Strontium (Sr), neodymium (Nd) and lead (Pb) isotope values from suspended and riverbed sediments of the Plata River and from two marine sediment cores in the western subtropical South Atlantic@en

Sediments transported in rivers reflect the geology of the catchment area and its radiogenic isotopic composition. These isotopic signatures are not significantly altered by weathering, transport or deposition and hence can document variations in sediment supply. Here we present strontium (Sr), neodymium (Nd) and lead (Pb) isotope values from suspended and riverbed sediments of the Plata River drainage basin, the second largest on the South American continent, and from two marine sediment cores collected off the mouth of the Plata River in the western subtropical South Atlantic. Our modern riverine data suggest that the basin has three main sediment source areas, namely the upper Paraná River, the Uruguay River and the Andean draining rivers. Sediments from the Andean draining rivers (Salado and Bermejo Rivers) have the most radiogenic Pb (i.e. >18.5 for 206Pb/204Pb) and Sr (average of 0.726 ± 0.031; 2SD) isotopic signatures, and least radiogenic εNd values (average of -10.5 ± 3.1; 2SD). The upper Paraná and Uruguay Rivers have less radiogenic Pb (i.e. <18.5 for 206Pb/204Pb) and Sr (average of 0.715 ± 0.003; 2SD) isotopic signatures. While the upper Paraná River has average εNd values of -7.8 ± 1.1 (2SD), the Uruguay River has average εNd values of -5.9 ± 0.2 (2SD). The modern isotopic signature at the mouth of the Plata River is largely dominated by the Andean draining rivers. To reconstruct changes in sediment supply to the western subtropical South Atlantic, we used this new riverine dataset together with literature values from the Argentine continental margin to interpret the data from two marine sediment cores. The downcore records cover the last ~ 30 kyr and show two short-lived excursions (i.e. Heinrich Stadial 1 and the Younger Dryas) and two long-term trends (i.e. late Pleistocene (i.e. ~29 to 11 cal ka BP for Sr and ~29 to 25 cal ka BP for Nd), and early and mid-Holocene). We suggest that the short-lived excursions result from increased precipitation in the headlands of the Andean draining rivers during Heinrich Stadial 1 and the Younger Dryas. The late Pleistocene long-term change showed an increase in the contribution of material from the Plata River drainage basin in relation to material from the Argentine continental margin most probably due to low sea-level. The early and mid-Holocene long-term trend showed the opposite change in sediment input and was related to decreased precipitation over most of the Plata River drainage basin.

河流输运的沉积物能够反映流域汇水区的地质特征及其放射成因同位素组成。这类同位素指纹不会因风化、搬运或沉积作用发生显著改变，因此可用于示踪沉积物供给的变化。本研究针对南美大陆第二大流域——拉普拉塔河（Plata River）流域的悬浮沉积物与河床沉积物，以及采自南大西洋亚热带西部海域拉普拉塔河河口外的两个海洋沉积物岩芯，获取了锶（Sr）、钕（Nd）与铅（Pb）的同位素数据。现代河流沉积物数据表明，该流域主要存在三大沉积物物源区，分别为上巴拉那河、乌拉圭河以及源自安第斯山脉的河流。其中源自安第斯山脉的河流（萨拉多河与贝尔梅霍河）的沉积物具有最高的放射成因Pb（即206Pb/204Pb比值>18.5）与Sr（平均值为0.726±0.031；2SD）同位素指纹，同时其εNd值的放射成因程度最低（平均值为-10.5±3.1；2SD）。上巴拉那河与乌拉圭河的沉积物则具有相对较低放射成因的Pb（即206Pb/204Pb比值<18.5）与Sr（平均值为0.715±0.003；2SD）同位素指纹。其中上巴拉那河的εNd平均值为-7.8±1.1（2SD），乌拉圭河的εNd平均值为-5.9±0.2（2SD）。拉普拉塔河河口的现代同位素指纹主要由源自安第斯山脉的河流贡献主导。为重建南大西洋亚热带西部海域的沉积物供给变化，本研究结合本次获取的河流同位素数据集与阿根廷大陆边缘的已有文献数据，对两个海洋沉积物岩芯的数据进行解译。这两个海洋沉积物岩芯的沉积序列覆盖了过去约30千年（kyr），揭示出两次短期偏移事件（即海因里希冰阶1与新仙女木事件）以及两类长期演化趋势：其一为晚更新世阶段（Sr同位素对应约29~11 cal ka BP，Nd同位素对应约29~25 cal ka BP），其二为全新世早中期阶段。我们认为，在海因里希冰阶1与新仙女木事件期间，源自安第斯山脉的河流河源区降水量增加，是导致这两次短期偏移事件的成因。晚更新世的长期演化趋势表现为，相较于阿根廷大陆边缘物质，拉普拉塔河流域来源物质的贡献占比上升，这一现象大概率由低海平面环境所致。而全新世早中期的长期演化趋势则呈现出相反的沉积物输入变化，这与拉普拉塔河流域大部分区域的降水量减少相关。