Large Perturbation Of Marine Carbon-sulfur Cycles And Redox Conditions During The Early Mesozoic

The Palaeozoic-Mesozoic transition witnessed the largest mass extinction and the longest biotic recovery of Earth life during the Phanerozoic. Recently, an increasing number of geologists focus on the relationship between the delayed marine ecosystem recovery and perturbed ocean conditions. The marine carbon-sulfur cycles and oceanic anoxic events across the Permian-Tirassic boundary have been long concerned, but little has been published on the same extreme events in the aftermath of the Great Dying. To reconstruct marine carbon-sulfur cycles and determine the redox conditions of the Early Mesozoic oceans, several important Lower and Middle Triassic sections (including the Dajiang, Guandao, Mingtang and Bianyang sections in the Nanpanjiang Basin; the Meishan, Daxiakou, Hushan, West Pingdingshan, and South Majiashan sections in the Yangtze platform) in South China have been sampled to study several important gepchemical proxies (including carbon isotope, sulfur isotope, iron species, and distribution of pyrite framboids).In this study, eight Lower Triassic sections from the northern Yangtze area and Nanpanjiang basin, representing several different depositional settings with various water depths, have been sampled to reconstruct δ813C (Δδ13C=δ13Cshallow water-δ13Cdeep water) of the Early Trissic seas. Of these,δ13Ccarb recorded in shallow marine facies sections are enriched in13C. These sections have recorded distinct δ13Ccarb excursions throughout the Early Triassic. However,δ13Ccarb recorded in deep-water facies sections are delepted in13C. These sections also lost some parts of the δ13Ccarb excursions. The δ13Ccarb excursions exbihit three pronounced negative-positive shifting cycles from the early Griesbachian to early Spathian (<2Ma), implying the large perturbation of the contemporaneous marine carbon cycles. There is a positive-negative cycle of δ13Ccarb during middle-late Spatian (-3Ma), indicating the mitigative carbon cycle. Both the Yangtze platform and Nanpanjiang Basin have recorded some large Δδ13C (3‰～8‰) during the Griesbachian to late Smithian. Compared with the high primary productivity, the stratified and stagnant ocean conditions were likely the primary cause of the large Δδ13C. The stratified and anoxic ocean conditions were attributed to the persistent high temperature and high content of pCO2. The strong volcanic activity might have resulted in the high pCO2and high seawater temperature. The large Δδ13C values during the early Griesbachian and late Dienerian to late Smithian, respectively imply the enhanced volcanic activity and seriously stratified ocean conditions during these periods. During the Smithian-Spathian transition, the overturned Δδ13C value indicates the stratified ocean began to circulate and it is agree with the cooling event in that time. An oceanographic model throughout the Early Triassic has been developed by integrating the δ13Ccarb, Δδ13C and chemical weathering results.The Lower and Middle Triassic strata of the Dajiang, Upper Guandao and Lower Guandao sections, located at the Great Bank of Guizhou, have been sampled to undertake the δ34SCAS study during the Early Mesozoic. The δ34SCAS compositions of the Early Triassic (average:～26‰) are more positive than the Late Permian (～22‰) and Middle Triassic (～17‰) counterparts and reflect the enhanced ocean anoxia and bacteria sulfate reduction during the Early Triassic. Based on the conodont stratigraphy and radiometric dating, the relationship between the δ13Ccarb and δ34SCAS has been established. The δ34SCAS compositions show great change rates and strong positive relationship with the δ13Ccarb values through the Griesbachian-early Spathian. These phenomenona indicate that the sulfate concentrations of the contemporaneous seawater were very low and the residence time of the sulfate was equal to the carbonate. Simple modeling shows that the sulfate concentrations during the Early Triassic decreased to one tenth of the modern ocean (<3Ma). During the Spathian, the change rates of δ13Ccarb and δ34SCAS decreased abruptly, and there is a negative relationship between them. These changes may be attributed to the enhanced ocean circulation. Both δ13Ccarb and δ34SCAS exhibit distinct positive excursion during the early Griesbachian, Dienerian-Smithian boundary and Smithian-Spathian boundary, respectively. Both enhanced organic carbon