Weathering model for the quantification of atmospheric oxygen evolution during the Paleoproterozoic
Weathering model for the quantification of atmospheric oxygen evolution during the Paleoproterozoic
复制标题
古元古代大气氧气演化定量的风化模型
DOI：
10.1016/j.gca.2013.03.015
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发表时间：
2013
影响因子：
5
通讯作者：
T.
中科院分区：
文献类型：
--
作者：
Yokota;K.;Kanzaki;Y.;Murakami;T.
一个风化模型已被开发，以量化在古元古代大气氧演化。该风化模型计算了在一定的大气氧分压（Po 2）下，风化过程中含Fe 2+原生矿物中溶解的Fe 2+和溶解的Fe 2+中氧化的Fe 3+的浓度，并建立了Po 2与Fe 3+之间的关系，其中Fe 3+是整个风化剖面中原生矿物溶解的Fe 2+中氧化沉淀的Fe 3+与溶解的Fe 2+的比值。风化模型考虑了风化过程中Fe再分配的控制因素，即含Fe ~（2+）原生矿物的溶解速率、Fe ~（2+）的氧化速率和地下水流量。通过将该模型应用于低氧条件下橄榄石溶解的实验数据，证实了该模型的有效性。模型的敏感性分析表明，风化作用形成时间、矿物溶解速率和O2向风化剖面的扩散对Po 2的变化没有影响或影响很小，形成时间的4个数量级变化、矿物溶解速率的10多个数量级变化和O2扩散的假定变化导致log（Po 2）的变化分别为0.0、0和0.3。分别另一方面，温度、pH值和地下水流速对PO 2有中等至大的影响：温度变化5 °C，pH值变化0.5，地下水流速变化一个数量级，log（Po 2）分别变化0.6、1.4和1.5。利用文献中估算的地表温度、pH值和地下水流速，计算了古土壤的δ-Po 2关系，并将其应用于2.5 ~ 1.8Ga形成的古土壤（化石风化剖面）的δ-Po 2值。考虑到硫同位素和其他地质代用指标（即，在2.45 Ga之前<10−6atm，在2.32 Ga> 10−6atm，在2.0 Ga> 10−3atm），我们的模型表明，Po 2水平在2.46 Ga <10−6atm，但>10−9atm，在2.25-2.0 Ga为10−4.5-10−2atm，在2.85 Ga>10−2atm。本风化模型能较好地反映古元古代热期的Po 2水平，但不能反映温度<0 °C的冰期的Po 2水平。研究发现，年龄为2.5- 2.1Ga的库珀湖、Pronto/NAN、哈博罗内和Drakenstein古土壤不是在极端条件下形成的，而是在温和条件下形成的。
A weathering model has been developed to quantify atmospheric oxygen evolution during the Paleoproterozoic. The weathering model calculates the concentrations of Fe2+dissolved from Fe2+-bearing primary minerals and oxidized Fe3+out of the dissolved Fe2+at a given partial pressure of atmospheric oxygen (Po2) during weathering and establishes the relationships betweenPo2andϕ, whereϕis the ratio of oxidized and then precipitated Fe3+out of the Fe2+dissolved from primary minerals to the dissolved Fe2+in a whole weathering profile. The weathering model considers controlling factors of the redistribution of Fe during weathering, that is, the dissolution rate of Fe2+-bearing primary minerals, the oxidation rate of Fe2+, and the groundwater flow rate. The validity of the model was confirmed by applying the model to the experimental data of olivine dissolution carried out under low O2conditions. The sensitivity analysis of the model has revealed that the formation time of weathering, the mineral dissolution rate and the diffusion of O2into a weathering profile have no or slight influence onϕ, resulting in ∼0, 0 and 0.3 changes in log(Po2) caused by four orders of magnitude change of the formation time, more than 10 orders change of the mineral dissolution rate, and assumed change of the O2diffusion, respectively. On the other hand, the temperature, the pH and the groundwater flow rate have moderate to large effects onϕ: 0.6, 1.4 and 1.5 changes in log(Po2) for changes of 5 °C in temperature, 0.5 in pH, and one order of magnitude in groundwater flow rate, respectively. Using possible surface temperature, pH and groundwater flow rate estimated from the literature, we calculated theϕ-Po2relationships which were then applied to theϕvalues of paleosols (fossil weathering profiles) formed between 2.5 and 1.8 Ga. Taking account of the constraints given by the records of mass independent fractionation in sulfur isotopes and other geological proxies (i.e., <∼10−6atm prior to 2.45 Ga, >∼10−6atm at 2.32 Ga and >∼10−3atm at 2.0 Ga), our model implies that thePo2levels were <10−6atm but >10−9atm at ∼2.46 Ga, 10−4.5–10−2atm at ∼2.25–2.0 Ga and >10−2atm at ∼1.85 Ga. The present weathering model can constrainPo2levels during the possible Paleoproterozoic hot periods but not those during the glacial periods when temperature was <0 °C. It has been found that the Cooper Lake, Pronto/NAN, Gaborone and Drakenstein paleosols with ages of ∼2.5–2.1 Ga did not form under extreme conditions such as those in glacial and hot periods but formed under moderate conditions.