Mineralogy and major element oxide composition of sediments from ODP Hole 172-1063D

Dansgaard-Oeschger (D-O) cycles in sediment at Site 1063 are characterized by distinct fluctuations in physical properties. Stadials are marked by low bulk density and interstadials by high bulk density. Compressional (P-)wave velocity is in phase with bulk density over some but not all depth intervals. Four of the D-O cycles straddling the oxygen isotope Stage 4/5 boundary have been studied in detail to understand the origin of the physical properties changes. Sediment on the Bermuda Rise is comprised of three main components: calcite, aluminosilicate minerals, and biogenic silica. Calcite concentrations vary from 1% to 43% of bulk sediment and are highest during interstadials. Aluminosilicate concentrations vary from 52% to 92% of bulk sediment and are highest during stadials. The major element ratios Al2O3/TiO2 and K2O/Al2O3 show increases across bulk density cycles, suggesting a change in the composition of aluminosilicates. This interpretation is supported by mineralogical analyses, which show a subtle change in clay composition. Biogenic silica concentrations vary from 0% to 23% of bulk sediment and are also highest during stadials. However, the abundance of silica varies significantly from one D-O cycle to another. Silt and fine sand abundance also increase during the first of the four stadials. This coarsening of sediment coincides with the increase in biogenic silica. The low grain density and high porosity associated with biogenic silica result in intervals of low bulk-sediment density. The abundance of biogenic silica closely matches P-wave velocity, suggesting that silica imparts a greater rigidity to the sediment.1063站位（Site 1063）沉积物中的丹斯果-奥什格尔旋回（Dansgaard-Oeschger, D-O）以显著的物理性质波动为典型特征。冰阶（Stadials）以低体积密度为特征，间冰阶（interstadials）则以高体积密度为特征。纵波（P-wave）速度在部分而非全部深度区间内与体积密度呈同相位变化。针对横跨氧同位素4/5阶段（oxygen isotope Stage 4/5）边界的4个D-O旋回开展了详细研究，以阐明沉积物物理性质变化的成因。百慕大隆起（Bermuda Rise）处的沉积物主要由三大组分构成：方解石（calcite）、铝硅酸盐矿物（aluminosilicate minerals）以及生物硅（biogenic silica）。方解石含量占沉积物总体的1%~43%，在间冰阶时达到峰值。铝硅酸盐矿物含量占比为52%~92%，在冰阶时含量最高。Al₂O₃/TiO₂与K₂O/Al₂O₃这两组主量元素比值随体积密度旋回出现升高，表明铝硅酸盐的组分发生了变化。矿物学分析结果佐证了这一解释，该分析显示黏土组分存在细微变化。生物硅含量占沉积物总体的0%~23%，同样在冰阶时达到最高值。不过，不同D-O旋回间的硅丰度存在显著差异。在4个冰阶中的首个冰阶内，粉砂与细砂的丰度也出现升高。沉积物的这种粗化现象与生物硅含量的增加相吻合。生物硅对应的低颗粒密度与高孔隙度，造就了沉积物体积密度偏低的层段。生物硅的丰度与纵波速度高度匹配，表明硅赋予了沉积物更强的刚性。