Faint young Sun and the carbon cycle: implication for the Proterozoic global glaciations
Faint young Sun and the carbon cycle: implication for the Proterozoic global glaciations
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DOI：
10.1016/s0012-821x(03)00396-0
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发表时间：
2003-09-30
影响因子：
5.3
通讯作者：
Tajika, E
中科院分区：
文献类型：
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作者：
Tajika, E
在元古代，地球可能多次被全球性的冰层覆盖。虽然新元古代和古元古代的冰川作用可能是“滚雪球”式的地球事件，但没有证据表明这种冰川作用发生在中生代。可以假设，在地球历史的早期，较暗的太阳可能使地球更容易受到全球冰川的影响。本文利用一个简单的碳地球化学循环模型和一维能量平衡气候模型，研究了太阳辐射通量和土壤生物活动在元古宙碳循环和气候中的作用。结果表明，也许与直觉相反，太阳较暗的元古代地球并不更容易受到“雪球冰川”的影响。变质作用和火山作用产生的CO2通量在大气和海洋中积累，直到这些输入量被硅酸盐风化作用、碳酸盐沉淀和净有机碳埋藏所平衡。由于风化速率依赖于气候条件，地质CO2输入的变化对气候条件有很大的影响。相比之下，太阳通量的缓慢变化对气候的长期影响相对较小，因为大气CO2水平的补偿性变化很大。二氧化碳输入量的减少降低了大气中的二氧化碳水平，最终引发了全球冰川作用。在元古宙，处于全球冰雪覆盖状态的临界条件下的大气CO2水平会很高。然而，元古宙和古生界所需的CO2通量减少量大致相同。这基本上是因为临界条件下的温度非常低，因此硅酸盐风化率（在稳定状态下应与净CO2输入率平衡）也非常低，而不管太阳通量的变化。此外，在临界条件下，较低的太阳辐射通量对CO2输入速率的影响将在很大程度上被较低的硅酸盐风化速率的效率所抵消，这是由于元古代土壤生物活性较低。因此，启动全球冰川作用的CO2通量条件在元古宙和中生代可能是相似的。因此，元古代地球对“滚雪球”条件的敏感性的解释不能简单地取决于较暗的太阳;我们必须寻找这两个时代之间碳循环和气候行为的其他差异。(C)2003 Elsevier B. V.保留所有权利。
The Earth may have been globally ice-covered several times during the Proterozoic. While the Neoproterozoic and the Paleoproterozoic glaciations may have been 'snowball' Earth events, there is no evidence for such glaciation during the Phanerozoic. It might be hypothesized that a dimmer Sun earlier in Earth's history may have made the Earth more susceptible to global glaciation. In this paper, the roles of solar flux and soil biological activity in the carbon cycle and the climate during the Proterozoic are investigated using a simple carbon geochemical cycle model with a one-dimensional energy balance climate model. The results indicate, perhaps counterintuitively, that the Proterozoic Earth, with its dimmer Sun, was not more susceptible to 'snowball glaciation'. Metamorphic and volcanic CO2 fluxes accumulate in the atmosphere and ocean until such time that those inputs are balanced by silicate weathering followed by carbonate precipitation and net organic carbon burial. Because of the dependence of weathering rates on climatic conditions, changes in geologic CO2 inputs have a large influence on climatic conditions. In contrast, slow variation in solar flux has relatively little long-term impact on climate, because of large compensating changes in atmospheric CO2 level. A reduction in CO2 inputs lowers atmospheric CO2 level, which finally initiates global glaciation. The atmospheric CO2 level at the critical condition for a globally ice-covered state would have been high during the Proterozoic. However, roughly the same amount Of CO2 flux reduction is required for both the Proterozoic and the Phanerozoic. This is essentially because the temperatures at the critical condition are very low, hence the silicate weathering rate (which should balance with a net CO2 input rate in a steady state) is also very low, regardless of the variation in solar flux. Furthermore, the effect of the lower solar flux on the CO2 input rate at the critical condition would have been largely canceled by a lower efficiency of the silicate weathering rate due to lower soil biological activity during the Proterozoic. As a result, CO2 flux conditions for initiating the global glaciation may be similar during both the Proterozoic and the Phanerozoic. Therefore, the explanation for the susceptibility of the Proterozoic Earth to 'snowball' conditions cannot hinge simply on the dimmer Sun; we must look to other differences in behaviors of the carbon cycle and the climate between these two ages. (C) 2003 Elsevier B.V. All rights reserved.