Paleoclimatic interpretation of the past 30 ka from isotopic studies of the deep confined aquifer of the North China plain

The δ 18O and δD values in the deep confined aquifer beneath the North China Plain which is located at 112°30′E–119°30′E and 34°46′N–40°25′N, reflect differences in paleoclimatic conditions between the Holocene and the late Pleistocene. Groundwater samples whose 14C ages are between 12 and 25 ka B.P have ranges of −9.4 to −11.7‰ for δ 18O and −76‰ to −85‰ for δD values. These very negative δ 18O and δD values reflect the cold and arid climate in the last glacial period. The temperature estimated in this period is 6–9 °C cooler than that of the present. The entire ranges of δ 18O and δD values for samples with 14C dating from 7 ka B.P to present are −7.7‰ to −10.2‰ and −63‰ to −73‰, respectively. The greater δ 18O and δD enrichments of these samples indicate a period of relatively humid and warm climate in the Holocene. However, the wide ranges of δ 18O (−9.0‰ to −11.1‰) and δD (−66‰ to −80‰) values for samples with 14C age ranging from 12 to 7 ka B.P. imply an unstable climatic condition of rapidly increasing temperature, which marks the transition from the Pleistocene to the Holocene.