弱,从而导致阿拉伯海深层水体流通性减弱,氧含量下降[81]。综合数据显示,除LGM到HS1阶段外,其余各阶段阿拉伯海深层水体的氧含量同NADW的生成速率同步变化(图3和图4)。当NADW生成速率增加时,阿拉伯海深层水氧含量增加(LGM—早全新世, HS1—B/A, YD—早全新世);当NADW生成速率降低时,深层水氧含量降低(B/A—YD)。 B/A—YD阶段只有水深大于3 000 m 的站位氧含量变化比较显著,说明水深越大,NADW对阿拉伯海深层水的影响越强。
与其他阶段不同,在LGM到HS1的转变过程中,NADW强度显著降低,在HS1时NADW为末次冰期以来最弱阶段(图4a),但深层水氧含量却相对增加(图3b),如本次研究的CJ06-13岩芯以及收集的TN047/6GGC和SK304A/05岩芯记录都显示在HS1早期氧含量明显增加,说明在这个过程中存在其他因素主导了阿拉伯海深层水的氧含量变化。前人研究指出,在LGM期间NADW被限制在2 000 m以浅的深度,深层水主要由AABW贡献[82-83]。此时的南大洋广泛受到海冰的覆盖并且上升流减弱[84-86],导致深层水体被隔离,氧气持续消耗,该阶段AABW的氧含量相对较低,相对低氧的AABW可能导致了阿拉伯海LGM期间氧含量相对偏低。另外,最近利用Tl同位素研究发现,末次冰消期全球海洋氧含量变化与NADW变化脱钩,在HS1等阶段虽然NADW减弱,但全球海洋氧含量却显著升高,暗示了南大洋通风强度可能控制了全球海洋氧含量的变化[87]。本次研究发现的LGM到HS1阶段深层水氧含量的增加进一步证实了南大洋深层水体通风在该阶段的显著增强,表现在南大洋表层生产力[85]和大气CO2[88]在同期的快速升高。
虽然本次研究的CJ06-6站位的水深与CJ06-13、TN047/6GGC和SK304A/05相近,但CJ06-6在LGM—HS1阶段开始氧化时间明显滞后,推测该现象受到CDW(该时期主要由AABW组成)进入阿拉伯海的路径特点所控制。如图1a所示,CJ06-6、CJ06-13和TN047/6GGC受到CDW西部分支的影响,当CDW进入阿拉伯海时应该首先影响到CJ06-13和TN047/6GGC,随后才影响到CJ06-6站位,因此CJ06-6站位的氧化时间相对滞后。虽然SK304A/05受到CDW中部分支的影响,但该站位的氧化时间与CJ06-13和TN047/6GGC站位一致。由于AABW所处水深较大,其流径受到海底地形的显著影响,导致了只有在主要受到AABW影响的LGM—HS1阶段产生了氧化滞后现象。为了证实以上推测,下一步工作中将增加相关岩芯的年龄控制点,并增加其他有关氧含量变化等指标的测试,完善上述研究。
4. 结论
(1)阿拉伯海中层水氧含量变化在千年尺度上主要受到中层水流通性、表层生产力和水体氧溶解度的控制。在B/A到YD的转变阶段,中层水氧含量受表层生产力的控制,导致中层水氧含量变化出现显著的空间差异。
(2)阿拉伯海深层水体氧含量主要受到CDW的流通性以及水体性质的影响,整体上受NADW强弱的控制;在LGM到HS1阶段主要受南大洋通风增强的影响,氧含量增加。
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图 1 区域水文和研究站位
a:印度洋表层洋流(黑色实线指示夏季表层流,黑色虚线指示冬季表层流。SC:索马里洋流。SMC:夏季风环流;WMC:冬季风环流;WICC:西印度沿岸流;EICC:东印度沿岸流)、中层水(棕色虚线)以及深层水(紫色实线)示意图(灰色虚线框指示图b范围)改自[36-37];b:站位分布(红色三角形为本次研究站位,黑点为收集站位);c:现代阿拉伯海水体氧含量剖面图,数据来源于World Ocean Atlas 2018[38]。
Figure 1. Regional hydrography and research stations
a: Indian Ocean surface currents (solid black lines indicate summer surface currents, dashed black lines were winter surface currents. SC: Somali Current. SMC: summer monsoon circulation; WMC: winter monsoon circulation; WICC: West Indian Coastal Current; EICC: East India Coastal Current), intermediate water (brown dashed line), and deep water (purple solid line) (gray dashed box indicating range in Fig.1b) adapted from [36-37]; b: station distribution (red triangles are the current study stations and black dots are collected stations); c: Modern Arabian Sea water column oxygen content profiles with data from World Ocean Atlas 2018[38].
图 3 阿拉伯海水体氧含量从LGM到早全新世不同阶段的变化
a:LGM—早全新世,b:LGM—HS1,c:HS1—B/A,d:B/A—YD,e:YD—早全新世。其中蓝色填充代表氧含量降低,黄色填充代表氧含量增加,灰色填充代表无明显变化。正方形代表氮同位素数据、三角形为氧化还原敏感元素数据,圆点代表有孔虫数据。水平虚线代表现代OMZ的影响深度,竖直虚线代表阿拉伯海东西部的分界。各站位详细信息见表2。
Figure 3. Variation of oxygen content in Arabian Sea water in different periods from LGM to Early Holocene
a: LGM-Early Holocene, b: LGM-HS1, c: HS1-B/A, d: B/A-YD, e: YD-Early Holocene. Blue: decrease in oxygen content; yellow: increase in oxygen content; gray: ambiguous variation. Squares: nitrogen isotope data; triangles: redox-sensitive element data; dots: foraminiferal data. Dashed line: the depth of influence of the modern OMZ; vertical dotted line: the boundary between east and west of the Arabian Sea. Details of each station are shown in Table 2.
