Chapter 3.6 - Coral Reefs and Sea-Level Change

Abstract

The last deglaciation was characterized by a rapid sea-level rise and coeval abrupt environmental changes. The Integrated Ocean Drilling Program Expeditions 310 “Tahiti Sea Level” and 325 “Great Barrier Reef Environmental Changes” aimed to provide the most comprehensive last deglacial sea-level and climatic records by conducting offshore drilling of fossil coral reefs now preserved at 40–130 m below present sea level. These expeditions have provided unparalleled reef records of the last and penultimate deglaciations and brought a wealth of new information in various scientific fields. They have demonstrated the pivotal importance of coral reef records to reconstruct the amplitude and timing of sea-level rise caused by the melting of land ice after the Last Glacial Maximum ∼23,000 years ago. In particular, short periods of dramatic sea-level rise indicating varying rates of melting imply the presence of thresholds within the dynamic behavior of ice sheets and provide a challenging test for ice sheet models used to predict future sea-level rise. The sea-level jump during meltwater pulse 1A, newly constrained by Expedition 310 with a magnitude of 16 ± 2 m in ∼350 years (14.6–14.3 ka), induced the retrogradation of shallow-water coral assemblages, gradual deepening, and incipient reef drowning. Reefs accreted continuously between 16 and 10 ka, mostly through aggradational processes, at growth rates averaging 10 mm/year. No cessation of reef growth, even temporary, was observed during this period at Tahiti. Post-cruise analyses are still in progress but initial results confirm that Expedition 325 will extend this record of sea level, environmental changes, and reef response in the South Pacific back to ∼30,000 years ago. Drilling of multiple sites around the globe, spanning several glacial–interglacial transitions, is needed to better constrain the timing and amplitude of sea-level change that resulted in the disintegration of large ice sheets, and to evaluate the response of ocean meridional overturning circulation to freshwater inputs.