A reef that just vanishes has a subtle, unsettling quality. Not overfished, not bleached, not destroyed by development—just gone. As if the ocean had chosen to hold its breath, it was absent from the fossil record for over a millennium. Scientists are just now starting to figure out why, but that is precisely what occurred some 5,000 years ago deep beneath the waters off the Galápagos Islands.
One of the most comprehensive ecological timelines ever put together for deep-sea ecosystems was created by researchers at the University of Bristol and published in PNAS. The study reconstructed 117,000 years of deep-water coral history in the Galápagos region. Using uranium-thorium dating, the team examined more than 900 fossil stony corals that were taken from depths of up to 1,000 meters. This allowed them to piece together an incredibly accurate picture of what lived there and when it died. Their findings appear to contradict the long-held beliefs of climate scientists regarding coral vulnerability.
These corals had survived the previous Ice Age. The subsequent warming had not affected them. The deep-sea reef systems in this Pacific region survived tens of thousands of years of planetary turmoil. Then they disappeared about five millennia ago. Over a millennium passed before the corals gradually and cautiously reappeared. It appears that recovery is feasible, but it happens on a schedule that does not allow for impatience.
The research found that a warming event was not the cause. It was not El Niño, the climate phenomenon most frequently associated with coral stress, whose warm Pacific surface water surges have decimated shallow tropical reefs in recent decades. The damage seems to have been caused by La Niña, the cooler, opposite phase of the El Niño Southern Oscillation. Stronger ocean circulation brought deep, nutrient-rich water to the surface during an extended La Niña period. The oxygen minimum zone, a layer of the ocean where oxygen levels drop sharply, was probably made worse by that upwelling, making it harder for deep-sea corals to survive. Dr. Joseph Stewart, the lead author, put it simply: large-scale ecosystem collapse occurs far below the surface due to subtle changes in global climate patterns. It’s difficult to ignore that for a little while.
The human-operated submersible Alvin and the remotely operated vehicle SuBastian, which were deployed from research vessels Atlantis and Falkor during recent expeditions to the archipelago, were used to recover the samples themselves. The image of a submersible plunging into almost complete darkness and collecting remnants of an ecosystem that collapsed prior to the construction of the first Egyptian pyramid has an almost cinematic quality. It was impossible for the researchers on those ships to predict exactly what the dating would ultimately show.
The implications of this finding for conservation are especially troubling. Shallow coastal ecosystems have traditionally been taken into consideration when designing Marine Protected Areas, the designated ocean zones intended to protect biodiversity. Due in part to its invisibility and in part to the significant effort and cost involved in studying it, the deep ocean has been largely ignored. According to this research, there might have been a serious oversight. The Charles Darwin Foundation’s co-author Stuart Banks noted that deep-sea ecosystems throughout the Pacific react to climate forces in ways that scientists are only now starting to map, and that future Marine Protected Area network design will need to include deep seafloor protection.
The exact way that ENSO behavior will change as the planet warms is still unknown; there is significant uncertainty in climate projections. However, it is challenging to ignore the directional concern. If an entire deep-sea coral ecosystem could be destroyed for a millennium in the past by a prolonged La Niña phase, then a future climate that intensifies or prolongs such conditions raises clear and grave concerns. Once, these reefs rebuilt themselves. Despite its depth, the fossil record is still unable to provide an answer to the question of whether they would have the opportunity to do so again.
