The Paleoproterozoic Baraboo paleosol revisited: Quantifying mass fluxes of weathering and metasomatism, chemical climofunctions, and atmospheric pCO2 in a chemically heterogeneous protolith
The Paleoproterozoic Baraboo paleosol revisited: Quantifying mass fluxes of weathering and metasomatism, chemical climofunctions, and atmospheric pCO2 in a chemically heterogeneous protolith
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DOI：
10.1016/j.precamres.2017.06.010
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发表时间：
2017-10
影响因子：
3.8
通讯作者：
L. Medaris;S. Driese;G. Stinchcomb
中科院分区：
文献类型：
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作者：
L. Medaris;S. Driese;G. Stinchcomb
摘要古元古代Baraboo古土壤是苏必利湖地区化学最成熟的前寒武纪古土壤，是在地势温和潮湿的气候影响下形成的。我们测定了6个花岗岩原岩样品的化学变异性，发现这种变异性在风化剖面中是遗传的。为了减少样品不均质性对各种地球化学计算的影响，通过对风化剖面中的SiO_2、TiO_2、Al_2O_3和Zr浓度进行线性最小二乘拟合并求和为100%，得到腐泥岩和风化岩的转换成分。幸运的是，在计算质量通量时，使用测量的或转换的成分几乎没有什么不同。然而，转换后的成分允许在古土壤之间进行更清晰的比较，并且对于确定某些古土壤参数是必不可少的，例如化学耗竭分数。总质量通量和折合质量通量、化学蚀变指数、长石风化指数和化学亏损分数的计算表明，Baraboo古土壤在化学上比古元古代Pronto和寒武纪Timna古土壤成熟，但不如现代亚热带和热带土壤成熟。巴拉布的年平均温度(MAT PPM)和年平均降水量(MAP PPM)估计分别为14.3 19.2 9.4°C和1177 1695 658 mm yr−1，证实了先前对温暖、潮湿气候的解释。尽管由于风化持续时间的不确定性和风蚀去除风化层的可能性，大气PCO2水平的约束很差，但计算出100000年的风化PCO2为≥4.8×工业前大气水平(2 80ppm)，与弱太阳模型预测的1.7Ga时的PCO2水平一致。
Abstract The Paleoproterozoic Baraboo paleosol is the most chemically mature Precambrian paleosol in the Lake Superior region, having formed under the influence of a warm, humid climate in a region of subdued topography. We have determined the chemical variability among six samples of granitic protolith and found that such variability is inherited in the weathering profile. To minimize the effect of sample heterogeneity on various geochemical calculations, transformed compositions of saprolite and regolith were generated by obtaining linear least squares fits to the concentrations of SiO 2, TiO 2, Al 2 O 3, and Zr through the weathering profile and summing the totals to 100%. Fortunately, use of measured or transformed compositions makes little difference in calculation of mass fluxes. However, transformed compositions allow for clearer comparisons among paleosols and are essential for determining certain paleosol parameters, such as Chemical Depletion Fraction. Calculations of Total and Reduced Mass Fluxes, Chemical Index of Alteration, Feldspar Index of Weathering, and Chemical Depletion Fraction demonstrate that the Baraboo paleosol is chemically more mature than the Paleoproterozoic Pronto and Cambrian Timna paleosols, but less mature than modern subtropical and tropical soils. Estimates of Mean Annual Temperature (MAT PPM) and Mean Annual Precipitation (MAP PPM) for Baraboo are 14.3 19.2 9.4° C and 1177 1695 658 mm yr− 1, confirming the previous interpretation of a warm, humid climate. Although the level of atmospheric pCO 2 is poorly constrained due to the uncertainty in duration of weathering and possibility of regolith removal by erosion, pCO 2 is calculated to be≥ 4.8× Pre-Industrial Atmospheric Level (280 ppm) for 100,000 years of weathering, which is consistent with the pCO 2 level at 1.7 Ga predicted by the weak sun model.