图 4 末次冰期以来NADW、AAIW以及南亚夏季风强度与阿拉伯海OMZ影响区表层生产力变化的对比
a:北大西洋GGC5岩芯沉积231Pa/230Th(棕色) [72]与ODP1063岩芯231Pa/230Th(绿色)指示NADW强度[73],b:南大西洋KNR159-36GGC岩芯εNd记录[67],c:印度东北部Mawmluh Cave 石笋δ 18O记录[71],d:阿拉伯海西部海域(WAS)岩芯NIOP905 Ba/Al 记录[14],e:阿拉伯海北部海域(NAS)NIOP464岩芯总有机碳(TOC)质量累积速率(MAR)[74],f:阿拉伯海东部海域(EAS)SK17岩芯富营养浮游有孔虫指数数据[53],g:阿拉伯海西北部海域(NWAS)MD00-2354岩芯初级生产力数据[9]。
Figure 4. Comparison among NADW, AAIW, and South Asian in summer monsoon intensity with changes in surface productivity in the OMZ (Minimum Oxygen Zone) affected area of the Arabian Sea since the last glacial period
a: 231Pa/230Th (brown) in core GGC5 (McManus et al., 2004) and 231Pa/230Th in core ODP1063 (green) of North Atlantic Ocean, indicating NADW intensity[72], b: the εNd record of KNR159-36GGC core in South Atlantic Ocean [67], c: δ 18O record of stalagmite in Mawmluh Cave on northeast of Indian [71], d: Ba/Al record in core NIOP905 of Western Arabian Sea (WAS) [14], e: Total Organic Carbon (TOC) Mass Accumulation Rate (MAR) in core NIOP464 of the Northern Arabian Sea (NAS) [74], Eutrophic planktonic foraminiferal index. Data are from core SK17 in the eastern Arabian Sea (EAS) [53], g: Primary productivity data are from core MD00-2354 in the northwestern Arabian Sea (NWAS) [9].
表 1 WIND-CJ06-6 和 WIND-CJ06-13孔AMS14C测年及日历年校正
Table 1 AMS14C dating and calendar year correction for cores WIND-CJ06-6 and WIND-CJ06-13
站位名称 深度/cm AMS14C 年龄/aBP 日历年龄/cal.aBP CJ06-6 4~5 8 900 ± 30 9 360(9 155~9 523) 24~25 11 770 ± 30 13 047(12 847~13 228) 44~45 17 220 ± 50 19 836(19 538~20 129) 64~65 24 420 ± 90 27 668(27 369~27 924) 84~85 30 360 ± 160 34 016(33 636~34 358) CJ06-13 3~4 4 040 ± 30 3 826(3 611~4 046) 23~24 11 910 ± 30 13 190(13 011~13 378) 43~44 20 180 ± 40 23 279(23 008~23 626) 63~64 27 920 ± 60 31 103(30 921~31 295) 83~84 33 610 ± 120 37 338(36 907~37 852) 表 2 研究站位汇总
Table 2 Information of research stations
站位 位置 水深/m 指标 来源 CJ06-13 14.54°N、65.8°E 3 909 Mn/Ti 本文 CJ06-6 16.3°N、65.8°E 3 680 Mn/Ti 本文 TN047/6GGC 17.38°N、58.8°E 3 652 有孔虫孔隙度 [44] SK304A/05 5.92°N、79.6°E 3 408 Mo/Ti [19] 3101G 6°N、74°E 2 680 Mn/Al [45] SK185-20 10°N、71.83°E 2 523 Uau [17] SK117/GC08 15.5°N、71.03°E 2 500 Mo [20] MD900963 5.05°N、73.88°E 2 446 Uau [46] SK129/CR05 9.33°N、71.98°E 2 300 U/Th [18] RC27-42 16.5°N、59.8°E 2 020 有孔虫孔隙度 [47] RC27-61 16.65°N、59.52°E 1 893 有孔虫孔隙度 [48] AAS9/21 14.51°N、72.65°E 1 807 U/Th [49] GeoB3004 14.61°N、52.92°E 1 803 有孔虫组合 [50] 3104G 12.9°N、71.9°E 1 680 Mn/Al [45] NIOP905 10.77°N、51.95°E 1 586 N同位素 [14] TN041/2PG 17.7°N、57.83°E 1 428 有孔虫孔隙度 [12] MD76-131 15.53°N、72.57°E 1 230 有孔虫组合 [51] NIOP455 23.56°N、65.95°E 1 002 Mn/Al [52] SK17 15.25°N、72.97°E 840 有孔虫组合 [53] MD04-2876 24.84°N、64°E 828 N同位素 [54] RC27-23 17.99°N、57.59°E 820 N同位素 [55] ODP723 18.05°N、57.61°E 808 N同位素 [15] SO90-111KL 23.1°N、66.49°E 774 N同位素 [56] TN041-8PG/JPC 17.81°N、57.51°E 761 有孔虫孔隙度 [57] RC27-14 18.25°N、57.66°E 596 N同位素 [55] NIOP478 24.21°N、65.66°E 565 Mn/Al [54] NIOP484 19.5°N、58.43°E 516 Mn/Al [54] -
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