The neodymium isotopic composition of waters masses in the eastern Pacific sector of the Southern Ocean
The neodymium isotopic composition of waters masses in the eastern Pacific sector of the Southern Ocean
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DOI：
10.1016/j.gca.2011.11.034
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发表时间：
2012-02-15
影响因子：
5
通讯作者：
Shoosmith, D. R.
中科院分区：
文献类型：
--
作者：
Carter, P.;Vance, D.;Shoosmith, D. R.
南极绕极洋流是海洋环流的主要组成部分之一，了解其同位素组成对于利用钕(ND)同位素追踪现在和过去的环流至关重要。本文介绍了57个新的南大洋东太平洋段水柱的Nd同位素组成分析，记录了这种变化在三维上的变化以及对这种变化的控制。水柱中部的Nd同位素数据表明，绕极深水(CDW)的均质性为-8.7+/-30.1。这种均质性反映了ACC流动中的大量ND库存，以及这种大库存在多大程度上缓冲了ACC的ND特征，使其免受来自大陆的海洋外输入，无论是来自大气的尘埃，还是来自邻近大陆的溶解和颗粒物质。CDW上升到阿蒙森海陆架，即使在这里，其ND同位素性质在水柱中部也接近保守(在600m处，End=-8.0+/-0.2)。然而，在陆架水柱的顶部和底部，Nd的同位素和浓度特征被强烈修改(最终高达-4.5)。这里获得的所有陆架水柱数据都与向底层和表层水中净添加的ND相一致，这些水的同位素组成与当地沉积物相匹配(END=-1至-2)，随后这些水保守地混合到中间水平。与这种陆架成分混合还导致向东流动时ACC中的公海表层水(ND同位素位移约为1埃单位)的显着变化。与沉积物相互作用对开阔洋底水域的影响较小，但在海底10m范围内，ND的非保守去除明显，同时伴随着同位素组成的显著变化。新的数据表明，在中层水柱中，特别是在ACC这样的大流量水流中，ND具有保守性。虽然边界交换型过程在该区域显然很重要，它们对陆架和公海表层水的影响也很大，但对CDW的主要核心没有明显的影响。这一发现很好地预示着，无论是现在还是过去，ND同位素都是一种保守的水团示踪剂。例如，这些数据表明，对通过上一次冰川周期向北输出到大西洋的南方成分深水的时间变异性的唯一可能控制是北大西洋深水在大西洋部分的输入的变化。另一方面，数据还表明，由于陆架底水的戏剧性变化和开阔大洋底水的更微妙的非保守行为，使用底水ND的沉积档案作为深水ND同位素特征的记录是谨慎的，这两个方面都表明沉积物对底层水的ND特征的修改。(C)2011爱思唯尔有限公司。保留所有权利。
The Antarctic Circumpolar Current is one of the key components of ocean circulation, and a knowledge of its isotopic composition is essential to the use of neodymium (Nd) isotopes to trace circulation now and in the past. Here we present 57 new analyses of the Nd isotopic composition of the water column in the eastern Pacific sector of the Southern Ocean, documenting both the variation in three dimensions as well as the controls on that variability.Nd isotopic data for the middle of the water column demonstrate the homogeneity of Circumpolar Deep Water (CDW) at an epsilon(Nd) value of -8.7 +/- 3 0.1. This homogeneity reflects the large Nd inventory in the ACC flow, and the degree to which this large inventory buffers the Nd characteristics of the ACC against extra-oceanic inputs from the continents, either via dust from the atmosphere or through dissolved and particulate material from the adjacent continents. CDW upwells onto the Amundsen Sea shelf and, even here, its Nd isotopic properties are close to conserved in the middle of the water column (eNd = -8.0 +/- 0.2 at 600 m). At the top and bottom of the shelf water column, however, the Nd isotopic and concentration characteristics are strongly modified (to eNd as high as -4.5). All the shelf water column data obtained here are consistent with net addition of Nd to bottom and surface waters with a contrasting isotopic composition that is matched by local sediment (eNd = -1 to -2), followed by conservative mixing of that water into intermediate levels. Mixing with this shelf composition also leads to significant modification of open ocean surface water (Nd isotopic shift around 1 epsilon unit) in the ACC as it flows eastwards. Modification of open ocean bottom waters by interaction with sediment is more subtle, but there is marked non-conservative removal of Nd accompanied by significant changes in isotopic composition in waters within 10 m of the seabed.The new data demonstrate the conservativity of Nd in the middle water column, especially for such large volume flows as the ACC. Though boundary exchange-type processes are clearly important in this region, and their imprint on both the shelf and open-ocean surface water is significant, there is no observable impact on the main core of CDW. This finding augurs well for the use of Nd isotopes as a conservative water-mass tracer now and in the past. For example, these data suggest that the only likely control on the temporal variability of southern-component deep water exported northwards into the Atlantic through the last glacial cycle is variations in the input of North Atlantic Deep Water in the Atlantic sector. On the other hand, the data also suggest caution in the use of sedimentary archives of bottom water Nd as records of deep water Nd isotopic characteristics, given the dramatic modification of bottom waters on the shelf and the more subtly non-conservative behaviour in the open ocean bottom water that are both suggestive of modification of the Nd characteristics of bottom waters by sediment. (C) 2011 Elsevier Ltd. All rights reserved.