The distribution and accumulation of rare earth elements (REE) in the labile fraction of sediment cores collected from salt marshes in the Patos Lagoon estuary from southern Brazil were investigated. Sediment cores (ca. 40 cm) were obtained from three locations within the estuary to capture possible changes in REE content across the salinity gradient (i.e., where saline, brackish, and freshwater dominate). Salt marsh sediments from all three coring locations were enriched in the light REE (LREE) over the heavy REE (HREE) when normalized to the North American shale composite (NASC). Shale normalized values for the LREE of marsh sediments from sites M2 and M3 in the mid- and upper estuary commonly approximates unity indicating these sediments chiefly exhibit a terrigenous signature. In contrast, all 14 naturally occurring REE are depleted in the sediments from the M1 coring location in the lower estuary compared to shale. Sediments from the mid-estuary (M2 core location) where typical salinity values are ca. 10 practical salinity units, exhibit the greatest shale normalized LREE enrichments. The higher LREE contents of the M2 sediments likely reflect preferential removal of LREE from the water column owing to salt-induced coagulation of river-borne colloids that occurs during estuarine mixing processes. Sediments samples collected from the salt marsh in the lower estuary nearer the Atlantic Ocean (i.e., M1), have substantially lower REE contents than salt marsh sediments from sites M2 and M3 in the mid- and upper estuary, respectively. The more sand-rich lower estuary sediments are exposed to higher salinity water from the South Atlantic compared to the generally finer grained sediments from the mid- and upper estuary, which are dominated by brackish and freshwater conditions, respectively. Negative Ce-anomalies (i.e., Ce/Ce* < 1) in marsh sediments from depths generally greater than 27 cm support the occurrence of early diagenetic reactions consistent with suboxic to anoxic conditions whereby reductive dissolution of ferromanganese oxide/oxyhydroxide carrier phases, and/or Ce(IV) phases, release Ce(III) to the marsh pore waters. The negative Ce anomalies suggests that a suboxic or anoxic environment predominates to a depth of 9 cm, and then again below 27 cm depth in sediments from the M2 salt marsh core, and below a depth of 13 cm in the upper estuary marsh sediments (i.e., M3). In contrast, positive Ce anomalies (Ce/Ce* > 1) for much of the sediment from the M1 core, the mid depths of core M2 (i.e., 9 cm to 27 cm), and shallower than 13 cm in sediments from core M3 support oxic conditions where Ce is enriched in sediments from these depths by preferential scavenging from pore waters onto Fe/Mn oxides/oxyhydroxides. These trends in Ce/Ce* with depth in each sediment core are also consistent with known bioturbation that occurs within these marsh sediments. The relatively high REE contents of the sediments from location M3 are also related to the fact that these sediments are dominantly composed of clay-silt deposits that are enriched in organic matter. Finally, we find no conclusive evidence that the studied salt marsh sediments have been impacted by addition of REE from the local fertilizer manufacturing industry.

研究了从巴西南部帕托斯泻湖河口盐沼收集的沉积岩芯不稳定部分中稀土元素 (REE) 的分布和积累。从河口内的三个位置获取沉积物核心（约 40 厘米），以捕捉整个盐度梯度（即盐水、半咸水和淡水占主导地位的地方）稀土元素含量的可能变化。当标准化为北美页岩复合材料 (NASC) 时，来自所有三个取芯位置的盐沼沉积物在重稀土 (HREE) 上都富含轻稀土 (LREE)。来自河口中上段 M2 和 M3 站点的沼泽沉积物的页岩标准化值通常接近统一，表明这些沉积物主要表现出陆源特征。相比之下，与页岩相比，所有 14 种天然存在的稀土元素都在来自河口下游 M1 取芯位置的沉积物中耗尽。来自河口中部（M2 核心位置）的沉积物，其中典型的盐度值为约。10 个实用的盐度单位，表现出最大的页岩归一化轻稀土富集。M2 沉积物的 LREE 含量较高可能反映了由于河口混合过程中发生的河流胶体的盐诱导凝结，LREE 优先从水体中去除。从靠近大西洋的下河口（即 M1）的盐沼收集的沉积物样品的稀土元素含量明显低于分别来自中河口和上河口的 M2 和 M3 地点的盐沼沉积物。与分别以咸水和淡水条件为主的河口中上和上河口的一般细粒沉积物相比，更富含沙子的下河口沉积物暴露于来自南大西洋的盐度更高的水。来自深度通常大于 27 cm 的沼泽沉积物中的负 Ce 异常（即 Ce/Ce* < 1）支持早期成岩反应的发生，与低氧到缺氧条件一致，其中氧化锰/羟基氧化物载体相的还原溶解，和/或 Ce(IV) 相，将 Ce(III) 释放到沼泽孔隙水中。Ce 负异常表明低氧或缺氧环境占主导地位，深度为 9 cm，然后在 M2 盐沼核心的沉积物中再次低于 27 cm 深度，在河口上游沼泽沉积物的深度低于 13 cm（即, M3）。相比之下，来自 M1 核心的大部分沉积物的 Ce 正异常（Ce/Ce* > 1）、核心 M2 的中间深度（即 9 厘米至 27 厘米）以及来自 M3 核心的沉积物的小于 13 厘米的正异常通过优先从孔隙水中清除到 Fe/Mn 氧化物/羟基氧化物上，支持有氧条件，其中 Ce 在这些深度的沉积物中富集。Ce/Ce* 随每个沉积物核心深度的这些趋势也与这些沼泽沉积物中发生的已知生物扰动一致。M3 位置沉积物的稀土元素含量相对较高也与这些沉积物主要由富含有机质的粘土-粉砂沉积物组成有关。最后，我们没有发现确凿的证据表明所研究的盐沼沉积物受到了当地肥料制造业添加 REE 的影响。