Spatial Variation In Carbonate Carbon Isotopes During The Cambrian SPICE Event Across The Eastern North China Platform

The Cambrian SPICE(Steptoean Positive Carbon Isotope Excursion)event is a large excursion(by?5‰)in marine carbon isotope(?13C)records during the late Miaolingian to the early Furongian,which is documented worldwide.The SPICE event is coincident with and ascribed to,most likely the second phase of,the Dresbachian(or end-Marjuman)extinction event that slashed approximately 40%of marine genera during the early stage of life evolution.The SPICE is hypothesized to be caused by a global carbon cycle perturbation(enhanced global carbon burial),which is accompanied with dramatic marine anoxia and euxinia.But recent studies imply that the synchroneity and magnitude of the SPICE may be controversial.The North China Platform,a typical sedimentary basin of a shallow epeiric sea in the late Cambrian,is an ideal region to research the spatial variation of SPICE because of the differences in relative sea level variation,paleostratigraphic and sedimentary facies between the eastern and northern margins of the platform.Six Cambrian sections including Suzhou Jiagou section in Anhui province,Jining Liangcheng section and Jinan Chengouwan section in Shandong province,Xishan Xiaweidian section in Beijing,Dalian Baijiashan and Shashan section in Liaoning province were measured and analyzed in paleontology,stratigraphy,sedimentology,and isotopic geochemisty to get the regional spatial variation characteristics of SPICE.Compared with the complete trilobite fossil belt from late Miaolingian to early Furongian in Liaoning region,the sedimentary discontinuity occurred in Shandong region due to the exposure erosion caused by the sea level drop,and the lower part of the trilobite Prochuangjia and Chuangia were missing.The main facies of the upper Gushan formation and lower part of Chaomidian formation were Shale,limestone and shale alternation,Thin-bedded lime mudstone,Bioturbated wackestone,Bioclastic pack-to grainstone,Cross-stratified oolitic grainstone,Crudely stratified bioclastic grainstone,Flat-pebble conglomerate,Microbialite.Based on this,four facies assemblage were established:shale-dominated facies association,thin-bedded limestone facies association,grainstone facies association and microbialite facies association,which respectively represent the sedimentary environments of deep subtidal,shallow subtidal,oolitic shoal facies and microbial flat facies.According to the vertical arrangement of the facies associations,the succession can be grouped into 3 third-order depositional sequences.Sea level subsidence during late Miaolingian followed by rapid uplift in early Furongian.In order to test the spatial variation of the SPICE,here we present 514 carbonate ?13C data from six outcrop sections across a?700 km transect along the eastern North China Platform.The SPICE is present in all the studied sections,but showing different features regarding the duration and magnitude.An abrupt increase in ?13C from?1.5‰ to 3.5‰ occurs in three Shandong sections,with maximum value present only in a thin(0.5-1.2 m thick)transgressive lag deposit(crudely wave stratified,bioclastic grainstone).In contrast,the ?13C record of the Baijiashan and Shashan sections in the Liaoning region displays a gradual increase(?20 m thick)from?1.5‰ to the maximum value of 4.7‰.Both facies analysis and trilobite collections suggest that the spatial variation of the SPICE in the North China Platform most likely resulted from missing of sedimentary record(with high ?13C values)in the Shandong sections as a result of erosion and non-deposition during sea-level lowstand,a possible coeval hiatus prior to the Sauk III transgression in the Laurentian basins.Compared with other areas,?18Odata in Beijing area is relatively negative,indicating that the diagenetic alteration in Beijing area is strong,which may lead to the lower value of ?13C than that in other areas.The SPICE is facies independent and synchronous based on the facies analysis.Our study shows significant variation in the duration and magnitude of the SPICE,but they can be reasonably correlated with sedimentary process.Further high-resolution,integrated studies of sedimentology,biostratigraphy,and sedimentary geochemistry on the SPICE event are required to better understand its nature.