Whale cemetery holds more than bones at the bottom of the sea

Scientists who recently took a submersible to a remote location in the southeast Indian Ocean have identified one of the largest and deepest whale burials, containing hundreds of fossils, including one from a previously unknown species. But not everything in this underwater necropolis is dead.

Tens of thousands of meters below the surface, dead or dying whales were swept by the current into the vast fossil graveyard, their bones jumbled together in an area approximately 1,200 kilometers wide. Alongside the older bones are modern skeletons, suggesting that the whale remains remained there continuously for at least 5 million years, based on the age of the fossils, researchers reported in the journal Nature.

Most of the remains belong to beaked whales, which have skulls that taper into slender snouts, similar to those of dolphins. These whales dive to great depths and spend little time near the surface, so they are rarely seen and very little is known about their habits.

Researchers observed some whale corpses sunk in the marine cemetery, recent enough to still have scavenging organisms attached to them; Known as whale carcasses, these bodies feed diverse communities of marine life, including bone-eating worms, snails, long-armed brittle sea stars, and bivalves that survive through chemosynthesis - using chemical energy to produce their food. Many of these species may be new to science, the study authors reported.

"Until now, whale deaths were mainly based on carcasses of large cetaceans, mainly baleen whales," Olivier Lambert, a vertebrate paleontologist at the Royal Belgian Institute of Natural Sciences in Brussels, told CNN in an email. "Here, the authors show that beaked whale carcasses may play a similar role in some specific deep ocean regions."

It's not at all surprising to learn that a graveyard of whale bones has been accumulating for millions of years, Lambert added. This oceanic region is known to be home to numerous species of beaked whales, and fossilized skulls of these whales have been discovered nearby, trapped by trawlers or long-line fishing boats in deep-water regions off the Iberian Peninsula, South Africa and the Kerguelen Islands.

However, "it's still really spectacular," said Lambert, who was not involved in the discovery.

Study was conducted by the Institute of Deepwater Science and Engineering of the Chinese Academy of Sciences - Reproduction/CNS
'It was a complete surprise'
Paleontologists found the cemetery in the Diamantina Fracture Zone, an area of ridges and marine trenches southwest of Australia. It formed between 30 and 40 million years ago, during the separation of the Australian and Antarctic continents, and reaches depths of around 5,000 to 7,000 meters below sea level.

"Although this is a truly gigantic whale graveyard, getting to it is extremely difficult due to the great depth," said Peng Zhou, co-lead author of the study and researcher at the Institute of Deepwater Science and Engineering (IDSSE) of the Chinese Academy of Sciences in Beijing. The expedition was part of the Global Hadal Trench Exploration Program, an international collaboration to explore some of the lesser-known areas in the deepest parts of Earth's oceans, Zhou told CNN in an email.

"That said, when we first observed this location, it was a complete surprise to everyone," Zhou added.

Zhou and his colleagues explored the zone from the Tan Suo Yi Hao research vehicle, using a submersible called Fendouzhe ("Striver" in Chinese), which had visited the bottom of the Mariana Trench - the deepest point on Earth - in 2020. In Zona Diamantina, they captured images of the necropolis and used the submersible's robotic arm to collect 43 fossils, as well as some of the scavenging animals, using a device called a suction sampler.

What surprised scientists most was the density of the fossils and the continuity of their distribution throughout the site, with some areas containing approximately 760 remains per square kilometer - "much more than anything previously documented" at similar sites, Zhou said.

"According to our estimates, there are more than 10 million whale remains on the ocean floor of this trench."

As impressive as that number may seem, even more bones could be buried there beneath seafloor sediments, he added.

The researchers carried out 32 dives between February and March 2023 and found 485 whale fossil deposits, as well as five modern whale carcasses. Scavengers were abundant in the more recent remains, with one whale carcass housing up to 2,840 organisms in a single square meter. Scientists identified one of the modern carcasses as that of a minke whale, or Balaenoptera acutorostrata, which measured about 3 meters in length. Remains of another modern species, Andrew's beaked whale - scientifically known as Mesoplodon bowdoini - were next to fossils of an extinct genus called Pterocetus. The oldest fossils, belonging to Pterocetus benguelae, were 5.3 million years old.

"Finding extinct genera like Pterocetus and living species like Mesoplodon bowdoini preserved together in the same region, along 1,200 kilometers of seafloor at such extreme depths - this was really unexpected," said Zhou.

Funnel for a cemetery
As for why so many beaked whale corpses ended up on this particular patch of seafloor, the answer may lie, in part, in the V-shaped topography of the Diamantina Zone, said Xikun Song, an IDSSE professor and co-lead author of the study.

"The area serves as a habitat or migratory corridor for cetaceans," Song told CNN in an email. Beaked whales dive to great depths, but once they exceed 3,000 meters (9,800 feet) they can reach their limit, "increasing the risk of fatal exhaustion or decompression sickness."

The zone then channels the carcasses to the sea floor, and the very little movement of sediment at these depths means the carcasses remain exposed to scavenging animals. Over time, deep-sea minerals such as ferromanganese oxide form crusts on bones and preserve them as fossils, Song said.

Another unexpected discovery at the site was a partial skull belonging to a previously unknown species, which scientists named Pterocetus diamantinae. Although the age of the fossil is not yet known, "its discovery helps document the evolutionary history of the group and shows that highly specialized beaked whales had already evolved by the time these fossils were deposited," said study co-lead author Giovanni Bianucci, associate professor in the department of Earth Sciences at the University of Pisa.

"It was certainly a fortuitous discovery," Bianucci told CNN in an email, "but it is likely that many other beaked whale fossils remain on the seabed of the Diamantina necropolis."

Source: CNN