Marine C, S and N biogeochemical processes in the redox-stratified early Cambrian Yangtze ocean

Abstract

The temporal change of redox conditions of the Yangtze ocean has been revealed by investigating the Ediacaran-Cambrian transition section at Zhalagou, South China. During the earliest Cambrian, cherts and shales were deposited under an anoxic and ferruginous bottom water with significantly increasing total organic carbon and P contents, and negative shift in kerogen δ 13 C values in the lowest part of the section. Euxinic bottom water conditions occurred during the earliest Cambrian Stage 2, with the surface water dominated by N 2 utilization by cyanobacteria or sulphur bacteria leading to negative kerogen δ 15 N values. During Stage 3, dissolved oxygen and sulphate concentrations were significantly increased, and thus the oxidized surface water and the redox transition zone overlying a euxinic bottom water may have been expanded, resulting in an increase in kerogen δ 15 N increasing to 2-4‰, a decrease in pyrite δ 34 S decreasing to as low as -24.6‰ and differences in δ 34 S values between kerogen and pyrite as high as 37‰. This period coincided with the abrupt appearance of large-body metazoans. Thus, the expanding oxic surface water may have reinforced the evolution of animals or vice versa. Interestingly, kerogen δ 34 S values show negative relationships to FePy/FeHR ratios and pyrite sulphur contents, indicating that they can be used to reflect redox conditions, with the lightest values being obtained from euxinic environments.

Formation, diagenesis and palaeoenvironmental significance of upper Ediacaran fibrous dolomite cements

“…In contrast, the low‐energy platform interior containing tidal flat and lagoonal facies, is dominated by dolo‐mudstone, with less common thrombolites, stromatolites and anhydrite (Shan et al ., ). Time‐equivalent sedimentary settings in the southern and eastern Yangtze Craton are dominated by slope and basin facies with chert succession (Chen et al ., ; Yuan et al ., ; Cai et al ., ).…”

“…Member III is typified as a marine transgression, and a subsequent regression developed during the deposition of Member IV. Transgressive black siliceous and carbonaceous mudstones were deposited during the earliest Cambrian in the middle and southern Sichuan Basin overlying the Dengying Formation (Li & He, ; Cai et al ., ).…”

Coupling of ocean redox and animal evolution during the Ediacaran-Cambrian transition

“…Nitrogen isotopic data were obtained from nine successions that represent shallow to deep marine environments (Fig. 1 ), including Xiaotan 26 , Zhongnan 27 , Sancha 28 , Nangao 27 , Zhalagou 29 , Longbizui 30 , Lijiatuo 30 , and Yuanjia 28 sections in the upper Yangtze region, as well as Yanjia section (see Supplementary Note 1 and Supplementary Fig. 1 for lithological description) in the lower Yangtze region.…”

Biomarkers and C and S Isotopes of the Permian to Triassic Solid Bitumen and Its Potential Source Rocks in NE Sichuan Basin

“…Small amounts of isotopically light H 2 S may have been incorporated into labile organic matter in water columns and sediment/water boundary with formation time overlapping with pyrite precipitation [49]. Most of the organic sulfur may have been formed during early diagenesis in sediments, a relatively closed system to sulfate supply, and thus possess significantly heavier 34 S values than the associated pyrite ( [14] and references therein).…”

“…TSR is found to result in negative shift in 13 C values [9] (Machel et al, 1995) and positive shift in 34 S due to incorporation of 12 C-rich alkanes and 34 Srich TSR sulfides [8,[10][11][12][13][14] (Cai et al, 2001); thus, 13 C and 34 S values can be used as effective tools for oil-source rock correlation only where there has been little or no TSR and oil generation occurred at a closed or semiclosed system in which the source rock was rapidly buried during the period [13][14][15][16]. In such a burial environment with oil cracking to solid bitumen and the associated gas, the solid bitumen and its parent kerogen are expected to have similar 13 C and 34 S values; thus it is possible to use 13 C and 34 S values for the purpose of the correlation between source rock and solid bitumen and consequently to determine the source rock for the gases.…”