Carbonate compensation depth

The carbonate compensation depth (abbreviated CCD , from English "carbonate compensation depth") denotes an interface in the deep sea , below which no carbonate sludge - the starting material for a certain type of limestone - is deposited. The cause is the dissolution of the particles of calcium carbonate ( calcite and aragonite ) that have sunk to the seabed as a result of the low temperatures, the high pressure and the relatively high carbon dioxide concentration in the deep water.

The carbonate solution already begins below the so-called lysocline , which is several hundred meters above the carbonate compensation depth. The carbonate compensation depth is that interface from which the dissolution rate of carbonates is just as great as the entry rate, or, to put it another way, from which the carbonate solution completely compensates for the carbonate entry. Between the lysocline and the carbonate compensation depth, the dissolution rate is even lower than the entry rate.

Physico-chemical background

Deep-sea carbonate sludge consists mainly of microscopic shells of planktonic single cells or the components of these shells (here the coccosphere of a coccolithophore composed of coccoliths ). However, these are completely dissolved below the carbonate compensation depth.

Both inland waters and the oceans usually have a gradient with a concentration of dissolved carbon dioxide that increases with increasing depth . In the seas, this gradient persists, whereas in inland waters it arises and disappears with the change of the seasons (see ecosystem lake ). Its cause is biogenic, i.e. H. Caused by life processes in the sea: Near the surface, carbon dioxide is bound as biomass through photosynthesis by algae and distributed to the food chain . The carbon that is organically bound in biomass can sink into the deeper layers of water as a rain of particles. There the biomass is usually destroyed again by bacteria and breathed into carbon dioxide . This gradually leads to an accumulation of CO ₂ in the depths and thus to the gradient mentioned.

In addition, the solubility of calcium carbonate is temperature and pressure dependent: the lower the temperature and the higher the pressure, the better the solubility. Because not only the CO ₂ concentration but also the pressure (see hydrostatic pressure ) increases with increasing depth in the ocean , but the temperature decreases, the deep water in the oceans is undersaturated with calcium carbonate, which leads to the calcium carbonate being in it Deep reaches, is resolved by reacting with the CO ₂ to form dissolved calcium hydrogen carbonate according to the following reaction equation:

{\ displaystyle \ mathrm {CaCO_ {3} + CO_ {2} + H_ {2} O \ \ rightleftharpoons \ Ca ^ {2 +} (aq) +2 \ HCO_ {3} ^ {-} (aq)}}

The depth of the carbonate compensation depth is neither the same in all oceans nor within a single ocean basin. It is deeper in the Atlantic than in the Pacific and at a lower depth at higher latitudes than at lower latitudes. This is due, among other things, to the fact that the CO ₂ concentration in deep water depends on the degree of mixing with water with less CO ₂ from lower depths, which in turn is influenced by the course of the ocean currents and the relief of the seabed. Also in lower latitudes the carbonate production in the uppermost area of the water column is greater than in high latitudes, that is, there the entry rate can exceed the dissolution rate of calcium carbonate even at a relatively great depth. The carbonate compensation depth in the Atlantic is 4,500 to 5,000 m at low latitudes, and 4,000 to 4,500 m in the Pacific.

Furthermore, the depth of the carbonate compensation depth also changed in the course of the earth's history . During the Cenozoic , it sank in the tropical Pacific from 3,000 to 3,500 m to 4,500 m today. This is primarily explained by the fact that in the early Cenozoic, larger amounts of CO _{2 were available} for the carbonate solution than today (see acidification of the seas ). Correspondingly, the lowering of the carbonate compensation depth correlates with an increasing cooling of the world climate and thus of the seas (see Cenozoic Ice Age ) as well as a proven general increase in the continental carbonic acid weathering of silicate minerals during the Cenozoic era.

Aragonite compensation depth

Stages in the dissolution of a pteropod housing, the main component of the aragonite fraction of carbonate deep-sea mud

Aragonite is more soluble in water than calcite . As a result, aragonite begins to dissolve at a shallower depth than calcite, which means that the interface from which the dissolution rate of aragonite is at least as great as the entry rate is higher up in the water column than the corresponding interface for calcite. This interface is called aragonite compensation depth (abbreviated ACD , from English "aragonite compensation depth"). At low latitudes, it is around 1,500 m deep on average, with 2,000 to 3,000 m in the Atlantic and between 500 and 1,500 m in the Ind and Pacific.

Calcite compensation depth

The calcite compensation depth (also abbreviated with CCD , from English "calcite compensation depth") is synonymous with the carbonate compensation depth, because calcite is the only other variety of calcium carbonate that is formed in significant amounts by planktonic marine life and form carbonatic deep-sea deposits can. As a result, only carbonate-free deep-sea sediments can be found below the CCD. These are primarily the red deep-sea clay and sludge made of low-water, amorphous silicon dioxide ( opal ). The latter arise from the housings of micro-organisms such as diatoms or Strahlentierchen (Diatomeenschlamm or Radiolarienschlamm).

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General

Paul R. Pinet: Invitation to Oceanography. 5th edition. Jones and Bartlett Publishers, 2009, ISBN 978-0-7637-5993-3 , pp. 118 ff., 165 ff.
Carbonate compensation depth in the spectrum online encyclopedia of geosciences
L. Bruce Railsback: Variation in concentration of solutes in the oceans IIIa: carbon dioxide and the carbonate compensation depth (CCD). Some Fundamentals of Mineralogy and Geochemistry. University of Georgia Sedimentary Geochemistry Laboratory, Athens (GA) 2008 ( PDF 20 kB, JPG 630 kB)

Individual evidence

↑ Heiko Pälike, Mitchell W. Lyle, Hiroshi Nishi and 62 other authors: A Cenozoic record of the equatorial Pacific carbonate compensation depth. Nature. Vol. 488, 2012, pp. 609–614, doi: 10.1038 / nature11360 (alternative full text access : NIO ).

^ WH Berger: Deep-sea carbonate: pteropod distribution and the aragonite compensation depth. Deep Sea Research. Vol. 25, No. 5, 1978, pp. 447-452, doi: 10.1016 / 0146-6291 (78) 90552-0 .

[1] Heiko Pälike, Mitchell W. Lyle, Hiroshi Nishi and 62 other authors: A Cenozoic record of the equatorial Pacific carbonate compensation depth. Nature. Vol. 488, 2012, pp. 609–614, doi: 10.1038 / nature11360 (alternative full text access : NIO ).

[2] WH Berger: Deep-sea carbonate: pteropod distribution and the aragonite compensation depth. Deep Sea Research. Vol. 25, No. 5, 1978, pp. 447-452, doi: 10.1016 / 0146-6291 (78) 90552-0 .