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Abrupt changes in deep Atlantic circulation during the transition to full glacial conditions

机译：过渡到完整的冰川条件期间，大西洋深层环流突然发生变化

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Six Ocean Drilling Program (ODP) sites, in the Northwest Atlantic have been used to investigate kinematic and chemical changes in the "Western Boundary Undercurrent" (WBUC) during the development of full glacial conditions across the Marine Isotope Stage 5a/4 boundary (～70,000 years ago). Sortable silt mean grain size SS? measurements are employed to examine changes in near bottom flow speeds, together with carbon isotopes measured in benthic foraminifera and % planktic foraminiferal fragmentation as proxies for changes in water-mass chemistry. A depth transect of cores, spanning 1.8-4.6 km depth, allows changes in both the strength and depth of the WBUC to be constrained across millennial scale events. SS? measurements reveal that the flow speed structure of the WBUC during warm intervals (" interstadials") was comparable to modern (Holocene) conditions. However, significant differences are observed during cold intervals, with higher relative flow speeds inferred for the shallow component of the WBUC (～2 km depth) during all cold "stadial" intervals (including Heinrich Stadial 6), and a substantial weakening of the deep component (～3-4 km) during full glacial conditions. Our results therefore reveal that the onset of full glacial conditions was associated with a regime shift to a shallower mode of circulation (involving Glacial North Atlantic Intermediate Water) that was quantitatively distinct from preceding cold stadial events. Furthermore, our chemical proxy data show that the physical response of the WBUC during the last glacial inception was probably coupled to basin-wide changes in the water-mass composition of the deep Northwest Atlantic. Key Points The Western Boundary Undercurrent is examined across the MIS 5a/4 transitionChanges in flow speed structure are linked to abrupt climate change eventsFull glacial conditions are associated with a shallower mode of circulation
机译：西北大西洋的六个海洋钻探计划（ODP）站点已被用来调查“海洋边界暗流”（WBUC）在海洋同位素5a / 4阶段跨全冰期发展过程中的运动学和化学变化（〜 70,000年前）。可分类淤泥的平均粒径SS？测量方法用于检查近底流速的变化，以及底栖有孔虫中测得的碳同位素和浮游有孔虫碎片百分率，作为水质化学变化的代表。岩心的深度横断面跨越1.8-4.6 km的深度，从而使WBUC强度和深度的变化在千年尺度事件中受到限制。 SS？测量表明，WBUC在温暖间隔（“星际”）期间的流速结构与现代（全新世）条件相当。但是，在寒冷的时间间隔中观察到了显着差异，在所有寒冷的“静止”时间间隔（包括海因里希第6步）中，WBUC的浅层部分（约2 km深度）的相对流速都较高，而在充分的冰川条件下（〜3-4 km）。因此，我们的研究结果表明，充分的冰川条件的发生与政权转向较浅的循环模式（涉及北大西洋中间水）有关，这与先前的寒冷恒星事件在数量上有所不同。此外，我们的化学代理数据表明，WBUC在最后一次冰期开始期间的物理响应可能与西北大西洋深部水质组成的全盆地变化有关。关键点在MIS 5a / 4过渡期间检查了西部边界暗流流速结构的变化与突变的气候变化事件有关全冰川条件与较浅的环流模式有关

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