The Hurst indexes of the four proxies are FeO 0.85, Fe 2O 3 0.76, Fe 2O 3/FeO 0.65 and organic carbon 0.74 respectively, indicating that they have obviously long run dependence persistence. This is in accordance with previous studies [13-15] .

• 4项指标的Hurst指数 (FeO 0 .；85;Fe2 O3 0