and pyrite in anoxic (euxinic) conditions resulted in the positive excursion events. Thus, the δ13Ccarb and δ34SCAS excursions indicate three super ocean anoxic (euxinic) events during these periods. The δ34SCAS values decreased to15‰in the Middle Triassic, indicate that the oxic conditions dominated the ocean. The increased sulfate concentration resulted in the small change rate of the δ34SCAS.The marine sulfur-carbon cycles coincide with the delayed biotic recovery. The large perturbed sulfur-carbon cycles during the Early Triassic represent the periodic ocean anoxic (euxinic) events that resulted in the mass extinctions and delayed recovery. The normal sulfur-carbon cycles in the Middle Triassic indicate the hospitable ocean conditions for the biotic radiation.The Smithian-Spathian boundary coincides with a second-order mass extinction, global cooling, and large positive C-isotope excursions. However, the causes of these biotic and environmental changes have not been resolved. In order to evaluate the changes in oceanic redox and environmental conditions, petrographic and geochemical analyses were undertaken in this study, including the distribution of pyrite framboids and carbon and sulfur isotopes, and iron species across the Smithian-Spathian boundary at the southern Majiashan section. An～6‰positive shift in δ13Ccarb across the Smith-Spathian boundary indicates a global event. The iron speciation, sulfur isotopes and pyrite framboid data provide unambiguous evidence for an intense but transient episode of euxinia around the Smithian-Spathian boundary. In the euxinic interval, the pyrite sulfur isotopes (δ34Spy) are negative and agminated. The difference between the δ34SCAS and δ34Spy implies the largest of the fractionation in the bacterial sulfur reduction. The δ34Spy in suboxic and oxic conditions are positive and scattered. The scattered compositions are attribute to the subsequent diagenesis. Based on the ocean anoxia, abrupt cooling and upwelling events during the Smithian-Spathian boundary, this study infers that these events were due to the reinvigorated oceanic thermohaline circulation and a resultant increase in marine primary productivity. Enhanced organic carbon burial and drawdown of atmospheric pCO2resulted in global cooling and concomitant changes in marine and terrestrial ecosystems. This hypothesis agrees with the Δδ13C and carbon-sulfur cycles.Mutil-geochemical proxies, including the total sulfur (TS), pyrite sulfur concentrations, Spyirte/Corg ratios, the degree of pyritization (DOP), and carbon isotopes through the Permian-Triassic boundary strata at the Dajiang section have been analysed. The results show that the TS, pyrite sulfur concentration, Spyirte/Corg ratio and DOP are in low values below the Late Permian mss extinction (LPME) horizon, indicating oxic condition dominated the shallow water facies. Besides, the geochemical data presented a negative excursion of carbon isotope and a hydrogen sulfide release event during this period. The increase of sulfur concentration, Spyirte/Corg ratio and DOP following the LPME demonstrates the dyoxic-anoxic condition in the shallow water facies. Consequently, the ocean anoxia developed in the Nanpanjiang Basin during Permian-Triassic transition. During the Late Permian, the enhanced volcanic activity released huge CO2and SO2, destroyed the terrestrial ecosystem, and resulted in the increased terrestrial weathering. Strong terrestrial weathering brought a large amount of organic carbon into the ocean and lead to the carbon isotopic negative excursion and the expansion of the oxygen minimum zone (OMZ). The H2S would diffuse into the shallow water periodically when the OMZ expanded into the euphotic zone, and further resulted in the pulse of enhanced pyrite burial. Nevertheless, the shallow water was still in oxic condition during this period. During the LPME and its aftermath, enhanced terrestrial weathering caused the further expansion of the OMZ and resulted in the dyoxic-anoxic condition of the shallow water. In addition, the microbialites that occurred in the immediate aftermath of the LPME were formed in anoxic condition.In conclusion, this study has established a high resolution δ34SCAS curve in the early Mesozoic, and clarified the unusual carbon-sulfur cycles and marine redox conditions. This provide important evidence for further understanding of the biotic and environmental events in the aftermath of the great end-Permian mass extinction.