An understanding of the processes that control the behavior of major elements with respect to weathering profile is essential to calculate the mobility, redistribution, and mass fluxes of elements. Hence, this study aims to determine the geochemical mass balance, strain, elemental correlation, and transport in weathering profiles. We constructed three weathering profiles for the black shale of Shujingtuo formation. As per the principal component analysis of major elements, density, and pH values, the first component represents the “elemental factor” and the second denotes the “external factor.” The “depletion” pattern is a mass transportation pattern, and Na, K, and Mg are depleted along transect relative to the composition of fresh rock. Fe is redeposited at the bottom half of the saprock zone, whereas Al is accumulated at the regolith zone. The Fe and Al patterns are attributed to the “depletion–addition” and “addition” patterns, respectively. The strain in profiles A and B demonstrates the expansion at the regolith zone and part of the saprock zone. In profile C, however, these zones collapsed at all depths. In chemical weathering, Na, K, Ca, Mg, and Si are depleted in the following order: valley (C) > near mountaintop (B) > ridge (A).
The chemical weathering of rocks is a major process that alters the Earth’s surface and is a critical process in the geochemical cycling of elements [
Black shale is widely distributed in Southern China [
The study was conducted in Chengkou County of Northeast Chongqing Province, which is located at the boundary between the Qinling Orogenic Belt and the Yangtze Platform. The site is characterized by rugged topography and elevations that are 750 m–1200 m above sea level. The latitude is 31°57′N-31°58′N, and the longitude is 108°37′E–108°39′E. Specifically, profiles A, B, and C are located at 31°57′27′′N and 108°38′10′′E (ridge), 31°57′46′′N and 108°37′42′′E (nearby mountaintop), and 31°57′39′′N and 108°38′39′′E (valley), respectively (Figure
Study site on Shuijingtuo Formation black shale in Chengkou county, Northeast Chongqing Province. The samples were collected from nearby mountaintop (B), midridge (A), and valley floor (C). Background color indicates altitude.
The profile material can be divided into regolith, saprock, weathered shale, and fresh shale (protolith) according to the field investigation. The regolith portions of profiles A, B, and C range from 0 m to 0.25 m, 0 m to 2 m, and 0 m to 1.5 m, respectively. The saprock portions in these profiles range from 0.25 m to 1.5 m, 2 m to 4.8 m, and 1.5 m to 6.2 m. The weathered shale portions range from 1.5 m to 3 m, 4.8 m to 7 m, and 6.2 m to 10.5 m. The fresh shale portions are below 3, 7, and 10.5 m. Samples were collected from these material zones for three profiles, and the sample number and depth are shown in Table
