Somewhere around 5,000 years ago, the deep-sea corals that had clung to seamounts off the Galápagos Islands for more than 100,000 years simply stopped growing. For the next millennium, the seafloor that once supported dense stony-coral communities sat empty. Then, just as quietly, the corals returned.
That striking pattern emerges from a peer-reviewed study published in Proceedings of the National Academy of Sciences in early 2025, in which researchers reconstructed a 117,000-year record of cold-water coral presence using uranium-thorium dating of more than 900 fossil specimens collected at depths reaching 1,000 meters. The gap represents one of the longest documented interruptions in deep-sea coral growth anywhere in the eastern Pacific, and the team behind the work says it was driven by a prolonged shift in the El Niño-Southern Oscillation (ENSO) system.
A record built from the seafloor up
Lead author Joseph Stewart and senior author Laura Robinson, both of the University of Bristol, assembled the timeline from fossil corals recovered during a series of deep-ocean expeditions to the Galápagos Marine Reserve. The earliest dives used the human-occupied submersible Alvin, launched from R/V Atlantis as part of the National Science Foundation’s deep-submergence program. Those missions discovered pristine living reefs on a previously unmapped seamount between 400 and 600 meters deep, the first such reefs documented inside the marine reserve since its establishment in 1998. Scientists from the Woods Hole Oceanographic Institution and the Charles Darwin Foundation joined the fieldwork, pairing geological sampling with biological surveys.
Later expeditions aboard the Schmidt Ocean Institute’s research vessel Falkor used the remotely operated vehicle SuBastian to locate two additional healthy cold-water reefs at depths between 370 and 420 meters. One stretched roughly 800 meters; the other spanned about 250 meters. Millimeter-scale laser scanning produced detailed maps of branching coral frameworks and surrounding rubble fields, giving researchers a high-resolution picture of reef architecture that could be compared directly with the fossil record.
The combination of thriving modern reefs and ancient fossils from the same region gave the team something rare: a before-and-after view spanning thousands of years. Across 117,000 years of record, the corals persisted through ice ages, interglacial warm periods, and repeated swings in ocean chemistry. The only interval in which they disappeared entirely was the roughly 1,000-year window beginning around 5,000 years ago.
Why the corals disappeared
Stewart and Robinson attribute the collapse to a well-documented period of unusually low ENSO variability. During normal conditions, the oscillation between El Niño and La Niña phases drives periodic upwelling that delivers nutrients, regulates water temperature, and refreshes dissolved oxygen at depth around the Galápagos. When that variability weakened for an extended stretch, the regular pulses of cold, nutrient-rich water slowed. The deep seafloor became less hospitable to the stony corals that depend on those conditions to feed and build their calcium carbonate skeletons.
The correlation between the coral gap and the low-ENSO interval is tight, but the precise kill mechanism has not been pinned down. The researchers have not yet quantified exactly how much temperature, dissolved oxygen, or carbonate saturation shifted during the hiatus. Whether the corals starved, suffocated in low-oxygen water, or lost the ability to calcify in more corrosive conditions remains an open question. Oxygen isotope and trace-element analyses from additional seamounts in the region could help resolve it, though no such expanded dataset has been published alongside this study.
The paleoclimate literature broadly supports the timing. Multiple independent proxy records, from lake sediments to cave formations across the tropical Pacific, confirm that ENSO variability dropped markedly during the mid-Holocene. What the Galápagos coral record adds is a biological consequence: a concrete example of an entire deep-sea ecosystem winking out in response to that climate reorganization and staying gone for over a thousand years.
Open questions and missing context
Several important uncertainties remain. The Galápagos record is, so far, a single-site story. Without comparable long fossil records from other eastern Pacific seamounts and continental margins, scientists cannot say whether the millennium-long absence was a basin-wide event or a collapse confined to the archipelago’s local circulation. That distinction matters: a regional die-off would suggest the corals were caught in a geographic bottleneck, while a basin-wide pattern would point to a more fundamental reorganization of deep-ocean conditions.
There is also the question of what today’s ENSO trajectory means for these reefs. Climate models assessed in the Intergovernmental Panel on Climate Change’s Sixth Assessment Report project that ENSO variability may increase under continued greenhouse warming, though confidence in that projection remains low. If variability intensifies, the specific conditions that caused the ancient collapse may not recur in the same form. But the broader lesson still applies: deep-sea corals are sensitive to sustained shifts in the oscillation patterns that govern their environment, and the threshold between persistence and disappearance can be crossed by changes that look modest from the surface.
On the policy side, there is no public indication that Ecuador’s Galápagos National Park has incorporated the fossil findings into marine reserve management. Conservation planning for the reserve has historically focused on fishing pressure, tourism, and warming impacts on shallow reefs. How deep-sea communities, which sit hundreds of meters below the zones most visitors and regulators think about, fit into that framework is an unresolved question as of May 2026.
What a thousand-year absence actually tells us
The strongest piece of evidence in this research is also the simplest: more than 900 dated coral fossils spanning 117,000 years, with one conspicuous hole. Uranium-thorium dating produces absolute ages with small error margins, and the volume of specimens makes it difficult to explain the gap as a fluke of sampling or preservation. When corals are present in every other interval across two full glacial cycles and absent only during one defined stretch, the signal is hard to dismiss.
The eventual return of the corals carries its own weight. It shows that deep-sea reef ecosystems can recover from prolonged collapse if the physical and chemical conditions that support them are restored. But “eventual” is the key word. The reefs did not bounce back in decades or even centuries. They were gone for more than a thousand years. For species that build slowly and reproduce infrequently, a disruption of that length is not a pause. It is a generational wipeout followed by a long, uncertain recolonization.
That timeline should inform how we think about the deep-sea corals alive today off the Galápagos and elsewhere. These organisms proved they could outlast ice ages. They could not outlast a sustained rearrangement of the tropical Pacific’s climate engine. As ocean warming, deoxygenation, and acidification accelerate under human influence, the fossil record from the Galápagos offers a blunt benchmark: deep-sea reefs are resilient on geological timescales, but once they cross a threshold, the road back is measured in centuries, not years.
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*This article was researched with the help of AI, with human editors creating the final content.
