Evolution of the oceanic circulation in the subtropical Atlantic across the Mid-Pleistocene Transition

现有的概念模型不能令人信服地解释北半球冰盖在中更新世过渡(MPT)期间的快速推进。1.2-0.7 Ma。我们的目标是调查北大西洋副热带环流的变化是否在这一关键时段内对启动和维持大冰盖的增长起到了关键作用。亚热带环流调节热量和水汽向高纬度的输送，这对冰盖在降温气候下的生长至关重要。代理数据和模型数据确实表明了副热带环流强度和位置的动态波动，这可能随着海洋同位素阶段22的开始而助长了冰川冰盖的增长。然而，现有的数据存在争议，到目前为止，观察到的波动的原因仍然难以捉摸。在拟议项目的框架内，副热带环流在MPT期间冰川向北热量和水汽输送中的作用将受到限制，方法是测试两种情景，将冰盖增长与副热带大洋环流的变化联系起来：(1)墨西哥湾流源区的气候变化改变了其盐度和温度，从而改变了向北的热湿平流。(2)冰盖增长导致副热带环流南移，导致西大西洋变暖，大气水分生产增加。这些水分被加强的西风带输送到欧洲中部，作为正反馈，进一步增加了积雪。因此，我们通过地球化学方法(有孔虫方解石上的镁/钙和稳定氧同位素)调查了北大西洋副热带环流表层水域在中更新世期间的温度和盐度演变，调查范围为1.21-0.54 Ma(MIS35-15)。其中一个地点将是ODP1058号地点，位于湾流主干道内的布莱克岭。此外，还将调查大西洋中部41°N的IODP站点U1313。后者的位置非常适合捕捉湾流和亚热带环流的纬向位移。作为方法，我们将对表层栖息有孔虫(Globigerinoides Ruber)进行d18O和镁/钙的联合分析，这允许通过冰量校正的d18O海水来计算海表面温度和盐度变化。对于U1313站点，我们将根据深水生物Globorotalia truncatulinoides(1058站点)和Globorotalia crosa formis(1313站点)进一步生成d18O和镁/钙记录，以重建反映副热带环流强度的温跃层结构。新获得的数据将有助于理清高纬和低纬气候过程之间的反馈，并为它们在MPT期间的作用提供新的见解。

Existing conceptual models fail to convincingly explain the rapid advance of northern hemisphere ice sheets during the mid-Pleistocene Transition (MPT), between approx. 1.2-0.7 Ma. We aim to investigate whether alterations in the North Atlantic Subtropical Gyre circulation played a key role for initiating and sustaining the growth of large glacial ice sheets during this critical interval. The Subtropical Gyre modulates heat and moisture transport into high latitudes, which is essential for the growth of ice sheets under a cooling climate. Proxy and model data indeed suggest dynamic fluctuations of the Subtropical Gyre’s strength and position, which might have fueled glacial ice shield growth with the onset of Marine Isotope Stage (MIS) 22 onwards. However, the existing data is controversial and the reasons for the observed fluctuations remained elusive so far. Within the framework of the proposed project, the role of the Subtropical Gyre for the glacial northward heat and moisture transport during the MPT will be constrained by testing two scenarios that link explain ice sheet growth to alterations in the subtropical oceanic circulation:(1) Climate changes in the source region of the Gulf Stream modify its salinity and temperature and thereby alter the northward heat and moisture advection.(2) Ice sheet growth causes a southward displacement of the Subtropical Gyre causing a warming in the western Atlantic and enhanced atmospheric moisture production. This moisture is transported into central Europe by intensified Westerlies and, as a positive feedback, further increases snow accumulation.Therefore, we constrain the temperature and salinity evolution in the surface waters of the North Atlantic Subtropical Gyre during the mid-Pleistocene, by investigating two sites by means of geochemical methods (Mg/Ca and stable oxygen isotopes on foraminiferal calcite), covering the interval 1.21 – 0.54 Ma (MIS 35-15). One Site will be ODP Site 1058 located on Blake Ridge within the main path of the Gulfstream. Further, IODP Site U1313 from the central Atlantic at 41°N will be investigated. The latter site is ideally located to capture the latitudinal displacement of Gulfstream and Subtropical Gyre. As methods we will apply the combined analyses of d18O and Mg/Ca on surface dwelling foraminifera (Globigerinoides ruber), which allows for the computation of sea surface temperature and salinity variations by means of the ice-volume corrected d18Oseawater. For Site U1313 we will further generate d18O and Mg/Ca records based on the deep dwellers Globorotalia truncatulinoides (Site 1058) Globorotalia crassaformis (Site 1313) to reconstruct the thermocline structure, which reflects the intensity of the Subtropical Gyre circulation. The newly obtained data will help to disentangle feedbacks between climatic processes in high and low latitudes and provide novel insights into their role during the MPT.