Major element concentration, bulk densities, strain values, and corresponding Tau values for black shale from profile A, B, and C in study area.
Sample |
Depth |
|
SiO2 |
Al2O3 |
TFe2O3 |
K2O |
Na2O |
CaO |
MgO |
MnO |
P2O5 |
BaO |
SO3 |
LOI |
Ti |
pH |
|
|
||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Si | Al | Fe | K | Na | Ca | Mg | ||||||||||||||||||
A-1 | 0.1 | 1.53 | 65.5 | 13.60 | 8.10 | 2.84 | 2.52 | 0.29 | 0.97 | 0.22 | 0.20 | 0.53 | 0.23 | 4.39 | 4970 | 6.03 | 0.63 | 0.18 | 0.09 | −0.23 | 0.01 | 0.07 | −0.93 | −0.63 |
A-2 | 0.25 | 1.79 | 65.7 | 13.70 | 7.78 | 2.87 | 2.63 | 0.28 | 0.96 | 0.05 | 0.25 | 0.47 | 0.30 | 4.41 | 5275 | 4.25 | 0.31 | 0.11 | 0.03 | −0.31 | −0.03 | 0.05 | −0.93 | −0.66 |
A-3 | 0.45 | 1.72 | 63.4 | 13.85 | 9.08 | 2.91 | 2.59 | 0.22 | 0.98 | 0.27 | 0.28 | 0.37 | 1.03 | 4.97 | 5138 | 3.28 | 0.40 | 0.10 | 0.07 | −0.17 | 0.01 | 0.06 | −0.95 | −0.64 |
A-4 | 0.55 | 1.92 | 69.8 | 14.60 | 1.35 | 3.13 | 2.74 | 0.19 | 1.09 | 0.01 | 0.03 | 0.28 | 1.10 | 5.69 | 5271 | 2.82 | 0.22 | 0.18 | 0.10 | −0.88 | 0.05 | 0.09 | −0.96 | −0.61 |
A-5 | 0.65 | 1.90 | 69.1 | 14.25 | 1.38 | 3.02 | 2.79 | 0.36 | 1.02 | BDL | 0.02 | 0.26 | 1.78 | 7.33 | 5818 | 2.68 | 0.12 | 0.06 | −0.03 | −0.89 | −0.08 | 0.01 | −0.92 | −0.67 |
A-6 | 0.75 | 1.98 | 67.8 | 14.20 | 1.63 | 3.05 | 2.94 | 0.19 | 0.96 | BDL | 0.02 | 0.29 | 3.36 | 8.21 | 5657 | 3.40 | 0.10 | 0.07 | 0.00 | −0.86 | −0.04 | 0.09 | −0.96 | −0.68 |
A-7 | 0.85 | 2.22 | 62.6 | 13.35 | 3.97 | 2.93 | 2.56 | 1.58 | 1.20 | 0.02 | 0.09 | 0.46 | 7.26 | 8.56 | 5340 | 4.24 | 0.04 | 0.05 | −0.01 | −0.65 | −0.03 | 0.01 | −0.64 | −0.58 |
A-8 | 1.1 | 2.42 | 55.7 | 11.70 | 4.93 | 2.51 | 2.28 | 5.94 | 2.16 | 0.05 | 0.17 | 0.32 | 8.31 | 10.64 | 5017 | 5.88 | 0.02 | −0.01 | −0.07 | −0.54 | −0.11 | −0.04 | 0.46 | −0.19 |
A-9 | 1.5 | 2.50 | 49.6 | 10.60 | 12.06 | 2.36 | 1.94 | 5.05 | 1.72 | 0.04 | 0.14 | 0.28 | 8.70 | 11.06 | 4767 | 6.12 | 0.04 | −0.07 | −0.12 | 0.19 | −0.12 | −0.14 | 0.30 | −0.32 |
A-10 | 2.1 | 2.61 | 57.2 | 11.10 | 4.45 | 2.41 | 2.08 | 6.79 | 1.72 | 0.05 | 0.15 | 0.37 | 7.75 | 10.94 | 4859 | 6.88 | −0.02 | 0.05 | −0.09 | −0.57 | −0.12 | −0.10 | 0.72 | −0.34 |
A-11 | 3.0 | 2.65 | 52.3 | 11.75 | 9.93 | 2.63 | 2.22 | 3.80 | 2.49 | 0.05 | 0.18 | 0.33 | 6.25 | 10.76 | 4670 | 6.52 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 |
|
||||||||||||||||||||||||
B-1 | 0.5 | 1.67 | 69.7 | 9.85 | 3.92 | 2.06 | 0.66 | 3.48 | 1.20 | 0.02 | 1.34 | 1.02 | 0.40 | 5.02 | 3542 | 7.04 | 0.34 | −0.06 | 0.00 | 0.04 | −0.13 | −0.66 | −0.28 | −0.17 |
