The Middle Eocene Climatic Optimum (MECO; ca. 40 Ma) was a prominent global warming event that lasted 400 kyr and was characterized by a 4–6 °C rise in high latitude surface and deep-water temperatures. As the radiolarian plankton response to this warming event is practically unknown, whole assemblage quantitative analyses were undertaken on well-preserved radiolarian assemblages from ODP Site 1051 (western North Atlantic). Although radiolarians have apparently benefited at this site from increase in oceanic fertility induced by the MECO, this event does not appear to have had a severe impact on subtropical radiolarian fauna. No prominent faunal turnover was found in the studied interval, suggesting that subtropical radiolarians are relatively resilient to transient warming events. Likewise, variations in radiolarian assemblage composition establish that ocean warming induced only a weak ecological response. One of the most striking faunal changes associated with the MECO is the clear increase in radiolarian diversity (taxic richness), as a result of the northward migration of warm tropical radiolarian species. Similarly, several typical middle Eocene tropical species are found to be more abundant in the warmest interval. In addition to these poleward migrations, we identified three radiolarian clusters composed of warm-water or cool-water species, as well as two abundant artostrobiid species which may represent nutrient opportunists.

中始新世气候最适期（MECO；约 40 Ma）是一次重要的全球变暖事件，持续了 400 kyr，其特点是高纬度地表和深水温度上升 4-6 °C。由于放射虫浮游生物对这种变暖事件的反应实际上是未知的，因此对 ODP 站点 1051（北大西洋西部）保存完好的放射虫组合进行了整体组合定量分析。尽管该地点的放射虫明显受益于 MECO 引起的海洋肥力增加，但这一事件似乎并未对亚热带放射虫动物区系产生严重影响。在研究期间没有发现明显的动物区系更替，这表明亚热带放射虫对短暂变暖事件的抵抗力相对较强。同样地，放射虫组合组成的变化表明海洋变暖仅引起微弱的生态反应。与 MECO 相关的最显着的动物区系变化之一是由于温暖的热带放射虫物种向北迁移，放射虫多样性（分类丰富度）明显增加。同样，一些典型的始新世中期热带物种被发现在最温暖的时期更为丰富。除了这些向极地的迁徙之外，我们还发现了三个由温水或冷水物种组成的放射虫簇，以及两个丰富的artostrobeid物种，它们可能代表营养机会主义者。这是温暖热带放射虫物种向北迁移的结果。同样，一些典型的始新世中期热带物种被发现在最温暖的时期更为丰富。除了这些向极地的迁徙之外，我们还发现了三个由温水或冷水物种组成的放射虫簇，以及两个丰富的artostrobeid物种，它们可能代表营养机会主义者。这是温暖热带放射虫物种向北迁移的结果。同样，一些典型的始新世中期热带物种被发现在最温暖的时期更为丰富。除了这些向极地的迁徙之外，我们还发现了三个由温水或冷水物种组成的放射虫簇，以及两个丰富的artostrobeid物种，它们可能代表营养机会主义者。