Carbonate Compensation Depth
Encyclopedia
Calcite compensation depth (CCD) is the depth in the oceans below which the rate of supply of calcite
(calcium carbonate
) lags behind the rate of solvation
, such that no calcite is preserved. Aragonite compensation depth (hence ACD) describes the same behaviour in reference to aragonitic
carbonates.
Calcium carbonate is essentially insoluble in sea surface waters today. Shells of dead calcareous plankton
sinking to deeper waters are practically unaltered until reaching the lysocline
where the solubility
increases dramatically. By the time the CCD is reached all calcium carbonate
has dissolved according to this equation:
Calcareous plankton and sediment
particles can be found in the water column above the CCD. If the sea bed is above the CCD, bottom sediment
s can consist of calcareous sediments called calcareous ooze
which is essentially a type of limestone
or chalk
. If the exposed sea bed is below the CCD tiny shells of CaCO3 will dissolve before reaching this level, preventing the deposition of carbonate sediment. An exception to this principle exists when carbonate sediment is deposited above the CCD, for example along a mid-ocean ridge
, and a layer of siliceous ooze
or abyssal
clay is deposited on top of the carbonate layer. As the sea floor spreads thermal contraction of the plate may bring the carbonate layer below the CCD; the carbonate layer may be prevented from chemically interacting with the sea water by the overlying sediments.
, pressure
and the chemical composition of the water - in particular the amount of dissolved CO2
in the water. Calcium carbonate is more soluble at lower temperatures and at higher pressures. It is also more soluble if the concentration of dissolved CO2 is higher. Adding a reactant to the above chemical equation pushes the equilibrium towards the right producing more products: Ca2+
and HCO3-
, and consuming more reactants CO2
and calcium carbonate
according to Le Chatelier's principle
.
At the present time the CCD in the Pacific Ocean
is about 4200 - 4500 metres except beneath the equatorial upwelling
zone, where the CCD is about 5000 m. In the temperate
and tropical Atlantic Ocean
the CCD is at approximately 5000 m. In the Indian Ocean
it is intermediate between the Atlantic and the Pacific. The variation in the depth of the CCD largely results from the length of time since the bottom water has been exposed to the surface; this is called the "age" of the water mass
. Consequently the CCD is, on average, deeper in the Atlantic Ocean, shallower in the Pacific Ocean and of intermediate depth in the Indian Ocean. Relative ages of these basins can be inferred from the pattern of ocean currents which form a global conveyor belt. Because organic material, such as fecal pellets from copepods, sink from the surface waters into deeper water, deep water masses tend to accumulate dissolved carbon dioxide as they age. The oldest water masses have the highest concentrations of carbon dioxide and therefore the shallowest CCD. The CCD is relatively shallow in high latitude
s with the exception of the North Atlantic and regions of Southern Ocean
where downwelling
occurs. This downwelling brings young, surface water with relatively low concentrations of carbon dioxide into the deep ocean, depressing the CCD.
In the geological past the depth of the CCD has shown significant variation. In the Cretaceous
through to the Eocene
the CCD was much shallower globally than it is today; due to intense volcanic activity during this period atmospheric carbon dioxide concentrations were much higher. Higher concentrations of carbon dioxide resulted in a higher partial pressure
of carbon dioxide over the ocean. This greater pressure of atmospheric carbon dioxide leads to increased dissolved carbon dioxide in the ocean mixed surface layer. This effect was somewhat moderated by the deep oceans' elevated temperatures during this period. In the late Eocene the transition from a greenhouse to an icehouse Earth
coincided with a deepened CCD.
Today, increasing atmospheric concentration of CO2 from combustion of fossil fuels may elevate the CCD to some degree, with zones of downwelling
first being affected.
John Murray
investigated and experimented on the dissolution of calcium carbonate and was first to identify the carbonate compensation depth in oceans.
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
(calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
) lags behind the rate of solvation
Solvation
Solvation, also sometimes called dissolution, is the process of attraction and association of molecules of a solvent with molecules or ions of a solute...
, such that no calcite is preserved. Aragonite compensation depth (hence ACD) describes the same behaviour in reference to aragonitic
Aragonite
Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...
carbonates.
Calcium carbonate is essentially insoluble in sea surface waters today. Shells of dead calcareous plankton
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...
sinking to deeper waters are practically unaltered until reaching the lysocline
Lysocline
The lysocline is a term used in geology, geochemistry and marine biology to denote the depth in the ocean below which the rate of dissolution of calcite increases dramatically....
where the solubility
Solubility
Solubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid, liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent. The solubility of a substance fundamentally depends on the used solvent as well as on...
increases dramatically. By the time the CCD is reached all calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
has dissolved according to this equation:
Calcareous plankton and sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
particles can be found in the water column above the CCD. If the sea bed is above the CCD, bottom sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
s can consist of calcareous sediments called calcareous ooze
Pelagic sediments
Pelagic sediment or pelagite is a fine-grained sediment that has accumulated by the settling of particles through the water column to the ocean floor beneath the open ocean far from land. These particles consist primarily of either the microscopic, calcareous or siliceous shells of phytoplankton or...
which is essentially a type of limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
or chalk
Chalk
Chalk is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. Calcite is calcium carbonate or CaCO3. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite plates shed from micro-organisms called coccolithophores....
. If the exposed sea bed is below the CCD tiny shells of CaCO3 will dissolve before reaching this level, preventing the deposition of carbonate sediment. An exception to this principle exists when carbonate sediment is deposited above the CCD, for example along a mid-ocean ridge
Mid-ocean ridge
A mid-ocean ridge is a general term for an underwater mountain system that consists of various mountain ranges , typically having a valley known as a rift running along its spine, formed by plate tectonics. This type of oceanic ridge is characteristic of what is known as an oceanic spreading...
