A Whale Necropolis in the Depths of the Indian Ocean Preserves a 5.3-Million-Year Record of Whale Falls

In the deep sea floor of the southeastern Indian Ocean, the Diamantina Zone lies off southwestern Australia, extending for about 1,200 km along the seabed at depths ranging from 4,616 to 7,001 m. Its deepest area is close to the Dordrecht Deep in the northwestern section. In 2023, after a research team explored this region aboard the human-occupied vehicle Fendouzhe, they discovered that this stretch of sea floor preserves a remarkable deep-sea whale necropolis.

Location of the Diamantina Zone（Image source：Peng X et al. (2026)， CC BY-NC-ND 4.0 ）

Across 32 deep-sea dives, the team recorded five whale falls with active biological communities along the Diamantina Zone, together with 476 fossil cetacean records. These whale falls are the remains of whale carcasses that sank into the deep ocean. Some of the remains are still undergoing decomposition and continue to serve as food sources for deep-sea organisms; others have already undergone long-term mineralization, are lightly coated with black Fe–Mn oxides, and lie partially buried in soft sediments. These remains are not isolated accidents. They are traces repeatedly accumulated within the same deep-sea terrain over millions of years. For this reason, the researchers regarded the area as a whale necropolis.

(a) A 3-m-long minke whale fall recorded at a depth of 5,610 m; (b) close-up view of the minke whale cranium, showing brittle stars and polynoids attached to the bone surface; (c) close-up view of the minke whale bone, showing numerous transparent tubes of the bone-eating worm Osedax emerging from the whale bone; (d) a beaked-whale fall at a depth of 5,609 m, consisting of eight ribs and several thoracic vertebrae, with brittle stars and chemosymbiotic bivalves associated with the bones; (e) close-up view of the beaked-whale fall, showing bivalves and brittle stars living together near the bones; (f) a whale bone recorded at a depth of 4,625.2 m, dominated by gastropods of the genus Phymorhynchus, polychaetes and Osedax; (g) the deepest whale fall recorded, at a depth of 6,788.7 m, consisting of three elongated vertebrae of a beaked whale, with small gastropods on the bone surface. The two red laser points are spaced 10 cm apart（Image source：Peng X et al. (2026)， CC BY-NC-ND 4.0 ）

Organic matter is scarce on the abyssal sea floor, which usually depends on the slow rain of debris from the upper ocean. A whale carcass, however, can deliver a large amount of fat, protein, and bone-bound organic material within a short period of time, drawing in different groups of deep-sea organisms in succession. All five active whale falls discovered in the Diamantina Zone are in the sulfophilic stage. Their bones are covered with dense, whitish microbial mats, and animals of the bone-eating worm genus Osedax, belonging to Polychaeta, bore into the whale bones. In Osedax, it is the females that penetrate the bones and feed on the organic matter inside, whereas the tiny dwarf males live inside the cavity of the female's gelatinous tube. The females extend ramifying roots into the bone and, with the help of acidification, enzymatic activity, and symbiotic bacteria, release and absorb nutrients such as lipids and collagen from the bone matrix.

Body structure of a female Osedax（Image source：Salathé RM and Vrijenhoek RC. (2012)， CC BY 2.0 ）

Rich benthic communities occur around these whale falls. From in situ imagery and collected specimens, the researchers recognized 35 macrofaunal taxa, including annelids, crustaceans, molluscs, cnidarians, and nematodes. Among the larger and more visible organisms, the communities are dominated by bone-eating worms, gastropods, chemosymbiotic bivalves, and brittle stars, with local densities reaching up to 2,840 individuals per square metre. These bivalves host sulfur-oxidizing microbial symbionts. When whale-bone decomposition allows reduced compounds such as sulfides to accumulate in the surrounding environment, these symbionts use chemical energy to fix inorganic carbon and synthesize organic matter, thereby nourishing their bivalve hosts.

Most previously documented whale falls have been found from depths of several tens of metres down to around 4,000 m. By contrast, the active whale falls in the Diamantina Zone occur at depths exceeding 6,700 m. The deepest one consists of three elongated vertebrae of a beaked whale and lies on the sea floor at a depth of 6,788.7 m.

