NOAA/WDS Paleoclimatology - Southern Ocean Eocene-Oligocene Foraminiferal d18O and Mg/Ca Data

Constraining the magnitude of high-latitude temperature change across the Eocene–Oligocene transition (EOT) is essential for quantifying the magnitude of Antarctic ice-sheet expansion and understanding regional climate response to this event. To this end, we constructed high-resolution stable oxygen isotope (δ18O) and magnesium/calcium (Mg/Ca) records from planktic and benthic foraminifera at four Ocean Drilling Program (ODP) sites in the Southern Ocean. Planktic foraminiferal Mg/Ca records from the Kerguelen Plateau (ODP Sites 738, 744, and 748) show a consistent pattern of temperature change, indicating 2–3 °C cooling in direct conjunction with the first step of a two-step increase in benthic and planktic foraminiferal δ18O values across the EOT. In contrast, benthic Mg/Ca records from Maud Rise (ODP Site 689) and the Kerguelen Plateau (ODP Site 748) do not exhibit significant temperature change. The contrasting temperature histories derived from the planktic and benthic Mg/Ca records are not reconcilable, since vertical δ18O gradients remained nearly constant at all sites between 35.0 and 32.5 Ma. Based on the coherency of the planktic Mg/Ca records from the Kerguelen Plateau sites and complications with benthic Mg/Ca paleothermometry at low temperatures, the planktic Mg/Ca records are deemed the most reliable measure of Southern Ocean temperature change. We therefore interpret a uniform cooling of 2–3 °C in both deep surface (thermocline) waters and intermediate deep waters of the Southern Ocean across the EOT. Cooling of Southern Ocean surface waters across the EOT was likely propagated to the deep ocean, since deep waters were primarily sourced on the Antarctic margin throughout this time interval. Removal of the temperature component from the observed foraminiferal δ18O shift indicates that seawater δ18O values increased by 0.6 ± 0.15‰ across the EOT interval, corresponding to an increase in global ice volume to a level equivalent with 60–130% modern East Antarctic ice sheet volume.

约束始新世-渐新世过渡期（Eocene–Oligocene transition, EOT）内高纬度温度变化的幅度，对于量化南极冰盖扩张规模、理解区域气候对该事件的响应至关重要。为此，我们从南大洋4个大洋钻探计划（Ocean Drilling Program, ODP）站位的浮游有孔虫与底栖有孔虫壳体中构建了高分辨率的稳定氧同位素（δ¹⁸O）与镁钙比（Mg/Ca）记录。凯尔盖朗高原（大洋钻探计划站位738、744与748）的浮游有孔虫Mg/Ca记录呈现出一致的温度变化模式，表明在EOT期间，伴随底栖与浮游有孔虫δ¹⁸O两阶段增长的第一阶段，发生了2~3℃的降温。与之形成对比的是，莫德海隆（大洋钻探计划站位689）与凯尔盖朗高原（大洋钻探计划站位748）的底栖Mg/Ca记录并未出现显著的温度变化。由于在35.0~32.5 Ma期间所有站位的垂直δ¹⁸O梯度几乎保持恒定，浮游与底栖Mg/Ca记录所反映的温度历史差异无法得到合理解释。基于凯尔盖朗高原站位的浮游Mg/Ca记录的一致性，以及低温环境下底栖Mg/Ca古温度测定法存在的复杂性，我们认为浮游Mg/Ca记录是反映南大洋温度变化最可靠的指标。因此我们推断，在EOT期间，南大洋的表层（温跃层）水与中层深水均发生了2~3℃的均匀降温。EOT期间南大洋表层水的降温可能传播至深海，因为该时段内深层水主要源自南极陆缘。从观测到的有孔虫δ¹⁸O偏移中去除温度分量后可知，海水δ¹⁸O值在EOT区间内上升了0.6±0.15‰，对应全球冰量增至相当于现代南极东部冰盖体积60%~130%的水平。