NOAA/WDS Paleoclimatology - Foraminifer Isotope Data (d15N, d13C, d18O) and Modeling of Nordic Seas Hydrography and Circulation during MIS 11

The Atlantic meridional overturning circulation (AMOC) is a critical element of Earth’s climate system and it is currently weakening. While this weakening is frequently explained by freshwater-driven disruptions to ocean convection, uncertainties about the impacts of prolonged freshening limit our capacity to predict its future state. For example, during the warm and unusually long marine isotope stage (MIS) 11 interglacial, ~424 to 374 ka, several lines of evidence suggest that a strong AMOC persisted concomitant with fresher-than-present conditions in the Nordic Seas, challenging our current understanding of convection. Here, we present new foraminifer-bound nitrogen isotope data along with multiple additional geochemical reconstructions of upper-ocean hydrography in the Nordic Seas during this anomalous interval. Our data suggest that a weak summer stratification was driven by the prolonged upper-ocean accumulation of freshwater beginning at the onset of the climatic optimum, ~410 to 407 ka, which could have helped precondition the region for convection. A box model constrained by paleo-proxy data additionally suggests that the density gradient between the subpolar North Atlantic and Nordic Seas was favorable for the onset of convection in the Nordic Seas during the climatic optimum. It is thus likely that the Nordic Seas became a locus of convection around this time. Enhanced northern-hemisphere heating driven by convection in the Nordic Seas may have been important for delaying glacial conditions, thereby driving the extended warming characteristic of MIS 11. Such findings may also be relevant for near-future changes under a relatively fresher high-latitude North Atlantic.

大西洋经向翻转环流（Atlantic meridional overturning circulation，AMOC）是地球气候系统的关键组成部分，目前正处于减弱状态。尽管这种减弱常被归因于淡水驱动的海洋对流扰动，但长期淡水化带来的影响存在诸多不确定性，这限制了我们对其未来状态的预测能力。例如，在约42.4万至37.4万年前的温暖且异常漫长的海洋同位素阶段（marine isotope stage，MIS）11间冰期内，多项证据表明，当时挪威海的海水较现代更淡的同时，大西洋经向翻转环流仍维持强劲，这对我们当前关于海洋对流的认知提出了挑战。本研究在此异常时段内，提供了挪威海区域的新型有孔虫结合态氮同位素数据，以及多组上层海洋水文状况的地球化学重建结果。我们的数据显示，约41.0万至40.7万年前气候最适宜期伊始，上层海洋长期累积淡水便导致夏季层结减弱，这或许为该区域的对流活动预先创造了有利条件。基于古代用指标约束的箱式模型进一步表明，气候最适宜期内，北大西洋副极地海域与挪威海之间的密度梯度有利于挪威海区域对流活动的启动。因此，挪威海在该时段左右很可能成为了对流活动的核心区域。挪威海对流活动引发的北半球加热增强，或许对延缓冰期到来起到了关键作用，进而造就了MIS 11异常漫长的暖期特征。此类研究结果对于理解未来北大西洋高纬度海域海水相对淡化背景下的气候变化同样具有参考价值。