Element And Isotope Geochemistry And Paleoenvironment Analysis Of Dinosaur Eggs From The Late Cretaceous In Xiuning Basin

Dinosaur eggs are richness and well preserved in the Late Cretaceous,Xiuning Basin.The research of elemental and isotopic characteristics of dinosaur eggshells and related paleoenvironment in the Late Cretaceous is very important.Therefore,this paper intends to study the newly discovered Umbellaoolithus xiuningensis and Parafaveoolithus wannanensis in the Xiuning Basin.The eggshell microstructures,recrystallization and pore development characteristics were observed by Polarizing Microscope（PLM）、Cathodoluminescence（CL）.The main and trace elements,carbon and oxygen isotopic composition of dinosaur eggshells were further analyzed using Inductively Coupled Plasma-Mass Spectrometry（ICP-MS）,Inductively Coupled Plasma-Optical Emission Spectrometer（ICP-OES）,MAT 253 Stable Isotope Mass Spectrometer and Laser ablation-Inductively Coupled Plasma-Mass Spectrometry（LA-ICP-MS）.Combined with the existing research results,the Late Cretaceous trace elemental anomalies,the elemental distribution on the eggshell microstructure,CO₂ concentration,temperature change,vegetation type,the relationship between isotopic content and pore development in the Xiuning Basin were discussed.The main results are as follows:（1）Umbellaoolithus xiuningensis and Parafaveoolithus wannanensis were observed by the polarizing microstructure.Results show that the radial section of these eggshell consist of eggshell units and pore channels while tangential sections show distinct honeycomb-like microstructures,which is similar to the microstructure of Similifaveoolithus qiyunshanensis.Radial sections of these three types dinosaur eggshells show different luminescence characteristics under cathodeluminescence.The dark brown and black luminescence regions represent primary calcite,which are mainly composed of the original eggshell and are less affected by recrystallization.The orange and red luminescence regions represent secondary calcite filled in the pores,which is greatly affected by recrystallization and a secondary product formed after eggshells are buried.（2）The main and trace elemental composition of dinosaur eggshell powder and surrounding rocks were tested.Results show that the main elements of eggshell mainly include Ca,Na,Mg,K,Fe and Al,among which Ca is the most abundant elements;the trace elements in eggshells mainly consist of Sr,Mn,Ti,Ba,As,La,Cr,Ni,Pb,Cu,Zn,Co and Ir,among which Sr and Mn contents in eggshells are much higher than those in surrounding rock.It is speculated that the Sr and Mn enrichments may entered into dinosaur body through food and water and then deposited in eggshells.The ultra-high anomaly of Ir element in surrounding rock is consistent with the global Ir anomaly in the Late Cretaceous.（3）The LA-ICP-MS trace elemental testing results of eggshell sections showed that Sr,Mn and Fe elements were obviously abnormal,which was consistent with the test results of powder samples.The content variation of trace element in the eggshell microstructures show that the excess Sr is mainly concentrated in the columnar layer of eggshells.The rare earth elements in eggshells are mainly composed of La,Ce,Pr,Nd,Sm,Eu,Gd,Tb,Dy,Ho,Er,Tm,Yb and Lu.The distribution diagram of rare earth element standardization curve shows obvious LREE enrichment,which is correspond with the values of（La/Yb）_N,（Yb/Nd）_N and（GD/Nd）_N.Eggshells show obvious negative Eu anomaly and partial positive Ce anomaly indicating neutral and partial oxidation conditions.ΣREE is positively correlated with other associated elements and rare earth elements,which indicate that the rare earth elements in eggshells may be come from terrigenous clastic materials.（4）The carbon isotopic values of dinosaur eggshells range from-5.730‰to-3.920‰with an average value of-4.749‰,which show the C3 vegetation type,the high atmospheric CO₂concentration and the low mean annual precipitation less than 10 mm/yr.The oxygen isotopic values of eggshell range from-7.520‰to-5.132‰with an average value of-6.266‰,which indicate that the drinking water of dinosaur is mainly composed of a large amount of plant leaf water and a small amount of atmospheric precipitation,and the temperature variation range is about 13-22℃,representing the relatively high temperature and low humidity and belong to semi-arid to arid climate.In addition,theδ¹⁸O value is relatively low and pore density is relatively high in three types of dinosaur eggshells,which also reflect the semi-arid to arid climate in the Late Cretaceous of Xiuning Basin.According to the microscopic structures,elemental and isotopic characteristics of dinosaur eggshells,the geochemical environment were characterized by Sr、Mn and Ir elemental anomalies;the vegetation types were dominated by C3 plants;the atmosphere CO₂ content was relatively high;the humidity and mean annual precipitation were relatively low;dinosaur drinking water mainly consists of most leaf water and less atmospheric precipitation in the Xiuning Basin during the Late Cretaceous.The related parameters such as Sr/Ba value,Ni/Co value and paleoclimate index in surrounding rocks indicate the terrestrial freshwater and oxidation environment in the Xiuning Basin.