B-2 | 1.0 | 1.90 | 69.9 | 6.98 | 2.87 | 1.62 | 0.56 | 3.65 | 0.94 | 0.01 | 0.60 | 0.65 | 0.26 | 8.80 | 2486 | 6.55 | 0.67 | 0.35 | 0.01 | 0.09 | −0.03 | −0.59 | 0.08 | −0.07 |
B-3 | 1.5 | 1.88 | 67.4 | 8.76 | 3.49 | 2.09 | 0.42 | 2.85 | 1.30 | 0.02 | 0.65 | 1.60 | 0.70 | 11.70 | 3234 | 6.62 | 0.30 | 0.00 | −0.02 | 0.02 | −0.03 | −0.76 | −0.35 | −0.01 |
B-4 | 2.0 | 1.75 | 74.7 | 9.34 | 3.50 | 2.16 | 1.41 | 0.66 | 0.86 | BDL | 0.15 | 0.48 | 0.33 | 5.91 | 4355 | 6.20 | 0.04 | −0.18 | −0.23 | −0.24 | −0.26 | −0.40 | −0.89 | −0.52 |
B-5 | 2.5 | 1.82 | 67.2 | 11.75 | 3.72 | 2.80 | 0.55 | 0.77 | 1.52 | 0.01 | 0.17 | 1.66 | 0.86 | 8.09 | 3834 | 6.02 | 0.13 | −0.16 | 0.11 | −0.09 | 0.09 | −0.74 | −0.85 | −0.03 |
B-6 | 3.2 | 1.72 | 66.0 | 12.10 | 5.94 | 2.80 | 1.96 | 0.12 | 1.03 | 0.01 | 0.12 | 0.41 | 8.24 | 8.88 | 4275 | 4.43 | 0.08 | −0.26 | 0.02 | 0.31 | −0.02 | −0.16 | −0.98 | −0.41 |
B-7 | 4.0 | 1.98 | 60.8 | 10.25 | 3.55 | 2.51 | 1.29 | 5.76 | 1.70 | 0.03 | 0.17 | 0.95 | 5.93 | 10.11 | 3764 | 6.50 | 0.06 | −0.22 | −0.02 | −0.11 | 0.00 | −0.37 | 0.13 | 0.11 |
B-8 | 4.8 | 2.14 | 59.7 | 13.00 | 4.42 | 3.11 | 1.84 | 3.74 | 2.04 | 0.03 | 0.20 | 0.52 | 7.00 | 9.01 | 4916 | 6.38 | −0.24 | −0.42 | −0.05 | −0.15 | −0.05 | −0.31 | −0.44 | 0.02 |
B-9 | 5.5 | 2.21 | 59.4 | 13.35 | 4.13 | 3.06 | 2.01 | 3.56 | 2.09 | 0.03 | 0.20 | 0.46 | 6.47 | 9.01 | 5010 | 6.51 | −0.28 | −0.43 | −0.04 | −0.22 | −0.09 | −0.26 | −0.48 | 0.02 |
B-10 | 6.2 | 2.38 | 59.7 | 13.35 | 4.13 | 3.01 | 2.29 | 4.00 | 1.94 | 0.04 | 0.19 | 0.50 | 5.83 | 8.37 | 4886 | 6.80 | −0.32 | −0.41 | −0.01 | −0.20 | −0.08 | −0.14 | −0.40 | −0.03 |
B-11 | 7.0 | 2.50 | 66.0 | 8.77 | 3.36 | 2.12 | 1.72 | 4.30 | 1.29 | 0.02 | 0.17 | 0.37 | 5.34 | 9.30 | 3167 | 7.12 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 |
|
||||||||||||||||||||||||
C-1 | 0.5 | 1.56 | 65.5 | 15.30 | 2.73 | 2.87 | 0.23 | 2.73 | 2.03 | 0.22 | 0.79 | 0.20 | 0.18 | 8.99 | 6345 | 6.54 | −0.44 | −0.58 | 0.07 | −0.71 | −0.22 | −0.62 | −0.94 | −0.75 |
C-2 | 1.0 | 1.49 | 70.5 | 16.60 | 1.33 | 3.19 | 0.30 | 0.31 | 0.74 | 0.02 | 0.04 | 0.83 | 0.30 | 5.01 | 5611 | 6.40 | −0.34 | −0.49 | 0.31 | −0.84 | −0.02 | −0.44 | −0.99 | −0.90 |
C-3 | 1.5 | 1.35 | 69.8 | 16.70 | 0.87 | 3.38 | 0.23 | 0.41 | 0.77 | 0.01 | 0.03 | 0.59 | 0.18 | 5.84 | 5693 | 6.16 | −0.29 | −0.50 | 0.30 | −0.90 | 0.02 | −0.57 | −0.99 | −0.90 |
C-4 | 2.0 | 1.43 | 68.7 | 17.50 | 1.48 | 3.55 | 0.21 | 0.31 | 0.86 | 0.01 | 0.04 | 0.55 | 0.16 | 5.42 | 5689 | 6.35 | −0.32 | −0.51 | 0.37 | −0.82 | 0.08 | −0.61 | −0.99 | −0.88 |
C-5 | 3.0 | 1.57 | 63.9 | 15.85 | 4.82 | 3.28 | 0.20 | 0.37 | 0.77 | 0.01 | 0.04 | 0.61 | 4.16 | 9.26 | 6329 | 3.85 | −0.48 | −0.59 | 0.11 | −0.48 | −0.11 | −0.67 | −0.99 | −0.91 |
