Trace element and isotope geochemistry of macroelongatoolithid eggs as an indicator of palaeoenvironmental reconstruction from the Late Cretaceous Xixia Basin, China

Abstract

The trace elements and isotopic compositions of macroelongatoolithid eggshells provide insights into the palaeoenvironment of these huge eggs preservation. We analysed the geochemical characteristics of newly discovered macroelongatoolithid eggs from the early Cretaceous Zhaoying Formation in the Xixia Basin, Henan Province, China. X-ray diffraction results reveal that the minerals of these eggs are predominately composed of calcite. The major elements in eggshells are Ca, Mg, K, Na, Al, Fe and Ti, and trace elements mainly include Sr, Ba, Mn, La, As, Sc, V, Cr, Co, Ni, Cu, Pb, Zn, Cd, U, and Ir. The high Sr and Ir concentrations could be caused by food materials and drinking water of dinosaurs. δ13C values of eggshells varying between −7.19‰ and −5.08‰ (average −5.69‰) indicate the C3 plants for dinosaurs with relatively high atmospheric CO2 concentration and low mean annual precipitation (MAP); further researches show that the carbon isotopic composition of atmospheric CO2 was around −4.11‰ to −2.19‰, and the MAP was lower than 10 mm/yr. δ18O values varying between −5.72‰ and −3.28‰ (average −4.53‰) show the temperature of nesting area were about 23 °C and the drinking water was dominated by leaf water with few meteoric water. The palaeoenvironment in which these huge eggs were preserved could be interpreted as continental deposit with extremely arid climate in the early Late Cretaceous Xixia Basin based on the high Fe contents of eggshells, the low palaeoclimate index of surrounding rocks, and the extremely low MAP shown by oxygen isotope.