Stable isotope record of planktonic foraminifera from sediment core MD96-2085

A high-resolution (~4-5cm/kyr) giant piston core record (MD962085) retrieved during an IMAGES II-NAUSICAA cruise from the continental slope of the southeast Atlantic Ocean reveals striking variations in planktonic foraminifer faunal abundances and sea-surface temperatures (SST) during the past 600 000 yr. The location and high-quality sedimentary record of the core provide a good opportunity to assess the variability of the Benguela Current system and associated important features of the ocean-climate system in the southeast Atlantic. The planktonic foraminifer faunal abundances of the core are dominated by three assemblages: (1) Neogloboquadrina pachyderma (right coiling) + Neogloboquadrina dutertrei, (2) Globigerina bulloides, and (3) Globorotalia inflata. The assemblage of N. pachyderma (right coiling) + N. dutertrei shows distinctive abundance changes which are nearly in-phase with glacial-interglacial variations. The high abundances of this assemblage are associated with major glacial conditions, possibly representing low SST/high nutrient level conditions in the southwestern Africa margin. In contrast, the G. bulloides and G. inflata assemblages show greater high-frequency abundance change patterns, which are not parallel to the glacial-interglacial changes. These patterns may indicate rapid oceanic frontal movements from the south, and a rapid change in the intensity of the Benguela upwelling system from the east. A single episode of maximum abundances of a polar water species N. pachyderma (left coiling) occurred in the beginning of stage 9 (~340-330 kyr). The event of the maximum occurrence of this species shown in this record may indicate instability in the Benguela coastal upwelling, or the Antarctic polar front zone position. A winter season SST estimate using transfer function techniques for this record shows primarily glacial-interglacial variations. The SST is maximal during the transitions from the major glacial to interglacial stages (Terminations I, II, IV, V), and is associated with the abundance maxima of a warm water species indicator Globigerinoides ruber. Cross-spectral analyses of the SST record and the SPECMAP stack reveal statistically significant concentrations of variance and coherencies in three major orbital frequency bands. The SST precedes changes in the global ice volume in all orbital frequency bands, indicating a dominant southern Hemispheric climate effect over the Benguela Current region in the southeast Atlantic.

于IMAGES II-NAUSICAA科考航次期间，从东南大西洋大陆坡获取的巨型活塞岩心（giant piston core）记录（MD962085）具有约4-5cm/千年的高分辨率，该记录揭示了过去60万年中浮游有孔虫（planktonic foraminifer）群落丰度与海表温度（Sea-Surface Temperature, SST）的显著变化。该岩心的取样位置与高质量沉积记录，为评估东南大西洋本格拉洋流（Benguela Current）系统的变化，以及该区域海洋-气候系统的相关关键特征提供了绝佳契机。
该岩心的浮游有孔虫群落丰度主要由三类组合构成：（1）右旋厚壳新抱球虫（Neogloboquadrina pachyderma, right coiling）+杜氏新抱球虫（Neogloboquadrina dutertrei），（2）布氏球室壳虫（Globigerina bulloides），（3）膨胀拟抱球虫（Globorotalia inflata）。其中右旋厚壳新抱球虫+杜氏新抱球虫组合的丰度变化特征显著，且与冰期-间冰期旋回几乎同步。该组合的高丰度时段与主要冰期环境相对应，可能指示非洲西南缘海域存在低海表温度、高营养盐的环境。
与之形成对比的是，布氏球室壳虫与膨胀拟抱球虫组合则呈现出更为显著的高频丰度变化模式，且与冰期-间冰期旋回并不耦合。这类变化模式可能指示了来自南方的海洋锋面快速移动，以及来自东部的本格拉上升流系统强度的快速变化。
左旋厚壳新抱球虫（Neogloboquadrina pachyderma, left coiling）作为极地水种，其丰度峰值仅出现于第9气候阶段初期（约34万~33万年）。本记录中该物种的丰度峰值事件，可能指示了本格拉沿岸上升流的不稳定性，或是南极极锋带（Antarctic polar front zone）位置的变化。
本研究采用转换函数法（transfer function techniques）对该记录开展的冬季海表温度估算结果，主要呈现出冰期-间冰期旋回特征。海表温度在主要冰期向间冰期的过渡阶段（终止期I、II、IV、V）达到峰值，且与暖水种指示种红拟抱球虫（Globigerinoides ruber）的丰度峰值相对应。
对海表温度记录与SPECMAP堆叠序列开展的交叉谱分析（cross-spectral analyses）结果显示，在三个主要轨道频率波段中，方差与相干性均存在统计学意义上的显著聚集。在所有轨道频率波段中，海表温度的变化均早于全球冰量（global ice volume）变化，这表明东南大西洋本格拉洋流区域的气候主导效应来自南半球。