Over 400 fossil sites discovered in the Diamantina Trench: research redefines the role of whale falls in the carbon cycle.

When a cetacean it dies, its body sinks to regenerate in the depths, giving rise to complex ecosystems that thrive for centuries in the darkness. The phenomenon of whale fall It has been known for a long time, but until now the evidence of these fossil oases had remained fragmentary.

A new international study has just documented the largest whale necropolis ever discovered, revealing an unprecedented biological and temporal breadth and redefining the role of the seafloor in the carbon cycle and life on our planet.

The largest and deepest whale burial site ever discovered

When a whale dies, its enormous body is dragged to the ocean floor, often hundreds of meters deep. Within a short time, the carcass attracts microorganisms, fish, and other deep-sea animals. Thus, the cetacean's body ends up creating a complex ecosystem which provides sustenance to deep-sea organisms for decades, sometimes centuries.

Among the organisms observed at the sites of whale fall in deep waters there are chordates, arthropods, cnidarians, echinoderms, molluscs, nematodes and annelids, but also new species, including some potentially specialized for life among whale carcasses. Furthermore, the bodies of cetaceans lying on the seabed play a very important role in long-term carbon sequestration and can help scientists understand theevolution of life and ecosystems in the deep, still partly unknown.

We have known for a long time about the existence of the phenomenon of whale fall, and we've begun studying the peculiar ecosystems that feed on the deaths of large cetaceans. The fossil record of these oases of abyssal biodiversity, however, remains sparse and fragmented. Or at least, it was until a few days ago. A study led by researchers from the Chinese Academy of Sciences, in collaboration with the University of Pisa and GNS Science in Wellington, just published in Nature, documents the largest and deepest aggregation of whale fossils and the largest active whale carcass ecosystems known to dateA veritable whale necropolis on the ocean floor of the Indian Ocean, just over a thousand kilometers from the western coast of Australia.

Whale carcasses: a carbon sink yet to be quantified

The abyssal site described in the study is located in the Diamantina Pit, between 4.616 and 7.001 meters deep, and extends for approximately 1.200 km. The whale necropolis, which contains evidence of cetacean carcasses dating back at least 5,3 millions of years ago, that is, the Early Pliocene, was discovered during a 2023 expedition aboard the research vessel Tansuoyihao. The team led by the Institute of Deep-Sea Science and Engineering (IDSSE) of the Chinese Academy of Sciences conducted 32 dives with the Fendouzhe underwater vehicle on a stretch of sea of ​​over a thousand kilometers, discovering five active whale carcasses and 476 fossil sites.

Researchers have documented that the density of whale remains reaches 759,5 individuals per square kilometer. Based on these data, it can be assumed that the Diamantina Trench contains over 10 million whale carcasses. The presence of so many bodies is due to several factors, they explain: the area is a foraging habitat for beaked whales (ziphiidae), which can die during deep dives. The V-shaped topography of the area also channels carcasses to the bottom of the trench, and low regional sedimentation rates contribute to the preservation of the bones.

These carcasses, the researchers point out, represent a gigantic carbon reservoir, yet to be quantified. Assuming a mass of two tons for each beaked whale specimen and a lipid content of 25%, we obtain approximately 6,7 million tons of carbon sequesteredThis is a carbon input equivalent to about 4.700 years of “marine snow,” the slow process that carries small organic debris (plankton, dead organisms, etc.) from the surface layers of the ocean to the deeper ones.

The huge abyssal ecosystem of the Whale Necropolis

Among the still active whale carcass sites, one in particular, consisting of three whale vertebrae from the beak to the depth of 6.789 meters, represents the deepest whale carcass ecosystem ever recorded. Another find, a five-meter-long carcass found at 5.610 meters deep, has been identified as belonging to an Antarctic minke whale (Balaenoptera bonaerensis). The researchers found that these skeletal remains hosted microbial mats and associated faunal communities, including bone-boring worms (Osedax) to chemosymbiotic bivalves.

Furthermore, they explain, they detected three species of brittle stars, or brittle stars, found exclusively on whale bones and first reported to be present Xyloplax sp., a rare wood-associated organism, on a whale carcass. Until now, this genus was only known from wooden carcasses and near hydrothermal vents: this could mean, the study says, that whale bodies contribute to the dispersal of chemosynthetic communities deep-sea.

This discovery allows us to better understand the biodiversity that characterizes extreme environments, which are still largely unexplored. These are 'laboratories of evolution': life forms particularly adapted to these extreme environments evolve there, unique to these environments.

ha told ANSA John Bianucci, full professor at the University of Pisa and co-author of the study led by Xiaotong Peng.

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