Oxidized micrometeorites suggest either high pCO2 or low pN2 during the Neoarchean
Oxidized micrometeorites suggest either high pCO2 or low pN2 during the Neoarchean
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氧化微陨石表明新太古代期间存在高 pCO2 或低 pN2
DOI：
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发表时间：
2020
影响因子：
11.1
通讯作者：
J. Kasting
中科院分区：
文献类型：
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作者：
R. Payne;D. Brownlee;J. Kasting
意义 古土壤（古代土壤）已被用来估计太古宙（4.0 至 2.5 Ga）期间的二氧化碳浓度。然而，不同的古土壤研究彼此不一致，并且与气候模型对古代二氧化碳水平的估计不一致。年代为 2.7 Ga 的氧化铁微陨石代表了一种可以与之比较的新二氧化碳替代物。这些陨石表明当时大气中的 25% 至 50% 是二氧化碳。这是最容易解释的，如果氮气分压比今天低，那么大气温室效应不大，气候凉爽，与同期冰川作用的证据一致。汤姆金斯等人。 [一个。 G. Tomkins et al., Nature 533, 235–238 (2016)]提出，2.7-Ga铁微陨石中含有的铁氧化物可用于测定太古宙高层大气中的O2浓度。具体来说，他们认为这些物体中磁铁矿的存在意味着，在微陨石在进入过程中熔化的高度范围内，氧气含量一定接近当今的水平（~21%）。在这里，我们使用一维光化学模型重新评估他们的数据。我们发现原子氧 O（而不是 O2）是高层大气中最丰富的强氧化剂。但激波管实验的数据表明，CO2 本身也可能充当氧化剂，在这种情况下，微陨石氧化实际上限制了 CO2/N2 比率，而不是总氧化剂丰度。对于含有 0.8 bar N2 的大气（如今天），CO2 混合比的下限为 ∼0.23。这将在 2.7 Ga 时产生约 300 K 的平均表面温度，考虑到大约在此时发生冰川作用的证据，这可能太高了。如果 pN2 是当前值的一半，并且甲烷等其他温室气体造成的变暖不是主要因素，则平均地表温度将降至约 291 K，与冰川作用一致。这表明新太古代的表面压力可能需要更低——接近0.6巴——二氧化碳才能氧化微陨石。最终，铁微陨石可能是古代大气二氧化碳和表面压力的指示器。并有助于解决气候模型与古土壤等现有二氧化碳替代指标之间的差异。
Significance Paleosols (ancient soils) have been used to estimate CO2 concentrations during the Archean Eon, 4.0 to 2.5 Ga. However, different paleosol studies disagree with each other and with climate model estimates for ancient CO2 levels. Oxidized iron micrometeorites dated at 2.7 Ga represent a new CO2 proxy with which to compare. These meteorites suggest that CO2 constituted 25 to 50% of the atmosphere at that time. This is easiest to explain if the N2 partial pressure was lower than today so that the atmospheric greenhouse effect was modest and the climate was cool, consistent with evidence for contemporaneous glaciation. Tomkins et al. [A. G. Tomkins et al., Nature 533, 235–238 (2016)] suggested that iron oxides contained in 2.7-Ga iron micrometeorites can be used to determine the concentration of O2 in the Archean upper atmosphere. Specifically, they argued that the presence of magnetite in these objects implies that O2 must have been near present-day levels (∼21%) within the altitude range where the micrometeorites were melted during entry. Here, we reevaluate their data using a 1D photochemical model. We find that atomic oxygen, O, is the most abundant strong oxidant in the upper atmosphere, rather than O2. But data from shock tube experiments suggest that CO2 itself may also serve as the oxidant, in which case micrometeorite oxidation really constrains the CO2/N2 ratio, not the total oxidant abundance. For an atmosphere containing 0.8 bar of N2, like today, the lower limit on the CO2 mixing ratio is ∼0.23. This would produce a mean surface temperature of ∼300 K at 2.7 Ga, which may be too high, given evidence for glaciation at roughly this time. If pN2 was half the present value, and warming by other greenhouse gases like methane was not a major factor, the mean surface temperature would drop to ∼291 K, consistent with glaciation. This suggests that surface pressure in the Neoarchean may need to have been lower—closer to 0.6 bar—for CO2 to have oxidized the micrometeorites. Ultimately, iron micrometeorites may be an indicator for ancient atmospheric CO2 and surface pressure; and could help resolve discrepancies between climate models and existing CO2 proxies such as paleosols.