Carbonate Compensation Depths

Carbonate compensation depths are the depths in ocean basins that separate calcium carbonate-containing seafloor sediments from carbonate-free sediments. The carbonate compensation depth (CCD) may be different in different ocean basins, and it may rise or fall at various times in the same ocean basin as a result of the balance between surface production of carbonate and deep-water carbonate dissolution. Carbonate solubility increases with increasing pressure and decreasing temperature.

Marine Sediments

Marine sediments are composed of a variety of materials of biological or mineral origin. Individual sediment grains in oceanic deposits may be either clastic or biogenic particles. Clastic sediments are materials derived from the weathering, erosion, and transportation of exposed continental rocks, and these grains are classified by particle diameter into gravels, sands, silts, and clays. Clastic sediment particles become less common with increasing distance from the continental landmasses and are nearly absent from deep-water sediments on the abyssal plains. Biogenic sediment particles are composed of skeletons and tests precipitated by planktonic plants and animals living in the shallowest waters of the ocean. Biogenic particles composed of calcium carbonate or opaline silica make up the majority of oceanic sediments and are deposited at great distances from land. Deposition of biogenic sediments is controlled by two factors: the biological productivity of surface waters and the dissolution of biogenic particles by corrosive bottom waters. Increased atmospheric carbon dioxide levels lead to an increase in the corrosiveness of bottom waters.

Biogenic sediment particles are produced in shallow, well-lighted surface waters as a result of the biochemical activity of microscopic plants and animals, which precipitate solid shells and tests formed from minerals dissolved in seawater. Biological productivity is a measure of the number of organisms present and their rate of reproduction. The productivity of planktonic organisms is directly related to chemical and physical conditions in the surface waters. High-productivity waters have abundant supplies of oxygen, with dissolved nutrients and chemicals needed for the formation of shells and tests, and they have enough light for photosynthesis by plants. Generally, high productivity is found in warm-water areas with abundant dissolved oxygen and nutrients.

When planktonic plants and animals die, their shells and tests fall through the water column to be deposited on the bottom of the ocean. This “planktonic rain” causes the deposition of seafloor sediments as the biogenic particles produced in the surface waters sink and accumulate on the sea floor, moving chemicals from surface waters to deep waters. The higher the productivity values in surface waters, the greater the supply of biogenic sediment particles to the sea floor. As biogenic particles sink through the water column to the sea floor, they may be dissolved by corrosive seawater. Surface waters are saturated with dissolved calcium carbonate, so most dissolution occurs in deep waters, which are undersaturated with carbonate.

Calcareous sediments are produced by the accumulation of biogenic particles of calcium carbonate that survive the fall through the water and are deposited and buried on the sea floor. Accumulation rates of biogenic sediments are controlled by the balance between surface productivity and deep-water dissolution: The higher the biological productivity in the surface, the greater the number of shells and tests that will sink to the ocean bottom. In certain high-productivity areas associated with the upwelling of cold, nutrient-loaded water masses to the surface, seafloor sediment accumulation rates may be as high as 3 to 5 centimeters (1 to 3 inches) per thousand years. In areas with higher dissolution rates, fewer calcareous particles will survive to be deposited on the ocean floor. In these low-productivity areas, all the carbonate produced at the surface may be dissolved, and sediment accumulation rates may be as low as 1 millimeter (0.03 inches) per million years.

Level of Carbonate Compensation Depth

The carbonate compensation depth (CCD) marks the boundary between carbonate-rich sediments (calcareous oozes and chalks) and carbonate-free sediments (red clays) in the oceans. It is the result of deep-water dissolution rates exceeding the rate of supply of calcium carbonate to the deep sea by surface productivity. The depth of the CCD marks the zone in which the supply of carbonate sinking in the planktonic rain from surface waters is exactly balanced by the rate of removal of carbonate dissolution in deep waters. Calcareous sediments will be deposited on the sea floor in shallower water depths than the CCD because in these areas, the rate of calcium carbonate supply is higher than the dissolution rate. Individual calcium carbonate particles will survive the trip through the water column in these areas and be deposited as biogenic sediments on the sea floor.

