XRF-scan (Si, K, Ca, Ti, Mn, Fe, Sr) and 14C-age of sediment core MD96-2094

We present a new index of carbonate fragmentation based on the size distribution of bulk sediments in core MD962094 from Walvis Ridge (SE Atlantic Ocean). The carbonate fragmentation index is constructed by taking a ratio of the two coarsest fractions in the grain size distributions of the bulk calcareous ooze. The coarsest two fractions (25–90 µm and >90 µm) of the bulk sediments consist primarily of complete shells and fragments of adult foraminifera shells, and juvenile foraminifera shells and fragments, respectively. The ratio of the proportions of the two fractions is interpreted as a measure of fragmentation of the foraminifera shells caused by carbonate dissolution. Downcore changes in our carbonate fragmentation index compare very well with those in the coarse-carbonate fragmentation index in sediments from a nearby core on Walvis Ridge. The latter commonly used fragmentation index is defined as a ratio of foraminifera fragments over whole foraminifera in the >150-µm fraction as seen with a light microscope. Fragmentation is relatively high during glacial stages and relatively low during interglacial stages during the last 300 kyr, caused by the combined effect of wind-driven upwelling of corrosive water and increased production of organic matter, decreasing the preservation potential of carbonates both during and after deposition. The carbonate fragmentation index we present here provides a precise and fast method to establish a downcore fragmentation record. It can be applied to bulk sediments that are carbonate-rich (CaCO3>68%) and to all other deep-marine sediments of which the grain size distribution of the carbonate-free fraction is available.

本研究基于南大西洋沃尔维斯脊（Walvis Ridge）MD962094岩芯的全沉积物粒度分布，构建了一种全新的碳酸盐破碎指数（carbonate fragmentation index）。该指数通过计算钙质软泥全样粒度分布中两个最粗粒级的占比比值得到。全沉积物中最粗的两个粒级（25~90 μm与>90 μm）分别以完整壳体与成年有孔虫（foraminifera）壳体碎片、幼年有孔虫壳体及其碎片为主要组成。这两个粒级的占比比值可用于表征由碳酸盐溶解作用引发的有孔虫壳体破碎程度。本研究构建的碳酸盐破碎指数的岩芯垂向变化趋势，与沃尔维斯脊附近另一岩芯沉积物中的粗粒碳酸盐破碎指数（coarse-carbonate fragmentation index）的变化趋势高度吻合。后者这一常用破碎指数的定义为：在光学显微镜下统计>150 μm粒级中，有孔虫碎片与完整有孔虫壳体的数量比值。在过去300千年（kyr）的地质记录中，冰期阶段的壳体破碎程度相对较高，间冰期则相对较低；这一现象由侵蚀性海水的风生上升流与有机质生产增强的共同作用所致，该作用会降低碳酸盐在沉积期及沉积后的保存潜力。本研究提出的碳酸盐破碎指数，为构建岩芯垂向破碎记录提供了一种精准且高效的方法。该指数可应用于碳酸盐含量较高（碳酸钙（CaCO₃）>68%）的全沉积物样本，以及所有可获取无碳酸盐组分粒度分布数据的深海沉积物样本。