Lacustrine Deposits And Their Response To Oceanic Anoxic Events In The Early Cretaceous Fuxin Basin,NE China

A series of important geological events occurred during the Cretaceous period,which is the best research example of an earth surface system,known as“The Cretaceous World”.Multiple black mudstones and Oceanic Anoxic Events（OAEs）are considered to be an important window for understanding the carbon cycle in Earth’s evolution.In this study,a variety of methods,including sedimentology,sequence stratigraphy,isotope geochemistry,element geochemistry and organic geochemistry,were employed to study lacustrine sediments of the Shahai and Fuxin formations from late Aptian to Albian in the Fuxin Basin,NE China.Based on a systematic analysis of sedimentary environments,lacustrine basin evolution and distribution of thick-bedded mudstones in a sequence framework,conclusions can be drawn concerning volcanic activity,degree of chemical weathering in the provenance area and palaeoclimate,atmospheric oxygen concentration（pO₂）,atmospheric carbon dioxide concentration（pCO₂）and wildfire events in the early Cretaceous Fuxin Basin.Moreover,the response mechanism of the early Albian OAE 1b in lacustrine basin were further studied.The significant contributions of this study are:（1）Based on observations of outcrop sections and borehole cores and analysis of thin sections and logging curves,five rock types,including conglomerate,sandstone,siltstone,mudstone and organic rock,were identified in the Lower Cretaceous Shahai and Fuxin formations in the Fuxin Basin.According to the comprehensive analysis of lithologic characteristics,sedimentary structures,sediment body geometries and fossils,16 lithofacies types were distinguished.Combined with the characteristics of probability cumulative grain size curves,five facies associations within the Shahai and Fuxin formations are interpreted as having formed in a continental basin setting,which include alluvial fan,braided fluvial,meandering fluvial,fan delta,and lacustrine depositional systems.Based on the recognition of sequence boundaries,initial flooding surfaces,maximum flooding surfaces and basal surfaces of forced regression,the Shahai and Fuxin formations are subdivided in five third-order sequences.The sequence stratigraphic framework of the Shahai and Fuxin formations throughout the basin is further established and consists of five third-order sequences S1 to S5.Based on the relative balance of rates of potential accommodation versus sediment and water fill,lacustrine basin evolution was analyzed.Sequences S1～S2 and sequences S4～S5 correspond to an overfilled lacustrine basin,and sequence S3 corresponds to a balanced-filled lacustrine basin.Wihtin sequence S3,three thick-bedded lacustrine mudstones were developed in the Dongliang area of the Fuxin Basin.（2）A procedure for estimating the degree of chemical weathering in the provenance area is based on i)elemental compositions of mudstones,ii)index of compositional variability（ICV）,iii)selection of samples with ICV>1 and using these values for CIA calculations,and iv)Al₂O₃-（CaO*+Na₂O）-K₂O（A-CN-K）ternary diagram or CIA_corr.formula to correct for potassium metasomatism.The corrected CIA values can then be used to estimate the degree of weathering to which the provenance region had been subjected.The chemical weathering trends of the Shahai and Fuxin formations from late Aptian to Albian in the Fuxin Basin reflected by CIA_corr.,Mafic Index of Alteration for Oxidative weathering environments(MIA_（O）)and Weathering Index of Parker（WIP）,permit the identification of four units vertically through the geological succession,which shows the characteristics of“fluctuation-rise-stability-decline”.The degree of chemical weathering calculated using these chemical weathering indices shows an increasing trend during the early Albian OAE 1b interval which developed during a relatively warm and humid palaeoclimate.（3）High levels of inertinite（24.6vol.%）and polycyclic aromatic hydrocarbons（3～5rings）in mudstone samples provide evidence for widespread wildfires during the early Albian.The main types of wildfires in the early Albian were ground and surface fires from the relationship between inertinite reflectance and burning temperature,and the frequency of surface fires was high.Inferred atmospheric pO₂ levels in the early Albian atmosphere,as estimated from inertinite contents,were～25%,which is much higher than the minimum required for sustained combustion（16%）.（4）Based on constraints of early Cretaceous strata and their astronomical age,a comprehensive analysis of the excursion characteristics of total organic carbon（TOC）contents and the comparative analysis of organic carbon isotopes(δ¹³C_org)with global change trends,terrestrial records of the early Albian OAE 1b were identified in the Fuxin lacustrine basin.Atmospheric pCO₂ recovered from C₃ plantδ¹³C_org compositions indicates an increasing trend in Unit C during the early Albian,and there were three short-term increases of atmospheric pCO₂ corresponding to the three sub-events of OAE 1b during this time interval.Mixed sources of terrestrial plants and lacustrine plankton demonstrated by pyrolysis analyses(HI vs.T_max and S₂ vs.TOC),indicate a terrestrial contribution to the organic-rich sediments of the Kilian,Paquier and Leenhardt sub-events of OAE 1b.（5）The increase of atmospheric pCO₂ under volcanic activity aggravated the greenhouse effect during the early Albian OAE 1b,which might have triggered the relatively warm and humid palaeoclimatic conditions,and consequently enhanced the hydrological cycle and chemical weathering.Wildfire activity resulted in the burning and destruction of both vegetation and soil structure,enhancing the post-fire erosion associated with intensified continental weathering under warmer and more humid conditions.In this context,the elevated levels of nutrients and organic matter that were flushed into lakes and oceans where nutrients may have increased productivity in surface waters.Ultimately,nutrients and organic matter were flushed into lakes and contemporaneous oceans,where decaying organic matter in surface waters,including terrestrial plants and lacustrine plankton,consumed oxygen during downward passage through the water column,which in turn led to anoxia in lacustrine and in contemporaneous oceanic systems.