Sporomorphs and quantitative analysis of the Eocene record from IODP Hole 318-U1356A

The early Eocene epoch was characterized by extreme global warmth, which in terrestrial settings was characterized by an expansion of near-tropical vegetation belts into the high latitudes. During the middle to late Eocene, global cooling caused the retreat of tropical vegetation to lower latitudes. In high-latitude settings, near-tropical vegetation was replaced by temperate floras. This floral change has recently been traced as far south as Antarctica, where along the Wilkes Land margin paratropical forests thrived during the early Eocene and temperate Nothofagus forests developed during the middle Eocene. Here we provide both qualitative and quantitative palynological data for this floral turnover based on a sporomorph record recovered at Integrated Ocean Drilling Program (IODP) Site U1356 off the Wilkes Land margin. Following the nearest living relative concept and based on a comparison with modern vegetation types, we examine the structure and diversity patterns of the Eocene vegetation along the Wilkes Land margin. Our results indicate that the early Eocene forests along the Wilkes Land margin were characterized by a diverse canopy composed of plants that today occur in tropical settings; their richness pattern was similar to that of present-day forests from New Caledonia. The middle Eocene forests were characterized by a canopy dominated by Nothofagus and exhibited richness patterns similar to modern Nothofagus forests from New Zealand.

始新世早期以极端全球暖化为典型特征，陆地环境中近热带植被带向高纬度地区扩张。至中晚始新世，全球变冷驱动热带植被向低纬度区域退缩，高纬度地区的近热带植被遂被温带植物区系取代。近期，这一植物群更替事件的分布范围被追溯至南极最南端：在威尔克斯地（Wilkes Land）陆缘，始新世早期发育繁盛的副热带森林，中始新世则形成了温带南山毛榉（Nothofagus）林。本文基于综合大洋钻探计划（Integrated Ocean Drilling Program, IODP）U1356站位在威尔克斯地陆缘海域获取的孢子花粉记录，提供了此次植物群更替事件的定性与定量孢粉学数据。基于最近现生亲缘类群（nearest living relative）概念，并结合现代植被类型开展对比分析，我们研究了威尔克斯地陆缘始新世植被的结构与多样性格局。结果表明，威尔克斯地陆缘的始新世早期森林拥有多样的冠层植被组成，其组成植物现今仅分布于热带区域；该森林的物种丰富度格局与现代新喀里多尼亚的森林相似。中始新世森林则以南山毛榉主导的冠层为典型特征，其物种丰富度格局与现代新西兰的南山毛榉林相近。