Iron-rich condition maintained throughout the mid-Proterozoic ocean: new evidence from the North China Craton

Abstract:

The redox characteristics of ocean during Earth’s middle age (1.7–0.75 Ga) are less well known, but it is conventionally assumed that the mid-Proterozoic was a period of a globally sulphidic (euxinic) in the deep ocean when the surface oceans were mildly oxygenated. Clay rich sedimentary rocks in Paleo-Mesoproterozoic strata deposited stably in the Yanshan-Taihangshan aulacogen (YTA), North China Craton (NCC), which created a unique opportunity to investigate the marine redox chemistry.

Our Fe speciation analysis involved totally 48 samples from the black shale of the Chuanlinggou Formation (~1650 Ma), the mud shale of the Gaoyuzhuang Formation (~1560 Ma) and the black shale of the Xiamaling Formation (~1380 Ma) cropped out in Hebei Province, North China. Of all the 48 samples, 19 of them from Chuanlinggou Formation have Fe_Py/Fe_HR ratios well below 0.8, while 14 of those have Fe_HR/Fe_T values above 0.15. For the 9 samples from Gaoyuzhuang Formation, their Fe_HR/Fe_T values are generally above 0.15, while 8 of them have Fe_Py/Fe_HR ratios well below 0.8. For the 21 samples from Xiamaling Formation, 19 of them have Fe_Py/Fe_HR values below 0.8, among which 13 samples have Fe_HR/Fe_T values above 0.15. Although a few samples analysed are more or less weathered, these data can still point to widespread ferruginous instead of sulfidic conditions over these three stratigraphic units.

Abundant Fe containing carbonate concretions have been found in the shale of the Xiamaling Formation, which are significant in revealing the early burial and diagenetic marine environment. Our X-ray diffraction and rock magnetism studies reveal that the major minerals in the concretions are ferron dolomite and siderite, without any signs of sulphide of iron. These results imply that the concretions in the Xiamaling Formation were formed in ferruginous.

Our analysis suggests a ferruginous condition was maintained throughout the Paleo-Mesoproterozoic (~1650 Ma to ~1380 Ma) in the YTA, a typical intracontinental rift basin developed in the interior of the Nuna supercontinent. Our results are in agreement with the previous study (Planavsky et al., 2011), the ferruginous conditions were both spatially and temporally extensive across the global ocean during the mid-Proterozoic.

Planavsky et al., 2011, Nature, 477(7365): 448-451.