In the North Pacific Subtropical Gyre, which is one of the largest oligotrophic regions, there is a lack of information regarding the sources and transport of trace metals through water mass mixing. Rare earth elements (REEs) are essential for tracing lithogenic sources and water mass transport. In this study, we present dissolved REE concentrations and the factors controlling their distributions in the northwest Pacific during a GEOTRACES cruise (GP09). In the surface water along 11°N, we observed input signals from the Philippine and the Hawaiian Islands, characterized by positive Eu anomalies and slightly elevated REE concentrations. By incorporating our data and published REE data from the northwest Pacific to the southeast Pacific (40°N–40°S), we demonstrated that the REE concentrations and Yb/Nd ratios can distinguish North Pacific Intermediate Water (NPIW), Antarctic Intermediate Water (AAIW), and modified AAIW. By estimating the ratio of water mass mixing, we suggest that heavy REEs are predominantly contributed by water mass mixing (e.g., 93 % ± 4% for Yb) and can be used as semi-conservative tracers to quantify the mixing of NPIW and modified AAIW at the potential density of 27.2 kg/m. At ∼800 m depth at stations K12, K13, and K14, weak negative Ce anomalies (>0.1) were observed, indicating the lateral transport of water masses imprinted with sediment signals from the Philippine Islands. In the deep waters (>1500 m) at the Luzon Strait (station K1), we propose that the weak Ce negative anomalies (>0.08) and low Yb/Nd ratios (<5), combined with the low beam transmission, are the result of particle resuspension and release. In deep water (>2000 m), combining high-resolution REE measurements with water mass analysis, our research reveals that dissolved REEs (except Ce) are dominantly controlled by water mass mixing (e.g., 80 %∼100 % for Yb and 70 %∼100 % for Nd). For the non-conservative behavior, which is not explained by water mass mixing, the residual fraction of heavy REEs (<20 % for Yb) originates from release of sinking particles (organic matter and siliceous particles), while the remaining concentrations of light REEs (<30 % for Nd) may be influenced by a combination of particle release and scavenging processes. These findings highlight valuable information about lithogenic sources and the proportions of REE distributions that are controlled by physical and biogeochemical processes. Moreover, it emphasizes the applicability of heavy REEs as effective tracers for understanding basin-scale water mass mixing in the northwest Pacific.

中文翻译：

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北太平洋副热带环流中溶解的稀土元素：岩成源和水团混合控制

北太平洋副热带环流是最大的贫营养区域之一，缺乏有关微量金属通过水团混合的来源和运输的信息。稀土元素 (REE) 对于追踪成石源和水体输送至关重要。在本研究中，我们介绍了 GEOTRACES 巡航 (GP09) 期间西北太平洋溶解的 REE 浓度以及控制其分布的因素。在北纬 11° 的地表水中，我们观察到来自菲律宾和夏威夷群岛的输入信号，其特征是 Eu 正异常和 REE 浓度略有升高。通过结合我们的数据和已发表的西北太平洋到东南太平洋（40°N–40°S）的稀土元素数据，我们证明稀土元素浓度和Yb/Nd比可以区分北太平洋中层水（NPIW）、南极中层水(AAIW)，以及修改后的AAIW。通过估计水质量混合的比率，我们建议重稀土元素主要由水质量混合贡献（例如，Yb 为 93% ± 4%），并且可以用作半保守示踪剂来量化 NPIW 和改性 AAIW 的混合势密度为 27.2 kg/m3。在 K12、K13 和 K14 站约 800 m 深度处，观测到弱负 Ce 异常（>0.1），表明带有来自菲律宾群岛沉积物信号的水团的横向输送。在吕宋海峡（K1站）深水区（>1500 m），我们提出弱Ce负异常（>0.08）和低Yb/Nd比值（<5），结合近波束传输，是颗粒再悬浮和释放的结果。在深水（>2000 m）中，结合高分辨率稀土元素测量和水质量分析，我们的研究表明，溶解的稀土元素（除了Ce）主要受水质量混合控制（例如，Yb为80%∼100%，Yb为70%） Nd 约 100%）。对于非保守行为，不能用水团混合来解释，重稀土元素的残余部分（Yb<20%）源自下沉颗粒（有机质和硅质颗粒）的释放，而轻稀土元素的剩余浓度（对于 Nd，<30%）可能会受到颗粒释放和清除过程组合的影响。这些发现突出了有关成岩来源以及受物理和生物地球化学过程控制的稀土元素分布比例的宝贵信息。此外，它强调了重稀土作为有效示踪剂的适用性，用于了解西北太平洋盆地规模的水团混合。