Abstract
Changes to the redox environment of seawater in the Late Ordovician affect the process of organic matter enrichment and biological evolution. However, the evolution of redox and its underlying causes remain unclear. This paper analyzed the vertical variability of main, trace elements and δ34Spy from a drill core section (well ZY5) in the Upper Yangtze Platform, and described the redox conditions, paleoproductivity and paleoclimate variability recorded in shale deposits of the P. pacificus zone and M. extraordinarius zone that accumulated during Wufeng Formation. The results showed that shale from well ZY5 in Late Ordovician was deposited under oxidized water environment, and there are more strongly reducing bottom water conditions of the M. extraordinarius zone compared with the P. pacificus zone. Excess silica (SiO2(exc)) and substitution index of paleoproductivity (Y) indicated that the P. pacificus zone had higher paleoproductivity whereas the M. extraordinarius zone was lower. The high productivity level controlled O2 release in the shallow water area as well as the oxidation degree of the P. pacificus zone. The decrease of productivity and the relatively stagnant water mass of the inner Yangtze Sea controlled the formation of relatively reduced water conditions in the M. extraordinarius zone. The chemical index of alteration (CIA) results suggested that palaeoclimatic conditions changed from warm and humid to cold and dry climate from the P. pacificus to the M. extraordinarius zones in the study area. A comparative analysis of the published Fe-S-C data for the Xiushan Datianba section showed that in the P. pacificus zone of the inner Yangtze Sea, warm and humid climate conditions drove high productivity, sulphate flux and low reactive iron flux, which promoted the expansion of oxic ocean-surface waters and mid-depth euxinic waters. In the M. extraordinarius zone, the cold and dry climate with significant uplift of the Xiang’e Submarine High led to the relative sea level decline, resulting in low productivity, sulfate flux and high reactive iron flux, which promoted the expansion of the mid-depth ferruginous waters and the shrinkage of oxic ocean-surface waters. The results offered new insights into the co-evolution of continents and oceans, and explained the role of continental weathering and uplift of the Xiang’e Submarine High in the exchange of sulfate flux and nutrients in the redox environment change of inner Yangtze Sea during the Late Ordovician.
Highlights
The Late Ordovician is an important transition period in geological history, accompanied by large-scale glaciation, volcanism, sea level change, local tectonic activities and other geological and biological extinction events [1,2,3]
Shale in well ZY5 was deposited in an oxic water column environment, and the reduction degree of water in the M. extraordinarius zone was stronger than in the P. pacificus zone
The decrease in productivity and the relatively restricted water mass of the inner Yangtze Sea controlled the formation of relatively reduced water conditions in the M. extraordinarius zone
Summary
The Late Ordovician is an important transition period in geological history, accompanied by large-scale glaciation, volcanism, sea level change, local tectonic activities and other geological and biological extinction events [1,2,3]. Recent studies have suggested that the Ordovician-Silurian oceans may have been made up of mid-depth euxinic waters dynamically sandwiched between surface oxic and deep ferruginous waters [3,8,9,10]. Mo exists in molybdate anion (MO42−) form in oxic water columns, and is converted to particle-reactive thiomolybdates (MoS4−xOx2−, x = 1–4) in euxinic environments. In the latter form, it can be taken up by Fe-Mo-S nano-minerals or organo-metallic ligands in organic matter, resulting in high abundances in the sediment [28]. Mo concentrations greater than 100 ppm represent an euxinia environment, concentrations between 25 and 100 ppm represent an intermittent euxinia environment, and concentrations less than 25 ppm represent a non-euxinic environment [29]
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