Bulk geochemistry and grain-size composition of surface sediments from the SW Atlantic

Surface sediments from the South American continental margin surrounding tbe Argentine Basin were studied with respect to bulk geochemistry (Caeo) and C ) and grain-size composition (sand/silt/clay relation and terrigenous silt grain-size distribution). The grain-size distributions of the terrigenous silt fraction were unmixed into three end members (EMs), using an end-member modelling algorithm. Three unimodal EMs appear to satisfactorily explain the variations in the data set of the grain-size distributions ofterrigenous silt. The EMs are related to sediment supply by rivers, downslope transport, winnowing, dispersal and re-deposition by currents. The bulk geochemical composition was used to trace the distribution of prominent water masses within the vertical profile. The sediments of the eastern South American continental margin are generally divided into a coarse-grained and carbonate-depleted southwestern part, and a finer-grained and carbonate-rich northeastern part. The transition of both environments is located at the position of the Brazil Malvinas Confluence (BMC). The sediments below the confluence mixing zone of the Malvinas and Brazil Currents and its extensions are characterised by high concentrations of organic carbon, low carbonate contents and high proportions of the intennediate grain-size end member. Tracing these properties, the BMC emerges as a distinct north-south striking feature centered at 52-54°W crossing the continental margin diagonally. Adjacent to this prominent feature in the southwest, the direct detrital sediment discharge of the Rio de la Plata is clearly recognised by a downslope tongue of sand and high proportions of the coarsest EM. A similar coarse grain-size composition extends further south along the continental slope. However, it displays bener sorting due to intense winnowing by the vigorous Malvinas Current. Fine-grained sedimentary deposition zones are located at the southwestern deeper part of the Rio Grande Rise and the southern abyssal Brazil Basin, both within the AABW domain. Less conspicuous winnowing/accumulation panerns are indicated north of the La Plata within the NADW level according to the continental margin topography. We demonstrate that combined bulk geochemical and grain-size properties of surface sediments, unmixed with an end-member algorithm, provide a powerful tool to reconstruct the complex interplay of sedimentology and oceanography along a time slice.本研究针对环绕阿根廷盆地的南美大陆边缘表层沉积物，开展了常量地球化学（bulk geochemistry，含碳（C）、氧化钙（CaO）等指标）与粒度组成（砂/粉砂/黏土占比关系及陆源粉砂粒度分布）的研究。研究人员采用端元模拟算法（end-member modelling algorithm），将陆源粉砂组分的粒度分布解混为三个端元（end members, EMs）。三个单峰端元可较好地解释陆源粉砂粒度分布数据集的变化特征。这些端元与河流沉积物补给、坡地搬运、流体力分选、输运及再沉积过程密切相关。常量地球化学组成被用于示踪垂向剖面内典型水团的分布特征。南美大陆边缘东部的沉积物通常可分为两个区域：西南部为粗粒且贫碳酸盐的区域，东北部为细粒且富碳酸盐的区域。两类沉积环境的过渡带位于巴西-马尔维纳斯汇合区（Brazil Malvinas Confluence, BMC）所处位置。马尔维纳斯海流与巴西海流汇合混合带及其延伸区域下方的沉积物，具有高有机碳浓度、低碳酸盐含量及高比例中等粒度端元的特征。通过示踪这些沉积特征，可发现巴西-马尔维纳斯汇合区呈现出一条清晰的南北走向特征，其中心位于西经52°~54°，斜穿大陆边缘。在该显著特征的西南部毗邻区域，拉普拉塔河（Rio de la Plata）的直接碎屑沉积物补给可通过一条沿坡向下的砂质舌状体及高比例最粗粒端元得到清晰识别。沿大陆坡向南延伸的区域也存在类似的粗粒粒度组成特征，但受强劲马尔维纳斯海流的强烈分选作用，其沉积物分选性更佳。细粒沉积区分布于里奥格兰德海隆（Rio Grande Rise）西南部深水区及巴西深海盆地南部，二者均位于南极底层水（Antarctic Bottom Water, AABW）影响范围内。根据大陆边缘地形特征，拉普拉塔河以北的北大西洋深层水（North Atlantic Deep Water, NADW）层位表现出相对不显著的分选/沉积模式。本研究表明，通过端元算法解混的表层沉积物常量地球化学与粒度组合特征，可为重建某一时间切片内沉积学与海洋学的复杂相互作用提供有力工具。