Beyond modern whale falls, the researchers collected and analysed 43 fossils from the region, identifying five species of beaked whales and one baleen-whale species. Most of the beaked-whale fossils consist of rostra, because these bones are exceptionally dense and mineral-rich, making them more likely than other skeletal elements to survive in the deep-sea environment. The researchers confirmed the presence of two extant species: Andrews' beaked whale, Mesoplodon bowdoini, and the strap-toothed whale, Mesoplodon layardii. Both still inhabit the southeastern Indian Ocean today. The fossils also include extinct beaked whales, such as the genera Pterocetus and Izikoziphius. One fossil represents a new species, named Pterocetus diamantinae. Baleen-whale fossils include the tympanic bulla of a sei whale, Balaenoptera borealis, as well as several cranial and postcranial fragments of baleen whales that are more difficult to identify precisely.

Fossil crania of Ziphiidae. (a, b) Andrews' beaked whale, Mesoplodon bowdoini; (c, d) strap-toothed whale, Mesoplodon layardii; (e–g) Pterocetus diamantinae; (h) Pterocetus benguelae; (i) Izikoziphius rossi（Image source：Peng X et al. (2026)， CC BY-NC-ND 4.0 ）

Mesoplodon bowdoini（Image source：Jörg Mazur， CC BY-SA 4.0 ）

Mesoplodon layardii（Image source：Jörg Mazur， CC BY-SA 4.0 ）

Balaenoptera borealis（Image source：NOAA，CC0 1.0 ）

To determine the ages of these fossils, the researchers analysed strontium isotope ratios in 33 bone specimens. Some samples showed isotopic signatures identical to modern seawater, indicating complete geochemical exchange after death. The remaining samples yielded ages between about 0.12 Ma and 5.26 Ma. The oldest sample belongs to Pterocetus benguelae, with an average age of about 5.26 Ma; Izikoziphius rossi is about 2.44 Ma old. This indicates that whale-fall events in this region have occurred since at least the Early Pliocene and have continued to the present day.

Why have so many cetacean remains accumulated here? The answer lies in the combined effects of whale behaviour, seafloor topography, and preservation conditions. The Antarctic minke whale, Balaenoptera bonaerensis, and the sei whale are migratory baleen whales that enter the southeastern Indian Ocean. They mainly feed in the upper ocean and do not need to dive thousands of metres below the surface. Their remains in the Diamantina Zone are therefore best explained by carcasses sinking to the deep sea floor beneath a migratory corridor. Beaked whales are different. They are highly specialized deep-diving predators that feed on deep-water squid and fish. Their diving ability allows them to exploit ecological resources unavailable to many other whales, but it also carries substantial risk. They routinely dive to depths greater than 1,000 m and can hold their breath for more than an hour. On the basis of lung collapse and oxygen storage, their maximum dive depth may exceed 3,000 m. Yet foraging beyond this range would be extremely demanding and may increase the risk of exhaustion or decompression-related problems. When beaked whales die naturally or suffer fatal accidents during deep dives, it is not surprising that their carcasses sink to this part of the sea floor.

Balaenoptera bonaerensis（Image source：Lycaon.cl， CC BY-SA 3.0 ）

Sedimentation rates near the Diamantina Zone are extremely low, meaning that bones are not quickly covered by large volumes of mud and do not rapidly disappear from the sea-floor surface. Some bones lying on slopes or exposed seabed may remain in contact with seawater for extended periods. As Fe–Mn oxides gradually accumulate on and within the bones, the skeletal remains become more robust and more effectively isolated from the surrounding environment. If some bones become buried in sediment, carbonates produced during organic degradation may also aid preservation, allowing whale bones to remain on the sea floor for millions of years. These remains support modern deep-sea ecosystems while also recording the evolutionary history of ancient cetaceans, forming a time archive written in whale bone.

Author: Shui-Ye You

References:

1. Peng X et al. (2026). A 5.3-million-year-old deep-sea whale necropolis in the Diamantina Zone. Nature.

2. Salathé RM and Vrijenhoek RC. (2012). Temporal variation and lack of host specificity among bacterial endosymbionts of Osedax bone worms (Polychaeta: Siboglinidae). BMC Evolutionary Biology.