Paleoclimate Records Derived From Noble Gases In Groundwater Over The Past 35 Ka In Songnen Plain

Climate change and its impact on the environment and social development have been increasingly concerned by the scientists,the public and the governments.Songnen Plain is located in an ecologically fragile zone and agro-pastoral ecotone with temperate continental semi-arid climate in Northern China,where was considerably sensitive to the climate change.In recent decades,the regional sustainable socio-economic development was severely restricted by the ecological environmental problems such as the land desertification,deterioration of groundwater quality and biodiversity degradation.To conduct paleoclimate records research since the late Pleistocene deduced form groundwater in this area will enable us to deep understand the regional atmospheric circulation patterns and variation of the monsoon in Northeast China,which is not only to provide a reliable basic scientific data for the global atmospheric circulation models(GCMs)of calibration and certification,but also scientific significance for the regional water resources management and applicability of climate change.40 surface water samples and 129 groundwater samples were collected in this study from the West Songnen Plain,including stable isotopes 2H and 18O,radioactive 3H,14C and 13C,noble gases,and water chemistry were analyzed.The origin of groundwater source recharge was identified with environmental isotope and water chemistry characteristics,and then the groundwater chronology was constructed with dating by multi-environmental tracers,with combining groundwater recharge temperature derived from noble gases,and thus paleoclimate records over the past 35 ka were reconstructed in the study area.Through comparing the noble gas paleoclimate records with the ?18O and pollen records,we were attemping to explore the relationship of paleodwater and the evolution of East Asia Monsoon.The main conclusions are as follows:(1)The source of groundwater recharge was identified using isotopic and hydrochemistry evidences.The Quaternary unconfined aquifer was mainly recharged by the local precipitation,while the Quaternary confined aquifers were recharged from two sources,one part was from the lateral flow of piedmont,and the other was from the upper unconfined aquifer leakage;The Neogene confined aquifers of Daan and Taikang groups were probably recharged from the lateral flow which was originated from the mountains precipitation with higher elevations.(2)The groundwater chronology since the late Pleistocene was established using a variety of environmental tracers(e.g.,3H,3H-3He,14C and 4He).The young groundwater was dated combing 3H and 3H-3He approaches and aged from 3.1 to 11.4a.And the old groundwater was dated with 14C and 4He methods and aged from 1.1 to 34.6 ka.(3)The noble gas Paleotemperature results indicated that the mean temperature during the late Pleistocene was 2.4 ?,which is about 4.6? cooler compared to the mean temperature of the Holocene(7.0?).And the average temperature of MIS3a period was 5.3?,which is about 1.7? cooler than the Holocene.(4)There were two significant climate changes occurred since the Late Pleistocene,one occurred at about 30 kaBP and the climate turned into cold and drier,which was probably due to the cold high pressure from Siberia and Mongolia increased suddenly and the summer wind began to shrink;The other presented at around 10 kaBP and the climate changed into warm and humidity probably because of the heavy rainfall caused by the summer monsoon.It is interesting that the interval of the two notable climate changes was about 20 ka,which is coincided with the precession period of climate change in the Quaternary.