Journal
CHEMICAL GEOLOGY
Volume 524, Issue -, Pages 259-271Publisher
ELSEVIER
DOI: 10.1016/j.chemgeo.2019.06.024
Keywords
Early Cambrian; Yangtze Block; South China; Iron speciation; Redox sensitive trace elements; Localized euxinic wedge; Outer shelf slope; Oceanic upwelling
Categories
Funding
- National Natural Science Foundation of China [91755210, 41472089]
- Open Research Fund Program of Hunan Provincial Key Laboratory of Shale Gas Resource Utilization, Hunan University of Science and Technology
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The early Cambrian was a critical interval in geological history and featured profound oceanic and biotic changes. To unravel the oceanic redox conditions, high-resolution analyses of iron speciation and redox-sensitive trace elements (Mo and U) within the framework of sequence stratigraphy were carried out on the Niutitang Formation (similar to 528-521 Ma). The two examined sections, the Daotuo and Bahuang sections, were respectively located in a mid-upper slope setting and a lower slope to basin settings behind a seaward submerged sill on the middle Yangtze Block, South China. At Daotuo, the Fe-py/Fe-HR values display moderate positive covariances with the total organic carbon contents (TOC) and the Mo/TOC ratios, notably in the basal part of the Niutitang Formation. Therefore, at this locality the euxinic water mass wedge was developed in association with a high primary organic productivity/burial rate, likely within a high-productivity zone. In contrast, at Bahuang, the Fe-py/Fe-HR values show weak to negative covariances with the Mo/TOC ratios and TOC contents in the basal part, indicating that the biogeochemical cycles of Fe, Mo with C were decoupled in a lower-productivity, ferruginous deeper basinal setting. These spatial changes in marine redox structures and biogeochemical cycles can be reasonably explained by the existence of oceanic upwelling in the presence of strong offshore currents and a seaward submarine sill, reconciling the oxygen minimum zone (OMZ) in modern oceanic margins to some extent. This spatial redox pattern also accounts well for previous data based on which the euxinic state intermittently invaded upward onto the shelf margin and evolved into a ferruginous-dominant anoxia in the inner shelf subbasins. In addition, the euxinic wedge dynamically fluctuated upslope and downslope along the transect from the ferruginous outer shelf slope to the basin in response to rise and fall of sea-level. Consequently, the temporal evolution of the redox conditions was driven in part by eustatic variations. Moreover, the paleogeographic position of the Yangtze Block in the mid-lower latitudes suggests that the block was associated with relatively strong offshore currents induced by trade winds, which notably enhanced the oceanic upwelling near the transgression maximum. On the other hand, the dominance of deep ferruginous waters with the local occurrence of a euxinic wedge on the outer shelf slope indicates relatively low concentrations of seawater sulfate and atmospheric oxygen during the deposition of the Niutitang black shales. This redox model thus highlights the important roles of paleogeographic, paleoclimatic and eustatic sea-level changes in controlling the spatio-temporal fluctuations in marine redox conditions and biogeochemical cycling in the early Cambrian ocean.
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