RECONSTRUCTION OF EARLY CRETACEOUS PALEOTEMPERATURES FROM δ18O OF PALEOSOL CARBONATE NODULES AND ITS IMPLICATIONS FOR THE HABITAT ENVIRONMENT OF HAMIPTERUS TIANSHANENSIS
The Lower Cretaceous strata in the Hami Basin contain abundant pterosaur,egg,and embryo fossils,providing crucial evidence for studying the details of pterosaur survival and reproduction.However,there are no published studies on the paleoclimate closely related to the living environment of the Hamipterus tianshanensis,which hinders our understanding of the formation mechanism of the"Eden"habitat for the Hamipterus tianshanensis and the reasons for their evolution.This paper reconstructs the near-surface atmospheric mean annual temperature(MAT)during the Barremian and early Aptian periods using oxygen isotopes(δ18O)from calcareous nodules of the Early Cretaceous paleosol in the Hami Basin.Sections PM03(42°44.5′N,92°36.1′E)and PM68(42°27.4′N,92°44.7′E)are located on the steep slopes of the terraced landforms on the north and south sides of the Hami Basin,respectively.The ca.52 m thick PM03 section is mainly composed of lacustrine light gray to yellow-gray fine to medium-grained sandstone,with more mud clasts and abundant pterosaur fossils at the bottom and intercalated with light brown-red siltstone and paleosol layers at the top.The ca.136 m thick PM68 section is mainly composed of fluvial-lacustrine light gray to yellow-gray fine to medium-grained sandstone and conglomerate,with an increase in brown-red siltstone and paleosol layers in the upper part.Under the constraints of the Hamipterus tianshanensis fossil at the bottom of the PM03 section and the youngest age of the detrital zircons,it is inferred that the age of the PM03 section is the Barremian period.Based on the horizontal strata and the altitude,we suggest that the age of the upper part of the PM68 section is the early Aptian period.In this study,calcareous nodule samples were collected from six paleosol layers on the PM03 section,and five paleosol layers on the upper ca.70 m of the PM68 section,with the diameter of the calcareous nodules being 1～3 cm.Three calcareous nodule samples were collected from each paleosol layer in the field and cut open in the laboratory.Carbonate powder was drilled using a dental drill under a magnifying glass and analyzed for isotopes using a stable gas isotope ratio mass spectrometer(model MAT235).The results show that the maximum δ18O value of the PM03 section is-7.66％o,the minimum value is-10.52％o,and the average value is-9.67％o.The maximum δ18O value of the PM68 section is-5.21％o,the minimum value is-5.85％o,and the average value is-5.51‰.Using the formula T=1.1415(δ18O)+20.805,the annual mean temperature(MAT)was calculated and the results show that the average MAT of the PM03 and PM68 sections is 9.04 ℃ and 13.79 ℃,respectively.Under the inferred stratigraphic ages,the reconstructed MAT based on the paleosol calcareous nodules in the Hamin Basin is consistent with that from terrestrial records in Central Asia,and the changing trend is also consistent with the marine record.A rapid warming of ca.5 ℃ during the early Aptian in the Hami Basin may correspond to the initial rapid seawater warming during the onset of the Oceanic Anoxic Event 1a(OAE1a)at ca.120 Ma.The Hamipterus tianshanensis mainly flourished during the Barremian period with a lower MAT and declined after the OAE 1a event,which is consistent with the evolution of the Jehol biota,possibly indicating that global warming events have a certain impact on biological evolution.
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