North Atlantic Midlatitude Surface-Circulation Changes Through the Plio-Pleistocene Intensification of Northern Hemisphere Glaciation

The North Atlantic Current (NAC) transports warm salty water to high northern latitudes, with important repercussions for ocean circulation and global climate. A southward displacement of the NAC and Subarctic Front, which separate subpolar and subtropical water masses, is widely suggested for the Last Glacial Maximum (LGM) and may have acted as a positive feedback in glacial expansion at this time. However, the role of the NAC during the intensification of Northern Hemisphere glaciation (iNHG) at similar to 3.5 to 2.5 Ma is less clear. Here we present new records from Integrated Ocean Drilling Program Site U1313 (41 degrees N) spanning similar to 2.8-2.4 Ma to trace the influence of Subarctic Front waters above this mid-latitude site. We reconstruct surface and permanent pycnocline temperatures and seawater delta O-18 using paired Mg/Ca-delta O-18 measurements on the planktic foraminifers Globigerinoides ruber and Globorotalia crassaformis and determine abundances of the subpolar foraminifer Neogloboquadrina atlantica. We find that the first significant glacial incursions of Subarctic Front surface waters above Site U1313 did not occur until similar to 2.6 Ma. At no time during our study interval was (sub)surface reorganization in the midlatitude North Atlantic analogous to the LGM. Our findings suggest that LGM-like processes sensu stricto cannot be invoked to explain interglacial-glacial cycle amplification during iNHG. They also imply that increased glacial productivity at Site U1313 during iNHG was not only driven by southward deflections of the Subarctic Front. We suggest that nutrient injection from cold-core eddies and enhanced glacial dust delivery may have played additional roles in increasing export productivity in the midlatitude North Atlantic from 2.7 Ma.

北大西洋暖流（NAC）将温暖的咸水输送到北半球高纬度地区，对海洋环流和全球气候具有重要影响。人们普遍认为，在末次盛冰期（LGM），分隔亚极地和亚热带水团的北大西洋暖流和亚北极锋向南移动，并且可能在当时的冰川扩张中起到了正反馈作用。然而，北大西洋暖流在距今约350万 - 250万年的北半球大冰期增强（iNHG）期间所起的作用尚不明确。在此，我们展示了来自综合大洋钻探计划U1313站点（北纬41度）的新记录，其时间跨度约为280万 - 240万年，以追踪该中纬度站点上方亚北极锋海水的影响。我们通过对浮游有孔虫红拟抱球虫（Globigerinoides ruber）和厚壁抱球虫（Globorotalia crassaformis）进行成对的镁/钙 - δ¹⁸O测量，重建了表层和永久性密度跃层的温度以及海水的δ¹⁸O，并确定了亚极地有孔虫大西洋新方球虫（Neogloboquadrina atlantica）的丰度。我们发现，直到约260万年前，U1313站点上方才首次出现亚北极锋表层海水的重大冰川入侵。在我们的研究区间内，北大西洋中纬度地区的（次）表层重组在任何时候都与末次盛冰期不同。我们的研究结果表明，不能严格按照末次盛冰期的过程来解释北半球大冰期增强期间的间冰期 - 冰期旋回放大。这也意味着在北半球大冰期增强期间，U1313站点冰川生产力的提高不仅仅是由亚北极锋向南偏转所驱动的。我们认为，从270万年前开始，冷核涡旋的营养注入以及冰川粉尘输送的增强可能在提高北大西洋中纬度地区的输出生产力方面起到了额外的作用。