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Interhemispheric controls on deep ocean circulation and carbon chemistry during the last two glacial cycles

机译：最近两个冰川周期对深海环流和碳化学的半球控制

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Changes in ocean circulation structure, together with biological cycling, have been proposed for trapping carbon in the deep ocean during glacial periods of the Late Pleistocene, but uncertainty remains in the nature and timing of deep ocean circulation changes through glacial cycles. In this study, we use neodymium (Nd) and carbon isotopes from a deep Indian Ocean sediment core to reconstruct water mass mixing and carbon cycling in Circumpolar Deep Water over the past 250 thousand years, a period encompassing two full glacial cycles and including a range of orbital forcing. Building on recent studies, we use reductive sediment leaching supported by measurements on isolated phases (foraminifera and fish teeth) in order to obtain a robust seawater Nd isotope reconstruction. Neodymium isotopes record a changing North Atlantic Deep Water (NADW) component in the deep Indian Ocean that bears a striking resemblance to Northern Hemisphere climate records. In particular, we identify both an approximately in-phase link to Northern Hemisphere summer insolation in the precession band and a longer-term reduction of NADW contributions over the course of glacial cycles. The orbital timescale changes may record the influence of insolation forcing, for example via NADW temperature and/or Antarctic sea ice extent, on deep stratification and mixing in the Southern Ocean, leading to isolation of the global deep oceans from an NADW source during times of low Northern Hemisphere summer insolation. That evidence could support an active role for changing deep ocean circulation in carbon storage during glacial inceptions. However, mid-depth water mass mixing and deep ocean carbon storage were largely decoupled within glacial periods, and a return to an interglacial-like circulation state during marine isotope stage (MIS) 6.5 was accompanied by only minor changes in atmospheric CO2. Although a gradual reduction of NADW export through glacial periods may have produced slow climate feedbacks linked to the growth of Northern Hemisphere ice sheets, carbon cycling in the glacial ocean was instead more strongly linked to Southern Ocean processes.
机译：已经提出了海洋循环结构的变化以及生物循环的作用，以便在晚更新世的冰川时期将碳捕集到深海中，但是通过冰川循环，深海循环变化的性质和时间仍存在不确定性。在这项研究中，我们使用来自印度洋深部沉积物核心的钕（Nd）和碳同位素，重建了过去25万年（包括两个完整的冰川周期，包括一个范围轨道强迫。在最近的研究的基础上，我们使用还原相沉积物浸出，并通过对孤立相（有孔虫和鱼齿）的测量来支持，以获得强大的海水Nd同位素重建。钕同位素记录了印度洋深处北大西洋深水（NADW）组分的变化，这与北半球的气候记录有着惊人的相似之处。特别是，我们确定了在旋进带中与北半球夏季日照的大致同相联系，以及在冰川周期过程中NADW贡献的长期减少。轨道时标变化可能会记录到日照强迫（例如通过NADW温度和/或南极海冰范围）对南大洋深层分层和混合的影响，从而导致全球深海与NADW源隔离。北半球夏季较低。该证据可能支持在冰川形成初期改变深海循环在碳储存中的积极作用。然而，在冰川期，中深度水体混合与深海碳存储在很大程度上脱钩，并且在海洋同位素阶段（MIS）6.5期间恢复到类似冰河间的循环状态的同时，大气二氧化碳的变化也很小。尽管整个冰川期NADW出口量的逐渐减少可能会产生与北半球冰盖生长有关的缓慢的气候反馈，但相反，冰川海洋中的碳循环与南部海洋过程之间的联系更为紧密。

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