Physical and biogeochemical controls on the distribution of dissolved cadmium and its isotopes in the Southwest Pacific Ocean
Physical and biogeochemical controls on the distribution of dissolved cadmium and its isotopes in the Southwest Pacific Ocean
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DOI：
10.1016/j.chemgeo.2018.07.021
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发表时间：
2019-04
期刊：
影响因子：
3.9
通讯作者：
M. Sieber;T. Conway;G. Souza;H. Obata;S. Takano;Y. Sohrin;Derek Vance
中科院分区：
文献类型：
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作者：
M. Sieber;T. Conway;G. Souza;H. Obata;S. Takano;Y. Sohrin;Derek Vance
镉稳定同位素比值（δ 114 Cd）已成为海洋学家研究影响生物活性痕量金属镉（Cd）在整个海洋中营养盐分布的地球化学和物理过程的有用工具。本文利用日本GEOTRACES剖面GP 19，沿沿着170°W，从南大洋64°S到赤道太平洋的溶解态Cd和δ 114 Cd的纬向样带。沿着GP 19剖面，深海（>1500 m）的溶解Cd变化很小（0.75-0.9 nmol kg−1），δ 114 Cd特征均匀（+0.26 ± 0.06‰，2SD，n= 60;相对于NIST SRM-3108）。将这些数据添加到先前发表的工作中，使我们能够计算出太平洋和南大洋深处（>1500 m）的δ 114 Cd平均值为+0.26 ± 0.10‰（2SD，n= 436）。在较高的水柱中，沿GP 19剖面的Cd沿着深度剖面表现出强烈的垂直梯度，从1500-2000 m处的最大值（高达0.9 nmol kg−1）到耗尽的表面沃茨（在赤道太平洋<0.001 nmol kg− 1）。溶解态Cd浓度的梯度与溶解态δ 114 Cd的变化有关，在中间深度（300-1500 m），溶解态δ 114 Cd的值高于深海平均值（+0.4至+0.6‰），然后在表层海洋中进一步增加到高δ 114 Cd值（高达+0.9‰）。这两种模式可以解释为一维的生物循环，包括优先吸收同位素轻镉浮游植物，这样的过程可能解释的表面图案。然而，在中间深度，观察到的强大的垂直Cd浓度和同位素梯度，而不是从南极中层水（AAIW）和亚南极模式水（SAMW）的横向等密度线的传输，这两个携带明显较低的预形成的Cd浓度和较高的δ 114 Cd值。这些在南大洋水团形成过程中形成的特征，在这些水团向北移动时明显地保留到低纬太平洋中。总体而言，Cd和δ 114 Cd沿着GP 19剖面的分布可以很好地解释为具有确定δ 114 Cd特征的水团端元的大尺度混合，强调了南大洋表层过程对西南太平洋次表层Cd等微量金属分布的重要性。然而，在区域范围内，另外两个过程可能会覆盖这种混合关系。首先，通过与附近的东南太平洋GP 16剖面的比较，我们发现赤道中间水团的δ 114 Cd特征在赤道太平洋上几乎没有纬向变化，尽管由于非均衡化而变得富含溶解Cd。我们建议，这种均匀性的解释是完全利用的镉在热带太平洋表面和镉与类似的中间沃茨的同位素签名，因此保存南方来源的同位素签名的矿化。类似地，从南太平洋到北太平洋深海Cd浓度增加约30%，这与δ 114 Cd信号几乎恒定有关。这些观测结果使我们能够限制净δ 114 Cd的Cd添加到深海，与警告，这样的信号是集成在整个太平洋，和不同的海洋制度下，如HNLC地区的矿化可能会添加Cd与不同的同位素组成的深沃茨。第二，在GP 19站靠近赤道，微妙的镉亏损（相对于磷酸盐）观察到与低氧地下沃茨，与其他研究从北.
Cadmium stable isotope ratios (δ114Cd) have become a useful tool for oceanographers investigating the biogeochemical and physical processes that affect the nutrient-like distribution of the bioactive trace metal cadmium (Cd) throughout the oceans. Here, we present a meridional transect of dissolved Cd and δ114Cd from Japanese GEOTRACES section GP19 along 170°W from 64°S in the Southern Ocean to the equatorial Pacific. Along the GP19 section, the deep ocean (>1500 m) shows small variability in dissolved Cd (0.75–0.9 nmol kg−1) and a homogeneous δ114Cd signature (+0.26 ± 0.06‰, 2SD,n= 60; relative to NIST SRM-3108). Adding these data to previously published work allows us to calculate a deep Pacific and Southern Ocean (>1500 m) mean δ114Cd of +0.26 ± 0.10‰ (2SD,n= 436). Higher in the water column, depth profiles of Cd along the GP19 section exhibit a strong vertical gradient from a maximum (up to 0.9 nmol kg−1) at 1500–2000 m up to depleted surface waters (<0.001 nmol kg−1in the equatorial Pacific). This gradient in dissolved Cd concentration is associated with changes in dissolved δ114Cd, with values higher (+0.4 to +0.6‰) than the deep ocean average at intermediate depths (300–1500 m), and then a further increase towards high δ114Cd values (up to +0.9‰) in the surface ocean. Both patterns could be explained by one-dimensional biological cycling including preferential uptake of isotopically light Cd by phytoplankton, and such processes likely explain the surface patterns. At intermediate depths, however, the observed strong vertical Cd concentration and isotopic gradients instead result from the lateral isopycnal transport of Antarctic Intermediate Water (AAIW) and Subantarctic Mode Water (SAMW), both of which carry distinctly lower pre-formed Cd concentrations and higher δ114Cd values. These pre-formed signatures, which are imparted during water-mass formation in the Southern Ocean, are clearly conserved into the lower latitude Pacific as these water masses travel northward.Overall, the distribution of Cd and δ114Cd along the GP19 section is remarkably well explained by large scale mixing of water mass endmembers with defined δ114Cd signatures, emphasizing the importance of surface Southern Ocean processes for the distribution of trace metals such as Cd in the subsurface Southwest Pacific. At the regional scale, however, two other processes may overprint this mixing relationship. First, by comparison with the nearby Southeast Pacific GP16 section, we find that the δ114Cd signature of equatorial intermediate water masses shows little zonal variation across the equatorial Pacific, despite becoming enriched in dissolved Cd due to remineralization. We propose that this uniformity is explained by complete utilization of Cd in the surface tropical Pacific and remineralization of Cd with an isotopic signature similar to intermediate waters, therefore conserving the southern-sourced isotopic signature. Similarly, the observed increase of about 30% in deep ocean Cd concentrations from the South to the North Pacific is associated with a near-constant δ114Cd signal. These observations enable us to constrain the net δ114Cd of Cd added by remineralization to the deep ocean, with the caveat that such a signal is integrated over the entire Pacific, and that remineralization under different oceanic regimes such as HNLC areas may add Cd with different isotopic compositions to deep waters. Second, at GP19 stations close to the equator, subtle Cd depletion (relative to phosphate) is observed associated with low-oxygen subsurface waters, consistent with other studies from the North …