C-6 | 4.0 | 1.79 | 60.5 | 15.20 | 6.73 | 3.08 | 0.20 | 0.63 | 1.02 | 0.14 | 0.14 | 0.63 | 5.25 | 10.60 | 4867 | 3.70 | −0.37 | −0.49 | 0.39 | −0.06 | 0.09 | −0.57 | −0.98 | −0.84 |
C-7 | 5.0 | 1.71 | 58.8 | 14.20 | 7.09 | 2.81 | 0.18 | 0.94 | 1.18 | 0.30 | 0.10 | 0.79 | 6.00 | 10.45 | 4643 | 4.74 | −0.30 | −0.48 | 0.36 | 0.04 | 0.04 | −0.59 | −0.97 | −0.80 |
C-8 | 6.2 | 1.86 | 56.9 | 14.85 | 9.91 | 3.02 | 0.20 | 0.35 | 0.78 | 0.03 | 0.08 | 0.46 | 6.10 | 11.61 | 4627 | 4.65 | −0.36 | −0.50 | 0.42 | 0.46 | 0.13 | −0.54 | −0.99 | −0.87 |
C-9 | 8.0 | 1.90 | 55.3 | 6.87 | 2.46 | 1.79 | 0.17 | 6.80 | 1.30 | 0.02 | 3.90 | 0.46 | 4.07 | 13.89 | 5055 | 6.83 | −0.42 | −0.55 | −0.40 | −0.67 | −0.39 | −0.65 | −0.76 | −0.80 |
C-10 | 9.0 | 2.45 | 53.6 | 6.45 | 2.71 | 1.66 | 0.18 | 8.00 | 1.84 | 0.02 | 1.74 | 0.35 | 4.18 | 17.53 | 2831 | 7.22 | −0.20 | −0.23 | 0.01 | −0.35 | 0.01 | −0.33 | −0.53 | −0.50 |
C-11 | 10.5 | 2.74 | 49.1 | 4.51 | 2.94 | 1.16 | 0.19 | 13.5 | 2.60 | 0.01 | 2.90 | 0.35 | 2.77 | 19.32 | 2002 | 7.26 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 |
UCC | 66.0 | 15.20 | 5.00 | 3.40 | 3.90 | 4.20 | 2.20 | 0.06 | 0.50 |
Note: BDL, below detection limit: <0.01%.
The samples were collected from the field at various depths to measure bulk density through the wax-sealing method. The samples were weighed after drying at 105°C in a drying oven for 24 h. They were then covered with wax and weighed in air and water. Given the known density of water and wax, the rock volumes were calculated according to the difference between the volumes of rocks covered with wax and the volumes of wax.
The major element concentrations, including the mean chemical composition of the upper continental crust (UCC), are listed in Table
The ratio of element contents to UCC can explain the variation in the weathering of protolith and of weathered rock [
Spider plots showing the UCC-normalized pattern of major elements for each sample of the Shuijintuo Formation black shale in Chengkou County.
Element mobility in the catchment is characterized by the mass transfer coefficient
The Tau values of corresponding major elements are listed in Table
Depth profiles for Tau values for major elements in profile A (left panel: (a), (d), and (g)), B (middle panel: (b), (e), and (h)), and C (right panel: (c), (f), and (i)). A = ridge; B = near mountaintop; C = valley.
PCA is a conventional multivariate technique that is applied to study geochemical data [
We examined chemical composition, density, and pH data in each individual sampling medium through Pearson correlation analysis (Table
Pearson correlation coefficients among major elements, density, and pH value for the profiles A, B, and C, respectively. Values greater than 0.80 are indicated in bold.
