Marked by the expansion of ice sheets in the high latitudes, the intensification of Northern Hemisphere glaciation across the Plio/Pleistocene transition at ~ 2.7 Ma represents a critical interval of late Neogene climate evolution. To date, the characteristics of climate change in North America during that time and its imprint on vegetation has remained poorly constrained because of the lack of continuous, highly resolved terrestrial records. We here assess the vegetation dynamics in northwestern North America during the late Pliocene and early Pleistocene (c. 2.8–2.4 Ma) based on a pollen record from a lacustrine sequence from paleo-Lake Idaho, western Snake River Plain (USA) that has been retrieved within the framework of an International Continental Drilling Program (ICDP) coring campaign. Our data indicate a sensitive response of forest ecosystems to glacial/interglacial variability paced by orbital obliquity across the study interval, and also highlight a distinct expansion of steppic elements that likely occurs during the first strong glacial of the Pleistocene, i.e. Marine Isotope Stage 100. The pollen data document a major forest biome change at ~ 2.6 Ma that is marked by the replacement of conifer-dominated forests by open mixed forests. Quantitative pollen-based climate estimates suggest that this forest reorganisation was associated with an increase in precipitation from the late Pliocene to the early Pleistocene. We attribute this shift to an enhanced moisture transport from the subarctic Pacific Ocean to North America, confirming the hypothesis that ocean-circulation changes were instrumental in the intensification of Northern Hemisphere glaciation.

以高纬度地区冰盖的扩张为标志，北半球冰川在约2.7 Ma的上里新世/更新世过渡期间的强化代表了新近纪晚期气候演化的关键间隔。迄今为止，由于缺乏连续的，高度解析的陆地记录，北美在那段时期的气候变化特征及其对植被的影响仍然受到限制。我们根据来自美国蛇河平原西部（美国）爱达荷州古湖湖相序列的花粉记录，评估了上新世晚期和更新世早期（约2.8-2.4 Ma）期间北美西北部的植被动态。在国际大陆钻探计划（ICDP）取芯活动的框架内检索到。我们的数据表明，在整个研究间隔中，森林生态系统对由轨道倾角变化的冰川/冰川间变化的敏感响应，还强调了在更新世的第一个强冰川时期（即海洋同位素第100期）可能发生的阶梯状元素的明显扩展。花粉数据记录了大约2.6 Ma的主要森林生物群落变化，其特征是开放混交林取代了以针叶树为主的森林。基于花粉的定量气候估计表明，这种森林重组与上新世晚期至更新世早期降水增加有关。我们将这一转变归因于水分的增加，从南亚太平洋到北美洲，