International Ocean Discovery Program
Expedition 389 Scientific Prospectus

Hawaiian Drowned Reefs¹

¹ Webster, J.M., Ravelo, A.C., and Grant, H.L.J., 2023. Expedition 389 Scientific Prospectus: Hawaiian Drowned Reefs. International Ocean Discovery Program. https://doi.org/10.14379/iodp.sp.389.2023

Abstract

Our understanding of the links and mechanisms that control eustatic sea level and global climate changes has been significantly hampered by a lack of appropriate fossil coral records over the last 500 ky, particularly into and out of the glacial periods. We propose to directly address this problem by drilling a unique succession of drowned coral reefs around Hawaii (USA) now at 134–1155 meters below sea level. Abundant observational and numerical modeling data indicate that the internal stratigraphy and tops of these reefs are highly sensitive to sea level and climate changes, thereby providing a firm template with which to conduct these operations. As a direct result of Hawaii’s rapid (2.5–2.6 m/ky) but nearly constant subsidence, a thick (100–200 m) expanded sequence of shallow coral reef dominated facies is preserved within the reefs. These reefs span important periods in the Earth’s climate history, and coral reef records are either not available or highly condensed on stable (Great Barrier Reef, Tahiti) and uplifted margins (Papua New Guinea, Barbados) because of a lack of accommodation space and/or unfavorable shelf morphology. Specifically, these data show that the reefs grew (for ~90–100 ky, albeit episodically) into, during, and out of the majority of the last five to six glacial cycles. Therefore, scientific drilling through these reefs will generate a new record of sea level and associated climate variability during several controversial and poorly understood periods over the last 500 ky.

The project has four major objectives. The first objective will be to constrain the timing, rate, and amplitude of sea level variability over the last 500 ky allowing a definitive test of Milankovitch climate theory and an assessment of controversial abrupt sea level events (meltwater pulses) that occur on suborbital frequencies associated with events occurring in the extratropics (i.e., Dansgaard–Oeschger ice core temperature events and related Heinrich ice-rafted debris events in North Atlantic sediment cores). The second objective will be to investigate processes that determine changes in mean climate and high-frequency (seasonal–interannual) climate variability using high-resolution coral proxy data from times with different climate forcing boundary conditions (e.g., ice sheet size, pCO₂, and solar forcing) over the last 500 ky. The third objective will be to determine the response of coral reef systems to abrupt sea level and climate changes, to test sedimentary models of reef evolution and ecological theories of coral reef resilience, and to establish the role of microbial communities in reef building. The fourth objective will be to refine the variation through space and time of the subsidence of Hawaii and contribute to understanding the volcanic evolution of the island.

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 Hawaii. Frequent and large volcanic eruptions formed and continue to grow the volcanic island of Hawaii, 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 Hawaii over time.