Coccoliths and carbonate grain-size compositions in sediment samples from the Mid-Atlantic Ridge@en

This study presents a differentiated carbonate budget for marine surface sediments from the Mid-Atlantic Ridge of the South Atlantic, with results based on carbonate grain-size composition. Upon separation into sand, silt, and clay sub-fractions, the silt grain-size distribution was measured using a SediGraph 5100. We found regionally characteristic grain-size distributions with an overall minimum at 8 µm equivalent spherical diameter (ESD). SEM observations reveal that the coarse particles (>8 µm ESD) are attributed to planktic foraminifers and their fragments, and the fine particles (<8 µm ESD) to coccoliths. On the basis of this division, the regional variation of the contribution of foraminifers and coccoliths to the carbonate budget of the sediments are calculated. Foraminifer carbonate dominates the sediments in mesotropic regions whereas coccoliths contribute most carbonate in oligotrophic regions. The grain size of the coccolith share is constant over water depth, indicating a lower susceptibility for carbonate dissolution compared to foraminifers. Finally, the characteristic grain-size distribution in fine silt (<8 µm ESD) is set into context with the coccolith assemblage counted and biometrically measured using a SEM. The coccoliths present in the silt fraction are predominantly large species (length > 4 µm). Smaller species (length < 4 µm) belong to the clay fraction (<2 µm ESD). The average length of most frequent coccolith species is connected to prominent peaks in grain-size distributions (ESD) with a shape factor. The area below Gaussian distributions fitted to these peaks is suggested as a way to quantitatively estimate the carbonate contribution of single coccolith species more precisely compared to conventional volume estimates. […]本研究针对南大西洋中脊的海洋表层沉积物，构建了差异化碳酸盐收支模型，其分析结果基于碳酸盐粒度组成特征。将沉积物分离为砂、粉砂与黏土亚组分后，采用SediGraph 5100粒度分析仪测定粉砂级组分的粒度分布。研究发现区域特征性粒度分布整体呈现出在等效球径（equivalent spherical diameter, ESD）8 μm处的最小值。扫描电子显微镜（Scanning Electron Microscope, SEM）观测显示，粒径大于等效球径8 μm的粗颗粒主要为浮游有孔虫及其碎屑，而粒径小于等效球径8 μm的细颗粒则以颗石为主。基于该粒度划分标准，本研究计算了浮游有孔虫与颗石对沉积物碳酸盐收支的贡献量区域差异。在中营养海域，沉积物碳酸盐以有孔虫来源为主；而在贫营养海域，颗石贡献的碳酸盐占比最高。颗石贡献组分的粒径不随水深发生显著变化，这表明相较于浮游有孔虫，颗石对碳酸盐溶蚀的敏感性更低。最后，本研究将细粉砂级组分（等效球径<8 μm）的特征粒度分布与通过扫描电子显微镜计数、生物计量分析得到的颗石组合进行关联分析。粉砂级组分中的颗石以体长大于4 μm的大型物种为主，体长小于4 μm的小型颗石物种则归于黏土级组分（等效球径<2 μm）。优势颗石物种的平均体长与粒度分布（等效球径）中的显著峰态存在关联，且该关联符合形状因子特征。相较于传统体积估算方法，通过拟合这些峰态的高斯分布下的面积，可更为精准地定量估算单一颗石物种的碳酸盐贡献量。……