The 4.2ka Event Recorded By Porites Lutea From Northern South China Sea

As an important climate event affecting the early human civilization,the detailed evolution and mechanism for 4.2ka event are still under debating.Revealing the high-resolution climate variability in tropic ocean is benefit for understanding the detailed evolution and mechanism for4.2ka event.In this study,we used 7 Porites lutea that sampled from Xisha Islands and Wenchang sea area,which covered the age from 4420-4312 a BP?YX10-60?,4189-4112 a BP?YX10-38?,4181-4177 a BP?WC45?,4052-4005 a BP?YX10-12?,3996-3978 a BP?WC92?,3800-3793 a BP?WC94?and 3775-3757 a BP?WC42?.We rebuilt the annual SST variation by skeleton growth rate and monthly SST and SSS by tested Sr/Ca and?¹⁸O.We then discovered the responded climatic change in northern South China Sea during the 4.2ka event by these SST and SSS data via interpolated process,filter analysis,power spectral analysis,and cross spectral analysis:?1?The SST documented by growth rate and Sr/Ca value indicated that the norther South China Sea?SCS?was in a cold period during the 4.2ka event.The SST displayed a downward tendency with a little decadal fluctuation from 4500-4100 a BP.Several large fluctuations that were higher than modern time were detected from 4100-3800 a BP at seasonal to decadal time scales.?2?We reconstructed the variation of rainy season by acquired SST and SSS during the 4.2ka event in Wenchang coast area,Hainan Island.The results showed that the rainy season mainly occurred from late summer to early winter?August-December?and from late autumn to early spring?November-March?at that time,which were different from the modern time.This change of the rainy season made the northern SCS present a climate characteristic of"summer dryness and winter wetness"around the4.2ka event.?3?We obtained the ENSO variability at multi-timescale during the4.2ka event by reconstructed SST series.The climate background during the 4.2ka event was dominated by continuous La Nina like condition.However,the ENSO activity increased sharply and the climate changed frequently between El Nino and La Nina like condition in a short time due to the sudden increase of the Eastern Pacific El Nino events?EP El Nino?around the 4000 a BP,which caused violet oscillations in SST at that time.?4?The anomalies in ENSO condition,represented by persistent La Nina like condition superimposed abrupt change to El Nino like condition,was an important mechanism for the climate change during the 4.2ka event.The continuous La Nina like condition not only led to the cooling background at that time,but also resulted the climate characteristic of"summer dryness and winter wetness"by affecting the change of the rainy season in norther SCS.These climatic features declined the social production by causing frequent flood and drought disasters,which resulted the collapse of numerous civilizations at that time eventually.