Below the compensation depth, dissolution exceeds the rate of supply, so all carbonate particles supplied from the surface become dissolved, and seafloor sediments are carbonate-free. Surface sediments in water depths below the CCD tend to be red clays, or combinations of fine-grained materials derived from continental sources and carried to the deep sea by wind, mixed with micrometeorites and other particles from extraterrestrial sources. Red clays generally lack fossils, as a result of complete dissolution of carbonate and opaline silica, so only a few dissolution-resistant fossils, such as phosphatic fish teeth and whale ear bones, are found in these sediments. The reddish-brown color of deep-sea clay deposits is a result of the presence of iron particles, which have reacted with oxygen in seawater to form the rust-brown color.

Much information on the past history of the compensation depth in different ocean basins has been provided by ocean-floor drilling programs. The level of the CCD has changed dramatically throughout geologic history, with fluctuations of up to 2,000 meters (6562 feet) being recorded in deep-sea sediments. Changes in the CCD are believed to be caused by changes in either the rate of supply of carbonate to the oceans or the rate of carbonate dissolution in the deep sea, which may be caused by changes in the shape of ocean basins, by changes in the location of carbonate deposition within different ocean basins, or by changes in the concentration of carbon dioxide in the atmosphere. During the last 100,000 years, carbonate sediments formed at greater depths during glacial intervals than during interglacials, indicating a less corrosive deep-water environment resulting from lower concentrations of atmospheric carbon dioxide.

Calcium Carbonate Deposition

Calcium carbonate is delivered to the oceans by rivers carrying particles eroded from continental rocks. Ocean sediments are the primary geochemical reservoir for calcium carbonate, so most of the calcium carbonate on Earth remains dissolved in ocean water or in the form of calcareous sediments on the sea floor. The amount of carbonate deposition on the sea floor depends on the input of calcium carbonate derived from continental weathering and delivered to the oceans by rivers. Because oceanic plankton can precipitate solid calcium carbonate at a much faster rate than the rate of input of dissolved carbonate from rivers, most of the calcium carbonate that is deposited in the oceans must dissolve in order to maintain the chemical balance between dissolved carbonate and solid carbonate in oceanic sediments.

Any changes in the locations of carbonate deposition may cause corresponding changes in the level of the CCD as the compensation depths in different oceans change as a result of bathymetric fractionation or basin-basin fractionation. In bathymetric fractionation, a balance is established between the rates of carbonate deposition in shallow-water and deep-water sedimentary basins. Greater deposition of calcium carbonate in shallow waters atop the continental shelves will cause a shallowing of the CCD as more deep-water carbonate deposits are dissolved so as to balance the shallow-water deposition. Similarly, the level of the compensation depth may vary by basin-basin fractionation of carbonate, which establishes a balance between the compensation depths in different ocean basins. For example, greater deposition of calcium carbonate in the Pacific Ocean will cause the Pacific CCD to become deeper, while at the same time the Atlantic compensation depth must become shallower because greater deposition of carbonate in Pacific sediments will leave less dissolved carbonate for precipitation in the Atlantic Ocean.

Even within an ocean basin, the level of the CCD may vary, depending on the balance between carbonate productivity and dissolution in a local area. For example, in the equatorial Pacific Ocean, the compensation depth is 500 to 800 meters (1640 to 2625 feet) deeper than in areas immediately to the north and south of this high-productivity area as a result of the greater supply of carbonate in the planktonic rain below these high-productivity surface waters. Also, the compensation depth tends to shoal near the edges of ocean basins because higher biological productivity in shallow water near the continents causes rapid sinking of large amounts of organic carbon produced by planktonic plants and animals. Breakdown of this organic carbon by seafloor bacteria produces increased amounts of dissolved carbon dioxide gas, which reacts with water molecules to form carbonic acid; carbonic acid is corrosive to solid calcium carbonate. Greater carbonic acid concentrations lead to increased carbonate dissolution in bottom waters and cause upward migration of the CCD into shallower waters.

