Magnetic extracts of three gravity cores located at the Ceará Rise, the Mid-Atlantic Ridge and the Sierra Leone Rise

The magnetic microparticle and nanoparticle inventories of marine sediments from equatorial Atlantic sites were investigated by scanning and transmission electron microscopy to classify all present detrital and authigenic magnetic mineral species and to investigate their regional distribution, origin, transport, and preservation. This information is used to establish source-to-sink relations and to constrain environmental magnetic proxy interpretations for this area. Magnetic extracts were prepared from sediments of three supralysoclinal open ocean gravity cores located at the Ceará Rise (GeoB 1523-1; 3°49.9'N/41°37.3'W), the Mid-Atlantic Ridge (GeoB 4313-2; 4°02.8'N/33°26.3'W), and the Sierra Leone Rise (GeoB 2910-1; 4°50.7'N/21°03.2'W). Sediments from two depths corresponding to marine isotope stages 4 and 5.5 were processed. This selection represents glacial and interglacial conditions of sedimentation for the western, central, and eastern equatorial Atlantic and avoids interferences from subsurface and anoxic processes. Crystallographic, elemental, morphological, and granulometric data of more than 2000 magnetic particles were collected by scanning and transmission electron microscopy. On basis of these properties, nine particle classes could be defined: detrital magnetite, titanomagnetite (fragmental and euhedral), titanomagnetite-hemoilmentite intergrowths, silicates with magnetic inclusions, microcrystalline hematite, magnetite spherules, bacterial magnetite, goethite needles, and nanoparticle clusters. Each class can be associated with fluvial, eolian, subaeric, and submarine volcanic, biogenic, or chemogenic sources. Large-scale sedimentation patterns are delineated as well: detrital magnetite is typical of Amazon discharge, fragmental titanomagnetite is a submarine weathering product of mid-ocean ridge basalts, and titanomagnetite-hemoilmenite intergrowths are common magnetic particles in West African dust. This clear regionalization underlines that magnetic petrology is an excellent indicator of source-to-sink relations. Hematite encrustations, magnetic spherules, and nanoparticle clusters were found at all investigated sites, while bacterial magnetite and authigenic hematite were only detected at the more oxic western site. At the eastern site, surface pits and crevices were seen on the crystal faces indicating subtle early diagenetic reductive dissolution. It was observed that paleoclimatic signatures of magnetogranulometric parameters such as the ratio of anhysteretic and isothermal remanent magnetizations can be formed either by mixing of multiple sources with separate, relatively narrow grain size ranges (western site) or by variable sorting of a single source with a broad grain size distribution (eastern site). Hematite, goethite, and possibly ferrihydrite nanoparticles occur in all sediment cores studied and have either high-coercive or superparamagnetic properties depending on their partly ultrafine grain sizes. These two magnetic fractions are generally discussed as separate fractions, but we suggest that they could actually be genetically linked.本数据集聚焦大西洋赤道海域多个站位的海洋沉积物中的磁性微粒与纳米颗粒清单，通过扫描电子显微镜（Scanning Electron Microscopy, SEM）与透射电子显微镜（Transmission Electron Microscopy, TEM）对所有现存的碎屑型与自生型磁性矿物物种进行分类，并探究其区域分布、成因、搬运过程与保存状态。相关成果被用于构建源到汇（source-to-sink）系统关系，并为该区域的环境磁学代用指标（environmental magnetic proxy）解译提供约束。研究人员从三处位于溶跃面以上（supralysoclinal）的开放洋区重力岩芯（gravity cores）的沉积物中制备了磁性提取物：塞阿拉海隆（Ceará Rise）站位GeoB 1523-1（3°49.9'N/41°37.3'W）、大西洋中脊（Mid-Atlantic Ridge）站位GeoB 4313-2（4°02.8'N/33°26.3'W）以及塞拉利昂海隆（Sierra Leone Rise）站位GeoB 2910-1（4°50.7'N/21°03.2'W）。选取了对应海洋同位素阶段（marine isotope stages, MIS）4与5.5的两个深度的沉积物进行处理。该选取方案覆盖了大西洋赤道西、中、东部的冰期与间冰期沉积环境，同时规避了地下过程与缺氧过程的干扰。通过扫描电子显微镜与透射电子显微镜，研究人员收集了超过2000颗磁性颗粒的晶体学、元素组成、形貌与粒度数据。基于上述特征，可将其划分为9类颗粒：碎屑型磁铁矿、（棱角状与自形的）钛磁铁矿（titanomagnetite）、钛磁铁矿-hemoilmentite共生体、含磁性包裹体的硅酸盐矿物、微晶赤铁矿、磁铁矿球粒、细菌成因磁铁矿、针铁矿针状体以及纳米颗粒团簇。每一类颗粒均可关联至河流、风成、陆上、海底火山、生物成因或化学成因的物源。研究同时明确了大型沉积格局：碎屑型磁铁矿是亚马孙河入海沉积物的典型组分，棱角状钛磁铁矿是洋中脊玄武岩海底风化的产物，而钛磁铁矿-hemoilmentite共生体则是西非风尘中常见的磁性颗粒。这一清晰的区域分异特征表明，磁性岩石学是源到汇关系的优良指示指标。在所有调查站位中均发现了赤铁矿结壳、磁铁矿球粒与纳米颗粒团簇；而细菌成因磁铁矿与自生赤铁矿仅在氧化性更强的西部站位被检出。在东部站位的晶体表面可见溶蚀坑与裂隙，指示了早期成岩作用过程中的轻微还原性溶解。研究观察到，磁粒度参数的古气候信号（如非滞后剩磁与等温剩磁的比值），既可由多个具有独立且相对狭窄粒径范围的物源混合形成（西部站位），也可由单一具有宽粒径分布的物源经历不同程度的分选作用形成（东部站位）。赤铁矿、针铁矿以及可能的水铁矿（ferrihydrite）纳米颗粒广泛分布于所有被研究的沉积岩芯中，其磁性特征可表现为高矫顽力或超顺磁性，这取决于其部分超细的晶粒尺寸。通常认为这两类磁性组分属于独立的组分，但本研究认为二者实际上可能具有成因联系。