Biogeochemical Cycles Of C-N-S And Paleoenvironmental Evolution In The Nenjiang Formation, Songliao Basin

The Cretaceous is the most important “green house” period and the oceanic anoxic events（OAEs） represent carbon cycling perturbations in the global atmosphere-ocean range over short terms（<1my）. Upper Cretacecous Nenjiang Formation is the main soure rock bearing layers in the Cretaceous large-scale inland lacustrine baisn, which is one of the best layers to research biogeochemistry of paleolimnology. In order to reveal terrestrial response influenced by OAEs, this study conduct the chemostratigraphy integrated by δ34S-δ13C-δ15N in high resolution in the Late Santonian-Early Campanian Nenjiang Formation. This study provides an indicator for the process of mechanism of environmental geological events and C-N-S cycles during the deoposition of Nenjiang Formation.Through variations in abundance in the trace elementals of different chemical properties and correlation analysis of elemental ratios, we reconstruct the trace elemental chemostratigraphy in high resolution and detect the significance in climatic conditions and water properties by trace elemental migration. All parameters vertify the transition from warm-humid climate to semi-humid conditions during the Nenjiang Formation. The variations in the climatic condition are in line with the lithological change, indicating terrestrial supplement promotes the primary productivity during the warm-humid conditions and the development of organic-rich shale. Two layers bearing oil shale both formed in this period. The fluctuated paleosalinity shows the organic-rich shale in the Nenjiang Member 1 and basla Nenjiang Member 2 formed in the saline conditions, while the upper Nenjiang Member 2 transited from brackish to fresh water. Consistent with paleosalinity variation, high salinity corresponds to anoxic conditions, which might be due to salinity stratigraphy in the semi-deep and deep lake zone. In turn oxygen-depleted environment creates favorable conditions for preservation of organic matter.Marine transgression of the Sonliao Basin is still a hot scientific hot issue and its scientific significance is associated with the formation mechanism of large-scale source rocks, even the theory on terrestrial oil-generation. Although some evidences of mineralogy, paleontology and orangic geochemistry were continuously mentioned, most of them are lack of marine specificity. Hence, more evidences in different perspectives are needed to be discovered. The data reveal a spike in pyrite sulfur abundance and a marked negative excursion in δ34S at the base of the Nenjiang Formation when the basin expanded to its maximal extent. The elemental and isotopic data suggest that flooding was associated with rapid marine transgression that enhanced sulfate concentrations, which promoted microbial sulfate reduction in anoxic bottom waters that were episodically euxinic. Subsequent restriction of the basin and a decline in marine influence is supported byprogressive upsection 34 S enrichment（up to 30‰） in Nenjiang Member I, which are interpreted to reflect the distillation of sulfate through enhanced pyrite burial, followed by a gradual return to lacustrine conditions that prevailed in overlying strata. In the Nenjiang Member 2, chemical weathering drove the terrestrial oxidized pyrite into the basin and led the δ34S to negative shift slowly, until recovered to the background values. Sulfate diffusive model and sulfate minimal zone model can interprete the superheavey pyrite inthe Nenjiang Member 1（δ34Spy>δ34Ssulfate）. Input of marine sulfate is favorable for remineralization of organic matter and elevates nutrient cycling and primary productivity, which provide the important biochemical pathway for the development of high-quality source rocks.The nitrogen cycles have a significant influence on the different spheres of earth surface. Preferential degradation of N-rich compounds during early diagenesis from shale deposited might be the results of high TOC/TN. The δ15N values in the organic-rich zone show a distinct positive excursion and then a steep negative excursion, indicating a big perturbation of nitrogen cycles in the lacustrine ecological systems. Based on the observation, we proposed an intense upwelling model of 15N-enriched nitrate from the underlying suboxic zoens of denitrification in the upper Nenjiang Member 1. The latter indicates the deficiency of nitrate consumed by denitrification and intense nitrogen fixation and also reflects prosperity of nitrogen-fixing geomicrobial functional groups. N2 O released by denitrifying process is the important greenhouse gas, which could influence the climatic changes and further strengen the anoxia of lake. Based on the comparison, the denitrification in the Songliao Basin is distinct stronger than seawater during the OAEs period, which might be the results of higher eutrophication degree.Many global marine sequences in the Upper Cretaceous Santonian-Campanian boundary record 0.3‰⁴.2‰ positive excursion in carbon isotope, which is defined as S-C Boundary Event. Although the bulk carbonate in the Nenjiang Formation is partly influenced by authigenesis, the δ13C reveals 4.2⁵.2‰positive anomaly near the S-C boundary, which is highly in line with +4.2‰ shift of the ostracode shells reported by previous study and indicates the event is global. Organic black shale bearing marine layers and high TOC of the Songliao Basin suggest increased organic burial sedimentation rate is the direct reason for the global carbon isotopic anomaly. The δ13Corg from TOC and the δ13CBC from terrestrial component reocord ⁴‰ and ³.5‰ negative shifts, respectively. It implies that the isotopic disturbance was not related to local limnological phenomena, but rather it recorded the behavior of entire exchangle carbon reservoir. Hence, we suggest the increase in atmospheric CO₂ is responsible for the amplified carbon isotopic fractionation.Superplume growth of lower mantle produce variations in heat flux across the core-mantle boundary that control the of geomagnetic polarity reversals. Hence, the reversal of Cretaceous Normal superchrons（CNS） in the Santonian-Campanian boundary implies the activities of superplume. Combined with the variations in δ34S of seawater sulfate, we suggest the rapid increase in CO₂ concentration might be from volcanic activities and further propose the superplume is the basic reason for the pertubations of different spheres during the Nenjiang epoch.