Deep-sea sediments are classified as “ooze” when they contain at least 30% skeletal remains from microscopic organisms. This soft, fine-grained material blankets vast stretches of the deep seafloor, far from continental sediments. Siliceous ooze is a specific pelagic deposit, distinguished by being overwhelmingly composed of silica skeletons. This biogenic sediment is a testament to the immense productivity of the upper ocean layers that slowly rains down to the abyssal plains.
Composition and Source Organisms
Siliceous ooze is formed primarily from the shells, or tests, of two major planktonic groups: diatoms and radiolarians. Both build their intricate exoskeletons from opal, a hydrated form of silicon dioxide (silica). This material defines siliceous ooze, separating it from calcareous ooze, which is made from calcium carbonate.
Diatoms are single-celled algae classified as phytoplankton, meaning they are photosynthetic primary producers found in sunlit surface waters. They construct a two-part shell called a frustule, which fits together like a pillbox and is marked by elaborate patterns. These organisms are typically about 25 micrometers in diameter, and their rapid reproduction makes them a fundamental component of the marine food web.
Radiolarians are unicellular organisms classified as zooplankton; they are tiny animals that prey on other plankton. Their skeletons are known for their beautiful, highly symmetrical, often spherical or helmet-like structures. They are significantly larger than diatoms, generally measuring between 0.1 and 0.2 millimeters. When these organisms die, their silica shells slowly sink through the water column, forming biogenic debris that eventually accumulates on the ocean floor.
Distribution and Preservation
The accumulation of siliceous ooze is not uniform but is concentrated where the production of diatoms and radiolarians is high. This high biological productivity is typically found in regions of strong oceanographic upwelling, where nutrient-rich deeper waters are brought to the surface. Consequently, the two main belts of siliceous ooze deposits are found in the Polar Regions and along the Equatorial Zones.
Diatomaceous ooze dominates high-latitude areas, such as the Southern Ocean, where cold temperatures and constant upwelling fuel massive diatom blooms. Radiolarian ooze, richer in the tests of larger radiolarians, tends to accumulate in the productive equatorial zones of the Pacific and Indian Oceans. The preservation of this silica on the seafloor is controlled by the chemical boundary known as the Silica Compensation Depth (SCD).
Unlike calcium carbonate, biogenic silica is more soluble in the shallower, warmer waters of the upper ocean, though it is subject to dissolution throughout the water column. The Silica Compensation Depth (SCD) is the depth where the rate of silica shell deposition exactly balances the rate of dissolution. Below the SCD, silica dissolution is so rapid that virtually no siliceous material accumulates. Significant siliceous ooze deposits only form where the surface production rate is high enough to overwhelm dissolution.
Geological Significance
Siliceous ooze holds value for Earth scientists because it represents a detailed archive of past ocean conditions and climate. The shells of diatoms and radiolarians are sensitive to changes in water temperature, nutrient availability, and ocean currents. The species composition locked within the ooze reflects the environment in which they lived. By analyzing deep-sea sediment cores, paleoceanographers can reconstruct ancient climate cycles, ocean productivity, and circulation patterns over millions of years.
Over geological timescales, the soft, silica-rich ooze undergoes diagenesis, transforming it into solid rock. As the sediment is buried deeper, increasing pressure and temperature cause the unstable, amorphous opal-A of the shells to dissolve and recrystallize. This transformation first produces a denser rock called porcellanite, and eventually leads to the formation of chert.
When the resulting rock is predominantly composed of radiolarian tests, it is termed radiolarite. These lithified deposits are hard and resistant to weathering. Their presence in continental rock sequences provides evidence of former deep-ocean basins. Ancient, fossilized diatomaceous ooze, known as diatomaceous earth, is mined for industrial uses, including filtration aid and mild abrasive.