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

现有的概念模型无法令人信服地解释北方半球冰盖在中更新世过渡期（MPT）的快速推进，大约在1990年至2000年之间。1.2-0.7 Ma。我们的目标是调查是否在北大西洋亚热带环流的变化在这一关键时期的启动和维持大型冰川冰盖的增长发挥了关键作用。副热带环流调节热量和水分向高纬度地区的输送，这对于在变冷气候下冰盖的生长至关重要。代理和模型数据确实表明，亚热带环流的强度和位置的动态波动，这可能会助长冰川冰盾的增长与海洋同位素阶段（MIS）22开始。然而，现有的数据是有争议的，观察到的波动的原因至今仍然难以捉摸。在拟议项目的框架内，副热带环流在MPT期间冰川向北输送热量和水分的作用将通过测试两种情景来限制，这两种情景将解释冰盖的增长与副热带海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量和水分平流。(2)冰盖的增长导致副热带环流向南移动，导致西大西洋变暖，增加了大气水分的产生。因此，我们通过地球化学方法对北大西洋副热带环流中更新世两个地点的表层沃茨的温度和盐度变化进行了研究（有孔虫方解石上的Mg/Ca和稳定氧同位素），覆盖时间为1.21 - 0.54 Ma（MIS 35-15）。其中一个地点是位于湾流主航道内布莱克山脊上的ODP 1058号地点。此外，还将对大西洋中部北纬41°的IODP站点U1313进行调查。后一个站点的位置非常理想，可以捕捉湾流和副热带环流的纬向位移。作为方法，我们将应用d18 O和Mg/Ca的组合分析的表面居住有孔虫（Globigerinoides ruber），这允许计算的海面温度和盐度的变化，通过冰量校正d18 O海水。 对于站点U1313，我们将根据深层居民Globorotalia truncatulinoides（站点1058）和Globorotalia crassaformis（站点1313）进一步生成d18 O和Mg/Ca记录，以重建温跃层结构，反映副热带环流的强度。新获得的数据将有助于解开高纬度和低纬度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.