Catling D C, Claire M W, Zahnle K J
Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol, UK.
Philos Trans A Math Phys Eng Sci. 2007 Jul 15;365(1856):1867-88. doi: 10.1098/rsta.2007.2047.
In modern marine sediments, the anoxic decomposition of organic matter generates a significant flux of methane that is oxidized microbially with sulphate under the seafloor and never reaches the atmosphere. In contrast, prior to ca 2.4Gyr ago, the ocean had little sulphate to support anaerobic oxidation of methane (AOM) and the ocean should have been an important methane source. As atmospheric O2 and seawater sulphate levels rose on the early Earth, AOM would have increasingly throttled the release of methane. We use a biogeochemical model to simulate the response of early atmospheric O2 and CH4 to changes in marine AOM as sulphate levels increased. Semi-empirical relationships are used to parameterize global AOM rates and the evolution of sulphate levels. Despite broad uncertainties in these relationships, atmospheric O2 concentrations generally rise more rapidly and to higher levels (of order approx. 10(-3) bar versus approx. 10(-4) bar) as a result of including AOM in the model. Methane levels collapse prior to any significant rise in O2, but counter-intuitively, methane re-rises after O2 rises to higher levels when AOM is included. As O2 concentrations increase, shielding of the troposphere by stratospheric ozone slows the effective reaction rate between oxygen and methane. This effect dominates over the decrease in the methane source associated with AOM. Thus, even with the inclusion of AOM, the simulated Late Palaeoproterozoic atmosphere has a climatologically significant level of methane of approximately 50ppmv.
在现代海洋沉积物中，有机物质的缺氧分解会产生大量甲烷通量，这些甲烷在海底被微生物与硫酸盐发生氧化反应，因而不会释放到大气中。相比之下，在大约24亿年前之前，海洋中几乎没有硫酸盐来支持甲烷的厌氧氧化（AOM），那时海洋应该是一个重要的甲烷源。随着早期地球大气中氧气和海水中硫酸盐水平的上升，AOM会越来越多地抑制甲烷的释放。我们使用一个生物地球化学模型来模拟随着硫酸盐水平增加，早期大气中氧气和甲烷对海洋AOM变化的响应。利用半经验关系对全球AOM速率和硫酸盐水平的演变进行参数化。尽管这些关系存在广泛的不确定性，但由于在模型中纳入了AOM，大气中氧气浓度通常上升得更快，达到更高水平（约为10⁻³巴，而之前约为10⁻⁴巴）。在氧气显著上升之前，甲烷水平会下降，但与直觉相反的是，当纳入AOM时，在氧气上升到更高水平之后，甲烷又会再次上升。随着氧气浓度增加，平流层臭氧对对流层的屏蔽作用减缓了氧气和甲烷之间的有效反应速率。这种效应超过了与AOM相关的甲烷源减少的影响。因此，即使纳入了AOM，模拟的古元古代晚期大气中甲烷含量仍达到约50 ppmv的气候学显著水平。