Study of Carbonate Compensation Depths

Carbonate compensation depths may be studied by obtaining a series of deep-sea sediment samples from different depths within an ocean basin to determine the relationship between sediment type and water depth. In 1891, a study was published describing the global patterns of seafloor sediment type in each ocean basin, based on sediment samples obtained on the HMS Challenger oceanographic expedition. It was discovered that virtually no calcium carbonate was present below depths of 4,500 meters (14,7645 feet) as a result of dissolution of carbonate.

The CCD in the modern ocean was first described in detail in 1935, based on sediment core transects taken across the South Atlantic Ocean by the 1925-1927 German Meteor expedition. Similar studies of sediment cores from different depths in the Pacific Ocean revealed that calcareous oozes are common seafloor sediments in water depths to 4,400 meters (14,436 feet), with noncalcareous red clays being found in surface sediments deeper than 4,400 meters (14,436 feet). Cores with calcareous oozes underlying red clays were obtained, however, in depths well below the present CCD, indicating that this geochemical boundary has migrated vertically throughout geologic history.

One innovative experiment to measure the rate of calcium carbonate dissolution with increasing water depth involved the placement of a stationary mooring for a period of months in deep water in the Pacific Ocean. Calcite spheres and calcareous microfossils were hung in permeable nylon bags at different water depths on the mooring; the nylon bags allowed seawater to come in contact with the calcium carbonate and thus permitted carbonate dissolution to occur. By measuring the weight loss of spheres suspended for a few weeks to months on the mooring, the rate of dissolution was determined for different water depths. In this experiment, little carbonate dissolution was observed at water depths shallower than 3,700 meters (12,139 feet), while a rapid transition from minimal dissolution to extreme dissolution was seen. Rapid loss of carbonate by dissolution occurred in carbonate spheres suspended between the lysocline (the depth at which carbonate dissolution first begins to occur) and the compensation depth. Below 4,500 meters (14,764 feet), the carbonate compensation depth, all carbonate was removed within a matter of weeks, demonstrating the ability of bottom waters to dissolve calcium carbonate.

Information on compensation depths may also be provided from microfossils preserved in seafloor sediments. Calcite dissolution can be measured by enrichment of dissolution-resistant forms of planktonic organisms, by benthic-planktonic foraminiferal ratios (foraminifera are one-celled animals that secrete a calcium carbonate internal test), by fragmentation indices (the percentage of broken planktonic tests compared to whole tests), or by the coarse-fraction ratios of seafloor sediments. Different microfossils will have differing susceptibilities to dissolution, depending on the thickness of the walls of the microfossil tests. Thin-walled plankton will be more dissolution-susceptible, while thicker-walled tests will resist dissolution. Thin-walled planktonic tests in seafloor sediments deposited in water depths between the lysocline and the CCD will be removed by dissolution more rapidly than thicker-walled tests, leading to greater enrichment of dissolution-resistant fossils with greater carbonate dissolution. Relative dissolution rates may be measured by the ratio between dissolution-susceptible and dissolution-resistant planktonic shells in seafloor sediments.

Similarly, the deeper-water bottom-dwelling benthic foraminifera tend to have thicker walls than the tests of planktonic foraminifera, which live floating in the shallow surface waters. Seafloor sediments may contain both benthic and planktonic varieties of foraminifera. Dissolution of calcium carbonate will preferentially remove the thinner-walled planktonic foraminifera, thus leading to enrichment in the proportion of benthic forms remaining in the sediment. The relative amount of carbonate dissolution in sediments may be determined by measuring the proportion between benthic and planktonic foraminifera in sediment samples.

