Something has been secretly recording in the submarine canyons off the southwest coast of Western Australia, at depths where light vanishes and pressure turns into its own form of violence. The mineral architecture of deep-sea corals, which grow incredibly slowly and construct their skeletons one thin layer at a time over decades and centuries, is found in calcium carbonate, not on paper or in pixels. It’s difficult not to imagine them as nature’s filing cabinet, methodically preserving the ocean’s chemistry, temperature, and disposition while the rest of the world was mainly ignorant of their existence.
A group of international scientists sailed into the frequently harsh Southern Ocean in late 2019 aboard the research vessel Falkor, an 83-meter former German fishery protection ship that had been converted into a floating laboratory. Three submarine canyon systems—Bremer, Leeuwin, and Perth—were the main focus of the expedition, which was headed by geochemist Dr. Julie Trotter of the University of Western Australia. Each is located along the shelf-edge off the coast of southwest Australia, where it was sculpted by ancient forces that are still up for debate among geologists. Their depths were truly unknown until this journey.
The shallower waters of the Bremer Canyon already had a certain reputation; killer whales gather there seasonally in some of the largest populations in the Southern Hemisphere, attracting both wildlife photographers and marine mammal researchers. However, the deep water? That was a whole other story. No one had actually looked. And considering what seemed to be waiting down there, that in and of itself is an odd thing to sit with.
The team made 17 dives throughout the canyon system using a remotely operated vehicle known as SuBastian, which can descend to a depth of 4,500 meters. What they experienced was a hybrid of childhood wonder and scientific jackpot. Deep-sea coral gardens clung to sheer cliffs. Cup corals had established themselves at a depth of 2,200 meters, which is far below the point at which the chemistry of seawater renders the construction of carbonate skeletons theoretically difficult, if not impossible. The specifics of how they handle it are still unknown. The team eventually discovered the Flabellum, a genus of tiny, solitary cup coral that was nestled in soft, muddy sediments throughout all three canyon systems. “It was like winning the lottery,” Dr. Trotter remarked. The reason is simple to comprehend.
These corals are significant in ways that go well beyond their novelty. In essence, every skeleton is a chemical ledger. Strontium, boron, and rare earth elements are among the trace elements and isotopes from the surrounding seawater that a coral polyp incorporates into its skeleton in proportions that reflect the pH, temperature, nutrient levels, and mixing of water masses. You can see decades or even centuries of oceanic history written in mineral form when you retrieve that skeleton, cut it, and examine the layers. Certain species have records that date back thousands of years. These are instruments that continued to function after everyone had left the room, not merely fossils.
This strategy is not wholly novel. Since at least the 1970s, when researchers X-rayed a coral slice from Enewetak Atoll in the Pacific and discovered distinct annual growth rings—lighter and darker bands laid down in seasonal rhythm—scientists have been analyzing coral skeletons for climate data. Although not perfect, the tree ring analogy is still helpful. A snapshot of the ocean’s activity during its formation is captured by each layer. Chemical signatures from warm and cold years differ. Water that is acidic leaves its mark. The coral matrix traps and preserves even monsoon dust that is blown across continents.
The Southern Ocean, which surrounds Antarctica and serves as something akin to the center of the global climate system, is what makes the Australian deep-sea corals so fascinating. Heat, carbon, and nutrients that control global conditions are carried by the cold, dense water that sinks around the Antarctic continent and flows northward into the Indian, Atlantic, and Pacific Oceans. Building reliable climate models requires a thorough understanding of how that circulation has changed over the past millennium, including whether it has slowed, strengthened, or changed character during warming or cooling events. One of the few natural archives that can provide a detailed account of that story may be the deep-sea corals off the coast of Australia.
Beneath the science, there’s also something more pressing. Coral calcification rates in shallow reef systems are already being measurably impacted by ocean acidification, which is caused by increasing CO2 absorption. Reduced growth rates in living coral colonies have been observed in the Great Barrier Reef. If scientists are able to fully decipher the deep-sea record, they may be able to determine how the current changes in the ocean compare to those that have occurred in the past. There is a chance that the current changes are truly unprecedented in the contemporary geological record. Even though it might be more uncomfortable to think about, it is equally possible that they are not. If scientists are able to accurately read the corals, they may be able to tell.
The labor is costly and time-consuming. In the deep ocean, equipment failures cannot be resolved with a fast order from a supplier. An electrical short brought on by seawater seepage during the Falkor expedition threatened to completely halt ROV operations. In just two days, the replacement part was transported by tender to the ship anchored offshore, driven from Seattle to a remote coastal town in Western Australia, and installed. Deep-sea research is characterized by this type of logistical improvisation. You begin to realize how much of what scientists know about the deep ocean is the result of stubbornness more than anything else when you watch this happen, even if you are only secondhand through accounts from expeditions.
Years of laboratory analysis follow, including isotope measurements, geochemical testing, and skeletal layer dating. When that work is finished, the results will provide a high-resolution window into Southern Ocean conditions over the past few centuries, anchored in direct physical evidence, something the climate record currently lacks. These records have been preserved by the corals for a very long time. They are just now starting to be read by scientists.