, and a layer of siliceous ooze
Siliceous ooze
Siliceous ooze is a soft siliceous pelagic sediment that covers large areas of the deep ocean floor. Siliceous oozes consist predominately of either diatoms or radiolarians. Sometimes, it contains lessor proportions of sponge spicules or silicoflagellates. Diatoms are golden-brown algae that...
or abyssal
Abyssal zone
The abyssal zone is the abyssopelagic layer or pelagic zone that contains the very deep benthic communities near the bottom of oceans. "Abyss" derives from the Greek word ἄβυσσος, meaning bottomless. At depths of 4,000 to 6,000 metres , this zone remains in perpetual darkness and never receives...
clay is deposited on top of the carbonate layer. As the sea floor spreads thermal contraction of the plate may bring the carbonate layer below the CCD; the carbonate layer may be prevented from chemically interacting with the sea water by the overlying sediments.
Variations in value of the CCD
The exact value of the CCD depends on the solubility of calcium carbonate which is determined by temperatureTemperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
, pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
and the chemical composition of the water - in particular the amount of dissolved CO2
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
in the water. Calcium carbonate is more soluble at lower temperatures and at higher pressures. It is also more soluble if the concentration of dissolved CO2 is higher. Adding a reactant to the above chemical equation pushes the equilibrium towards the right producing more products: Ca2+
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
and HCO3-
Bicarbonate
In inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...
, and consuming more reactants CO2
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
and calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
according to Le Chatelier's principle
Le Châtelier's principle
In chemistry, Le Chatelier's principle, also called the Chatelier's principle, can be used to predict the effect of a change in conditions on a chemical equilibrium. The principle is named after Henry Louis Le Chatelier and sometimes Karl Ferdinand Braun who discovered it independently...
.
At the present time the CCD in the Pacific Ocean
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
is about 4200 - 4500 metres except beneath the equatorial upwelling
Upwelling
Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water. The increased availability in upwelling regions results in high levels of primary...
zone, where the CCD is about 5000 m. In the temperate
Temperate
In geography, temperate or tepid latitudes of the globe lie between the tropics and the polar circles. The changes in these regions between summer and winter are generally relatively moderate, rather than extreme hot or cold...
and tropical Atlantic Ocean
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...
the CCD is at approximately 5000 m. In the Indian Ocean
Indian Ocean
The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...
it is intermediate between the Atlantic and the Pacific. The variation in the depth of the CCD largely results from the length of time since the bottom water has been exposed to the surface; this is called the "age" of the water mass
Water mass
An oceanographic water mass is an identifiable body of water with a common formation history which has physical properties distinct from surrounding water...
. Consequently the CCD is, on average, deeper in the Atlantic Ocean, shallower in the Pacific Ocean and of intermediate depth in the Indian Ocean. Relative ages of these basins can be inferred from the pattern of ocean currents which form a global conveyor belt. Because organic material, such as fecal pellets from copepods, sink from the surface waters into deeper water, deep water masses tend to accumulate dissolved carbon dioxide as they age. The oldest water masses have the highest concentrations of carbon dioxide and therefore the shallowest CCD. The CCD is relatively shallow in high latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...
s with the exception of the North Atlantic and regions of Southern Ocean
Southern Ocean
The Southern Ocean comprises the southernmost waters of the World Ocean, generally taken to be south of 60°S latitude and encircling Antarctica. It is usually regarded as the fourth-largest of the five principal oceanic divisions...
where downwelling
Downwelling
Downwelling is the process of accumulation and sinking of higher density material beneath lower density material, such as cold or saline water beneath warmer or fresher water or cold air beneath warm air. It is the sinking limb of a convection cell. Upwelling is the opposite process and together...
occurs. This downwelling brings young, surface water with relatively low concentrations of carbon dioxide into the deep ocean, depressing the CCD.
In the geological past the depth of the CCD has shown significant variation. In the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
through to the Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
the CCD was much shallower globally than it is today; due to intense volcanic activity during this period atmospheric carbon dioxide concentrations were much higher. Higher concentrations of carbon dioxide resulted in a higher partial pressure
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
of carbon dioxide over the ocean. This greater pressure of atmospheric carbon dioxide leads to increased dissolved carbon dioxide in the ocean mixed surface layer. This effect was somewhat moderated by the deep oceans' elevated temperatures during this period. In the late Eocene the transition from a greenhouse to an icehouse Earth
Greenhouse and Icehouse Earth
For the past millions of years, the earth has been fluctuating between two different states of dominant climate: Greenhouse and Icehouse. These two climate sets are complete opposites from each other and is on a continuing, uneven cycle between the two...
coincided with a deepened CCD.
Today, increasing atmospheric concentration of CO2 from combustion of fossil fuels may elevate the CCD to some degree, with zones of downwelling
Downwelling
Downwelling is the process of accumulation and sinking of higher density material beneath lower density material, such as cold or saline water beneath warmer or fresher water or cold air beneath warm air. It is the sinking limb of a convection cell. Upwelling is the opposite process and together...
first being affected.
John Murray
John Murray (oceanographer)
Sir John Murray KCB FRS FRSE FRSGS was a pioneering Scottish oceanographer, marine biologist and limnologist.-Early life:...
investigated and experimented on the dissolution of calcium carbonate and was first to identify the carbonate compensation depth in oceans.