The Piacenzian (3.60–2.58 Ma) covers the last stage of the Neogene just before the Earth's climate turned from relatively stable warm conditions to the cooler climate with high amplitude glacial‐interglacial oscillations of the Pleistocene. Even during this period early fluctuations towards cooler conditions occurred, and sea surface temperature (SST) reconstructions show stepwise increasing gradients. The zonal Pacific SST gradient which indicates the strength of the Walker circulation appears to have increased in two steps starting in the Piacenzian. We investigated vegetation and climate change in western equatorial South America under the influence of the Walker circulation and to detect signs for the onset of cooling in the tropics. We studied vegetation changes in western Ecuador using palynological analysis of 88 sediment samples from marine Ocean Drilling Program Site 1239 dated between 3.9 and 2.7 Ma. A general trend towards more open vegetation is observed. The climate changes towards cooler conditions, which is manifested by a lowering of the forest line from 3.3 Ma on. The increase of Amaranthaceae pollen after 3.1 Ma suggests drier conditions along the coast. A comparison with mid‐Piacenzian warm period (mPWP) modeling shows that data and models agree regarding a drier coastal climate during the mPWP. The isochronous occurrence of environmental changes in the presented record, that is, cooling and coastal drying, with the first major pulse of ice‐rafted debris and cooling temperatures in the Northern Hemisphere (between 3.28 and 3.31 Ma) suggests that these changes might have been a precursor of the intensification of the Northern Hemisphere glaciation.

中文翻译：

---

南美环境的Piacenzian环境变化和凉爽和干燥条件的开始

Piacenzian（3.60–2.58 Ma）涵盖了新近纪的最后阶段，就在地球气候从相对稳定的温暖条件转变为较凉爽的气候之前，这是更新世的高冰期-冰河间振荡。即使在此期间，也会出现朝着凉爽条件的早期波动，海表温度（SST）重建显示出逐步增加的梯度。指示沃克环流强度的太平洋太平洋海表温度梯度似乎从Piacenzian开始以两步增加。我们研究了沃克环流的影响下，赤道南美洲西部的植被和气候变化，并检测了热带降温的迹象。我们通过对来自海洋海洋钻探计划站点1239的3.9至2.7 Ma之间的88个沉积物样本进行了孢粉学分析，研究了厄瓜多尔西部的植被变化。观察到朝向更开放的植被的总体趋势。气候向凉爽的环境变化，这表现为森林线从3.3 Ma开始下降。3.1 Ma之后，A菜花粉的增加表明沿海地区的天气更干燥。与Piacenzian中暖期（mPWP）建模的比较表明，在mPWP期间，有关较干燥的沿海气候的数据和模型一致。在所提交的记录中，环境变化是同步发生的，即冷却和沿海干燥，这是北半球的第一个主要冰激凌碎片和冷却温度脉冲（介于3.28和3之间）。