“古新世-始新世极热” (PETM)是一个众所周知的突然变暖事件，发生在距今大约5580万年前，通常被认为是由温室气体的大量释放引起的（很可能来自水合甲烷），如在一次碳同位素大幅度偏移（异常）中所记录到的那样。
Ross Secord及其同事通过对来自美国怀俄明州Bighorn Basin的哺乳动物牙齿进行同位素分析发现，在那次同位素异常出现之前大陆气候变暖大约5 摄氏度。因此，PETM似乎是由至少两个单独的变暖事件引起的，每个都是由不同过程造成的。（生物谷Bioon.com）
生物谷推荐英文摘要：
Nature doi:10.1038/nature09441
Continental warming preceding the Palaeocene–Eocene thermal maximum
Ross Secord,rsecord2@unl.eduPhilip D. Gingerich,Kyger C. Lohmann& Kenneth G. MacLeod
Marine and continental records1 show an abrupt negative shift in carbon isotope values at ~55.8?Myr ago. This carbon isotope excursion (CIE) is consistent with the release of a massive amount of isotopically light carbon into the atmosphere and was associated with a dramatic rise in global temperatures termed the Palaeocene–Eocene thermal maximum (PETM). Greenhouse gases released during the CIE, probably including methane, have often been considered the main cause of PETM warming. However, some evidence from the marine record suggests that warming directly preceded the CIE2, 3, 4, raising the possibility that the CIE and PETM may have been linked to earlier warming with different origins. Yet pre-CIE warming is still uncertain. Disentangling the sequence of events before and during the CIE and PETM is important for understanding the causes of, and Earth system responses to, abrupt climate change. Here we show that continental warming of about 5?°C preceded the CIE in the Bighorn Basin, Wyoming. Our evidence, based on oxygen isotopes in mammal teeth (which reflect temperature-sensitive fractionation processes) and other proxies, reveals a marked temperature increase directly below the CIE, and again in the CIE. Pre-CIE warming is also supported by a negative amplification of δ13C values in soil carbonates below the CIE. Our results suggest that at least two sources of warming—the earlier of which is unlikely to have been methane—contributed to the PETM.
