Abstract The Congo River is the second largest river by discharge in the world and a major source of element inputs into the South Atlantic Ocean. Yet, the element fluxes and transport mechanisms across and beyond its estuary and their impacts on the marine distribution and cycling of many major and trace elements are not well understood. We present the first combined dissolved neodymium (Nd) and hafnium (Hf) isotope and rare earth element (REE) concentration distributions following the Congo River plume along its flow path off the West African coast and along a connected offshore latitudinal section at 3°S. The Congo River freshwater itself is characterized by extraordinarily high Nd and Hf concentrations of up to 4000 pmol/kg and 54 pmol/kg, and by Nd (eNd) and Hf (eHf) isotope compositions that range between -15.6 and -16.4 and between 0.35 and -1.4, respectively. Our near- and offshore data indicate that at salinities above 23 conservative mixing of Congo-derived Nd and Hf concentrations and isotopic signatures with ambient surface seawater occurs for at least 1000 km to the northwest of the river mouth. This demonstrates a large spatial extent of the influence of the Congo plume on trace metal distributions in the eastern south Atlantic surface waters. A comparison between dissolved Nd/Hf fluxes from the Congo River and the shelf zone estimated based on radium isotope compositions indicate that release from Congo-derived particulate phases likely balances strong estuarine REE/Hf removal in the low salinity zone. The combined riverine and shelf zone flux for Nd is almost twice as high as that estimated for the Amazon River, despite that the Amazon discharge is about five times higher than that of the Congo River. Even the offshore Nd flux estimated for the 3 °S transect based on radium isotope compositions still corresponds to ∼40 % of the Congo-shelf-zone flux and reaches 150±50 Mg/year for Nd. Moreover, intermediate waters below the plume are strongly affected by exchange with particulate inputs from the Congo River given that Nd isotope signatures are inconsistent with values expected from large-scale water mass mixing and instead support unradiogenic Nd release either from sinking or deposited Congo-derived detrital material. Deep and bottom water isotopic signatures are also slightly affected by interaction with particles and benthic Nd release.

中文翻译：

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刚果河羽流中溶解的钕和铪同位素以及稀土元素：追踪和量化进入东南大西洋的大陆输入

摘要 刚果河是世界第二大河流，也是南大西洋元素输入的主要来源。然而，人们对跨越和超出其河口的元素通量和运输机制及其对许多主要和微量元素的海洋分布和循环的影响尚不清楚。我们展示了第一个结合溶解的钕 (Nd) 和铪 (Hf) 同位素和稀土元素 (REE) 浓度分布，沿着刚果河羽流沿其远离西非海岸的流动路径和沿 3°S 的连接近海纬度剖面. 刚果河淡水本身的特点是 Nd 和 Hf 浓度非常高，高达 4000 pmol/kg 和 54 pmol/kg，并且 Nd (eNd) 和 Hf (eHf) 同位素组成范围在 -15.6 和 -16.4 之间以及介于0. 分别为 35 和 -1.4。我们的近海和近海数据表明，在盐度高于 23 时，刚果衍生的 Nd 和 Hf 浓度以及同位素特征与周围地表海水的保守混合发生在河口西北部至少 1000 公里处。这表明刚果羽流对南大西洋东部地表水中痕量金属分布的影响具有很大的空间范围。来自刚果河的溶解 Nd/Hf 通量与基于镭同位素组成估计的陆架区的比较表明，来自刚果的颗粒相的释放可能平衡了低盐度区强烈的河口 REE/Hf 去除。Nd 的河流和陆架带总通量几乎是亚马逊河估计值的两倍，尽管亚马逊河的流量比刚果河的流量高出约五倍。即使根据镭同位素组成估计的 3°S 断面的海上 Nd 通量仍然相当于刚果大陆架带通量的 40%，并且 Nd 达到 150±50 Mg/年。此外，考虑到 Nd 同位素特征与大规模水团混合的预期值不一致，而是支持来自下沉或沉积的刚果衍生的非放射性 Nd 释放，因此羽流下方的中间水域受到与来自刚果河的颗粒输入交换的强烈影响碎屑材料。深水和底水同位素特征也受到与颗粒相互作用和底栖 Nd 释放的轻微影响。即使是基于镭同位素组成估计的 3°S 断面的海上 Nd 通量仍然相当于刚果大陆架带通量的 40%，并且 Nd 达到 150±50 Mg/年。此外，考虑到 Nd 同位素特征与大规模水团混合的预期值不一致，而是支持来自下沉或沉积的刚果衍生的非放射性 Nd 释放，因此羽流下方的中间水域受到与来自刚果河的颗粒输入交换的强烈影响碎屑材料。深水和底水同位素特征也受到与颗粒相互作用和底栖 Nd 释放的轻微影响。即使根据镭同位素组成估计的 3°S 断面的海上 Nd 通量仍然相当于刚果大陆架带通量的 40%，并且 Nd 达到 150±50 Mg/年。此外，考虑到 Nd 同位素特征与大规模水团混合的预期值不一致，而是支持来自下沉或沉积的刚果衍生的非放射性 Nd 释放，因此羽流下方的中间水域受到与来自刚果河的颗粒输入交换的强烈影响碎屑材料。深水和底水同位素特征也受到与颗粒相互作用和底栖 Nd 释放的轻微影响。考虑到 Nd 同位素特征与大规模水团混合的预期值不一致，而是支持非放射性 Nd 从下沉或沉积的刚果衍生碎屑材料中释放出来，因此羽流下方的中间水域受到与来自刚果河的颗粒输入交换的强烈影响. 深水和底水同位素特征也受到与颗粒相互作用和底栖 Nd 释放的轻微影响。考虑到 Nd 同位素特征与大规模水团混合的预期值不一致，而是支持非放射性 Nd 从下沉或沉积的刚果衍生碎屑材料中释放出来，因此羽流下方的中间水域受到与来自刚果河的颗粒输入交换的强烈影响. 深水和底水同位素特征也受到与颗粒相互作用和底栖 Nd 释放的轻微影响。