Observation of manganese nodules and crusts recovered during the CHAIN 115 cruise from Nov 1973 until June 1974 in the Atlantic Ocean

Investigations of piston cores from the Vema Channel and lower flanks of the Rio Grande Rise suggest the presence of episodic flow of deep and bottom water during the Late Pleistocene. Cores from below the present-day foraminiferal lysocline (at ~4000 m) contain an incomplete depositional record consisting of Mn nodules and encrustations, hemipelagic clay, displaced high-latitude diatoms, and poorly preserved heterogeneous microfossil assemblages. Cores from the depth range between 2900 m and 4000 m contain an essentially complete Late Pleistocene record, and consist of well-defined carbonate dissolution cycles with periodicities of ~100,000 years. Low carbonate content and increased dissolution correspond to glacial episodes, as interpreted by oxygen isotopic analysis of bulk foraminiferal assemblages. The absence of diagnostic high-latitude indicators (Antarctic diatoms) within the dissolution cyclss, however, suggests that AABW may not have extended to significantly shallower elevations on the lower flanks of the Rio Grande Rise during the Late Pleistocene. Therefore episodic AABW flow may not necessarily be the mechanism responsible for producing these cyclic events. This interpretation is also supported by the presence of an apparently complete Brunhes depositional record in the same cores, suggesting current velocities insufficient for significant erosion. Fluctuations in the properties and flow characteristics of another water mass, such as NADW, may be involved. The geologic evidence in core-top samples near the present-day AABW/NADW transition zone is consistent with either of two possible interpretations of the upper limit of AABW on the east flank of the channel. The foraminiferal lysocline, at ~4000 m, is near the top of the benthic thermocline and nepheloid layer, and may therefore correspond to the upper limit of relatively corrosive AABW. On the other hand, the carbonate compensation depth (CDD) at ~4250 m, which corresponds to the maximum gradient in the benthic thermocline, is characterized by rapid deposition of relatively fine-grained sediment. Such a zone of convergence and preferential sediment accumulation would be expected near the level of no motion in the AABW/NADW transition zone as a consequence of Ekman-layer veering of the mean velocity vector in the bottom boundary layer. It is possible that both of these interpretations are in part correct. […]

对维玛海槽（Vema Channel）及里约格兰德海隆（Rio Grande Rise）下部侧翼的活塞岩芯（piston cores）开展的研究显示，晚更新世（Late Pleistocene）期间存在深层水与底层水的周期性流动。采自现今有孔虫溶跃面（foraminiferal lysocline，约4000米）以下的岩芯，其沉积记录并不完整，包含锰结核（Mn nodules）、结壳、半远洋黏土（hemipelagic clay）、迁移而来的高纬度硅藻（high-latitude diatoms）以及保存状态较差的非均质性微化石组合（microfossil assemblages）。采自2900米至4000米深度区间的岩芯则保存了近乎完整的晚更新世沉积记录，由周期约10万年的典型碳酸盐溶解旋回（carbonate dissolution cycles）构成。通过对有孔虫整体组合（bulk foraminiferal assemblages）开展氧同位素分析（oxygen isotopic analysis）可知，低碳酸盐含量与增强的溶解作用对应冰期事件。然而，这些溶解旋回中缺乏标志性高纬度指示物——南极硅藻（Antarctic diatoms），这表明晚更新世期间南极底层水（Antarctic Bottom Water, AABW）或许并未延伸至里约格兰德海隆下部侧翼的显著更浅区域。因此，周期性的南极底层水流动未必是形成这些旋回事件的成因。这一解释同样得到了下述发现的支持：同一批岩芯中存在近乎完整的布容期沉积记录（Brunhes depositional record），表明当时的水流速度不足以引发显著侵蚀。另一水团（例如北大西洋深层水（North Atlantic Deep Water, NADW））的属性与流动特征的波动或许与此有关。采自现今南极底层水/北大西洋深层水过渡带附近的岩芯表层样品（core-top samples）的地质证据，与维玛海槽东翼南极底层水上限的两种可能解释均相符。约4000米处的有孔虫溶跃面位于底栖温跃层（benthic thermocline）与悬沙层（nepheloid layer）的顶部附近，因此或许对应腐蚀性较强的南极底层水的上限。另一方面，约4250米处的碳酸盐补偿深度（carbonate compensation depth, CDD）对应底栖温跃层的最大梯度带，其特征为相对细粒沉积物的快速沉积。由于底边界层（bottom boundary layer）内平均流速矢量的埃克曼层转向（Ekman-layer veering），在南极底层水/北大西洋深层水过渡带的无运动层附近，理应存在这样的汇聚区与优先沉积区。这两种解释或许均部分成立。[…]