SiO2 | Al2O3 | Fe2O3 | K2O | Na2O | CaO | MgO | P2O5 | SO3 | LOI |
|
|
---|---|---|---|---|---|---|---|---|---|---|---|
Profile A | |||||||||||
Al2O3 |
|
||||||||||
Fe2O3 | −0.683* | −0.556 | |||||||||
K2O |
|
|
−0.583 | ||||||||
Na2O |
|
|
−0.651* |
|
|||||||
CaO |
|
|
0.275 |
|
|
||||||
MgO |
|
|
0.365 | −0.745** | −0.749** |
|
. | ||||
P2O5 | −0.359 | −0.286 | 0.760** | −0.387 | −0.394 | 0.141 | 0.160 | ||||
SO3 |
|
|
0.181 | −0.762** | −0.737** |
|
0.754** | −0.093 | |||
LOI | −0.780** |
|
0.102 | −0.725* | −0.678* |
|
|
−0.219 |
|
||
|
|
|
0.180 | −0.741** | −0.740** |
|
|
−0.093 |
|
|
|
pH |
|
|
−0.569 |
|
|
0.794** | 0.725* | 0.414 | 0.622* | 0.588 | 0.626* |
|
|||||||||||
Profile B | |||||||||||
Al2O3 | 0.710* | ||||||||||
Fe2O3 | −0.312 | 0.665* | |||||||||
K2O | −0.754** |
|
0.631* | ||||||||
Na2O | −0.593 | 0.674* | 0.548 | 0.672* | |||||||
CaO | −0.543 | −0.097 | −0.434 | −0.074 | 0.098 | ||||||
MgO |
|
−0.758** | 0.146 | 0.786** | 0.484 | 0.487 | |||||
P2O5 | 0.373 | −0.414 | −0.238 | −0.546 | −0.605* | 0.159 | −0.301 | ||||
SO3 | −0.781** | 0.663* | 0.665* | 0.711* |
|
0.218 | 0.563 | −0.573 | |||
LOI | −0.478 | −0.021 | −0.031 | 0.118 | 0.021 | 0.312 | 0.300 | −0.365 | 0.346 | ||
|
−0.626* | 0.264 | −0.157 | 0.314 | 0.595 | 0.575 | 0.590 | −0.426 | 0.509 | 0.367 | |
pH | −0.074 | −0.291 | −0.764** | −0.320 | −0.198 | 0.712* | 0.254 | 0.398 | −0.344 | −0.061 | 0.473 |
|
|||||||||||
Profile C | |||||||||||
Al2O3 |
|
||||||||||
Fe2O3 | −0.376 | 0.107 | |||||||||
K2O |
|
|
0.102 | ||||||||
Na2O | 0.740** | 0.561 | −0.383 | 0.501 | |||||||
CaO | −0.781** |
|
−0.249 |
|
−0.422 | ||||||
MgO | −0.639* | −0.745** | −0.190 |
|
−0.321 |
|
|||||
P2O5 | −0.691* |
|
−0.271 |
|
−0.477 |
|
0.641* | ||||
SO3 | −0.662* | −0.299 |
|
−0.266 | −0.668* | 0.072 | −0.044 | 0.110 | |||
LOI |
|
|
0.194 |
|
−0.670* |
|
0.750** | 0.736** | 0.513 | ||
|
|
|
0.108 |
|
−0.510 |
|
0.766** | 0.671* | 0.392 |
|
|
pH | −0.159 | −0.548 | −0.728* | −0.569 | 0.099 | 0.648* | 0.581 | 0.609* | −0.552 | 0.300 | 0.381 |
Data from the three profiles.
Subsequently, the average values of the different sampling media were examined through PCA. The results reported in Table
Correlations among major elements, density, and pH value with principal components.
Profile A | Profile B | Profile C | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
PC1 | PC2 | PC3 | PC4 | PC1 | PC2 | PC3 | PC4 | PC1 | PC2 | PC3 | PC4 | |
SiO2 | −0.967 | 0.137 | −0.193 | 0.077 | −0.892 | −0.345 | −0.050 | −0.265 | −0.921 | 0.378 | −0.042 | −0.037 |
Al2O3 | −0.989 | 0.051 | 0.080 | 0.101 | 0.862 | −0.253 | 0.384 | 0.079 | −0.983 | −0.078 | 0.091 | −0.126 |
Fe2O3 | 0.525 | −0.771 | 0.282 | −0.195 | 0.569 | −0.716 | 0.098 | 0.145 | −0.003 | −0.942 | 0.227 | 0.026 |
K2O | −0.955 | 0.160 | 0.193 | 0.082 | 0.906 | −0.223 | 0.232 | 0.102 | −0.984 | −0.108 | −0.014 | −0.105 |
Na2O | −0.946 | 0.206 | 0.095 | 0.119 | 0.833 | −0.095 | 0.015 | −0.470 | −0.618 | 0.529 | 0.439 | 0.367 |
CaO | 0.945 | 0.171 | −0.220 | 0.068 | 0.198 | 0.893 | 0.047 | 0.102 | 0.952 | 0.263 | 0.069 | −0.010 |
MgO | 0.878 | 0.128 | 0.238 | 0.371 | 0.806 | 0.403 | 0.280 | 0.222 | 0.813 | 0.283 | 0.322 | −0.349 |
P2O5 | 0.265 | −0.899 | −0.010 | 0.203 | −0.629 | 0.177 | 0.533 | 0.340 | 0.857 | 0.240 | −0.346 | 0.144 |
SO3 | 0.872 | 0.385 | 0.040 | −0.140 | 0.907 | −0.083 | −0.163 | −0.042 | 0.345 | −0.914 | −0.064 | 0.152 |
LOI | 0.852 | 0.495 | 0.100 | −0.041 | 0.332 | 0.315 | −0.731 | 0.472 | 0.968 | −0.203 | 0.091 | −0.001 |
|
0.883 | 0.397 | 0.146 | 0.068 | 0.588 | 0.635 | −0.165 | −0.382 | 0.941 | −0.086 | 0.252 | 0.065 |
pH | 0.864 | −0.242 | −0.266 | 0.139 | −0.216 | 0.899 | 0.306 | −0.156 | 0.490 | 0.808 | −0.051 | 0.043 |
Eigen value | 8.741 | 2.146 | 0.378 | 0.303 | 5.787 | 3.062 | 1.260 | 0.899 | 7.651 | 3.073 | 0.560 | 0.335 |
Variance explained (%) | 72.840 | 17.882 | 3.154 | 2.524 | 48.222 | 25.518 | 10.503 | 7.489 | 63.758 | 25.605 | 4.670 | 2.795 |
Cumulative variance (%) | 72.840 | 90.721 | 93.875 | 96.400 | 48.222 | 73.740 | 84.243 | 91.731 | 63.758 | 89.363 | 94.033 | 96.828 |
Mass fluxes (mol/m2) of major elements in three profiles.
