Episodes of pronounced climatic warming in the early Eocene (56–48 Ma) provide insights into how biological systems might respond to future climate changes. Eocene climate reconstructions for the Southwest (SW) Pacific have proved challenging due to some disparities between geochemical proxy-based estimates for sea temperatures and estimates derived from climate models. Changes in marine phytoplankton populations through the early Eocene provide a means to evaluate model and proxy-based approaches and also reveal how climatic changes affected the ocean ecosystem. This study documents early to middle Eocene calcareous nannofossil assemblages from legacy Deep Sea Drilling Project (DSDP) sites in the SW Pacific. We integrate nannofossil assemblage changes with foraminiferal-based stable isotopes and other microfossil data to provide updated age models for DSDP Sites 207 (Lord Howe Rise) and 277 (Campbell Plateau), and to infer past environmental change through the early to mid-Eocene in the SW Pacific. Although these sites record a short hiatus in the earliest Eocene, deposition resumed by ~54 Ma, prior to the onset of the Early Eocene Climate Optimum (EECO). Both sites show an increase in warm-water taxa from the onset of the EECO until approximately 50.5 Ma. Abundance of warm-water taxa was higher (~23%) at the EECO onset at the more northerly Site 207 and increased only slightly to ~27%, whereas at Site 277 warm-water taxa comprised only ~3% at the EECO onset but increase to ~31% by 50.5 Ma. Cool-water taxa at both sites were present in very sparse numbers (<3%) through much of the EECO and only began to increase in abundance after 50 Ma, concomitant with a decrease in warm-water taxa. These results are similar to other mid-latitude sites in the SW Pacific and Indian Ocean, including mid-Waipara River (Canterbury Basin, New Zealand) and ODP Site 762 (Exmouth Plateau), and indicate assemblages during the EECO are best described as warm temperate. Benthic δ¹⁸O records from both sites show good agreement with nannofossil trends and, despite the possibility of recrystallization at Site 277, sea surface temperatures (SSTs) indicated by planktic foraminifera also imply warm temperate conditions. This suggests that a warm proto-East Australian Current extended to at least 55°S during the EECO, displacing the proto-Tasman Front southward, which migrated northward after termination of the EECO possibly due to expansion of a proto-Ross Sea Gyre.

始于始新世（56-48 Ma）的明显气候变暖事件提供了有关生物系统如何响应未来气候变化的见解。由于基于地球化学代理的海水温度估算值与气候模型估算值之间存在差异，西南太平洋西南地区的始新世气候重建已被证明具有挑战性。始于始新世早期的海洋浮游植物种群变化为评估基于模型和基于代理的方法提供了一种手段，也揭示了气候变化如何影响海洋生态系统。这项研究记录了西南太平洋遗留的深海钻探项目（DSDP）站点的始新世至中新世钙质纳米化石组合。我们将纳米化石的组合变化与基于有孔虫的稳定同位素和其他微化石数据相结合，以提供DSDP站点207（豪勋爵山）和277（坎贝尔高原）的更新年龄模型，并推断出始于始新世中期至早期的环境变化西南太平洋。尽管这些地点在始新世最早就有短暂的裂隙，但在早始新世最佳气候（EECO）出现之前，沉积在〜54 Ma时恢复了。从EECO开始到直到大约50.5 Ma，这两个地点都显示出热水分类单元的增加。在偏北的207号站点，EECO发生时，热水分类单元的丰度较高（〜23％），而仅略微增加至〜27％，而在277号站点，EECO发生时，温暖分类单元仅占〜3％，但由50.5 Ma增加到〜31％。在整个EECO中，这两个地点的冷水分类单元都非常稀少（<3％），并且在50 Ma之后才开始增加丰度，与此同时，温水分类单元也减少了。这些结果与西南太平洋和印度洋的其他中纬度站点相似，包括怀帕拉河中部（新西兰坎特伯雷盆地）和ODP站点762（埃克斯茅斯高原），表明EECO期间的组合最好被描述为温暖的温带的。底栖生物 并指出EECO期间的组合最好被描述为温暖的温带。底栖生物 并指出EECO期间的组合最好被描述为温暖的温带。底栖生物两个站点的¹⁸ O记录都与纳米化石趋势很好地吻合，尽管有可能在站点277发生重结晶，但有孔有孔虫指示的海表温度（SST）也暗示着温暖的温带条件。这表明在EECO期间，一个温暖的东澳大利亚原始海流至少延伸至55°S，向塔斯曼前锋向南移动，该位置在EECO终止后向北迁移，这可能是由于罗斯海涡的原始扩张所致。