Carbon cycle change in the Eocene@en

The transition from the extreme global warmth of the early Eocene 'greenhouse' climate ~55 million years ago to the present glaciated state is one of the most prominent changes in Earth's climatic evolution. It is widely accepted that large ice sheets first appeared on Antarctica ~34 million years ago, coincident with decreasing atmospheric carbon dioxide concentrations and a deepening of the calcite compensation depth in the world's oceans, and that glaciation in the Northern Hemisphere began much later, between 10 and 6 million years ago. Here we present records of sediment and foraminiferal geochemistry covering the greenhouse-icehouse climate transition. We report evidence for synchronous deepening and subsequent oscillations in the calcite compensation depth in the tropical Pacific and South Atlantic oceans from ~42 million years ago, with a permanent deepening 34 million years ago. The most prominent variations in the calcite compensation depth coincide with changes in seawater oxygen isotope ratios of up to 1.5 per mil, suggesting a lowering of global sea level through significant storage of ice in both hemispheres by at least 100 to 125 metres. Variations in benthic carbon isotope ratios of up to ~1.4 per mil occurred at the same time, indicating large changes in carbon cycling. We suggest that the greenhouse-icehouse transition was closely coupled to the evolution of atmospheric carbon dioxide, and that negative carbon cycle feedbacks may have prevented the permanent establishment of large ice sheets earlier than 34 million years ago.距今约5500万年前的始新世早期极端温暖的"温室"全球气候，到现今冰期状态的气候转型，是地球气候演化历程中最显著的变化之一。学界普遍认为，南极大陆约3400万年前首次出现大型冰盖，这一事件与大气二氧化碳浓度下降以及全球海洋方解石补偿深度（calcite compensation depth）加深同步；而北半球的冰川作用则晚得多，发生于1000万至600万年前。本研究报道了覆盖该温室-冰室气候转型期的沉积与有孔虫地球化学（foraminiferal geochemistry）记录。研究发现，约4200万年前起，热带太平洋与南大西洋的方解石补偿深度同步加深，随后出现多次振荡，并在3400万年前发生永久性加深。方解石补偿深度最显著的变化，与海水氧同位素比值高达1.5‰的波动相吻合，这表明由于南北半球冰盖大规模储水，全球海平面至少下降了100至125米。与此同时，底栖碳同位素比值（benthic carbon isotope ratios）出现了最高约1.4‰的波动，这指示全球碳循环发生了剧烈变化。我们认为，温室-冰室气候转型与大气二氧化碳的演化紧密耦合，且碳循环负反馈可能阻止了大型冰盖在3400万年前之前永久形成。