Expedition 389 summary

Our understanding of the mechanisms controlling eustatic sea level and global climate changes has been hampered by a lack of appropriate fossil coral records over the last 500 ky, particularly into and out of the glacial periods. This problem was addressed by International Ocean Discovery Program Expedition 389, which drilled a unique succession of Hawaiian drowned coral reefs now at 110–1300 meters below sea level (mbsl). The four objectives are to investigate (1) the timing, rate, and amplitude of sea level variability to examine cryosphere and geophysical processes, including the assessment of abrupt sea level change events; (2) the processes that determine changes in mean and high-frequency (seasonal–interannual) climate variability from times with different boundary conditions (e.g., ice sheet size, pCO2, and solar forcing); (3) the response of coral reef systems to abrupt sea level and climate changes; and (4) the variations through space and time of the subsidence and the volcanic evolution of the island. To achieve these objectives, 35 holes at 16 sites in water depths ranging 131.9–1241.8 mbsl were drilled during the expedition. A total of 425 m of core was recovered, comprising reef (83%) and volcanic (17%) material. Average core recoveries were 66%, with recoveries >90% in numerous intervals characterized by very well preserved coralgal and microbialite frameworks. Some science-critical shallow sites were not drilled due to a failure to secure permits to operate in Hawaiian state waters. Furthermore, apart from one site, the target penetration depths were not achieved. Preliminary radiometric dates indicate that the recovered reef deposits are from 488 to 13 ka in age. The Onshore Science Party took place in February 2024. Cores were computed tomography (CT) scanned and then opened and hyperspectral scanned and described. Standard measurements were made, and samples were taken for postcruise research. Preliminary assessment of the age and quality of the reef and volcanic cores suggest that many of the expedition objectives will be met.