Rock dust bleeds through the cracks that form around the fossil coral sample I’m collecting as I hammer in the pouring rain. It pries loose and I pause to look down at the fierce waves smashing into the base of the cliff. The GPS says my current elevation is 8.65 meters, but the sandy beach we’d hoped to visit below has disappeared, and these storm waves feel much closer than that.
Global sea levels are projected to rise over the next century due to fossil fuel-driven global warming and melting of the polar ice caps. But the timing and full extent of that rise remain a subject of debate in the scientific community, due to uncertainty around the dynamics and extent of ice melt in West Antarctica—a region roughly the size of Mexico that affects sea levels across the planet. To get at some answers, a team of scientists based out of Lamont-Doherty Earth Observatory, which is part of the Climate School, traveled to the Cook Islands this past June, searching for evidence of past sea level change that might help unravel these questions. I joined them as a Ph.D. student studying geophysics and sea level change.
The Cook Islands are located far from Antarctica in the tropical South Pacific, but the makatea outcrops—former coral reefs that have fossilized and now form the bedrock on some islands—preserve clues to the history of Antarctica’s ice sheets and sea level rise around the world in their chemical composition and elevation.
The HISEAS project aims to understand why ice sheets melted significantly past their modern-day level during the Last Interglacial, a warm period about 125,000 years ago that scientists view as an analog for the climate conditions we may face in the near future.
When ice melts, sea level does not rise evenly like a bathtub. Ice sheets are large enough to interact with tectonic plates, alter the Earth’s gravity field, and change its rotation. These effects cause sea level to rise in some places and fall in others, producing a pattern of rise and fall around the world that depends on which ice sheet melted—and present-day observations of that pattern, evident in the elevation of fossilized coral, can point to where and when ice has melted in the past.
“We can use observations of sea level from different places to ‘fingerprint’ or identify which ice sheet melted,” said Jacqueline Austermann, associate professor of Earth and Environmental Sciences and a researcher at Lamont. Austermann is leading the HISEAS project in collaboration with modelers, geochronologists, geochemists and paleoceanographers from across the U.S., Canada and Germany. “The Cook Islands have recorded sea level through fossil corals, which are very good records of past sea level because we can date them very precisely…They tell us time and sea level, which are the two ingredients we need to reconstruct sea level change.”
Fossil corals from the Last Interglacial have been documented around the world, but there are limited records of Last Interglacial sea level from the Pacific. The team hopes that obtaining a new set of the data from the Cook Islands makatea will improve modeling, building a clearer picture of the processes that drive ice sheet melting and how quickly sea level may change in the future.
The HISEAS team visited five islands where Last Interglacial makatea has been previously identified, including Rarotonga, home to the capital, and several outer islands—Atiu, Mangaia, Mauke and Mitiaro—to collect more than 100 samples. Using hammers and chisels, the group extracted fist-sized fragments of carbonate rock, which have been sent to the University of Florida for dating in geochronologist Oana Dumitru’s lab. A mass spectrometer will measure uranium and thorium isotopes in the samples. Because the isotope ratios change in a predictable way over time due to radioactive decay, the results will provide precise ages for the coral skeletons and help constrain when the outcrops formed.
Interpreting the fossil record is not straightforward. Although they formed near sea level, the corals’ precise depth is uncertain. And their elevation above sea level today is the combined result of both sea level fall since the Last Interglacial and tectonic processes that have lifted the islands up out of the water. Understanding the impact of each of these processes will require careful study, but the group said that excellent preservation of the reef topography may provide new insights.
“The reef topography is preserved in such a remarkable way so you can actually see the structures created,” said Blake Dyer, an Earth scientist at University of Victoria. “On the reef today, waves are crashing…they create these spur and groove structures that actually show up basically frozen in time on the fossil record.…Because we have this beautiful geology, sort of unique exposure of the reefs, we can ask questions we’ve never been able to ask anywhere else.”
The 15 land masses that make up the Cook Islands are home to small communities, with just a few hundred people on most of the outer islands, spread out across hundreds of miles of ocean. Despite their geographical isolation, the research team said the experience working there was anything but lonely.
The group was welcomed with leis, treated to home cooked meals and invited to attend a school ceremony on one island. “Meeting the elders, the principals and the teachers, the kids at school, and the climate change agency on every island… It was the first time I felt so welcome, but also they were genuinely so interested and engaged with our research,” said Dumitru. “It was a new type of field work to me.”
The local hospitality included meetings with both government officials and traditional leaders, a high-level public seminar on Rarotonga and invitations to teach a high school class on each island, all arranged with the help of Climate Change Cook Islands.
On the outer islands, where a whole high school class may fill a single room, dedicated science teachers can be hard to come by, leaving some to fill several roles or requiring schools to seek help from overseas. Students were joined by curious community members who gathered to hear about how we conduct our research, inspect samples and tools, and discuss the questions we all share: How old is the makatea? How quickly will sea level rise? What does the future hold?
On Mangaia, students asked about the ages of higher makatea outcrops unique to the island, which document previous warm periods and higher sea levels some time before the Last Interglacial. The group collected several samples from this older section, but their ages may remain a mystery since extensive weathering could make dating difficult.
What the makatea reveals will depend in large part on what comes back from Dumitru’s lab. “I’m very, very excited and curious about the results. I’ve never sampled anything that we think is an older interglacial, so I’m very curious to see if any material was preserved well enough to date.”
Otillia Steadman is a Ph.D. candidate in Earth and Environmental Sciences, who studies geophysics and interglacial sea level at Lamont-Doherty Earth Observatory, which is part of Columbia Climate School.
Views and opinions expressed here are those of the authors, and do not necessarily reflect the official position of the Columbia Climate School, Earth Institute or Columbia University.
