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，当前正呈现减弱趋势。尽管该环流的减弱常被归因于淡水扰动引发的海洋对流异常，但长期淡水化影响的不确定性，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，多项证据显示，挪威海区域水体较现代更淡的同时，强劲的AMOC仍持续存在，这对我们当前的海洋对流认知构成了挑战。本研究提供了新的有孔虫结合态氮同位素数据，以及该异常时段挪威海上层海洋水文特征的多项地球化学重建结果。数据表明，在约41.0万至40.7万年前的气候适宜期伊始，上层水体长期累积的淡水导致夏季层结减弱，这或许为该区域的对流活动预先创造了有利条件。此外，受古代用数据约束的箱式模型（box model）进一步显示，亚极地北大西洋与挪威海之间的密度梯度，在气候适宜期内有利于挪威海对流活动的启动。由此可见，挪威海在该时段前后极有可能成为海洋对流活动的核心区域。挪威海对流活动驱动的北半球增强加热，可能对延缓冰期降临发挥了关键作用，进而造就了MIS 11延长的温暖气候特征。该研究发现或许也可为高纬度北大西洋水体相对偏淡背景下的近期气候变化研究提供参考。