Calcareous ooze is a common deep-sea sediment, coating vast areas of the ocean floor. It consists primarily of calcium carbonate, forming a soft, mud-like deposit. This widespread sediment offers insights into Earth’s past climate and ocean conditions.
Defining Calcareous Ooze
Calcareous ooze is a sediment primarily composed of calcium carbonate (CaCO3). This material originates from the skeletal remains of microscopic marine organisms. It is classified as a biogenous sediment, meaning it derives from biological activity in the ocean.
For a sediment to be termed an “ooze,” at least 30% of its dry weight must be composed of these biogenous materials. It is typically fine-grained and muddy, often white or pale in color. The calcium carbonate within the ooze is commonly in the form of calcite, though sometimes it can be aragonite.
Formation and Sources
Calcareous ooze forms from the accumulation of calcium carbonate shells and skeletons of tiny marine organisms. These organisms primarily include foraminifera (single-celled protozoa) and coccolithophores (microscopic phytoplankton). Pteropods, a type of marine snail, also contribute.
These organisms thrive in the sunlit upper layers of the ocean, known as the euphotic zone. Upon their death, their calcium carbonate shells, also called tests, sink to the seafloor. The continuous accumulation of these remains over vast geological timescales forms thick layers of calcareous ooze.
Distribution in the Ocean
Calcareous ooze is widely distributed across the ocean floor, particularly in shallower ocean basins and on mid-ocean ridges. It is generally found at depths less than about 4,000 to 5,000 meters. This depth restriction is due to the varying solubility of calcium carbonate in seawater, which changes with depth.
This distribution is governed by the Calcium Carbonate Compensation Depth (CCD). The CCD represents the depth where calcium carbonate dissolution equals its rate of accumulation. Below the CCD, calcium carbonate dissolves faster than it can accumulate, preventing ooze formation.
The lysocline is another important boundary, marking the depth where calcium carbonate begins to dissolve significantly, and the rate of dissolution increases dramatically. This occurs because seawater becomes more corrosive to calcium carbonate with increasing pressure, lower temperatures, and higher concentrations of dissolved carbon dioxide at greater depths. Consequently, below the CCD, sediments are typically devoid of calcium carbonate, consisting instead of red clays.
Significance and Role
Calcareous ooze holds importance in various scientific disciplines, offering a valuable record of Earth’s history. Its fossilized shells provide data for paleoceanography, allowing scientists to reconstruct past ocean conditions, temperatures, and marine life. Analyzing isotopic signatures within these shells helps understand ancient environmental changes.
This sediment also contributes to the global carbon cycle. The formation and accumulation of calcareous ooze sequester carbon dioxide from the atmosphere, acting as a natural long-term carbon sink in the deep sea. This process helps regulate ocean pH by removing CO2, which otherwise forms carbonic acid.
Geologically, calcareous ooze serves as a precursor to rock formations. Over immense periods, the compaction and cementation of ooze transform it into sedimentary rocks like limestone and chalk. The White Cliffs of Dover, for example, are a well-known formation composed primarily of ancient coccolithophore ooze.