Climatic and environmental indications of carbon and oxygen isotopes from the Lower Cretaceous calcrete and lacustrine carbonates in Southeast and Northwest China

摘要

Carbon and oxygen isotopic ratios were determined from ~100 Lower Cretaceous samples from four carbonate-bearing facies in Southeast (SE) and Northwest (NW) China for palaeoclimatic and palaeoenvironmental analyses. The samples were interpreted as sediments within sublithofacies of distal alluvial ponds (SF1), open shallow lakes (SF2), littoral lakes (SF3), and marshes (SF4). Results of analyses show in SE China, δ~(13)C values range between -5.0‰ and 3.0‰ with a negative trend through time, and δ~(18)O values are all negative (-19.3‰--7.4‰); in NW China, δ~(13)C values range from -4.0‰ to 4.0‰ with periodic change, and δ~(18)O values range between -18.0‰ and 1.5‰.Both relatively heavy values and pronounced covariances of most δ~(13)C and δ~(18)O indicate semiarid climate and domination of closed brackish lakes, marshes, and ponds in NW and SE China during the Early Cretaceous. Of them, positive δ~(13)C (0.0‰ to 4.0‰) and relatively heavy δ~(18)O (-5.0‰ to 1.5‰) values suggest interruptions of arid-evaporation in intermittences of the early Hauterivian in SW Fujian, of the middle Aptian in West Jiuquan basin, of the late Aptian in SW Ordos basin, and of the late Aptian-early Albian in Liupanshan basin. In a short interval of the early Aptian, the hot and humid climate occurred in local SW Zhejiang by high kaolinite content and in West Jiuquan basin by warm flora could be an exception. Particularly, extremely negative δ~(18)O values (mainly -19.0--9.0‰) indicate relatively low temperature in SE Fujian in the Berriasian-Barremian, in Liupanshan basin in the late Aptian, in SW Ordos basin in the late Albian, and in West Jiuquan basin in the mid-late Aptian, which is supposed to attribute to the presence of 2500m-4500m in elevation. More positive values and more covariance ratios of δ~(13)C and δ~(18)O in NW China than in SE China and in mainland China than in America indicate a arider climate in former than in later, probably attributing to the palaeogeography in distance to sea.A tentative correlation of the Early Cretaceous δ~(13)C value excursion of calcretes shows a good compatibility between marine OAEs and terrestrial carbonate sediments and between NW China and America continents, implying a global response to carbon cycle in both marine and terrestrial environments. The feature suggests the potential of global correlation and application of terrestrial calcrete δ~(13)C excursions in pre-Cenozoic.