World's biggest whale graveyard found in Indian Ocean off Australia

To come across a "whale fall" — the decomposing body of the marine mammal that has fallen to the sea floor — is a rare occurrence in the deep ocean.

So when a Chinese-led research expedition discovered a graveyard comprising five of these whale falls, along with hundreds of cetacean fossils from past falls, they knew it was globally significant.

On the one hand, that's because of the site's palaeontological record, which included the skull of a previously unknown extinct beaked whale species.

Some of the fossils found on the sea floor of the Diamantina Zone — a 1,200-kilometre-long area of underwater ridges and trenches in the Indian Ocean off Australia — were 5.3 million years old.

On the other hand, the discovery — published today in the journal Nature — is a biological wonder, with specialised communities of invertebrates and other marine creatures, many of them new to science, living on and devouring whale remains.

Together, these traits make up the deepest and largest collection of whale fossils and falls found to date in the ocean.

"These findings reshape our understanding of the limits and biogeography of whale-fall ecosystems and establish some deep-sea floors as a fossil archive for tracing cetacean evolution over geological time," study author and marine biodiversity researcher Xikun Song said.

How researchers found a whale graveyard

The whale graveyard was found during an expedition by the Chinese research vessel Tan Suo Yi Hao in March, 2023.

RV Tan Suo Yi Hao is often used for the Global Trench Exploration and Diving Program, a collaborative endeavour between China's Institute of Deep Sea Science and Engineering (IDSSE) and several countries, including New Zealand.

IDSSE deputy director Xiaotong Peng said the program aimed to investigate the biodiversity, ecosystem, pollution, and geological processes in the deepest parts of the ocean.

These deep waters between 6,000m and 11,000m are known as the hadal zone.

"The Diamantina Zone is one of the major hadal trenches in the Indian Ocean," Professor Peng said.

Australia is the closest country to the Diamantina Zone, which sits about 1,600km to the west.

A human-piloted submersible vehicle, Fendouzhe, capable of reaching depths of 11,000m, was used during the expedition to explore the zone.

These depths ranged from 4,200m to 7,002m, with the first fossils found in a geological feature called Dordrecht Deep.

"The first time we saw the whale fossils, we didn't recognise them as whale remains because they were coated with ferromanganese oxides," deep-sea geologist Peng Zhou, from IDSSE, said.

"Still, we could tell they were something special, so we collected the samples. Surprisingly, we found more and more similar whale fossils in later dives."

Thirty-three dives were conducted with Fendouzhe to map the extent of the fossils and whale falls.

There were 476 fossils of cetaceans found, including a new species.

What species of fossilised whale were found?

One fossil belonged to the baleen whale (Balaenoptera borealis), but five other fossil species were extinct beaked whales.

Three of the active whale falls belonged to species of modern beaked whales.

Dr Song, from IDSSE, said the beaked whale species still around today were primarily known from rare stranding events.

"Their [beaked whale] abundance, distribution and ecology remain poorly understood overall," Dr Song said.

Among the fossils was a skull fragment from an unknown species, which the new study named Pterocetus diamantinae.

Study author and palaeontologist Giovanni Bianucci, from the University of Pisa, said there was enough anatomical difference to distinguish the fossil from other species.

"The discovery improves our understanding of the evolutionary history of beaked whales and helps clarify how this highly specialised group evolved," he said.

UWA Oceans Institute marine mammal ecologist Kate Sprogis, who was not involved in the study, said that, with some of the fossils preserved for more than 5 million years, the research offered insights dating back to the Pliocene epoch.

But how did all these whales get there in the first place and stay so well preserved for so long?

How to make a whale graveyard

There are several reasons hypothesised for how the whale graveyard formed and has remained so well preserved.

Dr Song said the Diamantina Zone seemed to serve as either a habitat or a migratory corridor for several whale species.

This would mean a lot of the remains could have come from whales that died naturally.

But Dr Song said the high concentration of beaked-whale species among the fossils could also be behaviour-related.

"Deep-diving beaked whales exceeding 3,000m may reach physiological limits, increasing the risk of fatal exhaustion or decompression sickness," Dr Song said.

The V-shaped topography of the Diamantina Zone may also have helped funnel carcasses to the sea floor.

Dr Zhou said the preservation of the fossils for so long could come down to a combination of factors.

The first reason was that the fossils were mostly beaked whale rostra (noses), which were hyper-dense and mineral-rich, making them resistant to degradation.

"Second, the sedimentation rate here is extremely low … third, over time, ferromanganese oxide coatings form a protective crust on the bones," Dr Zhou said.

"[And] fourth, the deep-sea environment is cold and stable, so physical and chemical weathering are minimal."

Should we conserve whale fall sites?

While the Diamantina Zone is home to many dead whales, the seafloor there is teeming with life.

Whale falls happen in four stages, with different creatures taking advantage of a carcass:

• A whale's body sinks to the ocean floor, where large scavengers like hagfish and sleeper sharks eat its soft tissues over months.
• The "enrichment phase" is where organisms colonise the bones of the whale and the surrounding sediment, including bone-eating worms, lasting months or years.
• A "sulfophilic phase" is where bones and any remaining tissue decompose, and bacteria break down lipids (like fats) in the bones. Sulphur and methane are released. Bacterial mats form, which feed invertebrates like mussels. This stage can last for decades.
• With all the organic benefits from the whale exhausted, the remaining bone becomes a "reef" for filter feeders and other creatures to latch onto like sponges and sea anemones.

The largest active whale fall recorded for the study involved an Antarctic minke whale (Balaenoptera bonaerensis) covered in creatures, many likely to be new species.

University of Hawaii whale fall expert Craig Smith, who was not involved in the study, said the graveyard was a significant find.

He said it indicated that whale falls could promote biodiversity in some of the ocean's deepest parts.

"Clearly, many new species remain to be discovered on whale falls in other parts of the ocean," Dr Smith said.

"The results support the idea that deep-sea whale falls are biodiversity hotspots and provide stepping stones for sulphide-dependent taxa.

"This work suggests we should look for similar whale necropolises under feeding grounds for beaked whales."

Professor Peng said similar whale necropolises probably existed in other core beaked-whale habitats.

"It will be quite interesting to check these ocean regions," he said.

And sea trenches could be vital for ocean biodiversity, Dr Song said.

"However, they face threats from pollution," he said.

"Protecting trenches preserves Earth's last frontiers and their unique evolutionary heritage.

"Establishing marine protected areas in trench ecosystems should be considered in the future."