Mineral And Elemental Compositions Of Dinosaur Eggs And Associated Palaeoenvironmental Analysis From The Late Cretaceous Of Qiyunshan Area,Anhui Province

The Cretaceous has been recognized as a special episode with the most abrupt change of geochemical environment during the Late Cretaceous.The geochemical information within the dinosaur eggshells is the significant basis in revealing the paleoenvironmental and paleoclimatic inferences during the Late Cretaceous.To date,a large amount of dinosaur egg fossils have been discovered in the continental red beds of Upper Cretaceous worldwide.In recent years,two clutches of dinosaur eggs are found in Xiuning Basin,Anhui Province,great significance for discussing the dinosaurs living environment within Late Cretaceous in Xiuning Basin.This paper intends to study the newly discovered dinosaur eggs to analysis the compositions of macrominerals,macroelements,trace elements and stable carbon and oxygen isotopes together with the depositional environment of egg-bearing site in light of mineralogy,geochemistry and sedimentology,which could indicate the paleoenvironmental information including trace element anomalies,plant types,temperature and humidity.The main conclusions are showed as follows:(1)The mineralogical characteristics of dinosaur eggshell(Similifaveoloolithus qiyunshanensis)with its internal and external surrounding rocks reveal that the main minerals of eggshells are cryptocrystalline calcite(about 70%?80%),phenocrystalline calcite(about 20%?30%)and crystalline quartz.Among them,the cryptocrystalline calcite is the primary constituent of eggshell from lithogenesis,the phenocrystalline calcite is the secondary component produced by hydrothermal recrystallization in the buried dinosaur eggshells,and the crystalline quartz generated from clastic sediments around eggs.The minerals of the interior are similar to that of exterior of eggshells and are mainly composed of quartz and feldspar,which means the internal and external surrounding rocks have exchanged attributed to the break of dinosaur eggs during the burial process.(2)The data of major and trace elements of dinosaur eggshells,internal and external surrounding rocks and sedimentary rocks shows that the eggshell is mostly comprise calcium carbonate;the Sr and Ir contents with obvious anomalies reaches up to 6638.6X10'6 and 152.9×10-12,respectively,which could be due to the ingestion of food materials and water resources enriched in Sr or Ir.The stable carbon isotopic compositions of dinosaur eggshells range between-5.94‰?3.55‰,which suggest that C3 plants dominated in this area during Late Cretaceous together with relatively high pCO2;the oxygen isotopes fluctuate between-8.45‰?-4.95‰,indicative of high temperature and low humidity.The fluctuations of Sr/Ca ratios and ?18O of Similifaveoloolithus qiyunshanensis indicate that the calcification of eggshells depends on the decomposition of bone apatite,whereas the females consume various evaporated water during egg formation.The analysis of Sr/Ca ratios and ?18O of Umbellaoolithus xiuningensis shows that the feeding environment of dinosaurs is changed and the oxygen isotope composition could be related to the decomposition of biological apatite and drinking water in various environment.(3)The stratified sedimentary sequence of dinosaur egg-bearing site from the Xiuning Basin encompasses small scale tabular cross-bedding together with relatively low textural maturities,indicating weak hydrodynamic conditions and low-energy deposition.The lithologies of the sequence are continental clastic rocks comprised of terrigenous frag:ments and fillings while the dinosaur egg-bearing strata comprise pebbly medium-to-fine grained sandstone with Ca cement on the basis of microstructural features.The histogram for the whole profile with bimodal feature illustrates that the sequence is consistent with a fluvial interpretation;the characteristics of probability cumulative curves show that the profile consists of several sedimentary cycles,including lower point bar deposit and upper floodplain deposit;the grain size distribution characteristics of surrounding rocks show that the sedimentary cycles are attributed to the change of hydrodynamic conditions in periodic flood event episodes.(4)The mineralogical characteristics,element anomalies and distribution patterns of stable isotopes from dinosaur eggshells,as well as the sedimentary environment of measured profile,indicate that the nesting climate in the Xiuning Basin of Late Cretaceous is semi-arid to arid.The strong hydrodynamic condition within the egg-bearing site of Similifaveoloolithus qiyunshanensis and the preservation of the two nests reveal that dinosaurs build nests after the flood event,while the higher environmental temperature intensifies the evaporation of water,and the hydrodynamic condition gradually weakened,which is conducive to the rapid burial and preservation of dinosaur eggs.