Stable isotopic composition of dinosaur eggshells and pedogenic carbonates in the upper cretaceous seonso formation, South Korea: Paleoenvironmental and diagenetic implications

摘要

Stable isotopic compositions of the pedogenic calcites and calcitic dinosaur eggshells analyzed from the Seonso Formation in southern Korea provide information on the paleoenvironmental conditions that dominated the region during the Upper Cretaceous as well as on the nature of the diagenetic fluids. The 6180 values of the calcites recovered from the paleosols (-15.46 to -26.22 parts per thousand VPDB) indicate that they were precipitated and/or altered under a high temperature regime, whereas the delta C-13 values (-3.88 to -7.72 parts per thousand VPDB) suggest that these carbonates received contributions from hydrothermal fluids during shallow burial and possibly from C-3-type vegetation. The delta O-18 and delta C-13 of partially to well-preserved calcite eggshells are significantly different from those of the paleosols, indicating a departure from the earlier environmental conditions. The observed shift in delta O-18 values between the dinosaur eggshells (-3.43 to -14.09 parts per thousand) and pedogenic calcites indicates different environmental and thermal conditions and suggesting two possible scenarios; either that the timing of egg laying by the dinosaur and the soil formation were not synchronous, or that the dinosaur consumed water from the rivers and evaporated ponds. In contrast, isotopic compositions of vein calcites cross-cutting paleosols (-19.06 to -21.72 for delta O-18, and -3.78 to -4.79 for delta C-13) reflect their precipitation under hydrothermal conditions. The narrow range in the delta C-13 values of the eggshells (-7.04 to -8.69 parts per thousand) reflect a uniform source for carbon; mostly representing fresh water charged with CO2 from hydrothermal sources as dissolved carbon.