Since the middle Miocene climatic transition, the Earth's climate has steadily cooled. The late Miocene global cooling (LMGC) and the Northern Hemisphere Glaciation (NHG) were two key cooling events occurring during this time. To better understand the mechanisms underlying these cooling events, changes in radiolarian microfossil assemblages were examined in this study, aiming at the reconstructing of oceanographic changes that have occurred at Ocean Drilling Program site 1208 during the last 10 million years. Sea surface temperatures (SSTs) were reconstructed based on radiolarian species that were extant 0–10 million years ago. Reconstructed SSTs were then compared with previously published alkenone-based SSTs at site 1208, and it was found that overall, using SSTs based only on extant radiolarian species yielded a correct record for the last 10 million years. However, large discrepancies were observed between radiolarian- and alkenone-based SSTs during LMGC and the NHG. These discrepancies were attributed to the sustained influence of subsurface water (at depths from ∼50 to 100 m) on assemblages of radiolarians during extreme cooling events. Relative abundances of other radiolarian groups indicated that during LMGC, there was a reorganization of the regional oceanography that probably weakened the Pacific meridional overturning circulation, increased the meridional temperature gradient, and caused a southward migration of the subtropical front. Probably, the North Pacific intermediate water expanded southeastward during NHG.

中文翻译：

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过去一千万年来西北太平洋中部海洋学的演变，重点关注晚中新世全球变冷

自中新世中期气候转变以来，地球气候稳步变冷。中新世晚期全球变冷（LMGC）和北半球冰川作用（NHG）是这一时期发生的两个关键变冷事件。为了更好地了解这些冷却事件背后的机制，本研究检查了放射虫微化石组合的变化，旨在重建海洋钻探计划 1208 站点在过去 1000 万年中发生的海洋变化。海面温度 (SST) 是根据 0-1000 万年前现存的放射虫物种重建的。然后将重建的海表温度与之前在 1208 站点发布的基于烯酮的海表温度进行比较，结果发现，总体而言，使用仅基于现存放射虫物种的海表温度可以产生过去 1000 万年的正确记录。然而，在 LMGC 和 NHG 期间，观察到基于放射虫和基于烯酮的海表温度存在很大差异。这些差异归因于极端冷却事件期间地下水（深度约 50 至 100 m）对放射虫聚集的持续影响。其他放射虫类群的相对丰度表明，LMGC期间，区域海洋发生了重组，可能削弱了太平洋经向翻转环流，增加了经向温度梯度，并导致副热带锋向南迁移。很可能，北太平洋中间水在 NHG 期间向东南扩展。