Alvarez Walter
Department of Earth and Planetary Science, University of California, Berkeley 94720-4767, USA.
Astrobiology. 2003 Spring;3(1):153-61. doi: 10.1089/153110703321632480.
The five major mass extinctions identified in 1982 by Raup and Sepkoski have expanded to six, with the suggestion that the Permian-Triassic extinction was a double event. Is there a general explanation for great mass extinctions, or can they result from different triggers, or even from internal system instabilities? The two most-discussed candidates for a general extinction mechanism are impacts and flood-basalt eruptions. A compilation of evidence for impact at the times of mass extinctions shows that this cause is abundantly confirmed in the case of the Cretaceous-Tertiary extinction and the late Eocene, which is a time of minor and gradual extinction, but little or no evidence connects other major extinctions to impact. On the other hand, there is a remarkable time correlation between flood basalts and four major extinctions, but no other evidence that flood basalts cause mass extinctions. The evidence for an impact-extinction linkage is strikingly different from that for a connection between flood basalts and extinctions. Flood basalts cover larger areas than craters and their associated thick ejecta blankets, which are thus less likely to be found. Impacts distribute proxies globally at instantaneous time horizons, whereas flood-basalt events are extended in time, and no remote proxies have been recognized. Many global killing mechanisms have been proposed in the case of impacts, but few have been suggested for flood basalts. It is possible that flood basalts are triggered by impact, but it is not obvious how impacts could result from anything other than chance. The hypothesis that impacts are the general cause of mass extinctions has not received supporting evidence, but has not been falsified. The hypothesis that flood basalts are the general cause of mass extinctions is supported by evidence from timing, but is not susceptible to falsification. Other candidates for general extinction causes, especially sea-level changes and system instabilities, would require separate treatment. The question is still very much open.
1982年劳普和塞普科夫斯基确认的五次大灭绝事件已扩展为六次，有人认为二叠纪-三叠纪灭绝是一次双重事件。对于大灭绝是否存在一个普遍的解释，或者它们是否可能由不同的触发因素甚至内部系统不稳定导致？关于普遍灭绝机制，讨论最多的两个候选因素是撞击和洪流玄武岩喷发。一份关于大灭绝时期撞击证据的汇编表明，在白垩纪-第三纪灭绝以及始新世晚期（这是一个小规模渐进灭绝时期）的情况下，这一原因得到了充分证实，但几乎没有证据将其他主要灭绝事件与撞击联系起来。另一方面，洪流玄武岩与四次主要灭绝事件之间存在显著的时间相关性，但没有其他证据表明洪流玄武岩会导致大灭绝。撞击与灭绝之间联系的证据与洪流玄武岩和灭绝之间联系的证据截然不同。洪流玄武岩覆盖的面积比陨石坑及其相关的厚喷发物覆盖层更大，因此不太可能被发现。撞击在瞬间的时间范围内将代理指标分布在全球，而洪流玄武岩事件在时间上是扩展的，并且尚未识别出远程代理指标。在撞击的情况下已经提出了许多全球杀伤机制，但针对洪流玄武岩的却很少。洪流玄武岩有可能是由撞击引发的，但除了偶然因素外，撞击如何由其他因素导致并不明显。撞击是大灭绝普遍原因的假设尚未得到支持证据，但也未被证伪。洪流玄武岩是大灭绝普遍原因的假设在时间上的证据支持，但不容易被证伪。其他关于普遍灭绝原因的候选因素，特别是海平面变化和系统不稳定，需要单独探讨。这个问题仍然非常悬而未决。
