International Ocean Discovery Program
Expedition 389 Preliminary Report

Hawaiian Drowned Reefs

Platform operations 31 August–31 October 2023

Onshore Science Party 6–26 February 2024

Jody M. Webster, Ana Christina Ravelo, Hannah L.J. Grant, and the Expedition 389 Scientists

¹Webster, J.M., Ravelo, A.C., Grant, H.L.J., and the Expedition 389 Scientists, 2024. Expedition 389 Preliminary Report: Hawaiian Drowned Reefs. International Ocean Discovery Program. https://doi.org/10.14379/iodp.pr.389.2024

Abstract

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 by drilling 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 variation through space and time of the subsidence and the volcanic evolution of the island. To achieve these objectives, 35 holes at 16 sites 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 numerous intervals characterized by very well preserved mixtures of coralgal and microbialite frameworks with recoveries >90%. 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 CT 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 basalt cores suggest that many of the expedition objectives will be met.

Plain language summary

Shallow marine corals are highly sensitive to sea level and global climate change and preserve a reliable record of past sea level and climate conditions. Knowledge of sea level and global climate variations over the past half a million years is severely limited because of a lack of continuous fossil coral records over this time. To address the critical need for coral records, this project focuses on the submerged fossil reefs around the island of Hawaiʻi. Frequent and large volcanic eruptions formed and continue to grow the island of Hawaiʻi, and the island and surrounding shallow coral reefs are pushed down at a rapid and nearly constant rate because of the weight of the volcanic rock erupted onto the land. As the land and coral reefs subside, coral reef growth can match the subsidence rate, and changes in sea level and global climate are preserved in a unique and near-continuous fossil coral record covering the last half a million years. Scientific drilling of these reefs will provide a new record of climate and sea level change, including several key time periods where sea level and climate conditions are poorly known. The project has four major scientific objectives: (1) to measure the extent of sea level change over the past half a million years, (2) to investigate why sea level and climate change through time, (3) to investigate how coral reefs respond to abrupt sea level and climate changes, and (4) to improve scientific knowledge of the growth and subsidence of Hawaiʻi over time. Expedition 389 drilled 35 holes at 16 sites ranging from 131.9 to 1241.8 meters below sea level, recovering 425 meters of core, including very well-preserved mixtures of coralgal and microbialite reef frameworks as well as interlayered and basement volcanic rocks. Preliminary observations and radiometric data confirm that these deposits span the past ~500,000 years, including numerous key periods of major global ice sheet and sea level instability.