Taphonomy, the study of decomposition and preservation, reveals a complex fate for skeletal remains entering the ocean. The outcome is a dynamic interplay between biological consumption, chemical dissolution, and physical forces, dictated by depth, temperature, and location. Understanding what happens to bone requires examining a distinct series of events, starting with the initial descent and culminating in either complete consumption or burial in the seabed.
The Initial Descent and Soft Tissue Removal
The immediate fate of remains entering the water is determined by density and buoyancy. A body typically sinks initially because bone and muscle tissue are denser than water. Within a few days, bacterial activity generates gases like methane and hydrogen sulfide, causing the body to bloat and become positively buoyant. This gas buildup can cause the remains to refloat to the surface, exposing them to wave action and surface scavengers.
If the remains sink to the seafloor, or if the water is too cold for gas production, the immediate threat comes from generalist scavengers. Sharks, large fish, crabs, and crustaceans rapidly consume the soft tissues. This process leads to complete disarticulation and skeletonization in a matter of days or weeks. The swift removal of flesh exposes the bone’s surface to specialized organisms and environmental forces.
Biological Attack: Scavengers and Specialized Eaters
Once soft tissue is removed, the exposed bone becomes a nutrient source for organisms specializing in consuming the skeletal matrix itself. The most famous are the Osedax polychaetes, known as bone worms or zombie worms, which colonize bones in the deep ocean. These worms lack a mouth and a gut, instead using a specialized root-like system that burrows directly into the bone structure.
The root system secretes an acid to dissolve the inorganic mineral component of the bone, primarily calcium phosphate. This action allows the worms to access the rich organic components trapped inside the bone matrix, specifically lipids and collagen. The Osedax host symbiotic bacteria within their bodies, which metabolize these extracted nutrients, slowly hollowing out the bone from the inside.
Microbial activity is another biological agent, degrading the bone from the inside out through bioerosion. Bacteria and fungi penetrate microscopic channels within the bone structure, such as the Haversian canals, leaving behind characteristic tunnels. These microorganisms break down the organic collagen fibers, which provide the bone with its flexibility. The removal of this collagen weakens the bone and compromises the structural protection of the mineral crystals.
Environmental Factors Affecting Bone Structure
Beyond biological consumption, the bone structure is subject to degradation from non-living, or abiotic, environmental factors. One factor is hydrostatic pressure, which increases significantly with depth, though solid bone tissue is largely resistant to physical crushing. Pressure primarily affects decomposition rates by keeping temperatures low, which slows the bacterial activity that drives gas production and soft tissue decay.
A more destructive abiotic force is chemical dissolution, which targets the bone’s mineral component. Bone is composed primarily of hydroxyapatite, which is susceptible to being dissolved by acidic conditions. In deep-sea environments, the water can be slightly more acidic due to carbon dioxide concentration, slowly corroding the bone surface. Even in shallower waters, local microbial communities can create microenvironments of low pH when sulfur-cycling bacteria produce acids that accelerate demineralization.
The physical action of the water column also contributes to the bone’s breakdown, especially in high-energy, shallow-water environments. Currents and waves transport bones across the seabed, causing them to tumble and collide with abrasive sediments, rocks, and debris. This constant abrasion leads to the rounding of bone edges and surfaces. This process eventually exposes the porous inner cancellous bone, which is then more easily attacked by other agents.
Long-Term Preservation and Burial
For a bone to survive the long-term destructive forces of the marine environment, the most important factor is rapid burial beneath the sediment. If a bone is quickly covered by sand, silt, or clay, it is shielded from the physical abrasion of currents and the biological attack of scavengers and specialized borers like Osedax. Burial also isolates the bone from the open water chemistry that drives dissolution, significantly slowing the decay process.
If a bone is not buried, it is often subject to transport by deep-sea currents or underwater landslides. This movement can scatter a complete skeleton over a wide area.
Over vast spans of time, a buried bone may enter the process of fossilization. During this process, the original organic and inorganic components are slowly replaced by stable minerals from the surrounding sediment. This mineral replacement, known as diagenesis, is the final stage that can lead to the preservation of the skeletal form for millions of years.