The ocean floor is blanketed in the skeletal remains of microscopic, glass-like organisms

Nature · Published 3 October 2026 Swipe more like this →

In short: Deep beneath the waves, vast stretches of the seafloor are covered in siliceous ooze. Composed of the opal-like skeletons of tiny creatures like diatoms and radiolarians, these sediments act as a permanent record of oceanic productivity, revealing where life flourishes and where the ocean's chemistry dictates decay.

Siliceous ooze is a pelagic sediment defined by containing at least 30 percent skeletal material from microscopic organisms. Unlike carbonate oozes, which are restricted by the Calcite Compensation Depth—a threshold around 4,500 metres where calcium carbonate dissolves—siliceous oozes are composed of amorphous, hydrated silica known as opal. Because silica dissolves rapidly in seawater, the survival of these skeletons depends on high rates of primary productivity in surface waters. Only about four percent of the opal silica produced at the surface actually reaches the seafloor; the rest is recycled within the water column.

The distribution of these deposits is a direct map of ocean fertility. They are most prevalent in regions of intense upwelling, such as the Southern Ocean surrounding Antarctica, which accounts for 75 percent of the world's silica deposition, and near the Equator in the Pacific. In these zones, nutrient-rich waters fuel the growth of organisms that use biomineralization to construct their shells. To survive the journey to the abyss, these particles are often repackaged into larger fecal pellets, which protect them from immediate dissolution and accelerate their descent, a process that can take decades.

Once deposited, the fate of siliceous ooze is determined by the balance between accumulation and dissolution. If burial occurs faster than dissolution, the sediment can eventually undergo diagenesis to form chert, a process that may span tens of millions of years and effectively removes silica from the global cycle. Because only the most robust skeletons survive the harsh chemical environment of the deep ocean, these deposits provide an incomplete, yet vital, snapshot of the biological history of the surface waters above.

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