Materials | Depth (m) | Si | Al | Fe | K | Na | Ca | Mg |
---|---|---|---|---|---|---|---|---|
Profile A | ||||||||
Regolith | 0~0.25 | 285 | 47 | −45 | 0 | 7 | −191 | −100 |
Saprock | 0.25~1.5 | 334 | −57 | −563 | −44 | −10 | −662 | −476 |
Weathered rock | 1.5~3.0 | 194 | −167 | −433 | −78 | −82 | 815 | −349 |
0~3.0 | 813 | −177 | −1041 | −122 | −85 | −38 | −925 | |
Profile B | ||||||||
Regolith | 0~2.0 | −723 | −65 | −1 | −84 | −506 | −762 | −129 |
Saprock | 2.0~4.8 | −8750 | −19 | −17 | −22 | −536 | −1984 | −155 |
Weathered rock | 4.8~7.0 | −13312 | −91 | −180 | −90 | −280 | −1483 | −1 |
0~7.0 | −22785 | −175 | −198 | −196 | −1322 | −4229 | −285 | |
Profile C | ||||||||
Regolith | 0~1.5 | −10156 | 207 | −674 | −119 | −94 | −11479 | −2141 |
Saprock | 1.5~6.2 | −29304 | 1499 | −615 | 66 | −280 | −34617 | −6840 |
Weathered rock | 6.2~10.5 | −19906 | −173 | −497 | −185 | −174 | −19043 | −4169 |
0~10.5 | −59366 | 1533 | −1786 | −238 | −548 | −65139 | −13150 |
During chemical weathering, the changes in volume from the protolith (
Calculated strains of black shale by immobile element Ti in Chengkou County. (a) Profile A, (b) profile B, and (c) profile C.
The total mass flux of an element
Here, units for
The mass fluxes of major elements are listed in Table
Following the computation of mass flux, we evaluate the strain versus
Mass transfer coefficient
The conceptual model of the weathering mechanism of black shale is illustrated in Figure
Conceptual weathering model of Shuijingtuo Formation black shale, Lower Cambrian.
In this study, we analyzed the elemental transportation, elemental correlation, volumetric strain, mass fluxes, and weathering process of black shale from the Shuijingtuo Formation of the Lower Cambrian in Chengkou County, Chongqing Province. The following conclusions can be drawn from the discussion above. Alkali and alkaline earth elements are leached from fresh shale, and the mass transportation pattern is classified under the “depletion” pattern. Fe is depleted at the weathered shale zone and increases at the saprock zone. The mass transportation pattern is considered a “depletion-addition” pattern. Al is accumulated in the regolith zone, which is accumulated in profile C. This accumulation can then be classified under the “addition” pattern. In profiles A and B, expansion occurred in the regolith zone and in part of saprock zone, whereas compaction was observed in the weathered shale zone. Collapse occurred at all depths in profile C. As per PCA, elemental mobility and low pH enhance the weathering process. The mass fluxes are also related to degree of weathering. In chemical weathering, Na, K, Ca, Mg, and Si are depleted in the following sequence: valley (C) > near mountaintop (B) > ridge (A).
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by research funds awarded by the National Natural Science Foundation of China (no. 41172261), the Key Technology Research and Development Program of Sichuan Province, China (no. 2012SZ0051), and the National Railway Ministry Technology R&D Program, China (no. 2010G016-B).