Finally, fragmentation of planktonic tests may provide an indication of dissolution of carbonate from marine sediments. The percentage of broken tests to whole tests will increase with greater dissolution of carbonate. Also, coarse-fraction percentages of sediments provide information on carbonate dissolution, because unbroken foraminiferal tests are sand-sized, with particle diameters greater than 63 microns in size (1 millimeter is equal to 1,000 microns). As foraminifera are dissolved, they tend to break into smaller fragments, so dissolution tends to break down sand-sized particles into smaller silt-sized fragments. By measuring the proportion between sand-sized and silt-sized particles (the coarse-fraction percentage) in calcareous sediments, it is possible to obtain an indicator of the relative amount of dissolution that has affected those sediment deposits. The more dissolution that has occurred, the smaller will be the percentage of coarse (sand-sized) sediment particles.

Depth Estimates

All these methods provide similar depth estimates for the CCD at approximately 4,500 meters (14,764 feet) below the sea surface, about halfway between the crests of the midocean ridges and the abyssal plains. Individual compensation depths may vary in the different ocean basins of the world, however, as a result of basin-basin fractionation of carbonate. For example, in the Pacific Ocean, the CCD is typically between 4,200 and 4,500 meters (13,780 and 14,764 feet), while in the Atlantic and Indian Oceans the CCD is deeper, being found near a depth of 5,000 meters (16,404 feet). Even within an ocean basin, the level of the CCD may vary, depending on the balance between carbonate productivity and dissolution in a local area. In the centers of the North and South Pacific Oceans, the average compensation depth is between 4,200 and 4,500 meters (13,780 and 14,764 feet), while near the equator it is found near 5,000 meters (16,404 feet) because of the higher biological productivity of equatorial surface waters.

Analyses of deep-sea cores drilled by the Glomar Challenger have revealed that the level of the carbonate compensation depth has changed by up to 2,000 meters in the South Atlantic, Indian, and Pacific Ocean basins. For example, one of the results of Deep Sea Drilling Project Leg 2 was the discovery of significant vertical excursions in the compensation depth of the Atlantic Ocean. In seafloor boreholes drilled in the North Atlantic, 2- to 5-million-year-old calcareous ooze sediments were found atop older (5- to 23-million-year-old) red clays, which in turn were deposited atop carbonate deposits older than 23 million years. In order for these sediments to have accumulated in that order, large vertical changes must have occurred in the compensation depth, starting when the sea floor was shallower than the CCD more than 23 million years ago. Red clays were deposited between 23 and 5 million years ago, when the CCD became shallower. After these red clays accumulated, deepening of the compensation depth allowed the deposition of younger calcareous sediments atop the carbonate-free red clays.

In order to study the past history of the compensation depth within an ocean basin, it is necessary to obtain a series of cores that were deposited at different paleodepths at the same time in the past. (The paleodepth is the depth at which ancient seafloor sediments were deposited.) Paleodepth estimates for sea floor sediments are calculated by studying the cooling history of seafloor basement rocks. After new seafloor is produced by volcanic activity at the midocean ridge system, these rocks cool and contract, thus sinking to greater water depths as they move away from the ridge system. The older the sea floor, the greater its water depth will be, so sediments deposited atop volcanic sea floor will accumulate in progressively deeper water. Once a series of sediment cores deposited at the same time in the past has been obtained, it is possible for the oceanographer to determine the paleodepth of the compensation depth by finding the paleodepth below which no calcium carbonate is present in ancient sediment deposits.

Principal Terms

deposition: the process by which loose sediment grains fall out of seawater to accumulate as layers of sediment on the sea floor

paleodepth: an estimate of the water depth at which ancient seafloor sediments were originally deposited

plankton: microscopic marine plants and animals that live in the surface waters of the oceans; these floating organisms precipitate the particles that sink to form biogenic marine sediments

precipitation: the formation of solid mineral crystals from their chemical components that are dissolved in water

productivity: the rate at which plankton reproduce in surface waters, which in turn controls the rate of precipitation of calcareous or siliceous shells or tests by these organisms

red clays: fine-grained, carbonate-free sediments that accumulate at depths below the CCD in all ocean basins; their red color is caused by the presence of oxidized fine-grained iron particles

test: an internal skeleton or shell precipitated by a one-celled planktonic plant or animal

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