Whale falls form when whale carcasses sink to the seafloor, creating localized concentrations of biodiversity in the deep ocean. Besides playing a role in long-term carbon sequestration, whale falls help scientists understand the evolution and dispersal of deep-sea life as well as provide an unprecedented fossil record of whales and deep-sea ecosystems.
Researchers from the Institute of Deep-sea Science and Engineering (IDSSE) of the Chinese Academy of Sciences, in collaboration with the University of Pisa, Italy, and Earth Sciences New Zealand in Wellington, have now documented the world's deepest and largest known aggregation of whale fossils and active whale-fall ecosystems. This deep-sea site, referred to as a "whale necropolis" due to its vast size, is located in the Diamantina Zone of the southeastern Indian Ocean and contains evidence of cetacean falls for at least 5.3 million years.
Previously, most recorded whale falls were found at depths less than 4,000 meters, with the deepest active site at 4,204 meters in the Southwest Atlantic. No active whale-fall ecosystems had been reported from hadal depths exceeding 6,000 meters. These findings extend the recorded depth range and scale of these ecosystems and add new data to the understanding of deep-ocean life.
The study was published in the journal Nature on June 10.
In 2023, the IDSSE-led team, based aboard R/V Tansuoyihao, conducted 32 dives of the human-occupied vehicle (HOV) Fendouzhe along a 1,200-kilometer stretch of the Diamantina Zone. They discovered five active whale falls and 476 whale fossil sites at depths ranging from 4,616 to 7,001 meters.
The discovery includes a large number of whale remains. Researchers documented a density of whale remains reaching up to 759.5 individuals per square kilometer. Extrapolating this data suggests that the Diamantina Zone may contain over 10 million whale carcasses. This represents a large, previously unquantified carbon sink. Assuming an average beaked whale mass of two tons and 25% lipid content, this equates to approximately 6.7 million tons of sequestered carbon—a carbon input equivalent to roughly 4,700 years of the regional slow dispersal of small organic debris (e.g., plankton, dead organisms, etc.) from the upper ocean to the deep ocean, a phenomenon known as "marine snow."
Among the active whale falls, one site—comprising three beaked whale vertebrae at a depth of 6,789 meters—represents the deepest active whale-fall ecosystem recorded to date. Another, a five-meter-long carcass found at 5,610 meters, was identified as an Antarctic minke whale (Balaenoptera bonaerensis). The researchers discovered that these skeletal remains supported microbial mats and associated faunal communities, including brittle stars, bone-boring worms (Osedax), and chemosymbiotic bivalves.
They noted three brittle-star species found exclusively on whale bones, indicating their adaptation to this substrate. Additionally, they reported the first record of the wood-associated sea daisy (Xyloplax sp.) on a whale fall, representing the deepest known occurrence of this genus, which was previously known from wood falls and hydrothermal vents. The presence of shared taxa with cold seeps and hydrothermal vents supports the hypothesis that whale falls may facilitate the dispersal and connectivity of deep-sea chemosynthetic communities.
The fossil record in the necropolis spans at least 5.3 million years, confirmed by strontium isotope dating. This indicates that whale-fall events have occurred in the Diamantina Zone since the Early Pliocene. Identified fossils include both extant beaked whale species (e.g., Andrews' and strap-toothed beaked whales) and extinct taxa, including a newly described species, Pterocetus diamantinae.
This concentration of whale remains is attributed to several factors. The zone functions as a foraging habitat for beaked whales, some of which may die during deep dives. The V-shaped topography of the zone funnels carcasses onto the trench floor. Low regional sedimentation rates allow whale bones to remain exposed for extended periods, contributing to their preservation.
This study extends the known depth range of whale-fall ecosystems to nearly 7,000 meters. It provides information on the evolutionary history, paleoecology, and population dynamics of ancient whales, and expands knowledge of the distributional limits and biogeography of chemosynthetic life. The Diamantina Zone may constitute a previously unrecognized "whale-fall chemosynthetic life corridor" traversing the southeastern Indian Ocean, offering information on deep-sea connectivity.
The research was supported by the National Key R&D Program of China, the National Natural Science Foundation of China, the International Mega-Science Program of the Chinese Academy of Sciences, and the Global Hadal Exploration Programme (GHEP).
GHEP is a ten-year UN Ocean Decade-endorsed scientific programme initiated by IDSSE, dedicated to exploring the deepest regions of the global ocean and addressing fundamental questions regarding deep-sea geology, life, and environmental evolution.
