Feldspar Pb-isotope ratios for earliest Pleistocene sediments of the sub-polar North Atlantic Ocean

Relatively little is known in detail about the locations of the early Pleistocene ice-sheets responsible for ice-rafted debris (IRD) inputs to the sub-polar North Atlantic Ocean during intensification of northern hemisphere glaciation (iNHG). To shed new light on this problem, we present the first combined in-depth analysis of IRD flux and geochemical provenance of individual sand-sized IRD deposited in the sub-polar North Atlantic Ocean during the earliest large amplitude Pleistocene glacial, marine isotope stage (MIS) 100 (~2.52 Ma), arguably the key glacial during iNHG. IRD provenance is assessed using laser ablation lead (Pb) isotope analyses of single feldspar grains. We find that the Pb-isotope composition (206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb) of individual ice-rafted (>150 µm) feldspars deposited at DSDP Site 611A, ODP Site 981 and IODP Site U1308 during MIS 100 records a shift from predominantly Archaean-aged circum-North Atlantic Ocean continental sources during early glacial ice-rafting events to dominantly Palaeozoic and Proterozoic-aged sources during full glacial conditions. The distribution of feldspars in Pb-Pb space for full glacial MIS 100 more closely resembles that documented for feldspars deposited at the centre of the last glacial IRD belt (at IODP/DSDP Site U1308/609) during ambient (non-Heinrich-event) ice-rafting episodes of MIS 2 (~23.8 ka) than that documented for MIS 5d (~106 ka). Comparison of our early Pleistocene and last glacial cycle datasets suggests that MIS 100 was characterised by abundant iceberg calving from large ice-sheets on multiple continents in the high northern latitudes (not just on Greenland).

在北半球冰盖强化期（intensification of northern hemisphere glaciation, iNHG）内，向副极地北大西洋输入冰筏碎屑（ice-rafted debris, IRD）的早更新世冰盖的具体分布细节，迄今仍鲜有研究。为厘清这一科学问题，我们首次对早更新世最大振幅冰川期——海洋同位素阶段（marine isotope stage, MIS）100（约252万年）——期间沉积于副极地北大西洋的砂级冰筏碎屑的通量与单颗粒冰筏碎屑的地球化学物源开展联合深度分析。该冰川期被认为是北半球冰盖强化期（iNHG）的关键冰川事件。本研究通过对单颗粒长石开展激光剥蚀铅（Pb）同位素分析，对冰筏碎屑的物源进行示踪。我们发现，MIS 100时期沉积于深海钻探计划（Deep Sea Drilling Project, DSDP）611A站位、大洋钻探计划（Ocean Drilling Program, ODP）981站位以及综合大洋钻探计划（Integrated Ocean Drilling Program, IODP）U1308站位的单个冰筏搬运（粒径>150μm）长石的铅同位素组成（206Pb/204Pb、207Pb/204Pb与208Pb/204Pb），记录了一次物源转变：冰筏搬运早期以环北大西洋太古代陆壳物质为主，而盛冰期则以古生代与元古代陆壳物质为主。盛冰期MIS 100的长石在铅同位素二维空间中的分布特征，与MIS 2（约23.8万年）背景冰筏搬运期（非海因里希事件期）沉积于末次冰期冰筏碎屑带核心站位（IODP/DSDP U1308/609站位）的长石分布特征更为相似，而与MIS 5d（约10.6万年）的长石分布特征差异较大。对比本研究的早更新世与末次冰期数据集可知，MIS 100时期的冰筏碎屑主要来自高纬度多个大陆的大型冰盖（而非仅格陵兰岛），表现为大规模冰山崩解事件频发。