Composition of deep water in the northwestern Indian Ocean: Evidence from Nd isotopes and redox sensitive elements of surface sediments
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摘要:
作为全球大洋循环的重要组成部分，印度洋底层水是当前深水物质循环与环境变化研究的热点，主要由南极底层水（AABW）和北大西洋深层水（NADW）组成。本研究通过对西北印度洋表层沉积物Nd同位素、氧化还原敏感元素、TOC等指标的分析，探讨了底层水的氧化还原状态与水团组成。结果表明Mn、Mo和Ni等敏感元素的富集指示研究区整体为氧化环境，在洋脊南部存在强氧化区域。εNd的分布范围为−7.16～−8，指示该区底层水团主要由AABW组成。与邻近区域的Nd同位素记录相比更重，这主要是受到一定风尘输入的影响。研究区南部较重的εNd指示了AABW贡献的增加，这可能是受到了索马里盆地内赤道强东向流的影响。本研究初步揭示了区域氧化还原状态和εNd的空间分布变化特征及其可能的影响因素，为后续对底层水的深入研究提供了重要的参考依据。
Abstract:The Indian Ocean deep water, as an important part of the global ocean circulation, is composed of mainly the Antarctic Bottom Water (AABW) and North Atlantic Deep Water (NADW), which is currently a research hot topic of marine substance circulation and environmental changes. We discussed the deep water redox state and composition of the bottom water through the analyses of Nd isotopes, redox-sensitive elements, total organic carbon of surface sediments in the northwestern Indian Ocean. Results show that sensitive elements such as Mn, Mo, and Ni indicated that the study area was generally in an oxidizing environment, and there was a strongly oxidizing area in the south side of the ridge. The εNd ranged from −7.16 to −8, indicating that the bottom water were mainly composed of AABW, and almost no NADW. Due to a certain amount of dust input, the study area has heavier εNd records compared with adjacent areas. The heavier εNd is indicative of an increased AABW contribution in the south, which may be influenced by the strong equatorial eastward flow within the Somali Basin. We revealed the characteristics of redox-sensitive elements and Nd isotopes of surface sediment and their possible influencing factors, and especially provided an important reference for subsequent in-depth research on bottom water.
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表 1 经方差极大旋转后的沉积物元素因子荷载矩阵
Table 1. Sediment element factor-loading matrix after varimax rotation
F1 F2 F3 F4 TiO2 0.9716 − 0.0017 0.2228 − 0.0685 Fe2O3 0.9589 − 0.0027 0.2611 − 0.0992 MnO 0.8044 − 0.0130 0.1926 − 0.5572 CaO 0.3284 − 0.0884 0.9353 − 0.0834 TOC −0.002 − 0.9988 0.0488 0.0040 Mo 0.9042 − 0.0080 0.1071 − 0.3773 U 0.9499 − 0.0120 0.2934 − 0.0548 V 0.9553 − 0.0006 0.2863 − 0.0594 Cu 0.8289 − 0.0137 0.3937 − 0.3671 Ni 0.9132 − 0.0035 0.1966 − 0.3434 
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表 2 印度洋表层沉积物和海水的εNd数据统计
Table 2. εNd data of surface sediments and sea water collected in Indian Ocean
站位 深度/m 地理位置 εNd 样品类型 文献来源 02-09站 3483 中印度洋脊西侧 −7.77 铁锰氧化相 本研究 02-11站 3733 中印度洋脊东侧 −7.48 02-29站 3519 中印度洋脊西侧 −7.27 03-09站 4206 卡尔斯伯格脊东侧 −8.0 03-26站 3996 卡尔斯伯格脊西侧 −7.86 04-02站 4022 卡尔斯伯格脊东侧 −7.74 04-15站 4085 卡尔斯伯格脊西侧 −7.89 04-29站 4900 卡尔斯伯格脊西侧 −7.16 SK221-05站 2700 东南阿拉伯海 −8.8 Lathika[21] SK129-CR2站 3800 查戈斯-拉克代夫海岭东侧 −9.3 Piotrowski[19] V19-188站 − 马斯克林台地东北侧 −8.1 Pahnke[40] 5B站 3684 马达加斯加盆地 −8.32 Wilson[20] 30B站 3950 马斯克林盆地 −8.38 RC11-83站 4718 开普盆地 −8.39 Rutberg[52] 02-01站 3508 中印度洋脊东侧 −10.26 去碳酸盐残留相 何州天[41] SS-3101G站 − 查戈斯-拉克代夫海岭东侧 −13.0 Goswami[44] SS-3104G站 − 东南阿拉伯海 −9.9 803站 2504 查戈斯-拉克代夫海岭西北侧 −10.2 海水 Goswami[1] 804站 2350 印度大陆南端 −10.6 1504站 4500 马达加斯加盆地 −8.5 Bertram & Elderfield[17] 1507站 4000 索马里盆地 −8.6
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