Sea Surface Hydrological Changes And ENSO-like State In The Core Region Of The Western Pacific Warm Pool Over The Past 450 Ka

Western Pacific Warm Pool(WPWP)is the region with the most active air-sea exchange in the world,which plays a very important role in global climate evolution.Understanding the paleoenvironmental evolution of WPWP is of great significance to global climate change.In this study,the upper 31 m core composite depth below seafloor(CCSF)of IODP 363 Site U1486(02°22.34'S,144°36.08'E,water depth 1332 m),was used to analyze ?18O and Mg/Ca of planktonic foraminifera Trilobatus sacculife.Comparing with previous paleoclimate and paleocean studies,we reconstructed the hydrological evolution of the surface water in WPWP and thus to investigate the potential linkage of ENSO-like states between the warm/cold periods in the glacial/interglacial cycles.The ?18O record of core U1486 has shown obvious characteristics of glacial-interglacial cycles over the past 450 ka.The spectrum analysis shows typical periods of eccentricity(100 ka),obliquity(40 ka),and precession(23 ka),which means that and the changes of ?18O in WPWP may be driven both by high latitudes and tropical regions.The Mg/Ca-temperature results of T.sacculifer indicate that the Sea surface temperature(SST)in the WPWP during the last five glacial-interglacial cycles ranged between 25.3? and 30.8?.The minimum SST at the site during the Last Glacial Maximum(LGM)is 26.1?,nearly 2? lower than that during the late Holocene.The SST changes do not lead the changes of ?18O and exhibit a close similarity with atmospheric CO2 content for the whole study period,which also show characters of glacial-interglacial cycles.We speculate that greenhouse gas forcing is the primary driver SST changes in orbital sacles in WPWP region.The ?13C record of core U1486 T.sacculifer over the past 450 ka ranged between 0.71‰ and 30.8‰,and the variation of ?13C does not show obvious trends of glacial-interglacial cycles.Generally,the values in interglacial periods are slightly higher than it in glacial periods,and the change of ?13C may be controlled by various factors.There are obvious ?13C mimimum events during termination ?,?,?,? and MIS 8.We guess that the low ?13C signals may be from Southern Ocean and be related to CO2 air-sea exchange.?18Osw-iv of core U1486 was calculated by removing ice volume effect from?18Osw,which showed a positive relationship with local insolation since 450 ka.During interglacial periods,it corresponds well to the river runoff records of nearby core stations,which means it can be used to reflect the change of precipitation in the study area.?18Osw-iv shows an anti-relationship with east Asian monsoon precipetion records,which indicate a tight relationship between the Australian monsoon and the east Asian monsoon during interglacials.And the changes may indicate the meridional shifts of Intertropical Convergence Zone(ITCZ)in obitial time scales.By comparing the SST records derived from the cores located in the WPWP and the Eastern Pacific Cold Tongue(EPCT),it is found that the zonal temperature gradient of the tropical Pacific Ocean was more than 3?,along with deeper thermocline in the WPWP,during the glacial periods.In addition,larger zonal temperature gradients,with a maximum of 4?5?,were recorded in the major stadials of interglacial periods.The patterns of the zonal temperature gradient across the tropical Pacific Ocean imply that the ocean conditions therein showed more La Nina-like states in the colder periods and more El Nino-like states during the warming stages over the past few hundred thousand years.When the earth-surface temperature rises,the rapid increase of water vapor concentration in the lower troposphere could weaken the mass exchange between the boundary layer and the troposphere,and thus weaken the intensity of the Walker circulation and the eastliers in the tropical Pacific that may result in a decreased zonal temperature gradient with a more El Nino-like state.