Textural and stable isotopic records of a composite-type speleothem from Gwaneum Cave in the eastern part of the Korean peninsula show prominent paleoenvironmental changes since MIS (marine oxygen isotope stage) 5a. Based on 230Th/234U dating, the speleothem experienced growth from 90.9 ± 6.5 ka to 1.2 ± 0.5 ka with several hiatuses. Four growth phases (A, B, C and D) are recognized based on speleothem type and texture. Very irregular and laterally discontinuous growth laminae in Phases B and C indicate that the cave coralloids grew over the stalagmite (Phase A) when the supply of dripping water became limited. Variations within the δ13C time series of Phase A are interpreted as responses to millennial-scale fluctuations of the East Asian monsoon intensity during MIS 5a. The monsoonal interpretation is based on the idea that δ13C values reflect the isotopic composition of soil-derived CO2, which, in turn, should relate to monsoon-driven changes in terrestrial productivity above the cave during the MIS 5a. Our reconstruction reveals that the significant monsoonal changes on the Korean peninsula occurred in conjunction with changes in sea level and/or oceanic circulations during the transition period from MIS 5a to MIS 4.

In this study, I examine the effect of US foreign policy on the relationship between South Korea and North Korea. In particular, I analyze whether two different foreign policy approaches—the hard-line approach and the soft-line approach—have played a role in advancing or slowing steps toward peace in the Korean peninsula. I use the Integrated Data for Events Analysis dataset for the period 1990–2004. By employing a Vector Autoregression model, which analyzes the behavioral patterns of South and North Korea and the United States, I find that US foreign policy affects the relationship between the two Koreas by affecting North Korea's behavior toward South Korea. The triangular relationship among the United States, North Korea, and South Korea shows a reciprocal behavior pattern. This finding suggests that a soft-line and reciprocal US foreign policy toward North Korea is critical to maintaining peace in the Korean peninsula.