Two long and pronounced cold periods 3,000-5,000 and 6,600-8,400 years BP in East Asia and the southward migration of the westerly jet

To reconstruct the variations in sea surface temperature (SST) during the Holocene, the alkenone unsaturation index of marine sediments from two deep-drilled cores and 81 different core-top surface sediments recovered from the Heuksan Mud Belt, located off the southwestern coast of the Korean Peninsula, was measured. First, comparison of the alkenone temperature estimates of 81 core-top sediments with in situ temperatures indicated that the alkenone temperatures corresponded to the average SST in April to October. The spatial distribution pattern of the core-top alkenone SST showed a north south temperature gradient, which represented the in situ temperatures well. This indicated that the effects of the resuspension and lateral transport of sediments on the past temperature estimation might be insignificant. Based on the two deep-drilled cores, variations in the alkenone SST during the Holocene were reconstructed. In general, the alkenone SST decreased by less than 1 degrees C from the early to late Holocene. An interesting feature was the presence of two long and pronounced cold (approximately 2 degrees C) periods, which occurred at 3-5 kyr and 6.6-8.4 kyr B.P. These cold periods were also observed in other mid-latitude regions in East Asia, indicating that the cooling pattern was regional. The cold periods appeared to be associated with the southward migration of the westerly jet at that time. Investigations into the effects of local sea level changes and resultant changes in the tidal regime on changes in the SST during the Holocene period suggested that they were insignificant in the study area.