1. a State Key Laboratory of Earth Surface Processes and Resource Ecology , Beijing Normal University , Beijing , 100875 , China
2. b College of Resources Science & Technology , Beijing Normal University , Beijing , 100875 , China
3. c Department of Geology and Geography , Georgia Southern University , Statesboro , GA , 30460 , USA
4. d University of Waterloo , Waterloo , ON , Canada , N2L 3G1