Calcareous ooze is a fine-grained, deep-sea sediment that blankets vast areas of the ocean floor. It is defined as a pelagic deposit containing more than 30% calcium carbonate (\(text{CaCO}_3\)), derived almost entirely from biological sources. This soft, muddy material forms a continuous record of marine life and ocean chemistry over millions of years.
Composition and Biological Origin
The chemical basis for calcareous ooze is calcium carbonate (\(text{CaCO}_3\)), which is the same mineral found in seashells and limestone. This material is not precipitated inorganically from seawater but rather originates from the hard, protective shells of countless microscopic marine organisms. These organisms live in the sunlit surface waters of the ocean, and their remains drift slowly downward after they die.
Two groups of plankton are the primary contributors to this sediment layer. The first is the foraminifera, single-celled protozoans (zooplankton) that construct intricate, chambered shells known as tests. The second major group is the coccolithophores, tiny phytoplankton that cover themselves in microscopic, hubcap-shaped plates called coccoliths.
After the death of these planktonic organisms, their minute \(text{CaCO}_3\) shells begin their descent through the water column, a process often referred to as “marine snow.” The volume of these descending skeletal remains, particularly the coccoliths, creates the thick, widespread deposits of ooze on the seafloor. These biogenic particles must survive the journey through thousands of meters of water to accumulate as sediment.
The Carbonate Compensation Depth
The accumulation of calcareous ooze is limited by a specific oceanographic boundary known as the Carbonate Compensation Depth (CCD). This depth represents the level where the rate of calcium carbonate dissolution perfectly balances the rate of supply from the surface. Below the CCD, the dissolution rate exceeds the supply rate, meaning that no \(text{CaCO}_3\) can accumulate on the seafloor.
The increased solubility of calcium carbonate at greater depths is driven by physical and chemical factors. As depth increases, pressure rises and temperature drops, both increasing the mineral’s tendency to dissolve. The most significant factor is the increasing concentration of dissolved carbon dioxide (\(text{CO}_2\)) in the deep ocean water.
Deep waters naturally accumulate \(text{CO}_2\) from the decomposition of organic matter that sinks from the surface. When \(text{CO}_2\) dissolves in water, it forms carbonic acid, which makes the seawater more acidic and less saturated with carbonate ions. This chemical environment accelerates the dissolution of the sinking foraminifera tests and coccoliths before they can be preserved as sediment on the ocean floor.
Global Distribution and Accumulation
The Carbonate Compensation Depth dictates where calcareous ooze can accumulate across the global ocean floor. Because the CCD typically sits at depths between 4,000 and 5,000 meters, the ooze is restricted to the shallower features of the deep sea. These areas include the flanks of mid-ocean ridges, continental slopes, and oceanic plateaus that remain elevated above the corrosive deep water.
Calcareous ooze covers approximately 48% of the world’s deep seafloor, making it the most common pelagic sediment by area. Conversely, the deep abyssal plains, which often lie well below the CCD, are virtually devoid of \(text{CaCO}_3\) sediments. Here, the calcareous material dissolves entirely, leaving behind a residue composed primarily of fine terrestrial clays.
The depth of the CCD is not uniform, varying significantly between ocean basins. For example, the CCD is generally deeper in the Atlantic Ocean than in the Pacific Ocean. This difference is largely due to the Pacific’s older and more \(text{CO}_2\)-rich deep water, which makes it more corrosive to calcium carbonate and thus forces the compensation depth to a shallower level.
Ooze’s Geological Legacy
Over geological timescales, the persistent accumulation of calcareous ooze transforms into solid rock through a process called diagenesis. As the overlying sediment layers increase the pressure, the ooze undergoes compaction and cementation, reducing its porosity and solidifying the material. This process initially converts the soft ooze into a fine-grained sedimentary rock known as chalk, famously exemplified by the White Cliffs of Dover.
With continued burial, chalk transforms into hard, dense limestone, a rock type that constitutes a significant portion of Earth’s crustal history. The fossilized remains within the ooze, particularly the shells of foraminifera and coccolithophores, provide an invaluable archive. Researchers use the chemical and isotopic composition of these microfossils to reconstruct past ocean temperatures and the ancient concentration of atmospheric \(text{CO}_2\).
These sedimentary records are a primary tool for paleoclimatology, allowing scientists to look back millions of years to understand how Earth’s climate system naturally fluctuates. Analyzing the layers of ooze provides a continuous and highly detailed record of environmental conditions, offering context for current climate changes. The transformation of this deep-sea mud into massive rock formations highlights the long-term geological significance of these microscopic organisms.