Estimates of atmospheric O2 in the Paleoproterozoic from paleosols
Estimates of atmospheric O2 in the Paleoproterozoic from paleosols
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DOI：
10.1016/j.gca.2015.11.022
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发表时间：
2016-02-01
影响因子：
5
通讯作者：
Murakami, Takashi
中科院分区：
文献类型：
--
作者：
Kanzaki, Yoshiki;Murakami, Takashi
根据古土壤中的铁记录，建立了约束古元古代大气O-2 （P-O2）分压的风化模型。该模型描述了铁在风化剖面中的行为：含铁矿物的溶解/沉淀、溶解的铁（II）被氧氧化成铁（III）以及溶解的铁在稳定状态下被水流输送。该模型计算了风化过程中溶解的Fe（II）与溶解的Fe（II）析出的Fe(III)-（氧）氧化物的比值（phi），作为P-O2的函数。从文献中引入了Fe（II）被O-2氧化的先进动力学表达式，以计算精确的phi-P-O2关系。计算的ph - p - o2关系与文献的一致性支持了模型的有效性。该模型可以计算给定古土壤的P-O2，只要得到一个phi值和其他与风化有关的参数值，即孔隙水的pH值、二氧化碳分压（PCO2）、水流、温度和O-2向土壤的扩散。对单个古元古代古土壤进行了上述风化相关参数的考察。phi、temperature、pH和P-CO2的数值来源于古元古代古土壤的相关文献。根据水岩相硅的质量平衡以及含水饱和度与导水率的关系，对各古土壤的水流参数值进行了约束。根据文献中O-2输运参数与土壤O-2浓度的关系式，计算各古土壤中O-2向土壤扩散的参数值。然后，我们对单个古元古代古土壤进行了综合P-O2计算，反映了气候相关参数的所有不确定性。因此，得到了古元古代P-O2的可靠估计：在2.46 Ga附近10(-7.1)-10(-5.4)atm，在2.15 Ga附近10(-5.0)-10(-2.5)atm，在2.08 Ga附近10(-5.2)-10(-1.7)atm，在1.85 Ga附近超过10(-4.6)-10(-2.0)atm。与文献中P-O2的比较表明，在类似2.4 Ga的时期，氧的急剧上升不会发生，这表明在类似2.4 Ga的时期，氧的略微快速上升和长期的古元古代氧的逐渐上升。(c) 2015 Elsevier Ltd.版权所有。
A weathering model was developed to constrain the partial pressure of atmospheric O-2 (P-O2) in the Paleoproterozoic from the Fe records in paleosols. The model describes the Fe behavior in a weathering profile by dissolution/precipitation of Fe-bearing minerals, oxidation of dissolved Fe(II) to Fe(III) by oxygen and transport of dissolved Fe by water flow, in steady state. The model calculates the ratio of the precipitated Fe(III)-(oxyhydr) oxides from the dissolved Fe(II) to the dissolved Fe(II) during weathering (phi), as a function of P-O2. An advanced kinetic expression for Fe(II) oxidation by O-2 was introduced into the model from the literature to calculate accurate phi-P-O2 relationships. The model's validity is supported by the consistency of the calculated phi-P-O2 relationships with those in the literature. The model can calculate P-O2 for a given paleosol, once a phi value and values of the other parameters relevant to weathering, namely, pH of porewater, partial pressure of carbon dioxide (PCO2), water flow, temperature and O-2 diffusion into soil, are obtained for the paleosol.The above weathering-relevant parameters were scrutinized for individual Paleoproterozoic paleosols. The values of phi, temperature, pH and P-CO2 were obtained from the literature on the Paleoproterozoic paleosols. The parameter value of water flow was constrained for each paleosol from the mass balance of Si between water and rock phases and the relationships between water saturation ratio and hydraulic conductivity. The parameter value of O-2 diffusion into soil was calculated for each paleosol based on the equation for soil O-2 concentration with the O-2 transport parameters in the literature. Then, we conducted comprehensive P-O2 calculations for individual Paleoproterozoic paleosols which reflect all uncertainties in the weathering-relevant parameters. Consequently, robust estimates of P-O2 in the Paleoproterozoic were obtained: 10(-7.1) -10(-5.4) atm at similar to 2.46 Ga, 10(-5.0) -10(-2.5) atm at similar to 2.15 Ga, 10(-5.2)-10(-1.7) atm at similar to 2.08 Ga and more than 10(-4.6)-10(-2.0) atm at similar to 1.85 Ga. Comparison of the present P-O2 estimates to those in the literature suggests that a drastic rise of oxygen would not have occurred at similar to 2.4 Ga, supporting a slightly rapid rise of oxygen at similar to 2.4 Ga and a gradual rise of oxygen in the Paleoproterozoic in long term. (c) 2015 Elsevier Ltd. All rights reserved.