Eocene global warming events driven by ventilation of oceanic dissolved organic carbon

摘要

'Hyperthermals' are intervals of rapid, pronounced global warming known from six episodes within the Palaeocene and Eocene epochs (～65-34 million years (Myr) ago). The most extreme hyperthermal was the ～170 thousand year (kyr) interval of 5-7 ℃ global warming during the Palaeocene-Eocene Thermal Maximum (PETM, 56 Myr ago). The PETM is widely attributed to massive release of greenhouse gases from buried sedimentary carbon reservoirs, and other, comparatively modest, hyperthermals have also been linked to the release of sedimentary carbon. Here we show, using new 2.4-Myr-long Eocene deep ocean records, that the comparatively modest hyperthermals are much more numerous than previously documented, paced by the eccentricity of Earth's orbit and have snorter durations (～40 kyr) and more rapid recovery phases than the PETM. These findings point to the operation of fundamentally different forcing and feedback mechanisms than for the PETM, involving redistribution of carbon among Earth's readily exchangeable surface reservoirs rather than carbon exhumation from, and subsequent burial back into, the sedimentary reservoir. Specifically, we interpret our records to indicate repeated, large-scale releases of dissolved organic carbon (at least 1,600 giga-tonnes) from the ocean by ventilation (strengthened oxidation) of the ocean interior. The rapid recovery of the carbon cycle following each Eocene hyperthermal strongly suggests that carbon was re-sequestered by the ocean, rather than the much slower process of silicate rock weathering proposed for the PETM. Our findings suggest that these pronounced climate warming events were driven not by repeated releases of carbon from buried sedimentary sources, but, rather, by patterns of surficial carbon redistribution familiar from younger intervals of Earth history.%地球历史上最引人注目的全球变暖事件或称rn“hyperthermal”事件之一是“古新世-始新世rn极热事件”(PETM)。这个事件发生存距今约rn5,600九年前，很可能是甲烷从沉积物中释放rn造成的。另外几个程度较差的变暖时期出现在rnPETM之后约600万～800万年。Sexton等人发rn现，这些较小的事件是短暂的、且相当频繁rn的，其节奏是由地球轨道确定的。它们迅速的rn开始和恢复说明存在个机制，该机制主要取rn决于大气中的碳和溶解到海洋中的有机碳之间rn的转换，这与PETM事件中温室气体从埋藏的rn碳库中较慢释放的情形形成鲜明对比。这些发rn现有可能帮助回答关于极端变暖事件中全球碳rn周期怎样发挥作用的根本性问题。