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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Seasonal and provenance controls on Nd-Sr isotopic compositions of Amazon rivers suspended sediments and implications for Nd and Sr fluxes exported to the Atlantic Ocean
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Seasonal and provenance controls on Nd-Sr isotopic compositions of Amazon rivers suspended sediments and implications for Nd and Sr fluxes exported to the Atlantic Ocean

机译：亚马逊河悬浮沉积物中Nd-Sr同位素组成的季节和物源控制及其对出口到大西洋的Nd和Sr通量的影响

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This study reports daily water discharge and suspended sediment load concentrations and Sm-Nd and Rb-Sr isotopic systematics and major and trace elements concentrations of twelve monthly sampled suspended load sediments of the Solimoes and Madeira rivers, the two major Andean tributaries of the Amazon, during the year 2004. As observed in other long-term monitoring studies of Amazon Rivers, the maximum of suspended sediment load occurs before or during the rising water period (i.e. during the rainy season). In 2004, the Solimoes River exported similar to 289x10(6) tonnes of suspended sediments and the Madeira River 294x10(6) tonnes which correspond to erosion rates of 129 and 214 T/km(2)/yr for the Solimoes and Madeira rivers basins, respectively. Both the Solimoes and Madeira suspended sediments are enriched in LREE over HREE and exhibit similar MREE enrichment; the difference being that the Madeira sediments are more fractionated than the Solimoes ones. When plotted in Al2O3-CaO+Na2O-K2O diagram, the suspended sediments of the Solimoes and Madeira rivers exhibit two different weathering trends which suggests that these sediments evolved along two different trends starting from felsic rocks with different chemical compositions. The Nd isotopic compositions (epsilon Nd) of the Solimoes sediments (-8.9 to 9.9) are slightly more radiogenic than the corresponding values of the Madeira sediments (-10.8 to -12.1). The Sr-87/Sr-86 isotopic compositions of the Madeira sediments (0.728 to 0.740) are significantly more radiogenic than those of the Solimoes (0.713 to 0.717). Together with other major elements and REE evidences, these isotopic compositions suggest that the Solimoes sediments are more influenced by Andean volcanic arc detritus than the Madeira sediments that might be due to input of young basaltic products owing to strong volcanic activity in Ecuador. Mineralogical compositions as well as Nd isotopic compositions do not vary seasonally. Based on our data set and on previous published studies, we have calculated that the Nd suspended sediment flux dominates the Nd total flux (i.e., dissolved + suspended sediment) exported to the Atlantic Ocean by the Amazon River representing similar to 98% of the Nd total flux and having a global cNd isotopic composition of -10.3. Contrary to Nd isotopic compositions, Sr isotopic compositions vary seasonally in both rivers. The explanation for this variation remains unclear. We suspect increasing physical weathering during the rainy season to be the main cause of this seasonal control in favouring landslides and river bank erosion that might induce input of more radiogenic sediments not easily mobilized during low water level. We calculated that the Sr isotopic composition of the dissolved load exported by the Amazon River to the Atlantic Ocean is not seasonally dependent and remain fairly constant (0.715-0.716) By contrast, the Sr isotopic composition of the suspended load is strongly affected by the seasonal variation varying from 0.714 in the dry season to 0.730 in the rainy season. Consequently the total Sr isotopic composition (dissolved + suspended sediment) is also seasonally controlled varying from 0.716 to 0.722. We finally suggest that large seasonally controlled Sr isotopic variations of great river is a phenomenon that has been underestimated in previous paleo-climatic and paleo-oceanic studies and should be taken into account in further studies
机译：这项研究报告了亚马逊两个主要支流Solimoes和马德拉河的12个月采样悬浮负载沉积物的每日排水量和悬浮沉积物浓度，Sm-Nd和Rb-Sr同位素系统以及主要和微量元素浓度，如在2004年的Amazon Rivers的其他长期监测研究中所观察到的那样，最大的悬浮泥沙量出现在上升水期之前或期间（即雨季）。 2004年，Solimoes河的悬浮沉积物出口量约为289x10（6）吨，马德拉河的出口量为294x10（6）吨，对应于Solimoes和马德拉河流域的侵蚀速率分别为129和214 T / km（2）/年， 分别。 Solimoes和马德拉群岛的悬浮沉积物均在HREE之上的LREE中富集，并表现出相似的MREE富集。区别在于，马德拉群岛的沉积物比Solimoes沉积物更分散。当用Al2O3-CaO + Na2O-K2O图进行绘制时，Solimoes河和马德拉河的悬浮沉积物表现出两种不同的风化趋势，这表明这些沉积物从具有不同化学成分的长英质岩石开始沿两种不同的趋势演化。 Solimoes沉积物（-8.9至9.9）的Nd同位素组成（εNd）比马德拉沉积物的相应值（-10.8至-12.1）具有更高的放射成因。马德拉沉积物（0.728至0.740）的Sr-87 / Sr-86同位素组成比Solimoes（0.713至0.717）的放射性更高。与其他主要元素和REE证据一起，这些同位素组成表明，比起马德拉群岛的沉积物，索利莫斯沉积物受安第斯火山弧碎屑的影响更大，这可能是由于厄瓜多尔强烈的火山活动导致了年轻的玄武质产物的输入。矿物学组成以及Nd同位素组成没有季节性变化。根据我们的数据集和先​​前发表的研究，我们计算得出，钕悬浮泥沙通量主导着亚马逊河向大西洋输出的钕总量（即溶解+悬浮泥沙），约占钕的98％。总通量，总cNd同位素组成为-10.3。与Nd同位素组成相反，两条河流中Sr同位素组成随季节变化。这种变化的解释仍然不清楚。我们怀疑雨季期间物理天气的增加是这种季节性控制的主要原因，这有利于增加滑坡和河岸侵蚀，这可能导致输入更多的放射性沉积物，而这些沉积物在水位较低时不易动员。我们计算得出，亚马逊河向大西洋输出的溶解负荷的Sr同位素组成与季节无关，并且保持相当恒定（0.715-0.716）。相比之下，悬浮负荷的Sr同位素组成受季节的影响很大。变化从旱季的0.714到雨季的0.730不等。因此，总Sr同位素组成（溶解的+悬浮的沉积物）也受到季节性控制，范围从0.716到0.722。我们最后建议说，大河流的季节性受控Sr同位素大变化是以前的古气候和古海洋研究中低估的现象，应在进一步研究中予以考虑

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